Positive electrode active material and nonaqueous electrolyte secondary battery

Abstract

A positive electrode active material according to one embodiment of the present disclosure is characterized in comprising a lithium transition metal composite oxide, and a carbonaceous film and a first compound are provided on the surface of the lithium transition metal composite oxide, the carbonaceous film containing an alkali metal, and the first compound containing a polyacid and a heteroatom.

Description

Positive electrode active material and non-aqueous electrolyte secondary battery

[0001] This disclosure relates to a positive electrode active material and a non-aqueous electrolyte secondary battery.

[0002] In recent years, non-aqueous electrolyte secondary batteries, which comprise a positive electrode, a negative electrode, and a non-aqueous electrolyte and perform charging and discharging by moving Li ions and the like between the positive and negative electrodes, have been widely used as high-power, high-capacity secondary batteries. Patent Document 1 discloses a positive electrode active material included in non-aqueous electrolyte secondary batteries in which the surface of a lithium transition metal composite oxide is coated with a carbonaceous film containing S and Li for the purpose of improving electronic conductivity.

[0003] Japanese Patent Publication No. 2009-140876

[0004] Lithium transition metal composite oxides used as positive electrode active materials for non-aqueous electrolyte secondary batteries preferably contain Ni from the viewpoint of increasing battery capacity. However, increasing the Ni content presents a problem in that gas is more easily generated due to the decomposition of the non-aqueous electrolyte on the positive electrode surface. From the viewpoint of improving the reliability of non-aqueous electrolyte secondary batteries, it is important to suppress gas generation.

[0005] One aspect of the present disclosure is a positive electrode active material comprising a lithium transition metal composite oxide, wherein a carbonaceous film and a first compound are provided on the surface of the lithium transition metal composite oxide, the carbonaceous film containing an alkali metal, and the first compound comprising a polyacid and a heteroatom.

[0006] Furthermore, a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, is characterized by comprising a positive electrode containing the above-mentioned positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0007] According to one aspect of the present disclosure, the positive electrode active material can suppress the generation of gas due to the decomposition of the non-aqueous electrolyte on the positive electrode surface. As a result, a highly reliable non-aqueous electrolyte secondary battery can be provided.

[0008] This is an axial cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment.

[0009] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material and a non-aqueous electrolyte secondary battery using said positive electrode active material will be described in detail. Note that configurations obtained by selectively combining the components of the multiple embodiments and modified examples described below are included within the scope of this disclosure.

[0010] In the following, a cylindrical battery in which a wound electrode body 14 is housed in a bottomed cylindrical outer casing 16 is given as an example of a non-aqueous electrolyte secondary battery, but the battery outer casing is not limited to a cylindrical shape. The non-aqueous electrolyte secondary battery according to this disclosure may be, for example, a prismatic battery with a prismatic outer casing, a coin-type battery with a coin-type outer casing, or a pouch-type battery with an outer casing made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body is not limited to a wound type, and may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator in between. In addition, the design of the non-aqueous electrolyte secondary battery according to this disclosure is not limited to the example non-aqueous electrolyte secondary battery design, and known non-aqueous electrolyte secondary battery designs may be applied.

[0011] Figure 1 is an axial cross-sectional view of a cylindrical non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises a wound electrode body 14, a non-aqueous electrolyte, and an outer casing 16 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is closed by a sealing body 17. In the following, for convenience of explanation, the side of the battery with the sealing body 17 will be referred to as the top, and the bottom side of the outer casing 16 as the bottom.

[0012] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode body 14 are all rectangular elongated bodies, and are wound in a spiral shape in the longitudinal direction so as to be alternately laminated in the radial direction of the electrode body 14. The separator 13 isolates the positive electrode 11 and the negative electrode 12 from each other. Two separators 13 are arranged, for example, so as to sandwich the positive electrode 11. The electrode body 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode body 14, the longitudinal directions of the positive electrode 11 and the negative electrode 12 are the winding directions, and the short-side directions of the positive electrode 11 and the negative electrode 12 are the axial directions. That is, the end faces in the short-side direction of the positive electrode 11 and the negative electrode 👍 form the end faces in the axial direction of the electrode body 14.

[0013] Insulating plates 18 and 19 are respectively arranged above and below the electrode body 14. In the example shown in FIG. 1, the positive electrode lead 20 extends toward the sealing body 17 through the through-hole of the insulating plate 18, and the negative electrode lead 21 extends toward the bottom side of the outer package 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the outer package 16 by welding or the like, and the outer package 16 serves as the negative electrode terminal.

[0014] A gasket 28 is provided between the outer package 16 and the sealing body 17 to ensure the airtightness inside the battery. The outer package 16 is formed with a groove-in portion 22 that supports the sealing body 17, with a part of the side surface portion projecting inward. The groove-in portion 22 is preferably formed in an annular shape along the circumferential direction of the outer package 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the upper part of the outer package 16 by the groove-in portion 22 and the open end portion of the outer package 16 caulked to the sealing body 17.

[0015] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and an insulating member 25 is interposed between the peripheral portions of each. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 is deformed and broken so as to push up the upper valve body 26 toward the cap 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further rises, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0016] Hereinafter, the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte constituting the non-aqueous electrolyte secondary battery 10 will be described in detail, particularly the positive electrode active material constituting the positive electrode 11.

[0017] [Positive electrode] The positive electrode 11 has, for example, a positive electrode core and a positive electrode mixture layer formed on the surface of the positive electrode core. The positive electrode mixture layer is preferably formed on both sides of the positive electrode core. For the positive electrode core, a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, a film having the metal disposed on the surface layer, or the like can be used. The positive electrode mixture layer may contain a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. to the surface of the positive electrode core, drying the coating film, and then rolling the coating film using a roller or the like.

[0018] Examples of the conductive agent contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes, carbon nanofibers, and graphene. The content of the conductive agent is, for example, 0.01% by mass or more and 10% by mass or less, preferably 0.1% by mass or more and 5% by mass or less, based on the total mass of the positive electrode mixture layer.

[0019] Examples of binders included in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resins, and polyolefins. These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyethylene oxide (PEO), etc. The binder content is, for example, 0.1% by mass or more and 10% by mass or less, preferably 0.5% by mass or more and 5% by mass or less, relative to the total mass of the positive electrode mixture layer.

[0020] The positive electrode active material contained in the positive electrode mixture layer includes a lithium transition metal composite oxide. The lithium transition metal composite oxide includes, for example, secondary particles formed by the aggregation of primary particles.

[0021] A carbonaceous film containing alkali metals and a first compound containing a polyacid and heteroatoms are formed on the surface of the lithium transition metal composite oxide. Here, the surface of the lithium transition metal composite oxide refers to the surface of the secondary particles of the lithium transition metal composite oxide. That is, the carbonaceous film and the first compound exist on the surface of the secondary particles of the lithium transition metal composite oxide. The carbonaceous film and the first compound may exist, for example, in a dotted manner so as to cover at least a part of the surface of the secondary particles of the lithium transition metal composite oxide. Alternatively, the carbonaceous film and the first compound may exist so as to cover the entire surface of the secondary particles of the lithium transition metal composite oxide. Furthermore, the carbonaceous film and the first compound may exist at the interface where the primary particles of the lithium transition metal composite oxide come into contact with each other.

[0022] The particle size of the primary particles constituting the secondary particles of lithium transition metal composite oxides is, for example, between 0.02 μm and 5 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM). The average particle diameter of the secondary particles of lithium transition metal composite oxides is, for example, between 2 μm and 30 μm. Here, the average particle diameter refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of secondary particles of lithium transition metal composite oxides can be measured using a laser diffraction particle size distribution analyzer (for example, Microtrac-Bell MT3000II) with water as the dispersion medium.

[0023] Lithium transition metal composite oxides have a layered rock salt structure belonging to, for example, space group R-3m or space group C2 / m. From the standpoint of high capacity and crystal structure stability, it is preferable that lithium transition metal composite oxides have a layered rock salt structure belonging to space group R-3m. The layered rock salt structure of the lithium transition metal composite oxide may include a transition metal layer, a Li layer, and an oxygen layer. The charge-discharge reaction of the battery proceeds as Li ions present in the Li layer reversibly enter and exit.

[0024] Lithium transition metal composite oxides have the general formula Li ａ Ni １－ｘ－ｙ Mn ｘ M ｙ O ２＋ｂ It may also be expressed as follows. In this general formula, the following conditions are satisfied: 0.9 ≤ a ≤ 1.1, 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.1, 0.5 ≤ 1 - x - y ≤ 0.95, and -0.05 ≤ b ≤ 0.05, and element M is an element other than Li, Ni, Mn, and oxygen. The value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging. From the viewpoint of stabilizing the crystal structure of the lithium transition metal composite oxide, it is preferable that element M is at least one selected from the group consisting of Co, Nb, Al, Zr, B, Fe, Cu, Zn, Sn, W, Mo, Si, Ti, Fe, and Cr.

[0025] The Ni content (1-x-y) in the lithium transition metal composite oxide is preferably 50 mol% or more, and more preferably 70 mol% or more. Increasing the Ni content in the lithium transition metal composite oxide can increase the capacity of the non-aqueous electrolyte secondary battery. Furthermore, as the Ni content in the lithium transition metal composite oxide increases, gas is more easily generated due to the decomposition of the non-aqueous electrolyte on the surface of the positive electrode 11, making the effects of this disclosure even more pronounced.

[0026] The content of elements constituting lithium transition metal composite oxides can be measured using inductively coupled plasma atomic emission spectrometers (ICP-AES), electron beam microanalyzers (EPMA), etc.

[0027] The surface of the lithium transition metal composite oxide is provided with a carbonaceous coating containing alkali metals. By providing a carbonaceous coating on the surface of the lithium transition metal composite oxide, the electronic conductivity of the lithium transition metal composite oxide can be improved and its electrical resistance can be reduced. This can improve, for example, the charge-discharge efficiency and rate characteristics of non-aqueous electrolyte secondary batteries.

[0028] Furthermore, the inclusion of alkali metals in the carbonaceous coating improves the uniformity of the carbonaceous coating on the surface of the lithium transition metal composite oxide. This further enhances the electronic conductivity of the lithium transition metal composite oxide, thereby reducing its electrical resistance even more.

[0029] The carbonaceous coating preferably contains at least one of Na and K. In this case, the uniformity of the carbonaceous coating on the surface of the lithium transition metal composite oxide is further improved. The presence of alkali metals such as Na and K in the carbonaceous coating can be confirmed by X-ray photoelectron spectroscopy (XPS). The carbonaceous coating may also contain Li.

[0030] Furthermore, the carbonaceous coating may also contain sulfur (S). When the carbonaceous coating contains S, the electronic conductivity of the lithium transition metal composite oxide is further improved. The presence of S in the carbonaceous coating can be confirmed by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0031] The thickness of the carbonaceous coating is, for example, 30 nm or less, and may also be 25 nm or less. The lower limit of the carbonaceous coating thickness is, for example, 1 nm. The thickness of the carbonaceous coating is measured by an image of the cross-section of the positive electrode active material observed using a transmission electron microscope (TEM).

[0032] The surface of the lithium transition metal composite oxide is provided with a carbonaceous coating in addition to a compound (first compound) containing a polyacid and a heteroatom. The first compound may be formed on the carbonaceous coating or interposed between the surface of the lithium transition metal composite oxide and the carbonaceous coating. The presence of the first compound can be confirmed by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0033] Lithium carbonate (Li) is a gas-generating factor on the surface of lithium transition metal composite oxides. ２ CO ３ Compounds such as ) exist. As in this embodiment, when the first compound is present on the surface of the lithium transition metal composite oxide, the lithium carbonate is neutralized by the first compound. As a result, gas generation caused by lithium carbonate is suppressed. In addition, by covering the surface of the lithium transition metal composite oxide with the non-aqueous electrolyte on the surface of the lithium transition metal composite oxide, side reactions with the non-aqueous electrolyte on the surface of the lithium transition metal composite oxide are suppressed, and gas generation caused by these side reactions is suppressed.

[0034] Furthermore, gas generation can be specifically suppressed by forming a carbonaceous film on the surface of the lithium transition metal composite oxide while the first compound is present. This is presumed to be because the first compound has a carbonization-promoting effect, resulting in the formation of a high-quality carbonaceous film on the surface of the lithium transition metal composite oxide. The formation of a high-quality carbonaceous film further suppresses side reactions with non-aqueous electrolytes on the surface of the lithium transition metal composite oxide, thereby suppressing gas generation caused by these side reactions.

[0035] The polyacid contained in the first compound is preferably an oxide of any one of Group V elements and Group VI elements, more preferably an oxide of tungsten or molybdenum, and even more preferably an oxide of molybdenum. In this case, the effect of suppressing gas generation becomes remarkable. Examples of the heteroatom include B, Al, Si, P, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, and As.

[0036] Examples of the first compound include heteropolytungstic acids such as silicotungstic acid and phosphotungstic acid, heteropolymolybdic acids such as phosphomolybdic acid and silicomolybdic acid, and phosphovanadomolybdic acid, phosphotungtomolybdic acid, silicovanadomolybdic acid, silicotungtomolybdic acid, etc. Among them, it is preferably at least one selected from the group consisting of silicotungstic acid, phosphotungstic acid, and phosphomolybdic acid.

[0037] Further, the first compound may contain cations such as, for example, Li ＋ , Na ＋ , K ＋ , R , ４ N ＋ , R ４ , P ＋ (wherein R is H or a hydrocarbon group having 10 or less carbon atoms). Specifically, they are heteropolytungstic acid compounds such as sodium silicotungstate, sodium phosphotungstate, and ammonium phosphotungstate. Examples of the heteropolyacid compound also include heteropolymolybdic acid compounds such as sodium phosphomolybdate and ammonium phosphomolybdate.

[0038] The content of the first compound is preferably, for example, 0.01% by mass or more and 2% by mass or less based on the total mass of the positive electrode active material. When the content of the first compound is 0.05% by mass or more and 1% by mass or less, it becomes easy to suppress gas generation while achieving high capacity.

[0039] The presence of the first compound can be confirmed by the presence or absence of peaks of elements (for example, W, Mo, Si, P, etc.) derived from the first compound using X-ray photoelectron spectroscopy (XPS).

[0040] The positive electrode mixture layer may contain other positive electrode active materials in addition to the positive electrode active material described above. Examples of other positive electrode active materials include lithium transition metal composite oxides that do not have at least one of a carbonaceous coating and the first compound on their surface. The positive electrode active material of this disclosure is preferably 90% by mass or more, and more preferably 95% by mass or more, based on the total mass of the positive electrode active material.

[0041] Next, an example of a method for producing a positive electrode active material comprising a lithium transition metal composite oxide, a carbonaceous coating, and a first compound will be described.

[0042] The positive electrode active material can be prepared by, for example, the following methods. However, the methods for producing the positive electrode active material are not limited to these. <Method 1> (1) An aqueous solution is prepared by dissolving an anionic surfactant and a compound containing a polyacid and a heteroatom (first compound) in water. (2) A lithium transition metal composite oxide powder is added to the aqueous solution and stirred to prepare a suspension in which the lithium transition metal composite oxide is dispersed in the aqueous solution. (3) The residue obtained by filtering the suspension is calcined to obtain the positive electrode active material. <Method 2> (1) An aqueous solution is prepared by dissolving an anionic surfactant in water. (2) A lithium transition metal composite oxide powder is added to the aqueous solution and stirred to prepare a suspension in which the lithium transition metal composite oxide is dispersed in the aqueous solution. (3) The residue obtained by filtering the suspension is mixed with a compound containing a polyacid and a heteroatom (first compound) and calcined to obtain the positive electrode active material.

[0043] Anionic surfactants adsorb to the surface of lithium transition metal composite oxides in water, thus enabling the creation of a substantially uniform carbonaceous film. The surfactant is not particularly limited as long as it contains C and alkali metals. Preferably, the surfactant contains at least one of Na and K. The surfactant may also further contain S. Examples of surfactants include sodium lauryl sulfate, potassium lauryl sulfate, and lithium lauryl sulfate.

[0044] The concentration of the surfactant in the aqueous solution is, for example, 0.1% by mass or more and 30% by mass or less, preferably 1% by mass or more and 20% by mass or less. The concentration of the first compound in the aqueous solution is, for example, 0.05% by mass or more and 20% by mass or less, preferably 0.1% by mass or more and 10% by mass or less. The firing temperature is, for example, 100°C or more and 300°C or less.

[0045] [Negative Electrode] The negative electrode 12 may, for example, have a negative electrode core and a negative electrode mixture layer formed on the surface of the negative electrode core, or a metallic Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode core, and lithium metal may be deposited on the surface of the negative electrode core by charging. When the negative electrode 12 has a negative electrode mixture layer, it is preferable that the negative electrode mixture layer is formed on both sides of the negative electrode core. For the negative electrode core, a foil of a metal that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal arranged on the surface layer, can be used. The thickness of the negative electrode core is, for example, 5 μm or more and 30 μm or less. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the negative electrode core. The negative electrode 12 can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., to the surface of the negative electrode core, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0046] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si and Sn, or lithium titanium composite oxides may be used as the negative electrode active material. Furthermore, materials with a carbonaceous coating may also be used. For example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0047] Examples of binders included in the negative electrode mixture layer include styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0048] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator 13.

[0049] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides containing metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0050] [Non-aqueous electrolytes] Non-aqueous electrolytes are ionic conductive (for example, lithium ion conductive). Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0051] A liquid electrolyte (electrolyte solution) includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0052] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0053] Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methylphenyl ether. Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0054] The electrolyte salt is preferably a lithium salt. A suitable lithium salt is LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO4). ２ F ２Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. Of these, LiPF is used from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ It is preferable to use the following. The concentration of the lithium salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 1.8 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0055] Non-aqueous electrolytes may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0056] Examples of unsaturated cyclic carbonate esters include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. Unsaturated cyclic carbonate esters may be used individually or in combination of two or more. Some hydrogen atoms in the unsaturated cyclic carbonate esters may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by the intermolecular condensation of multiple carboxylic acid molecules, but it is preferable that it be an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0057] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0058] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanate methylcyclohexane (BIMCH). Examples of sultone compounds include propanesultone and propensultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethylborate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethylphosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethylphosphite and tris(trimethylsilyl)phosphite.

[0059] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0060] The present disclosure will be further illustrated below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0061] <Example 1> [Preparation of positive electrode active material] Sodium lauryl sulfate as a surfactant and tungstic acid as a first compound were dissolved in water at concentrations of 10% by mass and 2.5% by mass, respectively, to prepare aqueous solutions. A LiNi compound was added to this aqueous solution. ０．８ Mn０．２ O ２ A lithium transition metal composite oxide was added and stirred for 5 minutes to prepare a suspension with a concentration of 1000 g / L. The residue obtained by filtering this suspension was vacuum-dried at 80°C for 10 hours, and then vacuum-calcined at 200°C for 2 hours to obtain the positive electrode active material.

[0062] Observation using a transmission electron microscope (TEM) revealed that the carbonaceous coating had a thickness of 10 nm. Furthermore, energy-dispersive X-ray spectroscopy (TEM-EDX) of the carbonaceous coating showed a peak at around 2.2 keV, indicating the presence of sulfur (S) in the coating. X-ray photoelectron spectroscopy (XPS) of the carbonaceous coating also showed a peak, indicating the presence of sodium (Na). Finally, X-ray photoelectron spectroscopy (XPS) of the surface of the lithium transition metal composite oxide revealed peaks, indicating the presence of silicatungstic acid (Si).

[0063] [Preparation of the positive electrode] A positive electrode slurry was prepared by mixing 100 parts by mass of positive electrode active material, 1.1 parts by mass of acetylene black as a conductive agent, and 0.9 parts by mass of polyvinylidene fluoride as a binder, and then mixing this with N-methyl-2-pyrrolidone (NMP). Next, the positive electrode slurry was applied to one side of a positive electrode core made of 15 μm thick aluminum foil. After the coating film was dried, the coating film was rolled using a rolling roller and cut to a predetermined electrode size to produce a positive electrode for evaluation. The positive electrode had a 20 mm × 20 mm region to function as a positive electrode and a 5 mm × 5 mm region for connecting to the lead. Subsequently, the positive electrode slurry layer formed on the connection region was scraped off to expose the positive electrode core. Then, the exposed portion of the positive electrode core was connected to the positive electrode lead, and a predetermined region on the outer circumference of the positive electrode lead was covered with an insulating film.

[0064] [Fabrication of the negative electrode] The negative electrode was fabricated by attaching a lithium metal foil (300 μm thick) to one side of an electrolytic copper foil, which served as the negative electrode core. The negative electrode was then cut into the same shape as the positive electrode to obtain a negative electrode for evaluation. The lithium metal foil formed on the connection area, which was formed in the same way as the positive electrode, was peeled off to expose the negative electrode core. The exposed portion of the negative electrode core was then connected to the negative electrode lead, and a predetermined area on the outer circumference of the negative electrode lead was covered with an insulating film.

[0065] [Preparation of Non-Aqueous Electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:5:75. Lithium hexafluoride phosphate (LiPF) was added to this mixed solvent. ６ A non-aqueous electrolyte (non-aqueous electrolyte solution) was prepared by dissolving the substance in question to a concentration of 1.3 mol / liter.

[0066] [Preparation of Test Cells] An evaluation battery was prepared using evaluation positive and negative electrodes. First, the positive and negative electrodes were placed opposite each other with a separator in between so that the positive electrode mixture layer and the negative electrode mixture layer overlapped to obtain an electrode plate group. Next, a 60 x 90 mm rectangle of Al laminate film (100 μm thick) was folded in half, and the 60 mm long side end was heat-sealed at 230°C to form a 60 x 45 mm cylinder. Then, the prepared electrode plate group was placed inside the cylinder, and the end face of the Al laminate film was aligned with the position of the insulating film of each lead and heat-sealed at 230°C. Next, a non-aqueous electrolyte solution was poured 0.3 cm from the short side of the Al laminate film that was not heat-sealed. ３ The solution was injected, and after injection, the mixture was left to stand for 5 minutes under reduced pressure of 0.06 MPa to impregnate each mixture layer with the electrolyte. Finally, the end face of the Al laminate film on the injected side was heat-sealed at 230°C to prepare the test cell. The test cell was prepared in a dry environment with a dew point of -50°C or lower.

[0067] [Evaluation of Gas Generation] Test cells, whose volume was measured by the Archimedes method, were charged to 4.5V (vsLi) with a constant current of 0.2C at a temperature of 25°C. Then, the constant voltage of 4.5V (vsLi) was maintained for 3 days at a temperature of 45°C. The volume of the test cells after the test was measured by the Archimedes method, and the amount of gas generated was calculated from the difference between the volume before initial charging and the volume after the test.

[0068] <Example 2> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 1, except that the positive electrode active material was prepared using the method described below.

[0069] An aqueous solution was prepared by dissolving sodium lauryl sulfate, used as a surfactant, in water at a concentration of 10% by mass. In this aqueous solution, a compound with the composition LiNi was added. ０．８ Mn ０．２ O ２ A lithium transition metal composite oxide was added and stirred for 5 minutes to prepare a suspension with a concentration of 1000 g / L. This suspension was filtered to obtain a residue to which 0.5% by mass of silicatungstic acid as the first compound was added. The mixture was vacuum-dried at 80°C for 10 hours, and then vacuum-fired at 200°C for 2 hours to obtain the positive electrode active material.

[0070] Observation using a transmission electron microscope (TEM) revealed that the carbonaceous coating had a thickness of 10 nm. Furthermore, energy-dispersive X-ray spectroscopy (TEM-EDX) of the carbonaceous coating showed a peak at around 2.2 keV, indicating the presence of sulfur (S) in the coating. X-ray photoelectron spectroscopy (XPS) of the carbonaceous coating also showed a peak, indicating the presence of sodium (Na). Finally, X-ray photoelectron spectroscopy (XPS) of the surface of the lithium transition metal composite oxide revealed peaks, indicating the presence of silicatungstic acid (Si).

[0071] <Example 3> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 2, except that silicatungstic acid was added at a concentration of 0.2% by mass.

[0072] Observation using a transmission electron microscope (TEM) revealed that the carbonaceous coating had a thickness of 10 nm. Furthermore, energy-dispersive X-ray spectroscopy (TEM-EDX) of the carbonaceous coating showed a peak at around 2.2 keV, indicating the presence of sulfur (S) in the coating. X-ray photoelectron spectroscopy (XPS) of the carbonaceous coating also showed a peak, indicating the presence of sodium (Na). Finally, X-ray photoelectron spectroscopy (XPS) of the surface of the lithium transition metal composite oxide revealed peaks, indicating the presence of silicatungstic acid (Si).

[0073] <Example 4> In the preparation of the positive electrode active material, phosphotungstic acid was used instead of silicatungstic acid, but the test cell was prepared and evaluated in the same manner as in Example 1.

[0074] Observation using a transmission electron microscope (TEM) revealed that the carbonaceous coating had a thickness of 10 nm. Furthermore, energy-dispersive X-ray spectroscopy (TEM-EDX) of the carbonaceous coating showed a peak at around 2.2 keV, indicating the presence of sulfur (S) in the coating. X-ray photoelectron spectroscopy (XPS) of the carbonaceous coating also showed a peak, indicating the presence of sodium (Na). Finally, X-ray photoelectron spectroscopy (XPS) of the surface of the lithium transition metal composite oxide revealed peaks, indicating the presence of phosphotungstic acid (P) and water (W).

[0075] <Example 5> In the preparation of the positive electrode active material, phosphomolybdic acid was used instead of silicatungstic acid, but the test cell was prepared and evaluated in the same manner as in Example 1.

[0076] Observation using a transmission electron microscope (TEM) revealed that the carbonaceous coating had a thickness of 10 nm. Furthermore, energy-dispersive X-ray spectroscopy (TEM-EDX) of the carbonaceous coating showed a peak at around 2.2 keV, indicating the presence of sulfur (S) in the coating. X-ray photoelectron spectroscopy (XPS) of the carbonaceous coating also showed a peak, indicating the presence of sodium (Na). Finally, X-ray photoelectron spectroscopy (XPS) of the surface of the lithium transition metal composite oxide revealed peaks, indicating the presence of phosphomolybdic acid (Mo) and phosphorus (P).

[0077] <Comparative Example 1> LiNi ０．８ Mn ０．２ O ２ A test cell was prepared and evaluated in the same manner as in Example 1, except that the lithium transition metal composite oxide was not added to the aqueous solution of sodium lauryl sulfate and tungstic acid. In other words, no carbonaceous film or the first compound was formed on the surface of the lithium transition metal composite oxide of Comparative Example 1.

[0078] <Comparative Example 2> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that an aqueous solution was prepared by dissolving only sodium lauryl sulfate as a surfactant in water at a concentration of 10% by mass. In other words, the first compound was not formed on the surface of the lithium transition metal composite oxide of Comparative Example 1.

[0079] Observation using a transmission electron microscope (TEM) revealed that the carbonaceous coating had a thickness of 10 nm. Furthermore, energy-dispersive X-ray spectroscopy (TEM-EDX) of the carbonaceous coating showed a peak at around 2.2 keV, indicating the presence of sulfur (S) in the coating. Additionally, X-ray photoelectron spectroscopy (XPS) of the carbonaceous coating revealed a peak, indicating the presence of sodium (Na) in the coating.

[0080] <Comparative Example 3> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that an aqueous solution was prepared by dissolving only silicatungstic acid in water at a concentration of 2.5% by mass. In other words, no carbonaceous film was formed on the surface of the lithium transition metal composite oxide in Comparative Example 1. When the surface of the lithium transition metal composite oxide was measured by X-ray photoelectron spectroscopy (XPS), peaks attributable to W and Si were observed, confirming the presence of silicatungstic acid.

[0081] Table 1 shows the gas generation amount for each test cell. Note that in Table 1, the values ​​are shown relative to the gas generation amount of test cell in Comparative Example 1, which is set to 100. A smaller gas generation value indicates less gas generation and higher reliability.

[0082]

[0083] As shown in Table 1, the amount of gas generated in the example test cell is significantly reduced compared to the amount of gas generated in the comparative example test cell. Furthermore, comparing the test cell of Example 1, which has a carbonaceous coating and the first compound, with the test cell of Comparative Example 2, which has only a carbonaceous coating, and the test cell of Comparative Example 3, which has only the first compound, the amount of gas generated in the test cell of Example 1 is significantly reduced compared to the amount of gas generated in the test cells of Comparative Examples 2 and 3. From this, it can be said that gas generation can be specifically suppressed by forming a carbonaceous coating while having the first compound on the surface of the lithium transition metal composite oxide.

[0084] This disclosure is further illustrated by the following embodiments: Configuration 1: A positive electrode active material comprising a lithium transition metal composite oxide, wherein a carbonaceous film and a first compound are provided on the surface of the lithium transition metal composite oxide, the carbonaceous film contains an alkali metal, and the first compound comprises a polyacid and a heteroatom. Configuration 2: The positive electrode active material according to Configuration 1, wherein the polyacid is an oxide of any one element from Group V and Group VI. Configuration 3: The positive electrode active material according to Configuration 1 or 2, wherein the polyacid is an oxide of tungsten or molybdenum. Configuration 4: The positive electrode active material according to any one of Configurations 1 to 3, wherein the polyacid is an oxide of molybdenum. Configuration 5: The positive electrode active material according to any one of Configurations 1 to 4, wherein the first compound is at least one selected from the group consisting of silitungstic acid, phosphotungstic acid, and phosphomolybdic acid. Configuration 6: The positive electrode active material according to any one of Configurations 1 to 5, wherein the first compound is phosphomolybdic acid. Configuration 7: The carbonaceous film is a positive electrode active material according to any one of Configurations 1 to 6, wherein the carbonaceous film contains S. Configuration 8: The carbonaceous film is a positive electrode active material according to any one of Configurations 1 to 7, wherein the carbonaceous film has at least one of Na and K. Configuration 9: The carbonaceous film has a thickness of 30 nm or less, wherein the positive electrode active material according to any one of Configurations 1 to 8. Configuration 10: The carbonaceous film is a positive electrode active material according to any one of Configurations 1 to 9, wherein the carbonaceous film contains Li. Configuration 11: The lithium transition metal composite oxide is of the general formula Li ａ Ni １－ｘ－ｙ Mn ｘ M ｙ O ２＋ｂ A positive electrode active material according to any one of configurations 1 to 10, represented as (where 0.9 ≤ a ≤ 1.1, 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.1, 0.5 ≤ 1 - x - y ≤ 0.95, and -0.05 ≤ b ≤ 0.05, and element M is an element other than Li, Ni, Mn, and oxygen). Configuration 12: A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material according to any one of configurations 1 to 11, a negative electrode, and a non-aqueous electrolyte.

[0085] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket.

Claims

1. A positive electrode active material comprising a lithium transition metal composite oxide, wherein a carbonaceous film and a first compound are provided on the surface of the lithium transition metal composite oxide, the carbonaceous film contains an alkali metal, and the first compound contains a polyacid and a heteroatom.

2. The positive electrode active material according to claim 1, wherein the polyacid is an oxide of any one element from among the elements of Group V and the elements of Group VI.

3. The positive electrode active material according to claim 1, wherein the polyacid is an oxide of tungsten or molybdenum.

4. The positive electrode active material according to claim 1, wherein the polyacid is a molybdenum oxide.

5. The positive electrode active material according to claim 1, wherein the first compound is at least one selected from the group consisting of silicic acid, phosphotungstic acid, and phosphomolybdic acid.

6. The positive electrode active material according to claim 1, wherein the first compound is phosphomolybdic acid.

7. The positive electrode active material according to claim 1, wherein the carbonaceous coating contains S.

8. The positive electrode active material according to claim 1, wherein the carbonaceous coating contains at least one of Na and K.

9. The positive electrode active material according to claim 1, wherein the thickness of the carbonaceous coating is 30 nm or less.

10. The positive electrode active material according to claim 1, wherein the carbonaceous coating contains Li.

11. The lithium transition metal composite oxide has the general formula Li ａ Ni １－ｘ－ｙ Mn ｘ M ｙ O ２＋ｂ The positive electrode active material according to claim 1, wherein the following conditions are met: 0.9 ≤ a ≤ 1.1, 0 ≤ x ≤ 0.5, 0 ≤ y ≤ 0.1, 0.5 ≤ 1 - x - y ≤ 0.95, and -0.05 ≤ b ≤ 0.05, and element M is an element other than Li, Ni, Mn, and oxygen.

12. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material described in any one of claims 1 to 11, a negative electrode, and a non-aqueous electrolyte.

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