Nonaqueous electrolyte for secondary battery, and secondary battery

Abstract

This nonaqueous electrolyte for a secondary battery contains dichloromethane and an S-containing cyclic compound. The S-containing cyclic compound has a 5- or 6-membered S-containing ring that contains a sulfur atom.

Description

Non-aqueous electrolyte for secondary battery and secondary battery Cross-reference to related applications

[0001] This disclosure claims the benefit of priority for Japanese Patent Application No. 2025-006323 filed with the Japan Patent Office on January 16, 2025, and incorporates the entire contents of the said patent application herein by reference.

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

[0003] Patent Document 1 proposes a non-aqueous electrolyte secondary battery characterized by containing at least one 1,3-propene sultone derivative in a non-aqueous electrolyte and containing 2.0 wt% or less of a glycol sulfate derivative.

[0004] Japanese Patent Application Laid-Open No. 2003-142152

[0005] By using an S-containing cyclic compound such as a 1,3-propene sultone derivative as an additive for a non-aqueous electrolyte, a good solid electrolyte interface (SEI) is formed on the surface of the negative electrode. Due to the good SEI, the reduction reaction (gas generation reaction) of the non-aqueous electrolyte during high-temperature storage and charge-discharge cycles is suppressed.

[0006] However, the requirements for improving the performance of secondary batteries are increasing. Further improvement of the SEI is necessary to further improve the life characteristics of secondary batteries.

[0007] One aspect of this disclosure relates to a non-aqueous electrolyte for a secondary battery, which contains dichloromethane and an S-containing cyclic compound, and the S-containing cyclic compound has a 5-membered or 6-membered S-containing ring containing a sulfur atom.

[0008] Another aspect of this disclosure relates to a secondary battery including a positive electrode, a negative electrode, and the non-aqueous electrolyte described above.

[0009] By using dichloromethane and a sulfur-containing cyclic compound in combination as additives to a non-aqueous electrolyte, the lifespan characteristics of a secondary battery comprising that non-aqueous electrolyte are significantly improved. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of its structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description with reference to the drawings.

[0010] This is a partially cutaway perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure.

[0011] 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 ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". 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 is not greater than or equal to 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.

[0012] 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.

[0013] A non-aqueous electrolyte for a secondary battery according to one embodiment of the present disclosure comprises dichloromethane and a sulfur-containing cyclic compound. The sulfur-containing cyclic compound has a five-membered or six-membered sulfur-containing ring containing a sulfur atom.

[0014] S-containing cyclic compounds are compounds that, in one respect, have a structure similar to propensultones. In S-containing cyclic compounds, the sulfur atoms contained in the S-containing ring are, for example, in an O=S=O structure (hereinafter referred to as "SO"). ２It may also be called a "structure" and may form -O-SO ２ It may form a structure.

[0015] The sulfur atoms in the sulfur-containing ring are -O-SO ２ -O-structure, -O-SO ２ -C-structure or -O-SO ２ -C= may have a structure. That is, the carbon atoms in the S-containing ring may form carbon-carbon unsaturated bonds.

[0016] The sulfuric acid-containing cyclic compound may be at least one selected from the group consisting of cyclic sulfuric acid esters and sulfonic acid esters. The sulfuric acid ester is -O-SO ２ It has an -O- structure. Sulfonic acid esters are -O-SO ２ - It has a C structure.

[0017] Examples of cyclic sulfate esters include alkylene sulfates and alkenylene sulfates. Specific examples of cyclic sulfate esters include ethylene sulfate, propylene sulfate, trimethylene sulfate, butylene sulfate, and vinylene sulfate.

[0018] Examples of cyclic sulfonic acid esters include at least one selected from the group consisting of alkanesultones and alkenesultones. Specific examples of cyclic sulfonic acid esters include 1,3-propanesultone, 1,4-butanesultone, and 1,3-propensultone.

[0019] The sulfur-containing cyclic compound may have one or more hydrogen atoms substituted with substituents. Examples of substituents include alkyl groups, alkenyl groups, hydroxyalkyl groups, hydroxyl groups, alkoxy groups, and halogen atoms. The substituent may have 1 to 4 or 1 to 3 carbon atoms. Fluorine atoms are preferred as halogen atoms.

[0020] Since dichloromethane has a higher reduction potential than sulfur-containing cyclic compounds, it is thought to be preferentially reduced and decomposed on the negative electrode surface, forming a base layer for SEI. Subsequently, the sulfur-containing cyclic compounds undergo reduced decomposition on the negative electrode surface to form SEI. In this case, the base layer derived from dichloromethane has the effect of suppressing the excessive reduction reaction of sulfur-containing cyclic compounds.

[0021] Unlike the case where the non-aqueous electrolyte does not contain dichloromethane but contains a sulfur-containing cyclic compound, when the non-aqueous electrolyte contains both dichloromethane and a sulfur-containing cyclic compound, a low-resistance SEI (Surface-Emitting Isolate) is formed on the negative electrode surface, which significantly improves lifespan characteristics.

[0022] The underlying layer formed by the reductive decomposition of dichloromethane only needs to be formed to the extent that it can sufficiently cover the active sites of side reactions on the negative electrode surface. Therefore, the mass content of dichloromethane in the non-aqueous electrolyte may be, for example, 1 ppb to 3000 ppm, preferably 10 ppb to 1000 ppm, and more preferably 1 ppm to 100 ppm.

[0023] From the viewpoint of forming a better SEI, the ratio (Y / X) of the mass content Y of dichloromethane to the mass content X of sulfur-containing cyclic compounds in the non-aqueous electrolyte may be controlled. For example, the Y / X ratio may be 0.0001 to 0.01 or 0.0005 to 0.005. In this case, the components derived from dichloromethane and the components derived from sulfur-containing cyclic compounds in the SEI formed on the negative electrode surface are compounded in a suitable state, resulting in the formation of an SEI that can further improve its lifespan characteristics.

[0024] S-containing cyclic compounds include, for example, the following formula:

[0025]

[0026] It may be at least one compound selected from the group of compounds represented by (compound group A). ​​The nonaqueous electrolyte may contain only one sulfur-containing cyclic compound from compound group A, or it may contain two or more.

[0027] Here, X is independently a hydrocarbon group having one or two carbon atoms, or an oxygen atom.

[0028] The hydrocarbon group X, having one or two carbon atoms, can be a methylene group, an ethylene group, a halide methylene group, a halide ethylene group, etc. The halogen atom is preferably a fluorine atom.

[0029] To say that each X is independent means that any X is independent of all other X. Specifically, if a compound's formula contains two or more X's, those two or more X's may be the same or different from each other. Similarly, if multiple compounds' formulas each contain X's, the X's in the formulas of those multiple compounds may be the same or different from each other. The same applies to Y, R1, and R2 below.

[0030] Each Y element, when not bonded to a double bond, is either a C1 or C2 hydrocarbon group or an oxygen atom; when bonded to a double bond, it is a C1 or C2 hydrocarbon group. In other words, in the compound formula, when Y forms an S-Y= structure, Y is not an oxygen atom, but a C1 or C2 hydrocarbon group.

[0031] The hydrocarbon group Y, having one or two carbon atoms, can be a methylene group, an ethylene group, a halide methylene group, a halide ethylene group, etc. The halogen atom is preferably a fluorine atom.

[0032] It is preferable that at least one of X and Y is an oxygen atom, as this has a high SEI-forming ability. If X or Y is a hydrocarbon group, a C1 hydrocarbon group (e.g., a methylene group) is more preferable. That is, it is more preferable that the S-containing cyclic compound has a five-membered ring containing a sulfur atom.

[0033] R1 and R2 are each independently a hydrogen atom, a halogen atom, or a hydrocarbon group which may have an unsaturated bond with 1 to 4 carbon atoms. A fluorine atom is preferred as the halogen atom. The hydrocarbon group which may have an unsaturated bond with 1 to 4 carbon atoms may be a hydrocarbon group without an unsaturated bond, such as a methyl group, ethyl group, n-propyl group, iso-propyl group, n-butyl group, or isobutyl group, or a hydrocarbon group with an unsaturated bond, such as a vinyl group or allyl group.

[0034] S-containing cyclic compounds include, for example, the following formula:

[0035]

[0036] The nonaqueous electrolyte may contain at least one compound selected from the group of compounds represented by (compound group B). The nonaqueous electrolyte may contain only one sulfur-containing cyclic compound from compound group B, or it may contain two or more.

[0037] Among sulfur-containing cyclic compounds, 1,3-propensultone is suitable for good SEI formation and is also readily available. 50% or more by mass of the sulfur-containing cyclic compounds in the non-aqueous electrolyte may be 1,3-propensultone, or 60% or more by mass, 70% or more by mass, 80% or more by mass, 90% or more by mass, or even 100% of the sulfur-containing cyclic compounds may be 1,3-propensultone.

[0038] The content of the sulfur-containing cyclic compound in the non-aqueous electrolyte is not particularly limited as long as it is in an appropriate amount as an additive, but for example, 0.1% to 2% by mass is preferred, 0.3% to 2% by mass is also acceptable, 0.3% to 1.5% by mass is also acceptable, and 0.5% to 1.1% by mass is also acceptable. When the sulfur-containing cyclic compound is used within the above range, a good SEI is formed on the negative electrode surface and side reactions are significantly suppressed. Therefore, the capacity retention rate of the secondary battery can be maintained at a high level over a long period of time.

[0039] The content of sulfur-containing cyclic compounds in the non-aqueous electrolyte collected from secondary batteries may be 0.01% by mass or more, or 0.05% by mass or more. In secondary batteries, sulfur-containing cyclic compounds are used for film formation, so the concentration of sulfur-containing cyclic compounds in the non-aqueous electrolyte changes during storage or charge-discharge cycles. Therefore, the content of sulfur-containing cyclic compounds in the non-aqueous electrolyte collected from secondary batteries only needs to be above the detection limit.

[0040] When collecting non-aqueous electrolytes from secondary batteries, it is preferable to collect them from unused secondary batteries that have been assembled, undergone break-in charge-discharge (and aging as necessary), and have been in use for a period of 60 days or less.

[0041] The non-aqueous electrolyte is usually used in a liquid state, but may also be in a state where its fluidity is restricted by a gelling agent or the like. The non-aqueous electrolyte usually contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent, and may further contain an additive in addition to these. In the present disclosure, the non-aqueous electrolyte contains an S-containing cyclic compound as an additive. The non-aqueous electrolyte may further contain an additive other than the S-containing cyclic compound.

[0042] Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters.

[0043] Examples of the cyclic carbonate include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), and the like.

[0044] Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and the like.

[0045] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0046] Examples of the chain carboxylic acid ester include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and the like.

[0047] The non-aqueous electrolyte may contain one type of non-aqueous solvent or may contain a combination of two or more types.

[0048] Examples of the lithium salt include, for example, LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts.

[0049] As for phosphates, lithium difluorophosphate (LiPO) ２ F ２ Examples include lithium difluorobis(oxalato)phosphate (LiDFBOP), lithium tetrafluoro(oxalato)phosphate, etc.

[0050] Examples of borates include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB).

[0051] Examples of imide salts include bisfluorosulfonylimide lithium (LiN(FSO) ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ Examples include:

[0052] The non-aqueous electrolyte may contain one lithium salt or a combination of two or more lithium salts.

[0053] The concentration of lithium salt in the non-aqueous electrolyte is not particularly limited, but may be, for example, 0.5 mol / L or more and 2 mol / L or less.

[0054] The configuration of a secondary battery according to this disclosure will be described in more detail below. The secondary battery comprises a positive electrode, a negative electrode, and the non-aqueous electrolyte. The secondary battery may also include a separator placed between the positive electrode and the negative electrode. These components will be described below.

[0055] Aside from using the non-aqueous electrolyte mentioned above, various other components can be selected. For example, known components may be used for the other components.

[0056] (Negative electrode) The negative electrode includes at least a negative electrode current collector and may have a negative electrode mixture layer. The negative electrode mixture layer is a layered negative electrode mixture, and the negative electrode mixture includes at least a negative electrode active material. The negative electrode mixture may further include a binder, a thickener, a conductive additive, etc.

[0057] Examples of negative electrode active materials include carbonaceous materials and Si-containing materials. Since Si-containing materials expand and contract in volume during charging and discharging, if their 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 during charging and discharging.

[0058] Carbonaceous materials exhibit less expansion and contraction during charging and discharging than Si-containing materials. By using Si-containing materials and carbonaceous materials in combination, the contact state between negative electrode active material particles and between the negative electrode mixture and the negative electrode current collector can be maintained more effectively when charging and discharging is repeated. Therefore, by using Si-containing materials and carbonaceous materials that do not contain the Si phase in combination, it is easier to obtain excellent cycle characteristics while imparting a high capacity of the Si phase to the negative electrode.

[0059] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). A single carbonaceous material may be used, or two or more may be used in combination.

[0060] Graphite is preferred as a carbonaceous material due to its excellent charge-discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, easily graphitizable carbon, and difficult-to-graphitize carbon.

[0061] Graphite is a carbonaceous material with a well-developed graphite-type crystal structure. The average 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 average interplanar spacing d002 and crystallite size Lc(002) of the (002) planes of graphite are within the above ranges, high capacity is easily obtained.

[0062] The total ratio of Si-containing material and carbonaceous material (carbonaceous material without Si phase) in the negative electrode active material is preferably 90% by mass or more, and may be 95% by mass or more or 98% by mass or more. The total ratio of Si-containing material and carbonaceous material in the negative electrode active material is 100% by mass or less. The negative electrode active material may be composed only of Si-containing material and carbonaceous material.

[0063] Si-containing materials are, for example, composite particles in which a Si phase (fine Si phase) is dispersed within a lithium-ion conducting phase (matrix).

[0064] The Si-containing material may also be a carbon composite particle containing a carbon phase and a Si phase dispersed within the carbon phase. Because the carbon phase is electrically conductive, even if cracks occur in the carbon composite particle due to the expansion and contraction of the Si phase, it is less likely to become isolated, and it is easier to maintain contact between the carbon composite particle and its surroundings. Therefore, it is easier to suppress the deterioration of cycle characteristics.

[0065] The carbon phase can consist of, for example, amorphous carbon (i.e., crystalline carbon) and crystalline carbon. Amorphous carbon may be hard carbon, soft carbon, or something else. Generally, amorphous carbon refers to a carbonaceous material in which the average interplanar spacing d002 of (002) planes, as measured by X-ray diffraction, exceeds 0.340 nm. Examples of crystalline carbon include carbon with a graphite-type crystalline structure, such as graphite. Crystalline carbon such as graphite refers to a carbonaceous material in which d002 is 0.340 nm or less (for example, between 0.3354 nm and 0.340 nm).

[0066] The Si phase content in the carbon composite particles is, for example, 30% to 80% by mass, and may also be 40% to 70% by mass. Within this range, a higher initial capacity can be obtained, and the deterioration of cycle characteristics can be easily reduced. In addition, by including a relatively large amount of carbon phase, even if cracks occur in the particles due to charging and discharging, the carbon phase can easily penetrate into the resulting voids, and the conductive path in the negative electrode mixture can be easily maintained.

[0067] The content of carbon composite particles in the negative electrode active material may be, for example, 2% by mass or more, 3% by mass or more, 4% by mass or more, or 5% by mass or more. From the viewpoint of ensuring higher cycle characteristics, the content of carbon composite particles in the negative electrode active material may be, for example, 20% by mass or less, or 10% by mass or less.

[0068] Si-containing materials are SiO ２ Phase and SiO ２ These may also be composite particles containing a Si phase dispersed within the phase (SiO composite particles), or composite particles containing a silicate phase and a Si phase dispersed within the silicate phase (silicate composite particles).

[0069] SiO ２ The phase is an amorphous phase containing 95% or more by mass of silicon dioxide. SiO ２ SiO composite particles in which the Si phase is dispersed within the phase are SiO ｘ It is expressed as follows: x may be, for example, 0.5 ≤ x < 2, and 0.8 ≤ x ≤ 1.6. SiO ｘ For example, silicon monoxide is heat-treated and undergoes a disproportionation reaction to produce SiO ２ It is obtained by separating the phase and the fine Si phase. Using a transmission electron microscope (TEM), SiO ｘ When observing the particle cross-section, SiO ２ The Si phase dispersed within the phase can be observed.

[0070] The silicate phase preferably contains at least one of alkali metal elements (Group 1 elements other than hydrogen in the long-period periodic table) and Group 2 elements in the long-period periodic table. Alkali metal elements include lithium (Li), potassium (K), sodium (Na), etc. Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The lithium silicate phase has the formula: Li ２ｙ SiO ２＋ｙ The composition may be represented by (0 < y < 2), where y may be 1 / 2 or 1. Silicate composite particles in which the Si phase is dispersed within the silicate phase can be obtained, for example, by grinding a mixture of silicate 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.

[0071] The Si phase content dispersed within the silicate phase may be 30% to 95% by mass, or 35% to 75% by mass, relative to the total silicate composite particles.

[0072] The content of SiO composite particles in the negative electrode active material is, for example, 0.1% by mass or more, may be 0.5% by mass or more, or may be 1% by mass or more. The content of SiO composite particles in the negative electrode active material is, for example, 20% by mass or less, may be 10% by mass or less, or 5% by mass or less.

[0073] The content of silicate composite particles in the negative electrode active material is, for example, 0.1% by mass or more, may be 0.5% by mass or more, or may be 1% by mass or more. The content of silicate composite particles in the negative electrode active material is, for example, 20% by mass or less, may be 10% by mass or less, or 5% by mass or less.

[0074] Si-containing materials are typically particulate materials. The average particle size (D50) of the Si-containing material is, for example, 1 μm to 25 μm, and may also be 4 μm to 15 μm. Good battery performance is easily obtained within this range.

[0075] In this specification, the average particle size (D50) refers to the particle size (volume-average particle size) at which the volume integrated value in the particle size distribution measured by laser diffraction scattering method becomes 50%. For the measuring device, for example, the "LA-750" manufactured by HORIBA Ltd. may be used. The average particle size of the Si-containing material may also be determined from a cross-sectional sample of the negative electrode formed to obtain a backscattered electron image using FE-SEM. The equivalent circle diameter of the cross-sections of 10 or more Si-containing material particles is determined, and their average value is determined as the average particle size. Here, the equivalent circle diameter refers to the diameter of a circle having the same area as the area of ​​the particle observed in the cross-section of the negative electrode.

[0076] The Si phase dispersed within the carbon phase is usually composed of multiple crystallites. The crystallite size of the Si phase is, for example, 500 nm or less, and may also be 30 nm or less. The lower limit of the crystallite size of the Si phase is not particularly limited, but is, for example, 5 nm or more. The crystallite size is calculated from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the Si phase using Scherrer's formula.

[0077] The Si phase content in composite particles can be measured, for example, by Si-NMR.

[0078] (Binding agent) For example, resin materials can be used as binding agents. Examples of binding agents include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resin), 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 binding agent may be used, or two or more may be used in combination.

[0079] (Thickening agents) Examples of thickening agents include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include 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.

[0080] (Conductive additives) Examples of conductive agents include conductive fibers and conductive particles. Examples of conductive fibers include carbon fibers and metal fibers. Carbon fibers also include carbon nanotubes (CNTs). Examples of conductive particles include conductive carbon (such as carbon black) and metal powders. Conductive agents may be used individually or in combination of two or more types.

[0081] (Negative electrode current collector) 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-shaped current collectors. Metal foil may also be used as the current collector. A porous current collector may also be used. Examples of porous current collectors include mesh materials, perforated sheets, and expanded metal.

[0082] Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0083] The thickness of the negative electrode current collector is not particularly limited and may be, for example, 1 μm or more and 50 μm or less, or 5 μm or more and 30 μm or less.

[0084] (Positive Electrode) The positive electrode may comprise a positive electrode current collector and a positive electrode mixture layer held on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by coating a positive electrode slurry, obtained by dispersing the positive electrode mixture in a dispersion medium, onto the surface of the positive electrode current collector and drying it. The dried coating may be rolled if necessary. The positive electrode mixture may contain a positive electrode active material as an essential component and may contain binders, conductive agents, etc., as optional components. As the dispersion medium, for example, it can be selected from the dispersion media exemplified for the negative electrode.

[0085] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal is used. Examples of transition metals include Ni, Co, Mn, etc. Examples of composite oxides containing lithium and a transition metal include Li ａ CoO ２ Li ａ NiO ２ Li ａ MnO ２ Li ａ Co ｂ１ Ni １－ｂ１ O ２ Li ａ Co ｂ１ M １－ｂ１ O ｃ１ Li ａ Ni １－ｂ１ M ｂ１ O ｃ１ Li ａ Mn ２ O ４ Li ａ Mn ２－ｂ１ M ｂ１ O ４ The following are examples. Here, a = 0 to 1.2, b1 = 0 to 0.9, and c1 = 2.0 to 2.3. M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Note that the a value, which indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0086] Among them, Li ａ Ni ｂ２ M １－ｂ２ O ２ Lithium nickel composite oxides represented by (0 < a ≤ 1.2, 0.3 ≤ b² ≤ 1, and M is at least one selected from the group consisting of Mn, Co, and Al) are preferred. From the viewpoint of increasing capacity, it is more preferable that 0.8 ≤ b² ≤ 1 or 0.85 ≤ b² ≤ 1 is satisfied. From the viewpoint of crystal structure stability, Li ａ Ni ｂ２ Co ｃ２ Al ｄ O ２ (0 < a ≤ 1.2, 0.8 ≤ b² < 1, 0 < c² < 0.2 (or 0 < c² ≤ 0.18), 0 < d ≤ 0.1, b² + c² + d = 1) is even more preferable.

[0087] As a binder, resin materials such as those exemplified for the negative electrode can be used. As a conductive agent, for example, one can be selected from the conductive agents exemplified for the negative electrode. Graphite may also be used as the conductive agent.

[0088] The shape and thickness of the positive electrode current collector can be selected from the shapes and ranges described for the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0089] (Separator) Generally, 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. Examples of separators include microporous thin films, woven fabrics, and nonwoven fabrics. The separator may have a single-layer structure or a multilayer structure. A multilayer separator may be a laminate containing at least two layers selected from the group consisting of microporous thin films, woven fabrics, and nonwoven fabrics. Polyolefins (e.g., polypropylene, polyethylene) are preferred as the material for the separator.

[0090] One example of a secondary battery structure is one in which an electrode group, in which the positive and negative electrodes are wound around a separator, is housed together with a non-aqueous electrolyte in an outer casing. However, the structure of a non-aqueous electrolyte secondary battery is not limited to this structure. For example, the electrode group may be a stacked type in which the positive and negative electrodes are stacked with a separator in between. The form of a non-aqueous electrolyte secondary battery is also not limited, and may be cylindrical, prismatic, coin-type, button-type, or laminate-type, for example.

[0091] 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 10, which is an example of this embodiment. However, this disclosure is not limited to the following configuration.

[0092] The secondary battery 10 comprises an electrode group 18, an electrolyte (not shown), and a bottomed cylindrical battery case 22 that houses these components. 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 from the positive electrode 15, is connected to the metal plate 13. Therefore, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a, led from the negative electrode 16, is connected to the inner surface of the bottom of the battery case 22. An annular groove 22a is formed near the open end of the battery case 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.

[0093] [Note] The above description of embodiments discloses the following technologies: (Technology 1) A non-aqueous electrolyte for a secondary battery comprising dichloromethane and an S-containing cyclic compound, wherein the S-containing cyclic compound has a 5-membered or 6-membered S-containing ring containing a sulfur atom. (Technology 2) The non-aqueous electrolyte for a secondary battery according to Technology 1, wherein the mass content of dichloromethane is from 10 ppb to 1000 ppm. (Technology 3) The non-aqueous electrolyte for a secondary battery according to Technology 1 or 2, wherein the ratio (Y / X) of the mass content Y of dichloromethane to the mass content X of the S-containing cyclic compound is 0.0001 to 0.01. (Technology 4) The non-aqueous electrolyte for a secondary battery according to any one of Technology 1 to 3, wherein the S-containing ring has a carbon-carbon unsaturated bond. (Technology 5) The S-containing cyclic compound has the following formula:

[0094]

[0095] A non-aqueous electrolyte for a secondary battery according to any one of the technologies 1 to 4, wherein at least one compound is selected from the group of compounds represented by the formula, where X is independently a hydrocarbon group having 1 or 2 carbon atoms or an oxygen atom, Y is independently a hydrocarbon group having 1 or 2 carbon atoms or an oxygen atom if not bonded to a double bond, and a hydrocarbon group having 1 or 2 carbon atoms if bonded to a double bond, and R1 and R2 are independently a hydrogen atom, a halogen atom, or a hydrocarbon group having 1 to 4 carbon atoms which may have an unsaturated bond. (Technology 6) The S-containing cyclic compound is given by the following formula:

[0096]

[0097] (Technology 7) A non-aqueous electrolyte for a secondary battery according to any one of the technologies 1 to 5, wherein the S-containing cyclic compound is selected from the group of compounds represented by (Technology 7). A non-aqueous electrolyte for a secondary battery according to any one of the technologies 1 to 6, wherein the S-containing cyclic compound contains 1,3-propensultone. (Technology 8) A non-aqueous electrolyte for a secondary battery according to any one of the technologies 1 to 7, wherein the content of the S-containing cyclic compound is 0.1% by mass to 2% by mass. (Technology 9) A secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte according to any one of the technologies 1 to 8.

[0098] [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.

[0099] Examples 1-3 and Comparative Examples 1-2: Non-aqueous electrolyte secondary batteries were prepared and evaluated according to the following procedure.

[0100] (1) Preparation of the negative electrode As the negative electrode active material, graphite particles and SiO composite particles (SiO) were mixed in a mass ratio of graphite particles:SiO composite particles = 95:5. Graphite particles with an average particle size (D50) of 25 μm were used.

[0101] A negative electrode active material, a binder, and a conductive agent were mixed to form a negative electrode mixture, and an appropriate amount of water was added to the mixture to obtain a negative electrode slurry. Sodium polyacrylate (PAA-Na), sodium salt of CMC (CMC-Na), and SBR were used as the binder. CNTs (average diameter approximately 1.6 nm, average length approximately 5 μm) were used as the conductive agent.

[0102] The CNT content in the negative electrode mixture (dry solids) was set to 0.05% by mass. The PAA-Na, CMC-Na, and SBR content in the negative electrode mixture was set to 1% by mass, based on dry solids.

[0103] Next, the negative electrode slurry is applied to the surface of the copper foil, the coating is dried, and then it is rolled to create a negative electrode mixture layer (80 μm thick, 1.6 g / cm³ density) on both sides of the copper foil. ３ A negative electrode was obtained by forming a negative electrode.

[0104] (2) Preparation of the positive electrode Lithium-containing composite oxide (LiNi ０．８ Co ０．１８ Al ０．０２ O ２ 95 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of NMP were added and mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of the aluminum foil, the coating was dried, and then the foil was rolled to create a positive electrode mixture 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.

[0105] (3) Preparation of non-aqueous electrolytes A mixed solvent of EC and DMC (EC:DMC = 20:80 (volume ratio)) is mixed with LiPF ６ A non-aqueous electrolyte was prepared by dissolving dichloromethane (DCLM) and / or 1,3-propensultone (PRES) as needed. LiPF in the non-aqueous electrolyte ６ The concentration was set to 1.35 mol / L. The concentrations of DCLM and PRES in the non-aqueous electrolyte (concentrations during the preparation of the non-aqueous electrolyte) were as shown in Table 1 (mass%).

[0106] (4) Fabrication of a secondary battery An Al positive electrode lead was attached to the positive electrode, and a Ni negative electrode lead was attached to the negative electrode. In an inert gas atmosphere, the positive and negative electrodes were wound in a spiral shape via a polyethylene thin film (separator) to fabricate a wound electrode group. The electrode group was housed in a bag-shaped outer casing made of a laminate sheet with an Al layer, a predetermined amount of non-aqueous electrolyte was injected, and the outer casing was sealed to fabricate a secondary battery. Parts of the positive electrode lead and negative electrode lead were exposed to the outside of the outer casing.

[0107] 《Evaluation》 The following evaluation was performed using the obtained secondary battery. Under a 45°C environment, constant current charging was performed with a current of 0.5C (180mA) until the secondary battery voltage reached 4.2V, and then constant voltage charging was performed at a voltage of 4.2V until the current reached 0.05C (18mA). After a 10-minute break, constant current discharge was performed with a current of 0.7C (252mA) until the secondary battery voltage reached 2.5V. The discharge capacity (Ci) at this time was determined as the initial capacity.

[0108] To determine the discharge capacity Ci, the charging, resting, and discharging cycle was considered as one cycle, and this cycle was repeated 500 times. The discharge capacity (Cc) at the 500th cycle was then determined. The ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci (set as 100%) was calculated as the capacity retention rate Xr, and this was used as an indicator of the cycle characteristics. Capacity retention rate Xr (%) = (Cc / Ci) × 100

[0109] The results of the examples and comparative examples are shown in Table 1. In Table 1, A1 to A3 are Examples 1 to 3, and B1 to B2 are Comparative Examples 1 and 2.

[0110]

[0111] As shown in Table 1, the lifespan characteristics of batteries A1 to A3, which contained a sulfur-containing cyclic compound and dichloromethane in the non-aqueous electrolyte, were significantly improved.

[0112] The secondary battery comprising the non-aqueous electrolyte of this disclosure is useful in fields requiring a high level of lifespan characteristics, such as as a main power source for mobile communication devices, portable electronic devices, etc. However, the applications of the secondary battery are not limited in any way.

[0113] 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.

[0114] 10: Non-aqueous electrolyte 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: Annular groove, 23: First insulating plate, 24: Second insulating plate

Claims

1. A non-aqueous electrolyte for secondary batteries comprising dichloromethane and a sulfur-containing cyclic compound, wherein the sulfur-containing cyclic compound has a five-membered or six-membered sulfur-containing ring containing a sulfur atom.

2. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the mass content of dichloromethane is 10 ppb to 1000 ppm.

3. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the ratio (Y / X) of the mass content Y of dichloromethane to the mass content X of the sulfur-containing cyclic compound is 0.0001 to 0.

01.

4. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the sulfur-containing ring has a carbon-carbon unsaturated bond.

5. The sulfur-containing cyclic compound is at least one selected from the group of compounds represented by the following formula: wherein X is independently a hydrocarbon group having 1 or 2 carbon atoms or an oxygen atom, Y is independently a hydrocarbon group having 1 or 2 carbon atoms or an oxygen atom when not bonded to a double bond, and a hydrocarbon group having 1 or 2 carbon atoms when bonded to a double bond, and R1 and R2 are each independently a hydrogen atom, a halogen atom or a hydrocarbon group having 1 to 4 carbon atoms which may have an unsaturated bond. The non-aqueous electrolyte for a secondary battery according to claim 1.

6. The sulfur-containing cyclic compound is given by the following formula: The non-aqueous electrolyte for a secondary battery according to claim 1, which is at least one selected from the group of compounds represented by .

7. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the sulfur-containing cyclic compound comprises 1,3-propensultone.

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

9. A secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte as described in claim 1.

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