JP7628451B2 - Nonaqueous electrolyte and nonaqueous secondary battery - Google Patents

Description

The present invention relates to a non-aqueous electrolyte and a non-aqueous secondary battery.

Non-aqueous secondary batteries containing non-aqueous electrolytes are characterized by their light weight, high energy and long life, and are widely used as power sources for portable electronic devices such as notebook computers, mobile phones, digital cameras and video cameras. In addition, with the transition to a society with a low environmental impact, non-aqueous secondary batteries are attracting attention as power sources for hybrid electric vehicles (hereinafter abbreviated as "HEVs") and plug-in HEVs (hereinafter abbreviated as "PHEVs"), as well as in the field of power storage such as residential power storage systems.

When non-aqueous secondary batteries are installed in vehicles such as automobiles and residential power storage systems, the constituent materials of the batteries must have excellent chemical and electrochemical stability, strength, corrosion resistance, etc., from the viewpoint of cycle performance and long-term reliability in high-temperature environments. Furthermore, non-aqueous secondary batteries for vehicles such as automobiles and residential power storage systems have significantly different operating conditions than portable electronic device power sources, and must also operate in cold climates, so they are also required to have high output performance and long life performance in low-temperature environments.

In the above-mentioned circumstances, in order to realize a non-aqueous secondary battery with output performance, nitrile-based solvents with an excellent balance between viscosity and dielectric constant have been proposed as non-aqueous solvents. Among them, acetonitrile is known to be a solvent with outstanding performance, but these solvents containing nitrile groups have a fatal drawback of electrochemically undergoing reductive decomposition, and several measures to improve the performance have been reported. For example, the following Patent Documents 1 and 2 report electrolytes that reduce the effects of reductive decomposition of acetonitrile by combining the solvent acetonitrile with a specific electrolyte salt, additive, etc., and as a particularly suitable example, by using a sulfite ester compound as an additive.

Patent Document 3 also reports a good example of a system that uses acetonitrile as the solvent and combines an imide salt such as lithium bis(fluorosulfonyl)imide with a fluorine-containing inorganic lithium salt as the electrolyte salt.

Incidentally, with regard to a sulfite ester compound represented by ethylene sulfite, in order to contribute to further improvement of the performance of an electricity storage device, it has been proposed in Patent Document ₄ to use one in which the concentration of free acid _H2SO4 contained as an impurity is adjusted as low as possible.

International Publication No. 2012/057311 International Publication No. 2013/062056 JP 2015-65049 A JP 2010-120947 A

Secondary batteries that use acetonitrile as a solvent for the non-aqueous electrolyte solution have yet to fully resolve the issue of performance degradation when placed in a high-temperature environment for a long period of time, meaning that their output performance cannot be fully utilized.

The present invention has been made in consideration of the above circumstances, and therefore has an objective to provide a non-aqueous secondary battery that uses an acetonitrile-containing non-aqueous electrolyte solution and exhibits little performance degradation even in high-temperature environments.

The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by using a nonaqueous electrolyte solution or a nonaqueous secondary battery having the following configuration, thereby completing the present invention.
<1> A non-aqueous electrolyte solution containing a non-aqueous solvent and a lithium salt, wherein the non-aqueous solvent contains 5% by volume to 95% by volume of acetonitrile with respect to the total amount of the non-aqueous solvent, and the lithium salt contains lithium bisfluorosulfonylimide and LiPF ₆ , and a compound represented by the following general formula (I):
In the formula, R ₁ and R ₂ are each an alkyl group having 1 to 4 carbon atoms in which a hydrogen atom may be substituted with a fluorine atom, or R ₁ and R ₂ may be bonded to form a ring.
and a content of SO ₄ ^2- in the nonaqueous electrolyte is 10 ppm or more and 500 ppm or less.
<2> The nonaqueous electrolyte solution according to item 1, wherein the sulfite ester compound is ethylene sulfite.
<3> The nonaqueous electrolyte solution according to item 1 or 2, wherein the nonaqueous solvent contains a cyclic carbonate.
<4> The nonaqueous electrolyte solution according to item 3, wherein the cyclic carbonate includes at least one selected from the group consisting of vinylene carbonate and fluoroethylene carbonate.
<5> A nonaqueous secondary battery comprising: the nonaqueous electrolyte according to any one of items 1 to 4; a positive electrode having a positive electrode active material layer on one or both sides of a positive electrode current collector; a negative electrode having a negative electrode active material layer on one or both sides of a negative electrode current collector; and a separator.
<6> The positive electrode active material layer contains a lithium-containing metal oxide as a positive electrode active material,
6. The nonaqueous secondary battery according to item 5, wherein the negative electrode active material layer contains, as a negative electrode active material, a material capable of absorbing lithium ions at a potential lower than 0.4 V vs. Li/ ^{Li +} .

According to the present invention, by containing a sulfite compound and SO ₄ ^2- at specific concentrations in a nonaqueous electrolyte solution, it is possible to provide a nonaqueous secondary battery that can be used in a wider range of applications by improving the deterioration of battery characteristics in high-temperature environments while maintaining output performance.

1 is a plan view illustrating a schematic example of a nonaqueous secondary battery according to an embodiment of the present invention. 2 is a cross-sectional view of the nonaqueous secondary battery shown in FIG. 1 taken along line AA.

The following is a detailed description of an embodiment of the present invention (hereinafter, simply referred to as the "present embodiment"). The present invention is not limited to the following embodiment, and various modifications are possible without departing from the gist of the present invention. In this specification, a numerical range described using "~" includes the numerical values described before and after the range.

<Non-aqueous secondary battery>
The nonaqueous electrolyte of the present embodiment can be used in a nonaqueous secondary battery. There are no particular limitations on the negative electrode, positive electrode, separator, and battery exterior of the nonaqueous secondary battery of the present embodiment.

In addition, although not limited thereto, the non-aqueous secondary battery of this embodiment may be a lithium ion battery having a positive electrode containing a positive electrode material capable of absorbing and releasing lithium ions as a positive electrode active material, and a negative electrode containing a negative electrode material capable of absorbing and releasing lithium ions and/or metallic lithium as a negative electrode active material.

Specifically, the nonaqueous secondary battery of this embodiment may be the nonaqueous secondary battery shown in Figures 1 and 2. Here, Figure 1 is a plan view that shows a schematic representation of the nonaqueous secondary battery, and Figure 2 is a cross-sectional view taken along line A-A in Figure 1.

The nonaqueous secondary battery 100 shown in Figs. 1 and 2 is composed of a pouch-type cell. The nonaqueous secondary battery 100 contains a laminated electrode body formed by laminating a positive electrode 150 and a negative electrode 160 with a separator 170 between them, and a nonaqueous electrolyte (not shown) in a space 120 of a battery exterior 110 composed of two aluminum laminate films. The battery exterior 110 is sealed at its outer periphery by heat-sealing the upper and lower aluminum laminate films. The laminate formed by laminating the positive electrode 150, the separator 170, and the negative electrode 160 in this order is impregnated with a nonaqueous electrolyte. However, in Fig. 2, in order to avoid cluttering the drawing, the layers constituting the battery exterior 110 and the layers of the positive electrode 150 and the negative electrode 160 are not shown separately.

The aluminum laminate film constituting the battery exterior 110 is preferably an aluminum foil coated on both sides with a polyolefin-based resin.

The positive electrode 150 is connected to the positive electrode lead body 130 within the nonaqueous secondary battery 100. Although not shown, the negative electrode 160 is also connected to the negative electrode lead body 140 within the nonaqueous secondary battery 100. One end of each of the positive electrode lead body 130 and the negative electrode lead body 140 is pulled out to the outside of the battery exterior 110 so that they can be connected to external devices, and the ionomer portions of these are heat-sealed to one side of the battery exterior 110.

In the non-aqueous secondary battery 100 shown in Figs. 1 and 2, the positive electrode 150 and the negative electrode 160 each have one laminated electrode body, but the number of laminated positive electrodes 150 and negative electrodes 160 can be increased as appropriate depending on the capacity design. In the case of a laminated electrode body having multiple positive electrodes 150 and multiple negative electrodes 160, the tabs of the same electrode may be joined 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 above-mentioned tabs of the same electrode may be formed from the exposed part of the current collector, or may be formed by welding a metal piece to the exposed part of the current collector, etc.

The positive electrode 150 is composed of a positive electrode active material layer made from a positive electrode mixture and a positive electrode current collector. The negative electrode 160 is composed 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 the separator 170.

As these components, materials that are used in conventional lithium ion batteries can be used as long as they satisfy the requirements of this embodiment. Each component of the nonaqueous secondary battery will be described in more detail below.

<Positive electrode>
The positive electrode 150 is composed of a positive electrode active material layer made of a positive electrode mixture and a positive electrode current collector. The positive electrode 150 is not particularly limited as long as it acts as a positive electrode of a non-aqueous secondary battery, and may be a known positive electrode.

The positive electrode active material layer preferably contains a positive electrode active material and, if necessary, further contains a conductive additive and a binder.

The positive electrode active material layer preferably contains a material capable of absorbing and releasing lithium ions as the positive electrode active material. When such a material is used, it is preferable because it tends to be possible to obtain a high voltage and a high energy density.

The positive electrode active material may be, for example, a positive electrode active material containing at least one transition metal element selected from the group consisting of Ni, Mn, and Co. The lithium-containing metal oxide may be, for example, a lithium-containing metal oxide represented by the following general formula (a):
Li _p Ni _{q Cor} M n _s M _t O _u ... (a)
In the formula, M is at least one metal selected from the group consisting of Al, Sn, In, Fe, V, Cu, Mg, Ti, Zn, Mo, Zr, Sr, and Ba, and is within the ranges of 0<p<1.3, 0<q<1.2, 0<r<1.2, 0≦s<0.5, 0≦t<0.3, 0.7≦q+r+s+t≦1.2, 1.8<u<2.2, and p is a value determined by the charge/discharge state of the battery.
At least one selected from the lithium (Li)-containing metal oxides represented by the following formula (1) is suitable.

Specific examples of the positive electrode active material include lithium cobalt oxides such as _LiCoO2 ; _lithium manganese _oxides such as _LiMnO2 , _LiMn2O4 , and _Li2Mn2O4 ; lithium _nickel oxides such as _LiNiO2 ; _{and LizMO2} such _as LiNi1 _/ 3Co1 _{/3Mn1 / 3O2 , LiNi0.5Co0.2Mn0.3O2 , and LiNi0.8Co0.2O2 .} {in the formula, M contains at least one transition metal element selected from the group consisting of Ni, Mn, and Co, and represents two or more metal elements selected from the group consisting of Ni, Mn, Co, Al, and Mg, and z represents a number greater than 0.9 and less than 1.2}.

In particular, when the Ni content ratio q of the Li-containing metal oxide represented by the general formula (a) is 0.5<q<1.2, it is preferable because it is possible to reduce the amount of Co used, which is a rare metal, and to achieve a high energy density. _{Examples of} such _{positive electrode} active materials _{include lithium - containing composite metal oxides such as LiNi0.6Co0.2Mn0.2O2 , LiNi0.75Co0.15Mn0.15O2 , LiNi0.8Co0.1Mn0.1O2 , LiNi0.85Co0.075Mn0.075O2 , LiNi0.8Co0.15Al0.05O2 , LiNi0.81Co0.1Al0.09O2 , and LiNi0.85Co0.1Al0.05O2 .}

On the other hand, the higher the Ni content ratio, the more degradation tends to progress at low voltages. The positive electrode active material of the Li-containing metal oxide represented by the general formula (a) essentially has active sites that oxidize and deteriorate the non-aqueous electrolyte, but these active sites may unintentionally consume the compound added to protect the negative electrode on the positive electrode side. Among these, acid anhydrides tend to be easily affected. In particular, when acetonitrile is contained as the non-aqueous solvent, the effect of adding an acid anhydride is enormous, so the consumption of the acid anhydride on the positive electrode side is a fatal problem.

In addition, these additive decomposition products that are taken in and deposited on the positive electrode side not only increase the internal resistance of non-aqueous secondary batteries, but also accelerate the deterioration of the lithium salt, and furthermore, the protection of the negative electrode surface, which was the original purpose, becomes insufficient. In order to deactivate the active sites that essentially cause the oxidative deterioration of non-aqueous electrolytes, it is important to have a component that controls the Jahn-Teller distortion or plays a role as a neutralizer. 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.

For the same reason, the surface of the positive electrode active material is preferably coated with a compound containing at least one metal element selected from the group consisting of Zr, Ti, Al, and Nb. Also, the surface of the positive electrode active material is more preferably 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 ZrO ₂ , TiO ₂ , Al ₂ O ₃ , NbO ₃ , and LiNbO ₂ , since this does not inhibit the permeation of lithium ions.

The positive electrode active material may be a lithium-containing compound other than the Li-containing metal oxide represented by the general formula (a), and is not particularly limited as long as it contains lithium. Examples of such lithium-containing compounds include composite oxides containing lithium and a transition metal element, metal chalcogenides containing lithium, metal phosphate compounds containing lithium and a transition metal element, and metal silicate compounds containing lithium and a transition metal element. From the viewpoint of obtaining a higher voltage, the lithium-containing compound is preferably a metal phosphate compound 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.

More specifically, the lithium-containing compound is represented by the following formula (Xa):
_Liv M ^I D ₂ (Xa)
{wherein D represents a chalcogen element, M and ^I represent one or more transition metal elements, and the value of v is determined depending on the charge/discharge state of the battery and represents a number between 0.05 and 1.10}
The following formula (Xb):
Li _w M ^II PO ₄ (Xb)
{wherein M ^II represents one or more transition metal elements, and the value of w is determined depending on the charge/discharge state of the battery and represents a number from 0.05 to 1.10.}, and the following formula (Xc):
Li _t M ^III _u SiO ₄ (Xc)
In the formula, M ^III represents one or more transition metal elements, the value of t is determined depending on the charge/discharge state of the battery and represents a number from 0.05 to 1.10, and u represents a number from 0 to 2.
Examples of the compounds include those represented by the following formula:

The lithium-containing compound represented by the above formula (Xa) has a layered structure, and the compounds represented by the above formulas (Xb) and (Xc) have an olivine structure. These lithium-containing compounds may be those in which, for the purpose of stabilizing the structure, part of the transition metal element is replaced with Al, Mg, or other transition metal element, those in which these metal elements are contained in the crystal grain boundaries, those in which part of the oxygen atoms is replaced with fluorine atoms, etc., and those in which at least part of the surface of the positive electrode active material is coated with another positive electrode active material, etc.

In this embodiment, the positive electrode is preferably a lithium-containing metal oxide represented by Li _x FePO ₄ having an olivine structure among the positive electrode active materials represented by formula (Xb), in which 0<x<1.2, and more preferably lithium iron phosphate (LiFePO ₄ ). The olivine structure has excellent thermal stability, a stable crystal structure, and a small primary particle size, so that the positive electrode active material contains a lithium-containing metal oxide having the olivine structure, and thus the high-temperature durability tends to be improved. The content of the compound represented by Li _x FePO ₄ is preferably 40% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, and even more preferably 80% by mass to 100% by mass, based on the total mass of the positive electrode active material. The positive electrode active material may be Li _x FePO ₄ alone, or may contain other positive electrode active materials.

In this embodiment, the positive electrode active material may be the lithium-containing compound alone, or may be used in combination with other positive electrode active materials.

Other positive electrode active materials include, for example, metal oxides or metal chalcogenides having a tunnel structure and a layered structure; sulfur; conductive polymers, etc. Examples of metal oxides or metal chalcogenides having a tunnel structure and a layered structure include oxides, sulfides, and selenides of metals other than lithium, such as MnO ₂ , FeO ₂ , FeS ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , TiS ₂ , MoS ₂ , and NbSe _2. Examples of conductive polymers include conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole.

The above-mentioned other positive electrode active materials may be used alone or in combination of two or more, with no particular restrictions. However, it is preferable that the positive electrode active material layer contains at least one transition metal element selected from Ni, Mn, and Co, since it is possible to reversibly and stably store and release lithium ions and achieve a high energy density.

When a lithium-containing compound and another positive electrode active material are used in combination as the positive electrode active material, the ratio of the lithium-containing compound to the total positive electrode active material is preferably 80% by mass or more, and more preferably 85% by mass or more.

Examples of conductive additives include carbon black, such as graphite, acetylene black, and ketjen black, and carbon fiber. The content 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 the positive electrode active material.

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 the positive electrode active material.

The positive electrode active material layer is formed by dispersing a positive electrode mixture, which is a mixture of a positive electrode active material and, if necessary, a conductive assistant and a binder, in a solvent to form a positive electrode mixture-containing slurry, which is then applied to a positive electrode current collector, dried (solvent removal), and pressed if necessary. There are no particular limitations on the solvent, and any conventionally known solvent can be used. Examples of the solvent include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

The positive electrode current collector is composed of a metal foil such as aluminum foil, nickel foil, or stainless steel foil. The surface of the positive electrode current collector may be coated with carbon, or the positive electrode current collector 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. In the case of a mesh-shaped positive electrode current collector, the thickness may be measured at the non-perforated portion.

<Negative Electrode>
The negative electrode 160 is composed of a negative electrode active material layer made of a negative electrode mixture and a negative electrode current collector. The negative electrode 160 can function as a negative electrode of a non-aqueous secondary battery.

The negative electrode active material layer preferably contains a negative electrode active material and, if necessary, a conductive additive and a binder.

Examples of negative electrode active materials include amorphous carbon (hard carbon), graphite (e.g., artificial graphite, natural graphite, etc.), pyrolytic carbon, coke, glassy carbon, baked bodies of organic polymer compounds, mesocarbon microbeads, carbon fiber, activated carbon, carbon colloids, and carbon black, as well as metal lithium, metal oxides, metal nitrides, lithium alloys, tin alloys, silicon alloys, intermetallic compounds, organic compounds, inorganic compounds, metal complexes, organic polymer compounds, etc. The negative electrode active materials may be used alone or in combination of two or more.

From the viewpoint of increasing the battery voltage, the negative electrode active material layer preferably contains, as the negative electrode active material, a material capable of absorbing lithium ions at a potential lower than 0.4 V vs. Li/ ^{Li +} .

Examples of conductive additives include carbon black, such as graphite, acetylene black, and ketjen black, and carbon fiber. The content 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.

Examples of binders include carboxymethyl cellulose, PVDF, PTFE, polyacrylic acid, and fluororubber. Other examples include diene rubbers such as styrene butadiene rubber. 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.

The negative electrode active material layer is formed by dispersing a negative electrode mixture, which is a mixture of a negative electrode active material and, if necessary, a conductive assistant and a binder, in a solvent to form a negative electrode mixture-containing slurry, which is then applied to the negative electrode current collector, dried (solvent removal), and pressed if necessary. There are no particular limitations on the solvent, and any conventionally known solvent can be used. Examples of the solvent include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and water.

The negative electrode current collector is composed of a metal foil such as copper foil, nickel foil, or stainless steel foil. The surface of the negative electrode current collector may be coated with carbon, or the negative electrode current collector may 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. In the case of a mesh-shaped negative electrode current collector, the thickness may be measured at the non-perforated portion.

<Non-aqueous electrolyte>
In this specification, the term "nonaqueous electrolyte solution" (hereinafter, simply referred to as "electrolyte solution") refers to an electrolyte solution containing 1 mass % or less of water relative to the total amount of the electrolyte solution.

The electrolyte according to this embodiment preferably contains as little water as possible, but may contain a very small amount of water as long as it does not impede the solution of the problem of the present invention. The water content is 300 mass ppm or less, more preferably 200 mass ppm or less, based on the total amount of the nonaqueous electrolyte. As long as the nonaqueous electrolyte has a configuration for achieving the solution of the problem of the present invention, the other components can be appropriately selected and applied from the constituent materials of known nonaqueous electrolytes used in lithium ion batteries.

In this embodiment, the non-aqueous electrolyte contains acetonitrile as a non-aqueous solvent, lithium bisfluorosulfonylimide, i.e., LiN(SO ₂ F) ₂ and LiPF ₆ as a lithium salt, and a compound represented by the following general formula (I):
In the formula, R ₁ and R ₂ are each an alkyl group having 1 to 4 carbon atoms in which a hydrogen atom may be substituted with a fluorine atom, or R ₁ and R ₂ may be bonded to form a ring.
and a sulfite ester compound represented by the formula:

<Non-aqueous solvent>
Here, the non-aqueous solvent will be described. In the present embodiment, the "non-aqueous solvent" refers to the elements in the electrolyte excluding the lithium salt and various additives. When the electrolyte contains an additive for protecting an electrode, the "non-aqueous solvent" refers to the elements in the electrolyte excluding additives other than the lithium salt and the additive for protecting an electrode. Examples of the non-aqueous solvent include alcohols such as methanol and ethanol; aprotic solvents, etc. Among them, aprotic solvents are preferred. As long as the effect of the present invention is not impaired, the non-aqueous solvent may contain a solvent other than the aprotic solvent.

Specific examples of aprotic solvents include cyclic carbonates. Examples of cyclic carbonates include carbonates such as 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. Fluorinated cyclic carbonates such as cis-4,5-difluoro-1,3-dioxolane-2-one, trans-4,5-difluoro-1,3-dioxolane-2-one, 4,4,5-trifluoro-1,3-dioxolane-2-one, 4,4,5,5-tetrafluoro-1,3-dioxolane-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolane-2-one; lactones such as γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ-caprolactone, and ε-caprolactone;

Specific examples of aprotic solvents include sulfur compounds such as ethylene sulfite, propylene sulfite, butylene sulfite, pentene sulfite, sulfolane, 3-sulfolene, 3-methyl sulfolane, 1,3-propane sultone, 1,4-butane sultone, 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.

Specific examples of aprotic solvents include chain carbonates. Examples of chain carbonates include chain carbonates such as 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 fluorinated carbonates such as trifluorodimethyl carbonate, trifluorodiethyl carbonate, and trifluoroethyl methyl carbonate.

Specific examples of aprotic solvents include mononitriles such as acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, and acrylonitrile; alkoxy-substituted nitriles such as methoxyacetonitrile and 3-methoxypropionitrile; malononitrile, succinonitrile, glutaronitrile, adiponitrile, 1,4-dicyanoheptane, 1,5-dicyanopentane, 1,6-dicyanoheptane, 1,7-dicyanoheptane, 2,6-dicyanoheptane, 1,8-dicyanooctane, 2,7-dicyanooctane, 1,9-dicyanoheptane, 2,8-dicyanooctane, 2,9-dicyanooctane, 3,10-dicyanooctane, 3,11-dicyanooctane, 3,12-dicyanooctane, 3,13-dicyanooctane, 3,14-dicyanooctane, 3,15-dicyanooctane, 3,16-dicyanooctane, 3,17-dicyanooctane, 3,18-dicyanooctane, 3,19-dicyanooctane, 3,20-dicyanooctane, 3,21-dicyanooctane, 3,22-dicyanooctane, 3,23-dicyanooctane, 3,24-dicyanooctane, 3,25-dicyanooctane, 3,26-dicyanooctane, 3,27-dicyanooctane, 3,28-dicyanooctane, 3,29-dicyanooctane, 3,30-dicyanooctane, 3,31-dicyanooctane, 3,32-dicyanooctane, 3,33-dicyanooctane, 3,34-dicyanooctane Dinitriles such as nonane, 2,8-dicyanononane, 1,10-dicyanodecane, 1,6-dicyanodecane, and 2,4-dimethylglutaronitrile; cyclic nitriles such as benzonitrile; methyl acetate, methyl propionate, methyl isobutyrate, methyl butyrate, methyl isovalerate, methyl valerate, methyl pivalate, methyl hydroangelate, methyl caproate, ethyl acetate, ethyl propionate, ethyl isobutyrate, ethyl butyrate, ethyl isovalerate, ethyl valerate, ethyl pivalate, ethyl hydroangelate, ethyl caproate, propyl acetate, propyl propionate, and propyl isobutyrate. Pyrrole, Propyl butyrate, Propyl isovalerate, Propyl valerate, Propyl pivalate, Propyl hydroangelate, Propyl caproate, Isopropyl acetate, Isopropyl propionate, Isopropyl isobutyrate, Isopropyl butyrate, Isopropyl isovalerate, Isopropyl valerate, Isopropyl pivalate, Isopropyl hydroangelate, Isopropyl caproate, Butyl acetate, Butyl propionate, Butyl isobutyrate, Butyl butyrate, Butyl isovalerate, Butyl valerate, Butyl pivalate, Butyl hydroangelate, Butyl caproate, Isobutyl acetate, Isobutyl propionate, Isobutyl isobutyrate chain esters represented by butyl, isobutyl butyrate, isobutyl isovalerate, isobutyl valerate, isobutyl pivalate, isobutyl hydroangelate, isobutyl caproate, tert-butyl acetate, tert-butyl propionate, tert-butyl isobutyrate, tert-butyl butyrate, tert-butyl isovalerate, tert-butyl valerate, tert-butyl pivalate, tert-butyl hydroangelate, and tert-butyl caproate; chain ethers represented by dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme, and tetraglyme; fluorinated ethers represented by Rf ^A -OR ^B (wherein Rf ^A represents an alkyl group containing a fluorine atom, and R ^B represents an organic group which may contain a fluorine atom); and ketones represented by acetone, methyl ethyl ketone, and methyl isobutyl ketone.

Furthermore, examples of the compound in which some or all of the H atoms of the aprotic solvent are substituted with halogen atoms include compounds in which the halogen atoms are fluorine;
Examples include:

Examples of the fluorinated chain carbonate include methyl trifluoroethyl carbonate, trifluorodimethyl carbonate, trifluorodiethyl carbonate, trifluoroethylmethyl carbonate, methyl 2,2-difluoroethyl carbonate, methyl 2,2,2-trifluoroethyl carbonate, and methyl 2,2,3,3-tetrafluoropropyl carbonate. The fluorinated chain carbonate is represented by the following general formula:
R ^c -O-C(O)OR ^d
{In the formula, R ^c and R ^d are at least one selected from the group consisting of CH ₃ , CH ₂ CH ₃ , CH ₂ CH ₂ CH ₃ , CH(CH ₃ ) ₂ , and CH ₂ Rf ^e , Rf ^e is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom is substituted with at least one fluorine atom, and R ^c and/or R ^d contain at least one fluorine atom}
It can be expressed as:

Examples of fluorinated short-chain fatty acid esters include fluorinated short-chain fatty acid esters such as 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, and 2,2,3,3-tetrafluoropropyl acetate. Fluorinated short-chain fatty acid esters are represented by the following general formula:
R ^f -C(O)OR ^g
In the formula, R ^f is at least one selected from the group consisting of CH ₃ , CH ₂ CH ₃ , CH ₂ CH ₂ CH ₃ , CH(CH ₃ ) ₂ , CF ₃ CF ₂ H, CFH ₂ , CF ₂ Rf ^h , CFHRf ^h , and CH ₂ Rf ⁱ , R ^g is at least one selected from the group consisting of CH ₃ , CH ₂ CH ₃ , CH ₂ CH ₂ CH ₃ , CH(CH ₃ ) ₂ , and CH ₂ Rf ⁱ , R f ^h is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom may be substituted with at least one fluorine atom, R f ⁱ is an alkyl group having 1 to 3 carbon atoms in which a hydrogen atom is substituted with at least one fluorine atom, and R ^f and/or R ^g contain at least one fluorine atom, and R ^f is CF _2H , then R ^g is not _CH3 .
It can be expressed as:

These are used alone or in combination of two or more. Among these non-aqueous solvents, it is more preferable to use one or more of the cyclic carbonates and chain carbonates from the viewpoint of improving stability. Here, only one of the cyclic carbonates and chain carbonates exemplified above may be selected and used, or two or more (for example, two or more of the cyclic carbonates exemplified above, two or more of the chain carbonates exemplified above, or two or more of the cyclic carbonates exemplified above and one or more of the chain carbonates exemplified above) may be used. Among these, ethylene carbonate, propylene carbonate, vinylene carbonate, or fluoroethylene carbonate is more preferable as the cyclic carbonate, and ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate is more preferable as the chain carbonate. And, since the degree of ionization of the lithium salt that contributes to the charge and discharge of the non-aqueous secondary battery is increased, it is even more preferable to use the cyclic carbonate. When a cyclic carbonate is used, it is particularly preferred that the cyclic carbonate contains at least one of vinylene carbonate and fluoroethylene carbonate.

The nonaqueous solvent of the nonaqueous electrolyte contains acetonitrile, which improves the ionic conductivity of the nonaqueous electrolyte, thereby increasing the diffusibility of lithium ions in the battery. Therefore, when the electrolyte contains acetonitrile, lithium ions can be diffused well even to the area near the current collector that is difficult for lithium ions to reach during high-load discharge, especially in a positive electrode in which the positive electrode active material layer is thickened to increase the amount of positive electrode active material filled. Therefore, it is possible to extract sufficient capacity even during high-load discharge, resulting in a nonaqueous secondary battery with excellent load characteristics.

Furthermore, by containing acetonitrile in the nonaqueous electrolyte solution's nonaqueous solvent, the rapid charging characteristics of the nonaqueous secondary battery can also be improved. In constant current (CC)-constant voltage (CV) charging of a nonaqueous secondary battery, the capacity per unit time during the CC charging period is greater than the charge capacity per unit time during the CV charging period. When acetonitrile is used as the nonaqueous electrolyte solution's nonaqueous solvent, the range in which CC charging is possible can be expanded (CC charging time can be extended), and the charging current can also be increased, so that the time from the start of charging to fully charging the nonaqueous secondary battery can be significantly shortened.

On the other hand, acetonitrile is easily decomposed by electrochemical reduction. Therefore, when using acetonitrile, it is preferable to do at least one of the following: mix the acetonitrile with another solvent, or add an additive for electrode protection to form a protective film on the electrode.

The content of acetonitrile is preferably 5 to 95% by volume relative to the total amount of the non-aqueous solvent. The content of acetonitrile is more preferably 10% by volume or more, or 20% by volume or more, or 30% by volume or more, and even more preferably 35% by volume or more, relative to the total amount of the non-aqueous solvent. This value is more preferably 85% by volume or less, and even more preferably 66% by volume or less. When the content of acetonitrile is 5% by volume or more relative to the total amount of the non-aqueous solvent, the ion conductivity tends to increase and high output characteristics can be expressed, and further, the dissolution of the lithium salt can be promoted. Since the additive described later suppresses the increase in the internal resistance of the battery, when the content of acetonitrile in the non-aqueous solvent is within the above-mentioned range, the high-temperature cycle characteristics and other battery characteristics tend to be further improved while maintaining the excellent performance of acetonitrile.

<Lithium salt>
In this embodiment, the non-aqueous electrolyte contains LiN(SO ₂ F) ₂ and LiPF ₆ as lithium salts. LiN(SO ₂ F) ₂ is a lithium salt included in imide salts. In this specification, the imide salt is a lithium salt represented by the general formula LiN(SO ₂ C _m F _{2m +1} ) ₂ {wherein m is an integer of 0 to 8}, and specifically includes LiN(SO ₂ CF ₃ ) ₂ other than LiN(SO ₂ F) 2. The non-aqueous electrolyte may contain an imide salt other than these imide salts.

Since the saturation concentration of the imide salt in acetonitrile is higher than that of LiPF ₆ , it is preferable to include the imide salt at a molar concentration of LiPF ₆ ≦imide salt, since this can suppress the association and precipitation of the lithium salt and acetonitrile at low temperatures. In addition, it is preferable that the content of the imide salt is 0.5 mol to 3 mol per 1 L of non-aqueous solvent from the viewpoint of the amount of ions supplied. Preferably, LiN(SO ₂ F) ₂ is contained at 1.0 mol or more per 1 L of non-aqueous electrolyte, and LiPF ₆ is contained at 0.01 mol or more per 1 L of non-aqueous electrolyte. The upper limit of the content of LiN(SO ₂ F) ₂ is 2.5 mol or less per 1 L of non-aqueous electrolyte, and the upper limit of the content of LiPF ₆ is 1.0 mol or less per 1 L of non-aqueous electrolyte. By further containing LiN(SO ₂ F) ₂ and LiPF ₆ in the above quantitative relationship in addition to acetonitrile, the nonaqueous electrolyte can effectively suppress the decrease in ion conductivity in a low temperature range such as −10° C. or −30° C., and can obtain excellent low-temperature characteristics. In this way, by limiting the content, it is also possible to more effectively suppress the increase in resistance during high-temperature heating.

The lithium salt may also include a fluorine-containing inorganic lithium salt other than LiPF ₆ , such as LiBF ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiSbF ₆ , Li ₂ B ₁₂ F _b H _12-b (wherein b is an integer of 0 to 3). The term "inorganic lithium salt" refers to a lithium salt that does not contain a carbon atom in the anion and is soluble in acetonitrile. The term "fluorine-containing inorganic lithium salt" refers to a lithium salt that does not contain a carbon atom in the anion, contains a fluorine atom in the anion, and is soluble in acetonitrile. The fluorine-containing inorganic lithium salt is excellent in that it forms a passive film on the surface of the metal foil that is the positive electrode current collector, and inhibits the corrosion of the positive electrode current collector. These fluorine-containing inorganic lithium salts are used alone or in combination of two or more. As the fluorine-containing inorganic lithium salt, a compound that is a double salt of LiF and Lewis acid is preferable, and among them, when using a fluorine-containing inorganic lithium salt having a phosphorus atom, it is more preferable because it is easy to release free fluorine atoms. A representative fluorine-containing inorganic lithium salt is _LiPF6 , which dissolves and releases _PF6 anion. When using a fluorine-containing inorganic lithium salt having a boron atom as the fluorine-containing inorganic lithium salt, it is preferable because it is easy to capture excess free acid components that may cause battery deterioration, and from this viewpoint, _LiBF4 is particularly preferable.

The content of the fluorine-containing inorganic lithium salt in the non-aqueous electrolyte of this embodiment is not particularly limited, but is preferably 0.01 mol or more per 1 L of non-aqueous solvent, more preferably 0.1 mol or more, and even more preferably 0.25 mol or more. When the content of the fluorine-containing inorganic lithium salt is within the above-mentioned range, the ionic conductivity tends to increase and high output characteristics tend to be exhibited. In addition, it is preferably less than 2.8 mol per 1 L of non-aqueous solvent, more preferably less than 1.5 mol, and even more preferably less than 1 mol. When the content of the fluorine-containing inorganic lithium salt is within the above-mentioned range, the ionic conductivity tends to increase and high output characteristics can be exhibited, and the decrease in ionic conductivity due to the increase in viscosity at low temperatures tends to be suppressed, and the high-temperature cycle characteristics and other battery characteristics tend to be further improved while maintaining the excellent performance of the non-aqueous electrolyte.

The nonaqueous electrolyte of this embodiment may further contain an organic lithium salt. "Organic lithium salt" refers to a lithium salt that contains a carbon atom in the anion and is soluble in acetonitrile.

The organic lithium salt may be an organic lithium salt having an oxalic acid group. Specific examples of the organic lithium salt having an oxalic acid group include, for example, organic lithium salts represented by LiB(C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ), LiPF ₄ (C ₂ O ₄ ), and LiPF ₂ (C ₂ O ₄ ) ₂ , and the like, among which at least one lithium salt selected from the lithium salts represented by LiB(C ₂ O ₄ ) ₂ and LiBF ₂ (C ₂ O ₄ ) is preferred. It is more preferable to use one or more of these together with a fluorine-containing inorganic lithium salt. The organic lithium salt having an oxalic acid group may be added to a non-aqueous electrolyte solution or may be contained in the negative electrode (negative electrode active material layer).

From the viewpoint of better ensuring the effect of its use, the amount of the organic lithium salt having an oxalic acid group added to the non-aqueous electrolyte is preferably 0.005 mol or more, more preferably 0.02 mol or more, and even more preferably 0.05 mol or more, per 1 L of the non-aqueous solvent of the non-aqueous electrolyte. However, if the amount of the organic lithium salt having an oxalic acid group in the non-aqueous electrolyte is too large, there is a risk of precipitation. Therefore, the amount of the organic lithium salt having an oxalic acid group added to the non-aqueous electrolyte is preferably less than 1.0 mol, more preferably less than 0.5 mol, and even more preferably less than 0.2 mol, per 1 L of the non-aqueous solvent of the non-aqueous electrolyte.

It is known that organic lithium salts having an oxalic acid group are poorly soluble in organic solvents with low polarity, particularly in chain carbonates. Organic lithium salts having an oxalic acid group may contain a small amount of lithium oxalate, and when mixed to form a non-aqueous electrolyte solution, they may react with the small amount of moisture contained in other raw materials to generate new white precipitates of lithium oxalate. Therefore, the content of lithium oxalate in the non-aqueous electrolyte solution of this embodiment is not particularly limited, but is preferably 0 to 500 ppm.

In addition to the above, lithium salts generally used for non-aqueous secondary batteries may be added supplementarily as the lithium salt in this embodiment. Specific examples of other lithium salts include inorganic lithium salts that do not contain a fluorine atom in the anion, such as LiClO ₄ , LiAlO ₄ , LiAlCl ₄ , LiB ₁₀ Cl ₁₀ , and chloroborane Li; organic lithium salts, such as LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiC(CF ₃ SO ₂ ) ₃ , LiC _n F _(2n+1) SO ₃ (wherein n ≧ 2), lower aliphatic carboxylic acid Li, tetraphenylborate Li, and LiB(C ₃ O ₄ H ₂ ) ₂ ; and LiPF _n (C _p F _2p+1 ) _6-n, such as LiPF ₅ (CF ₃ ). an organic lithium salt represented by the formula: (wherein n is an integer of 1 to 5, and p is an integer of 1 to 8); an organic lithium salt represented by LiBF _q (C _s F _2s+1 ) _4-q , such as LiBF ₃ (CF ₃ ), (wherein q is an integer of 1 to 3, and s is an integer of 1 to 8); a lithium salt bonded to a polyvalent anion;
LiC( _SO2RA ⁾ ( _SO2RB ⁾ ( _SO2RC ⁾ (a)
{In the formula, R ^A , R ^B , and R ^C may be the same or different, and each represents a perfluoroalkyl group having 1 to 8 carbon atoms.}
The following formula (b):
LiN(SO ₂ OR ^D ) (SO ₂ OR ^E ) (b)
{wherein R ^D and R ^E may be the same or different and represent a perfluoroalkyl group having 1 to 8 carbon atoms.}, and the following formula (c):
LiN( _SO2RF ⁾ ( _SO2ORG ⁾ (c)
{In the formula, R ^F and R ^G may be the same or different and represent a perfluoroalkyl group having 1 to 8 carbon atoms.}
One or more of these can be used together with the fluorine-containing inorganic lithium salt.

<Sulfite Compound>
In this embodiment, the nonaqueous electrolyte solution is a compound represented by the following general formula (I):
In the formula, R ₁ and R ₂ are each an alkyl group having 1 to 4 carbon atoms in which a hydrogen atom may be substituted with a fluorine atom, or R ₁ and R ₂ may be bonded to form a ring.
The sulfite ester compound includes a sulfite ester compound represented by the formula:

In general formula (I), _R1 and _R2 may each independently be an unsubstituted alkyl group having 1 to 4 carbon atoms, or a fluoroalkyl group having 1 to 4 carbon atoms in which some or all of the hydrogen atoms have been substituted with fluorine atoms. When _R1 and _R2 are bonded to form a cyclic sulfite ester compound, the linkage between _R1 and _R2 may have 2 to 8 carbon atoms.

Specific examples of sulfite ester compounds include dimethyl sulfite, diethyl sulfite, dipropyl sulfite, dibutyl sulfite, diisopropyl sulfite, di(2-butyl) sulfite, ethylene sulfite, 1,2-propylene sulfite, 1,3-propylene sulfite, methyl-1,3-propylene sulfite, butylene sulfite, etc. Among these, ethylene sulfite is preferred from the viewpoint of forming a protective coating that has a high electrode protection effect.

The content of the sulfite ester compound is preferably 1% by volume or more and 5% by volume or less, and more preferably 2% by volume or more and 4% by volume or less, based on the total amount of the non-aqueous solvent. When the content of the sulfite ester compound is 1% by volume or more based on the total amount of the non-aqueous solvent, an electrode protection effect tends to be obtained. Also, when the sulfite ester compound is contained at 5% or less based on the total amount of the non-aqueous solvent, inhibition of the electrode reaction tends to be suppressed.

<Additives for electrode protection>
The electrolyte in this embodiment may also contain additives that protect other electrodes. There are no particular limitations on the additives for protecting the electrodes as long as they do not impede the problem-solving of the present invention. The additives may substantially overlap with the substance that plays the role of a solvent for dissolving lithium salts (i.e., the above-mentioned non-aqueous solvent). The additives for protecting the electrodes are preferably substances that contribute to improving the performance of the electrolyte and non-aqueous secondary battery in this embodiment, but also include substances that are not directly involved in electrochemical reactions.

Specific examples of electrode protection additives include, for example, 4-fluoro-1,3-dioxolane-2-one, 4,4-difluoro-1,3-dioxolane-2-one, cis-4,5-difluoro-1,3-dioxolane-2-one, trans-4,5-difluoro-1,3-dioxolane-2-one, 4,4,5-trifluoro-1,3-dioxolane-2-one, 4,4,5,5-tetrafluoro-1,3 fluoroethylene carbonates such as 4,4,5-trifluoro-5-methyl-1,3-dioxolane-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolane-2-one; unsaturated bond-containing cyclic carbonates such as vinylene carbonate, 4,5-dimethylvinylene carbonate, and vinylethylene carbonate; γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ- Examples of the acid anhydrides include lactones such as caprolactone and ε-caprolactone; cyclic ethers such as 1,4-dioxane; cyclic sulfur compounds such as sulfolane, 3-sulfolene, 3-methylsulfolane, 1,3-propane sultone, 1,4-butane sultone, 1-propene 1,3-sultone, and tetramethylene sulfoxide; chain acid anhydrides such as acetic anhydride, propionic anhydride, and benzoic anhydride; cyclic acid anhydrides such as malonic anhydride, succinic anhydride, glutaric anhydride, maleic anhydride, phthalic anhydride, 1,2-cyclohexanedicarboxylic anhydride, 2,3-naphthalene dicarboxylic anhydride, and naphthalene-1,4,5,8-tetracarboxylic dianhydride; and mixed acid anhydrides having a structure in which different types of acids, such as two different types of carboxylic acids or a carboxylic acid and a sulfonic acid, are dehydrated and condensed. These may be used alone or in combination of two or more types.

In this embodiment, the content of the electrode protection additive in the electrolyte is not particularly limited, but the content of the electrode protection additive relative to the total amount of the non-aqueous solvent is preferably 0.1 to 30 volume %, more preferably 0.3 to 15 volume %, and even more preferably 0.5 to 4 volume %.

In this embodiment, the greater the content of the electrode protection additive, the more the deterioration of the electrolyte is suppressed. However, the smaller the content of the electrode protection additive, the more the high-output characteristics of the non-aqueous secondary battery in a low-temperature environment are improved. Therefore, by adjusting the content of the electrode protection additive within the above-mentioned range, it tends to be possible to maximize the excellent performance based on the high ionic conductivity of the electrolyte without impairing the basic functions of the non-aqueous secondary battery. By preparing the electrolyte with such a composition, it tends to be possible to further improve all of the cycle performance of the non-aqueous secondary battery, the high-output performance in a low-temperature environment, and other battery characteristics.

In addition, since acetonitrile, which is one component of the non-aqueous solvent, is easily decomposed by electrochemical reduction, the non-aqueous solvent containing acetonitrile preferably contains one or more cyclic aprotic polar solvents as an electrode protection additive for forming a protective film on the negative electrode, and more preferably contains one or more unsaturated bond-containing cyclic carbonates.

As the unsaturated bond-containing cyclic carbonate, vinylene carbonate is preferred, and the content of vinylene carbonate in the non-aqueous electrolyte is preferably 0.1 vol.% or more and 4 vol.% or less, more preferably 0.2 vol.% or more and less than 3 vol.%, and even more preferably 0.5 vol.% or more and less than 2.5 vol.%. This makes it possible to more effectively improve low-temperature durability and provide a secondary battery with excellent low-temperature performance.

Vinylene carbonate is often essential as an electrode protection additive because it suppresses the reductive decomposition reaction of acetonitrile on the negative electrode surface, and a deficiency can lead to a rapid decline in battery performance. On the other hand, excessive film formation leads to a decline in low-temperature performance. Therefore, by adjusting the amount of vinylene carbonate added to fall within the above range, it is possible to keep the interface (film) resistance low and suppress cycle deterioration at low temperatures.

<Other optional additives>
In this embodiment, for the purpose of improving the charge/discharge cycle characteristics, high-temperature storage properties, and safety (e.g., prevention of overcharging) of the nonaqueous secondary battery, a nonaqueous electrolyte solution is used containing, for example, a sulfonic acid ester, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, tert-butylbenzene, a phosphoric acid ester [ethyl diethylphosphonoacetate (EDPA): (C ₂ H ₅ O) ₂ (P═O)—CH ₂ (C═O)OC ₂ H ₅ , tris(trifluoroethyl)phosphate (TFEP): (CF ₃ CH ₂ O) ₃ P═O, triphenyl phosphate (TPP): (C ₆ H ₅ O) ₃ P═O: (CH ₂ ═CHCH ₂ O) ₃ It is also possible to appropriately contain an optional additive selected from compounds such as cyclic compounds containing nitrogen with no steric hindrance around the unshared electron pair (e.g., pyridine, 1-methyl-1H-benzotriazole, 1-methylpyrazole), and derivatives of these compounds. Phosphate esters are particularly effective because they have the effect of suppressing side reactions during storage.

The content of the other optional additives in this embodiment is calculated as a mass percentage relative to the total mass of all components constituting the nonaqueous electrolyte. There are no particular limitations on the content of the other optional additives, but it is preferably in the range of 0.01% by mass to 10% by mass, more preferably 0.02% by mass to 5% by mass, and even more preferably 0.05 to 3% by mass, relative to the total amount of the nonaqueous electrolyte. By adjusting the content of the other optional additives within the above range, it tends to be possible to add even better battery characteristics without impairing the basic functions as a nonaqueous secondary battery.

<SO ₄ ^2- content adjustment process>
When a commercially available sulfite compound is used, a certain amount of SO ₄ ^2- is contained. The nonaqueous electrolyte of the present invention provides an effect of suppressing the deterioration of battery characteristics in a high-temperature environment while maintaining the output characteristics of the battery by adjusting the concentration (content) of SO ₄ ^2- within a specific range. Although the cause of this effect has not been fully elucidated, it is presumed that a substance containing ions such as SO ₄ ^2-, which is usually considered to decompose at the electrode interface and become electric resistance, is contained in the electrolyte within a specific concentration range, thereby forming a coating at the negative electrode interface that can suppress the decomposition of acetonitrile even at a high temperature of, for example, about 85°C, thereby improving high-temperature stability.

If the content of SO ₄ ^2- in the non-aqueous electrolyte is too low, a sufficient coating film formation effect cannot be obtained, and if the content is too high, the interface (coating) resistance increases, resulting in a decrease in output performance. The content of SO ₄ ^2- in the non-aqueous electrolyte is in the range of 10 ppm to 500 ppm, and preferably in the range of 20 ppm to 350 ppm, relative to the non-aqueous electrolyte.

As a method for adjusting the content of SO ₄ ^2- in the present invention, for example, after dissolving a sulfite ester compound in a non-aqueous solvent, a molecular sieve is added to the mixed solution and left for a certain period of time, so that SO ₄ ^2- can be reduced. Thereafter, the content of SO ₄ ^2- in the mixed solution is confirmed, and it is maintained as long as it is within the concentration range described above, but if it is too much, the contact time with the molecular sieve is extended, and if it is too little, it can be adjusted by adding a required amount of a salt containing the missing anion to the non-aqueous electrolyte. Lithium is preferable as the cation of the salt used in this case. The molecular sieve may be a commercially available one or may be a synthesized one. In addition, after adding the molecular sieve to the mixed solution, it may be heated or stirred as appropriate.

<Separator>
In the present embodiment, the non-aqueous secondary battery 100 is preferably provided with a separator 170 between the positive electrode 150 and the negative electrode 160 from the viewpoint of preventing short circuits between the positive electrode 150 and the negative electrode 160 and providing safety such as shutdown. The separator 170 is not limited, but may be the same as that provided in known non-aqueous secondary batteries, and is preferably an insulating thin film having high ion permeability and excellent mechanical strength. Examples of the separator 170 include woven fabric, non-woven fabric, and synthetic resin microporous membrane, and among these, synthetic resin microporous membrane is preferable.

As a synthetic resin microporous film, for example, a microporous film containing polyethylene or polypropylene as a main component, or a polyolefin-based microporous film such as a microporous film containing both of these polyolefins is preferably used. As a nonwoven fabric, for example, a porous film made of a heat-resistant resin such as glass, ceramic, polyolefin, polyester, polyamide, liquid crystal polyester, or aramid can be used.

The separator 170 may be configured as a single layer or multiple layers of one type of microporous film, or may be configured as a stack of two or more types of microporous films. The separator 170 may be configured as a single layer or multiple layers of a mixed resin material formed by melting and kneading two or more types of resin materials.

For the purpose of imparting functionality, inorganic particles may be present on the surface or inside of the separator, and other organic layers may be further coated or laminated. The separator may also include a crosslinked structure. In order to improve the safety performance of the nonaqueous secondary battery, these methods may be combined as necessary.

By using such a separator 170, it is possible to realize good input/output characteristics and low self-discharge characteristics required for lithium ion secondary batteries for high-power applications. The thickness of the microporous membrane used as the separator is not particularly limited, but is preferably 1 μm or more from the viewpoint of membrane strength, and is preferably 500 μm or less from the viewpoint of permeability. From the viewpoint of use in high-power applications where the amount of heat generated is relatively high and self-discharge characteristics higher than conventional ones are required, such as safety tests, and from the viewpoint of winding properties in a large battery winding machine, the thickness of the separator is preferably 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 25 μm or less. In addition, when attaching importance to both short-circuit resistance performance and output performance, the thickness of the separator is more preferably 15 μm or more and 25 μm or less, but when attaching importance to both high energy density and output performance, it is more preferably 10 μm or more and 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 output. In addition, the porosity of the separator is particularly preferably 50% to 70% when priority is given to improving output performance while ensuring safety, and is particularly preferably 40% to less than 50% when emphasis is placed on both short-circuit resistance and output performance. The air permeability of the separator is preferably 1 sec/100 cm ³ to 400 sec/100 cm ³ , more preferably 100 sec/100 cm ³ to 350/100 cm ³ , from the viewpoint of balance between the membrane thickness and the porosity. In addition, when the compatibility of short circuit resistance performance and output performance is emphasized, the air permeability of the separator is particularly preferably 150 seconds/100 cm ³ or more and 350 seconds/100 cm ³ or less, and when priority is given to improving output performance while ensuring safety, it is particularly preferable that it is 100/100 cm ³ seconds or more and less than 150 seconds/100 cm ^3. On the other hand, when a non-aqueous electrolyte solution with low ion conductivity is combined with a separator within the above range, the lithium ion migration rate is not determined by the structure of the separator, but by the high ion conductivity of the electrolyte solution, and the expected input/output characteristics tend not to be obtained. Therefore, the ion conductivity of the non-aqueous electrolyte solution is preferably 10 mS/cm or more, more preferably 15 mS/cm or more, and even more preferably 20 mS/cm or more. However, the membrane thickness, air permeability and porosity of the separator, and the ion conductivity of the non-aqueous electrolyte solution are not limited to the above examples.

<Battery exterior>
The configuration of the battery exterior 110 of the nonaqueous secondary battery 100 in this embodiment is not particularly limited, and for example, any of the battery exteriors of a battery can and a laminate film exterior body can be used. As the battery can, for example, a metal can such as a square, rectangular tube, cylindrical, elliptical, flat, coin, or button type made of steel, stainless steel, aluminum, clad material, etc. can be used. As the laminate film exterior body, for example, a laminate film made of a three-layer structure of a heat-melting resin/metal film/resin can be used.

The laminate film exterior body can be used as an exterior body by stacking two sheets with the heat-melting resin side facing inward, or by folding the laminate film exterior body so that the heat-melting resin side faces inward and sealing the ends with heat sealing. When using a laminate film exterior body, the positive electrode lead body 130 (or a lead tab connected to the positive electrode terminal and the positive electrode terminal) may be connected to the positive electrode current collector, and the negative electrode lead body 140 (or a lead tab connected to the negative electrode terminal and the negative electrode terminal) may be connected to the negative electrode current collector. In this case, the laminate film exterior body may be sealed with the ends of the positive electrode lead body 130 and the negative electrode lead body 140 (or the lead tabs connected to the positive electrode terminal and the negative electrode terminal, respectively) pulled out to the outside of the exterior body.

<Shape of non-aqueous secondary battery>
The shape of the nonaqueous secondary battery of this embodiment is not particularly limited, and may be, for example, a cylindrical shape, an elliptical shape, a rectangular tube shape, a button shape, a coin shape, a flat shape, a laminate shape, or the like.

<Method of manufacturing non-aqueous secondary battery>
The nonaqueous secondary battery 100 in this embodiment is produced by a known method using the above-mentioned nonaqueous electrolyte solution, a positive electrode 150 having a positive electrode active material layer on one or both sides of a current collector, a negative electrode 160 having a negative electrode active material layer on one or both sides of a current collector, a battery exterior 110, and, if necessary, a separator 170.

First, a laminate consisting of the positive electrode 150, the negative electrode 160, and, if necessary, the separator 170 is formed. For example, a laminate having a wound structure is formed by winding the long positive electrode 150 and the negative electrode 160 in a stacked state with the long separator interposed between the positive electrode 150 and the negative electrode 160; a laminate having a stacked structure is formed by alternately stacking the positive electrode sheets and the negative electrode sheets obtained by cutting the positive electrode 150 and the negative electrode 160 into a plurality of sheets having a certain area and shape with a separator sheet between them; a laminate having a stacked structure is formed by folding the long separator in a zigzag manner and alternately inserting the positive electrode sheets and the negative electrode sheets between the zigzag separators; and the like.

Next, the above-mentioned laminate is housed in a battery exterior 110 (battery case), the electrolyte solution according to this embodiment is poured into the battery case, and the laminate is immersed in the electrolyte solution and sealed, thereby producing the nonaqueous secondary battery according to this embodiment.

In another embodiment, a gel-state electrolyte membrane is prepared in advance by impregnating a substrate made of a polymer material with an electrolyte solution, and a laminate structure is formed using a sheet-like positive electrode 150, a negative electrode 160, and an electrolyte membrane, and a separator 170 as necessary, and then the laminate structure is housed in a battery exterior 110 to prepare a nonaqueous secondary battery 100.

If the electrodes are arranged so that there is a portion where the outer edge of the negative electrode active material layer overlaps with the outer edge of the positive electrode active material layer, or there is a portion where the width is too small in the non-facing portion of the negative electrode active material layer, the position of the electrodes may be shifted during battery assembly, which may result in a deterioration in the charge-discharge cycle characteristics of the non-aqueous secondary battery. Therefore, it is preferable to fix the positions of the electrodes in the electrode body used in the non-aqueous secondary battery in advance using tapes such as polyimide tape, polyphenylene sulfide tape, and polypropylene (PP) tape, adhesives, etc.

In this embodiment, when a non-aqueous electrolyte using acetonitrile is used, the lithium ions released from the positive electrode during the initial charge of the non-aqueous secondary battery may diffuse throughout the negative electrode due to its high ionic conductivity. In non-aqueous secondary batteries, the area of the negative electrode active material layer is generally made larger than that of the positive electrode active material layer. However, if lithium ions diffuse and are absorbed to a portion of the negative electrode active material layer that does not face the positive electrode active material layer, these lithium ions will not be released during the initial discharge and will remain in the negative electrode. Therefore, the contribution of the lithium ions that are not released becomes the irreversible capacity. For this reason, the initial charge/discharge efficiency may be low in non-aqueous secondary batteries using a non-aqueous electrolyte containing acetonitrile.

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 two are the same, current tends to concentrate at the edge of the negative electrode active material layer during charging, making it easier for lithium dendrites to form.

There is no particular restriction on the ratio of the area of the entire 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, but for the reasons described above, it is preferably 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 solution containing acetonitrile, the initial charge/discharge efficiency can be improved by reducing the ratio of the area of the entire 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.

Reducing the ratio of the area of the entire 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 ratio of the area of the portion 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 in the portion of the negative electrode active material layer that does not face the positive electrode active material layer among the lithium ions released from the positive electrode during the initial charge (i.e., the amount of lithium ions that are not released from the negative electrode during the initial discharge and become irreversible capacity). Therefore, by designing the ratio of the area of the entire 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 generation of lithium dendrites.

The non-aqueous secondary battery 100 in this embodiment can function as a battery by initial charging, but is stabilized by decomposing a part of the electrolyte during the initial charging. There is no particular restriction on the method of initial charging, but the initial charging 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. A preferable result is also obtained when the initial charging is performed via a constant voltage charge in the middle. The constant current for discharging the design capacity in one hour is 1 C. By setting the voltage range in which the lithium salt is involved in the electrochemical reaction to be long, a stable and strong SEI is formed on the electrode surface, which has the effect of suppressing an increase in internal resistance, and the reaction product is not firmly fixed only to the negative electrode 160, but has a good effect on the members other than the negative electrode 160, such as the positive electrode 150 and the separator 170, in some form. For this reason, it is very effective to perform the initial charging while taking into account the electrochemical reaction of the lithium salt dissolved in the non-aqueous electrolyte.

The nonaqueous secondary battery 100 in this embodiment can also be used as a battery pack in which multiple nonaqueous secondary batteries 100 are connected in series or in parallel. From the viewpoint of managing the charge/discharge state of the battery pack, the operating voltage range per battery is preferably 2V to 5V, more preferably 2.5V to 5V, and particularly preferably 2.75V to 5V.

The above describes the form for implementing the present invention, but the present invention is not limited to the above-mentioned embodiment. The present invention can be modified in various ways without departing from the gist of the invention.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

(1) Preparation of non-aqueous electrolyte solutions Various non-aqueous solvents, various sulfite compounds, and various additives were mixed in an inert atmosphere to give a predetermined concentration, and various lithium salts were added to the mixture to give a predetermined concentration, to prepare non-aqueous electrolyte solutions (S1) to (S6). The compositions of these non-aqueous electrolyte solutions are shown in Table 1.

The abbreviations for the non-aqueous solvent, lithium salt, sulfite compound, and additives in Table 1 have the following meanings.
(Non-aqueous solvent)
AcN: Acetonitrile DEC: Diethyl carbonate EC: Ethylene carbonate VC: Vinylene carbonate (sulfite ester compound)
ES: Ethylene sulfite (lithium salt)
_LiPF6 : Lithium hexafluorophosphate LiFSI: Lithium bis(fluorosulfonyl)imide (LiN( _SO2F ) ₂ )

After preparation, the non-aqueous electrolytes S1, S2, S3, S4, and S5 were added with 1 g of molecular sieve (5A 1/16 manufactured by Kanto Chemical Co., Ltd.) per 10 g of the non-aqueous electrolyte, and left for 24 hours with occasional stirring. The amount of SO ₄ ^2- in the obtained non-aqueous electrolyte was then measured using ion chromatography. The amount of SO ₄ ^2- was adjusted to about 20 ppm for S1, S3, and S4, and to about 300 ^ppm for S2, and _no adjustment was made for S5, which was used for battery evaluation.

In addition, non-aqueous electrolyte S6 was used for battery evaluation without being treated with molecular sieves after preparation.

(2) Preparation of coin-type non-aqueous secondary battery (2-1) Preparation of positive electrode (A) Lithium iron phosphate ( _LiFePO4 ) having an olivine structure as a positive electrode active material, (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.

N-methyl-2-pyrrolidone was added as a solvent to the obtained positive electrode mixture so that the solid content was 68 mass% and further mixed to prepare a positive electrode mixture-containing slurry. The positive electrode mixture-containing slurry was applied to one side of an aluminum foil having a thickness of 15 μm and a width of 280 mm, while adjusting the basis weight of the positive electrode mixture-containing slurry, so as to have a coating width of 240 to 250 mm, a coating length of 125 mm, and an uncoated length of 20 mm, using a three-roll transfer coater, and the solvent was dried and removed in a hot air drying oven. The obtained electrode roll was trimmed and cut on both sides, and dried under reduced pressure at 130 ° C. for ⁸ hours. Thereafter, the positive electrode consisting of the positive electrode active material layer and the positive electrode current collector was obtained by rolling with a roll press so that the density of the positive electrode active material layer was 1.9 g / cm 3. The basis weight excluding the positive electrode current collector was 17.5 mg / cm ² .

(2-2) Preparation of Negative Electrode Graphite powder as a negative electrode active material, carbon black powder as a conductive assistant, and carboxymethyl cellulose and styrene-butadiene rubber as binders were mixed in a solid mass ratio of 95.7:0.5:binder to 3.8:negative electrode active material:conductive assistant, to obtain a negative electrode mixture.

Water was added as a solvent to the obtained negative electrode mixture so that the solid content was 45% by mass, and further mixed to prepare a negative electrode mixture-containing slurry. The negative electrode mixture-containing slurry was applied to one side of a copper foil having a thickness of 8 μm and a width of 280 mm, while adjusting the basis weight of the negative electrode mixture-containing slurry, so as to have a coating width of 240 to 250 mm, a coating length of 125 mm, and an uncoated length of 20 mm, using a three-roll transfer coater, and the solvent was dried and removed in a hot air drying oven. The obtained electrode roll was trimmed and cut on both sides, and dried under reduced pressure at 80 ° C for 12 hours. Thereafter, the negative electrode active material layer was rolled with a roll press so that the density of the negative electrode active material layer was 1.5 g / cm ³ , and a negative electrode consisting of a negative electrode active material layer and a negative electrode current collector was obtained. The basis weight excluding the negative electrode current collector was 7.5 mg / cm ² .

(2-3) Assembly of coin-type non-aqueous secondary battery A polypropylene gasket was set in a CR2032 type battery case (SUS304/Al clad), and the positive electrode obtained as described above was punched into a disk shape with a diameter of 15.958 mm and set in the center with the positive electrode active material layer facing up. A glass fiber filter paper (GA-100, manufactured by Advantec Co., Ltd.) was punched into a disk shape with a diameter of 16.156 mm and set on top of it, and 150 μL of non-aqueous electrolyte was poured, and then the negative electrode obtained as described above was punched into a disk shape with a diameter of 16.156 mm and set with the negative electrode active material layer facing down. After further setting the spacer and spring, the battery cap was fitted and crimped with a crimping machine. The overflowing electrolyte was wiped off with a rag. The laminate was kept at 25 ° C for 12 hours, and the non-aqueous electrolyte was thoroughly soaked into the laminate to obtain a coin-type non-aqueous secondary battery.

(3) Evaluation of coin-type non-aqueous secondary batteries The coin-type non-aqueous secondary batteries obtained as described above were first subjected to an initial charge process and an initial charge/discharge capacity measurement according to the procedure (3-1) below. Next, each coin-type non-aqueous secondary battery was evaluated according to the procedures (3-2) to (3-3) below. Charging and discharging were performed using a charge/discharge device ACD-M01A (product name) manufactured by Asuka Electronics Co., Ltd. and a programmable thermostatic chamber IN804 (product name) manufactured by Yamato Scientific Co., Ltd.

Here, 1C means the current value at which a fully charged battery is expected to be discharged completely in 1 hour when discharged at a constant current.

(3-1) Initial charge/discharge treatment of coin-type nonaqueous secondary battery The ambient temperature of the coin-type nonaqueous secondary battery was set to 25° C., and the battery was charged at a constant current of 0.46 mA, which corresponds to 0.1 C, to reach 3.8 V, and then charged at a constant voltage of 3.8 V until the current attenuated to 0.05 C. Then, the battery was discharged to 2.5 V at a constant current of 1.38 mA, which corresponds to 0.3 C.

(3-2) -40°C discharge test For the coin-type non-aqueous secondary battery that had been subjected to the initial charge/discharge treatment according to the method described in (3-1) above, the ambient temperature of the battery was set to 25°C, and the battery was charged at a constant current of 2.3mA equivalent to 0.5C to reach 3.8V, and then charged at a constant voltage of 3.8V until the current attenuated to 0.05C, and discharged to 2.5V at a current value of 0.46mA equivalent to 0.1C. After that, the battery was charged at a constant current of 2.3mA equivalent to 0.5C to reach 3.8V, and then charged at a constant voltage of 3.8V until the current attenuated to 0.05C. Next, the ambient temperature of the battery was set to -40°C, and after a waiting time of 3 hours, the battery was discharged to 2.5V at a current value of 0.46mA equivalent to 0.1C.

The capacity retention rate was calculated as the discharge capacity at -40°C when the discharge capacity at 25°C was taken as 100%.

(3-3) 85°C storage test The small non-aqueous secondary battery that had been subjected to the initial charge/discharge treatment according to the method described in (3-1) above was charged at a constant current equivalent to 1C at an ambient temperature of 25°C until it reached a fully charged state, and then charged at a constant voltage for 1.5 hours. Next, this coin-type non-aqueous secondary battery was stored in a thermostatic chamber at 85°C for 2 weeks (336 hours). After that, the ambient temperature was returned to 25°C, and the battery was discharged to 2.5V at a current value of 0.46mA equivalent to 0.1C. The remaining discharge capacity at this time was taken as the remaining discharge capacity after 2 weeks. The remaining capacity retention rate was calculated based on the following formula as the measured value of the 85°C 2-week fully charged storage test.
Remaining capacity after 2 weeks = (remaining discharge capacity after 2 weeks / initial capacity) x 100 [%]

[Examples 1 and 2 and Comparative Examples 1 to 4]
A coin-type nonaqueous secondary battery was fabricated according to the method described in (2) above using the positive electrode, negative electrode, and nonaqueous electrolyte solution shown in Table 1. Next, each coin-type nonaqueous secondary battery was evaluated according to the procedures described in (3-1) to (3-3) above. The test results are shown in Table 2.

As shown in Table 2, Examples 1 and 2 used electrolytes containing ethylene sulfite and for which the SO ₄ ^2- content had been adjusted, and showed a capacity retention rate of 56% or more in a discharge test at -40°C and a remaining capacity retention rate of 48% or more in a storage test at 85°C.

Comparative Example 1 used an electrolyte solution containing neither acetonitrile nor LiFSI, but the capacity retention rate in the −40° C. discharge test was reduced to 20% or less, and therefore it is understood that the Examples have excellent low-temperature characteristics. In addition, Comparative Example 1 did not contain a sulfite ester compound, and although a step of adjusting the SO ₄ ^2- content was carried out, the remaining capacity retention rate in the 85° C. storage test also decreased to 20% or less.

On the other hand, Comparative Example 2 used an electrolyte solution containing acetonitrile and LiFSI but not a sulfite ester compound, and in which the amount of SO ₄ ^2- ions had been adjusted. Compared to Comparative Example 1, improved performance was observed in both the -40°C discharge test and the 85°C storage test, but no improvement in performance was observed, particularly in the 85°C storage test, where the improvement was only about 37%.

In Comparative Example 3, an electrolyte solution containing acetonitrile, LiFSI, and a sulfite ester compound was used in which the amount of SO ₄ ^2- ions was not adjusted after the molecular sieve treatment. In the −40° C. discharge test, the performance was equivalent to that of Examples 1 and 2, but in the 85° C. storage test, the capacity was about 41%, and high performance was not obtained.

In addition, in Comparative Example 4, an electrolyte solution containing acetonitrile, LiFSI, and a sulfite compound, but without adjusting the amount of SO ₄ ^2- ions, was used. In the −40° C. discharge test, the electrolyte showed performance equivalent to that of Examples 1 and 2, but in the 85° C. storage test, the electrolyte showed a loss of approximately 45%, which was not as good as that of the Examples.

The results of this experiment demonstrated that the use of the nonaqueous electrolyte according to the present invention makes it possible to achieve both long-term storage performance in a harsh high-temperature environment of 85°C and battery performance at low temperatures for nonaqueous secondary batteries.

The nonaqueous secondary battery of the present invention is expected to be used, for example, as a power source for idling stop systems; rechargeable batteries for hybrid vehicles, plug-in hybrid vehicles, electric vehicles, and the like; low-voltage power sources such as 12V class power sources, 24V class power sources, and 48V class power sources; and residential power storage systems.

REFERENCE SIGNS LIST 100 non-aqueous secondary battery 110 battery exterior 120 space in battery exterior 110 130 positive electrode lead body 140 negative electrode lead body 150 positive electrode 160 negative electrode 170 separator

Claims (6)

A non-aqueous electrolyte solution containing a non-aqueous solvent and a lithium salt, wherein the non-aqueous solvent contains 5 vol% or more and 95 vol% or less of acetonitrile with respect to a total amount of the non-aqueous solvent, and the lithium salt contains lithium bisfluorosulfonylimide and _LiPF6 , and a compound represented by the following general formula (I):
In the formula, R ₁ and R ₂ are each an alkyl group having 1 to 4 carbon atoms in which a hydrogen atom may be substituted with a fluorine atom, or R ₁ and R ₂ may be bonded to form a ring.
and a content of SO ₄ ^2- in the nonaqueous electrolyte is 10 ppm or more and 500 ppm or less. The nonaqueous electrolyte solution according to claim 1, wherein the sulfite ester compound is ethylene sulfite. The nonaqueous electrolyte solution according to claim 1 or 2, wherein the nonaqueous solvent contains a cyclic carbonate. The nonaqueous electrolyte solution according to claim 3, wherein the cyclic carbonate contains at least one selected from the group consisting of vinylene carbonate and fluoroethylene carbonate. A nonaqueous secondary battery comprising the nonaqueous electrolyte according to any one of claims 1 to 4, a positive electrode having a positive electrode active material layer on one or both sides of a positive electrode current collector, a negative electrode having a negative electrode active material layer on one or both sides of a negative electrode current collector, and a separator. the positive electrode active material layer contains a lithium-containing metal oxide as a positive electrode active material,
6. The nonaqueous secondary battery according to claim 5, wherein the negative electrode active material layer contains, as a negative electrode active material, a material capable of absorbing lithium ions at a potential lower than 0.4 V vs. Li/ ^{Li +} .

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