JP7590126B2 - Non-aqueous secondary battery - Google Patents

Description

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

In recent years, development has been underway with the aim of installing lithium-ion secondary batteries in electric vehicles, hybrid vehicles, and the like. Among these, there is a movement to replace the lead-acid batteries used in idling stop systems and starters with lithium-ion secondary batteries. Specifications required for the above applications include long-term reliability in the high-temperature environment assumed for installation in the engine compartment, as well as output performance in low-temperature environments, since the engine needs to be driven in cold regions.

However, in a high-temperature environment, the solvent contained in the nonaqueous electrolyte is oxidized and reduced to decompose, and the internal resistance increases due to the accumulation of the decomposition products, and the problem of degrading the battery performance due to gas generation remains. In particular, when lithium cobalt oxide (LiCoO ₂ ) or a ternary positive electrode (Li(Ni/Co/Mn)O ₂ ) is used as the positive electrode active material, it is known that transition metals are eluted from the positive electrode active material by performing charge/discharge cycles in a high-temperature environment, and the eluted metals are precipitated on the surface of the negative electrode, causing various battery deteriorations. On the other hand, for power tool applications, lithium-containing compounds having an olivine structure with excellent thermal stability have been used as the positive electrode active material for some time. This positive electrode active material has excellent thermal stability and high output performance because of its stable crystal structure and small primary particle size. Furthermore, it is low cost and has a small environmental impact because it is mainly made of iron, which is abundant in resources. On the other hand, in a low temperature environment (−40° C. or lower), freezing of the nonaqueous electrolyte and precipitation of lithium salts make it difficult to achieve the output performance required to drive the engine.

Patent Document 1 reports that when a positive electrode has a positive electrode active material, specifically olivine-type lithium iron phosphate, whose operating potential is less than 4 V relative to metallic lithium, and a negative electrode active material, specifically graphite, whose operating potential is more than 1 V relative to metallic lithium, and an ether compound with a specific structure and a lithium salt are used as the electrolyte, the battery shows better rate characteristics than conventional carbonate-based electrolytes.

Patent document 2 reports that the use of a specific electrolyte solution results in excellent properties over a wide range of temperatures, from high to low.

Patent Document 3 reports that by including an ester compound and methylene bissulfonate in the electrolyte, the reductive decomposition of the electrolyte is suppressed and a good coating is formed, improving the high-temperature storage characteristics.

Patent Document 4 reports that excellent input/output performance can be obtained by using an electrolyte solution containing a fluorosulfonate represented by M(FSO ₃ ) _x and LiPF ₆ in a certain ratio.

Patent Document 5 reports a lithium-ion secondary battery that uses olivine-type lithium iron phosphate as the positive electrode active material on a metal foil with through holes, and uses a specific electrolyte and separator, resulting in excellent output performance in an environment of -30°C.

JP 2010-277958 A International Publication No. 2017/057588 International Publication No. 2012/017999 JP 2011-187440 A JP 2015-215977 A

However, although the lithium-ion secondary battery described in Patent Document 1 mentions its output performance at room temperature, it makes no mention of its output in low-temperature environments.

Although the lithium-ion secondary battery described in Patent Document 2 operates over a wide range of temperatures from high to low, its ionic conductivity at 25°C is no different from that of a typical carbonate electrolyte, and in an extremely low temperature environment, the ionic conductivity is further reduced, so that the battery does not achieve performance sufficient to meet the output required for in-vehicle applications.

The lithium-ion secondary battery described in Patent Document 3 can improve high-temperature durability by strengthening the negative electrode protective coating, but because too much of the strong protective coating is formed on the negative electrode to suppress the reductive decomposition of the electrolyte, the internal resistance increases and it is not possible to achieve sufficient output performance in a low-temperature environment.

In a lithium ion battery using an electrolyte solution containing LiFSO ₃ as a lithium salt as described in Patent Document 4, it was confirmed that the FSO ₃ anion (FSO ₃ ⁻ ) was reduced and decomposed at the negative electrode in a high-temperature environment, causing an increase in internal resistance, and good cycle performance was not obtained.

Lithium ion secondary batteries using specific positive electrodes, negative electrodes, and separators as described in Patent Document 5 exhibit excellent output performance in low-temperature environments, but make no mention of the reductive decomposition of the electrolyte in the negative electrode in high-temperature environments.

In view of the above problems, the present invention aims to provide a non-aqueous secondary battery that suppresses the decrease in capacity retention rate when performing charge/discharge cycles in a high-temperature environment and also has excellent output performance in a low-temperature environment.

The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that the above-mentioned problems can be solved by using a nonaqueous electrolyte or a nonaqueous secondary battery having the following configuration, and have completed the present invention. That is, the present invention is as follows.
[1]
A non-aqueous secondary battery having a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator, and a non-aqueous electrolyte solution,
The positive electrode contains a lithium-containing metal oxide as the positive electrode active material,
The lithium-containing metal oxide includes a compound having an olivine structure represented by Li _x FePO ₄ , where 0<x<1.2;
The non-aqueous electrolyte contains 1 volume % to 85 volume % of acetonitrile, and LiN(SO ₂ F) ₂ and LiPF ₆ as lithium salts, based on the total mass of the non-aqueous solvent, and the content of FSO ₃ anions is 100 ppm or less based on the total mass of the non-aqueous electrolyte.
[2]
2. The nonaqueous secondary battery according to item 1, wherein the propionitrile contained in the nonaqueous electrolyte solution is less than 5.0 ppm based on the total mass of the nonaqueous electrolyte solution.

By using lithium iron phosphate having an olivine structure as the positive electrode active material, various types of battery deterioration in high temperature environments can be suppressed, and by controlling the content of _FSO3 anions in the non-aqueous electrolyte solution containing acetonitrile, LiN( _SO2F ) ₂ , and _LiPF6 within a predetermined range, an increase in the interface resistance of the negative electrode can be suppressed, and excellent output performance can be achieved in low temperature environments.

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 non-aqueous secondary battery of this embodiment includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator, and a non-aqueous electrolyte. In addition, although not limited thereto, the non-aqueous secondary battery of this embodiment includes a lithium ion battery including 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 FIGS. 1 and 2. Here, FIG. 1 is a plan view showing a schematic representation of a nonaqueous secondary battery, and FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1. The nonaqueous secondary battery 100 shown in FIGS. 1 and 2 is 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 interposed therebetween, and a nonaqueous electrolyte (not shown) in a space 120 of a battery exterior 110 formed 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 order is impregnated with a nonaqueous electrolyte. However, in FIG. 2, in order to avoid complicating 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 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 contains a lithium-containing metal oxide as a positive electrode active material. Typically, the positive electrode is composed of a positive electrode current collector and a positive electrode active material layer made of a positive electrode mixture on one or both sides of the positive electrode current collector. The positive electrode is not particularly limited as long as it acts as a positive electrode of a non-aqueous secondary battery.

The positive electrode active material layer preferably contains a positive electrode active material and, if necessary, further contains a conductive assistant and a binder. The positive electrode active material layer preferably contains, as the positive electrode active material, a material capable of absorbing and releasing lithium ions. 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, and may be represented by the following general formula (a):
Li _p Ni _{q Cor} Mn _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 in 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, and 1.8<u<2.2, and p is a value determined by the charge/discharge state of the battery.
At least one Li-containing metal oxide selected from the Li-containing metal oxides represented by the following formula (I) 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 .} (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 can unintentionally consume compounds added to protect the negative electrode on the positive electrode side. Acid anhydrides in particular tend to be susceptible to this effect. 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, the decomposition products of these additives 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. 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 control the Jahn-Teller distortion or to have a component that acts 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, 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 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 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 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)
{In the formula, D represents a chalcogen element, M and ^I represent one or more transition metal elements, the value of v 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.}
The following formula (Xb):
Li _w M ^II PO ₄ (Xb)
{wherein D represents a chalcogen element, M ^II represents one or more transition metal elements, 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 u represents a number from 0 to 2.}, and the following formula (Xc):
Li _t M ^III _u SiO ₄ (Xc)
{In the formula, D represents a chalcogen element, 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 are 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 contains a lithium-containing metal oxide represented by Li _x FePO ₄ having an olivine structure, among the positive electrode active materials represented by (Xb), where 0<x<1.2. Preferably, it is lithium iron phosphate (LiFePO ₄ ). Since the olivine structure has excellent thermal stability, a stable crystal structure, and a small primary particle size, 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 the present embodiment, the positive electrode active material may be only the lithium-containing compound as described above, or may be used in combination with other positive electrode active materials together with the lithium-containing compound. Examples of such other positive electrode active materials include 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 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 positive electrode current collector may have a carbon coating on the 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.

[Negative electrode]
The negative electrode has a negative electrode active material. The negative electrode is generally composed of a negative electrode active material layer made of a negative electrode mixture and a negative electrode current collector. The negative electrode can act as a negative electrode of a non-aqueous secondary battery. The negative electrode active material layer contains the negative electrode active material, and preferably contains a conductive assistant and a binder as necessary.

Examples of negative electrode active materials include amorphous carbon (hard carbon), artificial graphite, natural graphite, graphite, pyrolytic carbon, coke, glassy carbon, baked bodies of organic polymer compounds, mesocarbon microbeads, carbon fiber, activated carbon, graphite, carbon colloids, and 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 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 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 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.

[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.

In this embodiment, the non-aqueous electrolyte 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 content of such water is preferably 300 mass ppm or less, and more preferably 200 mass ppm or less, relative to the total amount of the non-aqueous electrolyte. As long as the non-aqueous electrolyte has a configuration for achieving the solution of the problem of the present invention, the other components can be appropriately selected and used from the constituent materials of known non-aqueous electrolytes used in lithium ion batteries.

In this embodiment, the non-aqueous electrolyte contains acetonitrile and LiN(SO ₂ F) ₂ and LiPF ₆ as lithium salts.

<Non-aqueous solvent>
In the present embodiment, the term "non-aqueous solvent" refers to components other than lithium salts and various additives among the components contained in the non-aqueous electrolyte. However, when the non-aqueous electrolyte contains an electrode protection additive, the electrode protection additive that serves as a solvent for dissolving the lithium salt is also treated as a "non-aqueous solvent". Examples of non-aqueous solvents include alcohols such as methanol and ethanol; aprotic solvents, etc. Among them, aprotic solvents are preferred as non-aqueous solvents. The non-aqueous solvent may contain a solvent other than the aprotic solvent as long as it does not hinder the problem solving of the present invention.

The non-aqueous solvent for the non-aqueous electrolyte contains acetonitrile as an aprotic solvent. The non-aqueous solvent contains acetonitrile, which improves the ionic conductivity of the non-aqueous electrolyte, thereby increasing the diffusibility of lithium ions in the battery. Therefore, when the non-aqueous electrolyte contains acetonitrile, lithium ions can be well diffused even to the area near the current collector where lithium ions are difficult to reach during high-load discharge. Therefore, it is possible to obtain sufficient capacity even during high-load discharge, and a non-aqueous secondary battery with excellent load characteristics can be obtained. This effect tends to be obtained especially in a positive electrode with a thick positive electrode active material layer and a high loading amount of positive electrode active material.

In addition, by containing acetonitrile in the non-aqueous solvent, the rapid charging characteristics of the non-aqueous secondary battery can be improved. In constant current (CC)-constant voltage (CV) charging of a non-aqueous 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 non-aqueous solvent for the non-aqueous electrolyte, the range in which CC charging can be performed 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 non-aqueous secondary battery can be significantly shortened.

Acetonitrile is easily decomposed by electrochemical reduction. Therefore, when using acetonitrile, it is preferable to use another solvent (e.g., an aprotic solvent other than acetonitrile) together with acetonitrile as a non-aqueous solvent, and/or to add an electrode protection additive to form a protective film on the electrode.

The content of acetonitrile is 1% to 85% by volume based on the total mass of the non-aqueous solvent. The lower limit of the content of acetonitrile is more preferably 5% by volume or more or 10% by volume or more, and even more preferably 20% by volume or more, based on the total mass of the non-aqueous solvent. The upper limit is more preferably 80% by volume or less, and even more preferably 70% by volume or less. When the content of acetonitrile is 5% by volume or more based on 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 range, the high temperature cycle characteristics and other battery characteristics tend to be further improved while maintaining the excellent performance of acetonitrile.

Examples of aprotic solvents other than acetonitrile include cyclic carbonates, fluoroethylene carbonates, lactones, organic compounds having sulfur atoms, linear fluorinated carbonates, cyclic ethers, mononitriles other than acetonitrile, alkoxy-substituted nitriles, dinitriles, cyclic nitriles, short-chain fatty acid esters, linear ethers, fluorinated ethers, ketones, and compounds in which some or all of the H atoms of the aprotic solvents have been replaced with halogen atoms.

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.

Examples of fluoroethylene carbonate include 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-dioxolane-2-one, and 4,4,5-trifluoro-5-methyl-1,3-dioxolane-2-one.

Lactones include gamma-butyrolactone, alpha-methyl-gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone, delta-valerolactone, delta-caprolactone, and epsilon-caprolactone.

Examples of organic compounds containing sulfur atoms include 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.

Examples of chain carbonates include ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, and ethyl propyl carbonate.

Examples of cyclic ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and 1,3-dioxane.

Examples of mononitriles other than acetonitrile include propionitrile, butyronitrile, valeronitrile, benzonitrile, and acrylonitrile.

Examples of alkoxy-substituted nitriles include methoxyacetonitrile and 3-methoxypropionitrile.

Examples of dinitriles include malononitrile, succinonitrile, glutaronitrile, 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, 1,6-dicyanodecane, and 2,4-dimethylglutaronitrile.

An example of a cyclic nitrile is benzonitrile.

Examples of short-chain fatty acid esters include 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, propyl isobutyrate, propyl butyrate, propyl isovalerate, propyl valerate, propyl pivalate, propyl hydroangelate, propyl caproate, isopropyl acetate, isopropyl propionate, isopropyl isobutyrate, isopropyl butyrate, isopropyl isovalerate, isopropyl valerate, and 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, 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.

Examples of chain ethers include dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme, and tetraglyme.

An example of the fluorinated ether is Rf ²⁰ -OR ²¹ (Rf ²⁰ is an alkyl group containing a fluorine atom, and R ²¹ is an organic group which may contain a fluorine atom).

Examples of ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone.

Examples of the compounds in which some or all of the H atoms of the aprotic solvent are replaced with halogen atoms include compounds in which the halogen atoms are fluorine.

Examples of fluorinated chain carbonates include methyl trifluoroethyl carbonate, trifluorodimethyl carbonate, trifluorodiethyl carbonate, trifluoroethyl methyl carbonate, methyl 2,2-difluoroethyl carbonate, methyl 2,2,2-trifluoroethyl carbonate, and methyl 2,2,3,3-tetrafluoropropyl carbonate. The above-mentioned fluorinated chain carbonates are represented by the following general formula:
R ²⁹ -O-C(O)OR ³⁰
(wherein R ²⁹ and R ³⁰ 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 ³¹ , Rf ³¹ 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 ²⁹ and/or R ³⁰ contain at least one fluorine atom).

In this embodiment, the aprotic solvent other than acetonitrile may be used alone or in combination of two or more.

In the present embodiment, it is preferable to use acetonitrile in combination with one or more of a cyclic carbonate and a chain carbonate as the nonaqueous solvent from the viewpoint of improving the stability of the nonaqueous electrolyte. From this viewpoint, it is more preferable to use acetonitrile in combination with a cyclic carbonate as the nonaqueous solvent in the present embodiment, and it is even more preferable to use acetonitrile in combination with both a cyclic carbonate and a chain carbonate.

When a cyclic carbonate is used with acetonitrile, it is particularly preferred that the cyclic carbonate comprises ethylene carbonate, vinylene carbonate and/or fluoroethylene carbonate.

<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, in addition to LiN(SO ₂ F) ₂ , LiN(SO ₂ CF ₃ ) ₂ can be mentioned. 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 contain a fluorine-containing inorganic lithium salt other than LiPF ₆ , for example, 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. } and the like. "Inorganic lithium salt" refers to a lithium salt that does not contain a carbon atom in the anion and is soluble in acetonitrile. Also, "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. Fluorine-containing inorganic lithium salts are excellent in that they form a passive film on the surface of the metal foil that is the positive electrode current collector, and suppress 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. An "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 (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 ). {wherein n is an integer of 1 to 5, and p is an integer of 1 to 8.}; organic lithium salts 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.}; lithium salts bonded to polyvalent anions;
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 each represents 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.

<Additives for electrode protection>
The electrolyte in this embodiment may contain an additive for protecting the electrode (referred to as an "electrode protection additive" in this specification). The electrode protection additive is not particularly limited as long as it does not impede the problem solving by the present invention. It may substantially overlap with a substance that plays a role as a solvent for dissolving lithium salts (i.e., the above-mentioned non-aqueous solvent). The electrode protection additive is preferably a substance that contributes to improving the performance of the electrolyte and non-aqueous secondary battery in this embodiment, but also includes a substance that is not directly involved in the electrochemical reaction.

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, and 4,4,5,5-tetrafluoro-1,3-dioxolane-2-one. fluoroethylene carbonates such as 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-caprolactone, and ε-caprolactone. lactones represented by 1,4-dioxane; cyclic ethers represented by 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, and tetramethylene sulfoxide; chain acid anhydrides represented by acetic anhydride, propionic anhydride, and benzoic anhydride; cyclic acid anhydrides represented by 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 can be used alone or in combination of two or more types.

There are no particular limitations on the content of the electrode protection additive in the electrolyte in this embodiment, but the content of the electrode protection additive relative to the total amount of the non-aqueous electrolyte is preferably 0.01% to 30% by volume, more preferably 0.3% to 15% by volume, and even more preferably 0.5% to 4% by 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.

Acetonitrile, which is one component of the non-aqueous solvent, is easily decomposed by electrochemical reduction, so 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, an additive for electrode protection, can improve battery performance by suppressing the reductive decomposition reaction of acetonitrile on the negative electrode surface. On the other hand, excessive film formation can lead to a decrease in low-temperature performance. Therefore, when vinylene carbonate is included, by adjusting the amount added within the above range, it is possible to suppress cycle deterioration at low temperatures while keeping the interface (film) resistance low.

< _FSO3 anion reduction process>
When LiN(SO ₂ F) _2, which is generally commercially available as a lithium salt, is used, a certain amount of FSO ₃ anion is contained as a raw material impurity. Although it is difficult to completely remove the FSO ₃ anion in the electrolyte, if the content of the FSO ₃ anion is too high, a reduction decomposition reaction occurs on the negative electrode surface in the non-aqueous secondary battery, and the accumulated decomposition product increases the interface (film) resistance and leads to a decrease in output performance. The content of the FSO ₃ anion is in the range of 100 ppm or less based on the total mass of the non-aqueous electrolyte, preferably in the range of 50 ppm or less, and more preferably in the range of 30 ppm or less. On the other hand, a small amount of FSO ₃ anion acts to strengthen the negative electrode protective film, so as long as the above upper limit is satisfied, it may contain 0.01 ppm or more, 0.1 ppm or more, or 1 ppm or more.

In the present invention, the content of _FSO3 anion can be reduced by, for example, dissolving LiN( _SO2F ) ₂ in a non-aqueous solvent, adding an adsorbent capable of adsorbing _FSO3 anion to the mixed solution, and leaving the mixture for a certain period of time. Examples of the adsorbent include molecular sieves. The molecular sieves may be commercially available or may be synthesized. The time required for the content of _FSO3 anion to be reduced to the range of _the present invention is not particularly limited, but is preferably 5 hours or more, more preferably 12 hours or more, after the molecular sieves are added. In addition, after the molecular sieves are added, heating or stirring may be added as appropriate.

<Propionitrile>
The non-aqueous electrolyte may contain propionitrile. Propionitrile reacts quickly during overcharging and contributes to alpha hydrogen abstraction. This allows the battery to swell and burst before burning. On the other hand, the addition of excessive propionitrile promotes the alpha hydrogen abstraction reaction even in the normal voltage range, causing gas accumulation in the electrode and capacity degradation. For this reason, by making the propionitrile contained in the non-aqueous electrolyte 5 ppm or less, the safety of the battery can be improved without impairing the battery performance in the normal voltage range.

The upper limit of the propionitrile content is preferably less than 5 ppm, and more preferably 4 ppm or less, relative to the nonaqueous electrolyte. The lower limit is preferably 0.001 ppm or more, and more preferably 0.01 ppm or more. The propionitrile content in the present invention is a value measured, for example, by gas chromatography mass spectrometry.

<Other optional additives>
In this embodiment, for the purpose of improving the charge/discharge cycle characteristics, high-temperature storage properties, and safety (e.g., preventing overcharging) of the nonaqueous secondary battery, the nonaqueous electrolyte may contain an optional additive selected from, for example, sulfonic acid esters, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, tert-butylbenzene, phosphate esters, nitrogen-containing cyclic compounds with no steric hindrance around the unshared electron pair, and derivatives of these compounds. Examples of phosphate esters include ethyl diethyl phosphonoacetate (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, and triphenyl phosphate (TPP): (C ₆ H ₅ O) ₃ P═O. Triallyl phosphate: (CH ₂ ═CHCH ₂ O) ₃ P═O, etc. Nitrogen-containing cyclic compounds include pyridine, 1-methyl-1H-benzotriazole, 1-methylpyrazole, etc. 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 mass% to 10 mass% relative to the total amount of the nonaqueous electrolyte, more preferably 0.02 mass% to 5 mass%, and even more preferably 0.05 mass% to 3 mass%. 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.

[Separator]
In the present embodiment, the non-aqueous secondary battery is preferably provided with a separator between the positive electrode and the negative electrode from the viewpoint of preventing short circuits between the positive electrode and the negative electrode, and providing safety such as shutdown. The separator 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 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 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 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. Also, a crosslinked structure may be included. In order to improve the safety performance of the non-aqueous secondary battery, these methods may be combined as necessary. By using such a separator, it is possible to achieve the good input/output characteristics and low self-discharge characteristics required for the lithium ion secondary battery for high-power applications described above.

The thickness of the microporous membrane 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-output applications such as safety tests where the amount of heat generated is relatively high and self-discharge characteristics better than conventional ones are required, and from the viewpoint of winding properties in large battery winding machines, it is preferably 5 μm to 30 μm, and more preferably 10 μm to 25 μm. Note that, when emphasis is placed on achieving both short-circuit resistance and output performance, it is even more preferable that it be 15 μm to 25 μm, but when emphasis is placed on achieving both high energy density and output performance, it is even more preferable that it be 10 μm to less than 15 μm.

From the viewpoint of following the rapid movement of lithium ions during high output, the porosity is preferably 30% or more and 90% or less, more preferably 35% or more and 80% or less, and even more preferably 40% or more and 70% or less. If priority is given to improving output performance while ensuring safety, 50% or more and 70% or less is particularly preferable, and if emphasis is placed on achieving both short circuit resistance and output performance, 40% or more and less than 50% is particularly preferable.

From the viewpoint of the balance between the film thickness and the porosity, the air permeability is preferably 1 sec/100 cm3 or ^more and 400 sec/100 cm3 or ^less , and more preferably 100 sec/100 cm3 ^or more and 350/100 ^cm3 or less. When emphasis is placed on achieving both short circuit resistance and output performance, the air permeability is particularly preferably 150 sec/100 cm3 or ^more and 350 sec/100 ^cm3 or less, and when priority is given to improving output performance while ensuring safety, the air permeability is particularly preferably 100/100 cm3 ^seconds or more and less than 150 sec/100 ^cm3 .

On the other hand, when a non-aqueous electrolyte with low ionic conductivity is combined with a separator within the above range, the lithium ion migration rate is determined not by the separator structure but by the high ionic conductivity of the electrolyte, and the expected input/output characteristics tend not to be obtained. Therefore, the ionic conductivity of the non-aqueous electrolyte is preferably 10 mS/cm or more, more preferably 15 mS/cm, and even more preferably 20 mS/cm. However, the membrane thickness, air permeability, and porosity of the separator, and the ionic conductivity of the non-aqueous electrolyte are not limited to the above examples.

[Battery exterior]
The structure of the battery exterior of the nonaqueous secondary battery in this embodiment is not particularly limited, and for example, either a battery can or a laminate film exterior 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, 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 (or the positive electrode terminal and the lead tab connected to the positive electrode terminal) may be connected to the positive electrode current collector, and the negative electrode lead body (or the negative electrode terminal and the lead tab connected to 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 and the negative electrode lead body (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 for manufacturing non-aqueous secondary battery>>
The nonaqueous secondary battery in this embodiment can be manufactured using the above-mentioned nonaqueous electrolyte, a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator, and, if necessary, a battery exterior.

First, a laminate consisting of positive and negative electrodes and a separator can be formed. For example, a method is possible in which a laminate is made by forming long positive and negative electrodes with a long separator between them, and then winding the laminate to form a wound laminate; a method is possible in which the positive and negative electrodes are cut into multiple sheets having a certain area and shape, and the positive and negative sheets are alternately laminated with a separator sheet between to form a laminate with a laminate structure; a method is possible in which a long separator is folded zigzag, and positive and negative electrode sheets are alternately inserted between the zigzag separators to form a laminate with a laminate structure; etc. are possible.

Then, the above-mentioned laminate is housed in a battery exterior (battery case), the electrolyte is poured into the battery case, and the laminate is immersed in the electrolyte and sealed, thereby producing the nonaqueous secondary battery of this embodiment. Alternatively, a gel-state electrolyte membrane can be produced in advance by impregnating a substrate made of a polymer material with the electrolyte, and a laminate having a laminate structure is formed using a sheet-like positive electrode, a negative electrode, an electrolyte membrane, and a separator, and then the laminate is housed in the battery exterior to produce a nonaqueous secondary battery.

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 decrease 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 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 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. It is also preferable that the initial charging is performed via a constant voltage charge in the middle, which gives a favorable result. The constant current at which the designed capacity is discharged 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, but has a good effect on the components other than the negative electrode, such as the positive electrode and the separator, 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 in this embodiment can also be used as a battery pack in which multiple nonaqueous secondary batteries are connected in series or parallel. From the viewpoint of managing the charge/discharge state of the battery pack, the operating voltage range per battery is preferably 2 to 5 V, more preferably 2.5 to 5 V, and particularly preferably 2.75 V to 5 V.

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 In an inert atmosphere, various non-aqueous solvents and various lithium salts were added to give predetermined concentrations to prepare non-aqueous electrolyte solutions (S1) to (S4). The compositions of these non-aqueous electrolyte solutions are shown in Table 1.

The abbreviations for the non-aqueous solvents and lithium salts in Table 1 have the following meanings.
(Non-aqueous solvent)
AcN: Acetonitrile EMC: Ethyl methyl carbonate DFA: Difluoroacetate GBL: γ-butyrolactone EC: Ethylene carbonate VC: Vinylene carbonate

(Lithium salt)
_LiPF6 : Lithium hexafluorophosphate LiFSI: Lithium bis(fluorosulfonyl)imide (LiN( _SO2F ) ₂ )

For the non-aqueous electrolyte solutions (S1) and (S2), 1 g of molecular sieve (5A 1/16 manufactured by Kanto Chemical Co., Ltd.) was added to 10 g of the non-aqueous electrolyte solution after preparation, and the solution was left for 24 hours with occasional stirring. After that, the amount of _FSO3 anion in the obtained non-aqueous electrolyte solution was measured using ^19F -NMR.

(2) Preparation of coin-type non-aqueous secondary battery (2-1) Preparation of positive electrode (A) Lithium iron phosphate (LiFePO ₄ ) having an olivine structure as a positive electrode active material, (B) Carbon black powder as a conductive assistant, 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. While adjusting the basis weight of this positive electrode mixture-containing slurry on one side of an aluminum foil having a thickness of 15 μm and a width of 280 mm to be a positive electrode current collector, the slurry was applied using a three-roll transfer coater so as to have a coating pattern of 240 to 250 mm in coating width, 125 mm in coating length, and 20 mm in uncoated length, and the solvent was dried and removed in a hot air drying oven. The obtained electrode roll was trimmed on both sides and dried under reduced pressure at 130° C. for 8 hours. Then, the electrode roll was rolled with a roll press so that the density of the positive electrode active material layer was 1.9 g/cm ³ to obtain a positive electrode consisting of the positive electrode active material layer and the positive electrode current collector. The basis weight excluding the positive electrode current collector was 17.5 mg/cm ² .

(2-2) Preparation of negative electrode As a negative electrode active material, graphite powder, carbon black powder as a conductive assistant, and carboxymethyl cellulose and styrene butadiene rubber as a binder were mixed in a solid content mass ratio of 95.7:0.5:3.8 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 mass% and further mixed to prepare a negative electrode mixture-containing slurry. While adjusting the basis weight of this negative electrode mixture-containing slurry on one side of a copper foil having a thickness of 8 μm and a width of 280 mm to be a negative electrode current collector, it was coated using a three-roll transfer coater so as to have a coating pattern of a coating width of 240 to 250 mm, a coating length of 125 mm, and an uncoated length of 20 mm, 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 became 1.5 g/cm ³ , thereby obtaining a negative electrode composed of the negative electrode active material layer and the negative electrode current collector. 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 clean 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 in (3-1) below. Next, each coin-type non-aqueous secondary battery was evaluated according to the procedure in (3-2). The charge/discharge was performed using a charge/discharge device ACD-M01A (product name) manufactured by Asuka Electronics Co., Ltd. and a program thermostatic chamber IN804 (product name) manufactured by Yamato Scientific Co., Ltd. Here, 1C means a current value at which a battery in a fully charged state is discharged at a constant current and discharge is expected to be completed in one hour. It means a current value at which a battery in a fully charged state is discharged at a constant current and discharge is expected to be completed in one hour.

(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 4.6 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 decayed 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) 25°C Cycle Test 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 was charged at an ambient temperature of 25°C, and after reaching 3.8V at a constant current of 2.3mA equivalent to 0.5C, it was charged at a constant voltage of 3.8V until the current attenuated to 0.05C. It was then discharged to 2.5V at a current value of 2.3mA equivalent to 0.5C. This process of charging and discharging once each was counted as one cycle, and 100 cycles of charging and discharging were performed. In addition, at the 1st cycle, the 50th cycle, and the 100th cycle, it was charged at a constant current of 1.38mA equivalent to 0.3C, and after reaching 3.8V, it was charged at a constant voltage of 3.8V until the current attenuated to 0.05C, and then it was discharged to 2.5V at a constant current of 1.38mA equivalent to 0.3C. The discharge capacity at the 99th cycle in the cycle test was calculated as the capacity retention rate, assuming that the discharge capacity at the first cycle in the initial charge/discharge treatment was 100%.

(3-3) 50°C Cycle Test 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 was charged at an ambient temperature of 50°C, and after reaching 3.8V at a constant current of 6.9mA equivalent to 1.5C, it was charged at a constant voltage of 3.8V until the current attenuated to 0.05C. It was then discharged to 2.5V at a current value of 6.9mA equivalent to 1.5C. This process of charging and discharging once each was counted as one cycle, and 100 cycles of charging and discharging were performed. In addition, at the 1st cycle, the 50th cycle, and the 100th cycle, it was charged at a constant current of 4.6mA equivalent to 1C, and after reaching 3.8V, it was charged at a constant voltage of 3.8V until the current attenuated to 0.05C, and then it was discharged to 2.5V at a constant current of 1.38mA equivalent to 0.3C. The discharge capacity at the 99th cycle in the cycle test was calculated as the capacity retention rate, assuming that the discharge capacity at the first cycle in the initial charge/discharge treatment was 100%.

(3-4) -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 until it reached 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 until it reached 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 discharge capacity at -40°C when the discharge capacity in a 25°C environment was taken as 100% was calculated as the capacity retention rate.

[Examples 1 to 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 (3-4) above. The test results are shown in Tables 2 and 3.

As shown in Table 2, Example 1 used an electrolyte solution that had undergone the _FSO3 anion reduction process, and the capacity retention rate in the 25°C cycle test was 85% or more, and the capacity retention rate in the -40°C discharge test was also 58% or more. On the other hand, Comparative Example 1 used an electrolyte solution that had not undergone the _FSO3 anion reduction process, and although the capacity retention rate in the -40°C discharge test was 57% or more, which showed excellent low-temperature performance, the capacity retention rate in the 25°C cycle test was reduced to 69% or less.

As shown in Table 3, Example 2 uses an electrolyte solution that has undergone an FSO ₃ anion reduction process, and the capacity retention rate in the 50 ° C. cycle test is 88% or more, and the capacity retention rate in the −40 ° C. discharge test is also 61% or more. On the other hand, since Comparative Example 2 does not use LiFSI as a lithium salt, the amount of FSO ₃ anions is within the range of the present invention (0 ppm) regardless of the presence or absence of an FSO ₃ anion reduction process. Comparative Example 2 showed excellent high-temperature durability with a capacity retention rate of 87% or more in the 50 ° C. cycle test, but since LiFSI was not used, the capacity retention rate in the −40 ° C. discharge test was 13% or less, which did not satisfy the low-temperature performance. Comparative Example 3 uses an electrolyte solution that does not contain acetonitrile and has not undergone an FSO ₃ anion reduction process, so the capacity retention rate in the 50 ° C. cycle test was reduced to 70% or less. The capacity retention rate in the −40° C. discharge test was 27% or less, which did not satisfy the low-temperature performance, because it did not contain acetonitrile and the LiFSI content was 0.7 mol or less. In Comparative Example 4, the FSO ₃ anion reduction step was not performed, and the electrolyte used had a propionitrile content of 5.5 ppm or more, so the capacity retention rate in the 50° C. cycle test was reduced to 15% or less. For Comparative Example 4, the capacity retention rate in the 50° C. cycle test was significantly reduced, so the −40° C. discharge test was not performed.

As shown by these results, it has become clear that by using a non-aqueous electrolyte solution and a specific positive electrode active material within the scope of the present invention, it is possible to improve cycle performance in high-temperature environments and low-temperature performance.

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.

100 Non-aqueous secondary battery 110 Battery exterior 120 Space 130 Positive electrode lead body 140 Negative electrode lead body 150 Positive electrode 160 Negative electrode 170 Separator

Claims (2)

A non-aqueous secondary battery having a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator, and a non-aqueous electrolyte solution,
The positive electrode contains a lithium-containing metal oxide as the positive electrode active material,
The lithium-containing metal oxide includes a compound having an olivine structure represented by Li _x FePO ₄ , where 0<x<1.2;
The nonaqueous electrolyte contains 1 volume % to 85 volume % of acetonitrile, and LiN(SO ₂ F) ₂ and LiPF ₆ as lithium salts, based on the total volume of the nonaqueous solvent, and the content of FSO ₃ anions is 1 ppm or more and 30 ppm or less, based on the total mass of the nonaqueous electrolyte. 2. The nonaqueous secondary battery according to claim 1, wherein the propionitrile contained in the nonaqueous electrolyte solution is 0.01 ppm or more and less than 5.0 ppm based on the total mass of the nonaqueous electrolyte solution.

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