Nonaqueous electrolyte for secondary battery, and secondary battery

Abstract

The disclosed nonaqueous electrolyte for a secondary battery contains a cyclic carboxylic acid anhydride and a sulfur-containing compound, the sulfur-containing compound contains at least one compound selected from the group consisting of hexavalent and tetravalent sulfur compounds, the cyclic carboxylic acid anhydride is represented by general formula (1), the hexavalent sulfur compound has a structure represented by general formula (2), the tetravalent sulfur compound has a structure represented by general formula (3), and the tetravalent sulfur compound has a structure represented by general formula (4). (In the formula, R1 to R4 are each independently a hydrogen atom, a fluorine atom, or a hydrocarbon group, X1 is a fluorine atom, a hydrocarbon group, or an oxyhydrocarbon group, X2, X3, and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal, and at least one hydrogen atom in the hydrocarbon group may be substituted by a halogen atom. # imgabs0 #

Description

[0001] Cross - reference to related applications

[0002] This disclosure claims the benefit of the priority of Japanese Patent Application No. 2022 - 187291, filed with the Japan Patent Office on November 24, 2022, and incorporates by reference the entire contents of the foregoing patent application into this specification. Technical field

[0003] This disclosure relates to a non - aqueous electrolyte for a secondary battery and a secondary battery. Background art

[0004] Patent Document 1 proposed a non - aqueous electrolyte solution containing an unsaturated sultone having a specified structure, a non - aqueous solvent, and an electrolyte, and the addition amount of the unsaturated sultone was 0.001 to 10% by mass relative to the whole non - aqueous electrolyte solution.

[0005] Prior art documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent No. 4190162 Gazette Summary of the invention

[0008] Problems to be Solved by the Invention

[0009] Patent Document 1 reported that: by using an unsaturated sultone, the reduction decomposition of the non - aqueous electrolyte solution during high - temperature storage can be significantly suppressed. On the other hand, in the case where the non - aqueous electrolyte contains a sulfur - containing compound, when the secondary battery reaches an over - discharged state, there is a drawback that the corrosion of the battery can progresses rapidly. It is speculated that in the battery in the over - discharged state, the sulfur - containing compound generates sulfate ions or sulfite ions, and these ions corrode the battery can.

[0010] Solutions for Solving the Problems

[0011] One aspect of this disclosure relates to a non - aqueous electrolyte for a secondary battery, which comprises: a non - aqueous solvent, a salt dissolved in the foregoing non - aqueous solvent, and an additive dissolved in the foregoing non - aqueous solvent, the foregoing additive includes a cyclic carboxylic anhydride and a sulfur - containing compound, the foregoing sulfur - containing compound includes at least 1 kind selected from the group consisting of a hexavalent sulfur compound and a tetravalent sulfur compound, and the foregoing cyclic carboxylic anhydride is represented by the general formula (1),

[0012] [Chemical formula 1]

[0013]

[0014] The foregoing hexavalent sulfur compound has a structure represented by the general formula (2),

[0015] [Chemical formula 2]

[0016]

[0017] The tetravalent sulfur compound described above has a structure represented by the general formula (3).

[0018] [Chemical formula 3]

[0019]

[0020] R1 to R4 are each independently a hydrogen atom, a fluorine atom, or a hydrocarbon group, X1 is a fluorine atom, a hydrocarbon group, or a hydrocarbyloxy group, X2, X3, and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal, at least one hydrogen atom of the aforementioned hydrocarbon group is optionally substituted by a halogen atom, X1 and X2 optionally form a ring, and X3 and X4 optionally form a ring.

[0021] Another aspect of the present disclosure relates to a secondary battery including: a positive electrode; a separator; a negative electrode facing the positive electrode with the separator interposed therebetween; a non-aqueous electrolyte; and a battery can housing the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte, wherein the non-aqueous electrolyte is the non-aqueous electrolyte for the secondary battery described above.

[0022] Effects of the Invention

[0023] According to the present disclosure, even when the non-aqueous electrolyte contains a sulfur-containing compound, corrosion of the battery can during over-discharge of the secondary battery can be suppressed.

[0024] The novel features of the present invention are described in the appended claims, but the present invention relates to both the constitution and the content, and together with other objects and features of the present invention, should be better understood by the following detailed description with reference to the drawings. Description of the Drawings

[0025] Figure 1 is a longitudinal sectional view of a secondary battery according to an embodiment of the present disclosure. Detailed Description

[0026] Hereinafter, examples will be given to illustrate the embodiments of the present disclosure, 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 also be applicable as long as the effects of the present disclosure can be obtained. It should be noted that components other than the characteristic parts of the present disclosure may also be applicable to the components of known secondary batteries. In this specification, in the case of "the range of numerical value A to numerical value B", this range includes numerical value A and numerical value B. For example, in the case of "A to B mol%", it is synonymous with "not less than A mol% and not more than B mol%". In the following description, when the lower limit and the upper limit of numerical values related to specific physical properties, conditions, etc. are exemplified, as long as the lower limit is not greater than or equal to the upper limit, any one of the exemplified lower limits and any one of the exemplified upper limits can be arbitrarily combined. When multiple materials are exemplified, one of them can be selected and used alone, or two or more of them can be combined and used.

[0027] In addition, the present disclosure includes combinations of matters recited in two or more claims arbitrarily selected from a plurality of claims recited in the appended claims. That is, as long as there is no technical contradiction, matters recited in two or more claims arbitrarily selected from a plurality of claims recited in the appended claims can be combined.

[0028] Non-aqueous electrolyte secondary batteries include: lithium ion secondary batteries that use at least a material that reversibly stores and releases lithium ions as a negative electrode active material; lithium secondary batteries in which lithium metal precipitates in the negative electrode during charging and lithium metal dissolves during discharging; solid batteries containing a gel electrolyte, etc.

[0029] The non-aqueous electrolyte secondary battery of the present disclosure includes: a positive electrode, a negative electrode, a non-aqueous electrolyte, and a battery can for housing these. A separator is usually disposed between the positive electrode and the negative electrode. The non-aqueous electrolyte usually has lithium ion conductivity.

[0030] It should be noted that in this specification, the "over-discharge characteristics" can be evaluated by the content rate of the eluted components eluted from the battery can in the non-aqueous electrolyte in the battery in an over-discharge state formed by short-circuiting the positive electrode and the negative electrode. In addition, the "high-temperature cycle characteristics" can be evaluated by the capacity retention rate during constant current constant voltage charging (CCCV charging) of the secondary battery at a specified number of cycles in a 45°C environment.

[0031] [Non-aqueous electrolyte]

[0032] The non-aqueous electrolyte contains: a non-aqueous solvent, a salt, and an additive. The non-aqueous electrolyte containing the non-aqueous solvent is usually a liquid electrolyte, but it can also be in a state where its fluidity is restricted using 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, but as long as the effects of the invention are not significantly impaired, a part of the salt or the additive may not be dissolved but may precipitate or separate. The additive defined by the general formula described later may also be a salt. In this case, other salts are dissolved in the non-aqueous solvent as a supporting electrolyte.

[0033] It should be noted that the non-aqueous electrolyte recovered from the secondary battery may also contain almost no additive. In this case, the oxidation product or reduction product of the additive is contained in the form of a film-forming component on the surface of the positive electrode or the surface of the negative electrode. Even in such a case, the non-aqueous electrolyte collected from the secondary battery usually has an additive remaining above the detection limit, so it can be confirmed that the non-aqueous electrolyte contains an additive.

[0034] (Additive)

[0035] In the present specification, cyclic carboxylic anhydrides and sulfur-containing compounds are classified as additives. The sulfur-containing compound contains at least one selected from the group consisting of hexavalent sulfur compounds and tetravalent sulfur compounds.

[0036] The cyclic carboxylic anhydride is represented by the general formula (1).

[0037] [Chemical formula 4]

[0038]

[0039] R1 to R4 are each independently a hydrogen atom, a fluorine atom, or a hydrocarbon group. At least one hydrogen atom of the hydrocarbon group is optionally substituted by a halogen atom.

[0040] The hexavalent sulfur compound has a structure represented by the general formula (2).

[0041] [Chemical formula 5]

[0042]

[0043] X1 is a fluorine atom, a hydrocarbon group, or an oxyhydrocarbon group, and X2 is a hydrocarbon group, a silyl group, or an alkali metal. X1 and X2 optionally form a ring. That is, the hexavalent sulfur compound can be a cyclic sulfur compound. At least one hydrogen atom of the hydrocarbon group is optionally substituted by a halogen atom.

[0044] The tetravalent sulfur compound has a structure represented by the general formula (3).

[0045] [Chemical formula 6]

[0046]

[0047] X3 and X4 are each independently a hydrocarbon group, a silyl group, or an alkali metal. X3 and X4 may optionally form a ring. That is, the tetravalent sulfur compound may be a cyclic sulfur compound. At least one hydrogen atom of the hydrocarbon group is optionally substituted with a halogen atom.

[0048] Among acid anhydrides, the cyclic carboxylic anhydride represented by the general formula (1) (hereinafter also referred to as "cyclic carboxylic anhydride (1)") does not have an effect of inhibiting battery can corrosion when used alone.

[0049] On the other hand, when at least one selected from the group consisting of the hexavalent sulfur compound represented by the general formula (2) (hereinafter also referred to as "sulfur compound (2)") and the tetravalent sulfur compound represented by the general formula (3) (hereinafter also referred to as "sulfur compound (3)") is used together with the cyclic carboxylic anhydride (1), an effect of inhibiting battery can corrosion can be exhibited. It is considered that the decomposition product of the cyclic carboxylic anhydride has a role of protecting the surface of the battery can and can hinder the reaction between the ionic species derived from the sulfur-containing compound and the battery can.

[0050] In addition, when at least one selected from the group consisting of sulfur compound (2) and sulfur compound (3) is used together with cyclic carboxylic anhydride (1), the cycle characteristics are improved. It is considered that this is because when cyclic carboxylic anhydride (1) is used together with sulfur compound (2) and / or (3), a strong and lithium-ion-permeable mixed coating film is formed as a protective coating film on both the positive electrode and the negative electrode. The mixed coating film has the effect of highly suppressing side reactions to improve the high-temperature cycle characteristics and reducing the charge transfer resistance of the positive electrode and the negative electrode. By reducing the charge transfer resistance, the resistance of the secondary battery is significantly reduced.

[0051] It is speculated that the coating film components derived from cyclic carboxylic anhydride (1) and sulfur compounds (2) and (3) constituting the mixed coating film have the following effects: hindering the excessive reaction between the transition metal element of the positive electrode active material contained in the positive electrode and the electrolyte and suppressing the deterioration of the positive electrode active material. In particular, when the positive electrode active material contains a Ni-containing lithium composite oxide at a high content rate, the effect of suppressing deterioration is significant.

[0052] In addition, it is speculated that the coating film components derived from cyclic carboxylic anhydride (1) and sulfur compounds (2) and (3) constituting the mixed coating film have the following effects: suppressing the excessive reaction between the negative electrode active material (such as graphite or a silicon-containing material) contained in the negative electrode and the electrolyte, thereby suppressing the deterioration of the negative electrode active material. In particular, when the negative electrode active material contains a silicon-containing material, the effect of suppressing deterioration is significant.

[0053] When one or more of R1 to R4 and X1 to X4 are aliphatic hydrocarbon groups or aliphatic oxyhydrocarbon groups (such as alkoxy groups), those with small steric hindrance are preferred. For example, it can be C with 1 to 5 carbon atoms 1-5A hydrocarbyl group or an oxyhydrocarbyl group (such as an alkoxy group). When one or more of R1 to R4 and X1 to X4 are aromatic hydrocarbyl groups (aryl groups), there may be one aromatic ring. When X1 and X2 or X3 and X4 form a ring, the ring can be a 5-membered ring, a 6-membered ring or a 7-membered ring. When one or more of X2 to X4 are alkali metals, the alkali metal can be Li, K, Na, etc. It should be noted that when X1 to X4 form a ring, X1 and X2 or X3 and X4 combine to form, for example, an alkylene group, an alkenylene group, an ether group, etc.

[0054] In cyclic carboxylic anhydride (1), all of R1 to R4 can be hydrogen atoms, or one or more of them can be fluorine atoms and the rest can be hydrogen atoms. In addition, at least one of R1 to R4 can be an alkyl group, an alkenyl group or an aryl group, and the rest can independently be hydrogen atoms or fluorine atoms. The alkyl group preferably has a small steric hindrance. For example, it can be a C alkyl group having 1 to 5 carbon atoms. 1-5 alkyl group. Similarly, the alkenyl group can be a C alkenyl group having 2 to 5 carbon atoms. 2-5 alkenyl group. Such hydrocarbyl groups can be, for example, methyl group, ethyl group, ethylene group, propyl group, propylene group, etc.

[0055] In cyclic carboxylic anhydride (1), one or more hydrogen atoms of the above-exemplified compounds are optionally substituted by substituents. Examples of the substituents include: an alkyl group, a hydroxyalkyl group, a hydroxy group, an alkoxy group, a halogen atom, etc. The carbon number of the substituents can be 1 to 3. As the halogen atom, a fluorine atom is preferred.

[0056] As a representative example of the cyclic carboxylic anhydride, diglycol anhydride can be cited. Diglycol anhydride preferably accounts for 50% by mass or more, more preferably 80% by mass or more, of cyclic carboxylic anhydride (1).

[0057] The additive may also contain an acid anhydride other than cyclic carboxylic anhydride (1) (hereinafter also referred to as "acid anhydride (2)"). However, acid anhydride (2) preferably accounts for less than 50% by mass, more preferably less than 30% by mass, of the total amount of acid anhydrides. Examples of acid anhydride (2) include: maleic anhydride, succinic anhydride, acetic anhydride, phthalic anhydride, benzoic anhydride, etc. Among them, maleic anhydride, succinic anhydride, etc. are preferred.

[0058] When X1 of sulfur compound (2) is a fluorine atom, X2 is preferably an alkyl group, an alkenyl group, an aryl group, a silyl group or an alkali metal. X1 and X2 of sulfur compound (2) can form an alkylene group, an alkenylene group, an ether group, etc. to form a ring. In addition, sulfur compound (2) can contain two hexavalent sulfur atoms. In this case, X1 can be an ether group sharing two sulfur atoms, and X2 can be an alkylene group sharing two sulfur atoms.

[0059] X3 and X4 of the sulfur compound (3) can be alkyl, alkenyl or aryl. X3 and X4 can form an alkylene group, an alkenylene group, an ether group, etc. to form a ring. In addition, the sulfur compound (3) can contain two tetravalent sulfur atoms. In this case, X3 and X4 can be an alkylene group or an ether group sharing two sulfur atoms.

[0060] It should be noted that the sulfur compound (2) includes: sulfate esters having an -O-S(=O)2-O- structure, and sulfonate esters having an -S(=O)2-O- structure. The sulfur compound (3) is a sulfite ester having an -O-S(=O)-O- structure. Therefore, the sulfur-containing compound can also be said to be at least one selected from the group consisting of sulfate esters, sulfite esters and sulfonate esters.

[0061] As the sulfate ester, preferably a C 2-4 alkyl sulfate ester. Specifically, examples include: ethylene sulfite, propylene sulfite, trimethylene sulfite, butylene sulfite, vinyl sulfite, ethyl sulfate, methyl sulfate, etc.

[0062] As the sulfite ester, preferably a C 2-4 alkylene sulfite. Specifically, examples include: ethylene sulfite (ES), propylene sulfite, trimethylene sulfite, butylene sulfite, vinyl sulfite, etc.

[0063] As the sulfonate ester, preferably at least one selected from the group consisting of C 3-5 alkane sultones and C 3-5 alkene sultones. Specifically, examples include: 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, etc.

[0064] For the sulfur-containing compound, one or more hydrogen atoms of the above-exemplified compounds are optionally substituted by substituents. Examples of the substituents include: alkyl, hydroxyalkyl, hydroxy, alkoxy, halogen atom, etc. The carbon number of the substituent can be 1 to 3. As the halogen atom, a fluorine atom is preferred.

[0065] Among the hexavalent sulfur compounds, in particular, at least one selected from the group consisting of lithium fluorosulfonate (LiFSO3), 1-propene-1,3-sultone (PRS), ethylene sulfite (DTD) and 1,5,2,4-dioxadithiolane-2,2,4,4-tetraoxide (MMDS) is easily obtained, and has a good effect of reducing resistance and improving high-temperature cycle characteristics, and thus is preferred. One or more of these are preferably 50% by mass or more, and further 80% by mass or more, of the hexavalent sulfur compound.

[0066] Among the sulfur(IV) compounds, in particular, at least one selected from the group consisting of ethylene sulfite (ES) and vinyl ethylene sulfite (VES) is easily available, and has a good effect of reducing resistance and improving high-temperature cycle characteristics, and thus is preferred. It is preferred that 50% by mass or more, and further 80% by mass or more, of the sulfur(IV) compound is occupied by one or more of these.

[0067] The content rate of the cyclic carboxylic anhydride (1) contained in the non-aqueous electrolyte is, for example, 2.5% by mass or less, and can be 0.01% by mass to 2.5% by mass, can also be 0.1% by mass to 2.0% by mass, and can further be 0.5% by mass to 1.5% by mass. It is considered that in this case, since a mixed coating film is moderately formed and the charge-discharge reaction easily proceeds uniformly, the effect of suppressing side reactions is improved. However, in the secondary battery, the cyclic carboxylic anhydride (1) is consumed to form a coating film. Therefore, it is sufficient that the non-aqueous electrolyte collected from the secondary battery contains the cyclic carboxylic anhydride (1) at a concentration above the detection limit.

[0068] The content rate of the sulfur-containing compound contained in the non-aqueous electrolyte is, for example, 5% by mass or less, and can be 0.01% by mass to 5% by mass, can also be 0.01% by mass to 2.5% by mass, can further be 0.1% by mass to 2.0% by mass, and can further be 0.5% by mass to 1.5% by mass. It is considered that in this case, the viscosity of the non-aqueous electrolyte does not increase excessively, and a mixed coating film is moderately formed and the charge-discharge reaction easily proceeds uniformly, so the effect of suppressing side reactions is improved. However, in the secondary battery, the sulfur-containing compound is consumed to form a coating film. Therefore, it is sufficient that the non-aqueous electrolyte collected from the secondary battery contains the sulfur-containing compound at a concentration above the detection limit.

[0069] The non-aqueous electrolyte may further contain other additives other than those described above. Examples of such additives include at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and vinyl carbonate.

[0070] (Non-aqueous solvent)

[0071] Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, cyclic ethers, chain ethers, etc. The non-aqueous electrolyte may contain one non-aqueous solvent or may contain two or more in combination.

[0072] Examples of the cyclic carbonate include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), etc.

[0073] Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc.

[0074] Examples of the cyclic carboxylic esters include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0075] Examples of the linear carboxylic esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and the like.

[0076] Examples of the cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, and the like.

[0077] Examples of the linear ethers 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.

[0078] (Salt)

[0079] In a lithium ion secondary battery, a lithium metal secondary battery, etc., a lithium salt is used as the salt. For example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 , lithium salts of lower aliphatic carboxylic acids, LiCl, LiBr, LiI, phosphates, borates, imide salts. Examples of the phosphates include lithium difluorophosphate (LiPO2F2), lithium difluoro bis(oxalato)phosphate (LiDFBOP), lithium tetrafluoro(oxalato)phosphate, and the like. Examples of the borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), and the like. As the imide salts, lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonyl nonafluorobutanesulfonyl imide (LiN(CF3SO2)(C4F9SO2)), lithium bis(pentafluoroethylsulfonyl)imide (LiN(C2F5SO2)2), and the like are used. The non-aqueous electrolyte may contain one kind of lithium salt or may contain two or more kinds in combination.

[0080] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0081] The content of each component in the non-aqueous electrolyte is determined, for example, by gas chromatography under the following conditions.

[0082] Measuring device: GC-2010Plus manufactured by Shimadzu Corporation

[0083] Column: HP-1 (1 μm × 60 m) manufactured by J&W

[0084] Linear velocity: 30.0 cm / sec

[0085] Inlet temperature: 270 °C

[0086] Detector: FID 290 °C (sens.10 1 )

[0087] Hereinafter, other components of the non-aqueous electrolyte secondary battery of the present disclosure will be specifically described.

[0088] [Positive electrode]

[0089] The positive electrode includes: 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 loaded 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 single surface of the positive electrode current collector. The positive electrode mixture contains a positive electrode active material as an essential component.

[0090] The positive electrode mixture layer may contain a conductive agent as an optional component. Examples of the conductive agent include: carbon-based materials such as carbon black (CB), acetylene black (AB), Ketjen black, carbon nanotubes (CNT), graphene, and graphite. These can be used alone or in combination of two or more.

[0091] The positive electrode mixture layer may contain a binder. Examples of the binder include: fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF); polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc. These can be used alone or in combination of two or more.

[0092] As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a sieve, a net, a punched sheet, etc.) is used. Examples of the material of the positive electrode current collector include: aluminum, aluminum alloy, titanium, titanium alloy, etc.

[0093] As the positive electrode active material, a lithium-containing composite oxide can be used. 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, space group C2 / m, etc. Among them, from the viewpoints of high capacity and high stability of the crystal structure, a layered rock salt structure belonging to space group R-3m is preferred. The layered rock salt structure of the lithium-containing composite oxide may also include a transition metal layer, a Li layer, and an oxygen layer.

[0094] From the perspective of high capacity, the proportion of Ni (Ni content) among the metal elements other than Li contained in the lithium-containing composite oxide can be 50 atomic% or more, can also be 80 atomic% or more, and can further be 90 atomic% or more.

[0095] From the perspective 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 can be 0 atomic% or more and 16 atomic% or less, and can also be 1.5 atomic% or more and 16 atomic% or less.

[0096] Similarly, from the perspective of stabilizing the crystal structure of the lithium-containing composite oxide and improving the heat resistance of the battery, the proportion of Al (Al content) among the metal elements other than Li contained in the lithium-containing composite oxide can be 0 atomic% or more and 18.5 atomic% or less, and can also be 4 atomic% or more and 10 atomic% or less.

[0097] From the perspective of cost reduction, the proportion of Mn (Mn content) among the metal elements other than Li contained in the lithium-containing composite oxide can be 0 atomic% or more and 50 atomic% or less, and can also be 0 atomic% or more and 30 atomic% or less.

[0098] The content rate of each metal element contained in the lithium-containing composite oxide is measured, for example, by inductively coupled plasma (ICP) emission spectrometry.

[0099] The lithium-containing composite oxide can be, for example, a composite oxide represented by the general formula Li a Ni x Co y Al z M1 w O 2-b (where 0.8 ≤ a ≤ 1.2, 0.80 ≤ x ≤ 0.95, 0.015 ≤ y ≤ 0.16, 0.04 ≤ z ≤ 0.185, 0 ≤ w ≤ 0.145, 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, and Zn). In this case, M1 is preferably Mn.

[0100] [Negative electrode]

[0101] The negative electrode includes 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 loaded on one or both surfaces of the negative electrode current collector. The negative electrode mixture layer is usually a layer or film made of a negative electrode mixture. The thickness of the negative electrode mixture layer is, for example, 10 μm to 150 μm per single surface of the negative electrode current collector. The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a conductive agent, a thickener, etc. as optional components. As the binder, conductive agent, and thickener, known materials can be used.

[0102] The negative electrode active material includes: materials that electrochemically absorb and release lithium ions, lithium metal, lithium alloys, etc. As the materials that electrochemically absorb and release lithium ions, carbon materials, alloy-based materials, etc. are used.

[0103] As the carbon materials, for example, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. can be exemplified. Among them, graphite with excellent charge-discharge stability and less irreversible capacity is preferred.

[0104] Graphite refers to a carbonaceous material with a well-developed graphite-type crystal structure. The interplanar spacing d002 of the graphite (002) plane measured by X-ray diffraction method can be, for example, 0.340 nm or less, or can be 0.3354 nm or more and 0.340 nm or less. In addition, the crystallite size Lc(002) of graphite can be, for example, 5 nm or more, or can be 5 nm or more and 200 nm or less. The crystallite size Lc(002) is measured by the Scherrer method, for example. When the interplanar spacing d002 and the crystallite size Lc(002) of the graphite (002) plane are within the above ranges, high capacity is easily obtained.

[0105] The alloy-based material refers to a material containing at least one metal that can form an alloy with lithium. As such materials, silicon, tin, silicon alloys, tin alloys, silicon oxides, tin oxides, silicon-containing materials, etc. can be cited.

[0106] The silicon-containing material contains, for example, a lithium ion conductive phase and a silicon phase dispersed in the lithium ion conductive phase. As the lithium ion conductive phase, for example, a silicon oxide phase, a silicate phase, a carbon phase, etc. can be used. The content rate of the silicon phase dispersed in the lithium ion conductive phase can be, for example, 30 mass% or more and 95 mass% or less, or can be 35 mass% or more and 75 mass% or less. The silicon-containing material can be used alone as one kind, or two or more kinds can be used in combination.

[0107] The main component (for example, 95 to 100 mass%) of the silicon oxide phase can be silicon dioxide. The silicon-containing material containing the silicon oxide phase and the silicon phase dispersed in the silicon oxide phase is represented by SiO xRepresentation. For example, x can be 0.5 ≤ x < 2, or can be 0.8 ≤ x ≤ 1.6. The silicon oxide phase can be an amorphous phase. SiO x For example, it can be obtained by the disproportionation reaction of silicon monoxide.

[0108] The silicate phase is preferred due to its low irreversible capacity. Among them, as a lithium-ion conductive phase with high initial charge-discharge efficiency, a lithium-containing silicate phase (hereinafter also referred to as lithium silicate phase) can be preferably used.

[0109] The lithium silicate phase only needs to be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and other elements can also be included. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. O / Si is preferably 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 can have the formula: Li 2z SiO 2+z (0 < z < 2) shown composition. z preferably satisfies the relationship of 0 < z < 1, and more preferably z = 1 / 2. As elements other than Li, Si, and O that may be contained in the lithium silicate phase, for example, the following can be cited: iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), etc.

[0110] The carbon phase can be composed of, for example, low-crystallinity amorphous carbon (i.e., amorphous carbon). The amorphous carbon can be, for example, hard carbon, can be soft carbon, or can be others.

[0111] A silicon-containing material in which a silicon phase is dispersed in the carbon phase can be obtained, for example, by using a ball mill or the like to pulverize a mixture of a carbon source and raw material silicon while stirring to make it microparticulate, and then performing heat treatment on the mixture in an inert atmosphere. As the carbon source, for example, the following can be used: sugars such as carboxymethyl cellulose (CMC), and water-soluble resins such as polyvinylpyrrolidone.

[0112] As the negative electrode active material, a silicon-containing material and a carbon material can be used together. The volume of the silicon-containing material expands and contracts with charge and discharge. Therefore, when the ratio it occupies in the negative electrode active material becomes large, it is easy to cause poor contact between the negative electrode active material and the negative electrode current collector during charge and discharge. On the other hand, by using a silicon-containing material and a carbon material together, it is possible to achieve excellent cycle characteristics while imparting a high capacity to the negative electrode.

[0113] The proportion of the silicon-containing material in the total of the silicon-containing material and the carbon material is, for example, preferably 0.5 to 15% by mass, and more preferably 1 to 10% by mass. Thereby, it is easy to balance high capacity and improvement of cycle characteristics.

[0114] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a sieve, a net, a punched sheet, etc.) is used. As the material of the negative electrode current collector, examples include: stainless steel, nickel, nickel alloy, copper, copper alloy, etc.

[0115] The composition of the silicon-containing material can be obtained, for example, by the following method: obtaining a backscattered electron image of the cross-section of the negative electrode mixture layer using a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), observing the particles of the silicon-containing material, and performing elemental analysis on the observed particles of the silicon-containing material. Elemental analysis is performed, for example, using an electron probe microanalyzer (EPMA: Electron Probe Micro Analyzer) analysis, etc.

[0116] The negative electrode mixture layer may contain a binder. As the binder, for example, the following can be cited: fluororesin (such as polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resin (such as polyethylene, polypropylene), polyamide resin (such as aramid resin), polyimide resin (such as polyimide, polyamideimide), acrylic resin (such as polyacrylic acid, polymethacrylic acid, acrylic acid-methyl methacrylic acid copolymer, ethylene-acrylic acid copolymer, or their salts), vinyl resin (such as polyvinyl acetate), rubber-like material (such as styrene-butadiene copolymer rubber (SBR)). The binder can be used alone as one kind or in combination of two or more kinds.

[0117] The negative electrode mixture layer may contain a thickener. As the thickener, for example, the following can be cited: cellulose derivatives such as cellulose ether. As the cellulose derivative, the following can be cited: CMC and its modified products, methyl cellulose, etc. The modified product of CMC also includes the salt of CMC. As the salt, the following can be cited: alkali metal salts (such as sodium salts), ammonium salts, etc. The thickener can be used alone as one kind or in combination of two or more kinds.

[0118] The negative electrode mixture layer may contain a conductive agent. As the conductive agent, for example, the following can be cited: carbon nanotubes (CNT), conductive particles. As the conductive particles, the following can be cited: conductive carbon (such as carbon black), metal powders, etc. The conductive agent can be used alone as one kind or in combination of two or more kinds.

[0119] The negative electrode current collector can be selected according to the type of non-aqueous electrolyte secondary battery. As the negative electrode current collector, for example, a sheet-like current collector can be cited. As the current collector, a metal foil or the like can be used. In addition, as the current collector, a porous current collector can also be used. As the porous current collector, for example, the following can be cited: a net-like current collector, a punched sheet, a metal net.

[0120] Examples of the material for the negative electrode current collector include: stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0121] [Separator]

[0122] A separator is preferably interposed between the positive electrode and the negative electrode. The separator has high ion permeability and has appropriate mechanical strength and insulation properties. As the separator, for example, a microporous film, a woven fabric or a non-woven fabric, or a laminate of at least two selected from these can be used. As the material of the separator, a polyolefin (for example, polypropylene, polyethylene) is preferable.

[0123] As an example of the structure of the non-aqueous electrolyte secondary battery, a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolytic solution are housed in an outer package can be cited. However, it is not limited thereto, and other types of electrode groups can also be applied. For example, it can also be a laminated electrode group in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween. The form of the non-aqueous electrolyte secondary battery is also not limited, and for example, it can be a cylindrical type, a square type, a coin type, a button type, a laminated type, etc.

[0124] Next, with reference to Figure 1 the structure of the non-aqueous electrolyte secondary battery will be described. Figure 1 is a longitudinal sectional view of a cylindrical secondary battery as an example of the present embodiment. However, the present disclosure is not limited to the following configuration.

[0125] The non-aqueous electrolyte secondary battery (hereinafter, battery 10) includes: an electrode group 18, a non-aqueous electrolyte (not shown), and a bottomed cylindrical battery can 22 that houses these. The battery can 22 is made of iron, stainless steel, etc. The inner surface of the battery can 22 can also be nickel-plated or the like. At the opening of the battery can 22, a sealing body 11 is press-fitted and fixed with a gasket 21 interposed therebetween. Thus, the inside of the battery is sealed. The sealing body 11 includes: 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 central portions. The positive electrode lead 15a led out from the positive electrode 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal of the positive electrode. The negative electrode lead 16a led out from the negative electrode 16 is connected to the inner surface of the bottom of the battery can 22. A ring-shaped groove portion 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the ring-shaped groove portion 22a. A second insulating plate 24 is disposed 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 interposed therebetween.

[0126] (Supplementary Note)

[0127] The following technology is disclosed according to the above description.

[0128] (Technology 1)

[0129] A non-aqueous electrolyte for a secondary battery, comprising: a non-aqueous solvent, a salt dissolved in the non-aqueous solvent, and an additive dissolved in the non-aqueous solvent,

[0130] The additive includes a cyclic carboxylic anhydride and a sulfur-containing compound,

[0131] The sulfur-containing compound includes at least one selected from the group consisting of a hexavalent sulfur compound and a tetravalent sulfur compound,

[0132] The cyclic carboxylic anhydride is represented by the general formula (1),

[0133] [Chemical formula 7]

[0134]

[0135] The hexavalent sulfur compound has a structure represented by the general formula (2),

[0136] [Chemical formula 8]

[0137]

[0138] The tetravalent sulfur compound has a structure represented by the general formula (3),

[0139] [Chemical formula 9]

[0140]

[0141] R1 to R4 are each independently a hydrogen atom, a fluorine atom or a hydrocarbon group,

[0142] X1 is a fluorine atom, a hydrocarbon group or an oxyhydrocarbon group,

[0143] X2, X3 and X4 are each independently a hydrocarbon group, a silyl group or an alkali metal,

[0144] At least one hydrogen atom of the hydrocarbon group is optionally substituted by a halogen atom,

[0145] X1 and X2 optionally form a ring,

[0146] X3 and X4 optionally form a ring.

[0147] (Technology 2)

[0148] The non-aqueous electrolyte for a secondary battery according to Technology 1, wherein the content of the cyclic carboxylic anhydride is 2.5% by mass or less.

[0149] (Technology 3)

[0150] The non-aqueous electrolyte for a secondary battery according to Technology 1 or 2, wherein the cyclic carboxylic anhydride includes diglycol anhydride.

[0151] (Technology 4)

[0152] The non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 3, wherein the content rate of the sulfur-containing compound is 5% by mass or less.

[0153] (Technology 5)

[0154] The non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 4, wherein the hexavalent sulfur compound contains at least one selected from the group consisting of lithium fluorosulfonate (LiFSO3), 1-propene-1,3-sultone (PRS), ethylene sulfite (DTD), and 1,5,2,4-dioxadithiolane-2,2,4,4-tetraoxide (MMDS).

[0155] (Technology 6)

[0156] The non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 5, wherein the tetravalent sulfur compound contains at least one selected from the group consisting of ethylene sulfite (ES) and vinyl ethylene sulfite (VES).

[0157] (Technology 7)

[0158] A secondary battery, comprising: a positive electrode; a separator; a negative electrode opposed to the positive electrode with the separator therebetween; a non-aqueous electrolyte; and a battery can housing the positive electrode, the separator, the negative electrode, and the non-aqueous electrolyte, wherein the non-aqueous electrolyte is the non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 6.

[0159] Although the presently preferred embodiments of the present invention have been described, such disclosure should not be construed in a limiting sense. Various modifications and changes will be apparent to those skilled in the art within the technical field of the present invention upon reading the above disclosure. Therefore, the appended claims should be construed to cover all modifications and changes without departing from the true spirit and scope of the present invention.

[0160] [Examples]

[0161] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0162] "Examples 1 to 5 and Comparative Examples 1 to 9"

[0163] A non-aqueous electrolyte secondary battery was fabricated through the following steps and evaluated.

[0164] (1) Fabrication of the positive electrode

[0165] In 95 parts by mass of a lithium-containing composite oxide (LiNi 0.8 Co 0.18 Al 0.02 O2), 2.5 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) are added and mixed to obtain a positive electrode paste. Then, the positive electrode paste is coated on the surface of an aluminum foil, and after the coating film is dried, it is calendered to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm 3 ) on both sides of the aluminum foil to obtain a positive electrode.

[0166] (2) Fabrication of the negative electrode

[0167] An appropriate amount of water is added to the negative electrode mixture and mixed to obtain a negative electrode paste. The negative electrode mixture is a mixture of a negative electrode active material, a binder, and a conductive agent. The negative electrode active material is a mixture of a silicon-containing material and graphite (average particle size (D50) 25 μm). The silicon-containing material is SiO x particles (x = 1, average particle size (D50) 5 μm) coated with a conductive layer of conductive carbon on the surface. In the negative electrode active material, the mass ratio of the silicon-containing material other than the conductive layer to graphite is set to 6:94. The binder uses sodium polyacrylate (PAA-Na), the sodium salt of CMC (CMC-Na), and SBR. The content rates of PAA-Na, CMC-Na, and SBR in the negative electrode mixture are each set to 1% by mass. Then, the negative electrode paste is coated on the surface of a copper foil, and after the coating film is dried, it is calendered to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm 3 ) on both sides of the copper foil to obtain a negative electrode.

[0168] (3) Preparation of the non-aqueous electrolyte

[0169] LiPF6 and additives shown in Table 1 as required are dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl acetate (MA) (EC:DMC:MA = 20:60:20 (volume ratio)) to prepare a non-aqueous electrolyte. The concentration of LiPF6 in the non-aqueous electrolyte is set to 1.35 mol / L. The concentration (initial concentration) of the additives in the non-aqueous electrolyte is set to the values shown in Table 1 (mass%).

[0170] (4) Fabrication of the non-aqueous electrolyte secondary battery

[0171] An aluminum positive electrode lead is attached to the obtained positive electrode above, and a nickel negative electrode lead is attached to the obtained negative electrode above. In an inert gas atmosphere, the positive electrode and the negative electrode are wound spirally with a polyethylene film (separator) in between to fabricate a wound-type electrode assembly. A first insulating plate is disposed on the lower end face of the electrode assembly, the electrode assembly is inserted into a battery can, and the negative electrode lead is resistance-welded to the bottom of the battery can. An iron can with its inner surface nickel-plated is used as the battery can. A second insulating plate is disposed on the upper end face of the electrode assembly, and then an annular groove portion is formed near the open end of the battery can. Next, the positive electrode lead is connected to a metal plate of a safety mechanism included in a sealing body, the above non-aqueous electrolyte is injected into the battery can, and then it is supported on the annular groove portion formed on the battery can with a gasket interposed therebetween, and the open end of the battery can is press-fitted to the periphery of the sealing body to complete the lithium ion secondary battery.

[0172] [Evaluation 1] Over-discharge characteristics

[0173] A 1 kΩ resistor is connected between the positive electrode and the negative electrode of the battery, and the battery is stored in a constant-temperature bath at 60 °C for 30 days while keeping the battery in a discharged state. Then, the battery is disassembled, and the content rate (mass basis) of Fe element in the non-aqueous electrolyte is quantified by ICP (Inductively Coupled Plasma) emission spectrometry.

[0174] [Evaluation 2] High-temperature cycle characteristics

[0175] In an environment at 45 °C, constant-current charging is performed at a current of 0.3It until the voltage of the non-aqueous electrolyte secondary battery reaches 4.2V, and then constant-voltage charging is performed at a voltage of 4.2V until the current reaches 0.05It. After a 20-minute interruption, constant-current discharging is performed at a current of 0.5It until the voltage of the non-aqueous electrolyte secondary battery reaches 2.5V. The discharge capacity (Ci) at this time is obtained. Such a cycle of charging, interruption, and discharging is regarded as one cycle, and the cycle is repeated 300 times, and the discharge capacity (Cc) of the 300th cycle is obtained. The ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci taken as 100% is obtained as the capacity retention rate.

[0176] The results of the examples and comparative examples are shown in Table 1. In Table 1, E1 to E4 are Examples 1 to 4, and C1 to C6 are Comparative Examples 1 to 6.

[0177] The additives in the table are represented as follows.

[0178] DGA: Diethylene glycol anhydride

[0179] SUC: Succinic anhydride

[0180] LiFSO3: Lithium fluorosulfonate

[0181] PRS: 1 - Propene - 1,3 - sultone

[0182] DTD: Ethylene sulfite

[0183] MMDS: 1,5,2,4 - Dioxadithiolane - 2,2,4,4 - tetraoxide

[0184] [Table 1]

[0185]

[0186] As shown in Table 1, it can be seen that, except for some exceptions, acid anhydrides have the effect of improving the high - temperature cycle characteristics (comparison between C1 and C4 - C6). It can also be seen that diglycol anhydride has a good effect on improving the high - temperature cycle characteristics (comparison between C1 and C4 - C6). On the other hand, it can be seen that sulfur - containing compounds increase the dissolution of Fe in the over - discharged state (comparison between C1 and C2 - C3).

[0187] In contrast, in batteries E1 - E4, high high - temperature cycle characteristics are obtained, and the amount of Fe dissolution in the over - discharged state is significantly reduced.

[0188] Industrial Applicability

[0189] The non - aqueous electrolyte secondary battery of the present disclosure can be used as the main power source for mobile communication devices, portable electronic devices, etc. However, the uses of the non - aqueous electrolyte secondary battery are not limited to these.

[0190] Explanation of Reference Signs

[0191] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 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 portion, 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 dissolved in the non-aqueous solvent, and an additive dissolved in the non-aqueous solvent, wherein the additive comprises a cyclic carboxylic anhydride and a sulfur-containing compound, the sulfur-containing compound comprises at least one selected from the group consisting of a hexavalent sulfur compound and a tetravalent sulfur compound, the cyclic carboxylic anhydride is represented by the general formula (1), [Chemical formula 1] the hexavalent sulfur compound has a structure represented by the general formula (2), [Chemical formula 2] the tetravalent sulfur compound has a structure represented by the general formula (3), [Chemical formula 3] R1 to R4 are each independently a hydrogen atom, a fluorine atom or a hydrocarbon group, X1 is a fluorine atom, a hydrocarbon group or an oxyhydrocarbon group, X2, 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 is optionally substituted by a halogen atom, X1 and X2 optionally form a ring, X3 and X4 optionally form a ring.

2. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein, The content of the cyclic carboxylic anhydride is 2.5% by mass or less.

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

4. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein, The cyclic carboxylic anhydride comprises diglycolic anhydride.

5. 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, 1-propene-1,3-sultone, ethylene sulfite and 1,5,2,4-dioxadithiolane-2,2,4,4-tetraoxide.

6. 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.

7. A secondary battery, comprising: a positive electrode; a separator; a negative electrode facing the positive electrode with the separator therebetween; a non-aqueous electrolyte; and a battery can housing the positive electrode, the separator, the negative electrode and the non-aqueous electrolyte, wherein the non-aqueous electrolyte is the non-aqueous electrolyte for a secondary battery according to claim 1.

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