Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

This positive electrode for nonaqueous electrolyte secondary batteries is provided with: a composite oxide that contains lithium and a transition metal; and an additive that covers at least a part of the surface of the composite oxide. The additive contains a cyclic inorganic phosphoric acid compound.

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, have high energy density and high output, and are considered promising as power sources for mobile devices such as smartphones, power sources for vehicles such as electric vehicles, storage devices for natural energy such as solar energy, etc. A composite oxide containing lithium and a transition metal is used as the positive electrode active material of non-aqueous electrolyte secondary batteries.

[0003] Patent Document 1 proposes forming a coating layer containing a phosphorus compound on the surface of a composite oxide containing lithium and manganese, which is the positive electrode active material of a non-aqueous electrolyte secondary battery. The phosphorus compound is at least one selected from the group consisting of Li3PO4, Li4P2O7, and LiPO3 (hereinafter referred to as Li3PO4, etc.). The coating layer contains, in addition to the phosphorus compound, an oxide or fluoride containing at least one element selected from the group consisting of Mg, Al, and Cu.

[0004] Patent Document 2 proposes that an organic phosphate compound be attached to the particle surface of a spinel-structured composite oxide containing lithium, manganese, and nickel, which is a positive electrode active material for non-aqueous electrolyte secondary batteries. The organic phosphate compound is a phosphate triester represented by PO(OR)3 (R is an organic group such as an alkyl group or an aryl group). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-187193 [Patent Document 2] International Publication No. 2016 / 084966 Brochure Summary of the Invention

[0006] Li3PO4 and other compounds described in Patent Document 1 may aggregate and become distributed in islands during coating using a liquid-phase method. This is due to the density difference (sintering) between the raw materials and the final product when the final product has a higher density than the raw materials during the coating process (heat drying step), or the effects of gas generation associated with the reaction of the raw materials. For example, when using (NH4)2HPO4 and Li2CO3 as raw materials to produce Li3PO4, which has a higher density than (NH4)2HPO4, the large density difference between (NH4)2HPO4 and Li3PO4 can produce NH3 and CO2 gas during the reaction, making the Li3PO4 prone to aggregation. Furthermore, the organophosphate compound described in Patent Document 2 is prone to leaking into the non-aqueous electrolyte.

[0007] If Li3PO4 or the like is distributed in an island-like pattern or if the organic phosphate compound flows into the non-aqueous electrolyte, the coating of the composite oxide becomes insufficient, and the cycle characteristics may deteriorate due to decomposition caused by contact between the non-aqueous electrolyte and the composite oxide.

[0008] In view of the above, one aspect of the present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery, including a composite oxide containing lithium and a transition metal, and an additive that covers at least a portion of a surface of the composite oxide, wherein the additive includes a cyclic inorganic phosphate compound.

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

[0010] According to the present disclosure, the cycle characteristics of a non-aqueous electrolyte secondary battery can be improved. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic perspective view, with a portion cut away, of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Positive electrode for non-aqueous electrolyte secondary batteries] A positive electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a composite oxide (positive electrode active material) containing lithium and a transition metal, and an additive that covers at least a portion of the surface of the composite oxide, and the additive includes a cyclic inorganic phosphate compound (hereinafter also referred to as compound A).

[0013] When the additive contains compound A, the surface of the composite oxide is easily covered with the additive sufficiently and stably, thereby suppressing decomposition of the non-aqueous electrolyte due to contact with the composite oxide and improving cycle characteristics.

[0014] Compound A is less likely to aggregate during the coating process using the liquid phase method. In the case of compound A, a raw material that has a small density difference from the final product and is less likely to generate gas during the reaction can be used in the coating process using the liquid phase method. Furthermore, compound A may have a ring structure containing multiple P atoms, and multiple oxygen atoms bonded to each of the multiple P atoms may be anionized to form O - In this case, the anion of compound A easily interacts with and bonds with the P of other surrounding inorganic phosphate compounds. Therefore, when the additive contains compound A, the surface of the composite oxide can be widely covered with the additive in a layer.

[0015] Compound A is easily anionized and bonds with the transition metal of the composite oxide, making it less likely to leak into the non-aqueous electrolyte. Therefore, when the additive contains compound A, the surface of the composite oxide can be stably covered with the additive. Compound A also has excellent oxidation resistance and exists stably in a high-potential positive electrode, which is advantageous for increasing the output of the battery. Compound A has excellent lithium ion conductivity, allowing lithium ions to move smoothly between the composite oxide and the non-aqueous electrolyte via the coating layer containing compound A.

[0016] A cyclic inorganic phosphate compound (e.g., cyclic polyphosphate) is less likely to become dense, have a lower density, and are less likely to aggregate than a chain inorganic phosphate compound (e.g., chain polyphosphate) in terms of molecular structure. When a raw material having a lower density than the cyclic inorganic phosphate compound and the chain inorganic phosphate compound is used, the cyclic inorganic phosphate compound has a smaller density difference with the raw material than the chain inorganic phosphate compound and is less likely to aggregate.

[0017] The additive contains at least compound A and may contain an inorganic phosphate compound other than compound A. The inorganic phosphate compound other than compound A may contain LiPO, LiPO, LiPO, or the like, and may contain a chain polyphosphate such as tetrapolyphosphate. The content of phosphorus (P) derived from compound A is, for example, 0.01% by mass or more, or may be 0.01% by mass or more and 0.5% by mass or less, or may be 0.1% by mass or more and 0.5% by mass or less, based on the total of the composite oxide and the additive.

[0018] The additive is substantially free of organic phosphate compounds that easily leak into the non-aqueous electrolyte. For example, when an aqueous solution containing H3PO4 and LiOH is used as the raw material solution, the additive does not contain any organic phosphate compounds. Even when an organic phosphate compound is contained, the amount of phosphorus derived from the organic phosphate compound attached to 100 parts by mass of the composite oxide is, for example, 0.001 parts by mass or less. This prevents the organic phosphate compound from leaking into the non-aqueous electrolyte, resulting in insufficient coverage of the composite oxide with the additive. The amount of organic phosphate compounds contained in the additive leaking into the non-aqueous electrolyte in the battery can be estimated by determining the content of the organic phosphate compounds in the non-aqueous electrolyte when the non-aqueous electrolyte is prepared (before the non-aqueous electrolyte is injected into the battery). The content of the organic phosphate compounds in the non-aqueous electrolyte can be determined by gas chromatography-mass spectrometry (GC / MS) or the like.

[0019] Compound A preferably contains at least one selected from the group consisting of cyclic polyphosphates and salts thereof. The salts of cyclic polyphosphates include, for example, alkali metal salts such as lithium salts. The anion of cyclic polyphosphates is O bonded to P.- The cyclic polyphosphate has a plurality of groups, and easily bonds with transition metals in the composite oxide. n The compound may have a composition represented by the formula: where n is, for example, 3 or more and 6 or less. Among these, the cyclic polyphosphate is hexametaphosphate (HPO 18 ) is preferably included.

[0020] Compound A becomes anionized and reacts with the transition metal in the composite oxide and Li in the non-aqueous electrolyte. + and H + The anion of compound A can easily interact with and bond to P in the surrounding inorganic phosphate compound (hereinafter referred to as transition metal in the composite oxide, etc.). When the anion of compound A bonds with P in the surrounding inorganic phosphate compound, the surface of the composite oxide is easily covered in a layer of additives. When the anion of compound A bonds with the transition metal in the composite oxide, the surface of the composite oxide is stably covered with additives. When the anion of compound A bonds with Li in the non-aqueous electrolyte, the surface of the composite oxide is easily covered in a layer of additives. + The ease of bonding with the compound oxide allows smooth movement of lithium ions between the compound oxide and the non-aqueous electrolyte.

[0021] The anion of hexametaphosphate has a structure represented by the following formula (I): O bonded to P in formula (I) - O can bond with transition metals in the complex oxide. - and easily forms many bonds with transition metals and the like in the composite oxide.

[0022] [ka]

[0023] The additive component (compound A) coating the surface of the composite oxide can be identified, for example, by the following method.

[0024] The battery is disassembled and the positive electrode is removed. The positive electrode is washed with a non-aqueous solvent to remove the non-aqueous electrolyte adhering to the positive electrode, and the non-aqueous solvent is then removed by drying. The positive electrode mixture layer is collected from the positive electrode, crushed appropriately, and dispersed in water. The positive electrode mixture dispersion is filtered to obtain the filtrate as a sample solution. Alternatively, the positive electrode material (composite oxide particles whose surface is coated with an additive) may be dispersed in water, and the positive electrode material dispersion may be filtered to obtain the filtrate as a sample solution. The components contained in the sample solution obtained above are analyzed by X-ray diffraction (XRD). If the additive coating the surface of the complex oxide contains compound A, compound A will be dissolved in the sample solution (water), and a peak due to compound A will be observed in the XRD pattern. The sample solution may also be analyzed by nuclear magnetic resonance (NMR) spectroscopy.

[0025] Furthermore, the coating material on the surface of the composite oxide may be analyzed based on an XRD pattern obtained by XRD of the positive electrode material or an electron beam diffraction pattern obtained by a transmission electron microscope (TEM).

[0026] The phosphorus (P) content in the positive electrode (the amount of P derived from the additive) may be 0.1% by mass or more and 0.75% by mass or less, or 0.2% by mass or more and 0.55% by mass or less, based on the total of the complex oxide and the additive. When the P content is 0.1% by mass or more based on the total of the complex oxide and the additive, the complex oxide is sufficiently coated with the additive, which tends to improve the cycle characteristics. When the P content is 0.75% by mass or less based on the total of the complex oxide and the additive, the complex oxide is sufficiently secured in the positive electrode, which tends to increase the capacity of the battery.

[0027] The content of P in the positive electrode (mass ratio to the total of the composite oxide and the additive) can be determined by the following method.

[0028] The battery is disassembled and the positive electrode is removed. The positive electrode is washed with a non-aqueous solvent to remove any non-aqueous electrolyte adhering to the positive electrode, and the non-aqueous solvent is then removed by drying. A positive electrode mixture is collected from the positive electrode and its mass, W1, is measured. The positive electrode mixture is dissolved in a specified acid and filtered to remove any residue of carbon material (acetylene black) and resin material (polyvinylidene fluoride) to obtain a sample solution. The mass, W2, of the residue after drying is measured. (W1 - W2) is calculated as the total mass of the complex oxide and additives. The resulting sample solution is used to determine the mass, W3, of P in the sample solution by inductively coupled plasma (ICP) atomic emission spectroscopy. Using the obtained (W1 - W2) and W3, W3 / (W1 - W2) x 100 is calculated, which is the P content.

[0029] Alternatively, after measuring the mass WA of the positive electrode material (composite oxide particles whose surfaces are coated with an additive), the positive electrode material may be dissolved in a predetermined acid to obtain a sample solution, and the mass WB of P in the sample solution may be determined by ICP atomic emission spectroscopy, and the P content may be calculated as WB / WA × 100.

[0030] The positive electrode may include a positive electrode material including composite oxide particles and an additive that covers the surfaces of the composite oxide particles and includes compound A. The positive electrode may include a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector, and the positive electrode mixture layer may include the above-mentioned positive electrode material.

[0031] The state of P distribution in the positive electrode material can be confirmed by performing elemental analysis (element mapping) on ​​a cross section of the positive electrode mixture layer or the positive electrode material using an electron probe microanalyzer (EPMA) or an energy dispersive X-ray (EDX) analyzer.

[0032] A method for producing a positive electrode material includes, for example, a first step of adhering a raw material solution to the surfaces of composite oxide particles, and a second step of heating and drying the composite oxide particles with the raw material solution adhering to their surfaces.

[0033] The composite oxide is synthesized using a coprecipitation method or the like, for example, by mixing a lithium compound with a compound containing a metal other than lithium (Me, a transition metal) obtained by the coprecipitation method or the like, and then firing the resulting mixture under predetermined conditions. The composite oxide usually consists of secondary particles formed by aggregation of multiple primary particles. The average particle size (D50) of the composite oxide particles is, for example, 3 μm or more and 25 μm or less. The average particle size (D50) of the composite oxide particles refers to the particle size (volume-average particle size) at which the volume-integrated value is 50% in the volume-based particle size distribution measured by a laser diffraction scattering method. The first step may also serve as a step of washing the synthesized composite oxide particles. This is advantageous in terms of improving productivity.

[0034] In the first step, for example, composite oxide particles are added to a raw material solution and stirred to disperse the composite oxide particles in the raw material solution. The raw material solution is, for example, an aqueous solution containing H3PO4 and LiOH, and is obtained by adding an appropriate amount of LiOH aqueous solution to an H3PO4 aqueous solution. When the raw material solution contains an acid component such as H3PO4, adding an alkaline component such as LiOH neutralizes part of the acid component, thereby reducing the effect of the acid component on the composite oxide. From the perspective of easy preparation of the raw material solution, it is desirable to adjust the amount of LiOH aqueous solution to be added so that the pH of the raw material solution is less than 8. The amount of composite oxide particles added is, for example, 500 g or more and 2000 g or less per 1 L of raw material solution.

[0035] The raw material composition in the raw material solution (aqueous solution of H3PO4 and LiOH) is Li z H (3-z) When expressed in terms of PO4, z may be 1.0 or more and 1.8 or less, or 1.2 or more and 1.8 or less. In this case, the pH of the raw material solution can be easily adjusted to a range of about 6 or more and less than 8. The influence of acid components on the composite oxide is avoided, allowing the composite oxide to fully fulfill its role as a positive electrode active material. The raw material solution is easy to prepare, and compound A can be efficiently obtained.

[0036] If the alkaline component (e.g., LiOH) used in the synthesis of the composite oxide remains in the composite oxide, the z value will shift slightly in the upward direction when the raw material solution adheres to the surface of the composite oxide due to the influence of the alkaline component. For example, if a raw material solution with a raw material composition having a z value of 1 adheres to a composite oxide with residual alkaline components, the z value will be greater than 1. The z value varies depending on the molar ratio of LiOH to H3PO4. For example, if no LiOH aqueous solution is added, z = 0.

[0037] The second step (heating step) serves both as a step of removing the dispersion medium adhering to the surface of the composite oxide particles by heating and drying, and a step of reacting the raw materials adhering to the surface of the composite oxide particles to produce compound A. The heating temperature is, for example, 180°C or higher and 450°C or lower. In this case, drying of the surface of the composite oxide particles and production of compound A on the surface are efficiently carried out. During heating in the second step, the produced compound A can bond as an anion to the metal Me (transition metal, etc.) of the composite oxide particles.

[0038] In the second step, for example, heating reduces the amount of water in the raw material solution (aqueous solution containing H3PO4 and LiOH) adhering to the surface of the composite oxide particles, resulting in the production and precipitation of Li3PO4 and LiH2PO4. Furthermore, Li3PO4 and LiH2PO4 react to produce hexametaphosphoric acid. The water produced during the reaction also evaporates with heating. Small amounts of cyclic polyphosphoric acids other than hexametaphosphoric acid, such as tetrametaphosphoric acid, and chain polyphosphoric acids, such as tetrapolyphosphoric acid, may also be produced. Small amounts of unreacted components, such as Li3PO4, may also remain. Hexametaphosphoric acid has a lower density than Li3PO4 and LiH2PO4, making it less likely to aggregate.

[0039] The positive electrode active material includes a composite oxide containing lithium and a metal other than lithium, Me. The metal Me includes at least a transition metal. The transition metal may include at least one element selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), copper (Cu), chromium (Cr), titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V), tantalum (Ta), and molybdenum (Mo).

[0040] The metal Me may contain a metal other than a transition metal. The metal other than a transition metal may contain at least one selected from the group consisting of aluminum (Al), magnesium (Mg), calcium (Ca), strontium (Sr), zinc (Zn), and silicon (Si). In addition to the metal, the composite oxide may further contain boron (B) or the like.

[0041] From the viewpoint of increasing capacity, the transition metal preferably contains at least Ni. The metal Me may contain Ni and at least one selected from the group consisting of Co, Mn, Al, Ti, and Fe. From the viewpoint of increasing capacity and power output, the metal Me preferably contains Ni and at least one selected from the group consisting of Co, Mn, and Al, and more preferably contains Ni, Co, and Mn and / or Al. When the metal Me contains Co, the phase transition of the composite oxide containing Li and Ni during charge and discharge is suppressed, the stability of the crystal structure is improved, and the cycle characteristics are likely to be improved. When the metal Me contains Mn and / or Al, thermal stability is improved.

[0042] From the viewpoint of facilitating high capacity, the atomic ratio of Ni to metal Me in the composite oxide: Ni / Me is preferably 0.3 or more and less than 1, more preferably 0.5 or more and less than 1, and even more preferably 0.75 or more and less than 1.

[0043] From the viewpoint of improving cycle characteristics and increasing output, the positive electrode active material may contain a composite oxide having a layered rock-salt crystal structure and containing Ni and / or Co, or may contain a composite oxide having a spinel crystal structure and containing Mn. Among these, from the viewpoint of increasing capacity, a composite oxide having a layered rock-salt crystal structure, containing Ni, and having an atomic ratio of Ni to metal Me, Ni / Me, of 0.3 or more (hereinafter also referred to as nickel-based composite oxide) is preferred.

[0044] The additive containing compound A that coats the surface of the composite oxide has excellent lithium ion conductivity, allowing the composite oxide to smoothly absorb and release lithium ions. Furthermore, by adding an alkaline component to the raw material solution when coating with the additive containing compound A, deterioration of the composite oxide due to the acid component in the raw material solution is suppressed. Therefore, when the surface of a nickel-based composite oxide is coated with an additive containing compound A, the high capacity of a positive electrode containing a nickel-based composite oxide can be fully utilized.

[0045] Nickel-based composite oxides have a relatively unstable crystal structure and are prone to deterioration due to Ni elution and other factors resulting from contact with the nonaqueous electrolyte at the high-potential positive electrode, resulting in a decrease in cycle characteristics. Therefore, in the case of nickel-based composite oxides, coating the surface of the composite oxide with an additive containing compound A significantly improves cycle characteristics. Furthermore, Ni-based composite oxides can exhibit alkaline properties due to residual alkaline components used in their synthesis, which makes it easier to suppress deterioration of the composite oxide due to acid components in the raw material solution used to coat the surface of the composite oxide with the additive.

[0046] The composite oxide has a layered rock salt type crystal structure and is represented by the general formula (1): LiNi α M 1-α O2 (wherein 0.3≦α<1 is satisfied and M is at least one element selected from the group consisting of Co, Mn, Al, Ti, and Fe). When α is within the above range, the effect of Ni and the effect of element M are obtained in a well-balanced manner.

[0047] From the viewpoints of improving cycle characteristics, increasing capacity, and increasing output, the composite oxide has a layered rock salt-type crystal structure and may have a composition represented by the general formula (2): LiNi x Co y M 1-x-y O₂. In the general formula (2), 0.3 ≦ x < 1, 0 < y ≦ 0.5, and 0 < 1 - x - y ≦ 0.35 are satisfied, and M is at least one element selected from the group consisting of Al and Mn. In this case, the effects of Ni, the effects of Co, and the effects of the element M can be obtained in a good balance. When the surface of the composite oxide represented by the general formula (2) is coated with an additive containing the compound A, the high capacity possessed by the composite oxide can be sufficiently extracted. Among them, M in the general formula (2) is preferably Al. The x value may be in the range of 0.5 ≦ x < 1. The y value may be in the range of 0 < y ≦ 0.35.

[0048] From the viewpoints of improving cycle characteristics and increasing output, the composite oxide has a spinel-type crystal structure and may have a composition represented by the general formula (3): LiMn β Ni 2-β O₄ (0.1 ≦ β < 2). Also, in the general formula (3), β may be 0.5 or more and less than 2.

[0049] [Non-aqueous electrolyte secondary battery] The non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the positive electrode is the above positive electrode.

[0050] Hereinafter, the configuration of the non-aqueous electrolyte secondary battery will be described in detail.

[0051] (Positive electrode) The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture contains the above-mentioned positive electrode material as an essential component. The positive electrode mixture may contain, as optional components, a binder, a conductive agent, etc. N-methyl-2-pyrrolidone (NMP), etc., is used as the dispersion medium.

[0052] Examples of binders include resin materials such as fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, and vinyl resins. Examples of fluororesins include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). One type of binder may be used alone, or two or more types may be used in combination.

[0053] Examples of the conductive agent include carbon blacks such as acetylene black, conductive fibers such as carbon fibers and metal fibers, and carbon fluoride. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0054] The positive electrode current collector may be, for example, a metal foil. Examples of metals constituting the positive electrode current collector include aluminum (Al), titanium (Ti), alloys containing these metal elements, and stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is, for example, 3 to 50 μm.

[0055] (Negative electrode) The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed, for example, by applying a negative electrode slurry in which the negative electrode mixture is dispersed in a dispersion medium to the surface of the negative electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector. Examples of the dispersion medium include water and NMP.

[0056] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. The binder and conductive agent may be the same as those exemplified for the positive electrode. The binder may be a rubber material such as styrene-butadiene copolymer rubber (SBR). Examples of thickeners include carboxymethyl cellulose (CMC) and its modified form (such as the Na salt).

[0057] The negative electrode active material may contain a carbon material that absorbs and releases lithium ions. Examples of carbon materials that absorb and release lithium ions include graphite (natural graphite and artificial graphite), easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity.

[0058] The negative electrode active material may include an alloy material. The alloy material is a material containing at least one metal capable of forming an alloy with lithium, such as silicon, tin, a silicon alloy, a tin alloy, or a silicon compound. The silicon compound may be a composite material having a lithium ion conductive phase and silicon particles dispersed in the phase. The lithium ion conductive phase may be a silicate phase such as a lithium silicate phase, a silicon oxide phase containing 95% or more by mass of silicon dioxide, or a carbon phase.

[0059] The negative electrode active material may be a combination of an alloy material and a carbon material. In this case, the proportion of the carbon material in the total of the alloy material and the carbon material is, for example, preferably 80 mass % or more, and more preferably 90 mass % or more.

[0060] The shape and thickness of the negative electrode current collector can be selected from the shape and range corresponding to those of the positive electrode current collector. Examples of metals constituting the negative electrode current collector include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements.

[0061] (non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0062] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of cyclic carbonates include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), and cyclic carbonates having a carbon-carbon unsaturated bond such as vinylene carbonate (VC) and vinylethylene carbonate. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL), γ-valerolactone (GVL), and examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more.

[0063] Known lithium salts can be used as the lithium salt. Preferred lithium salts include, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10Examples of the lithium salts include lithium cations of lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of the borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonyl imide (LiN(CF3SO2)(CF9SO2)), lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2), etc. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0064] (separator) It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0065] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a nonaqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0066] FIG. 1 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away.

[0067] The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a non-aqueous electrolyte housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0068] One end of a negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. In other words, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is closed by a sealing plug 8 .

[0069] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0070] Examples 1 to 5 [Preparation of cathode material] The following procedure is used to prepare layered rock salt LiNi 0.9 Co 0.05 Al 0.05 O The surface of composite oxide particles (average particle size (D50) 11.1 μm) having the composition of 2 (NCA) was coated with an additive containing compound A.

[0071] First, a LiOH solution (concentration 1 mol / L) was added to a H3PO4 solution (concentration 1 mol / L) to obtain a raw solution (aqueous solution containing H3PO4 and LiOH). z H (3-z) When expressed in terms of PO4, the amount of LiOH aqueous solution added to the H3PO4 aqueous solution was adjusted so that z would have the value shown in Table 1. The amount of LiOH aqueous solution added was set so that the pH of the raw material solution would be less than 8.

[0072] The composite oxide particles were added to the raw material solution. The amount of composite oxide particles added was 1250 g per 1 L of raw material solution. The raw material solution containing the composite oxide particles was stirred for 15 minutes to disperse the composite oxide particles in the raw material solution. The composite oxide particles were then filtered out from the dispersion, and the composite oxide particles with the raw material solution attached to their surfaces were heated at 450°C for 3 hours and dried. In this way, a positive electrode material was obtained in which the surfaces of the composite oxide particles were coated with an additive. The P content in the positive electrode material, determined by the method described above, was 0.21 mass%.

[0073] [Preparation of positive electrode] N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture and stirred to prepare a positive electrode slurry. The positive electrode mixture was a mixture of the positive electrode material obtained above, acetylene black (AB), and polyvinylidene fluoride (PVDF). The mass ratio of the positive electrode material, AB, and PVDF in the positive electrode mixture was 100:2:2.

[0074] The positive electrode slurry was applied to the surface of the aluminum foil serving as the positive electrode current collector, and the coating was dried and then rolled to form a positive electrode mixture layer (thickness: 40 μm, density: 3.6 g / cm) on both sides of the aluminum foil. 3 In Table 1, the positive electrodes of Examples 1 to 5 are designated a1 to a5, respectively.

[0075] [Preparation of negative electrode] Water was added to the negative electrode mixture and stirred to prepare a negative electrode slurry. The negative electrode mixture was a mixture of artificial graphite (average particle size 20 μm), styrene-butadiene rubber (SBR), and carboxymethyl cellulose sodium (CMC-Na). The mass ratio of the artificial graphite, SBR, and CMC-Na in the negative electrode mixture was 100:1:1. The negative electrode slurry was applied to the surface of copper foil, the coating was dried, and then rolled to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm) on both sides of the copper foil. 3 ) was formed on the negative electrode.

[0076] [Preparation of non-aqueous electrolyte] LiPF6 was dissolved in a mixed solvent of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) (volume ratio 2:8) at a concentration of 1 mol / L to obtain a non-aqueous electrolyte.

[0077] [Fabrication of non-aqueous electrolyte secondary battery] An Al positive electrode lead was attached to the positive electrode obtained above. A Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween in an inert gas atmosphere to produce a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet having an Al layer, and the nonaqueous electrolyte was injected, and then the exterior body was sealed to produce a nonaqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, a portion of the positive electrode lead and the negative electrode lead were each exposed to the outside from the exterior body. In Table 1, the batteries of Examples 1 to 5 are designated A1 to A5, respectively.

[0078] Examples 6 to 10 Positive electrodes a6 to a10 of Examples 6 to 10 were fabricated in the same manner as positive electrodes a1 to a5 of Examples 1 to 5, respectively, except that the concentration of the raw material solution (aqueous solution containing H3PO4 and LiOH) was changed to set the P content in the positive electrode material to the value shown in Table 1. The P content in the positive electrode material, determined by the method described above, was 0.1 mass%. Batteries A6 to A10 of Examples 6 to 10 were fabricated in the same manner as battery A1 of Example 1, respectively, except that positive electrodes a6 to a10 were used instead of positive electrode a1.

[0079] Examples 11 to 13 Positive electrodes a11 to a13 of Examples 11 to 13 were fabricated in the same manner as positive electrodes a3 to a5 of Examples 3 to 5, respectively, except that the heating temperature of the composite oxide particles filtered out from the dispersion was 180°C. The P content in the positive electrode material, determined by the method described above, was 0.2 mass%. Batteries A11 to A13 of Examples 11 to 13 were fabricated in the same manner as battery A1 of Example 1, respectively, except that positive electrodes a11 to a13 were used instead of positive electrode a1.

[0080] Comparative Example 1 A positive electrode b1 of Comparative Example 1 was produced in the same manner as the positive electrode a1 of Example 1, except that the surface of the composite oxide was not coated with an additive in the production of the positive electrode. A battery B1 of Comparative Example 1 was produced in the same manner as the battery A1 of Example 1, except that the positive electrode b1 was used instead of the positive electrode a1.

[0081] The batteries A1 to A13 and battery B1 obtained above were evaluated as follows.

[0082] [Evaluation: Charge / discharge cycle test] The battery was charged at a constant current of 0.3 C until the voltage reached 4.4 V, then charged at a constant voltage of 4.4 V until the current reached 0.05 C. It was then discharged at a constant current of 0.5 C until the voltage reached 2.5 V. The rest time between charge and discharge was 10 minutes. Charging and discharging were performed at 25°C.

[0083] The above charge / discharge cycle was counted as one cycle and the battery was subjected to 100 cycles. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate. The initial capacity in Table 1 is the discharge capacity at the first cycle.

[0084] The evaluation results are shown in Table 1.

[0085] [Table 1]

[0086] The components of the additives coating the surfaces of the composite oxide particles of the positive electrodes a1 to a13 were investigated by the method described above. As a result, in all cases, the compound A, hexametaphosphate (Li6P6O 18 ), lithium orthophosphate (Li3PO4), and lithium pyrophosphate (Li4P2O7).

[0087] Batteries A1 to A13 had high initial capacities and higher capacity retention rates than battery B1. Battery A4 was charged and discharged at 45°C in the same manner as above, and the initial capacity was determined. The initial capacity was 219.7 mAh / g, which was higher than the initial capacity when charged and discharged at 25°C. This confirmed that the composite oxide was fully functioning as an active material. This confirmed that adding the alkaline component LiOH during the preparation of the raw material solution sufficiently reduced the effect of the acid component on the composite oxide.

[0088] Examples 14 to 16 In the preparation of the positive electrode, layered rock salt type LiNi 0.5 Co 0.2 Mn 0.3 Positive electrodes a1, a2, and a5 of Examples 1, 2, and 5 were fabricated in the same manner as positive electrodes a1, a2, and a5 of Examples 14 to 16, respectively, except that composite oxide particles (average particle size (D50) 13 μm) having a composition of (NCM) were obtained. The P content in the positive electrode material, determined by the method described above, was 0.23 mass%. Batteries C1 to C3 of Examples 14 to 16 were fabricated in the same manner as battery A1 of Example 1, respectively, except that positive electrodes c1 to c3 were used instead of positive electrode a1.

[0089] Example 17 Positive electrode c4 of Example 17 was prepared in the same manner as positive electrode c2 of Example 15, except that in preparing the positive electrode, the concentration of the raw material solution (aqueous solution containing HPO and LiOH) was changed to the value shown in Table 2. The P content in the positive electrode material, determined by the method described above, was 0.52 mass%. Battery C4 of Example 17 was prepared in the same manner as battery A1 of Example 1, except that positive electrode c4 was used instead of positive electrode a1.

[0090] Example 18 A positive electrode c5 of Example 18 was prepared in the same manner as for the positive electrode c2 of Example 15, except that the concentration of the raw material solution (aqueous solution containing HPO and LiOH) was changed to set the P content in the positive electrode material to the value shown in Table 2. The P content in the positive electrode material, determined by the method described above, was 0.73 mass%. A battery C5 of Example 18 was prepared in the same manner as for the battery A1 of Example 1, except that the positive electrode c5 was used instead of the positive electrode a1.

[0091] Comparative Example 2 A positive electrode d1 of Comparative Example 2 was produced in the same manner as the positive electrode c1 of Example 14, except that the surfaces of the composite oxide particles were not coated with an additive in the production of the positive electrode. A battery D1 of Comparative Example 2 was produced in the same manner as the battery A1 of Example 1, except that the positive electrode d1 was used instead of the positive electrode a1.

[0092] The batteries C1 to C5 of Examples 14 to 18 and the battery D1 of Comparative Example 2 obtained above were evaluated in the same manner as the battery A1 of Example 1. The evaluation results are shown in Table 2.

[0093] [Table 2]

[0094] For positive electrodes c1 to c5, the components of the additive coating the surface of the composite oxide particles were investigated by the method described above. As a result, in all cases, compound A, hexametaphosphate (Li6P6O 18), lithium orthophosphate (Li3PO4), and lithium pyrophosphate (Li4P2O7).

[0095] The batteries C1 to C5 had a high initial capacity and a higher capacity retention rate than the battery D1.

[0096] Examples 19 and 20 In the preparation of the positive electrode, LiMn 1.5 Ni 0.5 Except for using composite oxide particles (average particle size (D50) 9 μm) having a composition of O4 (MnNi-based spinel structure), e1 to e2 of Examples 19 and 20 were fabricated in the same manner as for positive electrode a2 of Example 2. The P content in the positive electrode material, determined by the method described above, was 0.21 mass%. Except for using positive electrodes e1 to e2 instead of positive electrode a1, batteries E1 to E2 of Examples 19 and 20 were fabricated in the same manner as for battery A1 of Example 1.

[0097] Comparative Example 3 A positive electrode f1 of Comparative Example 3 was produced in the same manner as for the positive electrode e1 of Example 19, except that the surfaces of the composite oxide particles were not coated with an additive in the production of the positive electrode. A battery F1 of Comparative Example 3 was produced in the same manner as for the battery A1 of Example 1, except that the positive electrode f1 was used instead of the positive electrode a1.

[0098] The batteries E1 and E2 of Examples 19 and 20 and the battery F1 of Comparative Example 3 obtained above were evaluated in the same manner as the battery A1 of Example 1. The evaluation results are shown in Table 3.

[0099] [Table 3]

[0100] The components of the additives coating the surfaces of the composite oxide particles of the positive electrodes e1 and e2 were investigated by the method described above. As a result, in all cases, the compound A, hexametaphosphate (Li6P6O 18), lithium orthophosphate (Li3PO4), and lithium pyrophosphate (Li4P2O7).

[0101] Batteries E1 and E2 exhibited higher capacity retention rates than battery F1. [Industrial Applicability]

[0102] The nonaqueous electrolyte secondary battery according to the present disclosure is suitable for use, for example, as a power source for mobile devices such as smartphones, a power source for vehicles such as electric cars, and a storage device for natural energy such as sunlight. [Explanation of symbols]

[0103] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal

Claims

1. A composite oxide containing lithium and a transition metal, and an additive that covers at least a part of the surface of the composite oxide, the additive comprises a first inorganic phosphate compound and a second inorganic phosphate compound; the first inorganic phosphate compound is a cyclic inorganic phosphate compound, the cyclic inorganic phosphate compound includes at least one selected from the group consisting of cyclic polyphosphoric acid and salts thereof; the second inorganic phosphate compound is a chain inorganic phosphate compound, The chain inorganic phosphate compound includes Li 3 PO 4 , Li 4 P 2 O 7 , or LiPO 3 ; The composite oxide has a layered rock salt type crystal structure, and General formula: LiNi ｘ Co ｙ M １－ｘ－ｙ O ２ wherein in the general formula, 0.3≦x<1, 0<y≦0.5, and 0<1−x−y≦0.35 are satisfied, and M is at least one selected from the group consisting of Al and Mn; a phosphorus content of the composite oxide and the additive in a total amount of 0.2 mass % or more and 0.75 mass % or less;

2. A positive electrode for a non-aqueous electrolyte secondary battery as described in claim 1, wherein the cyclic inorganic phosphate compound includes at least one selected from the group consisting of hexametaphosphoric acid and salts thereof.

3. A positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode of claim 1 or 2, wherein the positive electrode is a non-aqueous electrolyte secondary battery.

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