Positive electrode active material for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries

Abstract

Provided is a non-aqueous electrolyte secondary battery that exhibits an improved charging / discharging efficiency at low cost. A positive electrode active material included in this non-aqueous electrolyte secondary battery includes a lithium-transition metal composite oxide containing 90 mol% or more of Ni and Mn with respect to the total molar amount of metal elements other than Li. The lithium-transition metal composite oxide is composed of single particles and / or secondary particles formed through aggregation of 2-100 of the single particles. The average particle size of the single particles is 0.5-1.5 μm. A boron compound is attached on the surface of the lithium-transition metal composite oxide.

Description

[Technical Field]

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

[0002] Lithium transition metal composite oxides are used as the positive electrode active material in lithium-ion secondary batteries, which are non-aqueous electrolyte secondary batteries. For example, Patent Document 1 discloses a method for improving the safety and high-temperature storage characteristics of lithium-ion secondary batteries by using in combination large-particle NCM-based lithium transition metal composite oxide coated with a metal oxide and small-particle NCM-based lithium transition metal composite oxide coated with lithium triborate. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6862503 [Overview of the Initiative]

[0004] However, NCM systems contain expensive Co, and the technology described in Patent Document 1 faces the challenge of cost reduction. Furthermore, while improvements in charge-discharge efficiency have been desired in recent years, the technology described in Patent Document 1 does not take charge-discharge efficiency into consideration, and there is still room for improvement.

[0005] The purpose of this disclosure is to provide a non-aqueous electrolyte secondary battery with improved charge and discharge efficiency at a low cost.

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery, according to one aspect of this disclosure, contains a lithium transition metal composite oxide containing 90 mol% or more of Ni and Mn relative to the total molar amount of metal elements other than Li, wherein the lithium transition metal composite oxide is composed of at least one of single particles and secondary particles formed by the aggregation of 2 to 100 single particles, the average particle size of the single particles being 0.5 μm to 1.5 μm, and a boron compound is attached to the surface of the lithium transition metal composite oxide. Single particles (primary particles of 0.5 μm or more) have good dielectric strength characteristics and exhibit good cycle characteristics even at high potentials of 4.3 V (lithium reference potential) or higher, but at high potentials it is difficult to suppress side reactions such as electrolyte decomposition, resulting in poor charge-discharge efficiency.

[0007] One embodiment of the present disclosure of a non-aqueous electrolyte secondary battery is characterized by comprising a positive electrode containing the above-mentioned positive electrode active material, a negative electrode, and a non-aqueous electrolyte.

[0008] According to one aspect of this disclosure, it is possible to achieve both cost reduction and high efficiency in a non-aqueous electrolyte secondary battery. [Brief explanation of the drawing]

[0009] [Figure 1] This is an axial cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment. [Figure 2] These are schematic cross-sectional views of the test cells prepared in the examples and comparative examples. [Figure 3] This is an SEM image of the cathode active material prepared in Example 3. [Modes for carrying out the invention]

[0010] In non-aqueous electrolyte secondary batteries, some of the Li ions that move from the positive electrode to the negative electrode during charging remain absorbed by the negative electrode active material and are not released from the negative electrode during discharge, resulting in a decrease in the battery's charge-discharge efficiency. In recent years, with the increasing capacity of batteries, there has been a demand for improved charge-discharge efficiency. Furthermore, there is a need to reduce the cost of non-aqueous electrolyte secondary batteries, and as a positive electrode active material, one mainly composed of relatively inexpensive Ni and Mn is preferred.

[0011] As a result of diligent research to solve the above problems, the present inventors have found that by forming a lithium transition metal composite oxide mainly composed of Ni and Mn into single particles with a predetermined average particle size, and by attaching a boron compound to the surface of the lithium transition metal composite oxide, the charge and discharge efficiency of the battery can be improved.

[0012] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to this disclosure will be described in detail. In the following, a cylindrical battery in which a wound electrode body is housed in a cylindrical outer casing will be given as an example, but the electrode body is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are stacked alternately one by one with a separator in between. The outer casing is not limited to the cylindrical shape and may be, for example, rectangular, coin-shaped, etc. The outer casing may also be a pouch type composed of a laminate sheet including a metal layer and a resin layer. Furthermore, in this specification, the description "numerical value (A) ~ numerical value (B)" means that it is greater than or equal to numerical value (A) and less than or equal to numerical value (B).

[0013] Figure 1 is an axial cross-sectional view of a cylindrical secondary battery 10, which is an example of an embodiment. In the secondary battery 10 shown in Figure 1, an electrode body 14 and a non-aqueous electrolyte (not shown) are housed in an outer casing 15. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13. For the sake of explanation, the side with the sealing body 16 will be referred to as "upper" and the bottom side of the outer casing 15 will be referred to as "lower".

[0014] The interior of the secondary battery 10 is hermetically sealed by closing the upper end of the outer package 15 with a sealing body 16. Insulating plates 17 and 18 are respectively provided above and below the electrode assembly 14. The positive electrode lead 19 extends upward through the through hole of the insulating plate 17, and is welded to the lower surface of the filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a cap 26, which is the top plate of the sealing body 16 electrically connected to the filter 22, serves as a positive electrode terminal. On the other hand, the negative electrode lead 20 passes through the through hole of the insulating plate 18, extends toward the bottom side of the outer package 15, and is welded to the inner surface of the bottom of the outer package 15. In the secondary battery 10, the outer package 15 serves as a negative electrode terminal. Note that when the negative electrode lead 20 is provided at the outer winding end portion, the negative electrode lead 20 passes outside the insulating plate 18, extends toward the bottom side of the outer package 15, and is welded to the inner surface of the bottom of the outer package 15.

[0015] The outer package 15 is, for example, a bottomed cylindrical metal outer can. A gasket 27 is provided between the outer package 15 and the sealing body 16, ensuring the airtightness of the interior of the secondary battery 10. The outer package 15 has, for example, a grooved portion 21 formed by pressing the side surface portion from the outside, the grooved portion 21 supporting the sealing body 16. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the outer package 15, and supports the sealing body 16 via the gasket 27 on the upper surface thereof.

[0016] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are sequentially stacked from the electrode assembly 14 side. Each member constituting the sealing body 16 has, for example, a disk shape or a ring shape, and the members other than the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective central portions, and the insulating member 24 is interposed between their respective peripheral edge portions. When the internal pressure of the battery rises due to abnormal heat generation, for example, the lower valve body 23 ruptures, which causes the upper valve body 25 to swell toward the cap 26 side and separate from the lower valve body 23, thereby cutting off the electrical connection between the two. When the internal pressure further rises, the upper valve body 25 ruptures, and gas is discharged from the opening 26a of the cap 26.

[0017] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the electrode body 14, with particular emphasis on the positive electrode 11.

[0018] [Positive electrode] The positive electrode 11 comprises a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. Preferably, the positive electrode mixture layer is formed on both sides of the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, a binder, etc. The thickness of the positive electrode mixture layer is, for example, 10 μm to 150 μm on one side of the positive electrode current collector. The positive electrode 11 is manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to the surface of the positive electrode current collector, drying the coating, and then rolling it to form the positive electrode mixture layer on both sides of the positive electrode current collector.

[0019] Examples of conductive agents included in the positive electrode mixture layer include carbon black (CB), acetylene black (AB), Ketjenblack, carbon nanotubes (CNT), graphene, graphite, and other carbon-based particles. These may be used individually or in combination of two or more types. The content of the conductive agent is, for example, 0.1% to 5.0% by mass per 100 parts by mass of positive electrode active material.

[0020] 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 cellulose derivatives such as carboxymethylcellulose (CMC) or its salts, and polyethylene oxide (PEO). The binder content is, for example, 0.1% to 5.0% by mass per 100 parts by mass of positive electrode active material.

[0021] The positive electrode active material contained in the positive electrode mixture layer includes a lithium transition metal composite oxide. A boron compound is attached to the surface of the lithium transition metal composite oxide. By attaching a boron compound to the surface of the lithium transition metal composite oxide with the configuration described later, the charge and discharge efficiency is significantly improved. It is presumed that the boron compound has Li-ion conductivity and therefore promotes the exchange of Li ions between the non-aqueous electrolyte and the positive electrode active material on the surface of the lithium transition metal composite oxide. A boron compound is a compound containing B (boron). In addition to the lithium transition metal composite oxide, the positive electrode active material may also contain LiF, Li2S, etc.

[0022] Lithium transition metal composite oxides consist of at least one of single particles and secondary particles formed by the aggregation of 2 to 100 single particles. In other words, lithium transition metal composite oxides contain single particles, and some of these single particles may aggregate in groups of 2 to 100 to form secondary particles. Note that single particles are primary particles in which no grain boundaries exist within the particle.

[0023] The proportion of single particles in the lithium transition metal composite oxide is preferably 20% or more by mass, more preferably 80% or more, particularly preferably 90% or more, and may be substantially 100%. The lithium transition metal composite oxide may also contain secondary particles formed by the aggregation of more than 100 primary particles.

[0024] The average particle size of a single particle is 0.5 μm to 1.5 μm. That is, the lithium transition metal composite oxide may contain single particles with an average particle size of 0.5 μm to 1.5 μm, and secondary particles formed by the aggregation of 2 to 100 single particles with an average particle size of 0.5 μm to 1.5 μm. In this specification, the average particle size means the volume-based median diameter (D50). D50 is 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 the positive electrode active material can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell MT3000II) with water as the dispersion medium.

[0025] Lithium transition metal composite oxides contain 90 mol% or more of Ni and Mn relative to the total molar amount of metal elements other than Li. This makes it possible to obtain lithium transition metal composite oxides at a relatively low cost and in high volume. Note that lithium transition metal composite oxides may also be composed of Ni and Mn alone.

[0026] Ni is preferably the most abundant of the metal elements other than Li that make up the lithium transition metal composite oxide. The Ni content in the lithium transition metal composite oxide is preferably 50 mol% or more, and more preferably 70 mol% or more, relative to the total molar amount of the metal elements other than Li. The upper limit of the Ni content may be 95 mol%, but is preferably 90 mol%.

[0027] Mn is preferably the second most abundant element (after Ni) among the metal elements other than Li that make up the lithium transition metal composite oxide. Mn can stabilize the crystal structure of the lithium transition metal composite oxide. The Mn content in the lithium transition metal composite oxide is, for example, 5 mol% to 50 mol% relative to the total molar amount of the metal elements other than Li.

[0028] The lithium transition metal composite oxide may further contain Fe. The Fe content in the lithium transition metal composite oxide is, for example, 0 mol% to 10 mol% relative to the total molar amount of the metal elements excluding Li.

[0029] In lithium transition metal composite oxides, the Co content is preferably less than 1 mol% relative to the total molar amount of the metal elements excluding Li. In other words, since Co is an expensive metal element, it is preferable that lithium transition metal composite oxides substantially contain no Co.

[0030] An example of a suitable lithium transition metal composite oxide is Li α Ni x Mn y M zO2 is a composite oxide represented by the formula O2 (wherein 0.9≦α≦1.2, 0.5≦x≦0.9, 0.05≦y≦0.5, 0≦z≦0.1, and M is at least one metallic element selected from Fe, Cu, Zr, Mg, Ca, W, Si, Cr, Sr, and Ti). The content of metallic elements in lithium-containing composite oxides is measured, for example, by inductively coupled plasma (ICP) emission spectroscopy.

[0031] The boron compound adhering to the surface of the lithium transition metal composite oxide is not particularly limited as long as it contains B, but examples include boron oxide, boron fluoride, boron chloride, and boron sulfide. The boron compound is preferably a boron oxide. Examples of boron oxides include boric acid (H3BO3), boron oxide (B2O3), and lithium borate (LiBO2, LiB3O5, Li2B4O7). The boron compound may be formed to cover the entire surface of the lithium transition metal composite oxide, or it may be scattered on the surface of the lithium transition metal composite oxide. The boron compound present on the surface of the lithium transition metal composite oxide can be confirmed by low-acceleration SEM, TEM-EDX, etc.

[0032] The amount of boron compound adhering to the surface of the lithium transition metal composite oxide is preferably 0.1 mol% to 7 mol% relative to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide. Increasing the amount of boron compound adhering to 0.1 mol% or more makes the effect of the boron compound more pronounced. If the amount of boron compound adhering to exceeds 7 mol%, the battery resistance may increase.

[0033] The positive electrode active material may contain at least one metal element selected from the group consisting of Ca, Sr, Fe, Cu, Zr, Mg, Si, Cr, Ti, and W. These metal elements may be contained in the lithium transition metal composite oxide, but it is preferable that they are attached to the surface of the lithium transition metal composite oxide. This suppresses side reactions between the lithium transition metal composite oxide and the electrolyte, thereby suppressing battery degradation. The positive electrode active material preferably contains these metal elements in amounts of 0 mol% to 5 mol%, and more preferably 0.01 mol% to 1 mol%, relative to the total amount of Ni, Mn, and Fe. The positive electrode active material contains these metal elements in amounts of, for example, 0.05 mol% to 0.2 mol%, relative to the total amount of Ni, Mn, and Fe.

[0034] Next, an example of a method for producing a positive electrode active material according to this embodiment will be described. The method for producing a positive electrode active material includes, for example, a synthesis step, a washing step, a drying step, and an addition step.

[0035] In the synthesis process, a metal hydroxide containing 90 mol% or more of Ni and Mn is mixed with a Li compound and calcined to obtain a lithium-containing composite oxide.

[0036] Metal hydroxides can be obtained, for example, by stirring a solution of a metal salt containing Ni, Mn, and any metal element (such as Fe), adding an alkaline solution such as sodium hydroxide dropwise to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), and allowing precipitation (coprecipitation). Alternatively, metal oxides obtained by heat-treating metal hydroxides may be used instead of metal hydroxides.

[0037] Examples of Li compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. The mixing ratio of the metal hydroxide and the Li compound is preferably such that, for example, the molar ratio of metal elements other than Li to Li is in the range of 1:0.98 to 1:1.1, as this facilitates adjusting each of the above parameters to the specified range. When mixing the metal hydroxide and the Li compound, Ca compounds, Sr compounds, W compounds, Fe compounds, Cu compounds, Zr compounds, Mg compounds, Si compounds, Cr compounds, Ti compounds, S compounds, fluorides, etc., may be added. Examples of Ca compounds include CaO, Ca(OH)2, and CaCO3. Examples of Sr compounds include SrO, Sr(OH)2, and SrCO3. Examples of W compounds include WO3, Li2WO4, Li4WO5, and Li6W2O9. Oxides, hydroxides, sulfides, and fluorides of other elements can also be used. In addition, alkaline earth metals, W, and Zr are highly effective in reducing resistance and are therefore useful.

[0038] A mixture of metal hydroxide and a Li compound, etc., is calcined, for example, under an oxygen atmosphere. The calcination conditions may be such that the heating rate is in the range of 1.0°C / min to 5.5°C / min from 450°C to 680°C, and the maximum temperature reached is in the range of 700°C to 850°C. The heating rate from above 680°C to the maximum temperature reached may be, for example, 0.1°C / min to 3.5°C / min. The holding time at the maximum temperature reached may be between 1 hour and 30 hours. Furthermore, this calcination process may be multi-stage calcination, and the first heating rate and second heating rate may be set multiple times for each temperature range, as long as they are within the ranges specified above.

[0039] In the washing step, the lithium-containing composite oxide obtained in the synthesis step is washed with water and dehydrated to obtain a cake-like composition. The washing and dehydration can be carried out by known methods and conditions. It should be done within a range that does not cause lithium to leach out from the lithium-containing composite oxide and degrade the battery characteristics. Ca compounds, Sr compounds, W compounds, Fe compounds, Cu compounds, Zr compounds, Mg compounds, Si compounds, Cr compounds, Ti compounds, S compounds, fluorides, etc. may be added to the cake-like composition. 1 g of the positive electrode active material after the washing step was mixed with 30 g of water for 20 seconds, and the alkali content of the filtrate was measured by neutralization titration. Monovalent and divalent alkalis were present in the positive electrode active material. The amount of monovalent alkali in the positive electrode active material was, for example, 50 μmol to 400 μmol per gram of positive electrode active material.

[0040] In the drying step, the cake-like composition obtained in the washing step is dried to obtain a powder-like composition. The drying step may be carried out under a vacuum atmosphere. The drying conditions are, for example, 150°C to 400°C for 0.5 to 15 hours.

[0041] In the addition step, a boron-containing compound, such as boric acid (H3BO3), is added to the powdered composition obtained in the drying step, and the temperature is raised to 200°C to 400°C. This allows the boron compound to adhere to the surface of the lithium-containing composite oxide. The amount of boron-containing compound added is, for example, 0.1 mol% to 7 mol% relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide.

[0042] [Negative electrode] The negative electrode 12 comprises a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. Preferably, the negative electrode mixture layer is formed on both sides of the negative electrode current collector. The negative electrode current collector can be made of a metal foil that is stable in the potential range of the negative electrode 12, such as copper, or a film with the metal arranged on its surface. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The negative electrode 12 is manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder to the surface of the negative electrode current collector, drying the coating, and then rolling it to form the negative electrode mixture layer on both sides of the negative electrode current collector.

[0043] The negative electrode 12 may contain boron. Some of the boron adhering to the surface of the positive electrode active material may migrate from the positive electrode 11 to the negative electrode 12. Even if metallic elements such as Ni precipitate on the negative electrode surface, the coexistence of boron and boron can suppress battery degradation. The amount of boron contained in the negative electrode is preferably 50 μg or more, and more preferably 400 μg to 1200 μg per gram of positive electrode active material. More than 35% of the boron added to the positive electrode precipitated on the negative electrode, and less than 55% of the boron remained on the positive electrode.

[0044] The negative electrode active material contained in the negative electrode mixture layer may include, for example, a carbon-based active material that reversibly intercepts and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lumpy graphite, and clay-like graphite, as well as artificial graphite such as lumpy artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, a Si-based active material composed of at least one of Si and a Si-containing compound may be used as the negative electrode active material, and carbon-based active materials and Si-based active materials may be used in combination.

[0045] As for the binder contained in the negative electrode mixture layer, as with the positive electrode 11, fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc. may be used, but styrene-butadiene rubber (SBR) is preferred. Furthermore, the negative electrode mixture layer preferably contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. Among these, the combination of SBR and CMC or a salt thereof, or PAA or a salt thereof is preferred. The negative electrode mixture layer may also contain a conductive agent.

[0046] [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 laminated structure. Furthermore, the surface of the separator 13 may be provided with a heat-resistant resin layer such as aramid resin, or a filler layer containing an inorganic compound filler.

[0047] [Non-aqueous electrolytes] Non-aqueous electrolytes include 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 of 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).

[0048] 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.

[0049] Examples of the above ethers include cyclic ethers such as 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, and crown ether; and linear ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl 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.

[0050] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1<x<6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, chloroborane lithium, lower aliphatic carboxylic acid lithium, borates such as Li2B4O7 and Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1Examples include imide salts such as SO2){l,m are integers greater than or equal to 0}. Lithium salts may be used individually or in mixtures of multiple types. Of these, LiPF6 is preferred from the viewpoint of ionic conductivity and electrochemical stability. The concentration of the lithium salt is preferably, for example, 0.8 moles to 1.8 moles per liter of non-aqueous solvent. [Examples]

[0051] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0052] <Example 1> [Fabrication of positive electrode active material] Ni with an average particle size of 6 μm obtained by LiOH and coprecipitation method. 0.8 Mn 0.2 (OH)2 powder was mixed with Li to obtain a mixture with a molar ratio of 1.1:1 of the total amount of Ni and Mn. This mixture was calcined under an oxygen stream with an oxygen concentration of 90% or more (flow rate of 0.15 to 0.2 L / min per 1 L of furnace volume) from room temperature to 650°C for 5 hours, then calcined to 850°C for 2 hours, and held for 9 hours to obtain a lithium-containing composite oxide (synthesis step). Excess lithium from this lithium-containing composite oxide was removed by washing with water, and after drying, the obtained positive electrode active material was crushed with a jet mill to obtain single particles with an average particle size of 1 μm. Powdered boric acid (H3BO3) was added to these single particles (addition step). The amount of boric acid added was 0.2 mol% relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide. By drying this, the positive electrode active material of Example 1 was obtained.

[0053] [Preparation of test cells] The test cell shown in Figure 2 was prepared using the following procedure. First, the above-mentioned positive electrode active material, acetylene black (conductive material), and polyvinylidene fluoride (binding agent) were mixed in a weight ratio of 80:10:10 and formed into a slurry using N-methyl-2-pyrrolidone. Next, this slurry was applied to an aluminum foil current collector, which served as the positive electrode current collector, and vacuum-dried at 110°C to produce the working electrode 30 (positive electrode).

[0054] Under dry air with a dew point of -50°C or lower, the electrode group, consisting of the working electrode 30, counter electrode 31 (negative electrode), and reference electrode 32, each fitted with an electrode tab 38 and with a separator 34 interposed between them, was housed in an outer casing 35. Then, an electrolyte 36 was injected into the outer casing 35, and the outer casing 35 was sealed to fabricate a test cell, which is a non-aqueous electrolyte secondary battery. Details of each component are as follows. Opposite electrode: Lithium metal Reference pole: Lithium metal Separator: Polyethylene separator Non-aqueous electrolyte: A non-aqueous solvent obtained by mixing 4-fluoroethylene carbonate (FEC) and methyl 3,3,3-trifluoropropionate (FMP) in a volume ratio of 25:75, to which LiPF6 is dissolved as an electrolyte salt to a concentration of 1.0 mol / l.

[0055] [Evaluation of charging capacity, discharging capacity, and charge / discharge efficiency] Under ambient temperature of 25°C, the test cell described above was charged to 4.5V (lithium reference) with a constant current of 0.2C, and then charged to 0.02C with a constant voltage of 4.5V. Subsequently, it was discharged to 2.5V with a constant current of 0.1C. The charging and discharging capacities at this time were measured, and the charge / discharge efficiency was calculated by dividing the discharge capacity by the charging capacity.

[0056] [Evaluation of cycle characteristics] Under an ambient temperature of 25°C, the above test cell was charged to 4.2V with a constant current of 0.2C, and then charged to 0.02C with a constant voltage of 4.2V. Afterward, it was discharged to 2.5V with a constant current of 0.1C. This charge-discharge cycle was considered one cycle. Measurements were taken under the above conditions for cycles 11, 21, and 31. For the remaining cycles, the charge-discharge procedure was the same as above, except that the constant current during discharge was set to 0.2C. The capacity retention rate was calculated using the following formula. Capacity retention rate = (Discharge capacity at cycle 31 / Discharge capacity at cycle 1) × 100

[0057] [Evaluation of IV resistance] Under an ambient temperature of 25°C, the above test cell was charged to 50% of its state of charge (SOC) with a constant current of 0.2C after 11 and 31 cycles. From this state, it was discharged for 10 seconds at currents of 0.05C, 0.1C, and 0.2C. The voltage values ​​after 10 seconds of discharge for each current value were plotted, and the IV resistance was determined from the slope of the linear approximation. The IV resistance after 11 cycles was defined as the initial IV resistance, and the IV resistance after 31 cycles was defined as the post-cycle IV resistance.

[0058] <Examples 2-7, Comparative Example 1> 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 amount of boric acid added was changed as shown in Table 1. In the test cell of Example 3, the amount of boron contained in the negative electrode was 430 μg per gram of positive electrode active material. 38% of the boron added to the positive electrode precipitated on the negative electrode. In addition, 1 g of the positive electrode active material after the washing process of Example 3 contained 226 μmoles of monovalent alkali and 120 μmoles of divalent alkali. In Comparative Example 1, 1 g of the positive electrode active material after the washing process contained 292 μmoles of monovalent alkali and 142 μmoles of divalent alkali. Furthermore, the positive electrode active material prepared in Example 3 was observed using a low-acceleration SEM. The SEM image of the positive electrode active material of Example 3 is shown in Figure 3. As shown in Figure 3, the positive electrode active material of Example 3 had a single-particle shape with no grain boundaries inside the particle.

[0059] <Example 8> In the preparation of the positive electrode active material, the water washing and drying processes were omitted, and secondary particles consisting of approximately 50 aggregated single particles were obtained. A test cell was then prepared and evaluated in the same manner as in Example 3. The amount of boron contained in the negative electrode was 460 μg per gram of positive electrode active material. 41% of the boron added to the positive electrode precipitated on the negative electrode. Furthermore, 1 gram of the positive electrode active material after the washing process contained 295 μmoles of monovalent alkali and 149 μmoles of divalent alkali.

[0060] <Comparative 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 8, except that the boron addition step was omitted. Furthermore, 1 g of the positive electrode active material after the washing step contained 414 μmoles of monovalent alkali and 157 μmoles of divalent alkali.

[0061] <Example 9> In the preparation of the positive electrode active material, the water washing, drying, and crushing processes using a jet mill were omitted, and secondary particles consisting of approximately 90 aggregated single particles were obtained. A test cell was then prepared and evaluated in the same manner as in Example 3. After the washing process, 1 g of the positive electrode active material contained 167 μmoles of monovalent alkali and 121 μmoles of divalent alkali.

[0062] <Comparative Example 3> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 9, except that the boron addition step was omitted. After the washing step, 1 g of the positive electrode active material contained 220 μmoles of monovalent alkali and 112 μmoles of divalent alkali.

[0063] <Example 10> Ni with an average particle size of 5 μm obtained by LiOH and coprecipitation method. 0.85 Mn 0.15(OH)2 powder was mixed with Li to obtain a mixture with a molar ratio of 1.1:1 of the total amount of Ni and Mn. This mixture was calcined under an oxygen stream with an oxygen concentration of 90% or higher (flow rate of 0.15-0.2 L / min per 1 L of furnace volume) from room temperature to 650°C for 5 hours, then calcined to 800°C for 2 hours, and held for 12 hours to obtain a lithium-containing composite oxide (synthesis step). The obtained positive electrode active material was crushed with a jet mill to obtain secondary particles consisting of approximately 50 aggregated single particles. Then, powdered boric acid (H3BO3) was added to these secondary particles (addition step). The amount of boric acid added was 1 mol% relative to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide. By drying this, the positive electrode active material of Example 10 was obtained. A test cell was prepared and evaluated in the same manner as in Example 1, except for the use of this positive electrode active material.

[0064] <Example 11> A secondary battery was prepared and evaluated in the same manner as in Example 10, except that in the preparation of the positive electrode active material, an additional 0.05 mol% of CuO was added relative to the total amount of Ni and Mn to obtain a mixture.

[0065] <Example 12> A secondary battery was prepared and evaluated in the same manner as in Example 10, except that 0.05 mol% of ZrO2 was added relative to the total amount of Ni and Mn to obtain a mixture in the preparation of the positive electrode active material.

[0066] <Example 13> A secondary battery was prepared and evaluated in the same manner as in Example 10, except that 0.05 mol% of Mg(OH)2 was added to the total amount of Ni and Mn to obtain a mixture in the preparation of the positive electrode active material.

[0067] <Example 14> A secondary battery was prepared and evaluated in the same manner as in Example 10, except that 0.05 mol% of Ca(OH)2 was added to the total amount of Ni and Mn to obtain a mixture in the preparation of the positive electrode active material.

[0068] <Example 15> A secondary battery was prepared and evaluated in the same manner as in Example 10, except that 0.05 mol% of WO3 was added to the total amount of Ni and Mn to obtain a mixture in the preparation of the positive electrode active material.

[0069] <Example 16> A secondary battery was prepared and evaluated in the same manner as in Example 10, except that 0.05 mol% of SiO was added to the total amount of Ni and Mn to prepare the positive electrode active material.

[0070] <Example 17> In preparing the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 10, except that 0.05 mol% of LiF was added to the lithium transition metal composite oxide obtained in Example 10 and this was used as the positive electrode active material.

[0071] <Example 18> In preparing the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 10, except that 0.05 mol% of Li2S was added to the lithium transition metal composite oxide obtained in Example 10 to use as the positive electrode active material.

[0072] <Comparative Example 4> A test cell was prepared and evaluated in the same manner as in Example 10, except that the boron addition step was omitted in the preparation of the positive electrode active material.

[0073] <Reference example 1> Ni with an average particle size of 6 μm obtained by LiOH and coprecipitation method. 0.85 Co 0.05 Mn 0.10(OH)2 powder was mixed with Li to obtain a mixture in a molar ratio of 1:1.05 for the total amount of Ni, Co, and Mn. This mixture was calcined under an oxygen stream with an oxygen concentration of 90% or higher (flow rate of 0.15 to 0.2 L / min per 1 L of furnace volume) from room temperature to 650°C for 5 hours, then calcined to 750°C for 2 hours, and held for 3 hours to obtain a lithium-containing composite oxide (synthesis process). This lithium-containing composite oxide was a secondary particle formed by the aggregation of single particles, and this was used as the positive electrode active material in Reference Example 1. Except for the use of this positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1.

[0074] <Reference example 2> A test cell was prepared and evaluated in the same manner as in Reference Example 1, except that powdered boric acid (H3BO3) was added to the secondary particles after the synthesis step for preparing the positive electrode active material, and the positive electrode active material was obtained by drying. The amount of boric acid added was 1 mol% relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide.

[0075] <Reference example 3> In the preparation of the positive electrode active material, the raw material, a metal hydroxide, is prepared using Ni with an average particle size of 11 μm. 0.83 Co 0.12 Mn 0.05 The test cells were prepared and evaluated in the same manner as in Reference Example 1, except that (OH)2 powder was used instead.

[0076] <Reference example 4> A test cell was prepared and evaluated in the same manner as in Reference Example 3, except that powdered boric acid (H3BO3) was added to the secondary particles after the synthesis step for preparing the positive electrode active material, and the positive electrode active material was obtained by drying. The amount of boric acid added was 1 mol% relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide.

[0077] <Reference example 5> In the preparation of the positive electrode active material, the raw material, a metal hydroxide, is prepared using Ni with an average particle size of 12 μm. 0.85 Co 0.05 Mn 0.10The test cells were prepared and evaluated in the same manner as in Reference Example 1, except that (OH)2 powder was used instead.

[0078] <Reference example 6> A test cell was prepared and evaluated in the same manner as in Reference Example 5, except that powdered boric acid (H3BO3) was added to the secondary particles after the synthesis step for preparing the positive electrode active material, and the positive electrode active material was obtained by drying. The amount of boric acid added was 1 mol% relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide.

[0079] Tables 1-5 show the evaluation results for the examples and comparative examples. Tables 1-3 show the composition of the lithium transition metal composite oxide and the amount of boric acid added. Table 4 shows the composition of the lithium transition metal composite oxide, the composition of the positive electrode active material other than the lithium transition metal composite oxide, and the amount of boric acid added. Table 5 shows the composition of the lithium transition metal composite oxide, the average particle size of the raw material metal hydroxide, and the amount of boric acid added.

[0080] [Table 1]

[0081] [Table 2]

[0082] [Table 3]

[0083] [Table 4]

[0084] [Table 5]

[0085] In all of Tables 1-3, the test cells of the examples showed higher charge / discharge efficiency and capacity retention rate, and lower IV resistance than the test cells of the comparative examples. Therefore, it can be seen that a low-cost, high-efficiency battery can be obtained by attaching a boron compound to the surface of a lithium transition metal composite oxide having a predetermined composition. In Table 4, the test cells of the examples also showed higher charge / discharge efficiency and capacity retention rate than the test cells of the comparative examples. This indicates that similar effects can be obtained even if the lithium transition metal composite oxide contains elements other than Ni, Mn, and Fe, and that similar effects can be obtained even if the positive electrode active material contains Li compounds other than lithium transition metal composite oxide, such as LiS and Li2F. In Table 5, no increase in discharge capacity was observed with boron addition, and the improvement in Coulomb efficiency was due to a decrease in charging capacity caused by boron addition. A decrease in charging capacity is undesirable from the viewpoint of high battery capacity. Furthermore, no clear improvement in IV resistance was observed after the cycle. [Explanation of Symbols]

[0086] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Outer casing, 16 Sealing body, 17,18 Insulating plate, 19 Positive electrode lead, 20 Negative electrode lead, 21 Grooved section, 22 Filter, 23 Lower valve body, 24 Insulating member, 25 Upper valve body, 26 Cap, 26a Opening, 27 Gasket, 30 Working electrode (Positive electrode), 31 Counter electrode (Negative electrode), 32 Reference electrode, 34 Separator, 35 Outer casing, 36 Electrolyte, 38 Electrode tab

Claims

1. It contains a lithium transition metal composite oxide containing 90 mol% or more of Ni and Mn relative to the total molar amount of metal elements other than Li. The lithium transition metal composite oxide is composed of at least one of a single particle and secondary particles formed by the aggregation of 2 to 100 of the single particles. The average particle size of the single particle is 0.5 μm to 1.5 μm. A boron compound is attached to the surface of the lithium transition metal composite oxide. The lithium transition metal composite oxide is a composite oxide represented by the compositional formula Li α Ni x Mn y M z O 2 (wherein 0.9 ≤ α ≤ 1.2, 0.5 ≤ x ≤ 0.9, 0.05 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1, and M is at least one metal element selected from Fe, Cu, Zr, Mg, Ca, W, Si, Cr, Sr, and Ti), and is a positive electrode active material for a non-aqueous electrolyte secondary battery.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide further contains Fe.

3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the Co content in the lithium transition metal composite oxide is less than 1 mol% relative to the total molar amount of metal elements other than Li.

4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the boron compound is a boron oxide.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the amount of the boron compound adhering to the surface of the lithium transition metal composite oxide is 0.1 mol% to 7 mol% with respect to the total molar amount of metal elements excluding Li in the lithium transition metal composite oxide.

6. Furthermore, the positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, comprising at least one selected from the group consisting of metal elements Ca, Sr, Fe, Cu, Zr, Mg, Si, Cr, Ti, and W, sulfur, and fluorides.

7. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery described in claim 1, a negative electrode, and a non-aqueous electrolyte.

8. The non-aqueous electrolyte secondary battery according to claim 7, wherein the negative electrode includes B.

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