Nonaqueous electrolyte for secondary battery, and secondary battery

Abstract

A nonaqueous electrolyte for a secondary battery, comprising: a nonaqueous solvent; a lithium salt soluble in the nonaqueous solvent; and an additive soluble in the nonaqueous solvent, wherein the additive comprises a carbodiimide compound, wherein the nonaqueous solvent comprises a carbonate ester, wherein the carbonate ester comprises at least a cyclic carbonate, wherein, when the cyclic carbonate comprises ethylene carbonate, the mass content CLi of the lithium salt and the mass content Cec of the ethylene carbonate satisfy 1.3 × Cec < CLi, and wherein the concentration of the lithium salt is 3.0 mol / L or less.

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. 2024-167358, 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 a secondary battery.

[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 a lithium secondary battery containing lithium nickel manganese cobalt composite oxide as the positive electrode active material.

[0008] Incidentally, in secondary batteries where the non-aqueous electrolyte contains a carbonate ester as the non-aqueous solvent, transition metals contained in the components of the positive electrode tend to dissolve into the non-aqueous electrolyte, and these dissolved transition metals can precipitate at the negative electrode, degrading the battery performance.

[0009] One aspect of this disclosure relates to a non-aqueous electrolyte for secondary batteries, comprising a non-aqueous solvent, a lithium salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound, the non-aqueous solvent comprises a carbonate ester, the carbonate ester comprises at least a cyclic carbonate, and if the cyclic carbonate comprises ethylene carbonate, the mass content of the lithium salt Cli and the mass content of the ethylene carbonate Cec satisfy 1.3 × Cec < Cli, and the concentration of the lithium salt is 3.0 mol / L or less.

[0010] Another aspect of this disclosure relates to a secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, and the above-mentioned non-aqueous electrolyte for a secondary battery.

[0011] According to this disclosure, it is possible to suppress the elution of transition metals into the non-aqueous electrolyte and the deposition of transition metals at the negative electrode in a secondary battery.

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

[0013] 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.

[0014] 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. In addition, components other than the characteristic parts of the present disclosure may be applied with the components of known secondary batteries. In this specification, when it is said "range of numerical value A to numerical value B", the range includes numerical value A and numerical value B. For example, when it is said "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 regarding specific physical properties, conditions, etc. are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be made 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.

[0015] In addition, 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.

[0016] Non-aqueous electrolyte secondary batteries include, for example, lithium-ion secondary batteries using, as a negative electrode active material, a material that can reversibly occlude and release at least lithium ions, 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.

[0017] A non-aqueous electrolyte secondary battery according to one aspect 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.

[0018] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent, a salt, and an additive. The non-aqueous solvent contains a carbonic acid ester. The carbonic acid ester contains at least a cyclic carbonate. The non-aqueous electrolyte may be an electrolytic 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.

[0019] In addition, 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.

[0020] (Additive) In this specification, the carbodiimide compound is classified as an additive. The carbodiimide compound is considered to have the effect of trapping transition metal ions eluted in the non-aqueous electrolyte. In addition, the carbodiimide compound can form a strong hybrid protective film that does not inhibit the permeation of lithium ions on both the positive electrode and the negative electrode.

[0021] The film derived from the carbodiimide compound can inhibit the excessive reaction of the transition metal element constituting the positive electrode active material contained in the positive electrode with the non-aqueous electrolyte. Thereby, the elution of the transition metal from the positive electrode is suppressed, and the deterioration of the positive electrode active material is suppressed. In particular, when the positive electrode active material contains a lithium-containing composite oxide containing Ni at a high content rate, the effect of suppressing deterioration is remarkable.

[0022] In addition, the film derived from carbodiimide can suppress the precipitation of transition metal in the negative electrode and suppress the excessive reaction of the negative electrode active material (for example, graphite or silicon-containing material) contained in the negative electrode with the non-aqueous electrolyte. Thereby, the deterioration of the negative electrode active material is suppressed. In particular, when the negative electrode active material contains a silicon-containing material, the effect of suppressing deterioration is remarkable.

[0023] The effect of carbodiimide compounds in suppressing the elution of transition metals from the positive electrode into the non-aqueous electrolyte and the precipitation of transition metals at the negative electrode becomes apparent when the cyclic carbonate contains ethylene carbonate, and the mass content of lithium salt (Cli) and the mass content of ethylene carbonate (Cec) in the non-aqueous electrolyte satisfy 1.3 × Cec < Cli, and the lithium salt concentration is 3.0 mol / L or less. Ethylene carbonate (EC) contains metal ions (M) as shown in the following formula. ＋ Because it readily coordinates with (), it has the effect of promoting the dissolution of transition metals.

[0024]

[0025] On the other hand, when the mass content of lithium salt (Cli) and ethylene carbonate (Cec) in the non-aqueous electrolyte satisfies 1.3 × Cec < Cli, lithium ions are coordinated to most of the ethylene carbonate, and there is very little uncoordinated ethylene carbonate. Therefore, the metal elution reaction of the transition metal from the positive electrode is significantly suppressed. In addition, by keeping the lithium salt concentration below 3.0 mol / L, the increase in viscosity of the non-aqueous electrolyte is suppressed, thus suppressing the increase in the internal resistance of the battery.

[0026] Carbodiimide compounds have the chemical formula (1):

[0027]

[0028] 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, 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.

[0029] Hydrocarbon groups with low steric hindrance are preferable, for example, C1 to C6 groups. １－６It 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.

[0030] 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.

[0031] The hydrocarbon groups R1 and R2 may be different from each other, but it is preferable that R1 and R2 are the same group. When the hydrocarbon groups R1 and R2 are the same, the molecular structure has good symmetry, resulting in excellent reactivity, which is thought to promote reactions with transition metal ions in non-aqueous electrolytes and film formation reactions.

[0032] Specific examples of carbodiimide compounds that exhibit favorable effects include N,N'-diisopropylcarbodiimide and N,N'-dicyclohexylcarbodiimide. These may be used individually or in combination of two or more. It is desirable that the carbodiimide compounds exemplified here constitute 50% or more, and more preferably 80% or more, of the total carbodiimide compound.

[0033] 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 film derived from the carbodiimide compound is appropriately formed, and the charge-discharge reaction proceeds more uniformly, which is thought to enhance the effect of suppressing side reactions.

[0034] (Non-aqueous solvent) The non-aqueous solvent contains carbonate ester. Carbonate ester is the main component of the non-aqueous solvent, accounting for, for example, 60% or more by volume, and may also account for 70% or more by volume, or 80% or more by volume, or even 90% or more by volume (for example, 100%). In non-aqueous electrolytes in which carbonate ester is the main component of the non-aqueous solvent, the elution of transition metals from the positive electrode is usually likely to occur, and the precipitation of transition metals at the negative electrode is likely to occur.

[0035] In contrast, when a carbodiimide compound is added to the non-aqueous electrolyte and the carbonate ester contains ethylene carbonate, the elution of transition metals from the positive electrode and the precipitation of transition metals at the negative electrode are significantly suppressed by controlling the mass content of the lithium salt (Cli) and the mass content of the ethylene carbonate (Cec) to satisfy 1.3 × Cec < Cli. Preferably, the mass content of the lithium salt (Cli) and the mass content of the ethylene carbonate (Cec) satisfy 2.0 × Cec < Cli, but may also satisfy 3.0 × Cec ≤ Cli ≤ 7 × Cec, or 3.0 × Cec ≤ Cli ≤ 5 × Cec.

[0036] The non-aqueous electrolyte preferably contains two or more non-aqueous solvents. The carbonate ester preferably contains both cyclic carbonate esters and linear carbonate esters.

[0037] Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0038] Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0039] However, from the viewpoint of satisfying the relationships 1.3 × Cec < Cli and further 2.0 × Cec < Cli < 10 × Cec, the volume content of ethylene carbonate in the non-aqueous solvent is preferably 10% by volume or less, and the volume content of ethylene carbonate in the non-aqueous solvent may be 5% by volume or less.

[0040] On the other hand, from the viewpoint of the balance of physical properties of the non-aqueous electrolyte and the ability to form a film on the electrode surface, the non-aqueous solvent preferably contains 2% or more by volume of ethylene carbonate, and more preferably contains 2.5% or more by volume of ethylene carbonate.

[0041] The non-aqueous solvent may contain fluoroethylene carbonate (FEC) as a cyclic carbonate. The carbonate group (-O-C=O-O- group) of fluoroethylene carbonate makes it difficult for metal ions to coordinate due to the electron-withdrawing effect of the fluorine atom. Therefore, using fluoroethylene carbonate instead of ethylene carbonate further reduces the elution of transition metals. From the viewpoint of balancing the physical properties of the non-aqueous electrolyte, it is preferable that the non-aqueous solvent contains both ethylene carbonate and fluoroethylene carbonate.

[0042] The volume content of fluoroethylene carbonate in the non-aqueous solvent is preferably 10% by volume or less, and may be 5% by volume or less. The volume content of fluoroethylene carbonate in the non-aqueous solvent may be 1% by volume or more, and may be 1.5% by volume or more.

[0043] From the viewpoint of more significantly suppressing the elution of transition metals from the positive electrode and the precipitation of transition metals at the negative electrode, it is preferable that the volume content of cyclic carbonate in the non-aqueous solvent be 15% by volume or less, and may also be 10% by volume or less.

[0044] On the other hand, from the viewpoint of the balance of physical properties of the non-aqueous electrolyte and the ability to form a film on the electrode surface, the non-aqueous solvent preferably contains 3% or more by volume of cyclic carbonate, and more preferably contains 5% or more by volume of cyclic carbonate.

[0045] The non-aqueous solvent may contain small amounts of other solvents in addition to the carbonate ester. Examples of other solvents include cyclic carboxylic acid esters, linear carboxylic acid esters, cyclic ethers, and linear ethers.

[0046] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. 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, etc. Examples of the cyclic ether include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, etc. 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, etc.

[0047] (Salt) In a lithium ion secondary battery, a lithium metal secondary battery, etc., a lithium salt is used as the salt. Examples of the lithium salt include, 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 ２Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) 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.

[0048] The concentration of the lithium salt is, for example, 1.3 mol / L or more and 2.5 mol / L or less, or 1.5 mol / L or more and 2.5 mol / L or less, from the viewpoint of increasing the proportion of carbonate esters coordinated with Li ions.

[0049] 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 )

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] The content of each metal element in lithium-containing composite oxides is measured, for example, by inductively coupled plasma (ICP) emission spectroscopy.

[0059] 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.

[0060] [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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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 ｘ It 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.

[0067] 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.

[0068] 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＋ｚThe 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).

[0069] The carbon phase may consist, for example, of amorphous carbon with low crystallinity. Amorphous carbon may be hard carbon, soft carbon, or something else.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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).

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0080] [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.

[0081] 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.

[0082] 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 battery case is made of a metal containing at least iron. The material of the battery case may be, for example, 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. The thickness of the nickel plating layer may be, for example, 0.1 μm or more and less than 2 μm, or 0.1 μm or more and 1 μm or less.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] (Note) The following technologies are disclosed in accordance with the above description. (Technology 1) A non-aqueous electrolyte for secondary batteries comprising a non-aqueous solvent, a lithium salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound, the non-aqueous solvent comprises a carbonate ester, the carbonate ester comprises at least a cyclic carbonate, and if the cyclic carbonate comprises ethylene carbonate, the mass content of the lithium salt Cli and the mass content of ethylene carbonate Cec satisfy 1.3 × Cec < Cli, and the concentration of the lithium salt is 3.0 mol / L or less. (Technology 2) The carbodiimide compound has chemical formula (1):

[0087]

[0088] (Technology 3) A non-aqueous electrolyte for a secondary battery according to Technology 1, having a structure represented by the formula (1), 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 or a nitrogen-containing group. (Technology 4) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 3, wherein the carbodiimide compound comprises at least one selected from the group consisting of N,N'-diisopropylcarbodiimide and N,N'-dicyclohexylcarbodiimide. (Technology 5) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 4, wherein the content of the carbodiimide compound is 0.1% by mass or more and 2% by mass or less. (Technology 6) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 5, wherein the concentration of the lithium salt is 1.3 mol / L or more and 2.5 mol / L or less. (Technology 7) A non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 6, wherein the concentration of the lithium salt is 1.5 mol / L or more and 2.5 mol / L or less. (Technology 8) A non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 7, wherein the volume content of the ethylene carbonate in the non-aqueous solvent is 10 volume% or less. (Technology 9) A non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 8, wherein the volume content of the ethylene carbonate in the non-aqueous solvent is 5 volume% or less. (Technology 10) A non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 9, wherein, when the cyclic carbonate contains ethylene carbonate, the mass content of the lithium salt Cli and the mass content of the ethylene carbonate Cec satisfy 2.0 × Cec < Cli < 10 × Cec. (Technology 11) A non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 10, wherein the cyclic carbonate further comprises fluoroethylene carbonate. (Technology 12) A non-aqueous electrolyte for a secondary battery according to any one of Technologies 1 to 11, wherein the volume content of the cyclic carbonate in the non-aqueous solvent is 15% by volume or less. (Technology 13) A secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte according to any one of Technologies 1 to 12.

[0089] [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.

[0090] Examples 1-12 and Comparative Examples 1-5: Non-aqueous electrolyte secondary batteries were prepared and evaluated according to the following procedure.

[0091] (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.

[0092] (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.

[0093] (3) Preparation of non-aqueous electrolyte A non-aqueous solvent containing ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in the volume content shown in Table 1, and also containing ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), to which LiPF ６ and bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ A non-aqueous electrolyte was prepared by dissolving the following and, if necessary, the additives shown in Table 1. The concentrations of each component in the non-aqueous electrolyte are shown in Table 1.

[0094] (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.

[0095] [Evaluation 1] Amount of metal deposition A non-aqueous electrolyte secondary battery was charged with a constant current of 0.3 It at a 25°C environment until the voltage reached 4.2 V, and then charged with a constant voltage of 4.2 V until the current reached 0.05 It. After charging, the battery was left at 60°C for 340 hours, then disassembled, and the amount of metal deposited per unit area of ​​the negative electrode was analyzed by inductively coupled plasma (ICP) emission spectroscopy.

[0096] The results of the examples and comparative examples are shown in Table 1. In Table 1, batteries E1 to E12 are from Examples 1 to 12, and batteries C1 to C5 are from Comparative Examples 1 to 5.

[0097] The additives in the table are indicated as follows: DIC: Diisopropylcarbodiimide DCC: Dicyclohexylcarbodiimide

[0098]

[0099] Table 1 shows that when the non-aqueous electrolyte contains a carbodiimide compound, the amount of metal deposition is significantly reduced (comparison of batteries C1-C5 and E1-E12). However, when the CLI / Cec ratio is 1.3 or less, the amount of metal deposition is high. Furthermore, when the CLI / Cec ratio is 2.0 or higher (especially 3.0 or higher), the amount of metal deposition is significantly reduced.

[0100] 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.

[0101] 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.

[0102] 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 secondary batteries comprising a non-aqueous solvent, a lithium salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound, the non-aqueous solvent comprises a carbonate ester, the carbonate ester comprises at least a cyclic carbonate, and if the cyclic carbonate comprises ethylene carbonate, the mass content of the lithium salt Cli and the mass content of ethylene carbonate Cec satisfy 1.3 × Cec < Cli, and the concentration of the lithium salt is 3.0 mol / L or less.

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 or a nitrogen-containing group.

3. The non-aqueous electrolyte for a secondary battery according to claim 2, wherein in the chemical formula (1), R1 and R2 are the same group.

4. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the carbodiimide compound comprises at least one selected from the group consisting of N,N'-diisopropylcarbodiimide and N,N'-dicyclohexylcarbodiimide.

5. 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.

6. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the concentration of the lithium salt is 1.3 mol / L or more and 2.5 mol / L or less.

7. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the concentration of the lithium salt is 1.5 mol / L or more and 2.5 mol / L or less.

8. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the volume content of ethylene carbonate in the non-aqueous solvent is 10% by volume or less.

9. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the volume content of ethylene carbonate in the non-aqueous solvent is 5% by volume or less.

10. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein, when the cyclic carbonate contains ethylene carbonate, the mass content of the lithium salt Cli and the mass content of the ethylene carbonate Cec satisfy 2.0 × Cec < Cli < 10 × Cec.

11. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the cyclic carbonate further comprises fluoroethylene carbonate.

12. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the volume content of the cyclic carbonate in the non-aqueous solvent is 15% by volume or less.

13. A secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, and the non-aqueous electrolyte described in claim 1.

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