Non-aqueous secondary batteries

Abstract

To provide a non-aqueous secondary battery that suppresses both degradation reactions within the non-aqueous secondary battery and self-discharge at high temperatures exceeding 60°C, while also improving output characteristics and cycle performance over a wide temperature range. [Solution] A positive electrode containing a lithium phosphorus metal compound with an olivine crystal structure containing iron (Fe), and a negative electrode containing graphite, It contains a non-aqueous solvent containing 5-95% by volume of acetonitrile, a lithium salt, and an electrode protection additive. 3. When CCCV charging is continued under conditions of 3.65V or higher and 60℃ or higher, the current value at which the current converges is 0.003C or less. Using the values ​​obtained from the XRD analysis results of the positive electrode after the CCCV charging test, the following formula is used: X(%) = (FP phase ratio * iron content of FP phase + LFP phase ratio * iron content of LFP phase) * 100 The iron occupancy rate X (%) of the positive electrode, calculated from this, is 85% or higher.

Description

[Technical Field]

[0001] This invention relates to a non-aqueous electrolyte and a non-aqueous secondary battery. [Background technology]

[0002] Non-aqueous secondary batteries, such as lithium-ion batteries (LIBs), are characterized by their light weight, high energy capacity, and long lifespan, and are widely used as power sources for various portable electronic devices. In recent years, non-aqueous secondary batteries have also expanded into industrial applications, such as power tools, as well as in-vehicle applications in electric vehicles and electric bicycles. Furthermore, they are attracting attention in the field of power storage, such as residential energy storage systems.

[0003] Typically, lithium-ion batteries use non-aqueous electrolytes. For example, solvents combining highly dielectric solvents such as cyclic carbonate esters with low-viscosity solvents such as lower chain carbonate esters are commonly used. However, conventional highly dielectric solvents not only have high melting points, but can also degrade the load characteristics (output characteristics) and low-temperature characteristics of non-aqueous secondary batteries.

[0004] Furthermore, among automotive batteries, lead-acid batteries are used as the power source for starting the engine. Lead-acid batteries have many drawbacks, including a high environmental impact, heavy weight, poor charging performance, and the need for periodic replacement due to degradation. Moreover, with the increasing electrification of automobiles, the high power load on batteries has increased, and lead-acid batteries have sometimes been unable to meet the demands for higher capacity, higher output, lighter weight, and longer lifespan. For this reason, efforts to replace lead-acid batteries with lithium-ion batteries have become more active in recent years. Thus, with the expansion of the large-scale energy storage industry, centered on electric vehicles, there is a strong demand for further improvements in the functionality of non-aqueous secondary batteries.

[0005] To address these challenges, nitrile-based solvents (such as acetonitrile), which offer an excellent balance between viscosity and dielectric constant, have been proposed as electrolyte solvents for non-aqueous secondary batteries. However, acetonitrile has the drawback of undergoing electrochemical reductive decomposition at the negative electrode, preventing it from achieving practical performance. Several improvements have been proposed to address this problem.

[0006] The main improvement measures proposed so far can be classified into the following three categories.

[0007] (1) A method for protecting the negative electrode and suppressing the reductive decomposition of acetonitrile by combining it with a specific electrolyte salt, additive, etc. For example, Patent Document 1 reports an electrolyte in which the reductive decomposition of acetonitrile is reduced by combining the solvent acetonitrile with a specific electrolyte salt and additives. Furthermore, Patent Document 2 reports an electrolyte containing a solvent in which acetonitrile is diluted with propylene carbonate and ethylene carbonate. However, simply diluting with ethylene carbonate and propylene carbonate is insufficient to suppress the reductive decomposition of an electrolyte containing acetonitrile.

[0008] (2) A method for suppressing the reductive decomposition of acetonitrile by using a negative electrode active material that intercepts lithium ions at a potential more noble than the reduction potential of acetonitrile. For example, Patent Document 3 reports that a battery can be obtained that avoids the reductive decomposition of acetonitrile by using a specific metal compound in the negative electrode. However, in applications where the energy density of lithium-ion batteries is important, applying the improvement measures described in Patent Document 3 is disadvantageous because it narrows the usable voltage range.

[0009] (3) A method for maintaining a stable liquid state by dissolving a high concentration of electrolyte salt in acetonitrile. For example, Patent Document 4 describes that when an electrolytic solution in which lithium bis(trifluoromethanesulfonyl)imide represented by the formula LiN(SO2CF3)2 is dissolved in acetonitrile at a concentration of 4.2 mol / L is used, reversible lithium insertion and desorption into a graphite electrode are possible.

[0010] On the other hand, for a lithium-ion battery to replace a lead-acid battery, it is necessary to highly satisfy both the output characteristics for starting an engine in a low-temperature environment and the durability in a high-temperature environment such as an engine room. In addition, since it is assumed to be used as a battery pack in which a plurality of non-aqueous secondary batteries are connected in series, stability against an unexpected high-voltage environment due to poor management or the like is also important. Patent Document 5 reports an attempt to achieve durability in a wide temperature range by using an acetonitrile-containing electrolytic solution excellent in viscosity and dielectric constant.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0012] In the technologies described in Patent Documents 1 to 4, a lithium-ion battery using an acetonitrile-containing electrolytic solution is inferior in high-temperature durability performance compared to an existing lithium-ion battery using an electrolytic solution containing a carbonate solvent.

[0013] On the other hand, the technology described in Patent Document 5 attempts to develop a non-aqueous electrolyte that can suppress self-discharge at high temperatures exceeding 60°C and improve output characteristics and cycle performance over a wide temperature range. However, Patent Document 5 does not describe durability against high voltage, and therefore, it is assumed that the technology described in Patent Document 5 cannot address short circuits and current leakage due to battery degradation.

[0014] The present invention aims to provide a non-aqueous secondary battery that suppresses both degradation reactions within the non-aqueous secondary battery and self-discharge at high temperatures exceeding 60°C, while also improving output characteristics and cycle performance over a wide temperature range. Another objective is to provide a non-aqueous secondary battery that can prevent short circuits caused by metal deposition in the event of unexpectedly high voltages. [Means for solving the problem]

[0015] The inventors of this invention have conducted extensive research and, as a result, have found that the above problems can be solved by using a non-aqueous electrolyte or non-aqueous secondary battery having the following configuration, and have completed the present invention. Examples of embodiments for carrying out the present invention are as follows. [1] The device comprises a positive electrode containing a lithium phosphorus metal compound with an olivine crystal structure containing iron (Fe), and a negative electrode containing graphite. It contains a non-aqueous solvent containing 5-95% by volume of acetonitrile, a lithium salt, and an electrode protection additive. 3. When CCCV charging is continued under conditions of 3.65V or higher and 60℃ or higher, the current value at which the current converges is 0.003C or less. Using the values ​​obtained from the XRD analysis results of the positive electrode after the CCCV charging test, the following formula is used: X(%) = (FP phase ratio * iron content of FP phase + LFP phase ratio * iron content of LFP phase) * 100 The iron occupancy rate X (%) of the positive electrode, calculated from this, is 85% or more. Non-aqueous secondary battery. [2] A non-aqueous secondary battery as described in item 1, wherein vinylene carbonate is present in an amount of 2% or more by volume relative to the total amount of the non-aqueous solvent. [3] The non-aqueous secondary battery according to item 1 or 2, wherein the lithium salt contains lithium hexafluoride phosphate (LiPF6) and a lithium-containing imide salt. [4] The non-aqueous secondary battery according to item 3, wherein the LiPF6 content is 0.01 mol / L or more and less than 0.5 mol / L relative to the non-aqueous solvent. [5] A non-aqueous secondary battery according to item 3 or 4, wherein the molar ratio of the lithium-containing imide salt to the LiPF6 is greater than 1. [6] A non-aqueous secondary battery according to any one of items 1 to 5, wherein the electrode protection additive is ethylene sulfite. [7] The non-aqueous secondary battery according to item 6, wherein the ethylene sulfite is 1% by volume or more relative to the total amount of the non-aqueous solvent. [8] Items 1-5 below: 1. It is a condensed polycyclic heterocyclic compound, 2. The condensed polycyclic heterocycle contains a pyrimidine skeleton, 3. The condensed polycyclic heterocycle contains three or more nitrogen atoms, 4. The condensed polycyclic heterocycle contains five or more sp2 carbons, 5. No hydrogen atoms are bonded to the nitrogen atoms within the fused polycyclic heterocycle. A non-aqueous secondary battery according to any one of items 1 to 7, comprising one or more compounds having a structure that satisfies the requirements. [9] The non-aqueous secondary battery according to item 7 or 8, wherein the condensed polycyclic heterocyclic compound is a purine derivative.

[10] The non-aqueous secondary battery according to item 9, wherein the purine derivative is caffeine.

[11] A non-aqueous secondary battery according to any one of items 1 to 10, wherein the content of the condensed polycyclic heterocyclic compound is 0.01% by mass or more and 10% by mass or less based on the total amount of the non-aqueous electrolyte.

[12] A non-aqueous secondary battery as described in any one of items 1 to 11, wherein the lower limit voltage for CCCV charging is 3.8V, 4.0V, or 4.2V. [Effects of the Invention]

[0016] The present invention aims to provide a non-aqueous secondary battery that suppresses both degradation reactions within the non-aqueous secondary battery and self-discharge at high temperatures exceeding 60°C, while also improving output characteristics and cycle performance over a wide temperature range. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic plan view showing an example of a non-aqueous secondary battery according to this embodiment. [Figure 2] Figure 1 is a cross-sectional view of a non-aqueous secondary battery along line AA. [Modes for carrying out the invention]

[0018] The following describes in detail embodiments for carrying out the present invention (hereinafter simply referred to as "this embodiment"). The present invention is not limited to the following embodiments, and various modifications are possible without departing from the spirit thereof.

[0019] The following describes non-aqueous electrolytes and non-aqueous secondary batteries containing them. The non-aqueous secondary battery according to this embodiment is The device comprises a positive electrode containing a lithium phosphorus metal compound with an olivine crystal structure containing iron (Fe), and a negative electrode containing graphite. It contains a non-aqueous solvent containing 5-95% by volume of acetonitrile, a lithium salt, and an electrode protection additive. 3. Continue CCCV charging under conditions of 3.65V or higher and 60℃ or higher, and the current value when the current converges is 0.003C or less. Using the values ​​obtained from the XRD analysis results of the positive electrode after the CCCV charging test, the following formula is used: X(%) = (FP phase ratio * iron content of FP phase + LFP phase ratio * iron content of LFP phase) * 100 The iron occupancy rate X (%) of the positive electrode, calculated from this, is 85% or higher. By using the non-aqueous secondary battery according to this embodiment, it is possible to provide a non-aqueous electrolyte and a non-aqueous secondary battery equipped therewith that suppress degradation reactions within the non-aqueous secondary battery and self-discharge at high temperatures exceeding 60°C, and improve output characteristics and cycle performance over a wide temperature range, by suppressing the decomposition reaction of non-aqueous electrolyte components by reactive oxygen species generated in conjunction with electrochemical reactions. Furthermore, by controlling the blending ratio of lithium (Li) salt and additives, it is possible to provide a non-aqueous secondary battery that avoids insufficient formation of negative electrode SEI or corrosion of aluminum (Al) current collector, widens the tolerable voltage threshold, and further improves safety (safety in unexpected high-voltage environments) by shutting down the current due to increased resistance of the positive electrode at unexpectedly high voltages of 4.2V or higher. In other words, the non-aqueous secondary battery according to this embodiment suppresses the decomposition reaction of non-aqueous electrolyte components by reactive oxygen species generated in conjunction with electrochemical reactions, thereby suppressing both degradation reactions within the non-aqueous secondary battery and self-discharge at high temperatures exceeding 60°C, and providing a non-aqueous secondary battery that can improve output characteristics and cycle performance over a wide temperature range. Furthermore, according to the non-aqueous secondary battery of this embodiment, by controlling the blending ratio of lithium (Li) salt and additives, it is possible to avoid insufficient formation of the negative electrode SEI or corrosion of the aluminum (Al) current collector, widen the tolerable voltage threshold, and prevent short circuits due to metal deposition in the event of unexpected high voltages, thereby providing a non-aqueous secondary battery that can improve safety.

[0020] <1.Non-aqueous electrolyte> In this embodiment, "non-aqueous electrolyte" refers to an electrolyte in which water is 1% by mass or less relative to the total amount of the non-aqueous electrolyte. While it is preferable that the non-aqueous electrolyte in this embodiment contains as little water as possible, it may contain a very small amount of water as long as it does not hinder the resolution of the problem of the present invention. Such water content is 300 ppm by mass or less, preferably 200 ppm by mass or less, per the total amount of the non-aqueous electrolyte. Regarding the non-aqueous electrolyte, as long as it has the configuration necessary to achieve the resolution of the problem of the present invention, other components can be appropriately selected and applied from known non-aqueous electrolyte materials used in lithium-ion batteries.

[0021] The non-aqueous electrolyte of this embodiment can be prepared by mixing a lithium salt and various additives (sometimes simply referred to as "additives" in this specification) in a non-aqueous solvent by any means. These various additives are a general term for electrode protection additives, condensed polycyclic heterocyclic compounds, and other optional additives, and their content is as shown below.

[0022] Unless otherwise specified, the content of each compound in the non-aqueous solvent is defined as follows: for each component listed in <2-1. Non-aqueous solvents> and the electrode protection additives listed in <2-3. Electrode protection additives>, the mixing ratio is defined as a volume percentage relative to the total amount of each component constituting the non-aqueous solvent; for the lithium salts listed in <2-2. Lithium salts>, the mixing ratio is defined as the number of moles per liter of non-aqueous solvent; and for <2-4. Other optional additives> and <2-5. Condensed polycyclic heterocyclic compounds>, ​​the mixing ratio is defined as parts by mass when the lithium salt and the entire non-aqueous solvent are considered to be 100 parts by mass.

[0023] Furthermore, in this embodiment, if the electrolyte contains compounds other than those specifically shown in items 2-1 to 2-4 below, if the compound is a liquid at room temperature (25°C), it shall be treated in the same manner as a non-aqueous solvent, and the mixing ratio shall be expressed as a volume % relative to the total amount of each component (including the compound) constituting the non-aqueous solvent. On the other hand, if the compound is a solid at room temperature (25°C), the mixing ratio shall be expressed as parts by mass when the lithium salt and the entire non-aqueous solvent are considered to be 100 parts by mass.

[0024] <2-1. Non-aqueous solvents> In this embodiment, "non-aqueous solvent" refers to the elements of a non-aqueous electrolyte excluding lithium salts and various additives. If the non-aqueous electrolyte contains electrode protection additives, "non-aqueous solvent" refers to the elements of the non-aqueous electrolyte excluding lithium salts and additives other than electrode protection additives.

[0025] The non-aqueous solvent in the non-aqueous electrolyte of this embodiment contains acetonitrile. Acetonitrile improves the ionic conductivity of the non-aqueous electrolyte, thereby increasing the diffusivity of lithium ions within the battery. Therefore, even in positive electrodes where the positive electrode active material layer is thickened and the amount of positive electrode active material filled is increased, lithium ions can diffuse well to areas near the current collector that are difficult for lithium ions to reach during high-load discharge. Thus, it becomes possible to extract sufficient capacity even during high-load discharge, and a non-aqueous secondary battery with excellent load characteristics can be obtained.

[0026] Furthermore, the inclusion of acetonitrile in the non-aqueous solvent can improve the rapid charging characteristics of non-aqueous secondary batteries. In constant current (CC)-constant voltage (CV) charging of non-aqueous secondary batteries, the capacity per unit time during the CC charging period is greater than the capacity per unit time during the CV charging period. Using acetonitrile as a non-aqueous solvent allows for a larger CC charging range (longer CC charging time) and also increases the charging current, thereby significantly shortening the time it takes to reach a fully charged state from the start of charging of a non-aqueous secondary battery.

[0027] Furthermore, since acetonitrile is readily reductively decomposed electrochemically, it is preferable to use another solvent (for example, an aprotic solvent other than acetonitrile) in combination with acetonitrile as a non-aqueous solvent, and / or to add an electrode protective additive for negative electrode SEI formation.

[0028] From the viewpoint of improving ionic conductivity, the acetonitrile content is 5 to 95% by volume of the total amount of the non-aqueous solvent. Preferably, the acetonitrile content is 20% or more by volume, or 30% or more by volume, and more preferably 40% or more by volume. On the other hand, this value is preferably 85% or less by volume, and more preferably 66% or less by volume. When the acetonitrile content is 5% or more by volume of the total amount of the non-aqueous solvent, the ionic conductivity tends to increase, leading to high-power characteristics, and furthermore, the dissolution of lithium salt can be promoted. Because the additives described later suppress the increase in the internal resistance of the battery, when the acetonitrile content in the non-aqueous solvent is within the above range, it tends to be possible to further improve high-temperature cycle characteristics and other battery characteristics while maintaining the excellent performance of acetonitrile. Furthermore, an acetonitrile content of 95% or less by volume of the total amount of the non-aqueous solvent is preferred from the viewpoint of electrode protection. When the acetonitrile content in the non-aqueous solvent is within the above range, a sufficient amount of electrode protection additive for negative electrode SEI formation can be added, and stable operation can be maintained while preserving the excellent performance of acetonitrile.

[0029] In addition to acetonitrile, other solvents such as methanol, ethanol, and other alcohols; and aprotic solvents can be used in combination. Among these, aprotic solvents are preferred as non-aqueous solvents. The non-aqueous solvent may contain solvents other than aprotic solvents, as long as it does not hinder the resolution of the problem of the present invention.

[0030] Examples of aprotic solvents that can be used other than acetonitrile include cyclic carbonates. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, trans-2,3-butylene carbonate, cis-2,3-butylene carbonate, 1,2-pentylene carbonate, trans-2,3-pentylene carbonate, cis-2,3-pentylene carbonate, vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate; 4-fluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolan-2-one Examples include fluorinated cyclic carbonates represented by cis-4,5-difluoro-1,3-dioxolan-2-one, trans-4,5-difluoro-1,3-dioxolan-2-one, 4,4,5-trifluoro-1,3-dioxolan-2-one, 4,4,5,5-tetrafluoro-1,3-dioxolan-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolan-2-one; and lactones represented by γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone.

[0031] Furthermore, specific examples of aprotic solvents include sulfur compounds such as ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methylsulfolane, 1,3-propanesultone, 1,4-butanesultone, 1-propene-1,3-sultone, dimethyl sulfoxide, tetramethylene sulfoxide, and ethylene glycol sulfite; and cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and 1,3-dioxane.

[0032] Furthermore, specific examples of aprotic solvents include, for example, chain-like carbonates. Examples of chain-like carbonates include chain-like carbonates represented by ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, and ethyl propyl carbonate; and chain-like fluorinated carbonates represented by trifluorodimethyl carbonate, trifluorodiethyl carbonate, and trifluoroethylmethyl carbonate.

[0033] Furthermore, specific examples of aprotic solvents include mononitriles such as propionitrile, butyronitrile, valeronitrile, benzonitrile, and acrylonitrile; alkoxy-substituted nitriles such as methoxyacetonitrile and 3-methoxypropionitrile; malononitrile, succinonitrile, glutalonitrile, adiponitrile, 1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1,9-dicyanononane, 2,8-dicyanononane, 1,10-dicyanodecane, 1 Examples include dinitriles represented by ,6-dicyanodecane and 2,4-dimethylglutalonitrile; cyclic nitriles represented by benzonitrile; linear esters represented by methyl propionate; linear ethers represented by dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme, and tetraglyme; fluorinated ethers represented by RfA-ORB (wherein RfA represents an alkyl group containing a fluorine atom, and RB represents an organic group which may contain a fluorine atom); ketones represented by acetone, methyl ethyl ketone, and methyl isobutyl ketone, and halides represented by their fluorinated products.

[0034] These can be used individually or in combination of two or more. Furthermore, among these non-aqueous solvents, it is more preferable from the viewpoint of improving stability to use one or more of cyclic carbonates and linear carbonates. Here, only one of the exemplified cyclic carbonates and linear carbonates may be selected and used, or two or more may be used (for example, two or more of the exemplified cyclic carbonates, two or more of the exemplified linear carbonates, or two or more consisting of one or more of the exemplified cyclic carbonates and one or more of the exemplified linear carbonates). Among these, ethylene carbonate, propylene carbonate, vinylene carbonate, or fluoroethylene carbonate are more preferable as cyclic carbonates, and ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate are more preferable as linear carbonates. Moreover, it is even more preferable to use cyclic carbonates because they increase the degree of ionization of lithium salts that contribute to the charging and discharging of non-aqueous secondary batteries. When using cyclic carbonates, it is particularly preferable that such cyclic carbonates include at least one of vinylene carbonate or fluoroethylene carbonate.

[0035] <2-2. Lithium Salts> The non-aqueous electrolyte according to this embodiment contains a lithium salt.

[0036] The lithium salt in this embodiment is of the formula LiN(SO2C m F 2m+1 It is preferable to include a lithium-containing imide salt represented by the formula 2{wherein m is an integer from 0 to 8}.

[0037] In this embodiment, the lithium salt includes LiPF6 and a lithium-containing imide salt. The lithium-containing imide salt is LiN(SO2C m F 2m+1The lithium salt is represented by )² [wherein m is an integer from 0 to 8], and more preferably contains at least one of LiN(SO₂F)₂ and LiN(SO₂CF₃)₂, and more preferably contains LiN(SO₂F)₂. In this art, LiN(SO₂F)₂ is also known as lithium bis(fluorosulfonyl)imide, and its abbreviation may be LiFSI. The lithium salt may further contain lithium-containing imide salts other than these lithium-containing imide salts.

[0038] The LiPF6 content in the non-aqueous electrolyte of this embodiment is preferably 0.01 moles or more, more preferably 0.020 moles or more, more preferably 0.030 moles or more, and even more preferably 0.045 moles or more per liter of non-aqueous solvent. If the LiPF6 content in the non-aqueous electrolyte is low, there is a concern that the lithium-containing imide salt may corrode the aluminum of the current collector, and that the electrolyte components may be reduced and decomposed due to insufficient formation of the negative electrode SEI. On the other hand, if there is sufficient LiPF6, hydrogen is extracted from the α position of acetonitrile, generating HF from the PF6 anion, which can promote the formation of the aluminum passivation state and the negative electrode SEI.

[0039] Furthermore, the LiPF6 content is 0.5 moles or less or less than 0.5 moles per liter of non-aqueous solvent, preferably 0.3 moles or less, and more preferably 0.1 moles or less. It is known that LiPF6 undergoes thermal decomposition at high temperatures, generating acidic components. This tendency is particularly pronounced in high-temperature environments exceeding 60°C, and even more pronounced in high-temperature environments exceeding 80°C. This tendency is also pronounced when the exposure time to high temperatures is 24 hours or more, more pronounced when it is 10 days or more, more pronounced when it is 20 days or more, and even more pronounced when it is 30 days or more. When the LiPF6 content is within the above range, the generation of acidic components due to the thermal decomposition reaction of LiPF6 can be suppressed even when exposed to high temperatures exceeding 60°C for 4 hours or more. Furthermore, the amount of HF generated from the PF6 anion by the abstraction of hydrogen from the α position of acetonitrile can also be reduced. If a large amount of HF is generated in the electrolyte, metal components will leach from the battery components, causing corrosion of the components. Furthermore, complex cations will be formed from the leached metal ions and acetonitrile, accelerating battery degradation. This tendency is particularly pronounced in high-temperature environments exceeding 60°C, and even more pronounced in high-temperature environments exceeding 80°C. This tendency is also pronounced when the exposure time to high temperatures is 24 hours or more, more pronounced when it is 10 days or more, more pronounced when it is 20 days or more, and even more pronounced when it is 30 days or more. However, if the LiPF6 content is within the above range, even when exposed to high temperatures exceeding 60°C for 4 hours or more, the amount of HF generated from the PF6 anion by abstracting hydrogen from the α position of acetonitrile can be reduced, thereby suppressing the progression of such degradation. As a result, the degradation of battery performance caused by corrosion of the positive electrode active material layer, positive electrode current collector, or electrolyte degradation reactions can be minimized.

[0040] Since the saturation concentration of lithium-containing imide salt relative to acetonitrile is higher than the saturation concentration of LiPF6, a molar concentration such that LiPF6 ≤ lithium-containing imide salt is preferred. From the viewpoint of suppressing the association and precipitation of lithium salt and acetonitrile at low temperatures, the molar ratio of lithium-containing imide salt to LiPF6 is preferably greater than 1 or greater than 3, preferably 10 or more, more preferably 20 or more, and even more preferably 25 or more. Furthermore, from the viewpoint of ion supply, the content of lithium-containing imide salt is preferably 0.5 moles or more and 3 moles or less per 1 L of non-aqueous solvent. An acetonitrile-containing non-aqueous electrolyte containing at least one of LiN(SO2F)2 and LiN(SO2CF3)2 can effectively suppress the reduction of ionic conductivity in low-temperature ranges such as -10°C or -40°C, and excellent low-temperature characteristics can be obtained. In this way, by controlling the content of each component, it is also possible to more effectively suppress the increase in resistance during high-temperature heating.

[0041] Furthermore, the lithium salt used in the non-aqueous electrolyte of this embodiment may also include fluorine-containing inorganic lithium salts other than LiPF6, such as LiBF4, LiAsF6, Li2SiF6, LiSbF6, and Li2B 12 F b H 12-b The formula may include fluorine-containing inorganic lithium salts such as [wherein b is an integer from 0 to 3]. Here, "inorganic lithium salt" refers to a lithium salt that does not contain carbon atoms as anions and is soluble in acetonitrile. Also, "fluorine-containing inorganic lithium salt" refers to a lithium salt that does not contain carbon atoms as anions, contains fluorine atoms as anions, and is soluble in acetonitrile. Fluorine-containing inorganic lithium salts are excellent in that they form a passive film on the surface of the aluminum foil, which is the positive electrode current collector, and suppress corrosion of the positive electrode current collector. These fluorine-containing inorganic lithium salts can be used individually or in combination of two or more. As the fluorine-containing inorganic lithium salt, a compound that is a double salt of LiF and a Lewis acid is preferred, and among these, a fluorine-containing inorganic lithium salt having a phosphorus atom is more preferred because it makes it easier to release free fluorine atoms. When a fluorine-containing inorganic lithium salt having a boron atom is used as the fluorine-containing inorganic lithium salt, it is preferred because it makes it easier to capture excess free acid components that may cause battery degradation, and from this viewpoint, LiBF4 is particularly preferred.

[0042] The fluorine-containing inorganic lithium salt content in the lithium salt used in the non-aqueous electrolyte of this embodiment is preferably 0.01 moles or more, more preferably 0.1 moles or more, and even more preferably 0.25 moles or more per liter of non-aqueous solvent. When the fluorine-containing inorganic lithium salt content is within the above range, the ionic conductivity tends to increase, and high-power characteristics can be achieved. Furthermore, this content is preferably less than 2.8 moles, more preferably less than 1.5 moles, and even more preferably less than 1 mole per liter of non-aqueous solvent. When the fluorine-containing inorganic lithium salt content is within the above range, the ionic conductivity increases, enabling high-power characteristics of the battery, and the decrease in ionic conductivity due to viscosity increase at low temperatures tends to be suppressed. This tends to improve high-temperature cycle characteristics and other battery characteristics while maintaining the excellent performance of the non-aqueous electrolyte. The non-aqueous electrolyte of this embodiment may further contain an organic lithium salt.

[0043] Examples of organolithium salts include organolithium salts having an oxalic acid structure. Specific examples of organolithium salts having an oxalic acid structure include, for example, organolithium salts represented as LiB(C2O4)2, LiBF2(C2O4), LiPF4(C2O4), and LiPF2(C2O4)2, respectively. Among these, at least one lithium salt selected from the lithium salts represented as LiB(C2O4)2 and LiBF2(C2O4) is preferred. Furthermore, it is more preferable to use one or more of these together with a fluorine-containing inorganic lithium salt. These organolithium salts having an oxalic acid structure may be added to a non-aqueous electrolyte or incorporated into the negative electrode active material layer.

[0044] The addition amount of the organic lithium salt having an oxalic acid structure to the non-aqueous electrolyte is preferably 0.005 mol or more, more preferably 0.02 mol or more, and still more preferably 0.05 mol or more per liter of the non-aqueous solvent of the non-aqueous electrolyte, from the viewpoint of ensuring better effects by its use. However, if the addition amount of the organic lithium salt having an oxalic acid structure to the non-aqueous electrolyte is too large, there is a risk of precipitation from the non-aqueous electrolyte. Therefore, the addition amount of the organic lithium salt having an oxalic acid structure to the non-aqueous electrolyte is preferably less than 1.0 mol, more preferably less than 0.5 mol, and still more preferably less than 0.2 mol per liter of the non-aqueous solvent of the non-aqueous electrolyte.

[0045] The organic lithium salt having an oxalic acid structure is known to be poorly soluble in non-polar organic solvents, particularly chain carbonates. The organic lithium salt having an oxalic acid structure may contain trace amounts of lithium oxalate, and further, when mixed as a non-aqueous electrolyte, it may react with trace amounts of moisture contained in other raw materials to newly generate a white precipitate of lithium oxalate. Therefore, the content of lithium oxalate in the non-aqueous electrolyte of the present embodiment is preferably 0 to 500 ppm.

[0046] As the lithium salt in the present embodiment, in addition to those described above, lithium salts generally used for non-aqueous secondary batteries may be additionally added as an auxiliary. Specific examples of other lithium salts include, for example, inorganic lithium salts without fluorine atoms in the anion such as LiClO4, LiAlO4, LiAlCl4, LiB 10 Cl 10 , chloro borane Li, etc.; organic lithium salts such as LiCF3SO3, LiCF3CO2, Li2C2F4(SO3)2, LiC(CF3SO2)3, LiC n F (2n+1) SO3 (where n≧2), lithium lower aliphatic carboxylate, lithium tetraphenylborate, LiB(C3O4H2)2, etc.; LiPFn(C p F 2p+1 ) 6-nOrganolithium salts represented by the formula [wherein n is an integer from 1 to 5 and p is an integer from 1 to 8]; LiBF such as LiBF3(CF3) q (CsF 2s+1 ) 4-q Organic lithium salts represented by the formula [wherein q is an integer between 1 and 3, and s is an integer between 1 and 8]; Lithium salts bound to polyvalent anions; The following formula (a): LiC(SO2RA)(SO2RB)(SO2RC) (a) {In the formula, RA, RB, and RC may be the same or different from each other, and represent a perfluoroalkyl group having 1 to 8 carbon atoms.} The following formula (b): LiN(SO ORD)(SO ORE) (b) {In the formula, RD and RE may be the same or different from each other, and represent a perfluoroalkyl group having 1 to 8 carbon atoms.}, and The following formula (c): LiN(SO RF)(SO ORG) (c) {In the formula, RF and RG may be the same or different from each other, and represent a perfluoroalkyl group having 1 to 8 carbon atoms.} Examples include organolithium salts represented by each of the above, and one or more of these can be used.

[0047] <2-3. Additives for electrode protection> The non-aqueous electrolyte according to this embodiment may contain an additive for protecting the electrodes (abbreviated as: electrode protection additive). The electrode protection additive may substantially overlap with the substance that acts as a solvent for dissolving the lithium salt (i.e., the non-aqueous solvent described above). The electrode protection additive is preferably a substance that contributes to improving the performance of the non-aqueous electrolyte and the non-aqueous secondary battery, but it also includes substances that do not directly participate in the electrochemical reaction.

[0048] Specific examples of electrode protection additives include, for example, Fluoroethylene carbonates, represented by 4-fluoro-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolan-2-one, cis-4,5-difluoro-1,3-dioxolan-2-one, trans-4,5-difluoro-1,3-dioxolan-2-one, 4,4,5-trifluoro-1,3-dioxolan-2-one, 4,4,5,5-tetrafluoro-1,3-dioxolan-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolan-2-one; Unsaturated bond-containing cyclic carbonates, such as vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate; Lactones, represented by γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone; Cyclic ethers, such as 1,4-dioxane; Cyclic sulfur compounds, such as ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methylsulfolane, 1,3-propanesultone, 1,4-butanesultone, 1-propene-1,3-sultone, and tetramethylene sulfoxide; These include, and they can be used individually or in combination of two or more types.

[0049] Ethylene sulfite is preferred as the electrode protection additive, and its content is preferably 1 to 30% by volume, more preferably 2 to 15% by volume, and even more preferably 3 to 8% by volume, based on the total amount of the non-aqueous solvent. In this embodiment, the higher the content of the electrode protection additive, the more the deterioration of the non-aqueous electrolyte is suppressed. However, the lower the content of the electrode protection additive, the better the high-power characteristics of the non-aqueous secondary battery in low-temperature environments. Therefore, by adjusting the content of the electrode protection additive within the above range, it is possible to exhibit excellent performance based on the high ionic conductivity of the electrolyte without impairing the basic functions of the non-aqueous secondary battery. Furthermore, by preparing a non-aqueous electrolyte with such a composition, the cycle performance, high-power performance in low-temperature environments, and other battery characteristics of the non-aqueous secondary battery can be further improved.

[0050] Acetonitrile is readily reductively decomposed electrochemically. Therefore, it is preferable to include one or more cyclic aprotic polar solvents as electrode protective additives for negative electrode SEI formation, and more preferably to include one or more unsaturated bond-containing cyclic carbonates.

[0051] As the unsaturated bond-containing cyclic carbonate, vinylene carbonate is preferred, and the vinylene carbonate content is preferably 2% to 10% by volume in the non-aqueous electrolyte, more preferably 2% to less than 7% by volume, and even more preferably 2% to 5% by volume. This makes it possible to more effectively improve low-temperature durability and provide a secondary battery with excellent low-temperature performance.

[0052] As an electrode protection additive, vinylene carbonate suppresses the reductive decomposition reaction of acetonitrile on the negative electrode surface, improving cycle performance under normal operating conditions around 3.6V. On the other hand, when high voltage is applied, it is thought to increase resistance by forming a film near the positive electrode, thereby shutting down current leakage and enhancing safety against unexpected high voltages.

[0053] <2-4. Other optional additives> In this embodiment, for the purpose of improving the charge-discharge cycle characteristics of the non-aqueous secondary battery, improving high-temperature storage capacity and safety (e.g., overcharge prevention), the non-aqueous electrolyte may appropriately contain optional additives selected from, for example, sulfonic acid esters, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, tert-butylbenzene, phosphate esters [ethyl diethyl phosphonoacetate (EDPA): (C2H5O)2(P=O)-CH2(C=O)OC2H5, tris(trifluoroethyl) phosphate (TFEP): (CF3CH2O)3P=O, triphenyl phosphate (TPP): (C6H5O)3P=O: (CH2=CHCH2O)3P=O, triallyl phosphate, etc.], nitrogen-containing cyclic compounds without steric hindrance around lone pairs [pyridine, 1-methyl-1H-benzotriazole, 1-methylpyrazole, etc.], and derivatives of these compounds. In particular, phosphate esters have the effect of suppressing side reactions during storage of non-aqueous electrolytes or batteries.

[0054] In the non-aqueous secondary battery according to this embodiment, a portion of the non-aqueous electrolyte decomposes during the initial charge, forming a negative electrode SEI on the negative electrode surface. To strengthen this negative electrode SEI, an acid anhydride can be added to the non-aqueous electrolyte as an additive. When acetonitrile is included as the non-aqueous solvent, the strength of the negative electrode SEI tends to decrease with increasing temperature, but the addition of an acid anhydride promotes the strengthening of the negative electrode SEI. Therefore, by using such an acid anhydride, the increase in internal resistance over time due to thermal history can be effectively suppressed.

[0055] Specific examples of acid anhydrides include, for example, chain-like acid anhydrides such as acetic anhydride, propionic anhydride, and benzoic anhydride; cyclic acid anhydrides such as malonic acid anhydride, succinic anhydride, glutaric acid anhydride, maleic anhydride, phthalic anhydride, 1,2-cyclohexanedicarboxylic acid anhydride, 2,3-naphthalenedicarboxylic acid anhydride, or naphthalene-1,4,5,8-tetracarboxylic dianhydride; and mixed acid anhydrides with structures formed by the dehydration condensation of two different carboxylic acids, or different types of acids such as a carboxylic acid and a sulfonic acid. These can be used individually or in combination of two or more.

[0056] In this embodiment, since it is preferable to strengthen the negative electrode SEI before the reductive decomposition of the non-aqueous solvent, it is preferable that the non-aqueous secondary battery contains at least one cyclic acid anhydride that acts early during the initial charge. These cyclic acid anhydrides may contain only one type, multiple types, or other cyclic acid anhydrides. Furthermore, it is preferable that the cyclic acid anhydride contains at least one of succinic anhydride, maleic anhydride, and phthalic anhydride.

[0057] A non-aqueous electrolyte containing at least one of succinic anhydride, maleic anhydride, and phthalic anhydride allows for the formation of a robust SEI on the negative electrode, more effectively suppressing the increase in resistance during high-temperature heating. The inclusion of succinic anhydride is particularly preferable. This allows for the more effective formation of a robust SEI on the negative electrode while suppressing side reactions.

[0058] When the non-aqueous electrolyte according to this embodiment contains an acid anhydride, its content is preferably in the range of 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the non-aqueous electrolyte, more preferably 0.05 parts by mass or more and 1 part by mass or less, and even more preferably 0.1 parts by mass or more and 0.5 parts by mass or less. When the acid anhydride content in the non-aqueous electrolyte is within the above range, the negative electrode SEI can be strengthened more effectively, and the high-temperature durability performance of the non-aqueous secondary battery using acetonitrile electrolyte can be improved more effectively.

[0059] The acid anhydride is preferably contained in a non-aqueous electrolyte. On the other hand, as long as the acid anhydride can act in a non-aqueous secondary battery, at least one battery component selected from the group consisting of a positive electrode, a negative electrode, and a separator may contain the acid anhydride. As for how to incorporate the acid anhydride into the battery component, for example, the acid anhydride may be incorporated into the battery component during its manufacture, or the acid anhydride may be impregnated into the battery component by post-treatment such as coating, immersion, or spray drying.

[0060] In this embodiment, the content of other optional additives is preferably in the range of 0.01% to 10% by mass, more preferably 0.02% to 5% by mass, and even more preferably 0.05 to 3% by mass, relative to the total amount of the non-aqueous electrolyte. By adjusting the content of other optional additives within the above range, it is possible to add even better battery characteristics without impairing the basic functions of the non-aqueous secondary battery.

[0061] <2-5. Condensed polycyclic heterocyclic compounds> The non-aqueous electrolytes in this embodiment are as follows: 1-5: 1. It is a condensed polycyclic heterocyclic compound, 2. The condensed polycyclic heterocycle contains a pyrimidine skeleton, 3. The condensed polycyclic heterocycle contains three or more nitrogen atoms, 4. The condensed polycyclic heterocycle contains five or more sp2 carbons, 5. No hydrogen atoms are bonded to the nitrogen atoms within the fused polycyclic heterocycle. The compound contains a compound having a structure that satisfies the following conditions (a fused polycyclic heterocyclic compound). A purine derivative is preferably used as such a fused polycyclic heterocyclic compound. Here, a purine derivative refers to a compound whose basic skeleton is a bicyclic heterocyclic structure in which an imidazole ring is bonded to a pyrimidine skeleton. More preferably, the fused polycyclic heterocyclic compound is one of the following general formulas (1) to (12): [ka] {In the formula, R2, R4, and R6, which form a double bond with the C atom in the fused polycyclic heterocycle, represent oxygen or sulfur atoms. R2, R4, and R6, which form a single bond with the C atom in the fused polycyclic heterocycle, and R1, R3, R5, and R7, which are bonded to the nitrogen atom in the fused polycyclic heterocycle, represent C1-C4 alkyl groups, C1-C4 haloalkyl groups, C1-C4 acylalkyl groups, allyl groups, propargyl groups, phenyl groups, benzyl groups, pyridyl groups, amino groups, and pyrrolidylmethyl groups. This represents a group, trimethylsilyl group, nitrile group, acetyl group, trifluoroacetyl group, chloromethyl group, methoxymethyl group, isocyanomethyl group, methylsulfonyl group, 2-(trimethylsilyl)-ethoxycarbonyloxy group, bis(N,N'-alkyl)aminomethyl group, bis(N,N'-alkyl)aminoethyl group, alkoxy group having 1 to 4 carbon atoms, fluorine-substituted alkoxy group having 1 to 4 carbon atoms, nitrile group, nitro group, halogen atom, sugar residue, or heterocyclic residue. However, R2, R4, and R6, which form single bonds with carbon atoms in fused polycyclic heterocyclic rings, may be hydrogen atoms. It contains at least one compound selected from the group consisting of compounds represented by and their isomers.

[0062] Specific examples of condensed polycyclic heterocyclic compounds in this embodiment are given below. These can be used individually or in combination of two or more. [ka] [ka]

[0063] Among the above formulas, the condensed polycyclic heterocyclic compound is preferably at least one selected from the group consisting of compounds represented by formulas (2), (5), (8), and (12), and their isomers, more preferably the compound represented by formula (2), and among the compounds represented by formula (2), caffeine is even more preferred.

[0064] In this embodiment, the content of the condensed polycyclic heterocyclic compound in the electrolyte is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more, based on the total amount of the electrolyte. In this embodiment, the condensed polycyclic heterocyclic compound suppresses the formation of complex cations formed from transition metals and acetonitrile. Therefore, a non-aqueous secondary battery containing the condensed polycyclic heterocyclic compound exhibits excellent load characteristics and suppresses the increase in internal resistance when repeated charge-discharge cycles are performed. Furthermore, in this embodiment, the content of the condensed polycyclic heterocyclic compound in the electrolyte is preferably 10% by mass or less, more preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less, based on the total amount of the electrolyte. By adjusting the content of the condensed polycyclic heterocyclic compound in this embodiment to within the above range, the formation reaction of complex cations on the electrode surface can be suppressed without impairing the basic functions of a non-aqueous secondary battery, and the increase in internal resistance associated with charging and discharging can be reduced. By preparing the electrolyte in this embodiment to within the specified range, the resulting non-aqueous secondary battery can exhibit even better cycle performance, high output performance in low-temperature environments, and other battery characteristics.

[0065] Although the detailed mechanism by which the condensed polycyclic heterocyclic compound in the electrolyte of this embodiment suppresses electrolyte degradation is unknown, it has been reported that in a polar solvent, three sp2 carbons in the five-membered ring within the imidazolyl ring adjacent to the pyrimidine skeleton act as reaction sites, thereby exhibiting antioxidant activity. Since reactive oxygen species are generated on the positive electrode surface in conjunction with electrochemical reactions, the common factor is that it is in an oxidative environment in a polar solvent, and similar effects can be expected as an additive to electrolytes for non-aqueous secondary batteries. One mechanism by which antioxidant activity is exhibited is that the reaction is promoted in the presence of iron (Fe) ions. Therefore, a high effect can be expected when using a positive electrode containing iron (Fe) ions, and a particularly high effect can be expected under conditions that cause iron (Fe) to leach from the positive electrode, such as when exposed to high temperatures of 60°C or higher for a long period of time. Furthermore, the condensed polycyclic heterocyclic compound has a pyrimidine skeleton and an imidazolyl ring, and possesses Lewis basicity derived from the lone pair of electrons on the nitrogen atom. Therefore, it is presumed that the condensed polycyclic heterocyclic compound can stabilize PF5 and suppress the formation of hydrofluoric acid (HF) from PF5 by interacting with strong Lewis acids such as PF5, which are produced when the electrolyte salt decomposes. In addition, it is presumed that it also interacts with strong Brønsted acids such as HF, thereby suppressing the leaching of metals from battery components.

[0066] <Overall configuration of a non-aqueous secondary battery> The non-aqueous electrolyte of this embodiment can be used in a non-aqueous secondary battery. There are no particular restrictions on the negative electrode, positive electrode, separator, and battery casing of the non-aqueous secondary battery of this embodiment.

[0067] Furthermore, an example of a non-aqueous secondary battery in this embodiment is a lithium-ion battery comprising a positive electrode containing a positive electrode material capable of intercalating and releasing lithium ions as a positive electrode active material, and a negative electrode containing a negative electrode material capable of intercalating and releasing lithium ions, and / or metallic lithium as a negative electrode active material.

[0068] The non-aqueous secondary battery of this embodiment may specifically be the non-aqueous secondary battery shown in Figures 1 and 2. Here, Figure 1 is a schematic plan view of the non-aqueous secondary battery, and Figure 2 is a cross-sectional view taken along line AA of Figure 1.

[0069] The non-aqueous secondary battery 100 shown in Figures 1 and 2 is composed of pouch-type cells. The non-aqueous secondary battery 100 houses a laminated electrode body, which is constructed by stacking a positive electrode 150 and a negative electrode 160 with a separator 170 in between, and a non-aqueous electrolyte (not shown) within a space 120 of a battery casing 110 made of two aluminum laminate films. The battery casing 110 is sealed at its outer periphery by heat-sealing the upper and lower aluminum laminate films together. The laminate, in which the positive electrode 150, separator 170, and negative electrode 160 are stacked in order, is impregnated with the non-aqueous electrolyte. However, in Figure 2, to avoid complexity in the drawing, the individual layers constituting the battery casing 110, as well as the individual layers of the positive electrode 150 and negative electrode 160, are not shown separately.

[0070] The aluminum laminate film constituting the battery casing 110 is preferably made by coating both sides of an aluminum foil with a polyolefin-based resin.

[0071] The positive electrode 150 is connected to the positive electrode lead 130 within the non-aqueous secondary battery 100. Although not shown in the diagram, the negative electrode 160 is also connected to the negative electrode lead 140 within the non-aqueous secondary battery 100. The positive electrode lead 130 and the negative electrode lead 140 each have one end extended outside the battery casing 110 so that they can be connected to external devices, and their ionomer portions are heat-sealed to one side of the battery casing 110.

[0072] The non-aqueous secondary battery 100 shown in Figures 1 and 2 has one stacked electrode body each for the positive electrode 150 and the negative electrode 160, but the number of stacked positive electrodes 150 and negative electrodes 160 can be increased as appropriate depending on the capacity design. In the case of a stacked electrode body having multiple positive electrodes 150 and negative electrodes 160, the tabs of the same electrode may be joined together by welding or the like and then joined to a single lead body by welding or the like and taken out of the battery. The tabs of the same electrode can be made from the exposed part of the current collector, or from a metal piece welded to the exposed part of the current collector, and so on.

[0073] The positive electrode 150 consists of a positive electrode active material layer made from a positive electrode mixture and a positive electrode current collector. The negative electrode 160 consists of a negative electrode active material layer made from a negative electrode mixture and a negative electrode current collector. The positive electrode 150 and the negative electrode 160 are arranged so that the positive electrode active material layer and the negative electrode active material layer face each other via a separator 170.

[0074] As long as these components satisfy the requirements of this embodiment, materials used in conventional lithium-ion batteries can be used. The components of the non-aqueous secondary battery will be described in more detail below.

[0075] <Positive electrode> The positive electrode 150 consists of a positive electrode active material layer made from a positive electrode mixture and a positive electrode current collector. The positive electrode mixture contains a positive electrode active material and, if necessary, a conductive additive and a binder.

[0076] The positive electrode active material layer uses a lithium phosphorus metal oxide having an olivine crystal structure containing iron (Fe) atoms, as shown in the following formula (Xba): Li w M II PO4(Xba) {In formula, M II This represents one or more transition metal elements, including at least one transition metal element containing Fe, and the value of w is determined by the battery's charge / discharge state, representing a number between 0.05 and 1.10. It is more preferable to use a lithium phosphate metal oxide having an olivine structure as represented by [formula].

[0077] These lithium-containing metal oxides may be modified in such ways as to stabilize the structure, by substituting some of the transition metal elements with Al, Mg, or other transition metal elements, incorporating these metal elements into the grain boundaries, substituting some of the oxygen atoms with fluorine atoms, or coating at least a portion of the surface of the positive electrode active material with another positive electrode active material.

[0078] To deactivate the active sites that essentially cause oxidative degradation of non-aqueous electrolytes, the presence of components that control Jahn-Teller strain or act as neutralizers is important. Therefore, it is preferable that the positive electrode active material contains at least one metal selected from the group consisting of Al, Sn, In, Fe, V, Cu, Mg, Ti, Zn, Mo, Zr, Sr, and Ba.

[0079] For similar reasons, it is preferable that the surface of the positive electrode active material is coated with a compound containing at least one metal element selected from the group consisting of Zr, Ti, Al, and Nb. It is even more preferable that the surface of the positive electrode active material is coated with an oxide containing at least one metal element selected from the group consisting of Zr, Ti, Al, and Nb. Furthermore, it is particularly preferable that the surface of the positive electrode active material is coated with at least one oxide selected from the group consisting of ZrO2, TiO2, Al2O3, NbO3, and LiNbO2, as this does not hinder the permeation of lithium ions.

[0080] Examples of positive electrode active materials include metal phosphate oxides containing lithium and a transition metal element, and metal silicate oxides containing lithium and a transition metal element. From the viewpoint of obtaining a higher voltage, metal phosphate oxides containing lithium and at least one transition metal element selected from the group consisting of Co, Ni, Mn, Fe, Cu, Zn, Cr, V, and Ti are particularly preferred as lithium-containing metal oxides, and metal phosphate oxides containing Li and Fe are more preferred from the viewpoint of lithium phosphate metal oxide represented by the above formula (Xba).

[0081] A lithium phosphate metal oxide different from the lithium phosphate metal oxide represented by the above formula (Xba) is given by the following formula (Xa): LivMID2(Xa) {In the formula, D represents a chalcogen element, MI represents one or more transition metal elements including at least one transition metal element, and the value of v is determined by the charge / discharge state of the battery and represents a number between 0.05 and 1.10.} Compounds represented by may also be used.

[0082] In this embodiment, the positive electrode active material may be lithium-containing metal oxides as described above, or other positive electrode active materials may be used in combination with the lithium-containing metal oxides.

[0083] Other positive electrode active materials include, for example, metal oxides or metal chalcogenides having tunnel and layered structures; sulfur; conductive polymers, etc. Examples of metal oxides or metal chalcogenides having tunnel and layered structures include MnO2, FeO2, FeS2, V2O5, V6O 13 Examples of conductive polymers include oxides, sulfides, and selenides of metals other than lithium, such as TiO2, TiS2, MoS2, and NbSe2. Examples of conductive polymers include polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0084] The other positive electrode active materials mentioned above can be used individually or in combination of two or more. It is preferable that the positive electrode active material layer contains at least one transition metal element selected from Ni, Mn, and Co, as this enables reversible and stable intercalation and release of lithium ions and achieves a high energy density.

[0085] When lithium-containing metal oxide and other positive electrode active materials are used in combination as positive electrode active materials, the ratio of lithium-containing metal oxide to the total positive electrode active material is preferably 80% by mass or more, and more preferably 85% by mass or more.

[0086] The positive electrode active material layer is formed by dispersing a slurry containing a positive electrode mixture, which is a mixture of the positive electrode active material and, if necessary, a conductive additive and a binder, in a solvent, onto a positive electrode current collector, drying (solvent removal), and pressing as necessary. Known solvents can be used for this purpose. Examples include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

[0087] Examples of conductive additives include acetylene black, carbon black such as Ketjenblack, carbon fibers, and graphite. The content ratio of the conductive additive is preferably 10 parts by mass or less, and more preferably 1 to 5 parts by mass, per 100 parts by mass of positive electrode active material.

[0088] Examples of binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid, styrene-butadiene rubber, and fluororubber. The binder content is preferably 6 parts by mass or less, and more preferably 0.5 to 4 parts by mass, per 100 parts by mass of positive electrode active material.

[0089] The positive electrode current collector is made of a metal foil such as aluminum foil, nickel foil, or stainless steel foil. The positive electrode current collector may have a carbon coating on its surface or may be processed into a mesh shape. The thickness of the positive electrode current collector is preferably 5 to 40 μm, more preferably 7 to 35 μm, and even more preferably 9 to 30 μm.

[0090] The basis weight per side of the positive electrode, excluding the positive electrode current collector, is set at 15 mg / cm³ from the viewpoint of improving the volumetric energy density in non-aqueous secondary batteries. 2 Preferably, the concentration is 17.5 mg / cm³. 2 It is more preferable that the above conditions are met. Furthermore, the basis weight per positive electrode side, excluding the positive electrode current collector, should be 100 mg / cm². 2 Preferably, it is 80 mg / cm³ 2More preferably, the following is 60 mg / cm³ 2 It is even more preferable that the following conditions are met: By limiting the basis weight per positive electrode side, excluding the positive electrode current collector, to the above range, it is possible to provide a non-aqueous secondary battery that achieves high output performance even when designing an electrode active material layer with a high volumetric energy density.

[0091] <Negative electrode> The negative electrode 160 consists of a negative electrode active material layer made from a negative electrode mixture and a negative electrode current collector. The negative electrode 160 can function as the negative electrode of a non-aqueous secondary battery.

[0092] The negative electrode mixture contains a negative electrode active material and, if necessary, a conductive additive and a binder.

[0093] Examples of anode active materials that can be used include amorphous carbon (hard carbon), graphite (artificial graphite, natural graphite), pyrolysis carbon, coke, glassy carbon, calcined organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, carbon colloids, and carbon materials such as carbon black, as well as metallic lithium, metal oxides, metal nitrides, lithium alloys, tin alloys, silicon alloys, intermetallic compounds, organic compounds, inorganic compounds, metal complexes, and organic polymer compounds. The anode active material can be used alone or in combination of two or more types.

[0094] In this embodiment, it is preferable to use graphite or a compound containing one or more elements selected from the group consisting of Ti, V, Sn, Cr, Mn, Fe, Co, Ni, Zn, Al, Si, and B as the negative electrode active material.

[0095] From the viewpoint of increasing the battery voltage, it is preferable that the negative electrode active material layer contains a material capable of intercepting lithium ions at a potential lower than 0.4V vs. Li / Li+ as the negative electrode active material.

[0096] The negative electrode active material layer is formed by dispersing a negative electrode mixture, which is a mixture of the negative electrode active material and, if necessary, a conductive additive and a binder, in a solvent, onto a negative electrode current collector, drying (solvent removal), and pressing as necessary. Known solvents can be used for this purpose. Examples include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

[0097] Examples of conductive additives include acetylene black, carbon black such as Ketjenblack, carbon fibers, and graphite. The content ratio of the conductive additive is preferably 20 parts by mass or less, and more preferably 0.1 to 10 parts by mass, per 100 parts by mass of the negative electrode active material.

[0098] Examples of binders include carboxymethylcellulose, PVDF, PTFE, polyacrylic acid, and fluororubber. Diene rubbers, such as styrene-butadiene rubber, are also acceptable. The binder content is preferably 10 parts by mass or less, and more preferably 0.5 to 6 parts by mass, per 100 parts by mass of the negative electrode active material.

[0099] The negative electrode current collector is made of a metal foil such as copper foil, nickel foil, or stainless steel foil. The negative electrode current collector may also have a carbon coating on its surface or be processed into a mesh shape. The thickness of the negative electrode current collector is preferably 5 to 40 μm, more preferably 6 to 35 μm, and even more preferably 7 to 30 μm.

[0100] <Separator> In this embodiment, the non-aqueous secondary battery 100 preferably includes a separator 170 between the positive electrode 150 and the negative electrode 160, from the viewpoint of providing safety such as preventing short circuits and shutting down the positive electrode 150 and the negative electrode 160. The separator 170 is preferably an insulating thin film with high ion permeability and excellent mechanical strength. Examples of the separator 170 include woven fabric, nonwoven fabric, and microporous membrane made of synthetic resin, and among these, a microporous membrane made of synthetic resin is preferred.

[0101] Suitable examples of synthetic resin microporous membranes include polyolefin-based microporous membranes, such as those containing polyethylene or polypropylene as the main component, or those containing both of these polyolefins. Examples of nonwoven fabrics include heat-resistant resin porous membranes made of glass, ceramic, polyolefin, polyester, polyamide, liquid crystal polyester, aramid, etc.

[0102] The separator 170 may have a configuration in which one type of microporous membrane is laminated in a single layer or multiple layers, or it may have two or more types of microporous membranes laminated together. The separator 170 may also have a configuration in which a mixed resin material obtained by melt-kneading two or more types of resin materials is laminated in a single layer or multiple layers.

[0103] For the purpose of imparting functionality, inorganic particles may be present on the surface or inside the separator, and other organic layers may be further coated or laminated. The separator may also include a cross-linked structure. These methods may be combined as needed to enhance the safety performance of non-aqueous secondary batteries.

[0104] By using such a separator 170, it is possible to achieve the good input / output characteristics and low self-discharge characteristics particularly required for lithium-ion batteries used in the high-power applications mentioned above. The thickness of the separator is preferably 1 μm or more from the viewpoint of separator strength, preferably 500 μm or less from the viewpoint of permeability, more preferably 5 μm to 30 μm, and even more preferably 10 μm to 25 μm. If short-circuit resistance is important, the thickness of the separator is even more preferably 15 μm to 20 μm, but if high energy density is important, it is even more preferably 10 μm to less than 15 μm. The porosity of the separator is preferably 30% to 90%, more preferably 35% to 80%, and even more preferably 40% to 70%, from the viewpoint of following the rapid movement of lithium ions at high power. Furthermore, when prioritizing improved output performance while ensuring safety, a ratio of 50% to 70% is particularly preferable, and when prioritizing a balance between short-circuit resistance and output performance, a ratio of 40% to less than 50% is particularly preferable. The air permeability of the separator should be 1 second / 100 cm, considering the balance with the separator's thickness and porosity. 3 More than 400 seconds / 100cm 3 The following is preferable: 100 seconds / 100 cm 3 A value of 350 / 100cm³ or less is preferable. However, if prioritizing both short-circuit resistance and output performance, the air permeability should be 150 seconds / 100cm. 3 More than 350 seconds / 100cm 3 The following is particularly preferable, and if prioritizing improved output performance while ensuring safety, 100 / 100cm 3 150 seconds / 100cm 3A value less than the specified value is particularly preferred. On the other hand, when a non-aqueous electrolyte with low ionic conductivity is combined with a separator within the above range, the rate of lithium ion migration is limited not by the separator's structure, but by the high ionic conductivity of the electrolyte, and the expected input / output characteristics tend not to be obtained. For this reason, the ionic conductivity of the non-aqueous electrolyte at 25°C is preferably 10 mS / cm or higher, more preferably 15 mS / cm or higher, and even more preferably 20 mS / cm or higher. However, the thickness, air permeability, and porosity of the separator, as well as the ionic conductivity of the non-aqueous electrolyte, are not limited to the above examples.

[0105] <Battery casing> The battery casing 110 of the non-aqueous secondary battery 100 in this embodiment can be made of either a battery can (not shown) or a laminate film casing. For example, a metal can made of steel, stainless steel, aluminum, or clad material can be used, such as a rectangular, rectangular tube, cylindrical, elliptical, flat, coin-shaped, or button-shaped can. For example, a laminate film casing can be made of a three-layer structure consisting of heat-melt resin / metal film / resin. The laminate film casing can be used by stacking two sheets with the heat-melt resin side facing inward, or by folding the film so that the heat-melt resin side faces inward, and sealing the ends with heat seal. When using the laminate film casing, the positive electrode lead body 130 (or positive electrode terminal and lead tab connected to the positive electrode terminal) may be connected to the positive electrode current collector, and the negative electrode lead body 140 (or negative electrode terminal and lead tab connected to the negative electrode terminal) may be connected to the negative electrode current collector. In this case, the laminate film casing may be sealed with the ends of the positive electrode lead body 130 and the negative electrode lead body 140 (or lead tabs connected to the positive electrode terminal and the negative electrode terminal, respectively) extended to the outside of the casing.

[0106] <How to make a battery> The non-aqueous secondary battery 100 in this embodiment is manufactured by a known method using the above-mentioned non-aqueous electrolyte, a positive electrode 150 having a positive electrode active material layer on one or both sides of the current collector, a negative electrode 160 having a negative electrode active material layer on one or both sides of the current collector, a battery casing 110, and a separator 170 if necessary.

[0107] First, a laminate is formed consisting of a positive electrode 150, a negative electrode 160, and, if necessary, a separator 170. For example, a laminate with a wound structure can be formed by winding long positive electrodes 150 and negative electrodes 160 in a laminated state with the long separator interposed between them; a laminate with a laminated structure can be formed by cutting the positive electrode 150 and negative electrode 160 into multiple sheets having a certain area and shape, and alternately stacking the resulting positive electrode sheets and negative electrode sheets via a separator sheet; a laminate with a laminated structure can be formed by folding a long separator in a zigzag pattern and alternately inserting positive electrode sheets and negative electrode sheets between the zigzag-folded separators; and so on.

[0108] Next, the laminate described above is housed in the battery casing 110 (battery case), the electrolyte according to this embodiment is poured into the battery case, and the laminate is immersed in the electrolyte and sealed to produce the non-aqueous secondary battery according to this embodiment. Alternatively, a gel-like electrolyte membrane can be prepared in advance by impregnating a substrate made of polymer material with the electrolyte, and a laminated structure can be formed using a sheet-like positive electrode 150, a negative electrode 160, and an electrolyte membrane, and a separator 170 if necessary, and then housed in the battery casing 110 to produce the non-aqueous secondary battery 100.

[0109] Furthermore, if the electrode arrangement is designed such that there is an overlap between the outer edge of the negative electrode active material layer and the outer edge of the positive electrode active material layer, or if there is a section of the non-opposing part of the negative electrode active material layer that is too narrow, then electrode misalignment may occur during battery assembly, potentially degrading the charge-discharge cycle characteristics of the non-aqueous secondary battery. Therefore, it is preferable to fix the electrode positions in advance using tapes such as polyimide tape, polyphenylene sulfide tape, or PP tape, or adhesives, when using the electrode body for the non-aqueous secondary battery.

[0110] In this embodiment, due to the high ionic conductivity of acetonitrile, lithium ions released from the positive electrode during the initial charge of a non-aqueous secondary battery may diffuse throughout the negative electrode. In non-aqueous secondary batteries, it is common to have a larger negative electrode active material layer than the positive electrode active material layer. However, if lithium ions diffuse and are intercalated in areas of the negative electrode active material layer that do not face the positive electrode active material layer, these lithium ions will remain in the negative electrode without being released during the initial discharge. As a result, the contribution of these unreleased lithium ions becomes irreversible capacity. For these reasons, non-aqueous secondary batteries using a non-aqueous electrolyte containing acetonitrile may have low initial charge-discharge efficiency.

[0111] On the other hand, if the area of ​​the positive electrode active material layer is larger than that of the negative electrode active material layer, or if the areas are the same, current concentration is more likely to occur at the edges of the negative electrode active material layer during charging, making it easier for lithium dendrites to form.

[0112] There are no particular restrictions on the ratio of the total area of ​​the negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other. However, for the reasons mentioned above, it is preferable that the ratio be greater than 1.0 and less than 1.1, more preferably greater than 1.002 and less than 1.09, even more preferably greater than 1.005 and less than 1.08, and particularly preferably greater than 1.01 and less than 1.08. In a non-aqueous secondary battery using a non-aqueous electrolyte containing acetonitrile, the initial charge-discharge efficiency can be improved by reducing the ratio of the total area of ​​the negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other.

[0113] Reducing the ratio of the total area of ​​the negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other means limiting the proportion of the negative electrode active material layer that does not face the positive electrode active material layer. This makes it possible to reduce as much as possible the amount of lithium ions absorbed into the portion of the negative electrode active material layer that does not face the positive electrode active material layer (i.e., the amount of lithium ions that are not released from the negative electrode during the first discharge and become irreversible capacity) from the lithium ions released from the positive electrode during the first charge. Therefore, by designing the ratio of the total area of ​​the negative electrode active material layer to the area of ​​the portion where the positive electrode active material layer and the negative electrode active material layer face each other within the above range, it is possible to improve the load characteristics of the battery by using acetonitrile, increase the initial charge-discharge efficiency of the battery, and further suppress the formation of lithium dendrites.

[0114] The non-aqueous secondary battery 100 in this embodiment can function as a battery through the first charge, but is stabilized by the decomposition of a part of the electrolytic solution during the first charge. There is no particular limitation on the method of the first charge, but the first charge is preferably performed at 0.001 to 0.3 C, more preferably at 0.002 to 0.25 C, and even more preferably at 0.003 to 0.2 C. It is also preferable that the first charge is performed via constant voltage charging. By setting a long voltage range in which the lithium salt participates in the electrochemical reaction, a stable and strong negative electrode SEI is formed on the electrode surface, suppressing an increase in internal resistance, and the reaction products are not firmly fixed only to the negative electrode 160, but also have a good effect on members other than the negative electrode 160, such as the positive electrode 150 and the separator 170. Therefore, it is very effective to perform the first charge in consideration of the electrochemical reaction of the lithium salt dissolved in the non-aqueous electrolytic solution.

[0115] The non-aqueous secondary battery 100 in this embodiment can also be used as a battery pack in which a plurality of non-aqueous secondary batteries 100 are connected in series or in parallel. From the viewpoint of managing the charge and discharge state of the battery pack, when using a positive electrode active material represented by (Xba) as the positive electrode, the operating voltage range per unit is preferably 1.5 to 4.0 V, and particularly preferably 2.0 to 3.8 V.

[0116] <CCCV Charge Test> The non-aqueous electrolytic solution of this embodiment is charged at a constant current of 0.5 C until the battery voltage reaches a predetermined voltage, then switched to a constant voltage current to be in a floating state, and a floating test is performed in a high-temperature constant temperature bath. The leakage current value when the current converges is 0.003 C or less. The predetermined voltage refers to a voltage higher than 3.6 V, which is the operating range of a general LFP electrode battery. It is preferably performed at 3.65 V or higher, more preferably at 3.8 V or higher, even more preferably at 4.0 V or higher, and even more preferably at 4.2 V or higher. In other words, it is preferable that the voltage of the CCCV charge is any one of 3.8 V, 4.0 V, and 4.2 V.

[0117] Here, the fact that current leakage during the float test can be suppressed means that electrochemical reactions and short circuits do not occur under high-temperature and high-voltage environments. The specific implementation method of the float test is as described in the examples.

[0118] <XRD analysis> For the non-aqueous electrolyte of this embodiment, regarding the XRD analysis result of the positive electrode after the above float test, the following formula (1-1): X(%) = (proportion of FP phase * iron occupancy of FP phase + proportion of LFP phase * iron occupancy of LFP phase) * 100 ··· (1-1) The residual iron ratio X (iron occupancy X of the positive electrode) in the positive electrode, represented by the above formula, is 85% or more. In the above formula, The LFP phase refers to the phase of lithium iron phosphate (LiFePO4), The FP phase refers to the phase in which Li has escaped from the above LFP.

[0119] Here, the fact that the residual iron ratio X in the positive electrode is 85% or more means that elution of iron from the positive electrode is suppressed under high-temperature and high-voltage environments, and it is possible to suppress the deterioration of the electrode due to precipitation on the negative electrode, etc., and the adverse effects due to short circuits. The specific implementation methods of XRD measurement and analysis are as described in the examples.

[0120] As described above, the embodiments for implementing the present invention have been described, but the present invention is not limited to the above-described embodiments. The present invention can be variously modified without departing from the gist thereof.

Examples

[0121] Hereinafter, the present invention will be described in more detail by way of examples. The present invention is not limited to these examples.

[0122] (1) Preparation of non-aqueous electrolyte Under an inert atmosphere, various non-aqueous solvents, various acid anhydrides, and various additives were mixed so that each had a predetermined concentration, and further, various lithium salts were added so that each had a predetermined concentration, thereby preparing non-aqueous electrolytes (S01) to (S07).

[0123] [Table 1] In the table, "LiPF6" is "lithium hexafluoride phosphate", "LiFSI" stands for "Lithium Bis(Fluorosulfonyl)imide," "AcN" stands for "acetonitrile". "EMC" stands for "ethyl methyl carbonate". "EC" stands for "ethylene carbonate". "VC" stands for "vinyl carbonate". "ES" stands for "ethylene sulfite". "CAF" stands for "caffeine," Each will be shown.

[0124] (2) Fabrication of non-aqueous secondary batteries (2-1) Preparation of the positive electrode (A) Lithium iron phosphate (LiFePO4) having an olivine-type structure as the positive electrode active material, and (B) carbon black powder as a conductive additive and polyvinylidene fluoride (PVDF) as a binder were mixed in a mass ratio of 84:10:6 to obtain a positive electrode mixture.

[0125] N-methyl-2-pyrrolidone was added as a solvent to the obtained positive electrode mixture to a solid content of 68% by mass, and the mixture was further mixed to prepare a positive electrode mixture-containing slurry. The slurry was applied to one side of a 15 μm thick, 280 mm wide aluminum foil, which would serve as the positive electrode current collector, using a three-roll transfer coater to create a coating pattern with a coating width of 240-250 mm, a coated length of 125 mm, and an uncoated length of 20 mm, while adjusting the basis weight of the slurry. The solvent was then dried and removed in a hot air drying oven. The resulting electrode rolls were trimmed on both sides and subjected to reduced-pressure drying at 130°C for 8 hours. After that, the density of the positive electrode active material layer was reduced to 1.9 g / cm³ using a roll press. 3 By rolling the material in this manner, a positive electrode consisting of a positive electrode active material layer and a positive electrode current collector was obtained. The basis weight excluding the positive electrode current collector was 17.5 mg / cm³. 2 That was the case.

[0126] (2-2) Fabrication of the negative electrode A negative electrode mixture was obtained by mixing graphite powder as the negative electrode active material, carbon black powder as a conductive additive, and carboxymethylcellulose and styrene-butadiene rubber as binders, in a solid content mass ratio of 95.7:0:3.8.

[0127] Water was added as a solvent to the obtained negative electrode mixture to a solid content of 45% by mass, and the mixture was further mixed to prepare a negative electrode mixture-containing slurry. The slurry was applied to one side of a copper foil 8 μm thick and 280 mm wide, which would serve as the negative electrode current collector, using a three-roll transfer coater, adjusting the basis weight of the slurry to create a coating pattern with a coating width of 240-250 mm, a coated length of 125 mm, and an uncoated length of 20 mm. The solvent was then dried and removed in a hot air drying oven. The resulting electrode rolls were trimmed on both sides and subjected to reduced-pressure drying at 80°C for 12 hours. After that, the density of the negative electrode active material layer was reduced to 1.5 g / cm³ using a roll press. 3 The material was rolled to obtain a negative electrode consisting of a negative electrode active material layer and a negative electrode current collector. The basis weight excluding the negative electrode current collector was 7.5 mg / cm³. 2 That was the case.

[0128] (3) Battery assembly The positive and negative electrode active materials were stacked in the following order from bottom to top: negative electrode, multilayer porous film, and positive electrode, so that their active material surfaces faced each other. This laminate was then placed in a stainless steel container with a lid that was insulated from the container body, so that the copper foil of the negative electrode and the aluminum foil of the positive electrode were in contact with the container body and lid, respectively, to obtain a cell. This cell was dried under reduced pressure at 65°C for 12 hours. After that, 0.4 mL of non-aqueous electrolyte was injected into the container in an argon box, and it was sealed to create an evaluation battery.

[0129] (4) Battery evaluation For the coin-type non-aqueous secondary batteries obtained as described above (Examples 1-3 and Comparative Examples 1-3), first, initial charging and initial charge / discharge capacity measurements were performed according to the procedure in (4-1) below. Next, each coin-type non-aqueous secondary battery was evaluated according to the procedures in (4-2) and (4-3). Charging and discharging were performed using the ACD-M01A charge / discharge device (product name) manufactured by Asuka Electronics Co., Ltd. and the IN804 programmable constant temperature bath (product name) manufactured by Yamato Scientific Co., Ltd.

[0130] In this specification, 1C refers to the current value at which a fully charged battery is expected to complete discharge in one hour when discharged at a constant current. Specifically, in this non-aqueous secondary battery, 1C refers to the current value at which discharge from a fully charged state of 3.6V to 2V at a constant current is expected to be completed in 1 hour. The non-aqueous secondary battery assembled according to the procedure in (3) above is a 5.5mAh class cell, with a fully charged battery voltage of 3.6V and a current value equivalent to 1C of 5.5mA. Hereafter, unless otherwise specified, the notation of current values ​​and voltages will be omitted for convenience.

[0131] (4-1) Initial charge and discharge process of non-aqueous secondary batteries The ambient temperature of the non-aqueous secondary battery was set to 25°C, and it was charged with a constant current equivalent to 0.1C until it reached full charge. Then, it was charged with a constant voltage until the current reached 0.05C (CCCV charging). After that, the battery was discharged to a predetermined voltage with a constant current equivalent to 0.2C (CC discharge). Subsequently, the 0.2C CCCV charging and 0.2C CC discharge cycles were repeated twice.

[0132] (4-2) Capacity test After discharging at 0.5C, the battery was charged with a constant current equivalent to 0.5C to a state of 3.6V, and then charged at a constant voltage until the current value reached 0.05C. Afterward, the battery was discharged to a predetermined voltage with a constant current equivalent to 0.5C. The discharge capacity at this point was defined as the initial capacity.

[0133] (4-3) High-temperature float test of non-aqueous secondary batteries For the non-aqueous secondary battery that underwent the initial charge-discharge treatment using the method described in (4-1) above, the ambient temperature was set to 25°C, and it was charged with a constant current equivalent to 0.5C until it reached 3.6V. Then, it was charged with a constant voltage until the current reached 0.05C. Next, this coin-type non-aqueous secondary battery was stored in a high-temperature constant-temperature bath for 4 hours. After that, it was charged to the test voltage with a current equivalent to 0.5C. After reaching the test voltage, a float test was performed at high temperature and constant voltage for one week (24 hours x 7 days). The current flowing through the battery after one week was defined as the leakage current. After the test, it was returned to room temperature, and the capacity test described in (4-2) was performed.

[0134] (4-4) Calculation of measurement values ​​for high-temperature float tests For non-aqueous secondary batteries subjected to high-temperature float testing using the method described in (4-3), the measured value of the float test is given by the following formula: Residual capacity retention rate = (Remaining discharge capacity after high-temperature float test / Initial capacity before high-temperature float test) × 100 [%] The remaining capacity retention rate was calculated based on the following. These results are shown in the table below.

[0135] [Examples 1-3 and Comparative Examples 1-3] Here, we will discuss the interpretation of each test result. The residual capacity retention rate and leakage current are indicators of self-discharge and electron consumption, respectively, during high-temperature float testing. A higher residual capacity retention rate indicates less self-discharge under high temperature and high voltage conditions, allowing for greater current utilization. However, since electrons are supplied to maintain a constant voltage during float testing, reactions may continue, and the residual capacity retention rate cannot be used to evaluate the safety of the battery. Therefore, leakage current is used as an indicator of safety. A lower leakage current indicates that side reactions and electron consumption due to deposition are not continuously occurring in the fully charged state, and thus indicates a higher level of battery safety. A leakage current value of 0.003C or less is preferred, 0.002C or less is more preferred, and 0.001C or less is even more preferred.

[0136] [Table 2]

[0137] Examples 1-5 all met the passing standard (leakage current value: 0.003C = 0.017mA) in all tests. Examples 3-5 showed high residual capacity retention rates and low leakage current values ​​in all tests, suggesting high resistance to high temperatures and high voltages. On the other hand, Examples 1 and 2 had lower residual capacity retention rates compared to Examples 3-5, but also lower leakage currents. This indicates that degradation occurred in response to high voltages outside the operating range without electrochemical reactions or short circuits due to metal deposition, demonstrating high safety due to the current shut-off function. On the other hand, in Comparative Examples 1 and 2, the leakage current values ​​were larger than those in Examples 1 to 5. Both Comparative Examples 1 and 2 were systems in which ethylene sulfite and condensed polycyclic compounds were not added. Since ethylene sulfite is expected to assist in film formation and condensed polycyclic compounds are expected to suppress the formation of complex cations consisting of transition metals and acetonitrile, it is thought that performance can be more effectively improved by combining them with a non-aqueous electrolyte as in this embodiment.

[0138] (4-5) XRD analysis of the cathode after high-temperature float testing For non-aqueous secondary batteries that underwent high-temperature float testing using the method described in (4-3) above, the positive electrode was removed after discharge, cleaned with DMC, air-dried, and then subjected to XRD analysis. The measurement device used was Rigaku Ultima-IV. The sample was placed on a non-reflective sample stage with the positive electrode surface facing upwards for measurement. The measurement conditions are shown in the table below. For analysis, Rigaku's SmartLab Studio II software was used to optimize the lattice constants of LFP and FP, the atomic coordinates and occupancy of iron, and the LFP / FP semi-quantitative values ​​using the Rietveld method. Furthermore, for some samples, due to experimental constraints, measurements were taken with the separator still attached to the positive electrode after testing. Because this resulted in strong diffraction peaks originating from the separator, the 20°-25° range where these peaks appeared was excluded from the data refined by the Rietveld analysis. Using the ratio of FP phase to LFP phase obtained from the Rietveld analysis, the following equation (1-1): X(%) = (FP phase proportion * iron content of FP phase + LFP phase proportion * iron content of LFP phase) * 100 ... (1 - 1) Based on this, the residual iron percentage X in the positive electrode (iron occupancy rate X in the positive electrode) was calculated. The X obtained from this formula is summarized in the table below.

[0139] [Table 3]

[0140] [Table 4]

[0141] As shown in the table above, all of Examples 1 to 5 showed an iron content of 85% or more in the positive electrode, indicating that iron elution is suppressed under high temperature and high voltage conditions. On the other hand, in Comparative Example 1, the positive electrode, separator, and negative electrode were all in close contact after the test, making XRD analysis impossible. Metal deposition such as black precipitate was also observed during disassembly, indicating that safety against high voltage was not met. It was confirmed that the non-aqueous electrolyte according to this embodiment can improve high temperature durability and high voltage durability, as well as enhance safety against unexpected high voltage.

[0142] These results confirm that by using the non-aqueous electrolyte described in this embodiment, it is possible to minimize the degradation of battery performance caused by corrosion of the positive electrode or current collector, or degradation of the electrolyte, over a wide temperature and voltage range, thereby improving the desired battery performance. Furthermore, it was confirmed that improved safety can be expected by shutting down the current at high voltages outside the operating range, thereby suppressing short circuits due to electrochemical reactions and metal deposition. [Industrial applicability]

[0143] The non-aqueous electrolyte and non-aqueous secondary battery of the present invention are expected to be used, for example, as rechargeable batteries for portable devices such as mobile phones, portable audio devices, personal computers, and IC (Integrated Circuit) tags; as rechargeable batteries for automobiles such as hybrid vehicles, plug-in hybrid vehicles, and electric vehicles; as low-voltage power supplies such as 12V, 24V, and 48V power supplies; as residential energy storage systems, IoT devices, and the like. Furthermore, the non-aqueous secondary battery of the present invention can also be applied to applications in cold regions and outdoor applications in summer. [Explanation of Symbols]

[0144] 100 Nonaqueous secondary battery 110 Battery casing 120 Battery enclosure space 130 Positive electrode lead body 140 Negative electrode lead body 150 positive electrode 160 negative electrode 170 Separator

Claims

1. The device comprises a positive electrode containing a lithium phosphorus metal compound with an olivine crystal structure containing iron (Fe), and a negative electrode containing graphite. It contains a non-aqueous solvent containing 5 to 95% by volume of acetonitrile, a lithium salt, and an electrode protection additive.

3. When CCCV charging is continued under conditions of 3.65V or higher and 60℃ or higher, the current value at which the current converges is 0.003C or less. Using the values ​​obtained from the XRD analysis results of the positive electrode after the CCCV charging test, the following formula is used: X (%) = (FP phase proportion * iron content of FP phase + LFP phase proportion * iron content of LFP phase) * 100 The iron occupancy rate X (%) of the positive electrode, calculated from this, is 85% or more. Non-aqueous secondary battery.

2. The non-aqueous secondary battery according to claim 1, wherein vinylene carbonate is present in an amount of 2% by volume or more relative to the total amount of the non-aqueous solvent.

3. The lithium salt is lithium hexafluoride phosphate (LiPF) ６ A non-aqueous secondary battery according to claim 1, comprising ) and a lithium-containing imide salt.

4. The LiPF ６ The non-aqueous secondary battery according to claim 3, wherein the content of is 0.01 mol / L or more and less than 0.5 mol / L relative to the non-aqueous solvent.

5. The LiPF ６ The non-aqueous secondary battery according to claim 3, wherein the molar ratio of the lithium-containing imide salt to is greater than 1.

6. The non-aqueous secondary battery according to claim 1, wherein the electrode protection additive is ethylene sulfite.

7. The non-aqueous secondary battery according to claim 6, wherein the ethylene sulfite is 1% by volume or more relative to the total amount of the non-aqueous solvent.

8. Items 1-5 below:

1. It is a condensed polycyclic heterocyclic compound, 2. The condensed polycyclic heterocycle contains a pyrimidine skeleton, 3. The fused polycyclic heterocycle contains three or more nitrogen atoms, 4. The fused polycyclic heterocycle contains five or more sp2 carbons, 5. No hydrogen atoms are bonded to the nitrogen atoms within the fused polycyclic heterocycle. A non-aqueous secondary battery according to claim 1, comprising one or more compounds having a structure that satisfies the requirements.

9. The non-aqueous secondary battery according to claim 7, wherein the condensed polycyclic heterocyclic compound is a purine derivative.

10. The non-aqueous secondary battery according to claim 9, wherein the purine derivative is caffeine.

11. The non-aqueous secondary battery according to claim 1, wherein the content of the condensed polycyclic heterocyclic compound is 0.01% by mass or more and 10% by mass or less based on the total amount of the non-aqueous electrolyte.

12. The non-aqueous secondary battery according to claim 1, wherein the lower limit voltage for CCCV charging is any of 3.8V, 4.0V, or 4.2V.

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