Positive electrode active material for non-aqueous electrolyte secondary battery and positive electrode for non-aqueous electrolyte secondary battery

Abstract

To provide a non-aqueous electrolyte secondary battery in which a reaction resistance of a cathode is low, and an output characteristic is improved.SOLUTION: A cathode active material for a non-aqueous electrolyte secondary battery, contains: a lithium inclusion transition metal oxide having a secondary particle formed by aggregating a primary particle; and a boron compound and a tungsten compound existed between at least the primary particles. The lithium inclusion transition metal oxide contains Co of 0 mol% or more and 3 mol% or less. A tungsten element amount dissolved when a cathode active material is washed with an aqueous sodium hydroxide of 0.01 mol / L for five minutes is 60% or less of the tungsten element amount detected when the cathode active material is solved with a mixed acid obtaining hydrofluoric acid, nitric acid, and hydrochloric acid. A boron element amount dissolved when the cathode active material is washed with an ion exchange water for a one minute is 80% or more of the boron element amount detected when the cathode active material is solved with hydrochloric acid.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Lithium-containing transition metal oxides are used as positive electrode active materials in lithium ion secondary batteries, which are non-aqueous electrolyte secondary batteries. For example, Patent Document 1 discloses a lithium-containing transition metal oxide having the general formula Li z Ni 1-x-y Co x M y A method for increasing the output of a lithium-ion secondary battery is disclosed in which a lithium tungstate compound is attached to the surface of primary particles of a lithium-containing transition metal oxide represented by O2 (0≦x≦0.35, 0≦y≦0.35, 0.95≦z≦1.30, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti, and Al), thereby reducing the reaction resistance of the positive electrode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-127004 Summary of the Invention

[0004] However, the method described in Patent Document 1 is unable to sufficiently reduce the reaction resistance of the positive electrode, and there is still room for improvement.

[0005] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery having a lower reaction resistance of the positive electrode and improved output characteristics than conventional batteries by reducing the resistance of the positive electrode active material.

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a lithium-containing transition metal oxide having secondary particles formed by aggregation of primary particles, and a tungsten compound and a boron compound present at least between the primary particles. The tungsten compound is present in an amount of 0.1 to 0.38 mass% in terms of elemental tungsten relative to the mass of the lithium-containing transition metal oxide, and the boron compound is present in an amount of 0.006 to 0.17 mass% in terms of elemental boron relative to the mass of the lithium-containing transition metal oxide. The amount of elemental tungsten eluted when the positive electrode active material is washed with a 0.01 mol / L aqueous sodium hydroxide solution for 5 minutes is 60% or less of the amount of elemental tungsten detected when the positive electrode active material is dissolved in a mixed acid containing hydrofluoric acid, nitric acid, and hydrochloric acid, and the amount of elemental boron eluted when the positive electrode active material is washed with ion-exchanged water for 1 minute is 80% or more of the amount of elemental boron detected when the positive electrode active material is dissolved in hydrochloric acid.

[0007] A positive electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure is characterized by including a positive electrode mixture layer containing the above positive electrode active material and a conductive material in an amount of 1 mass % or less relative to the mass of the above positive electrode active material.

[0008] According to one aspect of the present disclosure, a low-resistance positive electrode active material for a non-aqueous electrolyte secondary battery can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0009] In lithium-containing transition metal oxides, if a phenomenon called cation mixing occurs in which transition metals such as nickel enter the sites where lithium ions are inserted and extracted during charging and discharging, the resistance of the positive electrode increases, and high output cannot be obtained. For example, Patent Document 1 describes a lithium-containing transition metal oxide having the general formula Li z Ni 1-x-y Co x M yA method for reducing the resistance of a positive electrode has been disclosed in which a lithium tungstate compound is formed on the surface of primary particles of a lithium-containing transition metal oxide represented by O2 (where 0≦x≦0.35, 0≦y≦0.35, 0.95≦z≦1.30, and M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti, and Al), thereby forming a conductive path for lithium ions at the interface with the electrolyte. However, as a result of extensive studies by the present inventors, it has been found that simply securing a conductive path for lithium ions by adding tungsten is not enough to sufficiently reduce the resistance of the positive electrode. Therefore, the inventors conducted further studies and found that the resistance of a positive electrode active material can be reduced when a positive electrode active material is prepared by adding a tungsten compound and a boron compound to a lithium-containing transition metal oxide, and the amount of tungsten element eluted when the positive electrode active material is washed with a 0.01 mol / L aqueous sodium hydroxide solution for 5 minutes is 60% or less of the amount of tungsten element detected when the positive electrode active material is dissolved with a mixed acid containing hydrofluoric acid, nitric acid, and hydrochloric acid, and when the amount of boron element eluted when the positive electrode active material is washed with ion-exchanged water for 1 minute is 80% or more of the amount of boron element detected when the positive electrode active material is dissolved with hydrochloric acid.

[0010] The lithium-containing transition metal oxide, tungsten compound, and boron compound constituting the positive electrode active material according to this embodiment, and the method for producing the positive electrode active material will be described in detail below.

[0011] [Lithium-containing transition metal oxides] The lithium-containing transition metal oxide has secondary particles formed by aggregation of primary particles. In other words, in terms of improving the output characteristics of nonaqueous electrolyte secondary batteries, the lithium-containing transition metal oxide preferably comprises primary particles and secondary particles formed by aggregation of the primary particles, and the secondary particles preferably have voids and grain boundaries through which the electrolyte can penetrate. The average particle size of the primary particles is, for example, preferably 500 nm or less, more preferably in the range of 50 nm to 300 nm. The average particle size of the primary particles is determined by obtaining a backscattered electron image (hereinafter sometimes referred to as an SEM image) of the cross section of the particles and averaging the major axes of 10 primary particles. The median particle size (D50) of the secondary particles is, for example, preferably in the range of 1 μm to 50 μm, more preferably in the range of 5 μm to 20 μm. Here, the median particle size refers to the median particle size (D50) at which the volume-integrated value is 50% in the particle size distribution measured by laser diffraction scattering.

[0012] The composition of the lithium-containing transition metal oxide is represented by the general formula Li α Ni 1-w-x-y-z Co w Al x Mn y M z O2 (wherein 0.95≦α≦1.05, 0≦w≦0.09, 0≦x≦0.06, 0≦y≦0.1, 0≦z≦0.01, and M is at least one element selected from Mg, Sr, Si, Y, Mo, V, Ti, Fe, Zr, and Nb). From the viewpoint of increasing the energy density of nonaqueous electrolyte secondary batteries, a large Ni content is preferable, and therefore the sum of w, x, y, and z in the general formula preferably satisfies 0≦w+x+y+z≦0.15, and more preferably 0≦w+x+y+z≦0.12.

[0013] The content of Co contained in the lithium-containing transition metal oxide can be set to 0≦w≦0.03. In this case, the resistance value of the lithium-containing transition metal oxide increases as the content of Co decreases, and therefore the effect of reducing resistance by adding tungsten and boron becomes more pronounced.

[0014] The lithium-containing transition metal oxide can be synthesized as follows. (1) Formation of transition metal composite oxides First, a transition metal composite hydroxide such as nickel-cobalt-aluminum composite hydroxide obtained by coprecipitation is heat-treated to obtain a transition metal composite oxide. (2) Synthesis of transition metal composite oxides and lithium compounds Next, the transition metal composite oxide is mixed with a lithium compound such as lithium hydroxide or lithium carbonate, and the mixture is heat-treated and then pulverized to obtain lithium-containing transition metal oxide particles.

[0015] [Tungsten compounds] Examples of tungsten compounds (hereinafter sometimes referred to as W compounds) include tungsten oxide (WO), lithium tungstate (LiWO, LiWO, LiWO), and ammonium tungstate. At least a portion of the W compound is present between primary particles of the lithium-containing transition metal oxide. From an SEM image of the cross section of the secondary particles of the lithium-containing transition metal oxide contained in the positive electrode active material, it can be confirmed that elemental tungsten is present between the primary particles constituting the secondary particles. Furthermore, the amount of the W compound is 0.1 to 0.38 mass% in terms of elemental tungsten relative to the mass of the lithium-containing transition metal oxide. When the amount of the W compound is within this range, the resistance value of the positive electrode active material can be reduced.

[0016] The amount of tungsten element eluted when the positive electrode active material was washed with a 0.01 mol / L aqueous sodium hydroxide solution for 5 minutes was 60% or less of the amount of tungsten element detected when the positive electrode active material was dissolved in a mixed acid containing hydrofluoric acid, nitric acid, and hydrochloric acid (hereinafter, this value may be referred to as the tungsten elution rate (W elution rate)). If the W elution rate is within this range, it is assumed that most of the W compounds are present between the primary particles of the lithium-containing transition metal oxide, and the synergistic effect with the distribution state of the boron compounds described below significantly reduces the resistance of the positive electrode active material.

[0017] Specifically, the W elution rate is calculated as follows. (1) Measurement of the amount of elution into alkaline aqueous solution Immerse 1 g of the positive electrode active material in 10 mL of 0.01 M NaOH aqueous solution at room temperature and stir the solution at 200 rpm for 5 minutes using a magnetic stirrer. Then, separate the solution from the positive electrode active material by suction filtration. ICP analysis of the separated filtrate is performed to measure the amount of tungsten dissolved in the alkaline aqueous solution (NaOH). (2) Calculation of W elution rate (amount eluted in alkaline aqueous solution / total amount) To 1 g of positive electrode active material, add 10 mL of a 1:1:1 mixture of 46% hydrofluoric acid, 60% nitric acid, and 35% hydrochloric acid, heat at 80°C for 2 hours to dissolve, and then perform ICP analysis of the solution to measure the total amount of tungsten. The W elution rate is calculated by dividing the amount of tungsten eluted into the alkaline aqueous solution by the total amount of tungsten.

[0018] [Boron compounds] Examples of boron compounds (hereinafter sometimes referred to as B compounds) include boric acid (H3BO3), metaboric acid (HBO2), tetraboric acid (H2B4O7), etc. The B compound may be boric acid.

[0019] At least a portion of the compound B is present between the primary particles of the lithium-containing transition metal oxide, and the amount of the compound B is 0.006 to 0.17 mass %, and preferably 0.006 to 0.11 mass %, calculated as boron element, relative to the mass of the lithium-containing transition metal oxide.

[0020] The amount of boron dissolved when the positive electrode active material is washed with ion-exchanged water for one minute is 80% or more of the amount of boron detected when the positive electrode active material is dissolved in hydrochloric acid (hereinafter, this value may be referred to as the boron elution rate (B elution rate)). If the B elution rate is within this range, it is assumed that most of the B compounds are present on the surface of the secondary particles of the lithium-containing transition metal oxide. This synergistic effect with the distribution of the W compounds significantly reduces the resistance of the positive electrode active material. The B compounds possess Li-ion conductivity, which promotes the exchange of Li ions between the electrolyte and the positive electrode active material on the secondary particle surface. Furthermore, the presence of B compounds between the primary particles facilitates the transfer of Li ions received on the secondary particle surface to the primary particle surface. Furthermore, the presence of W compounds on the primary particle surface facilitates the smooth transfer of Li ions.

[0021] The B elution rate is more preferably 93% to 98%, and even more preferably 95% to 98%. When the W elution rate is 60% or less, the reaction resistance can be further reduced by setting the B elution rate within the above range.

[0022] Specifically, the B elution rate is calculated as follows. (1) Measurement of the amount of elution into water 1 g of the positive electrode active material was immersed in 30 mL of room temperature ion-exchanged water and stirred at 200 rpm for 1 minute using a magnetic stirrer. The solution and the positive electrode active material were then separated by suction filtration. The separated filtrate was subjected to ICP analysis to measure the amount of boron dissolved in the ion-exchanged water. (2) Calculation of B elution rate (amount eluted into water / total amount) 0.5g of positive electrode active material is dissolved in 10mL of 6mol / L hydrochloric acid by heating at 80℃ for 2 hours, and the solution is subjected to ICP analysis to measure the total amount of boron. The B elution rate is calculated by dividing the amount of boron eluted into the ion-exchanged water by the total amount of boron.

[0023] [Method of manufacturing positive electrode active material] <Cleaning process> First, the lithium-containing transition metal oxide is washed with water and dehydrated to obtain a cake-like composition. The lithium-containing transition metal oxide may be particulate, obtained in the synthesis step. By washing with water, unreacted lithium compounds added in the synthesis step of the lithium-containing transition metal oxide and impurities other than the lithium compounds can be removed. For example, 300 g to 5000 g of lithium-containing transition metal oxide can be added to 1 L of water. The washing can be repeated multiple times. Dehydration after washing can be performed using, for example, a filter press. By dehydration, the water content of the cake-like composition after washing can be reduced to 10% by mass or less. The water content of the cake-like composition is preferably 2% by mass to 10% by mass, more preferably 4% by mass to 8% by mass, from the viewpoint of facilitating the spreading of the tungsten compound or tungsten contained in the tungsten-containing solution added later on the surface of the lithium-containing transition metal oxide. The moisture content of the cake-like composition is calculated by drying 10 g of the cake-like composition by leaving it to stand in a vacuum at 120°C for 2 hours, and dividing the change in weight of the cake-like composition before and after drying by the weight of the cake-like composition before drying. The moisture content of the tungsten additive described below is calculated in the same manner.

[0024] <Tungsten addition process> Next, a tungsten compound or a tungsten-containing solution is added to the cake-like composition to obtain a tungsten additive. Even after washing, a portion of the lithium compound remains in the cake-like composition, and the remaining lithium compound dissolves in the water contained in the cake-like composition on the surface of the lithium-containing transition metal oxide contained in the cake-like composition to produce an alkaline aqueous solution.

[0025] When a tungsten compound is added to the cake-like composition, the tungsten compound dissolves in the alkaline aqueous solution and spreads over the entire surface of the lithium-containing transition metal oxide. Examples of tungsten compounds that can be directly added to the cake-like composition include tungsten oxide (WO), lithium tungstate (LiWO, LiWO, LiWO).

[0026] Alternatively, a tungsten-containing solution may be added to the cake-like composition. The tungsten concentration in the tungsten-containing solution is, for example, 0.05 mol / L or more, preferably 0.1 mol / L to 1 mol / L. The tungsten-containing solution is not particularly limited as long as it contains tungsten, but is preferably a solution in which a tungsten compound that is easily soluble in alkaline solutions, such as tungsten oxide, lithium tungstate, or ammonium tungstate, is dissolved in an aqueous solution of lithium hydroxide. In order to avoid increasing the water content of the tungsten additive, it is preferable to add a tungsten compound to the cake-like composition rather than a tungsten-containing solution.

[0027] Furthermore, the tungsten additive may be dried at 150 to 200°C. By drying, the moisture content of the tungsten additive can be adjusted. The drying atmosphere may be, for example, a vacuum. The drying time is not particularly limited, but may be 0.5 to 10 hours. After drying, the tungsten additive may be cooled in a drying furnace or the like.

[0028] <Boron addition process> Next, boron is added to the tungsten additive. By adding boron to the tungsten additive, the synergistic effect of tungsten and boron can ensure a conductive path for lithium ions, thereby further reducing the resistance of the positive electrode active material. Furthermore, by adhering to and coating the surface of the lithium-containing transition metal oxide, the surface structure of the lithium-containing transition metal oxide can be maintained during discharge and charge.

[0029] Examples of boron compounds that can be added directly to the tungsten additive include boric acid (H3BO3), metaboric acid (HBO2), and tetraboric acid (H2B4O7). The particle size of the boron compound is not particularly limited, but from the viewpoint of dispersibility, it is preferably 100 μm or less, more preferably 50 μm or less, and particularly preferably 10 μm or less. A boron-containing solution can also be added to the tungsten additive. The boron concentration in the boron-containing solution is, for example, 0.05 mol / L to 2 mol / L, preferably 0.1 mol / L to 1 mol / L. The boron-containing solution is not particularly limited as long as it contains boron, but examples include boric acid, metaboric acid, and tetraboric acid. Furthermore, it is preferable to add lithium hydroxide (LiOH), lithium carbonate (Li2CO3), or the like to the boron-containing solution to adjust the pH of the boron-containing solution to 7 or higher. In this case, deterioration of the surface of the tungsten additive due to the acid can be prevented.

[0030] The tungsten additive to which the boron compound has been added may be dried at 150 to 200°C. By drying, the amount of boron present between the secondary particles of the lithium-containing transition metal oxide can be increased. The drying atmosphere can be, for example, a vacuum. The drying time is not particularly limited, but is preferably 0.5 to 10 hours in order to diffuse the boron. The rate of temperature increase up to the predetermined heat treatment temperature is preferably 1°C / min to 10°C / min.

[0031] Next, the tungsten additive to which the boron compound has been added is heat-treated at a temperature higher than 180°C to 330°C to prepare a positive electrode active material. By performing the heat treatment at a temperature higher than 180°C to 330°C, which is higher than the melting point of the boron compound, the boron compound or the boron-containing solution can melt and spread over the entire surface of the lithium-containing transition metal oxide. The heat treatment time is not particularly limited, but is preferably 0.5 to 15 hours to fully melt the boron compound or the boron-containing solution.

[0032] After the heat treatment step, a cooling step can be performed. The cooling step can re-precipitate the boron-containing compound that was melted during the heat treatment. The cooling step is not particularly limited as long as it can lower the temperature of the tungsten additive. For example, rapid cooling can be performed to lower the temperature of the tungsten additive to 100°C or less in 30 minutes after the heat treatment step.

[0033] A non-aqueous electrolyte secondary battery to which the above-described positive electrode active material is applied can be obtained by, for example, housing an electrode assembly, which is formed by stacking or winding electrodes (positive electrode, negative electrode) and a separator, together with a non-aqueous electrolyte in a housing such as a battery can or a laminate. The positive electrode, negative electrode, separator, and non-aqueous electrolyte in this embodiment are, for example, as follows.

[0034] [Positive electrode] The positive electrode includes a positive electrode current collector such as a metal foil and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal such as aluminum that is stable within the potential range of the positive electrode, or a film with such a metal disposed on its surface.

[0035] The positive electrode composite layer preferably contains a positive electrode active material and also contains a conductive material, a binder, etc. The conductive material is preferably 1 mass% or less relative to the mass of the positive electrode active material. The positive electrode can be produced, for example, by applying a positive electrode composite slurry containing the positive electrode active material, the conductive material, the binder, etc., onto a positive electrode current collector, drying the coating, and then rolling to form positive electrode composite layers on both sides of the positive electrode current collector.

[0036] As the conductive material, carbon powder such as carbon black, acetylene black, ketjen black, graphite, and carbon nanotubes may be used alone or in combination of two or more kinds.

[0037] Examples of binders include fluorine-based polymers, rubber-based polymers, etc. For example, fluorine-based polymers include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and modified products thereof, and rubber-based polymers include ethylene-propylene-isoprene copolymers and ethylene-propylene-butadiene copolymers, and these may be used alone or in combination of two or more.

[0038] [Negative electrode] The negative electrode includes a negative electrode current collector such as a metal foil and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode current collector can be a foil of a metal such as copper that is stable within the potential range of the negative electrode, or a film with the metal disposed on its surface. The negative electrode composite layer contains a negative electrode active material, and preferably also contains a thickener, a binder, and the like. The negative electrode can be fabricated, for example, by applying a negative electrode composite slurry, in which a negative electrode active material, a thickener, and a binder are dispersed in water at a predetermined weight ratio, onto the negative electrode current collector, drying the coating, and then rolling the coating to form a negative electrode composite layer on both sides of the negative electrode current collector.

[0039] As the negative electrode active material, a carbon material capable of absorbing and releasing lithium ions can be used, and in addition to graphite, non-graphitizable carbon, graphitizable carbon, fibrous carbon, coke, carbon black, etc. Furthermore, as a non-carbon-based material, silicon, tin, and alloys and oxides mainly containing these can be used.

[0040] As in the case of the positive electrode, PTFE or the like can be used as the binder, but styrene-butadiene copolymer (SBR) or its modified form may also be used. Carboxymethyl cellulose (CMC) or the like can be used as the thickener.

[0041] [Non-aqueous electrolyte] The nonaqueous solvent (organic solvent) for the nonaqueous electrolyte may be a carbonate, lactone, ether, ketone, ester, or the like, or a mixture of two or more of these solvents. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, chain carbonates such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, and mixed solvents of cyclic carbonates and chain carbonates may be used.

[0042] The electrolyte salt for the non-aqueous electrolyte may be LiPF6, LiBF4, LiCF3SO3, or a mixture thereof. The amount of electrolyte salt dissolved in the non-aqueous solvent may be, for example, 0.5 to 2.0 mol / L.

[0043] [Separator] The separator may be a porous sheet or the like having ion permeability and insulating properties. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator may be coated with a material such as an aramid-based resin or ceramic. [Example]

[0044] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0045] [Preparation of positive electrode active material] Example 1 A positive electrode active material was prepared according to the following procedure.

[0046] (1) Preparation of lithium-containing transition metal oxides Ni obtained by coprecipitation 0.91 Co 0.05 Al0.04 A nickel-cobalt-aluminum composite hydroxide represented by (OH)2 was heat-treated at 500°C to obtain an oxide. LiOH and the oxide were mixed in an Ishikawa-type mortar so that the molar ratio of Li to the total transition metals was 1.02:1 to obtain a mixture. The mixture was placed in a firing furnace and heated under an oxygen flow (10 cm) with an oxygen concentration of 95%. 3 The mixture was fired at a temperature increase rate of 2.0°C / min from room temperature to 650°C, and then at a temperature increase rate of 0.5°C / min from 650°C to 710°C under a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture, to produce Li with an average secondary particle size of approximately 11 μm. 1.01 Ni 0.91 Co 0.05 Al 0.04 Particles of lithium nickel cobalt aluminum composite oxide (lithium-containing transition metal oxide) represented by O2 were obtained. The composition of the lithium-containing transition metal oxide particles was measured using an ICP emission spectrometer.

[0047] (2) Cleaning process 800 g of pure water was added to 1000 g of the lithium nickel cobalt aluminum composite oxide particles, and after stirring, the mixture was filtered, separated, and dehydrated to obtain a cake-like composition after a washing step. The water content of the cake-like composition was 4% by mass.

[0048] (3) Tungsten addition process Next, 0.19% by mass of WO powder, calculated as tungsten element relative to the lithium-containing transition metal oxide, was added to the cake-like composition, and the mixture was heated to 180°C in a vacuum and dried for 3 hours, and then cooled to room temperature in a furnace to obtain a tungsten additive with a moisture content of 0.08% by mass.

[0049] (4) Boron addition process To the tungsten additive, boric acid was added in an amount of 0.01% by mass, calculated as boron element, relative to the lithium-containing transition metal oxide, and the mixture was heat-treated in an air atmosphere at 250°C for 3 hours. After the heat treatment, the tungsten additive was cooled to 100°C or less within 30 minutes to obtain the positive electrode active material of Example 1. The W elution rate of the obtained positive electrode active material was 55%, and the B elution rate was 100%.

[0050] <Example 2> A positive electrode active material was produced in the same manner as in Example 1, except that in the boron addition step, after the addition of boric acid but before the heat treatment, the temperature was raised to 180°C in a vacuum and drying was performed for 4 hours. The resulting positive electrode active material had a W elution rate of 53% and a B elution rate of 97%. Because the drying was performed in a vacuum, more boric acid was present between the primary particles than in Example 1, and therefore the B elution rate was lower than in Example 1.

[0051] <Examples 3 to 7> A positive electrode active material was prepared in the same manner as in Example 1, except that the amounts of WO powder and boric acid added were changed as shown in Table 1. The W elution rate and B elution rate were as shown in Table 1.

[0052] Example 8 A positive electrode active material was prepared in the same manner as in Example 1, except for the boron addition step. In the boron addition step, a boron-containing aqueous solution was added to the tungsten additive instead of boric acid. The boron-containing aqueous solution was prepared by adding 0.045 g of boric acid to an aqueous solution in which 0.15 g of lithium hydroxide (LiOH) was dissolved in 10.5 g of pure water, and stirring the solution while heating at 60 °C. The prepared boron-containing aqueous solution was added to the tungsten additive so that the boron content was 0.01 mass% relative to the lithium-containing transition metal oxide. The tungsten additive to which the boron-containing aqueous solution had been added was held in a vacuum at room temperature for 24 hours, then heated to 180 °C and dried for 3 hours, and then furnace-cooled to room temperature to obtain a tungsten additive with a water content of 0.08 mass%. The tungsten additive was then heat-treated in an air atmosphere at 250 °C for 3 hours and cooled to 100 °C or less within 30 minutes to obtain the positive electrode active material of Example 8. The resulting positive electrode active material had a W elution rate of 57% and a B elution rate of 81%. By adding boric acid in the form of an aqueous solution and further holding the material in a vacuum at room temperature, more boric acid was present between the primary particles than in Example 2, and therefore the B elution rate was lower than in Example 2.

[0053] <Comparative Example 1> Except for not carrying out the boron addition step, a positive electrode active material was produced in the same manner as in Example 1. The W elution rate of the obtained positive electrode active material was 60%.

[0054] <Comparative Example 2> A positive electrode active material was produced in the same manner as in Example 1, except that the tungsten addition step was not performed, and instead the temperature was raised to 180°C in a vacuum, drying was performed for 3 hours, and then the boron addition step was performed after furnace cooling to room temperature. The B elution rate of the obtained positive electrode active material was 100%.

[0055] <Comparative Example 3> A positive electrode active material was produced in the same manner as in Example 1, except that the tungsten addition step and the boron addition step were not performed, and instead the temperature was raised to 180°C in a vacuum, dried for 3 hours, and then furnace-cooled to room temperature, thereby obtaining a cake-like composition with a water content of 0.08% by mass.

[0056] <Comparative Examples 4 and 5> A positive electrode active material was prepared in the same manner as in Example 1, except that the amounts of WO powder and boric acid added were changed as shown in Table 1. The W elution rate and B elution rate were as shown in Table 1.

[0057] <Comparative Example 6> A washing step was carried out after the preparation of the lithium-containing transition metal oxide in the same manner as in Example 1. Then, a boron-containing aqueous solution was added to the cake-like composition. The boron-containing aqueous solution was prepared by adding 0.045 g of boric acid to an aqueous solution in which 0.15 g of lithium hydroxide (LiOH) was dissolved in 10.5 g of pure water, and stirring the mixture while heating at 60°C. The prepared boron-containing aqueous solution was added to the cake-like composition so that the boron content was 0.01 mass% in terms of elemental boron relative to the lithium-containing transition metal oxide. The cake-like composition to which the boron-containing aqueous solution had been added was kept in a vacuum at room temperature for 24 hours, then heated to 180°C and dried for 3 hours, and then furnace-cooled to room temperature to obtain a boron additive with a water content of 0.08 mass%. Then, a heat treatment was carried out in an air atmosphere at 250°C for 3 hours.

[0058] Next, a tungsten-containing aqueous solution was sprayed onto the boron additive at a temperature of 200°C or higher. The tungsten-containing aqueous solution was prepared by adding 2.4 g of WO powder to an aqueous solution in which 1 g of lithium hydroxide (LiOH) was dissolved in 20 g of pure water, and stirring the mixture while heating at 60°C. The prepared tungsten-containing aqueous solution was added to the boron additive so that the tungsten content was 0.19 mass% in terms of elemental tungsten relative to the lithium-containing transition metal oxide. The boron additive to which the tungsten-containing aqueous solution had been added was heated to 180°C in a vacuum, dried for 3 hours, and then furnace-cooled to room temperature to obtain a positive electrode active material of Comparative Example 6. The W elution rate of the obtained positive electrode active material was 100%, and the B elution rate was 72%. By adding boric acid in the form of an aqueous solution and further maintaining the mixture at room temperature in a vacuum, more boric acid was present between the primary particles than in Example 2, resulting in a lower B elution rate than in Example 2. In addition, by spraying tungsten onto the boron additive at 200°C or higher, the water evaporated in a short time, and the tungsten compound remained on the surface of the secondary particles, resulting in a higher W elution rate than in Example 1.

[0059] <Comparative Example 7> Similar to Example 1, a washing process was carried out after the preparation of the lithium-containing transition metal oxide. The mixture was then heated to 180°C in a vacuum, dried for 3 hours, and then furnace-cooled to room temperature to obtain a cake-like composition with a moisture content of 0.08% by mass. Boric acid was added to the cake-like composition in an amount of 0.01% by mass, calculated as boron element relative to the lithium-containing transition metal oxide, and heat-treated in an air atmosphere at 250°C for 3 hours to obtain a boron additive. Next, a tungsten-containing aqueous solution was sprayed onto the boron additive at a temperature of 200°C or higher. The tungsten-containing aqueous solution was prepared by adding 2.4 g of WO powder to an aqueous solution in which 1 g of lithium hydroxide (LiOH) was dissolved in 20 g of pure water, and stirring the mixture while heating at 60°C. The prepared tungsten-containing aqueous solution was added to the boron additive so that the amount of tungsten element relative to the lithium-containing transition metal oxide was 0.19% by mass. The boron additive to which the tungsten-containing aqueous solution had been added was heated to 180°C in a vacuum, dried for 3 hours, and then furnace-cooled to room temperature to obtain a positive electrode active material of Comparative Example 7. The W elution rate of the obtained positive electrode active material was 100%, and the B elution rate was 96%. For the same reason as in Comparative Example 6, the W elution rate was higher than in Example 1. The B elution rate was slightly lower than in Example 1 because more tungsten was present on the surface of the secondary particles than in Example 1.

[0060] [Preparation of positive electrode] The positive electrode active materials prepared in Examples 1 to 8 and Comparative Examples 1 to 7, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 40:2:1, and then kneaded using an agate mortar and pestle to form thin pellets. After that, the mixture was rolled to a predetermined thickness using a roller, and then punched into a predetermined circular shape to form a positive electrode.

[0061] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:5:75. Lithium hexafluorophosphate (LiPF6) was dissolved in the mixed solvent to a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0062] [Test cell construction] The positive electrode and the negative electrode made of lithium metal were stacked facing each other with a separator interposed therebetween to prepare an electrode assembly. The electrode assembly and the nonaqueous electrolyte were then inserted into an aluminum coin-shaped outer casing and sealed by crimping with a press to prepare a test cell. Test cells for the other examples and comparative examples were prepared in the same manner.

[0063] [Measurement of reaction resistance] The test cell was subjected to constant current charging at 0.7 mA at 25°C until the cell voltage reached 4.3 V, followed by constant voltage charging at 4.3 V until the current reached 0.07 mA. Subsequently, a constant current discharge was performed at 0.7 mA until the cell voltage reached 2.5 V. Thereafter, again at 25°C, a constant current charge was performed at 0.7 mA until the cell voltage reached 4.3 V, followed by constant voltage charging at 4.3 V until the current reached 0.07 mA. The AC impedance of the test cell was then measured at 20 kHz to 0.01 Hz using an AC impedance meter. A Cole-Cole plot was drawn from the measured data, and the reaction resistance was calculated from the size of the arc between 10 Hz and 0.1 Hz. The reaction resistances shown in Table 1 are expressed relative to the reaction resistance of the test cell containing the positive electrode active material of Comparative Example 1, which is set to 100.

[0064] [Table 1]

[0065] Example 9 By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.88 Co 0.09 Al 0.03 Except for changing to O2, a positive electrode active material was produced in the same manner as in Example 1. The W elution rate of the obtained positive electrode active material was 59%, and the B elution rate was 100%.

[0066] <Comparative Example 8> By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.88 Co 0.09 Al 0.03 Except for changing to O2, a positive electrode active material was produced in the same manner as in Comparative Example 1. The W elution rate of the obtained positive electrode active material was 55%.

[0067] <Comparative Example 9> By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.88 Co 0.09 Al 0.03 A positive electrode active material was produced in the same manner as in Comparative Example 3, except that O2 was used.

[0068] Using the positive electrode active materials obtained in Example 9 and Comparative Examples 8 and 9, test cells were prepared and the reaction resistance was measured in the same manner as in Example 1. The reaction resistances shown in Table 2 are expressed relative to the reaction resistance of the test cell containing the positive electrode active material of Comparative Example 8, which is set to 100.

[0069] [Table 2]

[0070] Example 10 By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.92 Co 0.05 Al 0.03 Except for changing to O2, a positive electrode active material was produced in the same manner as in Example 1. The W elution rate of the obtained positive electrode active material was 58%, and the B elution rate was 100%.

[0071] <Comparative Example 10> By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.92 Co 0.05 Al 0.03 Except for changing to O2, a positive electrode active material was produced in the same manner as in Comparative Example 1. The W elution rate of the obtained positive electrode active material was 57%.

[0072] Using the positive electrode active materials obtained in Example 10 and Comparative Example 10, test cells were prepared and the reaction resistance was measured in the same manner as in Example 1. The reaction resistance shown in Table 3 is a relative expression of the reaction resistance of the test cell containing the positive electrode active material of Example 10, with the reaction resistance of the test cell containing the positive electrode active material of Comparative Example 10 set as 100.

[0073] [Table 3]

[0074] <Comparative Example 11> By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.82 Co 0.15 Al 0.03 Except for changing to O2, a positive electrode active material was produced in the same manner as in Example 1. The W elution rate of the obtained positive electrode active material was 52%, and the B elution rate was 100%.

[0075] <Comparative Example 12> By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.82 Co 0.15 Al 0.03 Except for changing to O2, a positive electrode active material was produced in the same manner as in Comparative Example 1. The W elution rate of the obtained positive electrode active material was 58%.

[0076] <Comparative Example 13> By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.82 Co 0.15 Al 0.03 A positive electrode active material was produced in the same manner as in Comparative Example 3, except that O2 was used.

[0077] Using the positive electrode active materials obtained in Comparative Examples 11 to 13, test cells were prepared and the reaction resistance was measured in the same manner as in Example 1. The reaction resistance shown in Table 4 is expressed relative to the reaction resistance of the test cell containing the positive electrode active material of Comparative Example 11, which is set to 100.

[0078] [Table 4]

[0079] Example 11 By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.92 Co 0.02 Al 0.02 Mn 0.04 Except for changing to O2, a positive electrode active material was produced in the same manner as in Example 1. The W elution rate of the obtained positive electrode active material was 56%, and the B elution rate was 100%.

[0080] <Comparative Example 14> By changing the composition of the nickel-cobalt-aluminum composite hydroxide, the composition of the lithium-containing transition metal oxide can be changed to Li 1.01 Ni 0.92 Co 0.02 Al 0.02 Mn 0.04 Except for changing to O2, a positive electrode active material was produced in the same manner as in Comparative Example 1. The W elution rate of the obtained positive electrode active material was 54%.

[0081] Using the positive electrode active materials obtained in Example 11 and Comparative Example 14, test cells were prepared and the reaction resistance was measured in the same manner as in Example 1. The reaction resistance shown in Table 5 is a relative expression of the reaction resistance of the test cell containing the positive electrode active material of Example 11, with the reaction resistance of the test cell containing the positive electrode active material of Comparative Example 14 set as 100.

[0082] [Table 5]

[0083] In Examples 1 to 11, it was confirmed that by adding not only tungsten but also boron and further setting the W elution rate and B elution rate within a predetermined range, the reaction resistance was lower than that of Comparative Example 1. On the other hand, in Comparative Examples 2 to 7, the reaction resistance was higher than that of Comparative Example 1.

[0084] Furthermore, it was confirmed from Examples 1, 9, 10, and 11 that the effect of adding boron increases as the amount of Co contained in the lithium-containing transition metal oxide decreases. On the other hand, from Comparative Examples 11 and 12, no effect of adding boron was obtained when the amount of Co was 15 mol%.

Claims

1. a lithium-containing transition metal oxide having secondary particles formed by aggregation of primary particles; A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a tungsten compound and a boron compound present at least between the primary particles, the lithium-containing transition metal oxide contains 0 mol % or more and 3 mol % or less of Co with respect to the total moles of metal elements excluding Li contained in the lithium-containing transition metal oxide, the tungsten compound is present in an amount of 0.1 to 0.38 mass% in terms of elemental tungsten relative to the mass of the lithium-containing transition metal oxide; the boron compound is present in an amount of 0.006 to 0.17 mass% in terms of elemental boron relative to the mass of the lithium-containing transition metal oxide; the amount of tungsten element eluted when the positive electrode active material is washed with a 0.01 mol / L aqueous sodium hydroxide solution for 5 minutes is 60% or less of the amount of tungsten element detected when the positive electrode active material is dissolved in a mixed acid containing hydrofluoric acid, nitric acid, and hydrochloric acid, a positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the amount of elemental boron eluted when the positive electrode active material is washed with ion-exchanged water for one minute is 80% or more of the amount of elemental boron detected when the positive electrode active material is dissolved in hydrochloric acid.

2. The composition of the lithium-containing transition metal oxide is represented by the general formula Li α Ni １－ｗ－ｘ－ｙ－ｚ Co ｗ Al ｘ Mn ｙ M ｚ O ２ 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein α is represented by the formula: (wherein 0.95≦α≦1.05, 0≦w≦0.03, 0≦x≦0.06, 0≦y≦0.1, 0≦z≦0.01, and M is at least one element selected from Mg, Sr, Si, Y, Mo, V, Ti, Fe, Zr, and Nb).

3. 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the boron compound contains boric acid.

4. 4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the boron compound is present in an amount of 0.006 to 0.11 mass% in terms of boron element relative to the mass of the lithium-containing transition metal oxide.

5. 5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the amount of boron element eluted when the positive electrode active material is washed with ion-exchanged water for 1 minute is 93% to 98% of the amount of boron element detected when the positive electrode active material is dissolved in hydrochloric acid.

6. The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, a conductive material in an amount of 1% by mass or less relative to the mass of the positive electrode active material;

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