Non-aqueous electrolyte and secondary battery

Abstract

This non-aqueous electrolytic solution contains: LiN(FSO2)2 serving as an electrolyte; and at least one compound selected from the group consisting of silicon-atom-containing compounds represented by general formula (2): Si(R5)2(OR6)2, boron-atom-containing compounds represented by general formula (3): B(OR6)3, carbon-atom-containing compounds represented by general formula (4): C(=O)(R5)(OR6), sulfur-atom-containing compounds represented by general formula (5): (S(=O)2(R5)y)x(OR6)x(2−y), and phosphorus-atom-containing compounds represented by general formula (6): P(=O)(R5)y(OR6)2, the at least one compound serving as an additive.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte and a secondary battery. [Background technology]

[0002] Various nonaqueous electrolytes for use in secondary batteries, such as lithium-ion secondary batteries, have been investigated to improve their performance. For example, Patent Document 1 proposes a nonaqueous electrolyte (nonaqueous electrolyte solution) containing an imide salt such as lithium bis(fluorosulfonylimide), glyme, and a fluorinated ether, and further containing lithium nitrate. Patent Document 2 proposes a nonaqueous electrolyte solution containing lithium bisfluorosulfonylimide and a trimethylsilyl phosphate (TMSPa) additive. Patent Document 3 proposes a nonaqueous electrolyte solution containing lithium hexafluorophosphate and a boroxine compound. Patent Document 4 proposes an electrolyte solution for a nonaqueous electricity storage device containing a nonaqueous solvent; a lithium salt; a sulfur-containing compound; and a compound in which at least one hydrogen atom of an acid selected from the group consisting of a protonic acid, a sulfonic acid, and a carboxylic acid having an atom selected from the group consisting of a phosphorus atom and a boron atom is substituted with a trimethylsilyl group or the like. Patent Document 5 proposes an electrolyte additive for a non-aqueous electricity storage device, which contains a compound that has 2 to 4 P atoms in one molecule and a structural unit represented by a specific formula. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-67501 A [Patent Document 2] Special Publication No. 2017-529671 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-152140 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-156372 [Patent Document 5] Japanese Patent Application Publication No. 2016-192401 Summary of the Invention [Problem to be solved by the invention]

[0004] Through previous studies, the present inventors have discovered that a non-aqueous electrolyte containing a sulfonylimide compound such as lithium bis(fluorosulfonyl)imide as an electrolyte salt improves the high-temperature durability and charge / discharge cycle performance of lithium-ion secondary batteries. Furthermore, they have found that a battery using a non-aqueous electrolyte containing this sulfonylimide compound has the advantageous effect of lowering the charge transfer resistance (impedance) and the direct current resistance (DCR) at low temperatures (e.g., −30° C.) compared to a battery using a non-aqueous electrolyte containing only a lithium compound other than a sulfonylimide compound (e.g., LiPF, LiBF, etc.) as the electrolyte.

[0005] However, in the nonaqueous electrolyte described in Patent Document 1, although the charge transfer resistance is reduced by adding lithium nitrate, there is still room for improvement in terms of high-temperature durability. In addition, in the nonaqueous electrolyte described in Patent Document 2, the addition of trimethylsilyl phosphate [tris(trimethylsilyl) phosphate] improves battery performance, but is insufficiently effective in reducing charge transfer resistance and battery DC resistance at low temperatures. Patent Documents 3 to 5 do not even consider nonaqueous electrolytes containing sulfonylimide compounds. In addition, in the electrolyte described in Patent Document 5 containing a phosphorus compound having a trimethylsilyl group, although the increase in DC resistance during high-temperature cycling is suppressed, the effect is insufficient, and there is also the problem of reduced battery performance during charge-discharge cycling at low temperatures.

[0006] As described above, the nonaqueous electrolytes described in Patent Documents 1 to 5 do not provide sufficient battery performance, and there is room for improvement.

[0007] The present disclosure has been made in view of the above points, and an object of the present disclosure is to provide a nonaqueous electrolyte solution containing a sulfonylimide compound that can improve battery performance, and a secondary battery including the nonaqueous electrolyte solution. [Means for solving the problem]

[0008] As a result of further investigation, the present inventors have found that batteries using a non-aqueous electrolyte containing a sulfonylimide compound have a larger self-discharge rate from a fully charged state than batteries using a non-aqueous electrolyte containing only a lithium compound other than a sulfonylimide compound (e.g., LiPF6, LiBF4, etc.) as the electrolyte salt, and that there is room for improvement in the storage characteristics of the batteries. The present inventors have also found that adding lithium nitrate to a non-aqueous electrolyte containing a sulfonylimide compound increases the self-discharge rate during battery storage.

[0009] These findings were first discovered by the present inventors, and Patent Documents 1 to 5 and other documents make no mention of the self-discharge of batteries using non-aqueous electrolytes containing sulfonylimide compounds (especially lithium bis(fluorosulfonyl)imide).

[0010] Even when trimethylsilyl phosphate (tris(trimethylsilyl) phosphate) described in Patent Document 2 was added to a non-aqueous electrolyte solution containing a sulfonylimide compound, the self-discharge suppression effect was insufficient.

[0011] Furthermore, Patent Document 4 describes the suppression of battery self-discharge by adding tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, or the like to a nonaqueous electrolyte containing LiPF6. Patent Document 4 uses the charge capacity during 240 hours of constant-voltage charging at 50°C and 4.55 V as an indicator of battery self-discharge. However, this indicator assumes that self-discharge occurs due to the side reaction of oxidative decomposition of the electrolyte at the positive electrode during 240 hours of continuous charging at 50°C and 4.55 V. This indicator evaluates the durability of the electrolyte and differs from the original indicator, which evaluates self-discharge as charge consumption in the absence of an electrical load between the positive and negative electrodes. When self-discharge was actually evaluated at high temperatures, the self-discharge suppression effect was insufficient.

[0012] In order to improve the storage characteristics and performance of the battery, the disclosed technology uses a specific compound in a non-aqueous electrolyte solution containing a sulfonylimide compound to suppress self-discharge of the battery and reduce the charge transfer resistance (impedance) and the battery direct current resistance (DCR).

[0013] The nonaqueous electrolyte solution of the present disclosure is LiN(FSO2)2 (sulfonylimide compound) as the electrolyte, As an additive Teichi General formula (14):Si(R 5 )4(14) (In formula (14), R 5 teeth an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt, or a hydrogen atom indicates.) a silicon atom-containing compound represented by the formula: General formula (3):B(OR 6 )3(3) (In formula (3), R 6 teeth R 5 ) a boron atom-containing compound represented by the formula: General formula (4):C(=O)(R 5 )(OR 6 ) (4) (In formula (4), R 5 and R6 teeth Same or different Indicates the same as above.) a carbon atom-containing compound represented by General formula (5):{S(=O)2(R 5 ) y} x (OR 6 ) x(2-y) (5) (In formula (5), R 5 and R 6 teeth Same or different It is the same as above, x is an integer of 1 or 2, and y is an integer of 0 or 1. When x is 2 and y is 1 (OR 6 )2 is (-OR 6 -O-), and formula (5) is S(=O)2(R 5 )-OR 6 -OS(=O)2(R 5 ) indicates. and a sulfur atom-containing compound represented by the formula: General formula (6):P(=O)(R 5 ) y (OR 6 )2(6) (In formula (6), R 5 and R 6 are the same or different This shows the same as above. y is the same as above. When the formula (6) is a polymerizable group, y is 0, and (OR 6 ) is (-O-), and formula (6) is [-P(=O)(OR 6 )O-] n indicates.) The non-aqueous electrolyte solution contains at least one phosphorus atom-containing compound selected from the group consisting of carbon dioxide (CO2), carbon monoxide (CO), bicarbonate ions (HCO3 - ) and carbonate ions (CO3 2- ) may be dissolved in the solution.

[0014] The nonaqueous electrolyte solution of the present disclosure is LiN(FSO2)2 as the electrolyte, As an additive Before A silicon atom-containing compound represented by the following general formula (14): a boron atom-containing compound represented by the general formula (3), a carbon atom-containing compound represented by the general formula (4), a sulfur atom-containing compound represented by the general formula (5), a phosphorus atom-containing compound represented by the general formula (6), General formula (7):P(OR 6 )3(7) (In formula (7), R 6 indicates the same as above.) a phosphorus atom-containing compound represented by General formula (8):P(=O)(OR 6 )3(8) (In formula (8), R 6 indicates the same as above.) and a phosphorus atom-containing compound represented by the formula: General formula (9):

[0015] [ka]

[0016] (In formula (9), R 7 are the same or different and represent an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), or a cycloalkyl group (which may have a substituent). and at least one boroxine compound selected from the group consisting of boroxine compounds represented by Carbon dioxide (CO2), carbon monoxide (CO), bicarbonate ion (HCO3 - ) and carbonate ions (CO3 2- ) is dissolved.

[0017] The non-aqueous electrolyte contains carbon dioxide (CO2), carbon monoxide (CO), bicarbonate ions (HCO3 - ) and carbonate ions (CO3 2- ) may be dissolved in a total amount of 20 mass ppm or more.

[0018] The nonaqueous electrolyte solution is such that the electrolyte is General formula (10): LiPF a (C m F 2m+1 ) 6-a (a:0≦a≦6, m:1≦m≦4) (10) A compound represented by General formula (11): LiBF b (C n F 2n+1 ) 4-b (b:0≦b≦4, n:1≦n≦4) (11) A compound represented by the formula: It may further contain at least one selected from the group consisting of LiAsF6.

[0019] The non-aqueous electrolyte is General formula (12):M 1 NO3(M 1 : indicates an alkali metal element.) (12) A compound represented by the formula: General formula (13):

[0020] [ka]

[0021] (In general formula (13), M 3 :B or P, A f+ : metal ion, H or onium ion, f: 1≦f≦3, g: 1≦g≦3, h: g / f, i: 1≦ i ≦3, j: 0≦j≦4, k: 0 or 1, R 3 : an alkylene group having 1 to 10 carbon atoms or a halogenated alkylene group having 1 to 10 carbon atoms, R 4 : F or a fluorinated alkyl group having 1 to 10 carbon atoms, T 1 , T 2 : each independently represents O or S. The compound may further contain at least one selected from the group consisting of compounds represented by the following formula:

[0022] The nonaqueous electrolyte solution of the present disclosure contains an electrolyte and at least one additive selected from the group consisting of ethyl polyphosphate, (triisopropylsilyl) polyphosphate, and (tert-butyl)dimethylsilyl] polyphosphate.

[0023] The secondary battery of the present disclosure uses the nonaqueous electrolyte solution described above. [Effects of the Invention]

[0024] According to the present disclosure, it is possible to provide a nonaqueous electrolyte solution containing a sulfonylimide compound that can improve battery performance, and a secondary battery including the nonaqueous electrolyte solution. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 shows the 31P-NMR spectrum of trimethylsilyl polyphosphate (Compound M), a reagent used in Example 5-2 series. [Figure 2] FIG. 2 shows the 31P-NMR spectrum of trimethylsilyl polyphosphate (compound M2) synthesized in Example 5-2(2) series. [Figure 3] FIG. 3 shows the 31P-NMR spectrum of trimethylsilyl polyphosphate (compound M3) synthesized in Example 5-2(3) series. DETAILED DESCRIPTION OF THE INVENTION

[0026]

[0023] The following detailed description of preferred embodiments of the present disclosure is merely exemplary in nature and is in no way intended to limit the present disclosure, its application, or its uses.

[0027] <First non-aqueous electrolyte> The first nonaqueous electrolyte solution according to this embodiment contains an electrolyte and a specific compound.

[0028] (electrolyte) The electrolyte has the general formula (1): [C3] LiN(R 1 SO2)(R 2 SO2) (1) The sulfonylimide compound (hereinafter referred to as "sulfonylimide compound (1)") is a fluorine-containing sulfonylimide salt.

[0029] In general formula (1), R 1 and R 2 are the same or different (independently of each other) and represent a fluorine atom, an alkyl group having 1 to 6 carbon atoms or a fluoroalkyl group having 1 to 6 carbon atoms.

[0030] Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a pentyl group, and a hexyl group. Among the alkyl groups having 1 to 6 carbon atoms, a linear or branched alkyl group having 1 to 6 carbon atoms is preferred, and a linear alkyl group having 1 to 6 carbon atoms is more preferred.

[0031] Examples of the fluoroalkyl group having 1 to 6 carbon atoms include alkyl groups having 1 to 6 carbon atoms in which some or all of the hydrogen atoms have been substituted with fluorine atoms. Examples of the fluoroalkyl group having 1 to 6 carbon atoms include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, and a pentafluoroethyl group. In particular, the fluoroalkyl group may be a perfluoroalkyl group.

[0032] Substituent R 1 and R 2 As the substituent R, a fluorine atom and a perfluoroalkyl group (for example, a perfluoroalkyl group having 1 to 6 carbon atoms such as a trifluoromethyl group, a pentafluoroethyl group, a heptafluoropropyl group, etc.) are preferred, a fluorine atom, a trifluoromethyl group, and a pentafluoroethyl group are more preferred, a fluorine atom and a trifluoromethyl group are even more preferred, and a fluorine atom is even more preferred. 1 and R 2 may be the same or different from each other.

[0033] Examples of sulfonylimide compounds (1) include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2, hereinafter also referred to as "LiFSI"), lithium bis(trifluoromethylsulfonyl)imide (LiN(CF3SO2)2, hereinafter also referred to as "LiTFSI"), lithium (fluorosulfonyl)(methylsulfonyl)imide, lithium (fluorosulfonyl)(ethylsulfonyl)imide, lithium (fluorosulfonyl)(trifluoromethylsulfonyl)imide, lithium (fluorosulfonyl)(pentafluoroethylsulfonyl)imide, lithium (fluorosulfonyl)(heptafluoropropylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, and lithium bis(heptafluoropropylsulfonyl)imide. The sulfonylimide compounds (1) may be used alone or in combination of two or more. Furthermore, commercially available sulfonylimide compounds (1) may be used, or those synthesized by conventional methods may be used.

[0034] Among the sulfonylimide compounds (1), LiN(FSO2)2 and LiN(CF3SO2)2 are preferred, and LiN(FSO2)2 is more preferred, from the viewpoints of reducing the impedance and DCR of the battery and improving the low-temperature charge-discharge characteristics and charge-discharge cycle characteristics. In other words, the nonaqueous electrolyte preferably contains at least one of LiN(FSO2)2 and LiN(CF3SO2)2, and more preferably contains LiN(FSO2)2.

[0035] The electrolyte may contain the sulfonylimide compound (1), but may also contain other electrolytes (electrolytes other than the sulfonylimide compound (1)). Examples of other electrolytes include imide salts and non-imide salts.

[0036] Examples of imide salts include fluorine-containing sulfonylimide salts other than sulfonylimide compound (1) (hereinafter referred to as "other sulfonylimide compounds"). Examples of other sulfonylimide compounds include non-lithium salts of the fluorine-containing sulfonylimides listed as sulfonylimide compound (1) (for example, salts in which the lithium (ion) in sulfonylimide compound (1) is substituted with a cation other than lithium ion). Examples of salts in which a cation other than lithium ion is substituted include alkali metal salts such as sodium salt, potassium salt, rubidium salt, and cesium salt; alkaline earth metal salts such as beryllium salt, magnesium salt, calcium salt, strontium salt, and barium salt; aluminum salt; ammonium salt; and phosphonium salt. The other sulfonylimide compounds may be used alone or in combination of two or more. In addition, commercially available products may be used as the other sulfonylimide compounds, or those synthesized by conventionally known methods may be used.

[0037] Examples of the non-imide salt include salts of non-imide anions and cations (lithium ions and the above-mentioned cations). [C4] LiPF a (C m F 2m+1 ) 6-a (a:0≦a≦6, m:1≦m≦4) (10) (hereinafter referred to as "fluorophosphate compound (10)"), a compound represented by general formula (11): [5] LiBF b (C n F 2n+1 ) 4-b (b:0≦b≦4, n:1≦n≦4) (11) (hereinafter referred to as "fluoroborate compound (11)"), lithium hexafluoroarsenate (LiAsF6), LiSbF6, LiClO4, LiSCN, LiAlF4, CF3SO3Li, LiC[(CF3SO2)3], LiN(NO2), LiN[(CN)2 ]and non-lithium salts (for example, salts in which the lithium (ion) in these lithium salts is substituted with the cations exemplified above (for example, NaBF4, NaPF6, NaPF3(CF3)3, etc.)). The non-imide salts may be used alone or in combination of two or more. Furthermore, the non-imide salts to be used may be commercially available products, or may be those obtained by synthesis by a conventionally known method.

[0038] Among the other electrolytes, non-imide salts are preferred from the viewpoints of ionic conductivity, cost, etc., and fluorophosphate compound (10), fluoroborate compound (11), and LiAsF are preferred, with fluorophosphate compound (10) being more preferred.

[0039] Examples of the fluorophosphate compound (10) include LiPF, LiPF(CF), LiPF(C,F), LiPF(C,F), LiPF(C,F), LiPF(C,F) etc. Among the fluorophosphate compounds (10), LiPF and LiPF(C,F) are preferred, with LiPF being more preferred.

[0040] Examples of the fluoroboric acid compound (11) include LiBF, LiBF(CF), LiBF(C,F), LiBF(C,F) and the like. Among the fluoroboric acid compounds (11), LiBF and LiBF(CF) are preferred, and LiBF is more preferred.

[0041] These electrolytes (sulfonylimide compound (1), other electrolytes, etc.) may be present (contained) in the form of ions in the non-aqueous electrolyte solution.

[0042] The concentration of sulfonylimide compound (1) in the nonaqueous electrolyte is preferably 0.01 mol / L or more, more preferably 0.05 mol / L or more, even more preferably 0.1 mol / L or more, even more preferably 0.2 mol / L or more, and even more preferably 0.5 mol / L or more from the viewpoint of reducing the impedance and DCR of the battery. In addition, the concentration is preferably 5 mol / L or less, more preferably 3 mol / L or less, and even more preferably 2 mol / L or less from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte.

[0043] From the viewpoint of reducing the impedance and DCR of the battery, the content of the sulfonylimide compound (1) in the non-aqueous electrolyte solution is preferably 10 mol % or more, more preferably 20 mol % or more, even more preferably 30 mol % or more, and particularly preferably 50 mol % or more, based on a total of 100 mol % of the electrolyte salts contained in the non-aqueous electrolyte solution.

[0044] The electrolyte salt composition may be an electrolyte salt having a simple salt composition of sulfonylimide compound (1), or an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and another electrolyte. When an electrolyte salt having a mixed salt composition is used, an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and fluorophosphate compound (10) is preferred, an electrolyte salt having a mixed salt composition containing at least one of LiN(FSO2)2 and LiN(CF3SO2)2 and LiPF6 is more preferred, and an electrolyte salt having a mixed salt composition containing LiN(FSO2)2 and LiPF6 is particularly preferred.

[0045] When an electrolyte salt having a mixed salt composition containing sulfonylimide compound (1) and other electrolytes is used, the concentration of each of the other electrolytes in the nonaqueous electrolyte is preferably 0.1 mol / L or more, more preferably 0.2 mol / L or more, and even more preferably 0.5 mol / L or more, from the viewpoint of reducing the impedance and DCR of the battery. Also, from the viewpoint of reducing the impedance and DCR of the battery, the concentration is preferably 1.5 mol / L or less, more preferably 1 mol / L or less, and even more preferably 0.8 mol / L or less.

[0046] The total concentration of the electrolyte in the nonaqueous electrolyte is preferably 0.8 mol / L or more, more preferably 1 mol / L or more, and even more preferably 1.2 mol / L or more from the viewpoint of reducing the impedance and DCR of the battery, and is preferably 5 mol / L or less, more preferably 3 mol / L or less, and even more preferably 2 mol / L or less from the viewpoint of suppressing a decrease in battery performance due to an increase in the viscosity of the electrolyte.

[0047] From the viewpoint of reducing the impedance and DCR of the battery, it is preferable to increase the concentration of sulfonylimide compound (1). The molar ratio of sulfonylimide compound (1) to other electrolytes (the molar ratio of sulfonylimide compound concentration to other electrolyte concentration) is preferably 1:25 or more, more preferably 1:10 or more, even more preferably 1:8 or more, even more preferably 1:5 or more, even more preferably 1:2 or more, particularly preferably 1:1 or more, and is preferably 25:1 or less, more preferably 10:1 or less, even more preferably 5:1 or less, and even more preferably 2:1 or less.

[0048] (specific compounds) The first nonaqueous electrolyte solution contains, as an additive, at least one of the various specific compounds shown below, together with the sulfonylimide compound (1). Thus, the first nonaqueous electrolyte solution contains the sulfonylimide compound (1) in combination with one or more specific compounds, thereby simultaneously suppressing the self-discharge of the battery and reducing the charge transfer resistance (impedance) and the battery direct current resistance (DCR). This means that the storage characteristics and performance of the battery can be improved.

[0049] <Classification by central atom> (Silicon atom-containing compounds) Specific compounds containing a silicon atom as the central atom include ,one General formula (14): [7] Si(R 5 )4(14) (hereinafter referred to as "silicon atom-containing compound (14)")。

[0050] one In general formula (14), R 5 teeth an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom (lithium atom, sodium atom, potassium atom, rubidium atom, etc.), an onium salt, or a hydrogen atom In general formula (14), R 5 is preferably a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), and even more preferably a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded. R 5 are preferably the same groups.

[0051] Specific examples of silicon-containing compounds (14) include tetrakis(trimethylsilyl)silane (R 5 represents a trimethylsilyl group.

[0052] By adding a silicon atom-containing compound (14) to a non-aqueous electrolyte containing a sulfonylimide compound (1), a battery using the electrolyte can reduce the impedance and DCR and also suppress self-discharge, a problem specific to non-aqueous electrolytes containing a sulfonylimide compound (1). Furthermore, by dissolving CO2 or the like in the electrolyte, the effect of adding the silicon atom-containing compound (14) can be further enhanced. In other words, the combined use of the silicon atom-containing compound (14) and the dissolved CO2 or the like can achieve a synergistic effect of suppressing battery self-discharge and reducing the impedance and DCR.

[0053] (Boron atom-containing compounds) Specific compounds containing a boron atom as a central atom include those represented by the general formula (3): [8] B(OR 6 )3(3) Examples of suitable boron-containing compounds include those represented by the following formula (hereinafter referred to as "boron-containing compound (3)").

[0054] In general formula (3), R 6 teeth an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom (lithium atom, sodium atom, potassium atom, rubidium atom, etc.), an onium salt, or a hydrogen atom In general formula (3), R 6 is preferably a linear or branched alkyl group having 1 to 6 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), or a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a linear alkyl group having 1 to 4 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 4 carbon atoms (which may have a substituent), or a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), even more preferably a methyl group, an ethyl group, a butyl group, a trifluoromethyl group, a trifluoroethyl group, or a trimethylsilyl group, and even more preferably a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded. 6 are preferably the same groups.

[0055] Specific examples of the boron atom-containing compound (3) include trimethyl borate (trimethyl borate, R 6 indicates a methyl group.), triethyl borate (triethyl borate. R 6 represents an ethyl group.), triisopropyl borate (triisopropyl borate. R 6 represents an isopropyl group.), tributyl borate (tributyl borate. R 6 represents a butyl group.), tris(2,2,2)trifluoroethyl borate [tris(2,2,2)trifluoroethyl borate. R 6 represents a trifluoroethyl group. ], tris(trimethylsilyl)borate [tris(trimethylsilyl) borate. R 6 represents a trimethylsilyl group.] The boron atom-containing compounds (3) may be used alone or in combination of two or more. Among the boron atom-containing compounds (3), tris(trimethylsilyl)borate is preferred.

[0056] By adding a boron atom-containing compound (3) to a non-aqueous electrolyte containing a sulfonylimide compound (1), a battery using the electrolyte has reduced impedance and DCR, and also suppresses self-discharge, a problem specific to non-aqueous electrolytes containing a sulfonylimide compound (1). Furthermore, by dissolving CO2 or the like in the electrolyte, as described below, the effect of adding the boron atom-containing compound (3) can be further enhanced. In other words, the combined use of the boron atom-containing compound (3) and the dissolved CO2 or the like provides a synergistic effect of suppressing battery self-discharge and reducing impedance and DCR.

[0057] (carbon-containing compounds) Specific compounds containing a carbon atom as a central atom include those of the general formula (4): [9] C(=O)(R 5 )(OR 6 ) (4) (hereinafter referred to as "carbon atom-containing compound (4)") represented by the following formula:

[0058] In general formula (4), R 5 and R 6 teeth Same or different In general formula (4), R 5 is preferably a linear or branched alkyl group having 1 to 6 carbon atoms (which may have a substituent) and a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a linear alkyl group having 1 to 3 carbon atoms (which may have a substituent) and a trifluoroalkyl group having 1 to 3 carbon atoms (which may have a substituent), even more preferably a methyl group and a trifluoromethyl group, and even more preferably a trifluoromethyl group.

[0059] In addition, in the general formula (4), R 6is preferably a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), and even more preferably a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded.

[0060] Specific examples of the carbon atom-containing compound (4) include trimethylsilyl acetate (R 5 is a methyl group, R 6 represents a trimethylsilyl group. ), trimethylsilyl trifluoroacetate (trimethylsilyl trifluoroacetate. R 5 is a trifluoromethyl group, R 6 represents a trimethylsilyl group.) The carbon atom-containing compound (4) may be used alone or in combination of two or more. Among the carbon atom-containing compounds (4), trimethylsilyl trifluoroacetate is preferred.

[0061] By adding a carbon-containing compound (4) to a nonaqueous electrolyte containing a sulfonylimide compound (1), a battery using the electrolyte has reduced impedance and DCR, and also suppresses self-discharge, a problem specific to nonaqueous electrolytes containing a sulfonylimide compound (1). Furthermore, by dissolving CO2 or the like in the electrolyte, as described below, the effect of adding the carbon-containing compound (4) can be further enhanced. In other words, the combined use of the carbon-containing compound (4) and the dissolved CO2 or the like provides a synergistic effect of suppressing battery self-discharge and reducing impedance and DCR.

[0062] (Sulfur atom-containing compounds) Specific compounds containing a sulfur atom as a central atom include those represented by the general formula (5): [C10] {S(=O)2(R 5 ) y} x (OR 6 ) x(2-y) (5) Examples include a sulfur atom-containing compound represented by the following formula (hereinafter referred to as "sulfur atom-containing compound (5)").

[0063] In general formula (5), R 5 and R 6 teeth Same or different x represents an integer of 1 or 2, and y represents an integer of 0 or 1.

[0064] In the general formula (5), when x is 2 and y is 1, (OR 6 )2 is (-OR 6 That is, the sulfur atom-containing compound (5) is represented by the general formula (5a): [C11] S(=O)2(R 5 )-OR 6 -OS(=O)2(R 5 ) (5a) It is also represented as

[0065] In the general formulas (5) and (5a), R 5 is preferably a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 3 carbon atoms (which may have a substituent), and more preferably a trifluoromethyl group.

[0066] In addition, in the general formula (5), R 6is preferably a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent) and a silyl group in which an alkyl group having 1 to 6 carbon atoms (which may have a substituent) is bonded to two alkyl groups having 1 to 6 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (linear, cyclic, etc.), etc.; more preferably a trialkylsilyl group having 1 to 4 carbon atoms (which may have a substituent) and a silyl group in which an alkyl group having 1 to 2 carbon atoms (which may have a substituent) is bonded to two alkyl groups having 1 to 4 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (linear, cyclic, etc.), etc.; even more preferably a trimethylsilyl group and a tertiary (tert-)butyldimethylsilyl group (which may have a substituent), and even more preferably a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 4 carbon atoms refers to a silyl group in which three alkyl groups having 1 to 6 or 1 to 4 carbon atoms are bonded. In general formula (5a), R 6 is preferably a linear or branched dialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a linear dialkylsilyl group having 1 to 4 carbon atoms (which may have a substituent), and even more preferably a ditertiary (tert-)butylsilyl group (which may have a substituent). In this specification, a dialkylsilyl group having 1 to 6 or 1 to 4 carbon atoms refers to a silyl group in which two alkyl groups having 1 to 6 or 1 to 4 carbon atoms are bonded.

[0067] A specific example of the sulfur atom-containing compound (5) is trimethylsilyl trifluoromethanesulfonate (trimethylsilyl trifluoromethanesulfonate). In the general formula (5), R 5 is a trifluoromethyl group, R 6 represents a trimethylsilyl group, and x and y represent 1.); bistrimethylsilyl sulfate (bis(trimethylsilyl) sulfate). In the general formula (5), R 6 represents a trimethylsilyl group, x represents 1, and y represents 0.]; di-tert-butylsilyl bistrifluoromethanesulfonate [di-tert-butylsilyl bis(trifluoromethanesulfonate). In the general formula (5a), R 5is a trifluoromethyl group, R 6 represents a ditertiary butylsilyl group. ]; tertiary butyldimethylsilyl trifluoromethanesulfonate (in general formula (5), R 5 is a trifluoromethyl group, R 6 is a tertiary butyldimethylsilyl group, and x and y are 1. The sulfur atom-containing compounds (5) may be used alone or in combination of two or more. Among the sulfur atom-containing compounds (5), bistrimethylsilyl sulfate and tertiary butyldimethylsilyl trifluoromethanesulfonate are preferred.

[0068] By adding a sulfur atom-containing compound (5) to a nonaqueous electrolyte containing a sulfonylimide compound (1), a battery using the electrolyte can reduce the impedance and DCR and also suppress self-discharge, a problem specific to nonaqueous electrolytes containing a sulfonylimide compound (1). Furthermore, by dissolving CO2 or the like in the electrolyte, as described below, the effect of adding the sulfur atom-containing compound (5) can be further enhanced. In other words, the combined use of the sulfur atom-containing compound (5) and the dissolved CO2 or the like can achieve a synergistic effect of suppressing battery self-discharge and reducing the impedance and DCR.

[0069] (Compound containing phosphorus atom) Specific compounds containing a phosphorus atom as the central atom include those represented by the general formula (6): [C12] P(=O)(R 5 ) y (OR 6 )2(6) a phosphorus atom-containing compound represented by the general formula (7): [C13] P(OR 6 )3(7) (hereinafter referred to as "phosphorus atom-containing compound (7)") represented by the following formula:

[0070] In general formula (6), R 5 and R6 are the same or different This shows the same as above. y is the same as above.

[0071] When the general formula (6) is a polymerizable group (polymer), y is 0 (R 5 There are two (OR 6 ) represents (—O—). That is, the phosphorus atom-containing compound (6) is represented by the general formula (6a): [C14] [-P(=O)(OR 6 )O-] n (6a) It is also represented as

[0072] In general formula (6), R 5 is preferably a hydrogen atom.

[0073] In addition, in the general formulas (6) and (6a), R 6is preferably a linear alkyl group having 1 to 6 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), and a silyl group in which an alkyl group having 1 to 6 carbon atoms (which may have a substituent) is bonded to two alkyl groups having 1 to 6 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (chain, cyclic, etc.), etc., and more preferably a linear alkyl group having 1 to 3 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 3 carbon atoms (which may have a substituent), Examples of the silyl group include a trialkylsilyl group having 1 to 4 carbon atoms (which may have a substituent), and a silyl group in which an alkyl group having 1 to 4 carbon atoms (which may have a substituent) is bonded to two alkyl groups having 1 to 6 carbon atoms (which may have a substituent) that differ in the number of carbon atoms, structure (chain, cyclic, etc.), etc., and more preferably, an ethyl group, a trifluoroethyl group, a trimethylsilyl group, a triethylsilyl group, a triisopropylsilyl group, a (tert-butyl)dimethylsilyl group, and a (tert-butyl)diphenylsilyl group, and even more preferably, a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 4 carbon atoms refers to a silyl group in which three alkyl groups having 1 to 6 or 1 to 4 carbon atoms are bonded. In addition, in general formulas (6) and (6a), R 6 may be a trialkoxysilyl group (-Si(-OR)3) in which three alkyl groups (R) having 1 to 6 carbon atoms or 1 to 4 carbon atoms are bonded to a silyl group via an oxygen atom. Examples of trialkoxysilyl groups include a trimethoxysilyl group, a triethoxysilyl group, a triisopropoxysilyl group, a (tert-butoxy)dimethoxysilyl group, and a (tert-butoxy)diphenoxysilyl group. The three alkyl or alkoxy groups may be the same or different. In general formula (6), R 6 are preferably the same groups.

[0074] In the general formula (6a), n represents the degree of polymerization, and preferably n=1 to 200.

[0075] Specific examples of the phosphorus atom-containing compound (6) include bis(2,2,2-trifluoroethyl) phosphite (in general formula (6), R 5 is a hydrogen atom, R 6 represents a trifluoroethyl group, and y represents 1.], polyethyl phosphate (in general formula (6a), R 6 represents an ethyl group), trimethylsilyl polyphosphate (TMS, in general formula (6a), R 6 represents a trimethylsilyl group), triethylsilyl polyphosphate (TES, in general formula (6a), R 6 represents a triethylsilyl group), poly(triisopropylsilyl)phosphate [TIPS, in general formula (6a), R 6 represents a triisopropylsilyl group.], polyphosphate [(tert-butyl)dimethylsilyl] [TBDMS, in general formula (6a), R 6 represents a (tert-butyl)dimethylsilyl group. ], polyphosphate [(tert-butyl)diphenylsilyl] [TBDPS, in general formula (6a), R 6 represents a (tert-butyl)diphenylsilyl group.], trimethoxysilyl polyphosphate (in general formula (6a), R 6 represents a trimethoxysilyl group), triethoxysilyl polyphosphate (in general formula (6a), R 6 represents a triethoxysilyl group), poly(triisopropoxysilyl)phosphate (in general formula (6a), R 6 represents a triisopropoxysilyl group.], polyphosphate [(tert-butoxy)dimethoxysilyl] [in general formula (6a), R 6 represents a (tert-butoxy)dimethoxysilyl group. ], polyphosphate [(tert-butoxy)diphenoxysilyl] [in general formula (6a), R 6 represents a (tert-butoxy)diphenoxysilyl group. ]. The phosphorus atom-containing compounds (6) may be used alone or in combination of two or more. Among the phosphorus atom-containing compounds (6), trimethylsilyl polyphosphate is preferred.

[0076] Trimethylsilyl polyphosphate is, for example,31 By measuring the presence or absence of peaks indicating bonds between phosphorus atoms and surrounding groups and their abundance ratios (integral ratios of each peak) using P-NMR or the like, structures such as a chain structure represented by structural formula (6a-1), a cyclic structure represented by structural formula (6a-2), and a branched structure represented by structural formula (6a-3) can be analyzed. 31 The P-NMR measurement conditions include those described in the Examples below.

[0077] [ka]

[0078] [ka]

[0079] [ka]

[0080] In structural formulas (6a-1) to (6a-3), "TMS" represents a trimethylsilyl group, n represents the degree of polymerization, and Pt, Pm, and Pb represent the peaks of phosphorus atoms. Pt represents the peak of a phosphorus atom at a terminal or side chain, specifically a phosphorus atom having one group in which the hydrogen atom of a hydroxyl group is substituted with another adjacent phosphorus atom (hereinafter also referred to as an "-OP group"). Pm represents the peak of a phosphorus atom with a linear structure, specifically a phosphorus atom having two "-OP groups." Pb represents the peak of a phosphorus atom with a branched structure, specifically a phosphorus atom having three "-OP groups." In this way, the presence or absence of branched structures, their abundance ratio, the degree of polymerization n, etc. can be estimated from the presence or absence of Pt, Pm, and Pb and their integral ratios. For example, if all trimethylsilyl polyphosphates have a chain structure (a structure without a Pb peak) represented by structural formula (6a-1), and the integral value of the Pt region is set to 1, the degree of polymerization (n) is expressed as 2 × {(integral value of Pt) + (integral value of Pm)} = 2 × {1 + (integral value of Pm)}. Furthermore, if the integral value of the Pt region is set to 2, the degree of polymerization (n) is expressed as (integral value of Pt) + (integral value of Pm) = 2 + (integral value of Pm). For example, in the case of trimethylsilyl polyphosphate used in the Example 5-2 series described below, the average degree of polymerization (n) is calculated to be 4.87. It should be noted that if trimethylsilyl polyphosphate contains a cyclic structure represented by structural formula (6a-2), the degree of polymerization (n) is estimated to be smaller than the above. Among trimethylsilyl polyphosphates, those with a Pb peak, i.e., those containing a branched structure represented by structural formula (6a-3), are preferred from the viewpoints of suppressing battery self-discharge and further improving battery performance. In this case, the integral ratio of Pt, Pm and Pb is preferably Pt:Pm:Pb=1.0:3.0:0.2 to 1.0:9.0:3.0, and more preferably Pt:Pm:Pb=1.0:6.0:0.7 to 1.0:7.0:1.2.

[0081] By adding a phosphorus atom-containing compound (6) to a non-aqueous electrolyte containing a sulfonylimide compound (1), a battery using the electrolyte can reduce the impedance and DCR and also suppress self-discharge, a problem specific to non-aqueous electrolytes containing a sulfonylimide compound (1). Furthermore, by dissolving CO2 or the like in the electrolyte, as described below, the effect of adding the phosphorus atom-containing compound (6) can be further enhanced. In other words, the combined use of the phosphorus atom-containing compound (6) and the dissolved CO2 or the like can achieve a synergistic effect of suppressing battery self-discharge and reducing the impedance and DCR.

[0082] In general formula (7), R 6 In the general formula (7), R 6 is preferably a linear or branched alkyl group having 1 to 6 carbon atoms (which may have a substituent) and a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a linear alkyl group having 1 to 4 carbon atoms (which may have a substituent) and a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), even more preferably a methyl group and a trimethylsilyl group, and even more preferably a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded. R 6 are preferably the same groups.

[0083] Specific examples of the phosphorus atom-containing compound (7) include trimethyl phosphite (R 6 represents a methyl group.), tris(trimethylsilyl) phosphite (tristrimethylphosphite). R 6 represents a trimethylsilyl group.] The phosphorus atom-containing compounds (7) may be used alone or in combination of two or more. Among the phosphorus atom-containing compounds (7), tris(trimethylsilyl) phosphite is preferred.

[0084] By adding a phosphorus atom-containing compound (7) to a nonaqueous electrolyte containing a sulfonylimide compound (1), a battery using the electrolyte can reduce the impedance and DCR and also suppress self-discharge, a problem specific to nonaqueous electrolytes containing a sulfonylimide compound (1). Furthermore, by dissolving CO2 or the like in the electrolyte, as described below, the effect of adding the phosphorus atom-containing compound (7) can be further enhanced. In other words, the combined use of the phosphorus atom-containing compound (7) and the dissolved CO2 or the like can achieve a synergistic effect of suppressing battery self-discharge and reducing the impedance and DCR.

[0085] Ho Specific compounds such as uran-atom-containing compound (3), carbon-atom-containing compound (4), sulfur-atom-containing compound (5), phosphorus-atom-containing compound (6), and phosphorus-atom-containing compound (7) may be used alone or in combination of two or more. When two or more compounds are used in combination, two or more compounds represented by the same general formula may be used, or multiple compounds represented by different general formulas may be used in combination.

[0086] The specific compound ( 3Among the compounds (3) to (7), boron atom-containing compounds (3), carbon atom-containing compounds (4), sulfur atom-containing compounds (5), phosphorus atom-containing compounds (6) and phosphorus atom-containing compounds (7) are preferred; trimethyl borate (trimethylborate), triethyl borate (triethylborate), triisopropyl borate, tributyl borate, tris(2,2,2)trifluoroethyl borate, tris(trimethylsilyl)borate, trimethylsilyl trifluoroacetate, trimethylsilyl trifluoromethanesulfonate, bistrimethylsilyl sulfate, ditertiarybutylsilyl bistrifluoromethanesulfonate, tertiarybutyldimethylsilyl trifluoromethanesulfonate, polytrimethylsilyl phosphate and tris(trimethylsilyl) phosphite are more preferred; tris(trimethylsilyl) More preferred are trimethylsilyl)borate, trimethylsilyl trifluoroacetate, trimethylsilyl trifluoromethanesulfonate, bis(trimethylsilyl) sulfate, ditert-butylsilyl bistrifluoromethanesulfonate, tert-butyldimethylsilyl trifluoromethanesulfonate, trimethylsilyl polyphosphate, and tris(trimethylsilyl) phosphite; even more preferred are trimethylsilyl trifluoroacetate, trimethylsilyl trifluoromethanesulfonate, bis(trimethylsilyl) sulfate, ditert-butylsilyl bistrifluoromethanesulfonate, tert-butyldimethylsilyl trifluoromethanesulfonate, trimethylsilyl polyphosphate, and tris(trimethylsilyl) phosphite; and especially preferred is trimethylsilyl polyphosphate.

[0087] Specific compounds in non-aqueous electrolytes ( 3 From the viewpoint of suppressing self-discharge of the battery and further improving battery performance, the content of each of (1) to (7) (the content of one specific compound) is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, and is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less.

[0088] A specific compound ( 3 When a plurality of compounds (1) to (7) are used, the total content of the specific compounds in the nonaqueous electrolyte is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less, from the viewpoint of suppressing self-discharge of the battery and further improving battery performance.

[0089] <Classification by the number of oxygen atoms surrounding the central atom> A specific compound ( 3 ) to (7) can also be classified as follows depending on the number of oxygen atoms around the central atom:

[0090] (Compounds with two oxygen atoms around the central atom) Compounds with two oxygen atoms around the central atom include those of the general formula (20): Q 1 (R 5 ) x (…OR 6 )2(20) (hereinafter also referred to as "O2-containing compound (20)").

[0091] In general formula (20), Q 1 represents a carbon atom or a silicon atom. 5 and R 6 are the same or different (and are independent of each other) and represent an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt, or a hydrogen atom. x represents an integer of 1 or 2. "..." represents at least one single bond and the rest represent double bonds, and when a double bond is formed (...OR 6 ) indicates (=O).

[0092] In general formula (20), R 5is preferably a linear or branched alkyl group having 1 to 6 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), and an aryl group (such as a phenyl group, tolyl group, or o-xylyl group), more preferably a linear alkyl group having 1 to 3 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 3 carbon atoms (which may have a substituent), and a phenyl group, even more preferably a methyl group, a trifluoromethyl group, or a phenyl group, and even more preferably a trifluoromethyl group. When x is 2, that is, R 5 When there are two of these, they are preferably the same group.

[0093] In addition, in the general formula (20), R 6 is preferably a linear or branched alkyl group having 1 to 6 carbon atoms (which may have a substituent) and a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a linear alkyl group having 1 to 3 carbon atoms (which may have a substituent) and a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), even more preferably a methyl group and a trimethylsilyl group, and even more preferably a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded. R 6 are preferably the same groups.

[0094] A specific example of the O2-containing compound (20) is trimethylsilyl trifluoroacetate (trimethylsilyl trifluoroacetate). In the general formula (20), "(...OR 6 )2" is "(-OR 6 ) and (=O)”, R 5 is a trifluoromethyl group, R 6 represents a trimethylsilyl group, and x represents 1.), diphenylsilanediol (in general formula (20), "(...OR 6 )2" is "(-OR 6 )2", R 5 is a phenyl group, R 6represents a hydrogen atom, and x represents 2.), dimethoxydiphenylsilane (in general formula (20), "(...OR 6 )2" is "(-OR 6 )2", R 5 is a phenyl group, R 6 represents a methyl group, and x represents 2.), trimethylsilyl acetate (in general formula (20), "(...OR 6 )2" is "(-OR 6 ) and (=O)”, R 5 is a methyl group, R 6 represents a trimethylsilyl group, and x represents 1. The O2-containing compounds (20) may be used alone or in combination of two or more. Among the O2-containing compounds (20), trimethylsilyl trifluoroacetate is preferred.

[0095] (Compounds with three oxygen atoms around the central atom) Compounds with three oxygen atoms around the central atom include those of the general formula (30): Q 2 (…OR 6 )3(30) (hereinafter also referred to as "O3-containing compound (30)"), a compound represented by the general formula (35): {Q 3 (R 5 )} x {(…OR 6 )3} x (35) (hereinafter also referred to as "O3-containing compound (35)").

[0096] In general formula (30), Q 2 represents a boron atom or a phosphorus atom. 6 and "..." indicate the same as above.

[0097] A preferred structure of the O3-containing compound (30) is represented by the general formula (31): Q 2 (OR 6 )3(31) etc.

[0098] When the general formula (30) is a polymerizable group (polymer), there are three (... OR 6 ) represents (—O—). That is, the O3-containing compound (30) is represented by the general formula (32): [-Q 2 [(…OR 6 )2]O-] n (32), More specifically, general formula (33): [-Q 2 (OR 6 )(=O)O-] n (33) It can also be expressed as:

[0099] In general formula (30), R 6 is preferably a linear or branched alkyl group having 1 to 6 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), or a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a linear alkyl group having 1 to 4 carbon atoms (which may have a substituent), a trifluoroalkyl group having 1 to 3 carbon atoms (which may have a substituent), or a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), and even more preferably a methyl group, an ethyl group, a butyl group, a trifluoroethyl group, or a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded. 6 are preferably the same groups.

[0100] In the general formulae (32) and (33), n represents the degree of polymerization, and preferably n=1 to 200.

[0101] A specific example of the O3-containing compound (30) is tris(trimethylsilyl) phosphite (tristrimethylphosphite). In the general formula (30), "(...OR 6 )3" is "(-OR 6 )3", R 6 represents a trimethylsilyl group.], polyethyl phosphate (in general formula (33), R6 represents an ethyl group), trimethylsilyl polyphosphate (TMS, in general formula (33), R 6 represents a trimethylsilyl group), triethylsilyl polyphosphate (TES, in general formula (33), R 6 represents a triethylsilyl group), poly(triisopropylsilyl)phosphate [TIPS, in general formula (33), R 6 represents a triisopropylsilyl group.], polyphosphate [(tert-butyl)dimethylsilyl] [TBDMS, in the general formula (33), R 6 represents a (tert-butyl)dimethylsilyl group. ], polyphosphate [(tert-butyl)diphenylsilyl] [TBDPS, in general formula (33), R 6 represents a (tert-butyl)diphenylsilyl group.], trimethoxysilyl polyphosphate (in general formula (33), R 6 represents a trimethoxysilyl group), triethoxysilyl polyphosphate (in general formula (33), R 6 represents a triethoxysilyl group), poly(triisopropoxysilyl)phosphate (in general formula (33), R 6 represents a triisopropoxysilyl group.], polyphosphate [(tert-butoxy)dimethoxysilyl] [in general formula (33), R 6 represents a (tert-butoxy)dimethoxysilyl group. ], polyphosphate [(tert-butoxy)diphenoxysilyl] [in general formula (33), R 6 represents a (tert-butoxy)diphenoxysilyl group. ], trimethyl borate (trimethyl borate). In general formula (30), "(...OR 6 )3" is "(-OR 6 )3", R 6 represents a methyl group.), triethyl borate (triethyl borate. In the general formula (30), "(...OR 6 )3" is "(-OR 6 )3", R 6 represents an ethyl group), triisopropyl borate (triisopropyl borate. In the general formula (30), "(...OR 6 )3" is "(-OR 6)3", R 6 represents an isopropyl group), tributyl borate (tributyl borate). In the general formula (30), "(...OR 6 )3" is "(-OR 6 )3", R 6 represents a butyl group.), tris(2,2,2)trifluoroethyl borate [tris(2,2,2)trifluoroethyl borate. In the general formula (30), "(...OR 6 )3" is "(-OR 6 )3", R 6 represents a trifluoroethyl group. ], tris(trimethylsilyl)borate [tris(trimethylsilyl) borate]. In general formula (30), "(...OR 6 )3" is "(-OR 6 )3", R 6 represents a trimethylsilyl group. ], trimethyl phosphite (in general formula (30), "(...OR 6 )3" is "(-OR 6 )3", R 6 represents a methyl group.) The O3-containing compounds (30) may be used alone or in combination of two or more. Among the O3-containing compounds (30), tris(trimethylsilyl) phosphite, trimethylsilyl polyphosphate, trimethyl borate, triethyl borate, tributyl borate, tris(2,2,2)trifluoroethyl borate, and tris(trimethylsilyl)borate are preferred, tris(trimethylsilyl) phosphite, trimethylsilyl polyphosphate, and tris(trimethylsilyl)borate are more preferred, tris(trimethylsilyl) phosphite and trimethylsilyl polyphosphate are even more preferred, and trimethylsilyl polyphosphate is even more preferred.

[0102] In general formula (35), Q 3 represents a phosphorus atom or a sulfur atom. x represents an integer of 1 or 2. R 5 , R 6 , and "..." indicate the same as above.

[0103] In general formula (35), when x=2, there are six (...OR 6 ) two of which are (-OR6 -O-). That is, O3-containing compound (35), general formula (36): Q 3 (R 5 )(…OR 6 )2-OR 6 -OQ 3 (R 5 )(…OR 6 )2(36), More specifically, general formula (37): Q 3 (R 5 )(=O)2-OR 6 -OQ 3 (=O)(OR 6 )2(37) It can also be expressed as:

[0104] In general formula (35), R 5 is preferably a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent) and a hydrogen atom, more preferably a trifluoroalkyl group having 1 to 3 carbon atoms (which may have a substituent) and a hydrogen atom, and even more preferably a trifluoromethyl group and a hydrogen atom. 5 are preferably the same groups.

[0105] In addition, in the general formula (35), R 6 R is a trifluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent) and a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a trifluoroalkyl group having 1 to 4 carbon atoms (which may have a substituent) and a trialkylsilyl group having 1 to 4 carbon atoms (which may have a substituent), and even more preferably a trifluoroethyl group, a trimethylsilyl group, and a tertiary (tert-)butylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 4 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 4 carbon atoms are bonded. 6 are preferably the same groups.

[0106] A specific example of the O3-containing compound (35) is trimethylsilyl trifluoromethanesulfonate (trimethylsilyl trifluoromethanesulfonate). In the general formula (35), R 5 is a trifluoromethyl group, "(...OR 6 )3" is "(-OR 6 ) and (=O)2”, R 6 represents a trimethylsilyl group, and x represents 1.), di-tert-butylsilyl bistrifluoromethanesulfonate [di-tert-butylsilyl bis(trifluoromethanesulfonate). In the general formula (37), R 5 is a trifluoromethyl group, R 6 represents a ditertiary butylsilyl group. ], tertiary butyldimethylsilyl trifluoromethanesulfonate (in general formula (35), R 5 is a trifluoromethyl group, "(...OR 6 )3" is "(-OR 6 ) and (=O)2”, R 6 represents a tertiary butyldimethylsilyl group), bis(2,2,2-trifluoroethyl) phosphite (in general formula (35), R 5 is a hydrogen atom, "(...OR 6 )3" is "(-OR 6 )2 and (=O)」, R 6 represents a trifluoroethyl group, and y represents 1. The O3-containing compounds (35) may be used alone or in combination of two or more. Among the O3-containing compounds (35), trimethylsilyl trifluoromethanesulfonate, ditertiarybutylsilyl bistrifluoromethanesulfonate, and tertiarybutyldimethylsilyl trifluoromethanesulfonate are preferred.

[0107] (Compounds with four oxygen atoms surrounding the central atom) Compounds with four oxygen atoms around the central atom include those of the general formula (40): S(…OR 6 )4(40) (hereinafter also referred to as "O4-containing compound (40)").

[0108] In general formula (40), R 6 and "..." are the same as above. In general formula (40), R 6 is preferably a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), and even more preferably a trimethylsilyl group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded. R 6 are preferably the same groups.

[0109] A specific example of the O4-containing compound (40) is bis(trimethylsilyl) sulfate (bis(trimethylsilyl) sulfate). In the general formula (40), "(...OR 6 )4" is "(-OR 6 )2 and (=O)2”, R 6 represents a trimethylsilyl group.] and the like.

[0110] Specific compounds such as O2-containing compound (20), O3-containing compound (30), O3-containing compound (35), and O4-containing compound (40) may be used alone or in combination of two or more. When two or more compounds are used in combination, two or more compounds represented by the same general formula may be used, or multiple compounds represented by different general formulas may be used in combination.

[0111] (additives) The first non-aqueous electrolyte contains a specific compound ( 3 ) to (7), as well as specific compounds ( 3 ) to (7) may further contain additives different from the above.

[0112] The additives include those represented by the general formula (12): [C18] M 1 NO3(12) (hereinafter also referred to as "nitric acid compound (12)"), a compound represented by the general formula (13):

[0113] [ka]

[0114] (hereinafter also referred to as "fluorooxalato compound (13)").

[0115] In general formula (12), M 1 Examples of the alkali metal element represented by the formula (I) include lithium, sodium, potassium, rubidium, cesium, etc. Among these, lithium is preferred.

[0116] Specific examples of the nitrate compound (12) include lithium nitrate (LiNO), sodium nitrate (NaNO), and potassium nitrate (KNO). The nitrate compound (12) may be used alone or in combination of two or more. Among the nitrate compounds (12), lithium nitrate is preferred.

[0117] The present inventors have found that adding a nitric acid compound (12) to a nonaqueous electrolyte containing a sulfonylimide compound (1) reduces the impedance and DCR of a battery using the electrolyte, but the battery exhibits significant self-discharge and thus has room for improvement in storage characteristics. The present inventors have also found that adding a nitric acid compound (12) to a nonaqueous electrolyte containing a sulfonylimide compound (1) and dissolving CO2 or the like (described below) improves the storage characteristics of the battery. Specifically, they have found that a battery using a nonaqueous electrolyte containing a sulfonylimide compound (1) in which LiNO3 is added and CO2 is dissolved in combination achieves a synergistic effect of reducing impedance and DCR and suppressing self-discharge.

[0118] From the above, when CO2 is dissolved in the non-aqueous electrolyte, the nitric acid compound (12) is a specific compound ( 3 ) to (7) may be used alone without being used in combination with the specific compound ( 3When the compounds (1) to (7) are used in combination with the nitric acid compound (12), a further reduction in impedance and DCR can be expected.

[0119] In general formula (13), M 3 indicates B (boron) or P (phosphorus).

[0120] In general formula (13), A f+ represents a metal ion, H (hydrogen) or onium ion. Examples of metal ions include alkali metal ions, alkaline earth metal ions, trivalent metal ions, etc. Examples of alkali metals and alkaline earth metals include those mentioned above. Examples of trivalent metals include boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl), etc. Among metal ions, Li + , Na + , Mg 2+ and Ca 2+ is preferred, and Li + is more preferred. Examples of onium ions include chain quaternary ammonium ions such as tetraethylammonium, tetrabutylammonium, and triethylmethylammonium; chain tertiary ammonium ions such as triethylammonium, tributylammonium, dibutylmethylammonium, and dimethylethylammonium; imidazolium ions such as 1-ethyl-3-methylimidazolium and 1,2,3-trimethylimidazolium; and pyrrolidinium ions such as N,N-dimethylpyrrolidinium and N-ethyl-N-methylpyrrolidinium. Among these, chain quaternary ammonium ions and imidazolium ions are preferred, and chain quaternary ammonium ions are more preferred. That is, in general formula (13), f, g, and h are preferably 1.

[0121] In general formula (13), R 3represents an alkylene group having 1 to 10 carbon atoms or a halogenated alkylene group having 1 to 10 carbon atoms. Examples of the alkylene group having 1 to 10 carbon atoms include a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, a hexylene group, a heptylene group, an octylene group, a nonylene group, and a decalene group, which may be branched. Examples of the halogenated alkylene group having 1 to 10 carbon atoms include groups in which part or all of the hydrogen atoms of an alkylene group having 1 to 10 carbon atoms have been replaced with F, Cl, Br, or I (among which, F is preferred, such as a fluoromethylene group or a fluoroethylene group). R 3 Among these, alkylene groups having 1 to 4 carbon atoms and fluorinated alkylene groups having 1 to 4 carbon atoms are preferred, and alkylene groups having 1 to 2 carbon atoms and fluorinated alkylene groups having 1 to 2 carbon atoms are more preferred. k is 0 or 1, and when k is 0, it represents a direct bond to a carbonyl group, and the compound of general formula (13) becomes an oxalatoborate or an oxalatophosphonium. k is preferably 0.

[0122] In general formula (13), R 4 represents F (fluorine) or a fluorinated alkyl group having 1 to 10 carbon atoms. Examples of the fluorinated alkyl group having 1 to 10 carbon atoms include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, a tetrafluoroethyl group, a perfluoroethyl group, a fluoropropyl group, a perfluoropropyl group, a perfluorobutyl group, and a perfluorooctyl group. R 4 Among these, a fluorinated alkyl group having 1 to 2 carbon atoms and F are preferred, and F is more preferred.

[0123] In general formula (13), T 1 and T 2 are each independently (the same or different) O (oxygen) or S (sulfur). 1 and T 2 In terms of availability, O is preferred for both.

[0124] In general formula (13), M 3When i is B (boron), i is preferably 1 or 2, and when i is 1, j is 2; 4 is more preferably F. When i is 2, j is 0. On the other hand, M 3 When is P (phosphorus), i is 1 to 3, when i is 1, j is 4, when i is 2, j is 2, and when i is 3, j is 0.

[0125] Examples of the fluorooxalate compound (13) include difluorooxalate borate salts, bisoxalate borate salts, tetrafluorophosphonium salts, difluorobisoxalate phosphonium salts, and trisoxalate phosphonium salts. More specifically, lithium salts having an oxalate skeleton, such as lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiDFOB), lithium difluorooxalate phosphanite (LIDFOP), lithium tetrafluorooxalate phosphate (LITFOP), lithium difluorobis(oxalate)phosphate (LiDFOP), and lithium tris(oxalate)phosphate, may be used. The fluorooxalate compounds (13) may be used alone or in combination of two or more. Among the fluorooxalate compounds (13), LiBOB and LiDFOB are preferred, and LiDFOB is more preferred from the viewpoint of reducing the impedance and DCR of the battery.

[0126] The additives such as the nitric acid compound (12) and the fluorooxalate compound (13) may be used alone or in combination of two or more. When two or more additives are used in combination, two or more of the nitric acid compound (12) or the fluorooxalate compound (13) may be used, or a plurality of the nitric acid compound (12) and the fluorooxalate compound (13) may be used in combination.

[0127] From the viewpoint of suppressing self-discharge of the battery and further improving battery performance, the content of each additive (content of one type of additive) in the nonaqueous electrolyte is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, even more preferably 0.2 mass % or more, even more preferably 0.3 mass % or more, and even more preferably 0.5 mass % or more, and is preferably 3 mass % or less, more preferably 2 mass % or less, and even more preferably 1 mass % or less.

[0128] When multiple additives are used, the total content of the additives in the nonaqueous electrolyte is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less, from the viewpoint of suppressing self-discharge of the battery and further improving battery performance.

[0129] (Other ingredients) The first non-aqueous electrolyte solution according to this embodiment may contain other additives (specific compounds ( 3) to (7) and additives (12) to (13) (compounds different from these). Examples of other additives include carboxylic acid anhydrides such as succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; sulfur-containing compounds such as ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide, and trimethylene glycol sulfate; nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide; saturated hydrocarbon compounds such as heptane, octane, and cycloheptane; and fluoroethylene carbonate. Examples of suitable additives include carbonate compounds such as ethylene carbonate (FEC), trifluoropropylene carbonate, phenylethylene carbonate, and erythritan carbonate; sulfamic acid (amidosulfuric acid, H3NSO3); sulfamate salts (alkali metal salts such as lithium salt, sodium salt, and potassium salt; alkaline earth metal salts such as calcium salt, strontium salt, and barium salt; other metal salts such as manganese salt, copper salt, zinc salt, iron salt, cobalt salt, and nickel salt; ammonium salt; guanidine salt; fluorophosphate compounds such as lithium monofluorophosphate (Li2PO3F) and lithium difluorophosphate (LiPO2F2); and fluorosulfonic acid compounds such as lithium fluorosulfonate (LiFSO3), sodium fluorosulfonate (NaFSO3), potassium fluorosulfonate (KFSO3), and magnesium fluorosulfonate (Mg(FSO3)2). These additives may be used alone or in combination of two or more.

[0130] The other additives are preferably used in an amount of 0.1% by mass to 10% by mass, more preferably 0.2% by mass to 8% by mass, and even more preferably 0.3% by mass to 5% by mass, based on 100% by mass of the non-aqueous electrolyte. If the amount of the other additives used is too small, it may be difficult to obtain the effects derived from the other additives. On the other hand, even if a large amount of the other additives is used, it may be difficult to obtain effects commensurate with the amount added. In addition, the viscosity of the non-aqueous electrolyte may increase, resulting in a decrease in conductivity.

[0131] (CO2, CO, HCO3 - and CO3 2- at least one of the following The first non-aqueous electrolyte according to this embodiment contains carbon dioxide (CO), carbon monoxide (CO), bicarbonate ions (HCO - ) and carbonate ions (CO3 2- ) (hereinafter also referred to as "CO2, etc.") may be dissolved therein.

[0132] As described above, the present inventors have found that a battery using a non-aqueous electrolyte containing sulfonylimide compound (1) exhibits a larger self-discharge rate from a fully charged state than a battery using a non-aqueous electrolyte containing only one of the other electrolytes (LiPF6, LiBF4, etc.). More specifically, it has been found that a battery using a non-aqueous electrolyte containing sulfonylimide compound (1) exhibits a larger self-discharge rate depending on the concentration of sulfonylimide compound (1). Then, as a result of extensive research to solve the specific problems associated with non-aqueous electrolytes containing sulfonylimide compound (1), the present inventors have found that a non-aqueous electrolyte containing sulfonylimide compound (1) exhibits a larger self-discharge rate from a fully charged state than a battery using a non-aqueous electrolyte containing sulfonylimide compound (1). 3 ) to (7), and by dissolving a predetermined amount of CO2 or the like in the electrolyte, a specific compound ( 3) to (7), the addition of sulfonylimide compound (1) further enhances the synergistic effects of suppressing self-discharge and reducing the charge transfer resistance and DC resistance of the battery. Furthermore, as will be shown in the Examples below, a non-aqueous electrolyte containing sulfonylimide compound (1) not only further suppresses self-discharge (excellent self-discharge suppression effect (storage characteristics)) due to the presence of CO2 dissolved in the electrolyte compared to a non-aqueous electrolyte containing LiPF6 alone, but also has a significant effect of improving various battery performances, such as reducing the DCR and impedance of the battery, improving low-temperature charge-discharge characteristics and charge-discharge cycle characteristics.

[0133] In this specification, dissolution of CO2 etc. in a non-aqueous electrolyte solution containing sulfonylimide compound (1) means intentionally dissolving CO2 etc. in the non-aqueous electrolyte solution, but does not exclude, for example, CO2 etc. contained in raw materials for the non-aqueous electrolyte solution, such as an electrolyte solvent, or CO2 etc. that is inevitably dissolved in the non-aqueous electrolyte solution during a normal manufacturing process for the non-aqueous electrolyte solution or a secondary battery. In other words, the total amount of dissolved CO2 etc., described below, may include intentionally dissolved CO2 etc. as well as CO2 etc. in raw materials and unavoidably dissolved CO2 etc.

[0134] The form of CO2 dissolved in the non-aqueous electrolyte is not particularly limited, and may be CO2, CO, HCO3 - and CO3 2- It is sufficient that the compound exists in at least one of the above forms, and it may exist in any one of the above forms or in a plurality of forms.

[0135] The total dissolved amount of CO2 and the like in the nonaqueous electrolyte is, for example, preferably 20 mass ppm or more, more preferably 50 mass ppm or more, even more preferably 100 mass ppm or more, even more preferably 150 mass ppm or more, still more preferably 200 mass ppm or more, and particularly preferably 250 mass ppm or more, relative to the electrolyte. The upper limit of the total dissolved amount is not particularly limited, but is, for example, equal to or less than the saturated concentration at 25°C. The total dissolved amount can be measured by the method described in the Examples below, for example, gas chromatography.

[0136] In this specification, the total amount of dissolved CO2 and the like in the non-aqueous electrolyte solution means: In the preparation process of the non-aqueous electrolyte, the total amount of dissolved CO2, etc. in the electrolyte immediately after preparation, or after an aging period (for example, one week) has elapsed to stabilize the amount of dissolved CO2, etc., as needed, or This refers to the total amount of dissolved CO2 and other substances in the electrolyte solution extracted from a secondary battery in, for example, a nitrogen atmosphere after the battery has been subjected to an aging process in the manufacturing process of the secondary battery. Examples of the aging process include the following processes and the conditions described in the Examples below. (I) After filling and partial charging, the battery is subjected to high-temperature treatment (storage) at 30°C or higher for at least 6 hours for a period of up to 28 days. After degassing and resealing, the battery is checked for initial performance defects through charging and discharging, and then kept at 50% charge for at least one week to check for defects due to self-discharge. (II) The same process as (I) except that the high temperature treatment is not performed after the partial charge. (III) The same process as (I) except that degassing is not performed after the high-temperature treatment. Examples of methods for dissolving CO2 or the like in a non-aqueous electrolyte solution containing the sulfonylimide compound (1) include (A) a method of dissolving CO2 or the like in a non-aqueous electrolyte solution in a preparation process of the non-aqueous electrolyte solution; and (B) a method of dissolving CO2 or the like in a non-aqueous electrolyte solution in a production process of a secondary battery.

[0137] In the (A) nonaqueous electrolyte preparation step, the method of dissolving CO2 or the like in the nonaqueous electrolyte is, in other words, a method of using a nonaqueous electrolyte containing sulfonylimide compound (1) and previously dissolving CO2 or the like at 20 ppm by mass or more (hereinafter also referred to as a "CO2 or the like-dissolved electrolyte" or "CO2-dissolved electrolyte") and injecting the electrolyte into a secondary battery. Examples of the method of dissolving CO2 or the like in the nonaqueous electrolyte (dissolving step) include a method of contacting the nonaqueous electrolyte with a gas containing CO2 or the like (contacting step), a method of blowing a gas containing CO2 or the like into the nonaqueous electrolyte (bubbling step), a method of stirring the nonaqueous electrolyte in a gas atmosphere containing CO2 or the like (stirring step), a method of contacting the nonaqueous electrolyte with a high-pressure gas containing CO2 or the like (a method of pressurizing a gas containing CO2 or the like into the nonaqueous electrolyte, pressurizing step), and a method of adding a substance that generates a gas containing CO2 or the like to the nonaqueous electrolyte (adding step). Examples of substances that generate gases containing CO2 or the like include bicarbonates, carbonates, and dry ice. Since CO2 or the like can be dissolved in electrolyte solvents generally used for nonaqueous electrolytes, a nonaqueous electrolyte may be prepared by dissolving sulfonylimide compound (1) in an electrolyte solvent in which CO2 or the like has been dissolved in advance. The same method as described above can be used to dissolve CO2 or the like in the electrolyte solvent. Another method includes placing a pre-prepared nonaqueous electrolyte into a sealed container to make up about 1 / 10 of its volume, creating a vacuum in the container, and then filling the container with CO2 or the like. This operation is repeated multiple times to replace the air in the container with CO2 or the like, and finally storing the container in a sealed state at a cool temperature for several days (replacement step). The dissolution step may include at least one of the steps described above, or a combination of multiple steps. The dissolving step preferably includes at least one of a pressurizing step, a liquid contacting step, a bubbling step, and a substitution step, more preferably includes at least one of a pressurizing step, a liquid contacting step, and a bubbling step, and further preferably includes a pressurizing step and a substitution step (a combination of a pressurizing step and a substitution step).

[0138] In the method (A), the secondary battery may be assembled in a CO atmosphere or an atmosphere containing CO, from the viewpoint of controlling the total amount of CO dissolved in the nonaqueous electrolyte to a constant level. Specifically, the step of injecting a nonaqueous electrolyte having CO dissolved therein into the battery and the steps after the injection may be performed in a CO atmosphere or an atmosphere containing CO. After the injection of the electrolyte, the battery may be exposed to a high-pressure CO atmosphere.

[0139] The CO2 and other dissolved electrolyte solution used in the method (A) is obtained by the method for producing a nonaqueous electrolyte solution according to this embodiment. This production method includes a dissolving step including at least one of the steps described above in order to dissolve 20 mass ppm or more of CO2 and other dissolved substances in a nonaqueous electrolyte solution containing a sulfonylimide compound (1).

[0140] In the manufacturing process of the secondary battery (B), examples of a method for dissolving CO2 or the like in the non-aqueous electrolyte include a method of assembling a secondary battery in a CO2 atmosphere and injecting the non-aqueous electrolyte into the battery (specifically, a method of creating a nearly vacuum inside the battery exterior sealed on three sides and then filling it with CO2, and then injecting the non-aqueous electrolyte into the one unsealed side and sealing it at normal pressure); and a method of injecting the non-aqueous electrolyte into the secondary battery and then replacing the air inside the battery with CO2. The method for replacing the air inside the battery with CO2 can be the same as the method for replacing the air inside the container with CO2. Specifically, the air inside the exterior can be replaced with CO2 by repeating the operation of creating a nearly vacuum inside the exterior into which the non-aqueous electrolyte has been injected and then filling it with CO2 multiple times.

[0141] The total amount of CO2 and other dissolved substances in the non-aqueous electrolyte solution varies depending on the temperature of the non-aqueous electrolyte solution, so it is preferable to control the temperature to a constant value during the preparation process of the non-aqueous electrolyte solution and / or the production process of the secondary battery.

[0142] (Electrolyte solvent) The first nonaqueous electrolyte solution according to this embodiment may contain an electrolyte solvent. The electrolyte solvent is not particularly limited as long as it can dissolve and disperse the electrolyte salt. Examples of the electrolyte solvent include nonaqueous solvents, and media such as polymers and polymer gels used in place of electrolyte solvents. Any solvent generally used in batteries can be used.

[0143] The non-aqueous solvent is preferably a solvent having a high dielectric constant, high solubility for the electrolyte salt, a boiling point of 60° C. or higher, and a wide electrochemical stability range, and more preferably an organic solvent with a low water content. Examples of such organic solvents include ether solvents such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2,6-dimethyltetrahydrofuran, tetrahydropyran, crown ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and 1,3-dioxolane; chain carbonate ester (carbonate) solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, diphenyl carbonate, and methyl phenyl carbonate; saturated cyclic carbonate ester solvents such as ethylene carbonate, propylene carbonate, 2,3-dimethyl ethylene carbonate, 1,2-butylene carbonate, and erythrityl carbonate; cyclic carbonate ester solvents having an unsaturated bond such as vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 2-vinyl ethylene carbonate, and phenyl ethylene carbonate; fluoroethylene carbonate, 4,5-difluoroethylene carbonate, and trifluoroethylene carbonate; Fluorine-containing cyclic carbonate ester solvents such as fluoropropylene carbonate; aromatic carboxylic acid ester solvents such as methyl benzoate and ethyl benzoate; lactone solvents such as γ-butyrolactone, γ-valerolactone, and δ-valerolactone; phosphate ester solvents such as trimethyl phosphate, ethyl dimethyl phosphate, diethyl methyl phosphate, and triethyl phosphate; acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile, 2-methylglutaronitrile, Examples include nitrile solvents such as valeronitrile, butyronitrile, and isobutyronitrile; sulfur compound solvents such as dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, sulfolane, 3-methyl sulfolane, and 2,4-dimethyl sulfolane; aromatic nitrile solvents such as benzonitrile and tolunitrile; nitromethane, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, and 3-methyl-2-oxazolidinone.These solvents may be used alone or in combination of two or more.

[0144] Among the electrolyte solvents, carbonate solvents such as chain carbonate ester solvents and cyclic carbonate ester solvents, lactone solvents, and ether solvents are preferred, with dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, γ-butyrolactone, γ-valerolactone, and the like being more preferred, and carbonate solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate being even more preferred.

[0145] When a polymer or polymer gel is used instead of the electrolyte solvent, the following methods may be employed: a method in which a solution of an electrolyte salt dissolved in a solvent is dropped onto a polymer film formed by a conventionally known method to impregnate and support the electrolyte salt and non-aqueous solvent; a method in which a polymer and an electrolyte salt are melted and mixed at a temperature equal to or higher than the melting point of the polymer, and then a film is formed and the film is impregnated with a solvent (these are referred to as gel electrolytes); a method in which a non-aqueous electrolyte in which an electrolyte salt has been dissolved in an organic solvent is mixed with a polymer, and then the mixture is formed into a film by a casting method or a coating method, and the organic solvent is volatilized; a method in which a polymer and an electrolyte salt are melted at a temperature equal to or higher than the melting point of the polymer, mixed, and molded (true polymer electrolyte); and the like.

[0146] Examples of polymers that can be used in place of the electrolyte solvent include polyethylene oxide (PEO), which is a homopolymer or copolymer of epoxy compounds (ethylene oxide, propylene oxide, butylene oxide, allyl glycidyl ether, etc.), polyether polymers such as polypropylene oxide, methacrylic polymers such as polymethyl methacrylate (PMMA), nitrile polymers such as polyacrylonitrile (PAN), fluorine-based polymers such as polyvinylidene fluoride (PVdF) and polyvinylidene fluoride-hexafluoropropylene, and copolymers thereof. These polymers may be used alone or in combination of two or more.

[0147] <Second non-aqueous electrolyte> Next, the second nonaqueous electrolyte will be described below. The second nonaqueous electrolyte according to this embodiment contains an electrolyte and a specific compound, and has CO2 and the like dissolved therein. Specifically, the second nonaqueous electrolyte differs in structure from the first nonaqueous electrolyte described above in terms of the type of specific compound and the dissolved CO2 and the like. Other aspects are similar to those of the first nonaqueous electrolyte described above, and therefore detailed descriptions will be omitted here. In other words, the electrolyte, additives, other components, electrolyte solvent, amount and method of dissolved CO2 and the like in the first nonaqueous electrolyte described above are all applicable to the second nonaqueous electrolyte. Furthermore, detailed descriptions of the same type of specific compound will be omitted.

[0148] (specific compounds) <Classification by central atom> The second non-aqueous electrolyte contains, as an additive, at least one of the various specific compounds shown below, together with the sulfonylimide compound (1). In this way, the second non-aqueous electrolyte contains the sulfonylimide compound (1) in combination with one or more specific compounds, thereby suppressing the self-discharge of the battery and reducing the charge transfer resistance (impedance) and the battery direct current resistance (DCR). In other words, the storage characteristics and performance of the battery can be improved. The second non-aqueous electrolyte contains ,Ke In addition to the boron atom-containing compound (14), the boron atom-containing compound (3), the carbon atom-containing compound (4), the sulfur atom-containing compound (5), the phosphorus atom-containing compound (6), the phosphorus atom-containing compound (7), and the like, the compounds shown below can also be used.

[0149] (Compound containing phosphorus atom) Specific compounds containing a phosphorus atom as the central atom include those represented by the general formula (8):

[20] P(=O)(OR 6 )3(8) Examples include a phosphorus atom-containing compound represented by the following formula (hereinafter also referred to as "phosphorus atom-containing compound (8)").

[0150] In general formula (8), R 6 In general formula (8), the three R6 are preferably identical trialkylsilyl groups having 1 to 6 carbon atoms (which may have a substituent), more preferably identical trialkylsilyl groups having 1 to 3 carbon atoms (which may have a substituent), and even more preferably identical trimethylsilyl groups. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group to which three alkyl groups having 1 to 6 or 1 to 3 carbon atoms are bonded.

[0151] Specific examples of the phosphorus atom-containing compound (8) include tris(trimethylsilyl) phosphate (tristrimethylsilyl phosphate).

[0152] As described above, adding a phosphorus-containing compound (8) such as tris(trimethylsilyl) phosphate to a nonaqueous electrolyte containing a sulfonylimide compound (1) does not sufficiently suppress self-discharge. However, by combining the addition of the phosphorus-containing compound (8) with the dissolved CO, etc., the addition of the phosphorus-containing compound (8) not only reduces the charge transfer resistance (impedance) and the direct current resistance (DCR) of the battery, but also suppresses self-discharge of the battery. In other words, the storage characteristics and performance of the battery can be improved.

[0153] (boroxine compounds) The boroxine compound includes compounds represented by the general formula (9):

[0154] [ka]

[0155] Examples thereof include a boroxine compound represented by the formula (hereinafter also referred to as "boroxine compound (9)").

[0156] In general formula (9), R 7 R are the same or different (independently of each other) and represent an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), or a cycloalkyl group (which may have a substituent). 7may contain halogen, nitrogen, sulfur, oxygen, etc. as necessary.

[0157] In general formula (9), there are three R 7 are preferably the same linear or branched alkyl group (which may have a substituent) having 1 to 6 carbon atoms, a fluoroalkyl group (which may have a substituent) and a cycloalkyl group (which may have a substituent) having 1 to 6 carbon atoms, and more preferably the same linear or branched alkyl group (which may have a substituent) and a cycloalkyl group (which may have a substituent) having 1 to 4 carbon atoms.

[0158] Specific examples of the boroxine compound (9) include 2,4,6-trimethoxyboroxine (trimethoxycyclotriboroxane), 2,4,6-triisopropoxyboroxine, 2,4,6-triphenylboroxine, etc. The boroxine compounds (9) may be used alone or in combination of two or more.

[0159] Although the addition of a boroxine compound (9) to a non-aqueous electrolyte containing a sulfonylimide compound (1) does not sufficiently suppress self-discharge, the addition of a boroxine compound (9) in combination with the dissolved CO2 or the like not only reduces the charge transfer resistance (impedance) and direct current resistance (DCR) of the battery, but also suppresses self-discharge of the battery, thereby improving the storage characteristics and performance of the battery.

[0160] Ho Specific compounds such as uran-atom-containing compounds (3), carbon-atom-containing compounds (4), sulfur-atom-containing compounds (5), phosphorus-atom-containing compounds (6), phosphorus-atom-containing compounds (7), phosphorus-atom-containing compounds (8), and boroxine compounds (9) may be used alone or in combination of two or more. When two or more compounds are used in combination, two or more compounds represented by the same general formula may be used, or multiple compounds represented by different general formulas may be used in combination.

[0161] Specific compounds in non-aqueous electrolytes (3 From the viewpoint of suppressing self-discharge of the battery and further improving battery performance, the content of each of (1) to (9) (the content of one specific compound) is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, even more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, and is preferably 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1% by mass or less.

[0162] A specific compound ( 3 When a plurality of compounds (1) to (9) are used, the total content of the specific compounds in the nonaqueous electrolyte is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less, from the viewpoint of suppressing self-discharge of the battery and further improving battery performance.

[0163] <Classification by the number of oxygen atoms surrounding the central atom> A specific compound ( 3 ) to (9) can also be classified as follows depending on the number of oxygen atoms around the central atom. Specifically, along with the above-mentioned O2-containing compound (20), O3-containing compound (30), O3-containing compound (35), and O4-containing compound (40), compounds having four oxygen atoms around the central atom include compounds of the general formula (45): P(…OR 6 )4(45) (hereinafter also referred to as "O4-containing compound (45)").

[0164] In general formula (45), R 6 and "..." are the same as above. In general formula (45), R 6 is preferably a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent), more preferably a trialkylsilyl group having 1 to 3 carbon atoms (which may have a substituent), and even more preferably a trimethylsilyl group. 6 are preferably the same group. In this specification, a trialkylsilyl group having 1 to 6 or 1 to 3 carbon atoms refers to a silyl group having three alkyl groups each having 1 to 6 or 1 to 3 carbon atoms bonded thereto.

[0165] A specific example of the O4-containing compound (45) is tris(trimethylsilyl) phosphate (tristrimethylsilyl phosphate). In the general formula (45), "(...OR 6 )4" is "(-OR 6 )3 and (=O)1”, R 6 represents a trimethylsilyl group.] and the like.

[0166] Specific compounds such as O2-containing compound (20), O3-containing compound (30), O3-containing compound (35), O4-containing compound (40), and O4-containing compound (45) may be used alone or in combination of two or more. When two or more compounds are used in combination, two or more compounds represented by the same general formula may be used, or multiple compounds represented by different general formulas may be used in combination.

[0167] <Third non-aqueous electrolyte> Next, the third nonaqueous electrolyte will be described below. The third nonaqueous electrolyte according to this embodiment contains an electrolyte and a specific compound. Specifically, the third nonaqueous electrolyte is different in structure from the first or second nonaqueous electrolyte described above in that the type of electrolyte is not specified and the type of specific compound is different. In other respects, the third nonaqueous electrolyte has the same structure as the first or second nonaqueous electrolyte described above, and therefore detailed description thereof will be omitted here.

[0168] (electrolyte) The electrolyte is not particularly limited, and can be, for example, the electrolyte used in the first or second non-aqueous electrolyte. Specific examples of the electrolyte include sulfonylimide compound (1), and electrolytes other than sulfonylimide compound (1), such as imide salts and non-imide salts (fluorophosphate compound (10), fluoroboric acid compound (11), LiAsF6, etc.). The concentration of the electrolyte is also not particularly limited, and the same concentration as that of the first or second non-aqueous electrolyte is used.

[0169] (specific compounds) The third nonaqueous electrolyte contains, as an additive, at least one of the specific compounds shown below among the phosphorus atom-containing compounds (6) described above, together with the electrolyte. Because the third nonaqueous electrolyte contains the electrolyte and one or more specific phosphorus atom-containing compounds (6) in combination, it is possible to simultaneously suppress battery self-discharge and reduce the charge transfer resistance (impedance) and the battery direct current resistance (DCR). This means that the storage characteristics and performance of the battery can be improved.

[0170] Specifically, the third nonaqueous electrolyte solution contains, as the phosphorus atom-containing compound (6) represented by general formula (6a), at least one compound selected from the group consisting of ethyl polyphosphate, (triisopropylsilyl) polyphosphate [TIPS], and [(tert-butyl)dimethylsilyl] polyphosphate [TBDMS].

[0171] (others) In addition to the above configuration, the additives, other components, electrolyte solvent, amount and method of dissolved CO2, etc., described for the first or second nonaqueous electrolyte also apply to the third nonaqueous electrolyte. For example, the third nonaqueous electrolyte may contain dissolved CO2, etc.

[0172] The first to third nonaqueous electrolyte solutions configured as described above are used, for example, in batteries (batteries having a charge / discharge mechanism), electricity storage (electrochemical) devices (or ion conductor materials constituting these), etc. Specifically, the electrolyte solutions can be used as electrolyte solutions constituting, for example, primary batteries, secondary batteries (e.g., lithium (ion) secondary batteries), fuel cells, electrolytic capacitors, electric double layer capacitors, solar cells, electrochromic display elements, etc. Hereinafter, a description will be given taking batteries (particularly secondary batteries) as an example.

[0173] <Secondary battery> The secondary battery according to this embodiment includes a positive electrode, a negative electrode, and a non-aqueous electrolyte solution, which is any one of the first to third non-aqueous electrolyte solutions described above.

[0174] (positive electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer, and the positive electrode mixture layer is formed on the positive electrode current collector and is usually formed into a sheet shape.

[0175] Examples of metals used for the positive electrode current collector include iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum. Among these, aluminum is preferred. The shape and dimensions of the positive electrode current collector are not particularly limited.

[0176] The positive electrode mixture layer is formed from a positive electrode mixture (positive electrode composition) that contains a positive electrode active material, a conductive additive, a binder, a solvent for dispersing these components, and the like.

[0177] In the secondary battery according to this embodiment, the positive electrode (positive electrode mixture) is preferably LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 Ternary positive electrode active materials such as O2; LiFePO4, LiFe 0.995 Mn 0.005 Suitable positive electrode active materials include iron phosphate-based positive electrode active materials having an olivine structure such as PO4, etc. These positive electrode active materials may be used alone or in combination of two or more.

[0178] The positive electrode preferably contains at least one of the above-mentioned ternary positive electrode active material and iron phosphate positive electrode active material, but may also contain other positive electrode active materials. The other positive electrode active materials may be any materials capable of absorbing and releasing various ions (lithium ions, sodium ions, etc.), and may be, for example, positive electrode active materials used in conventionally known secondary batteries (lithium ion secondary batteries, sodium ion secondary batteries), etc.

[0179] As the positive electrode active material used in a lithium-ion secondary battery, for example, lithium cobaltate; lithium nickelate; lithium manganate; LiNi 1-v-w Co x Al y O2 (0 ≦ v ≦ 1, 0 ≦ w ≦ 1), transition metal oxides such as ternary oxides other than the above-mentioned ternary positive electrode active materials; compounds having an olivine structure such as LiAPO4 (A = Mn, Ni, Co); solid solution materials incorporating a plurality of transition metals (solid solution of electrochemically inactive layered Li2MnO3 and electrochemically active layered LiMO2 (M = transition metal such as Co, Ni)); LiCo x Mn 1-x O2 (0 ≦ x ≦ 1); LiNi x Mn 1-x O2 (0 ≦ x ≦ 1); compounds having a fluorinated olivine structure such as Li2APO4F (A = Fe, Mn, Ni, Co); sulfur and the like can be used. These may be used alone or in combination of two or more.

[0180] As the positive electrode active material used in a sodium-ion secondary battery, for example, NaNiO2, NaCoO2, NaMnO2, NaVO2, NaFeO2, Na(Ni X Mn 1-X )O2 (0 < X < 1), Na(Fe X Mn 1-X )O2 (0 < X < 1), NaVPO4F, Na2FePO4F, Na3V2(PO4)3 and the like. These may be used alone or in combination of two or more.

[0181] Among other positive electrode active materials, particularly, a positive electrode active material capable of occluding and releasing lithium ions may be preferably used. Such a positive electrode active material is used, for example, in a lithium-ion secondary battery using a non-aqueous electrolyte. Such a non-aqueous system usually has lower ionic conductivity than an aqueous system, but in the present disclosure, even in such a case, the discharge capacity can be improved efficiently.

[0182] From the viewpoint of improving the output characteristics and electrical characteristics of the secondary battery, the content of the positive electrode active material (total content when multiple positive electrode active materials are included) is preferably 75% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more, relative to 100% by mass of the total amount of components included in the positive electrode composite, and is preferably 99% by mass or less, more preferably 98% by mass or less, and even more preferably 95% by mass or less.

[0183] Conductive additives are used to improve the output of lithium-ion secondary batteries. Conductive carbon is mainly used as the conductive additive. Examples of conductive carbon include carbon black, fibrous carbon, and graphite. Each conductive additive may be used alone, or two or more types may be used in combination. Among conductive additives, carbon black is preferred. Examples of carbon black include ketjen black and acetylene black. From the viewpoint of improving the output characteristics and electrical characteristics of lithium-ion secondary batteries, the content of the conductive additive in the non-volatile matter of the positive electrode mixture is preferably 1 to 20 mass %, more preferably 1.5 to 10 mass %.

[0184] Examples of binders include fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene; synthetic rubbers such as styrene-butadiene rubber and nitrile butadiene rubber; polyamide-based resins such as polyamideimide; polyolefin-based resins such as polyethylene and polypropylene; poly(meth)acrylic resins; polyacrylic acid; and cellulose-based resins such as carboxymethyl cellulose. Each binder may be used alone, or two or more types may be used in combination. Furthermore, the binder may be in a state of being dissolved in a solvent or dispersed in a solvent when used.

[0185] Examples of the solvent include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, tetrahydrofuran, acetonitrile, acetone, ethanol, ethyl acetate, and water. Each of the solvents may be used alone, or two or more of them may be used in combination. The amount of the solvent used is not particularly limited and may be determined appropriately depending on the production method and the materials used.

[0186] The positive electrode mixture may contain other components as needed, such as polymers such as non-fluorinated polymers such as (meth)acrylic polymers, nitrile polymers, and diene polymers, and fluorinated polymers such as polytetrafluoroethylene; emulsifiers such as anionic emulsifiers, nonionic emulsifiers, and cationic emulsifiers; dispersants such as polymer dispersants such as styrene-maleic acid copolymers and polyvinylpyrrolidone; thickeners such as carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyacrylic acid (salts), and alkali-soluble (meth)acrylic acid-(meth)acrylic acid ester copolymers; preservatives, etc. The content of other components in the non-volatile content of the positive electrode mixture is preferably 0 to 15% by mass, more preferably 0 to 10% by mass.

[0187] The positive electrode mixture can be prepared, for example, by mixing a positive electrode active material, a conductive additive, a binder, a solvent, and other components as necessary, and dispersing the mixture using a bead mill, a ball mill, an agitator mixer, or the like.

[0188] The method for forming the positive electrode (coating method) is not particularly limited, and examples thereof include: (1) a method in which a positive electrode composite is applied to a positive electrode current collector by a conventional coating method (e.g., a doctor blade method, etc.) (and then dried); (2) a method in which a positive electrode current collector is immersed in the positive electrode composite (and then dried); (3) a method in which a sheet formed from the positive electrode composite is bonded to a positive electrode current collector (e.g., bonded via a conductive adhesive) and pressed (and then dried); (4) a method in which a positive electrode composite to which a liquid lubricant has been added is applied or cast onto a positive electrode current collector, formed into a desired shape, and then the liquid lubricant is removed (and then stretched in uniaxial or multiaxial directions); and (5) a method in which a positive electrode composite (or a solid content forming a positive electrode composite layer) is slurried with an electrolyte, transferred in a semi-solid state to a current collector (positive electrode current collector), and used as an electrode (positive electrode) without drying.

[0189] The positive electrode mixture layer may be dried or pressed after being formed or coated (applied), as needed.

[0190] (Negative electrode) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer, and the negative electrode mixture layer is formed on the negative electrode current collector and is usually formed into a sheet shape.

[0191] Examples of metals used for the negative electrode current collector include iron, copper, aluminum, nickel, stainless steel (SUS), titanium, tantalum, gold, and platinum. Among these, copper is preferred. The shape and dimensions of the negative electrode current collector are not particularly limited.

[0192] The negative electrode mixture layer is formed from a negative electrode mixture (negative electrode composition) that contains a negative electrode active material, a conductive additive, a binder, a solvent for dispersing these components, and the like.

[0193] The negative electrode active material may be any conventionally known negative electrode active material used in various batteries (e.g., lithium secondary batteries), as long as it is capable of absorbing and releasing various ions (e.g., lithium ions). Specific negative electrode active materials that can be used include graphite materials such as artificial graphite and natural graphite, mesophase sintered bodies made from coal and petroleum pitch, carbon materials such as non-graphitizable carbon, Si-based negative electrode materials such as Si, Si alloys, and SiO, Sn-based negative electrode materials such as Sn alloys, lithium metal, and lithium alloys such as lithium-aluminum alloys. The negative electrode active materials may be used alone or in combination of two or more.

[0194] The negative electrode mixture may further contain a conductive additive (conductive substance), a binder, a solvent, etc. The conductive additive, binder, solvent, etc. may be the same components as those described above. The proportions used are also the same as those described above.

[0195] The negative electrode may be manufactured by the same method as the positive electrode.

[0196] (separator) The secondary battery may include a separator. The separator is disposed to separate the positive electrode from the negative electrode. There are no particular limitations on the separator, and any conventionally known separator can be used in the present disclosure. Specific examples of the separator include porous sheets made of polymers capable of absorbing and retaining an electrolyte solution (non-aqueous electrolyte solution) (e.g., polyolefin-based microporous separators, cellulose-based separators, etc.), nonwoven fabric separators, porous metal bodies, etc.

[0197] Examples of the material for the porous sheet include polyethylene, polypropylene, and a laminate having a three-layer structure of polypropylene / polyethylene / polypropylene.

[0198] Examples of materials for the nonwoven fabric separator include cotton, rayon, acetate, nylon, polyester, polypropylene, polyethylene, polyimide, aramid, and glass. Depending on the required mechanical strength, the above-mentioned materials may be used alone or in combination of two or more.

[0199] (battery exterior materials) A battery element including a positive electrode, a negative electrode, and a non-aqueous electrolyte (and a separator) is usually housed in a battery exterior material to protect the battery element from external impacts during battery use, environmental degradation, etc. The material of the battery exterior material is not particularly limited, and any conventionally known exterior material can be used.

[0200] If necessary, the battery exterior may contain expanded metal, an overcurrent prevention element such as a fuse or a PTC element, lead plates, etc. to prevent pressure buildup inside the battery and overcharging and discharging.

[0201] The shape of the battery (lithium ion secondary battery, etc.) is not particularly limited, and any of the conventionally known shapes of batteries (lithium ion secondary batteries, etc.) can be used, such as cylindrical, square, laminated, coin, large, etc. Furthermore, when used as a high-voltage power source (several tens to several hundreds of volts) to be mounted on electric vehicles, hybrid electric vehicles, etc., it can also be made into a battery module consisting of individual batteries connected in series.

[0202] The rated charging voltage of a secondary battery (lithium ion secondary battery, etc.) is not particularly limited, but when the secondary battery has a positive electrode containing the above-described ternary positive electrode active material as a main component, it may be 3.6 V or higher, preferably 4.0 V or higher, more preferably 4.1 V or higher, and even more preferably 4.2 V or higher. The higher the rated charging voltage, the higher the energy density can be, but from the viewpoint of safety, etc., the rated charging voltage may be 4.6 V or lower (e.g., 4.5 V or lower).

[0203] <Secondary battery manufacturing method> The secondary battery according to this embodiment can be easily manufactured by, for example, stacking a positive electrode and a negative electrode (with a separator interposed therebetween as necessary), placing the resulting laminate in a battery casing, injecting a nonaqueous electrolyte into the battery casing, and sealing the battery casing.

[0204] Here, in the method for producing a secondary battery according to this embodiment, in order to make the non-aqueous electrolyte solution contained in the secondary battery have dissolved therein CO2 or the like, as necessary, as described above, (A) in the step of preparing the non-aqueous electrolyte solution, CO2 or the like is dissolved in the non-aqueous electrolyte solution, specifically, the non-aqueous electrolyte solution according to this embodiment (electrolyte solution having dissolved therein CO2 or the like) is used as the non-aqueous electrolyte solution, or (B) in the step of producing the secondary battery, CO2 or the like is dissolved in the non-aqueous electrolyte solution, specifically, the non-aqueous electrolyte solution is injected into the battery in a CO2 atmosphere, or the air in the battery after the non-aqueous electrolyte solution is injected is replaced with CO2. [Example]

[0205] The present disclosure will be described below based on examples. Note that the present disclosure is not limited to the following examples, and the following examples can be modified or changed based on the spirit of the present disclosure, and such modifications are not excluded from the scope of the present disclosure.

[0206] <Reference Example Series> [Reference Examples 1 to 10] [Preparation of non-aqueous electrolyte] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Corporation) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent, to the concentrations shown in Table 1. Subsequently, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitric acid compound (12) was added to the obtained solution in the amount shown in Table 1, stirred for 1 day, and the insoluble matter was filtered through a membrane filter to prepare a nonaqueous electrolyte (hereinafter also simply referred to as "electrolyte"). In Reference Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the obtained solution in the amount shown in Table 1. In Reference Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the fluorooxalato compound (13) to the solution obtained above so that the content shown in Table 1 was achieved.

[0207] In the following description, a non-aqueous electrolyte in which CO2, etc. are not intentionally dissolved is referred to as a "reference electrolyte." This reference electrolyte is obtained without going through the dissolving step (before the dissolving step), and may contain CO2, etc. in the raw materials or CO2, etc. that is unavoidably dissolved.

[0208] [Dissolving CO2 in the electrolyte (dissolution process: pressurization process and substitution process)] Each reference electrolyte solution obtained above was placed in a sealed bottle to fill approximately 1 / 10 of the bottle's volume. The bottle was then placed in an autoclave in an open state with the opening facing upward, and the bottle was pressurized to 0.5 MPa with CO2. The release valve was then partially opened to reduce the internal pressure of the autoclave to 0.1 MPa (1 atmosphere). This process was repeated three times, and the air in the autoclave was replaced with CO2. After the replacement, the pressure was again increased to 0.5 MPa with CO2 and the autoclave was left standing for 30 minutes. After the 30-minute stand, the electrolyte solution was removed from the autoclave and left standing at room temperature (approximately 25°C) in a sealed state for an additional two weeks. After the two-week stand, a portion of the electrolyte solution (e.g., Reference Example 3 in Table 1) that had been left standing at 0.5 MPa for 30 minutes was taken and diluted with a reference electrolyte solution of the same salt composition to prepare electrolyte solutions with different dissolved CO2 concentrations (e.g., Reference Examples 7 to 10 in Table 1).

[0209] [Quantitative determination of dissolved CO2 in electrolyte] Each of the CO2-dissolved electrolyte solutions obtained above was analyzed by gas chromatography, and the amount of CO2 dissolved in the electrolyte solution (initial amount of CO2 dissolved) was quantified by the following method.

[0210] The amount of CO2 dissolved in the non-aqueous electrolyte was determined by the following method using gas chromatography (apparatus: GC-2010 plus, manufactured by Shimadzu Corporation; column: Micropacked ST, manufactured by Shinwa Kako Co., Ltd.).

[0211] (Gas chromatography measurement conditions) When measuring by gas chromatography, the electrolyte was directly introduced into the gas chromatography apparatus in a nitrogen purged state (under a nitrogen atmosphere) to prevent air from entering the measurement system. The specific measurement conditions for gas chromatography are as follows:

[0212] Column temperature program: 37°C hold (2.5 minutes from start) ⇒ 37°C to 250°C (heating at 20°C / min) ⇒ 250°C to 270°C (heating at 15°C / min) ⇒ 270°C hold (5.42 minutes) Vaporization chamber temperature: 130℃ Detector temperature: 300℃ (BID) Carrier gas: Helium (column flow rate 1.33 mL / min) Injection volume: 1 μL (split method, split ratio: 5.0) (Method for quantifying dissolved CO2) Several types of standard helium gas with known CO2 mixing ratios were analyzed under the same conditions as the gas chromatography measurement conditions described above, except that the injection volume (1 μL) was changed to 1 mL. From the peak area of ​​the obtained CO2 gas, a calibration curve showing the relationship between the amount of CO2 mixed (dissolved amount) and the peak area of ​​CO2 gas was created. Next, the nonaqueous electrolyte obtained in each Reference Example was analyzed by gas chromatography. Finally, the amount of CO2 dissolved in each nonaqueous electrolyte was quantified using the external standard method.

[0213] [Fabrication of Laminated Battery] (Preparation of positive electrode) LiNi, a ternary positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (manufactured by Umicore, product number: MX7h), acetylene black (AB, manufactured by Denka Co., Ltd., product name: Denka Black (registered trademark)), graphite (manufactured by Nippon Graphite Industries Co., Ltd., product number: SP270), and polyvinylidene fluoride (PVdF, manufactured by Kureha Corporation, product number: KF1120) were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode composite slurry (positive electrode active material: AB: graphite: PVdF = 93:2:2:3 (solid mass ratio)). Subsequently, the obtained positive electrode composite slurry was applied to an aluminum foil (positive electrode current collector, manufactured by Nippon Foil Co., Ltd., thickness 15 μm) so that the coating weight after drying was 19.4 mg / cm. 2 The mixture was coated on one side with an applicator so that the density was 3.1 g / cm , and then dried on a hot plate at 110°C for 10 minutes. It was then dried in a vacuum drying oven at 110°C for 12 hours. The density was then adjusted to 3.1 g / cm using a roll press. 3 The mixture was pressed into a sheet-like positive electrode (thickness: 83 μm) by molding until the sheet reached the desired thickness.

[0214] (Preparation of negative electrode) Graphite (natural graphite (Hitachi Chemical Co., Ltd., product number: SMG):artificial graphite (TIMCAL, product number: SFG15) = 85:15 (solid content mass ratio)) as the negative electrode active material, styrene-butadiene rubber (SBR, binder), and carboxymethyl cellulose (CMC, binder) were dispersed in ultrapure water to prepare a negative electrode composite slurry (negative electrode active material: SBR:CMC = 97.3:1.5:1.2 (solid content mass ratio)). Subsequently, the obtained negative electrode composite slurry was applied to a copper foil (negative electrode current collector, Fukuda Metal Foil & Powder Co., Ltd., thickness 15 μm) so that the coating weight after drying was 9.8 mg / cm. 2 The mixture was coated on one side with an applicator so that the density was 1.3 g / cm , and then dried on a hot plate at 80°C for 10 minutes. It was then dried in a vacuum drying oven at 100°C for 12 hours. The mixture was then pressed with a roll press to a density of 1.3 g / cm . 3 The mixture was pressed until it reached a thickness of 90 μm, thereby obtaining a sheet-shaped negative electrode (thickness: 90 μm).

[0215] (Making laminated batteries) The resulting positive and negative electrodes were cut, and the polarity leads were ultrasonically welded. The positive and negative electrodes were placed opposite each other with a 16 μm polyethylene (PE) separator interposed between them, and the three sides were sealed with a laminate exterior to prepare a non-filled battery. Subsequently, 700 μL of each electrolyte solution shown in Table 1 was added to one of the unsealed sides of the non-filled battery.

[0216] After the electrolyte injection, the battery was precharged at 0.2 C (6 mA) for 2 hours in an unpacked state. The battery was then vacuum-sealed and left at room temperature for 3 days. After 3 days, it was charged at 0.5 C (15 mA) for 5 hours to 4.2 V, followed by a charge-discharge cycle at 0.2 C (6 mA) with a termination at 2.75 V. One piece of the laminate was then opened, and the battery was vacuum-sealed again to allow degassing. After degassing, the battery was charged and discharged under the following conditions; this operation was considered the conditioning step. This resulted in the production of a 4.2 V, 30 mAh capacity laminate battery (cell). (Conditioning conditions) Charge: 0.5C (15mA), 4.2V for 5 hours (25℃) ⇒ Discharge: 1C (30mA), 2.75V termination (25℃) [Battery evaluation] (impedance) The cells after the conditioning process were subjected to a constant current charge of 4.2 V, 1 C (30 mA) for 30 minutes at room temperature using a charge-discharge tester (ASKA Electronics Co., Ltd., product number: ACD-01; the same applies hereinafter) until the state of charge (SOC) reached 50%. The cells were then subjected to impedance measurements at frequencies from 1 GHz to 1 mHz at -30°C using an impedance analyzer (Bio Logic, product number: VSP-300). The real axis resistance (interface resistance) was calculated from the frequency at which the arcs of the measured values ​​diverged. The frequency at which the arcs diverged refers to the frequency at which the imaginary axis value reached a minimum between 10 Hz and 0.001 Hz.

[0217] (Initial DCR) After impedance measurements, the cells were charged at room temperature at a constant current of 1 C (30 mA) and a constant voltage of 0.02 C (0.6 mA) at 4.2 V, resulting in a fully charged state (SOC 100%). After 30 minutes of rest, the cells were discharged at 6 mA for 10 seconds, then at 30 mA for 10 seconds, and then at 60 mA for 10 seconds. The difference in closed-circuit voltage (ΔV) between the start of discharge and 10 seconds after discharge at each discharge current was plotted on the horizontal axis, and the slope of the IV line was taken as the cell's "initial DCR."

[0218] (DCR after 300 cycles) After the initial DCR measurement, the cells were subjected to a cycle test at 45°C under the following charge / discharge conditions (cycle conditions) for a total of 300 cycles. After 300 cycles, the DCR at 25°C in a fully charged state ("DCR after 300 cycles") was measured in the same manner. (Cycle conditions) Charging: 4.2V, constant current / constant voltage charging at 1C (30mA), stopping at 0.02C (0.6mA), 10 minute rest Discharge: Constant current (CC) discharge at 1C (30mA), terminate at 2.75V, rest for 10 minutes.

[0219] (DCR increase rate after 300 cycles) Furthermore, using the measurement results of "initial DCR" and "DCR after 300 cycles", the "DCR increase rate after 300 cycles" was calculated by the following formula (1). [Number 1] "DCR increase rate after cycling" = "DCR after 300 cycles" / "initial DCR" (1).

[0220] (self-discharge) After the conditioning process, the cell was charged at room temperature with a constant current and constant voltage of 1 C (30 mA) and terminated at 0.02 C (0.6 mA) at 4.2 V until it reached a fully charged state (SOC 100%). After charging, the open circuit voltage (OCV) of the cell was measured at room temperature. This measured value was designated as the "initial OCV."

[0221] The cell was then stored for 4 weeks at 60° C. After storage, the cell was cooled to room temperature for 3 hours, and the OCV of the cell was measured at room temperature, and this measured value was defined as the "OCV after storage."

[0222] In addition, the measurement results of "initial OCV" and "OCV after storage" were used to calculate the "OCV difference (ΔV) before and after storage" using the following formula (2). [Number 2] "OCV difference before and after storage (ΔV)" = "initial OCV" - "OCV after storage" (2) The smaller the "OCV difference before and after storage (ΔV)", that is, the smaller the degree of decrease in "OCV after storage" compared to "initial OCV", the more suppressed the battery's self-discharge.

[0223] (DCR after 4 weeks of endurance at 60℃) After initial DCR measurement, the cells were charged at room temperature at 1C (30mA) to 4.2V with a cutoff voltage of 0.02C (0.6mA). After storing the cells at 60°C for 4 weeks in a fully charged state (SOC 100%), they were discharged at room temperature at 1C (30mA) with a cutoff voltage of 2.75V to measure the remaining capacity. Subsequently, they were charged at room temperature at 1C (30mA) with a cutoff voltage of 0.02C (0.6mA) to 4.2V, followed by a constant current discharge at 1C (30mA) with a cutoff voltage of 2.75V to measure the recovered capacity. The DCR of the cells after the recovered capacity measurement was measured under the same conditions as the initial DCR measurement, and this measurement was designated as the "DCR after 4 weeks at 60°C."

[0224] (DCR increase rate after 4 weeks at 60℃) In addition, using the measurement results of "initial DCR" and "DCR after 4 weeks of endurance at 60°C", the "DCR increase rate after 4 weeks of endurance at 60°C" was calculated by the following formula (3). [Number 3] "DCR increase rate after 4 weeks at 60°C" = "DCR after 4 weeks at 60°C" / "Initial DCR" (3).

[0225] [Quantitative determination of dissolved CO2 in electrolyte after conditioning process] The cells after the conditioning process were stored at room temperature for 4 weeks at an SOC of 50%. After storage, the cells were disassembled in argon gas, and the electrolyte was sampled. The sampled electrolyte was analyzed by gas chromatography using the same method as above to quantify the amount of CO2 dissolved in the CO2-dissolved electrolyte after the conditioning process ("amount of CO2 dissolved after the conditioning process").

[0226] From the above, the impedance, DCR before and after cycling (initial DCR, DCR after 300 cycles) and the DCR increase rate, DCR before and after 4 weeks of endurance at 60°C (initial DCR, DCR after 4 weeks of endurance at 60°C) and the DCR increase rate, OCV before and after storage (initial OCV, OCV after storage) and the OCV difference (ΔV), and the amount of CO2 dissolved in the non-aqueous electrolyte before and after the conditioning process (initial CO2 dissolved amount, CO2 dissolved amount after conditioning process) were measured and the results are shown in Table 1.

[0227] [Reference Comparative Examples 1 to 6, 8] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 1. In Reference Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Reference Comparative Example 5, LiBOB was further added to the solution obtained above to the content shown in Table 1, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Reference Comparative Example 6, LiDFOB was further added to the solution obtained above to the content shown in Table 1, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Reference Comparative Example 8, LiNO3 was further added to the solution obtained above to the content shown in Table 1, and the mixture was stirred for one day and filtered through a membrane filter to prepare a nonaqueous electrolyte. Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 1.

[0228] [Reference Comparative Example 7] A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 1. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 1.

[0229] [Table 1]

[0230] [Considerations on Table 1 (Reference Example Series)] Comparison of Reference Examples 1 to 4 (or Reference Comparative Examples 1 to 4) revealed that, depending on the concentration of LiFSI, the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased, while the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased. A comparison of Reference Comparative Example 3 and Reference Comparative Example 8, which have the same salt composition, showed that adding LiNO3 to the electrolyte significantly reduced impedance and DCR, and also reduced the rate of increase in DCR after 300 cycles and after 4 weeks at 60°C, while increasing self-discharge. By comparing Reference Examples 3, 7 to 10 and Reference Comparative Example 8, which have the same salt composition, it was found that by dissolving CO2 in the electrolyte, the difference in OCV (ΔV) before and after storage is small, that is, self-discharge caused by the addition of LiNO3 is suppressed, and the impedance and DCR are further reduced. In this way, it was found that batteries using electrolytes containing both LiNO3 and dissolved CO2 can achieve a synergistic effect of reducing impedance and DCR and suppressing self-discharge. Furthermore, it was found that this synergistic effect becomes more pronounced when the amount of dissolved CO2 in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). Comparison of Reference Examples 3, 5, and 6 with Reference Comparative Examples 3, 5, and 6, which have the same salt composition, revealed that the addition of LiBOB or LIDFOB to an electrolyte containing LiNO3 significantly increases the impedance and DCR (especially LiBOB), but the addition of LiNO3 and dissolving CO2 in the electrolyte suppresses the increase in impedance and DCR. By comparing Reference Examples 3, 5, and 6 (or Reference Comparative Examples 3, 5, and 6) with the same salt composition, it was found that by further adding LiBOB or LIDFOB to an electrolyte containing LiNO3, the rate of increase in DCR after 300 cycles was small, i.e., the increase in DCR after 300 cycles was suppressed, and self-discharge was also suppressed. 。

[0231] 〔 Battery evaluation (additional) (DCR increase rate with the same salt composition) The increase in "DCR after 4 weeks at 60°C" due to the addition of a specific compound to a non-aqueous electrolyte solution having the same salt composition (DCR increase rate with the same salt composition) was calculated using the following formula (4). The "DCR increase rate with the same salt composition" is shown in the column for each example or comparative example containing a specific additive. [Number 4] "DCR increase rate with the same salt composition" = "DCR after 4 weeks at 60°C with a non-aqueous electrolyte containing the same salt composition and specific compounds" / "DCR after 4 weeks at 60°C with a non-aqueous electrolyte containing the same salt composition and specific compounds" (4) For example, in Table 2, the DCR increase rate for the same salt composition of "1.2M LiPF6" can be calculated by dividing "DCR after 4 weeks at 60°C of Comparative Example 9" by "DCR after 4 weeks at 60°C of Comparative Example 1."

[0232] (Lithium electrodeposition amount after low-temperature cycle) After the conditioning process, each cell was charged at room temperature using a charge / discharge tester at a constant current of 4.2 V, 1 C (30 mA), and a constant voltage of 0.6 mA, followed by a constant current discharge of 0.2 C (6 mA) and a constant current of 2.75 V, and the initial capacity was measured. Subsequently, 200 cycles were performed under the conditions of a constant current charge of 4.2 V, 2 C (60 mA), followed by a constant current discharge of 1 C (30 mA) and a constant current of 2.75 V, with one cycle consisting of one cycle. The capacity after 200 cycles was measured at room temperature under the same conditions as when the initial capacity was measured. The capacity difference ΔC between the initial capacity and the capacity after 200 cycles was converted to per gram of positive electrode active material, and this converted value was defined as the "amount of lithium electrodeposited on the negative electrode after the low-temperature cycle" (amount of lithium electrodeposited after the low-temperature cycle). 。

[0233] < Example 2-1 Series> [Examples 1 to 4, 10, and 12] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 3. Trimethyl borate (Compound B, manufactured by Tokyo Chemical Industry Co., Ltd.) as the boron atom-containing compound (3) represented by general formula (3) was added to the solution obtained above to the content shown in Table 3, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). In Example 10, bistrimethylsilyl sulfate (Compound J, manufactured by Tokyo Chemical Industry Co., Ltd.) as the sulfur atom-containing compound (5) represented by general formula (5) was further added to the solution obtained above to the content shown in Table 3. In Example 12, tris(trimethylsilyl) phosphate (Compound N, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the phosphorus atom-containing compound (8) represented by general formula (8) to the solution obtained above so as to achieve the content shown in Table 3. Subsequently, cells were prepared using each of the reference electrolyte solutions obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 3.

[0234] [Examples 5 to 9, 11] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitric acid compound (12) to the solution obtained above to the content shown in Table 3, and the mixture was stirred for one day and then filtered through a membrane filter. In Example 11, bistrimethylsilyl sulfate (compound J) was further added to the solution obtained above to the content shown in Table 3. Subsequently, a dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the obtained CO2-dissolved electrolytes, and the cells were evaluated. The results are shown in Table 3.

[0235] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 3. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 3, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 3, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, LiNO3 was further added to the solution obtained above to the content shown in Table 3, and the mixture was stirred for one day and then filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, trimethyl borate (compound B) was further added to the solution obtained above to the content shown in Table 3, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 3.

[0236] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 3. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 3.

[0237] [Table 3]

[0238] [Discussion of Table 3 (Example 2-1 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding trimethyl borate (compound B) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding trimethyl borate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of trimethyl borate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it is clear that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is suppressed. It is clear that adding trimethyl borate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It is clear that adding trimethyl borate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding trimethyl borate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, which increases the effect of adding trimethyl borate, and that the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. By comparing Example 3 with Examples 5 to 8, which have the same salt composition, it was found that dissolving CO2 in an electrolyte containing trimethyl borate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. By comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing trimethyl borate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. Comparison of Example 3 with Examples 10 and 12 (without dissolved CO2) and Example 5 with Example 11 (with dissolved CO2) using the same salt composition revealed that the impedance and DCR were significantly reduced by further adding bistrimethylsilyl sulfate (Compound J) or tris(trimethylsilyl) phosphate (Compound N) to an electrolyte containing trimethyl borate. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of trimethyl borate using the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles using the same salt composition with the addition of trimethyl borate. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0239] <Example 2-2 Series> [Examples 1 to 4, 11, and 12] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 4. Tris(trimethylsilyl)borate (Compound C, manufactured by Tokyo Chemical Industry Co., Ltd.) as the boron atom-containing compound (3) represented by general formula (3) was added to the obtained solution to the content shown in Table 4, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). In Example 11, polytrimethylsilyl phosphate (Compound M, manufactured by Sigma-Aldrich Co., Ltd.) as the phosphorus atom-containing compound (6) represented by general formula (6) was further added to the obtained solution to the content shown in Table 4. In Example 12, tris(trimethylsilyl) phosphate (Compound N, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the phosphorus atom-containing compound (8) represented by general formula (8) to the solution obtained above so as to achieve the content shown in Table 4. Subsequently, cells were prepared using each of the reference electrolyte solutions obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 4.

[0240] [Examples 5 to 10] A non-aqueous electrolyte (reference electrolyte) was prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitrate compound (12) to the solution obtained above to the content shown in Table 4, and after stirring for one day, the solution was filtered through a membrane filter. In Example 10, LiNO3 and , Ji Methoxydiphenylsilane (Compound A) was further added to the solution to the contents shown in Table 4, and after stirring for one day, the solution was filtered through a membrane filter. Subsequently, the dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Using the obtained CO2-dissolved electrolyte, cells were prepared in the same manner as described above, and the cells were evaluated. The results are shown in Table 4.

[0241] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 4. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 4, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 4, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, LiNO3 was further added to the solution obtained above to the content shown in Table 4, and the mixture was stirred for one day and then filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, tris(trimethylsilyl)borate (compound C) was further added to the solution obtained above to the content shown in Table 4, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 4.

[0242] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 4. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 4.

[0243] [Table 4]

[0244] [Discussion of Table 4 (Example 2-2 Series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding tris(trimethylsilyl)borate (compound C) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding tris(trimethylsilyl)borate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of tris(trimethylsilyl)borate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it is clear that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It is clear that the addition of tris(trimethylsilyl)borate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It is clear that the addition of tris(trimethylsilyl)borate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding tris(trimethylsilyl)borate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a film derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing tris(trimethylsilyl)borate further enhances the impedance and DCR reduction effects, self-discharge suppression effects, and low-temperature electrodeposition suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. By comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing tris(trimethylsilyl)borate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. By comparing Example 3 with Examples 11 and 12 (without dissolved CO2) using the same salt composition, it was found that the impedance and DCR were reduced by further adding trimethylsilyl polyphosphate (compound M) or tris(trimethylsilyl) phosphate (compound N) to an electrolyte containing tris(trimethylsilyl)borate. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of tris(trimethylsilyl)borate to the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of tris(trimethylsilyl)borate to the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0245] <Example 2-3 Series> [Examples 1 to 4, 10] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 5. Triethyl borate (Compound D, manufactured by Tokyo Chemical Industry Co., Ltd.) as the boron atom-containing compound (3) represented by general formula (3) was added to the solution obtained above to the content shown in Table 5, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). In Example 10, bistrimethylsilyl sulfate (Compound J, manufactured by Tokyo Chemical Industry Co., Ltd.) as the sulfur atom-containing compound (5) represented by general formula (5) was further added to the solution obtained above to the content shown in Table 5. Next, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 5.

[0246] [Examples 5 to 9, 11] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitric acid compound (12) to the solution obtained above to the content shown in Table 5, and the solution was stirred for one day and then filtered through a membrane filter. In Example 11, bistrimethylsilyl sulfate (compound J) was further added to the solution obtained above to the content shown in Table 5. Subsequently, a dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the obtained CO2-dissolved electrolytes, and the cells were evaluated. The results are shown in Table 5.

[0247] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 5. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 5, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 5, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, LiNO3 was further added to the solution obtained above to the content shown in Table 5, and the mixture was stirred for one day and then filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, triethyl borate (compound D) was further added to the solution obtained above to the content shown in Table 5, and the mixture was stirred for one day to prepare a non-aqueous electrolyte solution. Subsequently, cells were prepared using each of the reference electrolyte solutions obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 5.

[0248] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF in the same mixed solvent as above to the concentration shown in Table 5. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 5.

[0249] [Table 5]

[0250] [Discussion of Table 5 (Examples 2-3 Series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding triethyl borate (compound D) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding triethyl borate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of triethyl borate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It can be seen that adding triethyl borate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that low-temperature charging accelerates deterioration of the negative electrode. It can be seen that adding triethyl borate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding triethyl borate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a film derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing triethyl borate further enhances the effects of reducing impedance and DCR, suppressing self-discharge, and suppressing low-temperature electrodeposition. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. By comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing triethyl borate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. A comparison between Example 3 and Example 10 (without dissolved CO2) and a comparison between Example 5 and Example 11 (with dissolved CO2) with the same salt composition showed that the impedance and DCR were reduced by further adding bistrimethylsilyl sulfate (Compound J) to an electrolyte containing triethyl borate. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of triethyl borate using the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles using the same salt composition with the addition of triethyl borate. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0251] <Example 2-4 Series> [Examples 1 to 4, 10] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 6. Tris(2,2,2)trifluoroethyl borate (Compound E, manufactured by Tokyo Chemical Industry Co., Ltd.) as the boron atom-containing compound (3) represented by general formula (3) was added to the obtained solution to the content shown in Table 6, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). In Example 10, trimethylsilyl trifluoromethanesulfonate (Compound I, manufactured by Tokyo Chemical Industry Co., Ltd.) as the sulfur atom-containing compound (5) represented by general formula (5) was further added to the obtained solution to the content shown in Table 6. Next, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 6.

[0252] [Examples 5 to 9, 11] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitric acid compound (12) to the solution obtained above to the content shown in Table 6, and the mixture was stirred for one day and then filtered through a membrane filter. In Example 11, trimethylsilyl trifluoromethanesulfonate (Compound I) was further added to the solution obtained above to the content shown in Table 6. Subsequently, a dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the obtained CO2-dissolved electrolytes, and the cells were evaluated. The results are shown in Table 6.

[0253] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 6. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 6, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 6, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, LiNO3 was further added to the solution obtained above to the content shown in Table 6, and the mixture was stirred for one day and then filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, tris(2,2,2)trifluoroethyl borate (Compound E) was further added to the solution obtained above to the content shown in Table 6, and the mixture was stirred for one day to prepare a non-aqueous electrolyte. Subsequently, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 6.

[0254] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 6. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 6.

[0255] [Table 6]

[0256] [Discussion of Table 6 (Examples 2-4 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR and DCR after 300 cycles) decreased depending on the concentration of LiFSI, the reduction effect became significant by adding tris(2,2,2)trifluoroethyl borate (compound E) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, but by adding tris(2,2,2)trifluoroethyl borate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparison of Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and comparison of Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of tris(2,2,2)trifluoroethyl borate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it is clear that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It is clear that the addition of tris(2,2,2)trifluoroethyl borate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It is clear that the addition of tris(2,2,2)trifluoroethyl borate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding tris(2,2,2)trifluoroethyl borate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be because a film derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing tris(2,2,2)trifluoroethyl borate further enhances the impedance and DCR reduction effects, self-discharge suppression effects, and low-temperature electrodeposition suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. By comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing tris(2,2,2)trifluoroethyl borate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. A comparison of Example 3 and Example 10 (without dissolved CO2) with the same salt composition, and a comparison of Example 5 and Example 11 (with dissolved CO2) with the same salt composition, revealed that the impedance and DCR were reduced by further adding trimethylsilyl trifluoromethanesulfonate (Compound I) to an electrolyte containing tris(2,2,2)trifluoroethyl borate. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of tris(2,2,2)trifluoroethyl borate with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of tris(2,2,2)trifluoroethyl borate with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0257] <Example 2-5 Series> [Examples 1 to 4, 10] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 7. Tributyl borate (Compound F, manufactured by Tokyo Chemical Industry Co., Ltd.) as the boron atom-containing compound (3) represented by general formula (3) was added to the solution obtained above to the content shown in Table 7, and the mixture was stirred for 1 day to prepare a nonaqueous electrolyte (reference electrolyte). In Example 10, bistrimethylsilyl sulfate (Compound J, manufactured by Tokyo Chemical Industry Co., Ltd.) as the sulfur atom-containing compound (5) represented by general formula (5) was further added to the solution obtained above to the content shown in Table 7. Next, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 7.

[0258] [Examples 5 to 9, 11] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitric acid compound (12) to the solution obtained above to the content shown in Table 7, and the solution was stirred for one day and then filtered through a membrane filter. In Example 11, bistrimethylsilyl sulfate (compound J) was further added to the solution obtained above to the content shown in Table 7. Subsequently, a dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the obtained CO2-dissolved electrolytes, and the cells were evaluated. The results are shown in Table 7.

[0259] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 7. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 7, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 7, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, LiNO3 was further added to the solution obtained above to the content shown in Table 7, and the mixture was stirred for one day and then filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, tributyl borate (compound F) was further added to the solution obtained above to the content shown in Table 7, and the mixture was stirred for one day to prepare a non-aqueous electrolyte solution. Subsequently, cells were prepared using each of the reference electrolyte solutions obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 7.

[0260] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 7. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 7.

[0261] [Table 7]

[0262] [Discussion of Table 7 (Examples 2-5 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, but the reduction effect became more pronounced by adding tributyl borate (compound F) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding tributyl borate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of tributyl borate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It can be seen that adding tributyl borate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It can be seen that adding tributyl borate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding tributyl borate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a film derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing tributyl borate further enhances the effects of reducing impedance and DCR, suppressing self-discharge, and suppressing low-temperature electrodeposition. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. Comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing tributyl borate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. A comparison between Example 3 and Example 10 (without dissolved CO2) and a comparison between Example 5 and Example 11 (with dissolved CO2) with the same salt composition revealed that the impedance and DCR were reduced by further adding bistrimethylsilyl sulfate (Compound J) to an electrolyte containing tributyl borate. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of tributyl borate to the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of tributyl borate to the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0263] <Example 3-1 Series> [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent, to the concentrations shown in Table 8. Trimethylsilyl acetate (Compound G, manufactured by Tokyo Chemical Industry Co., Ltd.) as the carbon-containing compound (4) represented by general formula (4) was added to the solution obtained above to the content shown in Table 8, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 8.

[0264] [Examples 5 to 8] A non-aqueous electrolyte (reference electrolyte) was prepared in the same manner as described above. Subsequently, the dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Using the resulting CO2-dissolved electrolyte, a cell was fabricated in the same manner as described above, and the cell was evaluated. The results are shown in Table 8.

[0265] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 8. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 8, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 8, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitric acid compound (12) was further added to the solution obtained above to the content shown in Table 8, and after stirring for one day, the mixture was filtered through a membrane filter to prepare a non-aqueous electrolyte. In Comparative Example 9, trimethylsilyl acetate (compound G) was further added to the solution obtained above to the content shown in Table 8, and the mixture was stirred for one day to prepare a non-aqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 8.

[0266] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 8. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 8.

[0267] [Table 8]

[0268] [Discussion of Table 8 (Example 3-1 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, but the reduction effect became more pronounced by adding trimethylsilyl acetate (compound G) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding trimethylsilyl acetate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of trimethylsilyl acetate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It can be seen that adding trimethylsilyl acetate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It can be seen that adding trimethylsilyl acetate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode is suppressed. The reason why the effect of adding trimethylsilyl acetate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a film derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Examples 1 to 8 revealed that the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI. However, by dissolving CO2 in an electrolyte containing trimethylsilyl acetate, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed, and the impedance, DCR, and low-temperature electrodeposition were further reduced. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. It was found that the impedance and DCR reduction effects and the self-discharge suppression effects become significant when the amount of CO2 dissolved in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of trimethylsilyl acetate using the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles using the same salt composition with the addition of trimethylsilyl acetate. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0269] <Example 3-2 Series> [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 as the electrolyte solvent, to the concentrations shown in Table 9. Trimethylsilyl trifluoroacetate (Compound H, manufactured by Tokyo Chemical Industry Co., Ltd.) as the carbon-containing compound (4) represented by general formula (4) was added to the solution obtained above to the content shown in Table 9, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 9.

[0270] [Examples 5 to 8] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Using the resulting CO2-dissolved electrolytes, cells were fabricated in the same manner as described above, and the cells were evaluated. The results are shown in Table 9.

[0271] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 9. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 9, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 9, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitric acid compound (12) was further added to the solution obtained above to the content shown in Table 9, and after stirring for one day, the solution was filtered through a membrane filter to prepare a non-aqueous electrolyte. In Comparative Example 9, trimethylsilyl trifluoroacetate (Compound H) was further added to the solution obtained above to the content shown in Table 9, and the solution was stirred for one day to prepare a non-aqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 9.

[0272] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 9. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 9.

[0273] [Table 9]

[0274] [Discussion of Table 9 (Example 3-2 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding trimethylsilyl trifluoroacetate (compound H) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding trimethylsilyl trifluoroacetate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance and self-discharge suppression effect and a higher DCR increase rate reduction effect due to the addition of trimethylsilyl trifluoroacetate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It can be seen that adding trimethylsilyl trifluoroacetate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It can be seen that adding trimethylsilyl trifluoroacetate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding trimethylsilyltrifluoroacetate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Examples 1 to 8 revealed that the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI. However, by dissolving CO2 in an electrolyte containing trimethylsilyl trifluoroacetate, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed, and the impedance and DCR were further reduced. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. It was found that the impedance and DCR reduction effects and the self-discharge suppression effects become significant when the amount of CO2 dissolved in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks of durability at 60°C with the addition of trimethylsilyl trifluoroacetate to the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the same salt composition with the addition of trimethylsilyl trifluoroacetate to the DCR increase rate after 300 cycles. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0275] <Example 4-1 Series> [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 10. Trimethylsilyl trifluoromethanesulfonate (Compound I, manufactured by Tokyo Chemical Industry Co., Ltd.) as the sulfur atom-containing compound (5) represented by general formula (5) was added to the solution obtained above to the content shown in Table 10, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 10.

[0276] [Examples 5 to 8] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the CO2-dissolved electrolytes obtained, and the cells were evaluated. The results are shown in Table 10.

[0277] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 10. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 10, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 10, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitric acid compound (12) was further added to the solution obtained above to the content shown in Table 10, and after stirring for one day, the mixture was filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, trimethylsilyl trifluoromethanesulfonate (Compound I) was further added to the solution obtained above to the content shown in Table 10, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 10.

[0278] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 10. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 10.

[0279] [Table 10]

[0280] [Discussion of Table 10 (Example 4-1 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding trimethylsilyl trifluoromethanesulfonate (Compound I) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding trimethylsilyl trifluoromethanesulfonate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance and self-discharge suppression effect and DCR increase suppression effect due to the addition of trimethylsilyl trifluoromethanesulfonate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It can be seen that adding trimethylsilyl trifluoromethanesulfonate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It can be seen that adding trimethylsilyl trifluoromethanesulfonate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding trimethylsilyl trifluoromethanesulfonate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Examples 1 to 8 revealed that the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI. However, by dissolving CO2 in an electrolyte containing trimethylsilyl trifluoromethanesulfonate, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed, and the impedance and DCR were further reduced. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. It was found that the impedance and DCR reduction effects and the self-discharge suppression effects become significant when the amount of CO2 dissolved in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of trimethylsilyl trifluoromethanesulfonate using the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles using the same salt composition with the addition of trimethylsilyl trifluoromethanesulfonate. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0281] <Example 4-2 Series> [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 11. Bistrimethylsilyl sulfate (Compound J, manufactured by Tokyo Chemical Industry Co., Ltd.) as the sulfur atom-containing compound (5) represented by general formula (5) was added to the solution obtained above to the content shown in Table 11, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 11.

[0282] [Examples 5 to 10] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitrate compound (12) to the solution obtained above to the contents shown in Table 11, and the mixture was stirred for one day and then filtered through a membrane filter. In Example 10, LiNO3 and lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) were further added to the solution obtained above to the contents shown in Table 11, respectively, and the mixture was stirred for one day and then filtered through a membrane filter. Subsequently, a dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated using the obtained CO2-dissolved electrolytes in the same manner as described above, and the cells were evaluated. The results are shown in Table 11.

[0283] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 11. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 11, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 11, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitric acid compound (12) was further added to the solution obtained above to the content shown in Table 11, and after stirring for one day, the mixture was filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, bistrimethylsilyl sulfate (compound J) was further added to the solution obtained above to the content shown in Table 11, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 11.

[0284] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 11. Subsequently, the dissolution process was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 11.

[0285] [Table 11]

[0286] [Discussion of Table 11 (Example 4-2 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding bistrimethylsilyl sulfate (compound J) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding bistrimethylsilyl sulfate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of bistrimethylsilyl sulfate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after low-temperature cycling is reduced depending on the LiFSI concentration, and that deterioration of the negative electrode is reduced. It can be seen that adding bistrimethylsilyl sulfate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during low-temperature cycling, and negative electrode deterioration progresses. It can be seen that adding bistrimethylsilyl sulfate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after low-temperature cycling, and that deterioration of the negative electrode is suppressed. The reason why the effect of adding bistrimethylsilyl sulfate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 with Examples 5 to 8, which have the same salt composition, shows that dissolving CO2 in an electrolyte containing bistrimethylsilyl sulfate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. Comparing Examples 5 and 9 and Comparative Example 8, which have the same salt composition, we found that adding LiNO3 to an electrolyte containing bistrimethylsilyl sulfate significantly reduced impedance and DCR. Meanwhile, dissolving CO2 in the electrolyte suppressed self-discharge due to the addition of LiNO3, although self-discharge increased. Comparing the DCR increase rate after 4 weeks of 60°C durability with the addition of bistrimethylsilyl sulfate using the same salt composition, the electrolyte containing a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte containing a single LiPF6 salt, demonstrating a greater suppression of the DCR increase rate. This is also true for the DCR increase rate after 300 cycles using the same salt composition with the addition of bistrimethylsilyl sulfate. This is likely due to the formation of an anionic LiFSI coating on the positive and negative electrodes.

[0287] <Example 4-3 Series> [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (volume ratio) as the electrolyte solvent to the concentrations shown in Table 12. Ditertiary butylsilyl bistrifluoromethanesulfonate (Compound K, manufactured by Tokyo Chemical Industry Co., Ltd.) as the sulfur atom-containing compound (5) represented by general formula (5) was added to the resulting solution in the amount shown in Table 12, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 12.

[0288] [Examples 5 to 9] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitric acid compound (12) to the solution obtained above to the content shown in Table 12, and after stirring for one day, the solution was filtered through a membrane filter. Subsequently, using each of the reference electrolytes obtained above, a dissolution step was carried out in the same manner as described above. Using the obtained CO2-dissolved electrolytes, cells were fabricated in the same manner as described above, and the cells were evaluated. The results are shown in Table 12.

[0289] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 12. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 12, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 12, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitric acid compound (12) was further added to the solution obtained above to the content shown in Table 12, and after stirring for one day, the mixture was filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, di-tert-butylsilyl bistrifluoromethanesulfonate (Compound K) was further added to the solution obtained above to the content shown in Table 12, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were prepared using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 12.

[0290] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 12. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 12.

[0291] [Table 12]

[0292] [Discussion of Table 12 (Example 4-3 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after endurance at 60°C) decreased depending on the concentration of LiFSI, but the reduction effect became more pronounced by adding di-tert-butylsilyl bistrifluoromethanesulfonate (compound K) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding di-tertiarybutylsilyl bistrifluoromethanesulfonate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance and DCR reduction effect and DCR fluctuation suppression effect due to the addition of di-tert-butylsilyl bistrifluoromethanesulfonate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it is clear that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It is clear that the addition of ditertiarybutylsilylbistrifluoromethanesulfonate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It is clear that the addition of ditertiarybutylsilylbistrifluoromethanesulfonate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding di-tert-butylsilyl bistrifluoromethanesulfonate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Examples 1 to 8 revealed that the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI. However, by dissolving CO2 in an electrolyte containing di-tert-butylsilyl bistrifluoromethanesulfonate, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was further suppressed, and the impedance and DCR were further reduced. The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. It was found that the impedance and DCR reduction effects and the self-discharge suppression effects become significant when the amount of CO2 dissolved in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. Comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing di-tert-butylsilyl bistrifluoromethanesulfonate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of ditertiarybutylsilylbistrifluoromethanesulfonate with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of ditertiarybutylsilylbistrifluoromethanesulfonate with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0293] <Example 4-4 Series> [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent, to the concentrations shown in Table 13. To the resulting solution, tertiary butyldimethylsilyl trifluoromethanesulfonate (Compound Z, manufactured by Tokyo Chemical Industry Co., Ltd.) was added as the sulfur atom-containing compound (5) represented by general formula (5) to the content shown in Table 13, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 13.

[0294] [Examples 5 to 9] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitrate compound (12) to the solution obtained above to the content shown in Table 13, and after stirring for one day, the solution was filtered through a membrane filter. Subsequently, using each of the reference electrolytes obtained above, a dissolution step was carried out in the same manner as described above. Using the obtained CO2-dissolved electrolytes, cells were fabricated in the same manner as described above, and the cells were evaluated. The results are shown in Table 13.

[0295] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 13. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 13, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 13, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitric acid compound (12) was further added to the solution obtained above to the content shown in Table 13, and after stirring for one day, the mixture was filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, tertiary butyldimethylsilyl trifluoromethanesulfonate (Compound Z) was further added to the solution obtained above to the content shown in Table 13, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 13.

[0296] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 13. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 13.

[0297] [Table 13]

[0298] [Discussion of Table 13 (Example 4-4 Series)] A comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the LiFSI concentration, adding tert-butyldimethylsilyl trifluoromethanesulfonate (Compound Z) to the electrolyte significantly reduced the impedance and DCR. Furthermore, a comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the difference in OCV (ΔV) before and after storage increased depending on the LiFSI concentration, i.e., self-discharge increased, adding tert-butyldimethylsilyl trifluoromethanesulfonate to the electrolyte reduced the difference in OCV (ΔV) before and after storage, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a greater effect of reducing impedance and DCR and suppressing DCR fluctuations due to the addition of tert-butyldimethylsilyl trifluoromethanesulfonate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It can be seen that adding tertiary butyldimethylsilyl trifluoromethanesulfonate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It can be seen that adding tertiary butyldimethylsilyl trifluoromethanesulfonate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding tertiary-butyldimethylsilyl trifluoromethanesulfonate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Examples 1 to 8 revealed that the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI. However, by dissolving CO2 in an electrolyte containing tertiary butyldimethylsilyl trifluoromethanesulfonate, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was further suppressed, and the impedance and DCR were further reduced. The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. It was found that the impedance and DCR reduction effects and the self-discharge suppression effects become significant when the amount of CO2 dissolved in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. Comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing tertiary butyldimethylsilyl trifluoromethanesulfonate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. When comparing the DCR increase rate after 4 weeks of use at 60°C with the addition of tertiary butyldimethylsilyl trifluoromethanesulfonate to the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of tertiary butyldimethylsilyl trifluoromethanesulfonate to the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0299] <Example 5-1 Series> [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent, to the concentrations shown in Table 14. To the resulting solution, bis(2,2,2-trifluoroethyl) phosphite (Compound L, manufactured by Tokyo Chemical Industry Co., Ltd.) as the phosphorus atom-containing compound (6) represented by general formula (6) was added in the amount shown in Table 14, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 14.

[0300] [Examples 5 to 9] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitrate compound (12) to the solution obtained above to the content shown in Table 14, and the solution was stirred for one day and then filtered through a membrane filter. Subsequently, a dissolution step was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the obtained CO2-dissolved electrolytes, and the cells were evaluated. The results are shown in Table 14.

[0301] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 14. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 14, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 14, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitrate compound (12) was further added to the solution obtained above to the content shown in Table 14, and the mixture was stirred for one day and then filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, bis(2,2,2-trifluoroethyl) phosphite (Compound L) was further added to the solution obtained above to the content shown in Table 14, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 14.

[0302] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 14. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 14.

[0303] [Table 14]

[0304] [Discussion of Table 14 (Example 5-1 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, but the reduction effect became more pronounced by adding bis(2,2,2-trifluoroethyl) phosphite (compound L) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding bis(2,2,2-trifluoroethyl) phosphite to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase rate suppression effect due to the addition of bis(2,2,2-trifluoroethyl) phosphite than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it is clear that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It is clear that adding bis(2,2,2-trifluoroethyl) phosphite to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It is clear that adding bis(2,2,2-trifluoroethyl) phosphite to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding bis(2,2,2-trifluoroethyl)phosphite is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing bis(2,2,2-trifluoroethyl) phosphite further enhanced the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects were more pronounced when the amount of CO2 dissolved in the electrolyte exceeded 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. By comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing bis(2,2,2-trifluoroethyl) phosphite significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of bis(2,2,2-trifluoroethyl) phosphite with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of bis(2,2,2-trifluoroethyl) phosphite with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0305] <Example 5-2 Series> [Example 1~ 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent, to the concentrations shown in Table 15. Polytrimethylsilyl phosphate (Compound M, manufactured by Sigma-Aldrich Co., Ltd.) as the phosphorus atom-containing compound (6) represented by general formula (6) was added to the solution obtained above to the content shown in Table 15, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15.

[0306] ( 31 P-NMR analysis) The above Sigma-Aldrich reagent trimethylsilyl polyphosphate (compound M) 31 Analysis by P-NMR confirmed two peaks: a peak (Pt) appearing at chemical shifts of -28 ppm to -33 ppm and a peak (Pm) appearing at chemical shifts of -35 ppm to -41 ppm. The integral ratio of the two peaks was Pt:Pm = 1.00:1.43. On the other hand, a peak (Pb) appearing at -41 ppm to -45 ppm was not confirmed. 31P-NMR measurements were performed using a JNM-ECA500 manufactured by JEOL (Japan Electronics Corporation) with a double sample tube as the sample tube, and the chemical shift was determined with the phosphorus peak of H3PO4 added to one of the tubes as 0 ppm.

[0307] [Examples 5 to 9] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. In Example 9, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as the nitrate compound (12) to the solution obtained above to the content shown in Table 15, and after stirring for one day, the solution was filtered through a membrane filter. Subsequently, using each of the reference electrolytes obtained above, a dissolution step was carried out in the same manner as described above. Using the obtained CO2-dissolved electrolytes, cells were fabricated in the same manner as described above, and the cells were evaluated. The results are shown in Table 15.

[0308] [Comparative Examples 1 to 6, 8, and 9] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 15. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 15, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 15, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitrate compound (12) was further added to the solution obtained above to the content shown in Table 15, and after stirring for one day, the mixture was filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Example 9, trimethylsilyl polyphosphate (compound M) was further added to the solution obtained above to the content shown in Table 15, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15.

[0309] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 15. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 15.

[0310] [Table 15]

[0311] [Discussion of Table 15 (Example 5-2 series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding trimethylsilyl polyphosphate (compound M) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding trimethylsilyl polyphosphate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 9 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase suppression effect due to the addition of trimethylsilyl polyphosphate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 4, 7, and 9 with Examples 2 to 9, it can be seen that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It can be seen that adding trimethylsilyl polyphosphate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging progresses. It can be seen that adding trimethylsilyl polyphosphate to a mixed salt composition containing LiFSI and LiPF6 reduces the amount of electrodeposition after the low-temperature cycle, and that deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding trimethylsilyl polyphosphate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing trimethylsilyl polyphosphate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. By comparing Examples 5 and 9 with Comparative Example 8, which have the same salt composition, it was found that the addition of LiNO3 to an electrolyte containing trimethylsilyl polyphosphate significantly reduced the impedance and DCR. On the other hand, although self-discharge increased, it was found that the self-discharge caused by the addition of LiNO3 was suppressed by dissolving CO2 in the electrolyte. The reason why the self-discharge suppression effect of dissolved CO2 is greater in electrolytes with a mixed salt composition containing LiFSI and LiPF6 than in electrolytes with a single salt composition containing LiPF6 is thought to be related to the order in which the film formed on the positive and negative electrodes during charging is made up of the anion component of FSI, and the film formed by the lithium carbonate component due to CO2. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of trimethylsilyl polyphosphate with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of trimethylsilyl polyphosphate with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0312] <Example 5-2(2) Series> (Synthesis of trimethylsilyl polyphosphate) 1.553 g of diphosphorus pentoxide was dispersed in 10 mL of methylene chloride solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring. The mixture was then stirred at room temperature for about a day. The solvent was then distilled off to synthesize trimethylsilyl polyphosphate (compound M2).

[0313] ( 31 P-NMR analysis) The synthesized trimethylsilyl polyphosphate (compound M2) 31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -28 ppm to -33 ppm, a peak (Pm) appearing at chemical shifts of -35 ppm to -41 ppm, and a peak (Pb) appearing at chemical shifts of -41 ppm to -45 ppm. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:6.73:1.00. From these analysis results, compound M2 is presumed to be polytrimethylsilyl phosphate containing many branched structures (branched structures represented by the above structural formula (6a-3)).

[0314] [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) as the electrolyte solvent to the concentrations shown in Table 15-2. Polytrimethylsilyl phosphate (compound M2) as the phosphorus atom-containing compound (6) represented by general formula (6) was added to the resulting solution to the content shown in Table 15-2, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-2.

[0315] [Examples 5 to 8] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the CO2-dissolved electrolytes obtained, and the cells were evaluated. The results are shown in Table 15-2.

[0316] [Comparative Examples 1 to 5] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 15-2. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, trimethylsilyl polyphosphate (compound M2) was further added to the solution obtained above to the content shown in Table 15-2, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-2.

[0317] [Table 15-2]

[0318] [Discussion of Table 15-2 (Example 5-2(2) Series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding trimethylsilyl polyphosphate (compound M2) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding trimethylsilyl polyphosphate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Furthermore, although the reason is unclear, among trimethylsilyl polyphosphate, 31The compound (compound M2) for which a Pb peak was confirmed by P-NMR analysis was found to have a greater suppression of self-discharge than the Sigma-Aldrich reagent (compound M) for which no Pb peak was confirmed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 5 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase suppression effect due to the addition of trimethylsilyl polyphosphate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 5 and Examples 2 to 8, it is found that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It is found that when trimethylsilyl polyphosphate is added to a simple salt composition containing only LiPF6, lithium electrodeposition on the negative electrode increases during the low-temperature cycle, and deterioration of the negative electrode due to low-temperature charging progresses. It is found that when trimethylsilyl polyphosphate is added to a mixed salt composition containing LiFSI and LiPF6, the amount of electrodeposition after the low-temperature cycle decreases, and deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding trimethylsilyl polyphosphate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing trimethylsilyl polyphosphate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of trimethylsilyl polyphosphate with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of trimethylsilyl polyphosphate with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0319] <Example 5-2(3) Series> (Synthesis of trimethylsilyl polyphosphate) 1.553 g of diphosphorus pentoxide was dispersed in 10 mL of toluene as a solvent, and 1.710 g of hexamethylenedisiloxane was gradually added dropwise while stirring. The mixture was then stirred at room temperature for about a day. The solvent was then distilled off to synthesize trimethylsilyl polyphosphate (compound M3).

[0320] ( 31 P-NMR analysis) The synthesized trimethylsilyl polyphosphate (compound M3) 31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -28 ppm to -33 ppm, a peak (Pm) appearing at chemical shifts of -35 ppm to -41 ppm, and a peak (Pb) appearing at chemical shifts of -41 ppm to -45 ppm. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:6.51:0.81. From these analysis results, compound M3 is presumed to be polytrimethylsilyl phosphate containing many branched structures (branched structures represented by the above structural formula (6a-3)).

[0321] [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent to the concentrations shown in Table 15-3. Polytrimethylsilyl phosphate (compound M3) as the phosphorus atom-containing compound (6) represented by general formula (6) was added to the resulting solution to the content shown in Table 15-3, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-3.

[0322] [Examples 5 to 8] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the CO2-dissolved electrolytes obtained, and the cells were evaluated. The results are shown in Table 15-3.

[0323] [Comparative Examples 1 to 5] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 15-3. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, trimethylsilyl polyphosphate (compound M3) was further added to the solution obtained above to the content shown in Table 15-3, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-3.

[0324] [Table 15-3]

[0325] [Discussion of Table 15-3 (Example 5-2(3) series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding trimethylsilyl polyphosphate (compound M3) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding trimethylsilyl polyphosphate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Furthermore, although the reason is unclear, among trimethylsilyl polyphosphate, 31 The compound (compound M3) for which a Pb peak was confirmed by P-NMR analysis was found to have a greater suppression of self-discharge than the Sigma-Aldrich reagent (compound M) for which no Pb peak was confirmed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 5 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase suppression effect due to the addition of trimethylsilyl polyphosphate than a single salt composition containing only LiPF6. Comparing Comparative Examples 1 to 5 and Examples 2 to 8, it is found that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It is found that when trimethylsilyl polyphosphate is added to a simple salt composition containing only LiPF6, lithium electrodeposition on the negative electrode increases during the low-temperature cycle, and deterioration of the negative electrode due to low-temperature charging progresses. It is found that when trimethylsilyl polyphosphate is added to a mixed salt composition containing LiFSI and LiPF6, the amount of electrodeposition after the low-temperature cycle decreases, and deterioration of the negative electrode due to low-temperature charging is suppressed. The reason why the effect of adding trimethylsilyl polyphosphate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing trimethylsilyl polyphosphate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of trimethylsilyl polyphosphate with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of trimethylsilyl polyphosphate with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0326] <Example 5-2(4) Series> (Synthesis of ethyl polyphosphate) 10 g of diphosphorus pentoxide, 10 g of chloroform as a solvent, and 20 g of diethyl ether were placed in a test tube and stirred at 900 rpm and 35°C for 3 days. Then, the solvent was removed using an evaporator, and the residue was dried in vacuum for 24 hours to synthesize ethyl polyphosphate (compound T).

[0327] ( 31 P-NMR analysis) The synthesized ethyl polyphosphate (compound T)31 Analysis by P-NMR in the same manner as above confirmed three peaks: a peak (Pt) appearing at chemical shifts of -12 ppm to -15 ppm, a peak (Pm) appearing at chemical shifts of -25 ppm to -31 ppm, and a peak (Pb) appearing at chemical shifts of -39 ppm to -46 ppm. The integral ratio of the three peaks was Pt:Pm:Pb = 1.00:3.59:0.42. From these analysis results, compound T is presumed to be polyethyl phosphate containing many branched structures (branched structures represented by the above structural formula (6a-3)).

[0328] [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent to the concentrations shown in Table 15-4. Ethyl polyphosphate (Compound T) as the phosphorus atom-containing compound (6) represented by general formula (6) was added to the resulting solution to the content shown in Table 15-4, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-4.

[0329] [Examples 5 to 8] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the CO2-dissolved electrolytes obtained, and the cells were evaluated. The results are shown in Table 15-4.

[0330] [Comparative Examples 1 to 5] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 15-4. In Comparative Examples 1 to 4, the solutions obtained above were used as non-aqueous electrolytes (reference electrolytes). In Comparative Example 5, ethyl polyphosphate (compound T) was further added to the solution obtained above to the content shown in Table 15-4, and the mixture was stirred for one day to prepare a non-aqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-4. Table 15-4 Shown below.

[0331] [Table 15-4]

[0332] [Discussion of Table 15-4 (Example 5-2(4) series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became more pronounced by adding ethyl polyphosphate (compound T) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding ethyl polyphosphate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons between Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and between Comparative Example 5 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase suppression effect due to the addition of ethyl polyphosphate than a single salt composition containing only LiPF6. The reason why the effect of adding ethyl polyphosphate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a film derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. By comparing Example 3 with Examples 5 to 8, which have the same salt composition, it was found that dissolving CO2 in an electrolyte solution containing ethyl polyphosphate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte solution exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of ethyl polyphosphate with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of ethyl polyphosphate with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0333] <Example 5-2(5) Series> (Synthesis of triisopropylsilyl polyphosphate) 0.53 g of indium(III) bromide was dissolved in 30 mL of tetrahydrofuran, followed by the addition of 4.75 g of triisopropylsilane and stirring at room temperature for one day. 30 mL of hexane was added to the reaction solution after the reaction, and the mixture was allowed to stand, resulting in separation into two layers. The separated upper layer was concentrated under reduced pressure to obtain 4.01 g of hexaisopropyldisiloxane, a colorless liquid. Next, 1.55 g of phosphorus pentoxide and 1.70 g of the hexaisopropyldisiloxane obtained above were added to 10 mL of dichloromethane, and the mixture was stirred at 35°C for three days to allow the reaction to proceed. The reaction solution was filtered and concentrated under reduced pressure to synthesize 2.10 g of triisopropylsilyl polyphosphate (Compound W) as a viscous liquid.

[0334] [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent, to the concentrations shown in Table 15-5. Poly(triisopropylsilyl)phosphate (Compound W) as the phosphorus atom-containing compound (6) represented by general formula (6) was added to the resulting solution in the amount shown in Table 15-5, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-5.

[0335] [Examples 5 to 8] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the CO2-dissolved electrolytes obtained, and the cells were evaluated. The results are shown in Table 15-5.

[0336] [Comparative Examples 1 to 5] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 15-5. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, poly(triisopropylsilyl)phosphate (Compound W) was further added to the solution obtained above to the content shown in Table 15-5, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-5.

[0337] [Table 15-5]

[0338] [Discussion of Table 15-5 (Example 5-2(5) series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became significant by adding poly(triisopropylsilyl)phosphate (compound W) to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding poly(triisopropylsilyl)phosphate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparison of Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and Comparison Example 5 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase suppression effect due to the addition of poly(triisopropylsilyl)phosphate than a single salt composition containing only LiPF6. The reason why the effect of adding ethyl polyphosphate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a film derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. By comparing Example 3 with Examples 5 to 8, which have the same salt composition, it was found that dissolving CO2 in an electrolyte containing poly(triisopropylsilyl)phosphate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 mass ppm (e.g., 20 mass ppm or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of triisopropylsilyl polyphosphate with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single LiPF6 salt composition, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of ethyl polyphosphate with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0339] <Example 5-2(6) Series> (Synthesis of (tert-butyl)dimethylsilyl polyphosphate) In Example 5-2(5) series, poly(triisopropylsilyl)phosphate (Compound W) was synthesized by carrying out the same procedure as in the synthesis thereof, except that the triisopropylsilane used in the synthesis was changed to tert-butyldimethylsilane, thereby synthesizing poly((tert-butyl)dimethylsilyl)phosphate (Compound X).

[0340] [Examples 1 to 4] An electrolyte salt having a mixed salt composition containing LiFSI (Nippon Shokubai Co., Ltd.) and LiPF6 (Stellar Chemifa Co., Ltd.) was dissolved in a mixed solvent (Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent to the concentrations shown in Table 15-6. Polyphosphate [(tert-butyl)dimethylsilyl] (Compound X) as the phosphorus atom-containing compound (6) represented by general formula (6) was added to the resulting solution in the amount shown in Table 15-6, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-6.

[0341] [Examples 5 to 8] Non-aqueous electrolytes (reference electrolytes) were prepared in the same manner as described above. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Cells were fabricated in the same manner as described above using the CO2-dissolved electrolytes obtained, and the cells were evaluated. The results are shown in Table 15-6.

[0342] [Comparative Examples 1 to 5] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 15-6. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, poly[(tert-butyl)dimethylsilyl] phosphate (Compound X) was further added to the solution obtained above to the content shown in Table 15-6, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were fabricated using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 15-6.

[0343] [Table 15-6]

[0344] [Discussion of Table 15-6 (Example 5-2(6) series)] Comparison of Examples 1 to 4 with Comparative Examples 2 to 4 revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decreased depending on the concentration of LiFSI, the reduction effect became significant when polyphosphate [(tert-butyl)dimethylsilyl] (compound X) was added to the electrolyte. Furthermore, by comparing Examples 1 to 4 with Comparative Examples 2 to 4, it was found that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the concentration of LiFSI, by adding [(tert-butyl)dimethylsilyl] polyphosphate to the electrolyte, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed. Comparisons of Examples 2 to 4 and Comparative Examples 2 to 4, which have the same salt composition, and Comparison Example 5 and Comparative Example 1, which have the same salt composition, revealed that the mixed salt composition containing LiFSI and LiPF6 has a higher impedance reduction effect, self-discharge suppression effect, and DCR increase suppression effect due to the addition of poly(tert-butyl)dimethylsilyl)phosphate than a single salt composition containing only LiPF6. The reason why the effect of adding poly(tert-butyl)dimethylsilyl]phosphate is greater in mixed salt compositions containing LiFSI and LiPF6 than in simple salt compositions containing only LiPF6 is thought to be that a coating derived from the anion component of LiFSI is formed at the interface between the positive and negative electrodes, and the high solubility of LiFSI reduces the viscosity of the electrolyte at low temperatures. A comparison of Example 3 and Examples 5 to 8, which have the same salt composition, revealed that dissolving CO2 in an electrolyte containing [(tert-butyl)dimethylsilyl] polyphosphate further enhances the impedance and DCR reduction effects and the self-discharge suppression effects. Furthermore, it was found that these effects become more pronounced when the amount of CO2 dissolved in the electrolyte exceeds 10 ppm by mass (e.g., 20 ppm by mass or more). The reason why these effects are improved by dissolving CO2 is thought to be that the formation of a CO2-derived coating on the positive and negative electrodes suppresses lithium ion intercurrent (intercalation reaction) in the positive electrode during storage, suppresses side reactions in the positive electrode, and improves low-temperature charging acceptance of the negative electrode. When comparing the DCR increase rate after 4 weeks at 60°C with the addition of polyphosphate [(tert-butyl)dimethylsilyl] with the same salt composition, the electrolyte with a mixed salt composition containing LiFSI and LiPF6 was significantly lower than the electrolyte with a single salt composition containing LiPF6, demonstrating a greater suppression effect on the DCR increase rate. This is also the case with the DCR increase rate after 300 cycles with the addition of polyphosphate [(tert-butyl)dimethylsilyl] with the same salt composition. This is thought to be due to the formation of an anionic coating of LiFSI on the positive and negative electrodes.

[0345] <Example 5-3 Series> [Examples 1 to 8] An electrolyte salt having a mixed salt composition containing LiFSI (manufactured by Nippon Shokubai Co., Ltd.) and LiPF6 (manufactured by Stella Chemifa Co., Ltd.) was dissolved in a mixed solvent (manufactured by Kishida Chemical Co., Ltd.) with a volume ratio of ethylene carbonate (EC):ethyl methyl carbonate (EMC) of 3:7 as the electrolyte solvent, to the concentrations shown in Table 16. Tris(trimethylsilyl) phosphate (Compound N, manufactured by Tokyo Chemical Industry Co., Ltd.) as the phosphorus atom-containing compound (8) represented by general formula (8) was added to the solution obtained above to the content shown in Table 16, and the mixture was stirred for one day to prepare a nonaqueous electrolyte (reference electrolyte). In Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitrate compound (12) was further added to the solution obtained above to the content shown in Table 16, and the mixture was stirred for one day and then filtered through a membrane filter. Subsequently, the dissolution process was carried out in the same manner as described above using each of the reference electrolytes obtained above. Using the obtained CO2-dissolved electrolyte, a cell was fabricated in the same manner as above, and the cell was evaluated. The results are shown in Table 16.

[0346] [Comparative Examples 1 to 6, 8 to 12] In the same mixed solvent as above, an electrolyte salt having a mixed salt composition containing LiFSI and LiPF6, or an electrolyte salt having a simple salt composition containing only LiPF6, was dissolved to the respective concentrations shown in Table 16. In Comparative Examples 1 to 4, the solution obtained above was used as a nonaqueous electrolyte (reference electrolyte). In Comparative Example 5, lithium bis(oxalato)borate (LiBOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 16, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 6, lithium difluorooxalatoborate (LiDFOB, manufactured by Tokyo Chemical Industry Co., Ltd.) as the fluorooxalato compound (13) was further added to the solution obtained above to the content shown in Table 16, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 8, lithium nitrate (LiNO3, manufactured by Tokyo Chemical Industry Co., Ltd.) as the nitrate compound (12) was further added to the solution obtained above to the content shown in Table 16, and the mixture was stirred for one day and then filtered through a membrane filter to prepare a nonaqueous electrolyte. In Comparative Examples 9 to 11, tris(trimethylsilyl) phosphate (Compound N) was further added to the solution obtained above to the content shown in Table 16, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. In Comparative Example 12, trimethylsilyl polyphosphate (Compound M) was further added to the solution obtained above to the content shown in Table 16, and the mixture was stirred for one day to prepare a nonaqueous electrolyte. Subsequently, cells were produced using each of the reference electrolytes obtained above in the same manner as above, except that the electrolyte dissolution step was not performed, and the cells were evaluated. The results are shown in Table 16.

[0347] Comparative Example 7 A nonaqueous electrolyte (reference electrolyte) was prepared by dissolving an electrolyte salt having a simple salt composition containing only LiPF6 in the same mixed solvent as above to the concentration shown in Table 16. Subsequently, the dissolution step was carried out in the same manner as above using the reference electrolyte obtained above. A cell was fabricated in the same manner as above using the obtained CO2-dissolved electrolyte, and the cell was evaluated. The results are shown in Table 16.

[0348] [Table 16]

[0349] [Discussion of Table 16 (Example 5-3 series)] Comparison of Comparative Examples 2-3 with Comparative Examples 10-11, which have the same salt composition, revealed that although the impedance and DCR (initial DCR, DCR after 300 cycles, and DCR after 4 weeks at 60°C) decrease depending on the concentration of LiFSI, the reduction effect becomes more pronounced by adding tris(trimethylsilyl) phosphate (compound N) to the electrolyte. A comparison of Examples 1 to 7 with Comparative Examples 10 and 11 revealed that although the difference in OCV (ΔV) before and after storage was large, i.e., self-discharge increased, depending on the LiFSI concentration, by dissolving CO2 in an electrolyte containing tris(trimethylsilyl) phosphate, the difference in OCV (ΔV) before and after storage was small, i.e., self-discharge was suppressed, and the impedance and DCR were further reduced. A comparison of Comparative Examples 1 to 3 and Comparative Examples 9 to 11 revealed that adding only tris(trimethylsilyl) phosphate did not have the effect of suppressing self-discharge. Comparing Comparative Examples 1 to 3, Comparative Examples 9 to 11, and Examples 2 to 7, it is clear that the amount of electrodeposition after the low-temperature cycle decreases depending on the LiFSI concentration, and that deterioration of the negative electrode due to low-temperature charging is reduced. It is clear that adding tris(trimethylsilyl)phosphate to a simple salt composition containing only LiPF6 increases lithium electrodeposition on the negative electrode during the low-temperature c...

Claims

1. LiN(FSO) as electrolyte ２ ) ２ and, A non-aqueous electrolyte solution containing, as an additive, at least one selected from the group consisting of a silicon atom-containing compound represented by general formula (14), a boron atom-containing compound represented by general formula (3), a sulfur atom-containing compound represented by general formula (5), and a phosphorus atom-containing compound represented by general formula (6). Si(R ５ ) ４ (14) (In formula (14), R ５ represents an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt, or a hydrogen atom. ５ are the same group.) B(OR ６ ) ３ (3) (In formula (3), R ６ is R ５ indicates the same thing as {S(=O) ２ (R ５ ) ｙ } ｘ (OR ６ ) ｘ（２－ｙ） (5) (In formula (5), R ５ and R ６ are the same or different and are the same as above, x is an integer of 1 or 2, and y is an integer of 0 or 1. When x is 2 and y is 1 (OR ６ ) ２ Is (-O-R ６ -O-), and formula (5) is S(=O) ２ (R ５ )-O-R ６ -O-S(=O) ２ (R ５ ) indicates. [-P(=O)(OR ６ )O-] ｎ (6) (In formula (6), R ６ represents the same as above. n represents the degree of polymerization.

2. Carbon dioxide (CO ２ ), carbon monoxide (CO), bicarbonate ion (HCO ３ － ) and carbonate ions (CO ３ ２－ 2. The non-aqueous electrolyte according to claim 1, wherein at least one of the following is dissolved therein:

3. LiN(FSO) as electrolyte ２ ) ２ and, The composition contains, as an additive, at least one selected from the group consisting of a silicon atom-containing compound represented by general formula (14), a boron atom-containing compound represented by general formula (3), a sulfur atom-containing compound represented by general formula (5), a phosphorus atom-containing compound represented by general formula (6), a phosphorus atom-containing compound represented by general formula (7), a phosphorus atom-containing compound represented by general formula (8), and a boroxine compound represented by general formula (9), Carbon dioxide (CO ２ ), carbon monoxide (CO), bicarbonate ion (HCO ３ － ) and carbonate ions (CO ３ ２－ ) dissolved in a non-aqueous electrolyte. Si(R ５ ) ４ (14) (In formula (14), R ５ represents an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), an aryl group (which may have a substituent), a silyl group (which may have a substituent), an alkali metal atom, an onium salt, or a hydrogen atom. ５ are the same group.) B(OR ６ ) ３ (3) (In formula (3), R ６ is R ５ indicates the same thing as {S(=O) ２ (R ５ ) ｙ } ｘ (OR ６ ) ｘ（２－ｙ） (5) (In formula (5), R ５ and R ６ are the same or different and are the same as above, x is an integer of 1 or 2, and y is an integer of 0 or 1. When x is 2 and y is 1 (OR ６ ) ２ Is (-O-R ６ -O-), and formula (5) is S(=O) ２ (R ５ )-O-R ６ -O-S(=O) ２ (R ５ ) indicates. [-P(=O)(OR ６ )O-] ｎ (6) (In formula (6), R ６ represents the same as above. n represents the degree of polymerization. P(OR ６ ) ３ (7) (In formula (7), R ６ indicates the same as above.) P(=O)(OR ６ ) ３ (8) (In formula (8), R ６ are the same and represent a trialkylsilyl group having 1 to 6 carbon atoms (which may have a substituent). 【Chemistry 1】 (In formula (9), R ７ are the same or different and represent an alkyl group having 1 to 6 carbon atoms (which may have a substituent), a fluoroalkyl group having 1 to 6 carbon atoms (which may have a substituent), or a cycloalkyl group (which may have a substituent).

4. The carbon dioxide (CO ２ ), carbon monoxide (CO), bicarbonate ion (HCO ３ － ) and carbonate ions (CO ３ ２－ 4. The nonaqueous electrolyte solution according to claim 2, wherein the total dissolved amount of at least one of the above is 20 ppm by mass or more.

5. The electrolyte is a compound represented by general formula (10), a compound represented by general formula (11), and LiAsF ６ The nonaqueous electrolyte solution according to claim 1 or 3, further comprising at least one selected from the group consisting of: LiPF ａ (C ｍ F ２ｍ＋１ ) ６－ａ (a:0≦a≦6、m:1≦m≦4) (10) LiBF ｂ (C ｎ F ２ｎ+１ ) ４－ｂ (b:0≦b≦4、n:1≦n≦4) (11)

6. The nonaqueous electrolyte solution according to claim 1 or 3, further comprising at least one selected from the group consisting of compounds represented by general formula (12) and compounds represented by general formula (13): M １ NO ３ (M １ : indicates an alkali metal element.) (12) 【Chemistry 2】 (In general formula (13), M ３ : B or P, A ｆ＋ : metal ion, H or onium ion, f: 1≦f≦3, g: 1≦g≦3, h: g / f, i: 1≦i≦3, j: 0≦j≦4, k: 0 or 1, R ３ an alkylene group having 1 to 10 carbon atoms or a halogenated alkylene group having 1 to 10 carbon atoms, R ４ F or a fluorinated alkyl group having 1 to 10 carbon atoms, T １ , T ２ : each independently represents O or S.

7. A secondary battery using the nonaqueous electrolyte solution according to any one of claims 1 to 3.

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