Nonaqueous electrolyte for secondary battery, and secondary battery

Abstract

This nonaqueous electrolyte for secondary batteries comprises a nonaqueous solvent, a salt that dissolves in the nonaqueous solvent, and an additive that dissolves in the nonaqueous solvent. The additive contains a carbodiimide compound and a sulfur-containing compound. The sulfur-containing compound contains at least one compound selected from the group consisting of hexavalent sulfur compounds and tetravalent sulfur compounds.

Description

Non-aqueous electrolytes for secondary batteries and secondary batteries Cross-reference of related applications

[0001] This disclosure claims priority with respect to Japanese Patent Application No. 2024-167357, filed with the Japan Patent Office on 26 September 2024, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to a non-aqueous electrolyte for secondary batteries and to secondary batteries.

[0003] Patent Document 1 proposes a non-aqueous electrolyte for batteries that contains an electrolyte containing lithium hexafluorophosphate, a non-aqueous solvent, and a predetermined carbodiimide compound which is a compound containing a nitrogen atom having a lone pair of electrons.

[0004] Patent Document 2 proposes a non-aqueous electrolyte for batteries containing a predetermined carbodiimide compound, which is used in lithium secondary batteries that include lithium nickel manganese cobalt composite oxide as the positive electrode active material.

[0005] Patent document 3 proposes a non-aqueous electrolyte characterized by containing carbodiimide and at least one of a sulfate ester and a boron compound.

[0006] Patent No. 7107491 Patent No. 7200465 Patent No. 5364890

[0007] Patent Document 1 aims to provide a non-aqueous electrolyte for batteries that contains an electrolyte containing lithium hexafluorophosphate, and in which the generation of hydrogen fluoride is suppressed when water is mixed in. Patent Document 2 aims to provide a non-aqueous electrolyte for batteries that can reduce the battery resistance in lithium secondary batteries containing lithium nickel manganese cobalt composite oxide as the positive electrode active material. Patent Document 3 aims to provide a non-aqueous electrolyte that contains a specific sulfate ester and / or a specific boron compound and a specific carbodiimide, thereby suppressing discoloration and an increase in acidity when the non-aqueous electrolyte is stored, and when a secondary battery is manufactured, the swelling of the battery during initial charging and discharging is small and the cycle characteristics are good.

[0008] On the other hand, it has been discovered that when the non-aqueous electrolyte of a secondary battery equipped with a battery case contains a specific sulfur-containing compound, a new problem arises: when the secondary battery enters an over-discharge state, corrosion of the battery case progresses rapidly. It is presumed that sulfate ions or sulfite ions are generated from the specific sulfur-containing compound inside the battery in an over-discharge state, and these ions corrode the battery case. This problem does not occur with the laminate-type batteries proposed in Patent Documents 1 and 2.

[0009] One aspect of this disclosure relates to a non-aqueous electrolyte for a secondary battery, comprising a non-aqueous solvent, a salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound and a sulfur-containing compound, and the sulfur-containing compound comprises at least one selected from the group consisting of hexavalent sulfur compounds and tetravalent sulfur compounds.

[0010] Another aspect of the present disclosure relates to a secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, the above-mentioned non-aqueous electrolyte for a secondary battery, and a battery case, wherein the battery case is made of a metal containing at least iron, and the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte are housed in the battery case.

[0011] According to this disclosure, corrosion of the battery case when a secondary battery is over-discharged is suppressed. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0012] This is a longitudinal cross-sectional view of a secondary battery according to one embodiment of the present disclosure.

[0013] Hereinafter, embodiments of the present disclosure will be described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure can be obtained. Note that components other than the characteristic parts of the present disclosure may be applied with components of known secondary batteries. In this specification, when referring to the "range of numerical value A to numerical value B", this range includes numerical value A and numerical value B. For example, when referring to "A to B mol%", it is synonymous with "A mol% or more and B mol% or less". In the following description, when the lower limit and the upper limit of a numerical value regarding a specific physical property, condition, etc. are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined as long as the lower limit is not more than the upper limit. When a plurality of materials are exemplified, one of them may be selected and used alone, or two or more of them may be used in combination.

[0014] Further, the present disclosure includes combinations of matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims. That is, as long as no technical contradiction occurs, matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims can be combined.

[0015] Non-aqueous electrolyte secondary batteries include, for example, lithium ion secondary batteries that use at least a material that reversibly occludes and releases lithium ions as a negative electrode active material, lithium metal secondary batteries in which lithium metal is deposited during charging at the negative electrode and dissolved during discharging, and solid batteries containing a gel electrolyte.

[0016] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a battery can that houses these. A separator is usually disposed between the positive electrode and the negative electrode. The non-aqueous electrolyte usually has lithium ion conductivity. The battery can is made of a metal containing at least iron.

[0017] Note that in this specification, the "over-discharge characteristic" can be evaluated by the content rate of iron (Fe) eluted from the battery can in the non-aqueous electrolyte solution in the battery when the positive electrode and the negative electrode are short-circuited to be in an over-discharge state.

[0018] Also, the "initial resistance" can be calculated from the voltage drop amount ΔV of the terminal voltage during discharge when a fully charged battery is discharged for, for example, 30 seconds in an environment of 25°C.

[0019] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent, a salt, and an additive. The non-aqueous electrolyte containing a non-aqueous solvent may be an electrolyte solution (liquid Electrolyte), or may be in a state where its fluidity is restricted by a gelling agent or the like. In the case of a lithium-ion secondary battery, a lithium metal secondary battery, etc., a lithium salt is used as the salt. The salt and the additive are basically dissolved in the non-aqueous solvent. As long as the effects of the invention are not significantly impaired, a part of the salt or the additive may precipitate or separate without being dissolved in the non-aqueous solvent. The additive defined by the general formula described later may be a salt. In that case, another salt is dissolved in the non-aqueous solvent as a supporting electrolyte.

[0020] Note that the non-aqueous electrolyte recovered from the secondary battery may contain almost no additive. In this case, the oxidation product or reduction product of the additive may be contained in the battery. Even in such a case, usually, an additive above the detection limit remains in the non-aqueous electrolyte collected from the secondary battery. Therefore, it can be confirmed that the non-aqueous electrolyte contains an additive.

[0021] (Additive) In this specification, the carbodiimide compound and the sulfur-containing compound are classified as additives. The sulfur-containing compound is a beneficial additive that has the effect of reducing the internal resistance and improving the high-temperature cycle characteristics. However, when a specific tetravalent or hexavalent sulfur-containing compound is used in the non-aqueous electrolyte, the corrosion of the battery can proceeds rapidly. On the other hand, it has been found that when a carbodiimide compound is co-added, sulfate ions or sulfite ions, which are corrosion-promoting components in the non-aqueous electrolyte, are trapped by the carbodiimide compound. Thereby, it is possible to achieve both the suppression of the corrosion of the battery can and the improvement of the high-temperature cycle characteristics.

[0022] The carbodiimide compound has the chemical formula (1):

[0023]

[0024] The structure may be represented by the following: R1 and R2 are each independently hydrocarbon groups, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom or a nitrogen-containing group, etc. The hydrocarbon group may be an aliphatic group or an aromatic group. The hydrocarbon group may be an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, etc. The halogen atom may be a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, etc. The nitrogen-containing group may be an amino group, a cyano group, etc.

[0025] Hydrocarbon groups with low steric hindrance are preferable, for example, C1 to C6 groups. １－６ It may be an alkyl group or a cycloalkyl group. Similarly, the alkenyl group may be a C2-C6 group. ２－６ It may be an alkenyl group or a cycloalkenyl group. The aryl group has 6 to 10 carbon atoms. ６－１０ It may also be an aryl group. In these alkyl groups, alkenyl groups, and aryl groups, at least one hydrogen atom may be substituted with a halogen atom, a nitrogen-containing group, etc.

[0026] Specific examples of hydrocarbon groups R1 and R2 may include methyl group, ethyl group, ethylene group, n-propyl group, isopropyl group, propylene group, n-butyl group, isobutyl group, tert-butyl group, pentyl group, hexyl group, cyclohexyl group, phenyl group, toluyl group, and the like.

[0027] The hydrocarbon groups R1 and R2 may be different from each other, but it is preferable that they be the same group. When the hydrocarbon groups R1 and R2 are the same, the molecular structure has good symmetry, resulting in excellent reactivity, and it is thought that the reaction with sulfate ions or sulfite ions, which are corrosion-promoting components in non-aqueous electrolytes, is promoted.

[0028] Specific examples of carbodiimide compounds that exhibit favorable effects include diisopropylcarbodiimide and dicyclohexylcarbodiimide. It is desirable that the carbodiimide compounds exemplified here constitute 50% or more, and more preferably 80% or more, of the total carbodiimide compound.

[0029] Next, the sulfur-containing compound includes at least one selected from the group consisting of hexavalent sulfur compounds and tetravalent sulfur compounds. A hexavalent sulfur compound contains a sulfur atom with a valence of 6. A tetravalent sulfur compound contains a sulfur atom with a valence of 4. The sulfur-containing compound may be a chain compound or a cyclic compound.

[0030] Hexavalent sulfur compounds have the general formula (2):

[0031]

[0032] The compound may have a structure represented by the formula (2). X1 is a halogen atom or a hydrocarbon group. X2 is a hydrocarbon group, a silyl group, or an alkali metal. At least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom. Among halogen atoms, fluorine (F) is preferred. X1 and X2 may form a ring. That is, the hexavalent sulfur compound may be a cyclic sulfur compound and may have two or more structures represented by general formula (2) in the molecule.

[0033] In general formula (2), if X1 of the hexavalent sulfur compound is a fluorine atom, it is desirable that X2 be an alkyl group, alkenyl group, aryl group, silyl group, or alkali metal. X1 and X2 of sulfur compound (2) may form a ring as alkylene groups, alkenylene groups, ether groups, etc. Also, the hexavalent sulfur compound may contain two hexavalent sulfur atoms. In that case, X1 may be an ether group shared by two sulfur atoms, and X2 may be an alkylene group shared by two sulfur atoms.

[0034] Among hexavalent sulfur compounds, hexavalent sulfur compounds other than cyclic sulfuric acid esters and cyclic sulfonic acid esters with a five-membered ring are preferred, and acyclic sulfonic acid compounds (such as linear sulfonic acid esters, linear sulfonates, linear fluorosulfonic acid esters, linear fluorosulfonates, etc.) or compounds with a six-membered ring structure are preferred. For example, lithium fluorosulfonate (LiFSO) ３At least one selected from the group consisting of methyl fluorosulfonate and 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide (MMDS) is desirable because it is readily available and has a significant effect in reducing internal resistance. The hexavalent sulfur compounds exemplified herein should preferably constitute 50% or more by mass, and more preferably 80% or more by mass, of the hexavalent sulfur compounds.

[0035] Tetravalent sulfur compounds have the general formula (3):

[0036]

[0037] The compound may have a structure represented by the formula shown. X3 and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal. At least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom. Among halogen atoms, fluorine (F) is preferred. X3 and X4 may form a ring. That is, the tetravalent sulfur compound may also be a cyclic sulfur compound.

[0038] In general formula (3), X3 and X4 of the tetravalent sulfur compound may be alkyl groups, alkenyl groups, or aryl groups. X3 and X4 may also form a ring as alkylene groups, alkenylene groups, ether groups, etc. Furthermore, the tetravalent sulfur compound may contain two tetravalent sulfur atoms. In that case, X3 and X4 may be alkylene groups or ether groups shared by two sulfur atoms.

[0039] The tetravalent sulfur compound may also be a sulfite ester having an -O-S(=O)-O- structure. As for sulfite esters, C ２－４ Alkylene sulfites are preferred. Specifically, examples include ethylene sulfite (ES) (ethylene sulfite), propylene sulfite (propylene sulfite), trimethylene sulfite, butylene sulfite, vinylene sulfite, and the like.

[0040] Among tetravalent sulfur compounds, at least one selected from the group consisting of ethylene sulfite (ES) and vinyl ethylene sulfite (VES) is particularly desirable because it is readily available and has a significant effect in reducing internal resistance. The tetravalent sulfur compounds exemplified here should preferably constitute 50% or more, and more preferably 80% or more, of the total tetravalent sulfur compounds.

[0041] Hereinafter, the hexavalent sulfur compounds and tetravalent sulfur compounds described above may be collectively referred to as "sulfur-containing compounds (S)." In sulfur-containing compounds (S), one or more hydrogen atoms of the compounds exemplified above may be substituted with substituents. Examples of substituents include alkyl groups, alkenyl groups such as vinyl groups, hydroxyalkyl groups, hydroxyl groups, alkoxy groups, and halogen atoms. The substituent may have 1 to 3 carbon atoms. Fluorine atoms are preferred as halogen atoms.

[0042] The effect of improving over-discharge characteristics by the carbodiimide compounds described above is particularly pronounced in sulfur-containing compounds (S) other than cyclic sulfuric acid esters and cyclic sulfonic acid esters (for example, cyclic sulfites, linear sulfites, linear sulfonic acid esters, linear sulfonates, etc.).

[0043] Carbodiimide compounds are thought to have the effect of trapping ionic species (such as sulfate ions and sulfite ions) derived from sulfur-containing compounds (S) in non-aqueous electrolytes, and forming a dense film on the inner surface of the battery casing. Such a film is thought to protect the surface of the battery casing and inhibit the reaction between ionic species derived from sulfur-containing compounds (S) and the battery casing.

[0044] Furthermore, it is presumed that the carbodiimide compound and the sulfur-containing compound (S) form a strong hybrid protective coating on both the positive and negative electrodes that does not inhibit lithium ion permeability. Therefore, it is thought that the charge transfer resistance of both the positive and negative electrodes is reduced.

[0045] The hybrid coating can, for example, inhibit excessive reactions between the transition metal elements constituting the positive electrode active material and the non-aqueous electrolyte. This suppresses the degradation of the positive electrode active material. The degradation suppression effect is particularly pronounced when the positive electrode active material contains a lithium-containing composite oxide with a high content of Ni.

[0046] Furthermore, the hybrid coating can suppress excessive reaction between the negative electrode active material (e.g., graphite or silicon-containing material) contained in the negative electrode and the non-aqueous electrolyte. This suppresses the degradation of the negative electrode active material. In particular, the effect of suppressing degradation is significant when the negative electrode active material contains silicon-containing material.

[0047] The mass content of the carbodiimide compound in the non-aqueous electrolyte is, for example, 2% or less, and may be 0.01% to 2.0%, 0.1% to 2.0%, 0.5% to 2.0%, or 0.5% to 1.5%. In this case, a sufficient effect is obtained to suppress corrosion of the battery can by sulfur-containing compounds (S). Furthermore, since a hybrid film is formed appropriately and the charge-discharge reaction proceeds uniformly, it is thought that the effect of suppressing side reactions is enhanced.

[0048] The mass content of sulfur-containing compounds (S) in the non-aqueous electrolyte is, for example, 2% or less, and may be 0.01% to 2.0%, 0.1% to 2.0%, 0.5% to 2.0%, or 0.5% to 1.5%. In this case, a sufficient effect of reducing the internal resistance due to sulfur-containing compounds (S) can be obtained. Furthermore, since a hybrid coating is formed appropriately and the charge-discharge reaction proceeds more uniformly, it is thought that the effect of suppressing side reactions is enhanced.

[0049] When the mass content of the carbodiimide compound is a and the mass content of the sulfur-containing compound (S) is b, for example, the following conditions may be met: 0.5 ≤ b / a ≤ 3.0. More preferably, b / a may satisfy 1.0 ≤ b / a ≤ 3.0, or 1.0 ≤ b / a ≤ 2.0. In this case, it is considered that the carbodiimide compound acts more effectively on the sulfur-containing compound (S). When the b / a ratio is controlled as described above, the carbodiimide compound is considered to be included at a concentration that does not inhibit the film formation of the sulfur-containing compound (S). Therefore, the effect of the sulfur-containing compound (S) can be enhanced. Furthermore, when the b / a ratio is controlled as described above, the carbodiimide compound effectively traps sulfate ions, sulfite ions, etc., released by the sulfur-containing compound (S), thus suppressing corrosion of the battery can.

[0050] The non-aqueous electrolyte may contain other additives besides those mentioned above. Examples of such additives include at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate.

[0051] (Non-aqueous solvents) Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, linear carboxylic acid esters, cyclic ethers, linear ethers, etc. The non-aqueous electrolyte may contain one type of non-aqueous solvent, or a combination of two or more types.

[0052] Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0053] Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0054] Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

[0055] Examples of the chain carboxylic acid ester include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and the like.

[0056] Examples of the cyclic ether include 1,3 - dioxolane, 4 - methyl - 1,3 - dioxolane, tetrahydrofuran, 2 - methyltetrahydrofuran, and the like.

[0057] Examples of the chain ether include 1,2 - dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2 - diethoxyethane, diethylene glycol dimethyl ether, 1,1,2,2 - tetrafluoroethyl - 2,2,2 - trifluoroethyl ether, 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether, and the like.

[0058] (Salt) In a lithium - ion secondary battery, a lithium - metal secondary battery, etc., a lithium salt is used as the salt. For example, LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lithium lower aliphatic carboxylate, LiCl, LiBr, LiI, phosphate, borate, imide salt. Examples of the phosphate include lithium difluorophosphate (LiPO ２ F ２ ), lithium difluoro - bis(oxalato)phosphate (LiDFBOP), lithium tetrafluoro(oxalato)phosphate, and the like. Examples of the borate include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), and the like. Examples of the imide salt include lithium bisfluorosulfonyl imide (LiN(FSO２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. The non-aqueous electrolyte may contain one lithium salt or a combination of two or more.

[0059] The concentration of lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0060] The content of each component in a non-aqueous electrolyte can be determined, for example, using gas chromatography under the following conditions: Measurement device: Shimadzu GC-2010 Plus Column: J&W HP-1 (1 μm x 60 m) Linear velocity: 30.0 cm / sec Inlet temperature: 270°C Detector: FID 290°C (sense 10°C) １ )

[0061] The following describes in detail the other components of the non-aqueous electrolyte secondary battery of this disclosure. [Positive electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer provided on the surface of the positive electrode current collector. The positive electrode current collector is made of a sheet-like conductive material. The positive electrode mixture layer is supported on one or both surfaces of the positive electrode current collector. The positive electrode mixture layer is usually a layer or film made of a positive electrode mixture. The thickness of the positive electrode mixture layer is, for example, 10 μm to 150 μm per side of the positive electrode current collector. The positive electrode mixture contains a positive electrode active material as an essential component.

[0062] The positive electrode mixture layer may contain a conductive agent as an optional component. Examples of conductive agents include carbon-based materials such as carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), graphene, and graphite. These may be used individually or in combination of two or more types.

[0063] The positive electrode mixture layer may contain a binder. Examples of binders include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These may be used individually or in combination of two or more types.

[0064] As the positive electrode current collector, non-porous conductive substrates (such as metal foil) or porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the positive electrode current collector include aluminum, aluminum alloys, titanium, and titanium alloys.

[0065] A lithium-containing composite oxide can be used as the positive electrode active material. The lithium-containing composite oxide may have a layered rock salt structure. The layered rock salt structure may belong to, for example, space group R-3m or space group C2 / m. Among these, the layered rock salt structure belonging to space group R-3m is preferred because it has high capacity and high crystal structure stability. The layered rock salt structure of the lithium-containing composite oxide may include a transition metal layer, a Li layer, and an oxygen layer.

[0066] From the viewpoint of increasing capacity, the proportion of Ni among the metal elements other than Li in the lithium-containing composite oxide (Ni content) may be 50 atomic percent or more, 80 atomic percent or more, or 90 atomic percent or more.

[0067] From the viewpoint of stabilizing the crystal structure of the lithium-containing composite oxide and improving the heat resistance of the battery, the proportion of Co (Co content) among the metal elements other than Li contained in the lithium-containing composite oxide may be set to 0 atomic% or more and 16 atomic% or less, or to 1.5 atomic% or more and 16 atomic% or less.

[0068] Similarly, from the viewpoint of stabilizing the crystal structure of the lithium-containing composite oxide and improving the heat resistance of the battery, the proportion of Al among the metal elements other than Li contained in the lithium-containing composite oxide (Al content) may be set to 0 atomic% or more and 18.5 atomic% or less, or to 4 atomic% or more and 10 atomic% or less.

[0069] From the standpoint of cost reduction, the proportion of Mn among metal elements other than Li in the lithium-containing composite oxide (Mn content) may be set to 0 atomic% or more and 50 atomic% or less, or to 0 atomic% or more and 30 atomic% or less.

[0070] The content of each metal element in lithium-containing composite oxides is measured, for example, by inductively coupled plasma (ICP) emission spectroscopy.

[0071] Lithium-containing composite oxides include, for example, those with the general formula LiaNi ｘ Co ｙ Al ｚ M1 ｗ O ２－ｂ It may also be a composite oxide represented by the formula (wherein 0.8 ≤ a ≤ 1.2, 0.5 ≤ x ≤ 0.95, 0 ≤ y ≤ 0.16, 0 ≤ z ≤ 0.185, 0 ≤ w ≤ 0.5, 0 ≤ b < 0.05, x + y + z + w = ​​1, and M1 is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, Sr, W, B, Ca, and Zn). In this case, M1 is preferably Mn.

[0072] [Negative Electrode] The negative electrode comprises a negative electrode current collector and may have a negative electrode mixture layer provided on the surface of the negative electrode current collector. The negative electrode current collector is made of a sheet-like conductive material. The negative electrode mixture layer is supported on one or both surfaces of the negative electrode current collector. The negative electrode mixture layer is usually a layer or film made of negative electrode mixture. The thickness of the negative electrode mixture layer is, for example, 10 μm to 150 μm per side of the negative electrode current collector. The negative electrode mixture contains a negative electrode active material as an essential component and may contain binders, conductive agents, thickeners, etc. as optional components. Known materials can be used as binders, conductive agents, and thickeners.

[0073] The negative electrode active material includes materials that electrochemically intercept and release lithium ions, lithium metals, lithium alloys, etc. Examples of electrochemically intercept and release lithium ions include carbon materials and alloy materials.

[0074] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Among these, graphite is preferred because it has excellent charge-discharge stability and low irreversible capacity.

[0075] Graphite is a carbonaceous material in which a graphite-type crystal structure is well-developed. The interplanar spacing d002 of the (002) planes of graphite, measured by X-ray diffraction, may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. The crystallite size Lc(002) of graphite may be, for example, 5 nm or more, or 5 nm or more and 200 nm or less. The crystallite size Lc(002) is measured, for example, by the Scherrer method. When the interplanar spacing d002 and crystallite size Lc(002) of the (002) planes of graphite are within the above ranges, high capacity is easily obtained.

[0076] Alloy materials are materials that contain at least one metal capable of forming an alloy with lithium. Examples of such materials include silicon, tin, silicon alloys, tin alloys, silicon oxide, tin oxide, and silicon-containing materials.

[0077] The silicon-containing material includes, for example, a lithium-ion conductive phase and a silicon phase dispersed in the lithium-ion conductive phase. Examples of the lithium-ion conductive phase include a silicon oxide phase, a silicate phase, a carbon phase, etc. The content of the silicon phase dispersed in the lithium-ion conductive phase is, for example, 30% by mass or more and 95% by mass or less, and may be 35% by mass or more and 75% by mass or less. The silicon-containing material may be used alone or in combination of two or more types.

[0078] The main component of the silicon oxide phase (e.g., 95-100% by mass) may be silicon dioxide. A silicon-containing material comprising a silicon oxide phase and a silicon phase dispersed in the silicon oxide phase is SiO ｘIt is expressed as follows: x may be, for example, 0.5 ≤ x < 2, and 0.8 ≤ x ≤ 1.6. The silicon oxide phase may be an amorphous phase. SiO ｘ This can be obtained, for example, by the disproportionation reaction of silicon monoxide.

[0079] The silicate phase is preferred because it has low irreversible capacity. In particular, a lithium-containing silicate phase (hereinafter also referred to as the lithium silicate phase) can be preferably used as a lithium-ion conductive phase with high initial charge-discharge efficiency.

[0080] The lithium silicate phase may be any oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may also contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase is given by the formula: Li ２ｚ SiO ２＋ｚ The composition may be represented by (0 < z < 2). Preferably, z satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2. Examples of elements other than Li, Si, and O that may be included in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0081] The carbon phase may consist, for example, of amorphous carbon with low crystallinity. Amorphous carbon may be hard carbon, soft carbon, or something else.

[0082] Silicon-containing materials, in which a silicon phase is dispersed within a carbon phase, can be obtained, for example, by grinding a mixture of a carbon source and raw silicon while stirring in a ball mill or the like to produce fine particles, and then heat-treating the mixture in an inert atmosphere. As the carbon source, for example, sugars such as carboxymethylcellulose (CMC) or water-soluble resins such as polyvinylpyrrolidone can be used.

[0083] A silicon-containing material and a carbon material may be used in combination as the negative electrode active material. Since the volume of the silicon-containing material expands and contracts with charging and discharging, if its proportion in the negative electrode active material is large, poor contact between the negative electrode active material and the negative electrode current collector is likely to occur with charging and discharging. On the other hand, by using a silicon-containing material and a carbon material in combination, it becomes possible to achieve excellent cycle characteristics while imparting high capacity to the negative electrode.

[0084] The proportion of silicon-containing material in the total of silicon-containing material and carbon material is preferably 0.5 to 15% by mass, and more preferably 1 to 10% by mass. This makes it easier to achieve both high capacity and improved cycle characteristics.

[0085] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) or porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0086] The composition of silicon-containing materials can be determined, for example, by obtaining a backscattered electron image of the cross-section of the negative electrode mixture layer using a field emission scanning electron microscope (FE-SEM), observing the silicon-containing material particles, and performing elemental analysis on the observed silicon-containing material particles. Elemental analysis can be performed using methods such as electron probe microanalyzer (EPMA).

[0087] The negative electrode mixture layer may contain a binder. Examples of binders include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resins), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)). A single binder may be used, or two or more may be used in combination.

[0088] The negative electrode mixture layer may contain a thickening agent. Examples of thickening agents include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethylcellulose (CMC) and its modified forms, and methylcellulose. Modified forms of CMC also include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts. A single thickening agent may be used alone, or two or more may be used in combination.

[0089] The negative electrode mixture layer may contain a conductive agent. Examples of conductive agents include carbon nanotubes (CNTs) and conductive particles. Examples of conductive particles include conductive carbon (such as carbon black) and metal powders. One conductive agent may be used alone, or two or more may be used in combination.

[0090] The negative electrode current collector is selected according to the type of non-aqueous electrolyte secondary battery. Examples of negative electrode current collectors include sheet-like materials. Metal foil may also be used as the current collector. Alternatively, a porous material may be used as the current collector. Examples of porous current collectors include mesh-like materials, perforated sheets, and expanded metal.

[0091] Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0092] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and appropriate mechanical strength and insulating properties. As the separator, for example, a microporous thin film, a woven fabric, or a nonwoven fabric, or a laminate of at least two selected from these can be used. Polyolefins (e.g., polypropylene, polyethylene) are preferred as the material of the separator.

[0093] One example of the structure of a non-aqueous electrolyte secondary battery is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, is housed together with a non-aqueous electrolyte in an outer casing such as a battery case. However, it is not limited to this, and other forms of electrode groups may be used. For example, the electrode group may be a stacked type in which the positive electrode and negative electrode are stacked with a separator in between.

[0094] A typical battery case is a bottomed cylindrical shape, but is not limited to this. A bottomed cylindrical battery case comprises a bottom and cylindrical sides rising from the bottom. The end opposite the bottom is open. The material of the battery case may be iron (Fe), Fe alloy, stainless steel, etc. It is preferable that a nickel plating layer is formed on the inner surface of the battery case.

[0095] Furthermore, the casing is not limited to a battery casing. The form of the non-aqueous electrolyte secondary battery is also not limited. Non-aqueous electrolyte secondary batteries may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0096] The structure of a non-aqueous electrolyte secondary battery will be described below with reference to Figure 1. Figure 1 is a longitudinal cross-sectional view of a cylindrical secondary battery, which is an example of this embodiment. However, this disclosure is not limited to the following configuration.

[0097] The non-aqueous electrolyte secondary battery (hereinafter referred to as battery 10) comprises an electrode group 18, a non-aqueous electrolyte, and a bottomed cylindrical battery case 22 that houses these. The battery case 22 is made of iron, stainless steel, or the like. The inner surface of the battery case 22 may be nickel plated or the like. A sealing body 11 is crimped and fixed to the opening of the battery case 22 via a gasket 21. This seals the inside of the battery. The sealing body 11 comprises a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. The positive electrode lead 15a, which is led out from the positive electrode 15, is connected to the metal plate 13. Therefore, the valve body 12 functions as an external terminal of the positive electrode. The negative electrode lead 16a, which is led out from the negative electrode 16, is connected to the bottom inner surface of the battery case 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is positioned between one end face of the electrode group 18 and the annular groove 22a. A second insulating plate 24 is positioned between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode 15 and a negative electrode 16 with a separator 17 in between.

[0098] (Note) The following technologies are disclosed in accordance with the above description. (Technology 1) A non-aqueous electrolyte for a secondary battery comprising a non-aqueous solvent, a salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound and a sulfur-containing compound, and the sulfur-containing compound comprises at least one selected from the group consisting of hexavalent sulfur compounds and tetravalent sulfur compounds. (Technology 2) The carbodiimide compound has chemical formula (1):

[0099]

[0100] A non-aqueous electrolyte for a secondary battery according to Technical 1, having a structure represented by, where R1 and R2 are each independently hydrocarbon groups, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom or a nitrogen-containing group. (Technical 3) The hexavalent sulfur compound is general formula (2):

[0101]

[0102] The tetravalent sulfur compound has a structure represented by the following formula: X1 is a halogen atom or a hydrocarbon group, X2 is a hydrocarbon group, a silyl group or an alkali metal, at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, and X1 and X2 may form a ring, and the tetravalent sulfur compound is general formula (3):

[0103]

[0104] (Technology 4) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 3, having a structure represented by, where X3 and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, and X3 and X4 may form a ring. (Technology 5) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 4, having a structure represented by, where X3 and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, and X3 and X4 may form a ring. (Technical 7) A non-aqueous electrolyte for a secondary battery according to any one of Technical 1 to 6, wherein the hexavalent sulfur compound comprises at least one selected from the group consisting of lithium fluorosulfonate, methyl fluorosulfonate, and 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide. (Technical 8) A non-aqueous electrolyte for a secondary battery according to any one of Technical 1 to 7, wherein the tetravalent sulfur compound comprises at least one selected from the group consisting of ethylene sulfite and vinyl ethylene sulfite. (Technical 9) A secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, a non-aqueous electrolyte according to any one of Technical 1 to 8, and a battery case, wherein the battery case is made of a metal containing at least iron, and the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte are housed in the battery case. (Technical 10) A non-aqueous solvent, a salt that dissolves in the non-aqueous solvent, and an additive that dissolves in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound and a sulfur-containing compound, and the carbodiimide compound has chemical formula (1):

[0105]

[0106] The compound has a structure represented by the formula (2): R1 and R2 are each independently hydrocarbon groups, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom or a nitrogen-containing group, and the sulfur-containing compound comprises at least one selected from the group consisting of hexavalent sulfur compounds and tetravalent sulfur compounds, and the hexavalent sulfur compound has a general formula (2):

[0107]

[0108] The tetravalent sulfur compound has a structure represented by (excluding cyclic sulfuric acid esters and cyclic sulfonic acid esters of a five-membered ring), where X1 is a halogen atom or a hydrocarbon group, and X2 is a hydrocarbon group, a silyl group or an alkali metal, where at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, and where X1 and X2 may form a ring, and the tetravalent sulfur compound has a general formula (3):

[0109]

[0110] A non-aqueous electrolyte for a secondary battery having a structure represented by the formula, wherein X3 and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, and X3 and X4 may form a ring.

[0111] [Examples] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0112] Examples 1-7 and Comparative Examples 1-6: Non-aqueous electrolyte secondary batteries were prepared and evaluated according to the following procedure.

[0113] (1) Preparation of the positive electrode An appropriate amount of N-methyl-2-pyrosidone (NMP) was added to the positive electrode mixture and mixed to obtain a positive electrode slurry. The positive electrode mixture was a lithium-containing composite oxide (LiNi ０．８ Co ０．１８ Al ０．０２ O ２) was obtained by adding 2.5 parts by mass of acetylene black and 2.5 parts by mass of polyvinylidene fluoride to 95 parts by mass of ) the cathode slurry. Next, the cathode slurry was applied to the surface of the aluminum foil, the coating was dried, and then it was rolled to create a cathode slurry layer (thickness 95 μm, density 3.6 g / cm³) on both sides of the aluminum foil. ３ A positive electrode was obtained by forming a positive electrode.

[0114] (2) Preparation of the negative electrode An appropriate amount of water was added to the negative electrode mixture and mixed to obtain a negative electrode slurry. The negative electrode mixture was a mixture of a negative electrode active material, a binder, and a conductive agent. The negative electrode active material was a mixture of a silicon-containing material and graphite (average particle size (D50) 25 μm). The silicon-containing material was SiO2 whose surface was coated with a conductive layer of conductive carbon. ｘ Particles (x=1, average particle size (D50) 5 μm) were used. In the negative electrode active material, the mass ratio of silicon-containing material excluding the conductive layer to graphite was 6:94. Sodium polyacrylate (PAA-Na), sodium salt of CMC (CMC-Na), and SBR were used as binders. The content of PAA-Na, CMC-Na, and SBR in the negative electrode mixture was 1% by mass each. Next, the negative electrode slurry was applied to the surface of the copper foil, the coating was dried, and then rolled to form negative electrode mixture layers (thickness 80 μm, density 1.6 g / cm³) on both sides of the copper foil. ３ A negative electrode was obtained by forming a negative electrode.

[0115] (3) Preparation of non-aqueous electrolyte A mixed solvent of ethylene carbonate (EC) and diethyl carbonate (EMC) (EC:EMC = 20:80 (volume ratio)) is mixed with LiPF ６ Non-aqueous electrolytes were prepared by dissolving the additives shown in Table 1 as needed. LiPF in non-aqueous electrolyte ６ The concentration was set to 1.20 mol / L. The concentration of the additive in the non-aqueous electrolyte (initial concentration) was set to the value (mass%) shown in Table 1.

[0116] (4) Fabrication of a non-aqueous electrolyte secondary battery An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. In an inert gas atmosphere, the positive electrode and negative electrode were wound in a spiral shape via a polyethylene thin film (separator) to fabricate a wound electrode group. A first insulating plate was placed on the lower end surface of the electrode group, the electrode group was inserted into a battery can, and the negative electrode lead was resistance welded to the bottom of the battery can. An iron can with nickel plating on the inner surface was used as the battery can. After placing a second insulating plate on the upper end surface of the electrode group, an annular groove was formed near the open end of the battery can. Next, the positive electrode lead was connected to the metal plate of the safety mechanism provided by the sealing body, the non-aqueous electrolyte was injected into the battery can, and the battery can was supported in the annular groove formed in the battery can via a gasket, and the open end of the battery can was crimped to the periphery of the sealing body to complete the lithium-ion secondary battery.

[0117] [Evaluation 1] Over-discharge characteristics A 1 kΩ resistor was connected between the positive and negative electrodes of each battery, and the batteries were stored in a constant temperature bath at 60°C for 30 days while remaining discharged. After that, the batteries were disassembled, and the content of element Fe in the non-aqueous electrolyte (by mass) was quantified by ICP (Inductively Coupled Plasma) emission spectrometry. The relative value (Index) was determined when the content of element Fe in the non-aqueous electrolyte in Comparative Example 1 was set to 100.

[0118] [Evaluation 2] Initial Resistance At a temperature of 25°C, each battery was charged with a constant current of 0.3 It until the voltage reached 4.2V, and then charged with a constant voltage of 4.2V until the current reached 0.05 It. Next, the batteries were discharged with a constant current of 0.3 It for 100 minutes to bring the State of Charge (SOC) to 50%. For batteries with an SOC of 50%, the voltage values ​​were measured when they were discharged for 10 seconds at current values ​​of 0A, 0.1A, 0.5A, and 1.0A. The DCIR (initial DCIR) was calculated from the absolute value of the slope when the relationship between the discharge current value and the voltage value after 10 seconds was approximated by a straight line using the least squares method. The relative value (Index) was determined when the initial resistance in Comparative Example 1 was set to 100.

[0119] The results of the examples and comparative examples are shown in Table 1. In Table 1, batteries E1 to E7 are from Examples 1 to 7, and batteries C1 to C6 are from Comparative Examples 1 to 6.

[0120] The additives in the table are indicated as follows: DIC: Diisopropylcarbodiimide DCC: Dicyclohexylcarbodiimide

[0121] LiFSO ３ : Lithium fluorosulfonate ES: Ethylene sulfite PRS: 1-propene-1,3-sultone MMDS: 1,5,2,4-dioxadithian-2,2,4,4-tetraoxide

[0122]

[0123] Table 1 shows that when the non-aqueous electrolyte contains a sulfur-containing compound (S), the initial resistance tends to decrease, except when PRS is used (comparison of batteries C1-C4 and C6). However, batteries C2-C4, in which the non-aqueous electrolyte contains a sulfur-containing compound (S), have a very large amount of Fe leaching from the battery casing during over-discharge, resulting in insufficient over-discharge characteristics.

[0124] On the other hand, when the non-aqueous electrolyte contains a sulfur-containing compound (S) and a carbodiimide compound, the leaching of Fe element from the battery can during over-discharge is significantly suppressed (compared to batteries E1 to E6). Furthermore, a comparison of batteries E5 and E6 shows that the initial resistance reduction effect is greater when the b / a ratio is 1 or greater. In battery E6, the sulfur-containing compound (S) (LiFSO4) ３ In this case, the b / a ratio is favorable, and the amount of carbodiimide compound (DIC) is relatively high, which may partially offset the effect of sulfur-containing compound (S) in reducing initial resistance. On the other hand, in the case of battery E5, the b / a ratio is favorable, the dissolution of Fe element is sufficiently suppressed, and a significant effect of reducing initial resistance is obtained.

[0125] Furthermore, when using 1-propene-1,3-sultone (PRS), a cyclic sulfonic acid ester (Battery E7), although the amount of Fe elution is relatively suppressed, the effect of reducing the initial resistance is not obtained, and on the contrary, there is a tendency for the initial resistance to increase.

[0126] The secondary battery comprising a non-aqueous electrolyte as described herein is useful as a main power source for mobile communication devices, portable electronic devices, etc. However, the applications of the non-aqueous electrolyte secondary battery are not limited to these.

[0127] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0128] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating material, 15: Positive electrode, 15a: Positive electrode lead, 16: Negative electrode, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove, 23: First insulating plate, 24: Second insulating plate, 16: Negative electrode

Claims

1. A non-aqueous electrolyte for a secondary battery, comprising a non-aqueous solvent, a salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound and a sulfur-containing compound, and the sulfur-containing compound comprises at least one selected from the group consisting of hexavalent sulfur compounds and tetravalent sulfur compounds.

2. The carbodiimide compound has the chemical formula (1): The non-aqueous electrolyte for a secondary battery according to claim 1, having a structure represented by R1 and R2, wherein R1 and R2 are each independently hydrocarbon groups, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom.

3. The hexavalent sulfur compound is defined by general formula (2): The tetravalent sulfur compound has a structure represented by the following formula: X1 is a halogen atom or a hydrocarbon group, X2 is a hydrocarbon group, a silyl group or an alkali metal, at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom or a nitrogen-containing group, and X1 and X2 may form a ring, and the tetravalent sulfur compound is general formula (3): The non-aqueous electrolyte for a secondary battery according to claim 1, having a structure represented by, where X3 and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, and X3 and X4 may form a ring.

4. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the content of the carbodiimide compound is 0.1% by mass or more and 2% by mass or less.

5. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the content of the sulfur-containing compound is 0.1% by mass or more and 2% by mass or less.

6. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the mass content of the carbodiimide compound is a and the mass content of the sulfur-containing compound is b, and the condition 0.5 ≤ b / a ≤ 3 is satisfied.

7. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the hexavalent sulfur compound comprises at least one selected from the group consisting of lithium fluorosulfonate, methyl fluorosulfonate, and 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide.

8. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the tetravalent sulfur compound comprises at least one selected from the group consisting of ethylene sulfite and vinyl ethylene sulfite.

9. A secondary battery comprising: a positive electrode; a separator; a negative electrode facing the positive electrode via the separator; a non-aqueous electrolyte as described in claim 1; and a battery casing, wherein the battery casing is made of a metal containing at least iron, and the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte are housed in the battery casing.

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