Nonaqueous electrolyte and clay-type lithium ion secondary battery using same

Abstract

The present invention addresses the problem of providing: a non-aqueous electrolyte solution which has excellent battery safety and excellent battery characteristics when used at high temperatures, which are important, in a vehicle-mounted secondary battery such as a recent electric vehicle; and a clay-type lithium ion secondary battery which uses the non-aqueous electrolyte solution. This nonaqueous electrolyte solution is obtained by dissolving an electrolyte salt in a nonaqueous solvent, and is characterized in that: the nonaqueous solvent contains at least one substance selected from among ethylene carbonate, propylene carbonate, and gamma-butyrolactone in a total amount of 80-100 vol%; and the non-aqueous electrolyte also contains trifluoroacetate of which the carbon chain length of an alcohol group is 6-8.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte having excellent safety of a battery when used at high temperatures, further having an initial capacity and a cycle characteristic, and a clay-type lithium ion secondary battery using the non-aqueous electrolyte. Background Art

[0002] In recent years, lithium ion secondary batteries have been widely used not only as power sources for small electronic devices and the like but also as power sources for electric vehicles and power storage. A lithium ion secondary battery mainly includes a positive electrode, a non-aqueous electrolyte, a separator, and a negative electrode. In particular, a lithium ion secondary battery using a lithium composite oxide containing Ni as the positive electrode and a carbon material or a titanium oxide as the negative electrode is preferably used. Further, as the electrolyte for the lithium ion secondary battery, a combination of a cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) and a chain carbonate such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC) is preferably used.

[0003] As a power source for in-vehicle secondary batteries such as electric vehicles, a non-combustible all-solid electrolyte has attracted attention, but the actual situation is that its performance is still very poor. Therefore, as a method for improving the thermal stability of in-vehicle secondary batteries using a solution-based electrolyte, there is a method of using EC, PC, or γ-butyrolactone (GBL) having a high flash point as the non-aqueous electrolyte. However, a non-aqueous electrolyte composed only of a cyclic compound cannot function as a battery or its performance deviates because it has no impregnability to a polyolefin separator. If a chain carbonate (DMC, EMC, DEC, etc.) is used to obtain impregnability to a polyolefin separator, there is a problem that the flash point of the entire electrolyte decreases to around 25°C due to the low flash point of the chain carbonate.

[0004] In Non-Patent Document 1, an electrolyte obtained by adding a trifluoroacetate to an EC / EMC-based electrolyte was proposed. Here, since EMC, a chain carbonate, is used, there is no problem with the impregnability of the polyolefin separator. In Document 1, the effect of the carbon chain length of the trifluoroacetate was focused on, and as a result, since the carbon chain length of n-hexyl trifluoroacetate is long, the dissociation ability of LiPF 6 is low, and the mobility of the solvated lithium ions is reduced. Therefore, methyl trifluoroacetate and ethyl trifluoroacetate having a shorter carbon chain length of the alcohol group of the trifluoroacetate are better.

[0005] Further, as a method for improving the impregnability of a polyolefin separator, in Patent Document 1, it was shown that by using an alkyl group (R) bonded to an oxygen atom added to a non-aqueous solvent (for example, EC, PC, GBL) 4An electrolyte solution composed of pivalic acid esters having 4 to 20 carbon atoms can improve the discharge capacity retention rate after 50 cycles at 20°C of the battery. In addition, in the examples, pivalic acid esters such as butyl pivalate, hexyl pivalate, octyl pivalate, and decyl pivalate with a carbon chain length of 4 or more in the alcohol group have a high affinity for the separator, and the microporous separator quickly penetrates into the porous structure of the separator, resulting in a shortening of the manufacturing time in the manufacturing process of the lithium-ion secondary battery. However, in the prior art disclosed in Patent Document 1, the viscosity of octyl pivalate (R 4 = n-octyl) at 25°C is 2.5 cP, which is higher than that of GBL (viscosity 1.8 cP), which has the lowest viscosity among the cyclic compounds used as the main solvent, and there is a concern about a decrease in battery characteristics accompanying the increase in viscosity.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent No. 4691871 Gazette.

[0009] Non-Patent Documents

[0010] Non-Patent Document 1: Journal of Fluorine Chemistry, 156, 136-143 (2013). Summary of the Invention

[0011] A clay-type lithium-ion secondary battery according to one embodiment of the present invention is a clay-type lithium-ion secondary battery having a positive electrode, a negative electrode, a polyolefin separator, and a non-aqueous electrolyte solution containing an electrolyte salt in a non-aqueous solvent. The non-aqueous electrolyte solution is a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent. The non-aqueous solvent is a non-aqueous solvent containing at least one selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone in a total amount of 80 to 100% by volume. The non-aqueous electrolyte solution further contains a trifluoroacetate having a carbon chain length of 6 to 8 in the alcohol group. Brief Description of the Drawings

[0012] Figure 1 It is a cross-sectional view showing the structure of the clay-type lithium-ion secondary battery. Detailed Description of the Invention

[0013] An object of the present invention is to provide a non-aqueous electrolyte solution that is excellent in battery safety during high-temperature use, which is important for recent in-vehicle secondary batteries such as electric vehicles, and further has excellent battery characteristics, and a clay-type lithium-ion secondary battery using the non-aqueous electrolyte solution.

[0014] The non-aqueous electrolyte of the present invention is a non-aqueous electrolyte obtained by dissolving an electrolyte salt in a non-aqueous solvent. The non-aqueous solvent is a non-aqueous solvent containing at least one selected from ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolactone (GBL) in a total amount of 80 to 100% by volume. The non-aqueous electrolyte further contains a trifluoroacetate having an alcohol-based carbon chain length of 6 to 8. The flash point of EC is 143°C, and the flash point of PC is 133°C. They are cyclic carbonates with a flash point of 80°C or higher. In addition, the flash point of GBL is 100°C, and it is a cyclic compound with a flash point of 80°C or higher. In the present invention, the non-aqueous solvent contains at least one selected from EC, PC, and GBL in a total amount of 80 to 100% by volume. Containing a total of 80 to 100% by volume means that when only one selected from EC, PC, and GBL is contained, 80 to 100% by volume of the selected one is contained, and when two or more selected from EC, PC, and GBL are contained, a total of 80 to 100% by volume of the selected two or more are contained. The above volume % is the ratio with respect to the volume of all non-aqueous solvents. In the present invention, it is preferred that the non-aqueous solvent contains at least one selected from EC, PC, and GBL in a total amount of 90 to 100% by volume. It is preferred that the non-aqueous solvent contains two or more selected from EC, PC, and GBL. The volume ratio of EC to PC in the non-aqueous solvent is preferably EC / PC = 10 / 90 to 100 / 0, more preferably 50 / 50 to 90 / 10, and further preferably 60 / 40 to 80 / 20. Since the melting point of EC is 36°C and it is a solid under normal temperature conditions, when the ratio of EC is high, EC is likely to precipitate under low temperature conditions, and there is a concern that the low temperature characteristics will deteriorate. In addition, when the ratio of PC increases, there is a concern that the ionic conductivity of the non-aqueous electrolyte will decrease. Therefore, when the mixing ratio of EC is higher than that of PC, it is preferred to add GBL, and the volume ratio of (EC+PC) / GBL is preferably 0 / 100 to 100 / 0, more preferably 20 / 80 to 70 / 30, and further preferably 30 / 70 to 50 / 50.

[0015] The non-aqueous electrolyte of the present invention may also contain a non-aqueous solvent other than EC, PC, and GBL. For the non-aqueous electrolyte of the present invention, when it contains a non-aqueous solvent other than EC, PC, and GBL, the flash point is also preferably 80 °C or higher, more preferably 100 °C or higher. In addition, a cyclic compound having a flash point of 80 °C or higher other than EC, PC, and GBL may also be contained. As such a cyclic compound, fluoroethylene carbonate (FEC, flash point 122 °C), vinylene carbonate (VC, flash point 80 °C), succinic anhydride (SA, flash point 157 °C), maleic anhydride (MA, flash point 102 °C), 1,3-propane sultone (PS, flash point > 110 °C), and ethylene sulfate (ES, flash point > 100 °C) can be cited. When PC or GBL is used, it is preferably one or more selected from these six, and is preferably added in a total amount of 1% by weight or more and 10% by weight or less in the non-aqueous electrolyte. In addition, for the non-aqueous electrolyte of the present invention, as the non-aqueous solvent, each chain carbonate of DMC, EMC, DEC, and dibutyl carbonate (DBC) may not be contained, or may be contained. When contained, their total content is preferably 5% by volume or less based on the volume of all non-aqueous solvents.

[0016] The non-aqueous electrolyte of the present invention contains a trifluoroacetate having an alcohol group with a carbon chain length of 6 to 8. The carbon chain length of 6 to 8 of the alcohol group means that the number of carbon atoms derived from the alcohol in the trifluoroacetate is 6 to 8. The content of the trifluoroacetate having an alcohol group with a carbon chain length of 6 to 8 is preferably 0.1 to 5% by mass, preferably 0.1 to 4% by mass, and more preferably 0.1 to 3% by mass based on the total mass of the non-aqueous electrolyte.

[0017] As the trifluoroacetate in the present invention, any trifluoroacetate having an alcohol group with a carbon chain length of 6 to 8 may be used, and there is no particular limitation. When the number of carbon atoms is 5 or less, the impregnation property (penetration property) of the spacer is insufficient, resulting in a decrease in battery performance. In addition, when the number of carbon atoms is 9 or more, the viscosity becomes high, resulting in a decrease in battery performance. As the trifluoroacetate having an alcohol group with a carbon chain length of 6 to 8 in the present invention, for example, as a linear ester, n-hexyl trifluoroacetate, n-heptyl trifluoroacetate, and n-octyl trifluoroacetate can be cited. As a branched ester, 2-ethylhexyl trifluoroacetate, 2-octyl trifluoroacetate, 3-octyl trifluoroacetate, and 4-octyl trifluoroacetate can be cited. One or more selected from them are preferred, and one or more selected from n-hexyl trifluoroacetate, n-heptyl trifluoroacetate, and 2-ethylhexyl trifluoroacetate are more preferred.

[0018] As the pivalate ester added to the non-aqueous electrolyte of the present invention, any pivalate ester with an alcohol-based carbon chain length of 6 to 8 is acceptable, without particular limitation. An alcohol-based carbon chain length of 6 to 8 means that the number of carbon atoms from the alcohol in the pivalate ester is 6 to 8. When the number of carbon atoms is 5 or less, the permeability to the spacer is insufficient, resulting in a decline in battery performance. Additionally, for hydrocarbon groups with 9 or more carbon atoms, the viscosity becomes high, leading to a decline in low-temperature battery performance. Relative to the total mass of the non-aqueous electrolyte, the content of the pivalate ester with an alcohol-based carbon chain length of 6 to 8 is preferably 0.1 to 5% by mass, more preferably 0.1 to 4% by mass, and still more preferably 0.1 to 3% by mass. As the preferred pivalate esters in the present invention, as straight-chain esters, n-hexyl pivalate, n-heptyl pivalate, and n-octyl pivalate can be cited; as branched-chain esters, 2-ethylhexyl pivalate, 2-octyl pivalate, 3-octyl pivalate, and 4-octyl pivalate can be cited. It is preferred to select one or more of them, and more preferably to select one or more from n-hexyl pivalate, n-heptyl pivalate, and 2-ethylhexyl pivalate.

[0019] In the non-aqueous electrolyte of the present invention, when the addition amounts of the above-mentioned trifluoroacetate ester and pivalate ester are large, the permeability to the spacer is excessive, which may reduce the cycle characteristics. Therefore, it is preferably as little as possible. On the other hand, when the amount is too small, the permeability to the spacer is insufficient, and thus the battery capacity may decline. Therefore, in the present invention, for the addition amount when the trifluoroacetate ester and pivalate ester are used in combination, relative to the total mass of the non-aqueous electrolyte, the total content of the trifluoroacetate ester and pivalate ester is preferably 0.5 to 5% by mass.

[0020] When the trifluoroacetate ester and pivalate ester are used in combination, for their respective addition ratios, as long as the total viscosity of the trifluoroacetate ester and pivalate ester at 25°C is not greater than 1.8 cP, they can be used in combination without limitation. In particular, the pivalate ester with an alcohol-based carbon chain length of 6 to 8 has a higher viscosity than the trifluoroacetate ester, but has a higher lithium ion dissociation ability, resulting in improved battery performance. Therefore, it is preferred.

[0021] When using a graphite negative electrode in the battery, in order to suppress the reductive decomposition of PC and GBL on the graphite negative electrode, it is preferred to add 2-propynyl methanesulfonate (PMS, flash point 124°C) to the non-aqueous solvent. Relative to the entire non-aqueous electrolyte, it is preferred to add these compounds in the range of 0.1% by weight to 5% by mass.

[0022] As the electrolyte salt in the present invention, any electrolyte salt that can be used in the electrolyte of a clay-type lithium ion secondary battery is acceptable, without particular limitation. For example, LiN(SO 2 F) 2 、Li N(SO 2 CF 3 ) 2, LiPF 6 , LiBF 4 , LiB(C 2 O 4 ) 2 , LiBF 2 (C 2 O 4 ), LiPF 2 (C 2 O 4 ). LiN(SO 2 F) 2 has high chemical thermal stability and can improve the battery performance under high temperature conditions, so it is preferred. In addition, LiPF 6 has the effect of assisting in improving the battery performance under low temperature conditions. Therefore, a specified amount of LiPF 6 is preferably added. It is presumed that this reason is attributed to the smoother movement of Li ions near the spacer. The total concentration of the electrolyte salts contained in the non-aqueous solvent is preferably 0.5 to 3 mol / L (that is, 0.5 to 3 mol of electrolyte salts relative to 1 L of non-aqueous solvent), and more preferably 1 to 2 mol / L. In addition, when LiN(SO 2 F) 2 is used alone and when LiN(SO 2 F) 2 and LiPF 6 are used in combination, for the weight ratio of LiN(SO 2 F) 2 to LiPF 6 , that is, LiN(SO 2 F) 2 / LiPF 6 , as a preferred range, 100 / 0 to 0 / 100, 90 / 10 to 50 / 50, 80 / 20 to 70 / 30 can be cited. In the non-aqueous electrolyte of the present invention, relative to the total mass of the non-aqueous electrolyte, it is preferably contained in a total of 1 to 10% by mass of fluoroethylene carbonate (FEC, flash point 122 °C), vinylene carbonate (VC, flash point 80 °C), succinic anhydride (SA, flash point 157 °C), maleic anhydride (MA, flash point 102 °C), 1,3-propane sultone (PS, flash point > 110 °C), ethylene sulfate (ES, flash point > 100 °C), LiBF 2 (C 2 O 4 ), LiB(C 2 O 4 ) 2and at least one selected from 2-propynyl methanesulfonate. By containing them, the battery characteristics are further improved. The non-aqueous electrolyte of the present invention can be prepared by dissolving an electrolyte and a trifluoroacetate ester having an alcohol group carbon chain length of 6 to 8 and / or a pivalate ester having an alcohol group carbon chain length of 6 to 8 in a non-aqueous solvent.

[0023] As the separator in the present invention, any separator that can be used in a clay-type lithium-ion secondary battery can be used without particular limitation. Most preferably, a separator composed of a microporous membrane formed of a polyolefin material such as polypropylene or polyethylene is used, and a non-woven fabric separator can also be used. They can be single-layer or multi-layer structures, and an oxide such as alumina can also be coated on the surface of the separator. In order to increase the volumetric energy density of the battery or improve the permeability of the electrolyte, it is preferable that the thickness of the separator is as thin as possible. Therefore, the thickness of the separator is preferably 20 μm or less, and more preferably 10 μm or less.

[0024] As the negative electrode in the present invention, any negative electrode that can be used in a clay-type lithium-ion secondary battery can be used without particular limitation. However, in order to increase the volumetric energy density, graphite materials such as natural graphite and artificial graphite, and carbon materials such as hard carbon and soft carbon are preferably selected. In addition, in order to improve rapid charge and discharge, it is preferable to use a lithium 4 Ti 5 O 12 titanium oxide having a spinel-type structure such as that, TiNb 2 O 7 、Ti 2 Nb 10 O 29 titanium oxide, and a titanium oxide having a spinel-type structure such as Li 4 Ti 5 O 12 is particularly preferred.

[0025] As the negative electrode composite material, a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), copolymer of styrene and butadiene (SBR), copolymer of acrylonitrile and butadiene (NBR), carboxymethyl cellulose (CMC) is kneaded in the negative electrode active material for use.

[0026] As the positive electrode active material of the positive electrode in the present invention, for example, LiCoO 2 、LiNiO 2 、LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O 2 、LiCo 0.15 Ni 0.8Al 0.05 O 2 、LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523), LiNi 0.8 Co 0.2 O 2 、LiNi 0.5 Mn 1.5 O 4 etc. To increase the volumetric energy density, as the positive electrode active material of the lithium composite oxide containing Ni at an atomic ratio of 50% or more, it is preferable to use LiNi 0.5 Co 0.2 Mn 0.3 O 2 、LiNi 0.6 Co 0.2 Mn 0.2 O 2 、LiNi 0.8 Co 0.1 Mn 0.1 O 2 and LiCo 0.15 Ni 0.80 Al 0.05 O 2 . In addition, to improve rapid charge and discharge, it is preferable to use LiMn 2 O 4 with a spinel structure, LiFePO 4 with an olivine structure.

[0027] As the positive electrode composite material, the positive electrode active material can use known or commercially available conductive aids such as acetylene black, Ketjen black, carbon nanotubes, carbon fibers, activated carbon, and graphite, and be kneaded with binders such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVFF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethyl cellulose (CMC) to form a paste-like positive electrode mixture. Then, the positive electrode material is coated on an aluminum foil as the current collector, dried, and pressure-molded. For example, it is heat-treated at 80 °C under vacuum conditions to fabricate.

[0028] As the combination of the positive electrode composite material and the negative electrode composite material used in the present invention, it is preferable to select the above combinations for increasing the volumetric energy density or the above combinations for improving rapid charge and discharge to fabricate a battery.

[0029] As the current collector used in the present invention, there is no particular limitation. Generally, it is an aluminum foil or a copper foil. To further improve the permeability of the electrolyte, a porous current collector can also be used.

[0030] In the present invention, the solvent for the binder is not particularly limited, and various solvents can be selected according to the active material or binder used. Specifically, when PVDF is used as the binder, N-methyl-2-pyrrolidone is preferably used as the solvent. On the other hand, when rubber-based binders such as styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, and carboxymethyl cellulose (CMC) are used, water is preferably selected as the solvent.

[0031] The clay-type lithium-ion secondary battery of the present invention includes a positive electrode and a negative electrode having clay-like properties. The clay-type lithium-ion secondary battery of the present invention can be manufactured by sandwiching each electrode layer of the clay-like positive electrode and negative electrode with a separator and injecting the above non-aqueous electrolyte into the separator. In the clay-type lithium-ion secondary battery of the present invention, from the viewpoints of performance and safety, it is preferable to use the above non-aqueous electrolyte.

[0032] Refer to Figure 1 An example of the structure of the clay-type lithium-ion secondary battery will be described. Figure 1 It is a cross-sectional view showing the structure of the lithium-ion secondary battery 14. As Figure 1 shown, in the lithium-ion secondary battery 14, the negative electrode active material layer 11b can be a layer of a negative electrode material that is a mixture of a negative electrode active material 11c, a negative electrode conductive additive 11d, and an electrolyte 11f. In addition, in the lithium-ion secondary battery 14, the positive electrode active material layer 12b can be a layer of a positive electrode material that is a mixture of a positive electrode active material 12c, a positive electrode conductive additive 12d, and an electrolyte 12f.

[0033] The negative electrode material can have a so-called clay-like property in which an electrolyte is mixed into a mixture composed of the negative electrode active material 11c and the negative electrode conductive additive 11d. The positive electrode material can have a so-called clay-like property in which an electrolyte is mixed into a mixture composed of the positive electrode active material 12c and the positive electrode conductive additive 12d. The negative electrode 11 can be an electrode in which the negative electrode material is coated on a negative electrode current collector 11a. The positive electrode 12 can be an electrode in which the positive electrode material is coated on a positive electrode current collector 12a.

[0034] The lithium-ion secondary battery 14 can also have a separator 13. The negative electrode 11, the positive electrode 12, and the separator 13 have a positional relationship such that the negative electrode active material layer 11b and the positive electrode active material layer 12b are in contact with the separator 13. That is, the lithium-ion secondary battery 14 can have a structure in which the negative electrode 11 and the positive electrode 12 are laminated via the separator 13. The separator 13 can function as an insulating member that insulates the negative electrode 11 and the positive electrode 12. The separator 13 can be, for example, a sheet-like non-woven fabric or a porous material. Therefore, the electrolyte can penetrate into the separator 13.

[0035] The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0036] Example

[0037] Hereinafter, examples of the present invention will be given to specifically illustrate the present invention. The technical scope of the present invention is not limited to these examples.

[0038] (Method for measuring viscosity)

[0039] The principle of the viscometer used is a rotational viscometer. The model is VISCOMETER DVI PrimeLV (manufactured by BROOKFIELD). At 25 °C, the viscosity of the trifluoroacetate used in Examples 1 and 2 and the viscosity of the mixture of trifluoroacetate and pivalate used in Examples 3 to 12 were measured. The viscosity was measured 5 times, and the average value was shown as the viscosity in Table 1.

[0040] (Preparation of electrolyte solution)

[0041] [Example 1]

[0042] 0.5 mol / L of LiPF 6 and 0.5 mol / L of LiN(SO 2 F) 2 (LiFSI) was dissolved in a non-aqueous solvent mixed at EC / PC / GBL = 2 / 1 / 7 (volume ratio). 3% by mass of n-hexyl trifluoroacetate was added to the solution thus obtained to prepare the electrolyte solution of Example 1. The addition amount (3% by mass) of n-hexyl trifluoroacetate represents the ratio relative to the total mass of the prepared electrolyte solution. In addition, mol / L of the electrolyte represents the respective ratios relative to the total volume of the prepared electrolyte solution.

[0043] [Examples 2 to 12]

[0044] Except that the respective contents of trifluoroacetate and pivalate are as described in Table 1, the electrolyte solutions of Examples 2 to 12 were prepared in the same manner as in Example 1. The addition amounts (mass%) of trifluoroacetate and pivalate in Table 1 represent the ratios relative to the total mass of the prepared electrolyte solution.

[0045] [Comparative Example 1]

[0046] In Comparative Example 1, the electrolyte solution was prepared in the same manner as in Example 1 except that trifluoroacetate and pivalate were not added.

[0047] (Fabrication of lithium ion secondary battery (LIB) and measurement of battery characteristics)

[0048] Mix 93% by mass of NCM523 (cathode active material), 3% by mass of acetylene black (conductive additive), and 4% by mass of polyvinylidene fluoride (binder), add 1-methyl-2-pyrrolidone thereto to form a slurry, and prepare a cathode composite material, which is then coated on an aluminum foil. Then, dry it and press-mold it to prepare a cathode. Similarly, mix 98% by mass of artificial graphite (anode active material), 1% by mass of a binder, i.e., a copolymer of styrene and butadiene, and 1% by mass of carboxymethyl cellulose in water to form a slurry, and prepare an anode composite material, which is then coated on a copper foil. Then, dry it, press-mold it, and heat-treat it to prepare an anode sheet. Then, use a three-layer 20-μm microporous membrane formed by sandwiching polyethylene with polypropylene as a separator, inject the electrolytes of Examples 1 to 12 and Comparative Example 1 respectively, and fabricate a coin-type battery (coin-type LIB: diameter 20 mm, thickness 3.2 mm).

[0049] For this coin-type battery, use a charge-discharge device ACD-MO1A (manufactured by Asuka Electronics Co., Ltd.), and at 25 °C, charge it at a constant current and a constant voltage at a rate of 1C to an upper limit voltage of 4.2V, and then discharge it at a rate of 1C to a lower limit voltage of 3.0V, and repeat the charge and discharge. The cycle characteristics (%) are obtained by numericalizing the value of the 50th cycle / the 1st cycle of the obtained capacity (mAh / g) × 100. The discharge capacity (initial capacity) of the 1st cycle is calculated as a relative ratio compared with the discharge capacity of the 1st cycle of the electrolyte prepared by dissolving 0.5 mol / L of LiPF 6 and 0.5 mol / L of LiFSI in a non-aqueous solvent mixed at a volume ratio of EC / DMC = 1:2. The results are shown in Table 1.

[0050] Table 1

[0051]

[0052] In the present invention, the non-aqueous electrolyte of Comparative Example 1 without trifluoroacetate does not penetrate into the separator, and the coin-type battery made using this non-aqueous electrolyte does not undergo charge and discharge. On the other hand, the non-aqueous electrolytes of all Examples containing trifluoroacetate in the present invention penetrate into the separator, and the coin-type battery made using this non-aqueous electrolyte can not only undergo charge and discharge, but also obtain excellent cycle characteristics. In addition, by using trifluoroacetate and pivalate in combination in the present invention, more excellent cycle characteristics are obtained (Examples 3 to 7 and 11), and even when the content of trifluoroacetate is reduced, equivalent cycle characteristics can be obtained (Examples 8 and 9). In addition, in Example 10, the initial capacity is increased.

[0053] As described above, the electrolytes containing trifluoroacetate and the electrolytes containing trifluoroacetate and pivalate of the present invention have low viscosities and excellent permeability to the separator. Therefore, it is also known that in the electrolyte injection process of the mass production process of lithium ion secondary batteries, the electrolyte quickly penetrates into the microporous separator, and the manufacturing time of lithium ion secondary batteries can also be shortened. In addition, it is known that it is also effective for permeability and safety in clay-type lithium ion secondary batteries in which clay-like positive and negative electrode layers are sandwiched by a separator.

[0054] 〔Preparation of electrolyte〕

[0055] [Example 13]

[0056] 0.5 mol / L of LiPF 6 , 0.5 mol / L of LiN(SO 2 F) 2 (LiFSI) was dissolved in a non-aqueous solvent mixed with EC / PC / GBL = 2 / 1 / 7 (volume ratio). 2% by mass of fluoroethylene carbonate (FEC), 1% by mass of succinic anhydride (SA), and 2% by mass of LiBF 2 (C 2 O 4 )(LiDFOB) were added to the electrolyte of Example 8 obtained by mixing and adding 2% by mass of n-hexyl trifluoroacetate and 0.5% by mass of n-octyl pivalate to the solution thus obtained. A lithium ion secondary battery (LIB) was fabricated in the same manner as in Example 8, and a charge-discharge test was conducted to measure the battery characteristics. The results are shown in Table 2.

[0057] [Example 14]

[0058] Except that 2% by mass of vinylene carbonate (VC) was used instead of FEC in Example 13, 1% by mass of maleic anhydride (MA) was used instead of SA, and 2% by mass of LiB(C 2 O 4 ) 2 (LiBOB), a charge-discharge test was conducted in the same manner as in Example 13. The results are shown in Table 2.

[0059] [Example 15]

[0060] Except that 2% by mass of 1,3-propane sultone (PS) was used instead of FEC in Example 13, 1% by mass of ethylene sulfate (ES) was used instead of SA, and 2% by mass of 2-propynyl methanesulfonate (PMS) was used instead of LiDFOB, a charge-discharge test was conducted in the same manner as in Example 13. The results are shown in Table 2.

[0061] Table 2

[0062] Initial capacity Cycle (%) Example 8 0.95 82 Example 13 0.98 97 Example 14 0.99 99 Example 15 0.98 99

[0063] From the results in Table 2, it can be seen that by adding FEC, VC, SA, MA, LiDFOB, LiBOB, PS, ES or PMS to the non-aqueous electrolyte, more excellent results can be obtained. Thus, it was found that the lithium ion secondary battery using the non-aqueous electrolyte of the present invention has excellent safety during high-temperature use, and also has excellent initial capacity and cycle characteristics.

[0064] [Example 16]

[0065] Dissolve 0.9 mol / L of LiPF 6 in a non-aqueous solvent mixed with EC / PC / GBL = 2 / 2 / 6 (volume ratio). Add 0.5% by mass of n-heptyl trifluoroacetate to the solution thus obtained to obtain the electrolyte of Example 16, and then add 2.8% by mass of vinylene carbonate (VC), 0.8% by mass of maleic anhydride (MA), and 1.4% by mass of LiB(C 2 O 4 ) 2 (LiBOB). The addition amount (0.5% by mass) of n-heptyl trifluoroacetate represents the proportion relative to the total mass of the prepared electrolyte. In addition, the mol / L of the electrolyte represents the respective proportions relative to the total volume of the prepared electrolyte.

[0066] [Examples 17 to 20, Comparative Examples 2 to 3]

[0067] Except that the respective contents of the trifluoroacetate and pivalate are as described in Table 3, the electrolytes of Examples 17 to 20 and Comparative Examples 2 to 3 were prepared in the same manner as in Example 16. The addition amounts (mass%) of the trifluoroacetate and pivalate in Table 13 represent the proportions relative to the total mass of the prepared electrolyte.

[0068] Mix LFP (positive electrode active material), carbon black (conductive assistant), and the electrolytes of Examples 16 to 20 and Comparative Examples 2 to 3 in a mass ratio of 74:1:25, form a viscous state, and then coat it on an aluminum foil to prepare a positive electrode. The coating weight of the positive electrode is 0.078 g / cm 2 . Mix artificial graphite (negative electrode active material), carbon black (conductive assistant), and the electrolytes of Examples 16 to 20 and Comparative Examples 2 to 3 in a mass ratio of 63:2:35, form a viscous state, and then coat it on a copper foil to fabricate a negative electrode. The coating weight of the negative electrode is 0.054 g / cm 2 . Use the positive electrode, the negative electrode, and a separator to make a clay-type battery.

[0069] When using this clay-type lithium-ion secondary battery, at 25 °C, it is charged at a constant current and a constant voltage at a rate of 0.3C to the upper limit voltage of 3.6V, and then discharged at a rate of 0.3C to the lower limit voltage of 3.0V to perform charge and discharge. The discharge capacity at this time is measured as the 25 °C capacity (mAh / g). Then, the temperature is lowered to 0 °C, and it is charged at a rate of 0.3C to the upper limit voltage of 3.6V. The value obtained by dividing the charging capacity at this time by the 25 °C capacity is calculated as the 0 °C charging rate (%). The results are shown in Table 3.

[0070] Table 3

[0071]

[0072] When using the non-aqueous electrolytes of Comparative Examples 2 and 3 that do not contain trifluoroacetate but only contain pivalate, the 0 °C charging rate is low. When using the non-aqueous electrolyte of Example 16 that contains trifluoroacetate, the 25 °C capacity and the 0 °C charging rate are improved. In addition, the 25 °C capacity and the 0 °C charging rate of Examples 17 to 20 in which trifluoroacetate and pivalate are used in combination are excellent.

[0073] Industrial Applicability

[0074] When manufacturing a clay-type lithium-ion secondary battery, by using the non-aqueous electrolyte of the present invention, it is possible to realize a clay-type lithium-ion secondary battery with excellent battery safety during high-temperature use, and further excellent initial characteristics and cycle characteristics. The contribution of the present invention to the industrial field is inestimable.

[0075] Explanation of Reference Numerals

[0076] 11 Negative electrode;

[0077] 11a Negative electrode current collector;

[0078] 11b Negative electrode active material layer;

[0079] 11c Negative electrode active material;

[0080] 11d Negative electrode conductive additive;

[0081] 11f Electrolyte;

[0082] 12 Positive electrode;

[0083] 12a Positive electrode current collector;

[0084] 12b Positive electrode active material layer;

[0085] 12c Positive electrode active material;

[0086] 12d Positive electrode conductive additive;

[0087] 12f Electrolyte;

[0088] 13 Spacer;

[0089] 14 Lithium-ion secondary battery.

Claims

1. A clay-type lithium-ion secondary battery, which is a clay-type lithium-ion secondary battery having a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte containing an electrolyte salt in a non-aqueous solvent, characterized in that, the non-aqueous electrolyte is a non-aqueous electrolyte obtained by dissolving an electrolyte salt in a non-aqueous solvent, the non-aqueous solvent is a non-aqueous solvent containing at least one selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone in a total amount of 80 to 100% by volume, and the non-aqueous electrolyte further contains a trifluoroacetate having an alcohol group carbon chain length of 6 to 8.

2. The clay-type lithium-ion secondary battery according to claim 1, characterized in that, the non-aqueous electrolyte contains 0.1 to 5% by mass of the trifluoroacetate having an alcohol group carbon chain length of 6 to 8.

3. The clay-type lithium-ion secondary battery according to claim 1 or 2, characterized in that, in the non-aqueous electrolyte, the trifluoroacetate having an alcohol group carbon chain length of 6 to 8 is at least one selected from n-hexyl trifluoroacetate, 2-ethylhexyl trifluoroacetate, n-heptyl trifluoroacetate, n-octyl trifluoroacetate, 2-octyl trifluoroacetate, 3-octyl trifluoroacetate, and 4-octyl trifluoroacetate.

4. The clay-type lithium-ion secondary battery according to any one of claims 1 to 3, characterized in that, the non-aqueous electrolyte contains 0.1 to 5% by mass of the trifluoroacetate having an alcohol group carbon chain length of 6 to 8 and contains 0.1 to 5% by mass of a pivalate having a carbon chain length of 6 to 8.

5. The clay-type lithium-ion secondary battery according to claim 4, characterized in that, in the non-aqueous electrolyte, the pivalate having a carbon chain length of 6 to 8 is at least one selected from n-hexyl pivalate, 2-ethylhexyl pivalate, n-heptyl pivalate, n-octyl pivalate, 2-octyl pivalate, 3-octyl pivalate, and 4-octyl pivalate.

6. The clay-type lithium-ion secondary battery according to any one of claims 1 to 5, characterized in that, The non-aqueous electrolyte contains at least one selected from vinyl fluorocarbonate, vinylene carbonate, succinic anhydride, maleic anhydride, 1,3-propane sultone, ethylene sulfate, LiBF 2 (C 2 O 4 )、LiB(C 2 O 4 ) 2 and 2-propynyl methanesulfonate, in a total amount of 1 to 10% by mass.

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