Nonaqueous electrolyte for secondary battery, and secondary battery

Abstract

This nonaqueous electrolyte for secondary batteries comprises a nonaqueous solvent, a salt that dissolves in the nonaqueous solvent, and an additive that dissolves in the nonaqueous solvent. The additive contains a carbodiimide compound, and the salt contains an imide salt.

Description

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

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2024-167356, filed with the Japan Patent Office on 26 September 2024, and the entirety of the said patent application is incorporated herein by reference.

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

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

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

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

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

[0007] Patent Document 1 aims to provide a non-aqueous electrolyte for batteries that contains an electrolyte containing lithium hexafluorophosphate, and in which the generation of hydrogen fluoride is suppressed when water is mixed in. Patent Document 2 aims to provide a non-aqueous electrolyte for batteries that can reduce the battery resistance in a lithium secondary battery containing lithium nickel manganese cobalt composite oxide as the positive electrode active material.

[0008] Incidentally, as electrolyte salts other than lithium hexafluorophosphate, much consideration has been given to the use of imide salts with high hydrolysis resistance and high thermal stability. However, it has been found that when a non-aqueous electrolyte contains an imide salt as a supporting electrolyte, there is a new problem that when the secondary battery is in an over-discharged state, the corrosion of the battery can proceeds rapidly. It is presumed that in the over-discharged battery, sulfate ions or sulfite ions are generated from the imide salt, and these ions are corroding the battery can.

[0009] One aspect of the present disclosure relates to a non-aqueous electrolyte for a secondary battery, which includes a non-aqueous solvent, a salt dissolved in the non-aqueous solvent, and an additive dissolved in the non-aqueous solvent, the additive includes a carbodiimide compound, and the salt includes an imide salt.

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

[0011] According to the present disclosure, the corrosion of the battery can when the secondary battery is in an over-discharged state is suppressed. The novel features of the present invention are described in the appended claims, but the present invention will be better understood from the following detailed description in combination with the drawings, with respect to both the structure and the content, as well as other objects and features of the present invention.

[0012] It is a longitudinal sectional view of a secondary battery according to an embodiment of the present disclosure.

[0013] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. In addition, components other than those characteristic of this disclosure may be replaced with components of known secondary batteries. In this specification, when "range of numerical value A to numerical value B" is used, the range includes numerical values ​​A and B. For example, "A to B mol%" is synonymous with "A mol% or more and B mol% or less". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0014] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0015] Non-aqueous electrolyte secondary batteries include lithium-ion secondary batteries that use a material that reversibly intercepts and releases lithium ions as the negative electrode active material, lithium metal secondary batteries in which lithium metal is deposited at the negative electrode during charging and dissolves during discharge, and solid batteries containing gel electrolytes.

[0016] The non-aqueous electrolyte secondary battery according to this disclosure comprises a positive electrode, a negative electrode, a non-aqueous electrolyte, and a battery case housing these. A separator is usually placed between the positive and negative electrodes. The non-aqueous electrolyte is usually lithium-ion conductive. The battery case is made of a metal containing at least iron.

[0017] In this specification, "over-discharge characteristics" can be evaluated by the iron (Fe) content in the non-aqueous electrolyte inside the battery, which has been put into an over-discharge state by short-circuiting the positive and negative electrodes, and the iron (Fe) content leached from the battery casing.

[0018] Furthermore, "high-temperature cycle characteristics" can be evaluated by the capacity retention rate when a secondary battery is subjected to a predetermined number of constant-current constant-voltage (CCCV) charging cycles in a 45°C environment.

[0019] [Non-aqueous electrolytes] Non-aqueous electrolytes include a non-aqueous solvent, a salt, and an additive. The non-aqueous electrolyte containing the non-aqueous solvent may be a liquid electrolyte, or its fluidity may be restricted by a gelling agent or the like. In the case of lithium-ion secondary batteries, lithium metal secondary batteries, etc., lithium salts are used as the salt. The salt and additive are basically dissolved in the non-aqueous solvent. If the effects of the invention are not significantly impaired, some of the salt or additive may precipitate or separate without dissolving in the non-aqueous solvent. The additive defined by the general formula described later may be a salt. In that case, another salt is dissolved in the non-aqueous solvent as a supporting electrolyte.

[0020] Furthermore, the non-aqueous electrolyte recovered from the secondary battery may contain very few additives. In this case, oxidation or reduction products of the additives may be present in the battery. Even in such cases, additives usually remain in the non-aqueous electrolyte collected from the secondary battery at levels above the detection limit. Therefore, it is possible to confirm that the non-aqueous electrolyte contains additives.

[0021] (Additives) In this specification, carbodiimide compounds are classified as additives, and imide salts are classified as salts. Imide salts are excellent salts in that they have high hydrolysis resistance and high thermal stability. However, when imide salts are used as the main electrolyte salt in non-aqueous electrolytes, corrosion of the battery can progress rapidly. In contrast, it has been found that when carbodiimide compounds are co-added, sulfate ions or sulfite ions, which are corrosion-promoting components in the non-aqueous electrolyte, are trapped by the carbodiimide compounds. This makes it possible to achieve two effects simultaneously: suppression of battery can corrosion and improvement of high-temperature cycle characteristics.

[0022] Carbodiimide compounds have the chemical formula (1):

[0023]

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

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

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

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

[0028] Specific examples of the carbodiimide compounds that exhibit good effects include N,N'-diisopropylcarbodiimide, N,N'-dicyclohexylcarbodiimide, and the like. These may be used alone or in combination of two or more. The carbodiimide compounds exemplified herein desirably account for 50% by mass or more, and further desirably 80% by mass or more, of the carbodiimide compounds.

[0029] Next, the imide salt may have a structure represented by the general formula (2):

[0030]

[0031] It may have two SO ２ X1 groups bonded to the nitrogen atom. That is, the imide salt may have the structure of N(SO ２ X1) ２ . In this case, the two X1s may be the same or different.

[0032] X1 is a halogen atom or a hydrocarbon group. At least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom. Among the halogen atoms, a fluorine atom (F) is preferable. 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, or the like.

[0033] It is desirable that the hydrocarbon group has less steric hindrance. For example, it may be a C １－６ alkyl group or a cycloalkyl group having 1 to 6 carbon atoms. Similarly, the alkenyl group may be a C ２－６ alkenyl group or a cycloalkenyl group having 2 to 6 carbon atoms. The aryl group may be a C ６－１０ aryl group having 6 to 10 carbon atoms. In these alkyl groups, alkenyl groups, and aryl groups, at least one hydrogen atom may be substituted with a halogen atom or the like. Among them, it is preferable that the hydrocarbon group X1 is a perfluoroalkyl group in terms of high stability.

[0034] Specific examples of perfluoroalkyl groups include trifluoromethyl, tetrafluoroethyl, and pentafluoropropyl groups.

[0035] As an imide salt, LiN(SO ２ Rx) (SO ２ Ry) (However, Rx and Ry are each independently C ｎ F ２ｎ＋１ It can be expressed as , where n is a non-negative integer. At least one of the imide salts selected from the group represented by ) may be used. Such imide salts have excellent lithium ion dissociation properties, low viscosity, and LiPF ６ It exhibits ionic conductivity of a similarly high degree. Furthermore, because imide anions coordinate relatively weakly to lithium ions, they have the advantage of easily forming a high-quality anion-derived coating on the surface of the electrodes (especially the negative electrode).

[0036] Imide salts are, for example, lithium bis(fluorosulfonyl)imide (LiN(SO) ２ F) ２ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO) ２ CF ３ ) ２ ), lithium trifluoromethanesulfonyl nonafluorobutanesulfonylimide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )) and lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO) ２ C ２ F ５ ) ２ It may be at least one selected from the group consisting of ) and lithium bis(fluorosulfonyl)imide (LiN(SO ２ F) ２ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO) ２ CF ３ ) ２ ) and lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO) ２ C ２ F５ ) ２ ) may be at least one selected from the group consisting of ). It is desirable that the imide salts exemplified herein make up 50% by mass or more, and more preferably 80% by mass or more, of the imide salt. Lithium bis(fluorosulfonyl)imide will also be referred to as LiFSI below, and lithium bis(trifluoromethanesulfonyl)imide will also be referred to as LiTFSI below.

[0037] Carbodiimide compounds are thought to have the effect of trapping ionic species (such as sulfate ions and sulfite ions) derived from imide salts in non-aqueous electrolytes, and forming a dense film on the inner surface of the battery case. Such a film has the effect of protecting the surface of the battery case and is thought to inhibit the reaction between ionic species derived from imide salts and the battery case.

[0038] Furthermore, it is presumed that the carbodiimide compound and imide salt form a strong hybrid protective coating on both the positive and negative electrodes that does not inhibit lithium ion permeability. Therefore, it is thought that the charge transfer resistance of both the positive and negative electrodes is reduced.

[0039] The hybrid coating can, for example, inhibit excessive reactions between the transition metal elements constituting the positive electrode active material and the non-aqueous electrolyte. This suppresses the degradation of the positive electrode active material. The degradation suppression effect is particularly pronounced when the positive electrode active material contains a lithium-containing composite oxide with a high content of Ni.

[0040] Furthermore, the hybrid coating can suppress excessive reaction between the negative electrode active material (e.g., graphite or silicon-containing material) contained in the negative electrode and the non-aqueous electrolyte. This suppresses the degradation of the negative electrode active material. In particular, the effect of suppressing degradation is significant when the negative electrode active material contains silicon-containing material.

[0041] The effect of the carbodiimide compounds described above on improving over-discharge characteristics is particularly pronounced when the concentration of the imide salt in the non-aqueous electrolyte is high enough to function as a supporting electrolyte (for example, 0.1 mol / L or higher).

[0042] The concentration of the imide salt can be, for example, 0.1 mol / L to 3 mol / L, 0.1 mol / L to 2 mol / L, 0.1 mol / L to 1 mol / L, or 0.2 mol / L to 1 mol / L. In this case, a sufficient effect of improving the high-temperature cycling characteristics due to the imide salt can be obtained. Furthermore, since a hybrid film is formed appropriately and the charge-discharge reaction proceeds more uniformly, it is thought that the effect of suppressing side reactions is enhanced.

[0043] From the standpoint of balancing performance and reducing the corrosiveness of non-aqueous electrolytes, imide salts may be used in combination with other salts. Other salts include LiPF ６ This is preferable. In this case, from the viewpoint of fully obtaining the effect of the imide salt (for example, the effect of improving high-temperature cycling characteristics), the proportion of the imide salt in the salt (supporting electrolyte) is preferably 15 mol% or more, and may be 20 mol% or more. The proportion of the imide salt in the salt (supporting electrolyte) may be 15 mol% or more and 50 mol% or less, or 15 mol% or more (for example, 20 mol% or more) and 30 mol% or less.

[0044] The mass content of the carbodiimide compound in the non-aqueous electrolyte is, for example, 2% or less, and may be 0.01% to 2.0%, 0.1% to 2.0%, 0.5% to 2.0%, or 0.5% to 1.5%. In this case, a sufficient effect in suppressing corrosion of the battery can by the carbodiimide compound can be obtained. Furthermore, since a hybrid film is formed appropriately and the charge-discharge reaction proceeds more uniformly, it is thought that the effect of suppressing side reactions is enhanced.

[0045] When the mass content of the imide salt is a and the mass content of the carbodiimide compound is b, for example, the following can be satisfied: 0.5 ≤ b / a ≤ 3.0. More preferably, b / a may satisfy 1.0 ≤ b / a ≤ 3.0, or 1.0 ≤ b / a ≤ 2.0. In this case, it is considered that the carbodiimide compound acts more effectively on the imide salt. When the b / a ratio is controlled as described above, the effect of the carbodiimide compound in trapping sulfate ions, sulfite ions, etc. released by the imide salt is sufficiently achieved, and corrosion of the battery can is suppressed more significantly.

[0046] The non-aqueous electrolyte may contain other additives besides those mentioned above. Examples of such additives include at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate.

[0047] (Non-aqueous solvents) Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, linear carboxylic acid esters, cyclic ethers, linear ethers, etc. The non-aqueous electrolyte may contain one type of non-aqueous solvent, or a combination of two or more types.

[0048] Examples of cyclic carbonate esters include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0049] Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0050] Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

[0051] Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0052] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran.

[0053] Examples of chain-like ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methylphenyl 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, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0054] (Salts) In lithium-ion secondary batteries, lithium metal secondary batteries, etc., lithium salts are used as salts. Imide salts and LiPF ６ Other salts include, for example, LiClO ４ LiBF ４ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, and borates. Examples of phosphates include lithium difluorophosphate (LiPO4). ２ F ２ Examples include lithium difluorobis(oxalato)phosphate (LiDFBOP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. The non-aqueous electrolyte may contain three or more lithium salts.

[0055] The concentration of lithium salt in the non-aqueous electrolyte is, for example, between 0.5 mol / L and 3 mol / L.

[0056] 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 )

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] From the standpoint of cost reduction, the proportion of Mn among metal elements other than Li in the lithium-containing composite oxide (Mn content) may be set to 0 atomic% or more and 50 atomic% or less, or to 0 atomic% or more and 30 atomic% or less.

[0066] The content of each metal element in lithium-containing composite oxides is measured, for example, by inductively coupled plasma (ICP) emission spectroscopy.

[0067] Lithium-containing composite oxides include, for example, those with the general formula LiaNi ｘ Co ｙ Al ｚ M1 ｗ O ２－ｂIt 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.

[0068] [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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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).

[0077] The carbon phase may consist, for example, of amorphous carbon with low crystallinity. Amorphous carbon may be hard carbon, soft carbon, or something else.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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).

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0088] [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.

[0089] 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.

[0090] 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. The thinner the nickel plating layer, the more easily corrosion of the battery case becomes apparent when the secondary battery is over-discharged, thus significantly suppressing corrosion by carbodiimide compounds.

[0091] Furthermore, the cost of forming the nickel plating layer increases with increasing thickness. Also, increasing the thickness of the nickel plating layer necessitates reducing the thickness of the battery casing, which tends to decrease the strength of the battery casing. On the other hand, increasing the thickness of the nickel plating layer while maintaining the thickness of the battery casing reduces the internal volume of the battery casing, leading to a decrease in capacity. Therefore, it is preferable to keep the thickness of the plating layer as small as possible while still achieving the desired effect of the plating layer.

[0092] 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.

[0093] 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.

[0094] 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 otherwise treated. 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. A positive electrode lead 15a, 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. A negative electrode lead 16a, 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.

[0095] (Note) The following technologies are disclosed in accordance with the above description. (Technology 1) A non-aqueous electrolyte for a secondary battery comprising a non-aqueous solvent, a salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound and the salt comprises an imide salt. (Technology 2) The carbodiimide compound has chemical formula (1):

[0096]

[0097] A non-aqueous electrolyte for a secondary battery according to Technical 1, having a structure represented by, where R1 and R2 are each independently hydrocarbon groups, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom or a nitrogen-containing group. (Technical 3) The imide salt is general formula (2):

[0098]

[0099] (Technology 4) A non-aqueous electrolyte for a secondary battery according to any one of the following technologies: (Technology 5) The salt is further comprising LiPF ６(Technology 6) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 4, wherein the proportion of the imide salt in the salt is 15 mol% or more. (Technology 7) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 5, wherein the content of the carbodiimide compound is 0.1% by mass or more and 2% by mass or less. (Technology 8) A non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 6, wherein when the mass content of the imide salt is a and the mass content of the carbodiimide compound is b, the condition 1.0 ≤ b / a ≤ 3.0 is satisfied. (Technical 9) The non-aqueous electrolyte for a secondary battery according to any one of Technical 1 to 8, wherein the carbodiimide compound comprises at least one selected from the group consisting of N,N'-diisopropylcarbodiimide and N,N'-dicyclohexylcarbodiimide. (Technical 10) A secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, the non-aqueous electrolyte according to any one of Technical 1 to 9, and a battery case, wherein the battery case is made of a metal containing at least iron, and the positive electrode, the separator, the negative electrode and the non-aqueous electrolyte are housed in the battery case. (Technical 11) The secondary battery according to Technical 10, wherein the inner surface of the battery case is provided with a nickel plating layer with a thickness of 0.1 μm or more and less than 2 μm.

[0100] [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.

[0101] Examples 1-6 and Comparative Examples 1-5: Non-aqueous electrolyte secondary batteries were prepared and evaluated according to the following procedure.

[0102] (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 Al０．０２ 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.

[0103] (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.

[0104] (3) Preparation of non-aqueous electrolyte A mixed solvent of ethylene carbonate (EC) and diethyl carbonate (EMC) (EC:EMC = 20:80 (volume ratio)) is mixed with LiPF ６ Non-aqueous electrolytes were prepared by dissolving LiFSI and, if necessary, the additives shown in Table 1. The concentrations of each component in the non-aqueous electrolyte are shown in Table 1.

[0105] (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 to the thickness shown in Table 1 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 in 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.

[0106] [Evaluation 1] Over-discharge characteristics A 1 kΩ resistor was connected between the positive and negative electrodes of each battery, and the batteries were stored in a constant temperature bath at 60°C for 30 days while remaining discharged. After that, the batteries were disassembled, and the content of element Fe in the non-aqueous electrolyte (by mass) was quantified by ICP (Inductively Coupled Plasma) emission spectrometry. The relative value (Index) was determined when the content of element Fe in the non-aqueous electrolyte in Comparative Example 1 was set to 100.

[0107] [Evaluation 2] High-Temperature Cycle Characteristics Under a 45°C environment, constant current charging was performed with a current of 0.3 It until the voltage of the non-aqueous electrolyte secondary battery reached 4.2 V, and then constant voltage charging was performed with a voltage of 4.2 V until the current reached 0.05 It. After a 20-minute pause, constant current discharge was performed with a current of 0.5 It until the voltage of the non-aqueous electrolyte secondary battery reached 2.5 V. The discharge capacity (Ci) at this time was determined. This charging, pausing, and discharging cycle was considered as one cycle, and it was repeated 300 times. The discharge capacity (Cc) at the 300th cycle was determined. The ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci, which was set to 100%, was determined as the 45°C cycle capacity retention rate, and the relative value (Index) was determined when the 45°C cycle capacity retention rate in Comparative Example 1 was set to 100.

[0108] The results of the examples and comparative examples are shown in Table 1. In Table 1, batteries E1 to E6 are from Examples 1 to 6, and batteries C1 to C5 are from Comparative Examples 1 to 5.

[0109] The additives listed in the table are as follows: DIC: Diisopropylcarbodiimide DCC: Dicyclohexylcarbodiimide LiFSI: Lithium bis(fluorosulfonyl)imide LiTFSI: Lithium bis(trifluoromethanesulfonyl)imide

[0110]

[0111] Table 1 shows that when the non-aqueous electrolyte contains an imide salt, the high-temperature cycle characteristics tend to improve (comparison of batteries C1 to C5). However, batteries C3 to C4, which contain an imide salt at a considerable concentration in the non-aqueous electrolyte, have very high Fe leaching amounts from the battery casing during over-discharge, resulting in insufficient over-discharge characteristics.

[0112] On the other hand, when the non-aqueous electrolyte contains an imide salt and a carbodiimide compound, the leaching of Fe element from the battery casing during over-discharge is significantly suppressed, and the high-temperature cycle characteristics are also improved compared to batteries C2-C5 which contain only an imide salt (compared to batteries E1-E6). Furthermore, a comparison between batteries E1 and E2 and batteries C3 and C4 shows that when the b / a ratio is large, the effect of suppressing the leaching of Fe element from the battery casing during over-discharge is significant, and the effect of improving high-temperature cycle characteristics is also large. On the other hand, as shown in battery E6, as the b / a ratio decreases, the effect of suppressing the leaching of Fe element tends to decrease.

[0113] Furthermore, in battery C5, despite not containing a carbodiimide compound, the leaching of Fe element from the battery casing during over-discharge is suppressed, and high-temperature cycle characteristics are also good. This is because in battery C5, the nickel plating layer applied to the inner surface of the battery casing is large at 2.0 μm, making corrosion of the battery casing less likely. However, it is thought that the disadvantages of battery C5 may arise if the over-discharge conditions in Evaluation 1 are made even more severe. On the other hand, in the example battery in which the non-aqueous electrolyte contains an imide salt and a carbodiimide compound, the leaching of Fe element from the battery casing during over-discharge can be significantly suppressed regardless of the thickness of the plating layer, and high-temperature cycle characteristics are also improved, which is an advantage. Moreover, it is superior in that the effect of suppressing the leaching of Fe element from the battery casing is enhanced even under severe over-discharge conditions.

[0114] 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.

[0115] 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.

[0116] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating material, 15: Positive electrode, 15a: Positive electrode lead, 16: Negative electrode, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove, 23: First insulating plate, 24: Second insulating plate, 16: Negative electrode

Claims

1. A non-aqueous electrolyte for a secondary battery, comprising a non-aqueous solvent, a salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive comprises a carbodiimide compound and the salt comprises an imide salt.

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 imide salt has the general formula (2): The non-aqueous electrolyte for a secondary battery according to claim 1, having a structure represented by, where X1 is a halogen atom or a hydrocarbon group, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom.

4. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the concentration of the imide salt is 0.2 mol / L or more and 3 mol / L or less.

5. The salt is further divided into LiPF ６ The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the proportion of the imide salt in the salt is 15 mol% or more.

6. 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.

7. A non-aqueous electrolyte for a secondary battery according to claim 1, wherein the mass content of the imide salt is a and the mass content of the carbodiimide compound is b, and the condition 1.0 ≤ b / a ≤ 3.0 is satisfied.

8. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the imide salt comprises at least one selected from the group consisting of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(pentafluoroethanesulfonyl)imide.

9. 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.

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

11. The secondary battery according to claim 10, wherein the inner surface of the battery case is provided with a nickel plating layer with a thickness of 0.1 μm or more and less than 2 μm.

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