Non-aqueous electrolyte and secondary battery

Abstract

This non-aqueous electrolyte for a secondary battery includes a non-aqueous solvent, a salt that dissolves into the non-aqueous solvent, and an additive that dissolves into the non-aqueous solvent. The additive includes a hetero ring compound and a sulfur-containing compound. The hetero ring compound is represented by general formula (1), where X1 through X4 are each independently an oxygen atom or elemental sulfur, R1 through R4 are each independently a C1-5 alkyl group in which at least one hydrogen atom is optionally substituted with a halogen atom.

Description

Nonaqueous electrolyte and secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2022-187287, filed on November 24, 2022, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a non-aqueous electrolyte and a secondary battery.

[0003] Patent Document 1 proposes "an electrolyte characterized by containing an electrolytic solution containing at least one member selected from the group consisting of paraldehyde and its derivatives."

[0004] JP 2007-141494 A

[0005] Paraldehyde forms a coating on the surface of the negative electrode in secondary batteries. This coating suppresses the decomposition reaction of the electrolyte at the negative electrode, improving the high-temperature storage characteristics of secondary batteries. However, the effects of paraldehyde on improving the high-temperature cycle characteristics and reducing the initial resistance of secondary batteries are insufficient.

[0006] One aspect of the present disclosure provides a composition 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 heterocyclic compound and a sulfur-containing compound, and the heterocyclic compound is represented by general formula (1):

[0007]

[0008] wherein X1 to X3 each independently represent an oxygen atom or a sulfur atom, and R1 to R3 each independently represent an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom may be substituted with a halogen atom.

[0009] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a separator, a negative electrode facing the positive electrode with the separator interposed therebetween, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte is the non-aqueous electrolyte for secondary batteries described above.

[0010] According to the present disclosure, it is possible to reduce the initial resistance of a secondary battery and improve the high-temperature cycle characteristics.

[0011] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0012] 1 is a longitudinal sectional view of a secondary battery according to an embodiment of the present disclosure.

[0013] The following describes embodiments of the present disclosure using 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. However, other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that components other than those characteristic of the present disclosure may be those of known secondary batteries. In this specification, when a "range of numerical value A to numerical value B" is used, the range includes numerical value A and numerical value 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 for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit. When multiple materials are exemplified, one material may be selected from the materials and used alone, or two or more materials may be used in combination.

[0014] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0015] Non-aqueous electrolyte secondary batteries include lithium ion secondary batteries that use a material that reversibly absorbs and releases at least lithium ions as a negative electrode active material, lithium secondary batteries in which lithium metal precipitates at the negative electrode during charging and dissolves during discharging, and solid-state batteries that contain a gel electrolyte.

[0016] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. A separator is typically disposed between the positive electrode and the negative electrode. The non-aqueous electrolyte typically has lithium ion conductivity.

[0017] In this specification, the "resistance" in the resistance increase of a secondary battery can be evaluated by the direct current resistance (DCR) of a secondary battery in an initial state after shipment (unused or after less than 10 charge / discharge cycles) in a 25°C environment. The "high-temperature cycle characteristics" can be evaluated by the capacity retention rate when a secondary battery is subjected to constant-current, constant-voltage charging (CCCV charging) for a predetermined number of cycles in a 45°C environment. The "amount of stored gas" can be evaluated by storing a charged battery at 80°C for three days and then measuring the amount of gas recovered from the battery after storage.

[0018] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent, a salt, and an additive. A non-aqueous electrolyte containing a non-aqueous solvent is usually a liquid electrolyte, but 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., a lithium salt is used as the salt. The salt and the additive are basically dissolved in the non-aqueous solvent, but as long as the effects of the invention are not significantly impaired, a portion of the salt or the additive may remain undissolved and precipitate or separate. The additive defined by the general formula described below may be a salt. In this case, another salt is dissolved in the non-aqueous solvent as a supporting electrolyte.

[0019] The nonaqueous electrolyte recovered from the secondary battery may contain almost no additive. In this case, an oxidation product or reduction product of the additive is contained as a coating component on the positive electrode surface or the negative electrode surface. Even in such a case, the additive usually remains in the nonaqueous electrolyte collected from the secondary battery at a level above the detection limit, so it is possible to confirm that the nonaqueous electrolyte contains the additive.

[0020] (Additives) In this specification, heterocyclic compounds and sulfur-containing compounds are classified as additives.

[0021] (Heterocyclic Compound) The heterocyclic compound is represented by the general formula (1):

[0022]

[0023] X1 to X3 are each independently an oxygen atom or a sulfur atom. R1 to R3 are each independently an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom may be substituted with a halogen atom. The halogen atom may be a fluorine atom, a chlorine atom, or a bromine atom, with a fluorine atom being preferred.

[0024] The heterocyclic compound represented by general formula (1) (hereinafter also referred to as "heterocyclic compound (1)") is unique among compounds having similar structures, and is less likely to increase the resistance of a secondary battery, and can reduce the resistance. Furthermore, the heterocyclic compound (1) can improve the high-temperature cycle characteristics of the secondary battery. However, even when the heterocyclic compound (1) is used alone, the resistance of the secondary battery remains high, and the effect of improving the high-temperature cycle characteristics is insufficient.

[0025] On the other hand, when the sulfur-containing compound is used in combination with the heterocyclic compound (1), the resistance of the secondary battery can be significantly reduced, and the high-temperature cycle characteristics of the secondary battery are significantly improved by using the heterocyclic compound (1) in combination with the sulfur-containing compound.

[0026] The reason for this synergistic effect is believed to be that when the heterocyclic compound (1) and the sulfur-containing compound are used in combination, a strong hybrid coating is formed as a protective coating on both the positive and negative electrodes. The hybrid coating highly suppresses side reactions at high temperatures, reduces gas generation, improves high-temperature cycle characteristics, and reduces the charge transfer resistance of the positive and negative electrodes.

[0027] The coating components derived from the heterocyclic compound (1) and the sulfur-containing compound constituting the hybrid coating are presumed to have an effect of suppressing deterioration of the positive electrode active material by, for example, interacting with a transition metal element constituting the positive electrode active material contained in the positive electrode. In particular, when the positive electrode active material contains a lithium-containing composite oxide containing a high content of Ni, the effect of suppressing deterioration is remarkable.

[0028] Furthermore, it is presumed that the coating components derived from the heterocyclic compound (1) and the sulfur-containing compound constituting the hybrid coating interact with elements constituting the negative electrode active material contained in the negative electrode, thereby suppressing deterioration of the negative electrode active material, and this effect is particularly pronounced when the negative electrode active material contains a silicon-containing material.

[0029] It is desirable that R1 to R3 have little steric hindrance, and may be, for example, a methyl group, an ethyl group, an ethylene group, a propyl group, or a propylene group. Typically, R1 to R3 are the same group, and may be a methyl group, an ethyl group, an n-propyl group, or an iso-propyl group. One or more hydrogen atoms of the alkyl group may be substituted with a halogen atom. The halogen atom is preferably a fluorine atom.

[0030] More specifically, the heterocyclic compound (1) may be at least one selected from the group consisting of paraldehyde, trithioacetaldehyde, 2,4,6-triethyl-1,3,5-trioxane, 2,4,6-tripropyl-1,3,5-trioxane, and 2,4,6-tris(propan-2-yl)-1,3,5-trioxane (hereinafter also referred to as "triheterocyclic compound (TH)").

[0031] Paraldehyde is represented by the following formula:

[0032]

[0033] Trithioacetaldehyde is represented by the following formula:

[0034]

[0035] 2,4,6-triethyl-1,3,5-trioxane is represented by the following formula:

[0036]

[0037] 2,4,6-tripropyl-1,3,5-trioxane is represented by the following formula:

[0038]

[0039] 2,4,6-tris(propan-2-yl)-1,3,5-trioxane is represented by the following formula:

[0040]

[0041] The content of the heterocyclic compound (1) in the non-aqueous electrolyte is, for example, 3% by mass or less, and may be 0.01% by mass to 3% by mass, 0.1% by mass to 2.5% by mass, or 0.5% by mass to 2% by mass. In this case, a hybrid film is appropriately formed, and the charge / discharge reaction is likely to proceed uniformly, which is thought to enhance the effect of suppressing side reactions. However, within the secondary battery, the heterocyclic compound (1) is consumed to form the film. Therefore, it is sufficient that the non-aqueous electrolyte sampled from the secondary battery contains the heterocyclic compound (1) at a concentration equal to or greater than the detection limit.

[0042] (Sulfur-containing compound) The sulfur-containing compound may be any compound containing an S=O structure. The sulfur-containing compound may contain, for example, a hexavalent or tetravalent sulfur atom. That is, the sulfur-containing compound may contain at least one selected from the group consisting of a hexavalent sulfur compound and a tetravalent sulfur compound.

[0043] The hexavalent sulfur compound is, for example, a compound represented by the general formula (2):

[0044]

[0045] Hereinafter, the hexavalent sulfur compound represented by general formula (2) will also be referred to as "sulfur compound (2)." In general formula (2), n is 0 or 1.

[0046] When n = 0, R4 may be a fluorine atom, a hydrocarbon group, an imide group, or a silyl group. When n = 1, R4 may be a hydrocarbon group, a silyl group, or an alkali metal. R5 may be a fluorine atom, a hydrocarbon group, an imide group, or a silyl group. R4 and R5 may form a ring. That is, the hexavalent sulfur compound may be a cyclic sulfur compound. The sulfur compound (2) may contain two hexavalent sulfur atoms. In that case, R5 may be an imide group, an alkylene group, or an ether group shared by two sulfur atoms.

[0047] The tetravalent sulfur compound is, for example, a compound represented by the general formula (3):

[0048]

[0049] Hereinafter, the tetravalent sulfur compound represented by general formula (3) will also be referred to as "sulfur compound (3)." In general formula (3), R6 and R7 may each independently represent a hydrocarbon group, a silyl group, or an alkali metal.

[0050] R6 and R7 may be an alkyl group, an alkenyl group, or an aryl group. R6 and R7 may be an alkylene group, an alkenylene group, an ether group, or the like to form a ring. That is, the tetravalent sulfur compound may be a cyclic sulfur compound. The sulfur compound (3) may contain two tetravalent sulfur atoms. In that case, R6 and R7 may be an alkylene group or an ether group shared by two sulfur atoms.

[0051] When one or more of R4 to R7 are aliphatic hydrocarbon groups, it is desirable that the steric hindrance is small. For example, C １－５ It may be a hydrocarbon group. When one or more of R4 to R7 are aromatic hydrocarbon groups (aryl groups), there may be one aromatic ring. When R4 and R5 or R6 and R7 form a ring, the ring may be a 5-membered ring, a 6-membered ring, or a 7-membered ring. When one or more of R4, R6, and R7 are alkali metals, the alkali metal may be Li, K, Na, or the like. Note that when R4 to R7 form a ring, R4 and R5 or R6 and R7 combine to form, for example, an alkylene group, an alkenylene group, an ether group, or the like.

[0052] It is desirable that the general formula (2) satisfies n = 1. That is, the sulfur compound (2) is a compound represented by the general formula (4):

[0053]

[0054] As described above, X1 is a fluorine atom, a hydrocarbon group, an imide group, or a silyl group, and X2 is a hydrocarbon group, a silyl group, or an alkali metal. In general formula (4), at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom.

[0055] When X1 in the sulfur compound (4) is a fluorine atom, X2 is preferably an alkyl group, an alkenyl group, an aryl group, a silyl group, or an alkali metal. When X1 is a hydrocarbon group, X1 and X2 may form a ring by combining an alkylene group, an alkenylene group, an ether group, or the like. The sulfur compound (4) may contain two hexavalent sulfur atoms. In this case, X1 may be an ether group shared by two sulfur atoms, and X2 may be an alkylene group shared by two sulfur atoms.

[0056] The sulfur compound (2) is —O—S(═O) ２ Sulfuric acid esters having the —O— structure and —S(═O) ２ The sulfur compound (3) includes a sulfonate ester having an —O— structure. The sulfur compound (3) is a sulfite ester having an —O—S(═O)—O— structure. Therefore, it can be said that the sulfur-containing compound is at least one selected from the group consisting of a sulfate ester, a sulfite ester, and a sulfonate ester.

[0057] As the sulfate ester, C ２－４ Alkyl sulfates are preferred, and specific examples thereof include ethylene sulfate, propylene sulfate, trimethylene sulfate, butylene sulfate, vinylene sulfate, ethyl sulfate, and methyl sulfate.

[0058] Sulfite esters include C ２－４ Alkylene sulfites are preferred, and specific examples include ethylene sulfite (ES), propylene sulfite, trimethylene sulfite, butylene sulfite, and vinylene sulfite.

[0059] The sulfonic acid esters include C ３－５ Alkanesultone and C ３－５ At least one selected from the group consisting of alkene sultones is preferred, specifically 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, etc.

[0060] The sulfur-containing compound may have one or more hydrogen atoms of the compounds exemplified above substituted with a substituent. Examples of the substituent include an alkyl group, a hydroxyalkyl group, a hydroxy group, an alkoxy group, and a halogen atom. The number of carbon atoms in the substituent may be 1 to 3. The halogen atom is preferably a fluorine atom.

[0061] Among hexavalent sulfur compounds, lithium fluorosulfonate (LiFSO ３ At least one selected from the group consisting of 1-propene-1,3-sultone (PRS), ethylene sulfate (DTD), lithium bisfluorosulfonylimide (LiFSI), and 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide (MMDS) is desirable because it is easily available and has a significant effect of reducing resistance and improving high-temperature cycle characteristics.

[0062] Among the tetravalent sulfur compounds, at least one selected from the group consisting of ethylene sulfite (ES) and vinyl ethylene sulfite (VES) is particularly desirable because it is easily available and has a significant effect of reducing resistance and improving high-temperature cycle characteristics.

[0063] The content of the sulfur-containing compound in the non-aqueous electrolyte is, for example, 5% by mass or less, and may be 0.01% by mass to 5% by mass, 0.01% by mass to 2.5% by mass, 0.1% by mass to 2.0% by mass, or 0.5% by mass to 1.5% by mass. In this case, the viscosity of the non-aqueous electrolyte does not increase excessively, a hybrid film is appropriately formed, and charge / discharge reactions tend to proceed uniformly, which is thought to enhance the effect of suppressing side reactions. However, within the secondary battery, the sulfur-containing compound is consumed to form the film. Therefore, it is sufficient that the non-aqueous electrolyte sampled from the secondary battery contains the sulfur-containing compound at a concentration above the detection limit.

[0064] The non-aqueous electrolyte may contain additives other than those described above, such as at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate.

[0065] (Non-aqueous Solvent) Examples of the non-aqueous solvent include cyclic carbonate esters, chain carbonate esters, cyclic carboxylic acid esters, chain carboxylic acid esters, cyclic ethers, chain ethers, etc. The non-aqueous electrolyte may contain one type of non-aqueous solvent or a combination of two or more types.

[0066] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0067] Examples of the chain carbonate ester include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0068] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

[0069] 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, and propyl propionate.

[0070] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran.

[0071] Examples of chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, diethylene glycol dimethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0072] (Salt) In lithium ion secondary batteries, lithium metal secondary batteries, etc., lithium salts are used as salts. For example, LiClO ４ , LiBF ４ , LiPF６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２ Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The non-aqueous electrolyte may contain one lithium salt or a combination of two or more lithium salts.

[0073] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0074] The content of each component in the non-aqueous electrolyte is determined, for example, by gas chromatography under the following conditions: Measuring device: GC-2010 Plus manufactured by Shimadzu Corporation Column: HP-1 (1 μm×60 m) manufactured by J&W Linear velocity: 30.0 cm / sec Injection port temperature: 270° C. Detector: FID 290° C. (sens. 10 １ )

[0075] Other components of the nonaqueous electrolyte secondary battery of the present disclosure will be specifically described below.

[0076] [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.

[0077] The positive electrode mixture layer may contain a conductive agent as an optional component. Examples of the conductive agent include carbon-based materials such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotubes (CNT), graphene, and graphite. These may be used alone or in combination of two or more.

[0078] 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-based resins, acrylic-based resins, and polyolefin-based resins. These may be used alone or in combination of two or more.

[0079] The positive electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the positive electrode current collector include aluminum, an aluminum alloy, titanium, and a titanium alloy.

[0080] 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, space group C2 / m, etc. Among these, a layered rock salt structure belonging to space group R-3m is preferred in terms of high capacity and highly stable crystal structure. The layered rock salt structure of the lithium-containing composite oxide may include a transition metal layer, a Li layer, and an oxygen layer.

[0081] From the viewpoint of increasing the capacity, the ratio of Ni to the metal elements other than Li contained in the lithium-containing composite oxide (Ni content) may be set to 50 atomic % or more, 80 atomic % or more, or 90 atomic % or more.

[0082] From the viewpoint of stabilizing the crystal structure of the lithium-containing composite oxide and improving the heat resistance of the battery, the ratio of Co to the metal elements other than Li contained in the lithium-containing composite oxide (Co content) may be set to 0 atomic % or more and 16 atomic % or less, or may be set to 1.5 atomic % or more and 16 atomic % or less.

[0083] Similarly, from the viewpoint of stabilizing the crystal structure of the lithium-containing composite oxide and improving the heat resistance of the battery, the ratio of Al to 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 may be set to 4 atomic % or more and 10 atomic % or less.

[0084] From the viewpoint of cost reduction, the ratio of Mn to the metal elements other than Li contained in the lithium-containing composite oxide (Mn content) may be set to 0 atomic % or more and 50 atomic % or less, or 0 atomic % or more and 30 atomic % or less.

[0085] The content of each metal element contained in the lithium-containing composite oxide is measured, for example, by inductively coupled plasma (ICP) emission spectrometry.

[0086] The lithium-containing composite oxide is, for example, a compound represented by the general formula Li ａ Ni ｘ Co ｙ Al ｚ M1 ｗ O ２－ｂ(wherein 0.8≦a≦1.2, 0.80≦x≦0.95, 0.015≦y≦0.16, 0.04≦z≦0.185, 0≦w≦0.145, 0≦b<0.05, x+y+z+w=1, and M1 is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn.) In this case, M1 is preferably Mn.

[0087] [Negative Electrode] The negative electrode includes a negative electrode current collector and may have a negative electrode mixture layer provided on the surface of the negative electrode current collector. The negative electrode current collector is made of a sheet-like conductive material. The negative electrode mixture layer is 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 a 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, as optional components, a binder, a conductive agent, a thickener, etc. Known materials can be used as the binder, conductive agent, and thickener.

[0088] The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions, lithium metal, lithium alloy, etc. As the material that electrochemically absorbs and releases lithium ions, a carbon material, an alloy-based material, etc. are used.

[0089] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and hard carbon, among which graphite is preferred because it has excellent charge / discharge stability and a small irreversible capacity.

[0090] Graphite is a carbonaceous material with a developed graphite crystal structure. The interplanar spacing d002 of the (002) plane 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 of the (002) plane of graphite and the crystallite size Lc(002) are within the above ranges, high capacity is easily obtained.

[0091] An alloy-based material is a material that contains at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, silicon oxide, tin oxide, and silicon-containing materials.

[0092] 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 that can be used include a silicon oxide phase, a silicate phase, and a carbon phase. 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. One type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0093] The silicon oxide phase may be mainly composed of silicon dioxide (e.g., 95 to 100% by mass). A silicon-containing material containing a silicon oxide phase and a silicon phase dispersed in the silicon oxide phase may be composed of SiO ｘ where x is, for example, 0.5≦x<2, and may be 0.8≦x≦1.6. The silicon oxide phase may be an amorphous phase. SiO ｘ can be obtained, for example, by the disproportionation reaction of silicon monoxide.

[0094] The silicate phase is preferred because it has a small irreversible capacity. Among them, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) can be preferably used as a lithium ion conductive phase having a high initial charge / discharge efficiency.

[0095] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may 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 has the formula: Li ２ｚ SiO ２＋ｚ(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 can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0096] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other.

[0097] A silicon-containing material in which a silicon phase is dispersed within a carbon phase can be obtained, for example, by pulverizing a mixture of a carbon source and raw silicon material while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere. The carbon source can be, for example, a sugar such as carboxymethyl cellulose (CMC) or a water-soluble resin such as polyvinylpyrrolidone.

[0098] A silicon-containing material and a carbon material may be used in combination as the negative electrode active material. Since the silicon-containing material expands and contracts in volume with charge and discharge, a large proportion of the silicon-containing material in the negative electrode active material is likely to cause poor contact between the negative electrode active material and the negative electrode current collector with charge and discharge. On the other hand, by using a silicon-containing material and a carbon material in combination, it is possible to achieve excellent cycle characteristics while imparting a high capacity to the negative electrode.

[0099] The proportion of the silicon-containing material in the total of the silicon-containing material and the carbon material is, for example, preferably 0.5 to 15 mass %, more preferably 1 to 10 mass %, which makes it easier to achieve both high capacity and improved cycle characteristics.

[0100] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0101] The composition of the silicon-containing material can be determined, for example, by obtaining a backscattered electron image of a 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. For the elemental analysis, for example, an electron probe microanalyzer (EPMA) analysis or the like is used.

[0102] 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)). One type of binder may be used alone, or two or more types may be used in combination.

[0103] The negative electrode mixture layer may contain a thickener. Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include CMC, its modified products, and methyl cellulose. Modified CMC also includes salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts. One type of thickener may be used alone, or two or more types may be used in combination.

[0104] The negative electrode mixture layer may contain a conductive agent. Examples of the conductive agent include carbon nanotubes (CNTs) and conductive particles. Examples of the conductive particles include conductive carbon (carbon black, etc.) and metal powder. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0105] The negative electrode current collector is selected depending on the type of nonaqueous electrolyte secondary battery. Examples of the negative electrode current collector include a sheet-like one. Metal foil or the like may also be used as the current collector. Alternatively, a porous current collector may also be used. Examples of porous current collectors include a mesh-like one, a punched sheet, and an expanded metal.

[0106] Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0107] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be, for example, a microporous thin film, a woven fabric, a nonwoven fabric, or a laminate of at least two selected from these. The separator is preferably made of polyolefin (e.g., polypropylene, polyethylene).

[0108] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an exterior body together with an electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the nonaqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0109] The structure of a nonaqueous electrolyte secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical secondary battery as an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0110] The nonaqueous electrolyte secondary battery (hereinafter referred to as battery 10) includes an electrode group 18, a nonaqueous electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode 15 and a negative electrode 16 with a separator 17 interposed therebetween.

[0111] (Additional Note) The above description discloses the following technology: (Technology 1) A non-aqueous solvent, a salt soluble in the non-aqueous solvent, and an additive soluble in the non-aqueous solvent, wherein the additive includes a heterocyclic compound and a sulfur-containing compound, and the heterocyclic compound is represented by the general formula (1):

[0112]

[0113] wherein X1 to X3 are each independently an oxygen atom or a sulfur atom, and R1 to R3 are each independently an alkyl group having 1 to 5 carbon atoms, at least one of whose hydrogen atoms may be substituted with a halogen atom. (Technology 2) The sulfur-containing compound includes at least one selected from the group consisting of a hexavalent sulfur compound and a tetravalent sulfur compound, and the hexavalent sulfur compound is represented by the general formula (2):

[0114]

[0115] The tetravalent sulfur compound has a structure represented by the general formula (3):

[0116]

[0117] The nonaqueous electrolyte for a secondary battery according to Technology 1 has a structure represented by the following formula: n is 0 or 1, R4 is, when n = 0, a fluorine atom, a hydrocarbon group, an imide group, or a silyl group, and when n = 1, a hydrocarbon group, a silyl group, or an alkali metal, R5 is a fluorine atom, a hydrocarbon group, an imide group, or a silyl group, R6 and R7 are each independently a hydrocarbon group, a silyl group, or an alkali metal, at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, R4 and R5 may form a ring, or R6 and R7 may form a ring. (Technology 3) The hexavalent sulfur compound is represented by the general formula (4):

[0118]

[0119] The non-aqueous electrolyte for secondary batteries according to Technology 2, wherein X1 is a fluorine atom, a hydrocarbon group, an imide group, or a silyl group, and X2 is a hydrocarbon group, a silyl group, or an alkali metal, and at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom. (Technology 4) The non-aqueous electrolyte for secondary batteries according to any one of Technology 1 to 3, wherein the content of the heterocyclic compound is 3 mass% or less. (Technology 5) The non-aqueous electrolyte for secondary batteries according to any one of Technology 1 to 4, wherein the content of the sulfur-containing compound is 5 mass% or less. (Technology 6) The non-aqueous electrolyte for secondary batteries according to any one of Technology 1 to 5, wherein the heterocyclic compound includes at least one selected from the group consisting of paraldehyde, trithioacetaldehyde, 2,4,6-triethyl-1,3,5-trioxane, 2,4,6-tripropyl-1,3,5-trioxane, and 2,4,6-tris(propan-2-yl)-1,3,5-trioxane. (Technology 7) The non-aqueous electrolyte for secondary batteries according to any one of Techniques 1 to 6, wherein the hexavalent sulfur compound comprises at least one selected from the group consisting of lithium fluorosulfonate (LiFSO3), 1-propene-1,3-sultone (PRS), ethylene sulfate (DTD), lithium bisfluorosulfonylimide (LiFSI), and 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide (MMDS). (Technology 8) The non-aqueous electrolyte for secondary batteries according to any one of Techniques 1 to 7, wherein the tetravalent sulfur compound comprises at least one selected from the group consisting of ethylene sulfite (ES) and vinyl ethylene sulfite (VES). (Technology 9) A secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte, wherein the non-aqueous electrolyte is the non-aqueous electrolyte for secondary batteries according to any one of Techniques 1 to 8.

[0120] While the present invention has been described with reference to presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all such changes and modifications that do not depart from the true spirit and scope of the invention.

[0121] EXAMPLES The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0122] Examples 1 to 8 and Comparative Examples 1 to 6 Non-aqueous electrolyte secondary batteries were fabricated and evaluated according to the following procedures.

[0123] (1) Preparation of the positive electrode: Lithium-containing composite oxide (LiNi ０．８ Co ０．１ 8Al ０．０２ O ２ To 95 parts by mass of aluminum foil, 2.5 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidinone (NMP) were added and mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm) on both sides of the aluminum foil. ３ ) was formed to obtain a positive electrode.

[0124] (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 SiOx particles (x = 1, average particle size (D50) 5 μm) whose surfaces were coated with a conductive layer of conductive carbon. In the negative electrode active material, the mass ratio of the silicon-containing material excluding the conductive layer to graphite was 6:94. The binders used were sodium polyacrylate (PAA-Na), a sodium salt of CMC (CMC-Na), and SBR. The contents of PAA-Na, CMC-Na, and SBR in the negative electrode mixture were each 1% by mass. Next, the negative electrode slurry was applied to the surface of the copper foil, the coating was dried, and then the copper foil was rolled to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm ) on both sides of the copper foil. ３ ) was formed to obtain a negative electrode.

[0125] (3) Preparation of non-aqueous electrolyte A non-aqueous electrolyte was prepared by dissolving LiPF6 and, if necessary, the additives shown in Table 1 in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 30:70 (volume ratio)). ６ The concentration of the additive in the non-aqueous electrolyte (initial concentration) was set to the value (mass %) shown in Table 1.

[0126] (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. The positive and negative electrodes were spirally wound in an inert gas atmosphere with a polyethylene thin film (separator) interposed between them to produce a wound electrode assembly. A first insulating plate was placed on the lower end surface of the electrode assembly, the electrode assembly 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. A second insulating plate was placed on the upper end surface of the electrode assembly, and an annular groove was formed near the open edge of the battery can. Next, the positive electrode lead was connected to the metal plate of the safety mechanism provided in the sealing body, and the non-aqueous electrolyte was poured into the battery can. The battery can was then supported in the annular groove formed in the battery can via a gasket, and the open edge of the battery can was crimped to the periphery of the sealing body to complete the lithium-ion secondary battery.

[0127] [Evaluation 1] Initial DC Resistance (DCR) In an environment of 25°C, the battery was charged at a constant current of 0.3 It until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 It. Next, the battery was discharged at a constant current of 0.3 It for 100 minutes to bring the state of charge (SOC) to 50%.

[0128] The battery with an SOC of 50% was discharged for 10 seconds at currents of 0 A, 0.1 A, 0.5 A, and 1.0 A, and the voltage was measured. The relationship between the discharge current and the voltage after 10 seconds was approximated to a straight line using the least squares method, and the DCR (initial DCR) was calculated from the absolute value of the slope. Table 1 shows the relative values ​​(index) when the DCR of the battery of Comparative Example 1 is set to 100.

[0129] [Evaluation 2] High-Temperature Cycle Characteristics In a 45°C environment, the nonaqueous electrolyte secondary battery was subjected to constant-current charging at a current of 0.5 It until the voltage reached 4.2 V, and then to constant-voltage charging at 4.2 V until the current reached 0.05 It. After a 10-minute rest, the nonaqueous electrolyte secondary battery was subjected to constant-current discharging at a current of 0.7 It until the voltage reached 2.5 V. The discharge capacity (Ci) at this time was determined. This cycle of charge, rest, and discharge was counted as one cycle, and 300 cycles were repeated to determine the discharge capacity (Cc) at the 300th cycle. The ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci, taken as 100%, was determined as the capacity retention rate.

[0130] [Evaluation 3] Storage Gas Amount (80°C / 3 days) In a 25°C environment, the nonaqueous electrolyte secondary battery was charged at a constant current of 0.5 It until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 It. The charged battery was stored at 80°C for 3 days, and gas was recovered from the battery after storage. Table 1 shows the relative value (index) when the gas amount of the battery of Comparative Example 1 is set to 100.

[0131] The results of the Examples and Comparative Examples are shown in Table 1. In Table 1, E1 to E8 are Examples 1 to 8, and C1 to C6 are Comparative Examples 1 to 6.

[0132] The additives in the table are indicated as follows: PAD: Paraldehyde TAD: Trithioacetaldehyde TEY: 2,4,6-triethyl-1,3,5-trioxane TPYT: 2,4,6-tripropyl-1,3,5-trioxane TPAT: 2,4,6-tris(propan-2-yl)-1,3,5-trioxane TOX: 1,3,5-trioxane

[0133] LiFSO ３ : Lithium fluorosulfonate PRES: 1-propene-1,3-sultone ES: Ethylene sulfite DTD: Ethylene sulfate

[0134]

[0135] As shown in Table 1, the heterocyclic compound (1) alone has almost no effect on improving high-temperature cycle performance (C1 and C5). It is also found that the heterocyclic compound (1) has a slight effect on reducing initial resistance (C1 and C4 to C7). On the other hand, the sulfur-containing compound alone actually reduces high-temperature cycle performance (C1 to C3). Batteries C1 to C6 are unable to achieve a good balance between low DCR and high high-temperature cycle performance.

[0136] In contrast, batteries E1 to E8 exhibit a good balance between low DCR and high high-temperature cycle performance. Furthermore, a comparison between battery C6 and battery E1 reveals that such a synergistic effect is specifically achieved by heterocyclic compound (1). Furthermore, the coating derived from the sulfur-containing compound and heterocyclic compound (1) has high thermal stability and a strong effect of suppressing the oxidation-reduction decomposition of the non-aqueous electrolyte, significantly reducing the amount of gas generated during high-temperature storage.

[0137] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, etc. However, the uses of the nonaqueous electrolyte secondary battery are not limited to these.

[0138] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode, 15a: Positive electrode lead, 16: Negative electrode, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove, 23: First insulating plate, 24: Second insulating plate, 16: Negative electrode

Claims

1. A non-aqueous electrolyte for a secondary battery, comprising a non-aqueous solvent, a salt dissolved in the non-aqueous solvent, and an additive dissolved in the non-aqueous solvent, wherein the additive includes a heterocyclic compound and a sulfur-containing compound, the heterocyclic compound is represented by the general formula (1): 【Chemical 1】 represented by, X1 to X3 are each independently an oxygen atom or a sulfur element, R1 to R3 are each independently an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom may be substituted with a halogen atom.

2. The sulfur-containing compound includes at least one selected from the group consisting of a hexavalent sulfur compound and a tetravalent sulfur compound, the hexavalent sulfur compound has a structure represented by the general formula (2): 【Chemical Formula 2】 having the structure represented by, the tetravalent sulfur compound has a structure represented by the general formula (3): [Chemical Formula 3] having the structure represented by, n is 0 or 1, R4 is, when n = 0, it is a fluorine atom, a hydrocarbon group, an imide group or a silyl group, when n = 1, it is a hydrocarbon group, a silyl group or an alkali metal, R5 is a fluorine atom, a hydrocarbon group, an imide group or a silyl group, R6 and R7 are each independently a hydrocarbon group, a silyl group or an alkali metal, at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom, R4 and R5 may form a ring, R6 and R7 may form a ring. The non-aqueous electrolyte for a secondary battery according to claim 1.

3. The hexavalent sulfur compound is represented by the general formula (4): 【Chemical Formula 4】 represented by, X1 is a fluorine atom, a hydrocarbon group, an imide group or a silyl group, X2 is a hydrocarbon group, a silyl group or an alkali metal, at least one hydrogen atom of the hydrocarbon group may be substituted with a halogen atom. The non-aqueous electrolyte for a secondary battery according to claim 2.

4. The content of the heterocyclic compound is 3% by mass or less. The non-aqueous electrolyte for a secondary battery according to claim 1.

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

6. The heterocyclic compound includes at least one selected from the group consisting of para-aldehyde, trithioacetaldehyde, 2,4,6-triethyl-1,3,5-trioxane, 2,4,6-tripropyl-1,3,5-trioxane, and 2,4,6-tris(propan-2-yl)-1,3,5-trioxane. The non-aqueous electrolyte for a secondary battery according to claim 1.

7. The hexavalent sulfur compound contains at least one selected from the group consisting of lithium fluorosulfonate, 1-propene-1,3-sultone, ethylene sulfate, lithium bis(fluorosulfonyl)imide, and 1,5,2,4-dioxadithiane-2,2,4,4-tetraoxide, and the non-aqueous electrolyte for a secondary battery according to claim 2.

8. The tetravalent sulfur compound contains at least one selected from the group consisting of ethylene sulfite and vinyl ethylene sulfite, and the non-aqueous electrolyte for a secondary battery according to claim 2.

9. A secondary battery comprising a positive electrode, a separator, a negative electrode facing the positive electrode via the separator, and a non-aqueous electrolyte. The non-aqueous electrolyte is the non-aqueous electrolyte for a secondary battery according to claim 1, and the secondary battery.

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