Cathode active material for lithium ion secondary battery, and cathode mixture slurry for lithium ion secondary battery

Abstract

To provide a cathode active material for a lithium ion secondary battery, which further reduces the moisture absorption and carbonation of a surface of a cathode active material particle, which can suppress an increase in viscosity when used to manufacture cathode mixture slurry, and which has a high initial discharge capacity.SOLUTION: A cathode active material for a lithium ion secondary battery contains a coating lithium metal composite oxide that has a lithium metal composite oxide particle including lithium, nickel, manganese and cobalt, and a coating layer for coating at least a part of a surface of the particle. The lithium metal composite oxide particle includes a secondary particle in which a plurality of primary particles clump together. In the secondary particle having a porous structure, a void ratio is more than 15% and 70% or less. The coating layer contains a high polymer compound. The coating lithium metal composite oxide contains chlorine.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery and a positive electrode mixture slurry for a lithium ion secondary battery. [Background technology]

[0002] With growing global awareness of energy and environmental issues, technological development to reduce dependence on fossil fuels and realize a low-carbon society is still underway. Examples of such technological development are extremely diverse, including clean vehicles such as hybrid and electric vehicles, natural energy generation such as solar and wind power, home energy storage devices, and large-scale energy storage facilities for building smart grids and smart communities.

[0003] The key device common to these technologies is the lithium-ion secondary battery. Lithium-ion secondary batteries have excellent performance, including high energy density for system miniaturization, high rate characteristics (input / output characteristics) that enable stable power supply regardless of the usage environment, and good cycle characteristics (durability characteristics) that enable long-term use, and efforts are being made to further improve and refine them.

[0004] Lithium ion secondary batteries are composed of a negative electrode, a positive electrode, an electrolyte, etc., and the active materials for the negative and positive electrodes are materials that can extract and insert lithium. Positive electrode active materials for lithium ion secondary batteries include lithium cobalt composite oxides (e.g., LiCoO2), lithium nickel metal composite oxides (e.g., LiNiO2), and lithium nickel cobalt manganese composite oxides (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), lithium manganese composite oxides (e.g., LiMn2O4), lithium nickel manganese composite oxides (e.g., LiNi 0.5 Mn 0.5 O2), and lithium nickel cobalt aluminum composite oxide (e.g., LiNi 0.75Co 0.15 Al 0.10 Examples of lithium metal composite oxides include lithium-metal composite oxides such as O2.

[0005] Among lithium metal composite oxides, lithium nickel manganese cobalt composite oxide, which contains nickel, manganese, and cobalt in a specific ratio, has attracted attention as a material that has excellent thermal stability, high capacity, good cycle characteristics, and can provide high output with low resistance.

[0006] However, when a lithium metal composite oxide is used as a positive electrode active material, problems such as a deterioration in the weather resistance of the positive electrode active material due to moisture absorption by the positive electrode active material or carbonation of the particle surface, which can lead to a decrease in battery characteristics, and an increase in viscosity when the positive electrode active material is used to produce a positive electrode mixture slurry, can occur. Therefore, for example, the following techniques are known as means for solving these problems.

[0007] Patent Document 1 discloses an aqueous binder composition for secondary battery positive electrodes, which contains a binder having (meth)acrylic acid ester monomer units, vinyl monomer units having an acidic group, and α,β-unsaturated nitrile monomer units, and 0.001 to 1.0 part by mass of an isothiazolinone compound and 0.001 to 1.0 part by mass of a chelate compound per 100 parts by mass of the binder. According to Patent Document 1, the use of the above composition improves the long-term storage stability of the binder composition, and prevents deterioration of the cycle characteristics (particularly high-temperature cycle characteristics) and safety of the secondary battery.

[0008] Patent Document 2 discloses, for example, an agent for preventing viscosity increase or an anti-gelling agent for a slurry for a secondary battery electrode, which contains a chelating agent, and in which the slurry for a secondary battery electrode contains, as a binder, a polymer containing a monomer unit having a phosphate group.

[0009] Patent Document 3 discloses a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: a mixing step of mixing lithium-nickel composite oxide particles with a lithium-free tungsten compound powder to obtain a tungsten mixture containing lithium-nickel composite oxide particles and tungsten compound particles; and a heat treatment step of heat-treating the tungsten mixture to disperse tungsten on the surfaces of primary particles and form lithium-nickel composite oxide particles having tungsten and a compound containing lithium on the surfaces of the primary particles. According to Patent Document 3, the above-mentioned production method is capable of producing a positive electrode active material for a non-aqueous electrolyte secondary battery that can achieve high capacity and high output when used as a battery positive electrode material.

[0010] Patent Document 4 discloses a coated positive electrode active material for a lithium-ion secondary battery, characterized by comprising: lithium metal composite oxide particles containing lithium, nickel, manganese, and cobalt, and primarily composed of secondary particles with a solid structure and a porosity of less than 20%; and a coating layer having an average thickness of 0.005 to 0.1 μm, formed on the surface of the lithium metal composite oxide particles, and made of an ion-conductive polymer containing electronically conductive particles. Patent Document 4 also discloses that the ion-conductive polymer may be one or more selected from polyethylene glycol, polyethylene oxide, and derivatives or salts thereof. Patent Document 4 discloses that the coated positive electrode active material has excellent weather resistance and long-term storage stability, is capable of suppressing gelation during kneading of the positive electrode composite paste, and is capable of maintaining a sufficiently high initial discharge capacity when incorporated into a lithium-ion secondary battery. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2012 / 029839 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-207346 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-134996 [Patent Document 4] Japanese Patent Application Publication No. 2019-140090 [Patent Document 5] Japanese Patent Application Laid-Open No. 2007-273108 Summary of the Invention [Problem to be solved by the invention]

[0012] However, Patent Documents 1 and 2 are technologies related only to viscosity control of the positive electrode mixture slurry, and do not disclose any improvement in the weather resistance of the positive electrode active material. Meanwhile, Patent Document 3 discloses that by mixing a lithium metal composite oxide with a tungsten compound in the presence of water and subjecting the mixture to heat treatment, the lithium hydroxide present on the particle surface of the lithium metal composite oxide, which contributes to moisture absorption and carbonation of the particle surface, reacts with tungsten, thereby reducing the amount of excess lithium, but does not disclose anything about the viscosity of the positive electrode mixture slurry.

[0013] Furthermore, while Patent Document 4 can simultaneously solve the problems of improving weather resistance and controlling the viscosity of the positive electrode composite slurry, when the coated positive electrode active material is kneaded with a conductive material and a binder to produce a positive electrode composite paste, the coating layer, which is made of an ion-conductive polymer such as polyethylene glycol, which has a thickening effect, dissolves in the paste, and in order to minimize the increase in viscosity that can occur, the average thickness of the coating layer must be controlled to be very thin, at 0.005 to 0.1 μm. Furthermore, with the recent demand for smaller and higher-performance lithium-ion secondary batteries, there is a demand for lithium-metal composite oxides as positive electrode active materials for lithium-ion secondary batteries that have higher weather resistance, can reduce the viscosity of the positive electrode composite slurry, and have high battery characteristics.

[0014] However, after calcination, lithium metal composite oxide particles have excess lithium, such as unreacted lithium compounds, on their particle surfaces. In particular, lithium metal composite oxide particles obtained by calcining a nickel-containing composite hydroxide and / or oxide with a lithium compound tend to have a large amount of excess lithium. Therefore, it is known to wash the calcined lithium metal composite oxide particles with water to remove excess lithium from the particle surface (see, for example, Patent Document 5). However, when lithium metal composite oxide is washed with water, some moisture may remain on the surface of the lithium metal composite oxide particles during the water-washing process and subsequent processes, or lithium hydroxide eluted from the particle surface may react with carbon dioxide in the air to form lithium carbonate. Lithium carbonate remaining on the particle surface may cause gas generation when the battery is charged under high-temperature conditions. Furthermore, washing with water not only removes excess lithium, but also removes lithium ions present near the surface (in the crystals) of the lithium metal composite oxide, which may result in a deterioration of battery performance.

[0015] The present invention has been made in view of the above problems, and an object of the present invention is to provide a positive electrode active material for lithium ion secondary batteries, which can further reduce moisture absorption and carbonation on the particle surfaces of the positive electrode active material, can suppress an increase in viscosity when used in the production of a positive electrode composite slurry, and can achieve a high battery capacity, and a positive electrode composite slurry for lithium ion secondary batteries. [Means for solving the problem]

[0016] According to one aspect of the present invention for solving the above problem, A first aspect of the present invention is The present invention relates to a coated lithium metal composite oxide, the coated lithium metal composite oxide comprising lithium metal composite oxide particles containing lithium, nickel, manganese, and cobalt, and a coating layer that coats at least a portion of the surface of the particles. the lithium metal composite oxide particles include secondary particles formed by aggregation of a plurality of primary particles, The secondary particles have a porous structure and a porosity of more than 15% but not more than 70%, the coating layer contains a polymer compound, the coated lithium metal composite oxide contains chlorine; It is a positive electrode active material for lithium-ion secondary batteries.

[0017] A second aspect of the present invention is The present invention relates to a coated lithium metal composite oxide, the coated lithium metal composite oxide comprising lithium metal composite oxide particles containing lithium, nickel, manganese, and cobalt, and a coating layer that coats at least a portion of the surface of the particles. the lithium metal composite oxide particles include secondary particles formed by aggregation of a plurality of primary particles, The secondary particles have a porous structure and a porosity of more than 15% but not more than 70%, the coating layer contains a polymer compound, The amount of lithium carbonate in the coated lithium metal composite oxide is 0.05% by mass or less. It is a positive electrode active material for lithium-ion secondary batteries.

[0018] A third aspect of the present invention is the first or second aspect, The lithium metal composite oxide particles have the general formula: Li a Ni 1-x-y-z Mn x Co y M z O 2+α (wherein 0.95≦a≦1.5, 0.01≦x≦0.5, 0.01≦y≦0.5, 0≦z≦0.2, 0<(1-xyz), and −0.1≦α≦0.2 are satisfied, and M is one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, S, Na, and K).

[0019] A fourth aspect of the present invention is the first or second aspect, The positive electrode active material for a lithium ion secondary battery includes at least one polymer compound selected from the group consisting of ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxyethyl cellulose, carboxymethyl cellulose, carboxymethyl ethyl cellulose, polyvinyl alcohol, and polyvinylpyrrolidone.

[0020] A fifth aspect of the present invention is the first or second aspect, The polymer compound has an average molecular weight of 3,000,000 or less, and is a positive electrode active material for a lithium ion secondary battery.

[0021] A sixth aspect of the present invention is a method for producing a composition comprising the steps of: The positive electrode active material for a lithium ion secondary battery has a chlorine content of 0.1 mass % or less.

[0022] A seventh aspect of the present invention is the first or second aspect, In the positive electrode active material for lithium ion secondary batteries, the content of the polymer compound contained in the coated lithium metal composite oxide is 0.01 mass % or more.

[0023] An eighth aspect of the present invention is The positive electrode active material includes a conductive material, a binder, a polymer compound, and an organic solvent, The positive electrode active material has a porous structure and a porosity of more than 15% and not more than 70%, The positive electrode mixture slurry for a lithium ion secondary battery includes one or more polymer compounds selected from ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxyethyl cellulose, carboxymethyl cellulose, carboxymethyl ethyl cellulose, polyvinyl alcohol, and polyvinylpyrrolidone. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a positive electrode active material that can further reduce moisture absorption and carbonation on the particle surfaces of the positive electrode active material, can suppress an increase in viscosity when used in the production of a positive electrode mixture slurry, and has a high initial discharge capacity when incorporated into a lithium ion secondary battery. Furthermore, the positive electrode mixture slurry of the present invention can reduce an increase in slurry viscosity, allowing for the production of positive electrodes for secondary batteries with high productivity. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a positive electrode active material for a lithium ion secondary battery. [Figure 2] FIG. 2 is an example of a scanning electron microscope (SEM) image of a polymer compound (before mixing). [Figure 3] FIG. 3 is a schematic cross-sectional view of a coin-type battery used to evaluate the battery characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0026] An example of a positive electrode active material for a lithium ion secondary battery according to the present invention will be described in detail below. The present invention is not limited to the following embodiment, and modifications can be made based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

[0027] In the drawings, some components are emphasized or simplified to make each component easier to understand, and the actual structure, shape, scale, etc. may differ. Furthermore, the present embodiment described below does not unduly limit the content of the present invention described in the claims, and not all of the components described in the present embodiment are necessarily essential as the solution of the present invention. Furthermore, in the following description, the expression "A to B" means "A or more and B or less." Furthermore, the phrase "A and / or B" means "one or both of A and B."

[0028] 1. Positive electrode active material for lithium-ion secondary batteries FIG. 1 is a diagram showing an example of a positive electrode active material for a lithium ion secondary battery according to this embodiment (hereinafter also simply referred to as "positive electrode active material"). As shown in FIG. 1, the positive electrode active material contains a coated lithium metal composite oxide 10. The coated lithium metal composite oxide 10 has a lithium metal composite oxide particle 1 that constitutes a core (base material) and a coating layer 2 that coats at least a portion of the particle surface. The positive electrode active material may be composed of the coated lithium metal composite oxide 10, or may contain a lithium metal composite oxide other than the coated lithium metal composite oxide 10.

[0029] The lithium metal composite oxide particle 1 includes secondary particles 1b formed by aggregation of multiple primary particles 1a. The secondary particles 1b of the lithium metal composite oxide particle 1 have a porous structure. Here, the porous structure refers to a structure having two or more pores inside the secondary particles 1b, as shown in the schematic cross-sectional view of FIG. 1, and may also be a structure in which multiple voids 1c are dispersed throughout the secondary particles 1b. The porosity in the cross section of the secondary particles 1b is more than 15% and not more than 70%.

[0030] The coating layer 2 is formed on at least a portion of the surface of the secondary particles 1b. The coating layer 2 contains a polymer compound. By having the coating layer 2, the coated lithium metal composite oxide 10 can impart excellent weather resistance and long-term storage stability to the positive electrode active material, and can dissolve in the slurry during the production of the positive electrode mixture slurry, thereby suppressing an increase in viscosity. Furthermore, when the coated lithium metal composite oxide 10 is incorporated into a lithium ion secondary battery as a positive electrode active material, the battery can have a high initial discharge capacity. Each component of the coated lithium metal composite oxide 10 will be described below.

[0031] [Coating layer] The coating layer 2 is formed on at least a portion of the surface of the secondary particle 1b of the lithium metal composite oxide particle 1. The coating layer 2 also contains a polymer compound. The presence of the coating layer 2 in the coated lithium metal composite oxide 10 reduces the effects of moisture, carbon dioxide, and the like when the coated lithium metal composite oxide 10 is stored or handled in an atmospheric environment, while the polymer compound in the coating layer dissolves and suppresses an increase in viscosity during the production of a positive electrode composite slurry. Conventional coating layers function as protective layers that cover the lithium metal composite oxide particles even after the production of the positive electrode composite slurry, and increasing the amount of coating can increase the resistance of the particle surface and lead to a decrease in battery performance. However, in the case of the coating layer 2, the polymer compound can dissolve in the positive electrode composite slurry, resulting in a high battery capacity when used as a positive electrode active material for a secondary battery.

[0032] It has been known that excess lithium (lithium hydroxide, lithium carbonate, etc.) and moisture remaining in a positive electrode active material cause problems such as increased viscosity of the positive electrode composite slurry and gas generation at high temperatures. While it is known that excess lithium can be reduced, for example, by washing a lithium metal composite oxide with water, there has been a need for a more stable method for suppressing the increase in viscosity of the positive electrode composite slurry and suppressing moisture absorption and carbonation on the surface of the positive electrode active material. Through extensive research, the present inventors have newly discovered that even if excess lithium is reduced by washing with water, the viscosity increase may occur due to moisture absorption and carbonation, and that not only the effects of excess lithium and moisture but also the presence of trace amounts of chlorine (chloride) further accelerates the viscosity increase. Furthermore, the present inventors have discovered that a coating layer containing a specific polymer compound functions very effectively as a means of solving the above problems, and have completed the present invention.

[0033] The reason why the inclusion of a polymer compound in the coating layer 2 suppresses gelation of the positive electrode mixture slurry is not clear, but is presumed to be as follows: The viscosity increases due to the formation of aggregates in the slurry due to the influence of excess lithium, moisture, and chlorine, and it is thought that chlorine in particular is involved in the formation of these aggregates, causing the viscosity increase to be more pronounced. Here, when the coated lithium metal composite oxide 10 is used as the positive electrode active material, it is thought that the dissolution of the polymer compound contained in the coating layer 2 in the positive electrode mixture slurry destroys the aggregates and also suppresses the formation of new aggregates.

[0034] (Type of polymer compound) The polymer compound contained in the coating layer 2 can be an organic polymer compound, such as a cellulose-based polymer compound and / or a vinyl-based polymer compound. Among these, it is preferable to include one or more selected from ethyl cellulose (EC), methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxymethyl cellulose (HMC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), carboxyethyl cellulose (CEC), carboxymethyl cellulose (CMC), carboxymethyl ethyl cellulose (CEMC), polyvinyl alcohol (PVA, PVAL), and polyvinylpyrrolidone (PVP). The polymer compound preferably includes hydroxyalkyl cellulose or carboxyl alkyl cellulose, and among these, hydroxyalkyl cellulose is preferred. Among hydroxyalkyl celluloses, hydroxyethyl cellulose (HEC) is preferred. The polymer compounds may be used alone or in combination of two or more. When the organic polymer compound described above is used as the coating layer 2, even if the coating amount is increased, gelation is not induced when the polymer compound dissolves, and gelation of the positive electrode mixture slurry can be more effectively suppressed.

[0035] (Particle shape of polymer compound) The method for forming the coating layer 2 is not particularly limited as long as a coating layer having the above-mentioned properties can be obtained, but for example, the coating layer can be formed by mixing particles of a polymer compound (e.g., FIG. 2) with particles of lithium metal composite oxide (containing water) after calcination and water washing to obtain a water-containing mixture, and then heat-treating the water-containing mixture. The shape of the particles of the polymer compound used as a raw material is not particularly limited, and may be, for example, spherical particles and / or fibrous particles, as shown in the scanning electron microscope (SEM) image in FIG.

[0036] Furthermore, the polymer compound particles may be difficult to dissolve in water at room temperature, but may be soluble in water by heat treatment (e.g., at 60 to 120°C for 1 to 15 hours). Note that the water contained in the aqueous mixture becomes alkaline when excess lithium dissolves in the water, so the polymer compound particles may be soluble in alkaline conditions.

[0037] (Weight-average molecular weight of polymer compound) The weight-average molecular weight (Mw) of the polymer compound may be 3,000,000 or less, preferably 1,000,000 or less, and may be 800,000 or less, or 500,000 or less. The lower limit of the weight-average molecular weight (Mw) of the high molecular weight compound may be, for example, 2,000 or more, 5,000 or more, or 10,000 or more. When the weight-average molecular weight (Mw) is within the above range, a high battery capacity can be obtained while further reducing the increase in viscosity of the positive electrode composite slurry. When the weight-average molecular weight (Mw) of the polymer compound is too high, the polymer compound may not be sufficiently dissolved into the slurry during the preparation of the positive electrode slurry, and the above-mentioned effects may not be achieved. The weight-average molecular weight of the polymer compound can be appropriately selected depending on the type and physical properties of the polymer compound used. The weight-average molecular weight (Mw) of the polymer compound can be measured, for example, by gel permeation chromatography (GPC) and calculated using a standard polymethyl methacrylate calibration curve.

[0038] (Polymer compound content) The lower limit of the polymer compound content may be 0.005% by mass or more, and preferably 0.01% by mass or more, relative to the positive electrode active material. The lower limit of the polymer compound content may be, for example, 0.05% by mass or more, or 0.1% by mass or more. The upper limit of the polymer compound content may be, for example, 3% by mass or less, 2% by mass or less, or 1% by mass or less.

[0039] Furthermore, the coating layer 2 preferably contains the above-mentioned polymer compound as a main component, and for example, the content ratio of the above-mentioned polymer compound relative to the total amount of the coating layer may be 50% by mass or more, 70% by mass or more, or 90% by mass or more. The coating layer 2 may be composed of the above-mentioned polymer compound. The coating layer 2 may also contain elements or compounds other than the above-mentioned polymer compound, as long as the effects of the present invention are not impaired. The coating layer 2 may not contain electronically conductive particles such as carbon particles.

[0040] The coating layer 2 is formed on the surface of the primary particles 1a and / or the surface of the secondary particles 1b of the lithium metal composite oxide, and on a part of or the entire surface thereof, and may also be formed on the areas exposed to the voids 1c, and its form may be a continuous layer, a discontinuous layer, or an aggregate of fine particles.

[0041] [Lithium metal composite oxide particles] (particle morphology, particle structure) The lithium metal composite oxide particle 1 includes secondary particles 1b formed by aggregation of multiple primary particles 1a. Alternatively, the lithium metal composite oxide particle 1 may include individual primary particles 1a that are not aggregated as secondary particles 1b. The shapes of the primary particles 1a constituting the secondary particles 1b and the individual primary particles 1a are not particularly limited and may take various shapes, such as spherical, plate-like, needle-like, rectangular, elliptical, and rhombohedral. The aggregation form of the multiple primary particles 1a is also not particularly limited as long as the above-mentioned porosity is satisfied. In addition to a form in which the primary particles 1a aggregate in random directions, various forms are possible, such as a form in which the primary particles aggregate uniformly and radially from the center to form substantially spherical or ellipsoidal secondary particles. The average particle size (longest diameter) of the primary particles 1a may be, for example, 0.01 μm to 1 μm, or 0.05 μm to 0.5 μm.

[0042] As described above, the lithium metal composite oxide particle 1 has a porous structure inside. The porosity is more than 15% and not more than 70%, and preferably more than 15% and not more than 60%. The lower limit of the porosity may be more than 15%, may be 16% or more, or may be 17% or more. The upper limit of the porosity may be 55% or less. This porosity can be determined, for example, by the following method.

[0043] After embedding the coated lithium metal composite oxide 10 particles in resin, they are cut using a cross-section polisher (CP) and argon sputtering to expose the cross-section of the particles, and the cross-section of the exposed particles is imaged using a scanning electron microscope (SEM) or the like. The cross-sectional image of the particles obtained is then analyzed using image analysis software to identify void areas as black regions and dense areas as white regions, and the porosity can be determined for the cross-sections of any 20 or more particles by calculating "area of ​​black regions / (area of ​​black regions + area of ​​white regions) × 100."

[0044] Thus, when used as a positive electrode active material, coated lithium metal composite oxide particles having an internal porous structure can maintain particle strength within an acceptable range without excessively reducing bulk density, and can ensure a sufficient contact area between the positive electrode active material and the electrolyte. Furthermore, in the coated lithium metal composite oxide described above, the shape of the primary particles, the morphology of the secondary particles, and the internal structure of the secondary particles can be determined, for example, by observing the cross-section of the particles using a scanning electron microscope. The method for producing lithium metal composite oxide particles 1 having a porous structure is not particularly limited, and known methods can be used. For example, lithium metal composite oxide particles may be produced by mixing a metal hydroxide (precursor) containing a transition metal (Ni, Mn, Co) obtained by a crystallization reaction with a lithium compound and calcining the mixture. Alternatively, the precursor may be heat-treated to convert at least a portion of the hydroxide into an oxide, and then mixed with the lithium compound. The porosity of the lithium metal composite oxide particles 1 can be controlled, for example, by adjusting the crystallization conditions of the precursor.

[0045] The positive electrode active material may contain other lithium metal composite oxides in addition to the coated lithium metal composite oxide 10 described above. For example, lithium metal composite oxides having a solid structure and lithium metal composite oxides having a hollow structure may be mixed. Furthermore, these lithium metal composite oxide particles may have a coating layer 2 containing a polymer compound. The mixing ratio of such lithium metal composite oxide particles having a porous structure, a solid structure, and a hollow structure can be controlled, for example, by appropriately adjusting the crystallization conditions of the metal composite hydroxide that serves as a source of transition metals such as nickel, manganese, and cobalt. The coated lithium metal composite oxide obtained in this manner has the advantage of reducing variation in composition and particle size compared to those produced by simply mixing porous, solid, and hollow particles.

[0046] (Volume average particle size MV) The volume average particle size MV of the coated lithium metal composite oxide 10 is preferably 2 μm or more and 20 μm or less, more preferably 3 μm or more and 19 μm or less, and particularly preferably 4 μm or more and 18 μm or less. When the volume average particle size MV of the coated lithium metal composite oxide 10 is within the above range, a secondary battery incorporating this in the positive electrode can have a large battery capacity per volume, improved safety, and good cycle characteristics.

[0047] If the volume average particle diameter MV of the coated lithium metal composite oxide 10 is less than 2 μm, the particle packing density may decrease when a positive electrode is fabricated, resulting in a decrease in the battery capacity per volume of the positive electrode. On the other hand, if the volume average particle diameter MV of the coated lithium metal composite oxide 10 exceeds 20 μm, the specific surface area of ​​the positive electrode active material may decrease, reducing the interface with the electrolyte of the secondary battery. As a result, the resistance of the positive electrode may increase and the output characteristics of the battery may deteriorate. The volume average particle diameter MV can be determined from the integrated volume measured using a laser diffraction / scattering method.

[0048] Furthermore, when a metal composite hydroxide (precursor) obtained by a crystallization reaction is used as a raw material, the volume average particle size MV of the coated lithium metal composite oxide can be controlled by the supply amount and pH of the raw material solution (metal compound) as well as the time of the crystallization step in the production process of the metal composite hydroxide.

[0049] (composition) The lithium metal composite oxide particles 1 contain lithium, nickel, manganese, and cobalt. The lithium metal composite oxide particles 1 may also optionally contain one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, S, Na, and K (hereinafter simply referred to as "element M" or "M"). For example, the lithium metal composite oxide particles 1 may have a ratio of the amounts of substances of the above elements expressed as Li:Ni:Mn:Co:M=a:(1-xyz):x:y:z (where 0.95≦a≦1.5, 0.01≦x≦0.5, 0.01≦y≦0.5, 0≦z≦0.2, and 0<(1-xyz)).

[0050] The composition of the lithium metal composite oxide particles 1 is, for example, a general formula: Li a Ni 1-x-y-z Mn x Co y M z O 2+α (wherein 0.95≦a≦1.5, 0.01≦x≦0.5, 0.01≦y≦0.5, 0≦z≦0.2, 0<(1-xyz), and −0.1≦α≦0.2 are satisfied, and M is one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, S, Na, and K).

[0051] In the above general formula, a, which indicates the lithium (Li) content, is preferably in the range of 0.95 to 1.5, and may be 0.95 to 1.3.

[0052] In the above general formula, 1-xyz, which indicates the nickel (Ni) content, may be greater than 0 and may be 0.3 or greater, and is preferably in the range of 0.3 to 0.98. When the nickel content is within the above preferred range, when used as a positive electrode active material, it is possible to achieve a high potential and a high capacity of a lithium ion secondary battery. When 1-xyz is less than 0.3, the above-mentioned high potential and high capacity may not be sufficiently achieved. On the other hand, when 1-xyz exceeds 0.98, the molar ratio of manganese (Mn) to cobalt (Co) decreases, and the effect may not be fully obtained. The upper limit of 1-xyz may be, for example, 0.9 or less, 0.8 or less, or 0.7 or less.

[0053] In the above general formula, x, which represents the manganese content, is preferably within the range of 0.01 to 0.5. When the manganese content is within the above range, the durability of the lithium ion secondary battery can be improved when used as a positive electrode active material. If x is less than 0.01, the effect of improving the durability may not be sufficiently obtained. If x exceeds 0.5, the amount of metal elements that contribute to the oxidation-reduction reaction (redox reaction) decreases, which may result in a decrease in battery capacity.

[0054] In the above general formula, y, which represents the cobalt content, is preferably in the range of 0.01 to 0.5. When the cobalt content is in the above range, when used as a positive electrode active material, the cycle characteristics of a lithium-ion secondary battery can be improved and the expansion and contraction behavior of the crystal lattice due to the insertion and extraction of lithium during charge and discharge can be reduced. If y is less than 0.01, the effect of reducing the expansion and contraction behavior of the crystal lattice may not be obtained. If y exceeds 0.5, the amount of cobalt added is too large, resulting in a significant decrease in initial discharge capacity and may be disadvantageous in terms of cost.

[0055] In the above general formula, the element M represents one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, S, Na, and K. The element M may be one element or multiple elements. Furthermore, by using an element M such as magnesium (Mg) or aluminum (Al), it is possible to further improve battery characteristics such as cycle characteristics and output characteristics.

[0056] In the above general formula, z, which indicates the content of element M, is preferably 0 to 0.2, and may be 0 to 0.1. If z, which indicates the content of M, exceeds 0.2, the metal elements contributing to the oxidation-reduction reaction (redox reaction) decrease, which may result in a decrease in battery capacity. The element M, which is optionally added, may be added either during the production of the metal composite hydroxide (precursor) or during the production of the lithium calcined product (base material). The lithium metal composite oxide particles 1 may or may not contain the element M. The element M may be distributed inside the lithium metal composite oxide particles 1 or on the surface.

[0057] (chlorine content) The chlorine (Cl) content of the coated lithium metal composite oxide 10 is preferably 0.1% by mass or less, and may be 0.05% by mass or less, or may be 0.04% by mass or less. The coated lithium metal composite oxide 10 may also be substantially chlorine-free (e.g., a chlorine content of less than 0.001% by mass). When the chlorine content is within the above range, the diffusion of lithium in the solid phase of the positive electrode active material is not hindered, and when incorporated into a lithium-ion secondary battery, a sufficiently high initial discharge capacity can be maintained. On the other hand, when the chlorine content exceeds 0.1% by mass, a sufficient initial discharge capacity may not be obtained, and the viscosity may increase significantly when used to produce a positive electrode mixture slurry. When the coated lithium metal composite oxide 10 is produced using a chloride (a compound containing chlorine) as a raw material, the lower limit of the chlorine content of the coated lithium metal composite oxide 10 is not particularly limited, but may be, for example, 0.005% by mass or more, or 0.01% by mass or more.

[0058] The chlorine content can be evaluated by directly analyzing the coated lithium metal composite oxide 10 by X-ray fluorescence analysis (XRF) or by separating the chloride contained in the coated lithium metal composite oxide 10 in the form of silver chloride by distillation and analyzing it by X-ray fluorescence analysis (XRF).

[0059] (Lithium hydroxide content) The lithium hydroxide content of the coated lithium metal composite oxide 10 is preferably 0.05% by mass or less. When the lithium hydroxide content is within the above range, excellent weather resistance can be imparted to the positive electrode active material. On the other hand, when the lithium hydroxide content exceeds 0.05% by mass, the lithium hydroxide is likely to be carbonated to form lithium carbonate, which may deteriorate the weather resistance and prevent a sufficient initial discharge capacity from being obtained. The lower limit of the lithium hydroxide content is not particularly limited, but may be, for example, 0.005% by mass or more, or 0.01% by mass or more.

[0060] (Lithium carbonate content) The lithium carbonate content of the coated lithium metal composite oxide 10 is preferably 0.05% by mass or less, and may be 0.02% by mass or less. When the lithium carbonate content is within the above range, excellent weather resistance can be imparted to the positive electrode active material, and when incorporated into a lithium ion secondary battery, a sufficiently high initial discharge capacity can be maintained. On the other hand, if the lithium carbonate content exceeds 0.05% by mass, a sufficient initial discharge capacity may not be obtained. The lower limit of the lithium carbonate content is not particularly limited, but may be, for example, 0.005% by mass or more, or 0.01% by mass or more.

[0061] (Method for measuring lithium hydroxide content and lithium carbonate content) The excess lithium present on the surface of the positive electrode active material, lithium hydroxide and lithium carbonate, dissolve in water and form hydroxide ions (OH - ), carbonate ions (CO3 2- ) in the form of lithium ion (Li + ) ionizes. Therefore, lithium hydroxide and lithium carbonate can be separately quantified by titrating these ionized anions with an inorganic acid (e.g., hydrochloric acid). In the neutralization titration, the first endpoint (pH: approximately 8.3) is the pH change when all of the lithium hydroxide and half of the lithium carbonate react to form lithium bicarbonate, and the second endpoint (pH: approximately 3.8) is the pH change when the remaining half of the lithium carbonate reacts. Specifically, the lithium hydroxide content and lithium carbonate content can be measured by detecting the alkali content eluted when the coated lithium metal composite oxide 10 (positive electrode active material) is dispersed in water using the neutralization titration method (RB Warder method). For example, 20 g of powder of the coated lithium metal composite oxide 10 (positive electrode active material) is added with ultrapure water up to 100 ml, stirred for 1 minute, and then titrated with 1 mol / L hydrochloric acid until the second titration point is reached. The amount of lithium hydroxide and the amount of lithium carbonate eluted are calculated from the amount of hydrochloric acid required until the end of each reaction.

[0062] (Other element content) The coated lithium metal composite oxide 10 (as a whole) may contain sodium (Na) (for example, when the element M contains Na, or when Na is contained as an impurity). The sodium content of the coated lithium metal composite oxide 10 may be, for example, 0.46% by mass or less, less than 0.2% by mass, 0.1% by mass or less, or 0.05% by mass or less. The lower limit of the sodium amount may be, for example, 0% by mass or more, or 0.005% by mass or more. Furthermore, when a compound containing, for example, chlorine or nitrogen is not used as a raw material, the coated lithium metal composite oxide 10 (as a whole) may not contain these elements (Cl, N). Furthermore, when a compound containing one or more elements of the element M is not used as a raw material, the coated lithium metal composite oxide 10 (as a whole) may not contain this compound. For example, when potassium hydroxide is used as a neutralizing agent, the coated lithium metal composite oxide 10 may not contain sodium.

[0063] (moisture content) The moisture content of the coated lithium metal composite oxide 10 is preferably 0.1% by mass or less. The moisture content is an important indicator of weather resistance, and a high moisture content has a negative effect on the charge / discharge reaction of the secondary battery. Therefore, it is preferable to remove as much moisture as possible to reduce the moisture content. The moisture content can be evaluated by analysis at 300°C using Karl Fischer titration.

[0064] [Viscosity of positive electrode mixture slurry] The viscosity of the positive electrode mixture slurry produced using the coated lithium metal composite oxide 10 (positive electrode active material) is preferably 6000 mPa·s or less, more preferably 5000 mPa·s or less. This makes the positive electrode mixture slurry easy to handle and allows the initial discharge capacity to be maintained sufficiently high when incorporated into a lithium ion secondary battery. The viscosity of the positive electrode mixture slurry may be 4000 Pa·s or less, or may be 3000 Pa·s or less. The positive electrode mixture slurry can be produced, for example, by the method described in the Examples below. The viscosity of the positive electrode mixture slurry can be measured, for example, 90 days after the positive electrode active material is produced and stored in the air. The viscosity of the positive electrode mixture slurry produced using the coated lithium metal composite oxide 10 can maintain the above viscosity, for example, even 90 days after the positive electrode active material is produced and stored in the air. [Example]

[0065] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to these examples. The methods for analyzing elements contained in the positive electrode active material in the examples and comparative examples and the various methods for evaluating the positive electrode active material are as follows.

[0066] [Analysis and evaluation method] (particle structure) The particle structure of the secondary particles was confirmed using a scanning electron microscope (SEM).

[0067] (porosity) The porosity of secondary particles was evaluated as follows. The positive electrode active material (secondary particles) was embedded in resin or other materials, and a cross-section polisher IB-19530CP (manufactured by JEOL Ltd.) was used to prepare samples suitable for cross-sectional observation of the secondary particles. The cross-sections of the secondary particles were then observed using a Schottky field emission scanning electron microscope (SEM) JSM-7001F (manufactured by JEOL Ltd.). Specifically, the SEM images were analyzed using WinRoof 6.1.1 (manufactured by Mitani Corporation), an image analysis and measurement software. The void areas of the cross-sections of the secondary particles were designated as black areas, and the dense areas (presence areas) of the secondary particles were designated as white areas. For each of 20 randomly selected particles, the ratio of the area of ​​the black area to the area of ​​the white area was calculated, and the porosity was calculated by averaging the calculated values.

[0068] (Volume average particle size MV) The volume average particle size (MV) of secondary particles was determined from the volume distribution measured by the laser diffraction / scattering method using a Microtrac MT3300EXII (Microtrac Bell Co., Ltd.).

[0069] (composition) The composition was evaluated by acid decomposition-ICP (inductively coupled plasma) atomic emission spectrometry using a multi-type ICP atomic emission spectrometer, ICPE-9000 (Shimadzu Corporation). From the obtained composition of each element, Li / Me (the ratio of the number of lithium (Li) atoms to the sum of the numbers of metal elements (Ni, Co, Mn) and element M contained in the coated lithium metal composite oxide (positive electrode active material)) can be determined.

[0070] (chlorine content) The chlorine content was evaluated by X-ray fluorescence analysis (XRF) using an X-ray fluorescence analyzer, Axios (manufactured by Spectris Inc.), either directly on the sample or after separating the chloride contained in the sample in the form of silver chloride by distillation.

[0071] (Lithium hydroxide content and lithium carbonate content) 20 g of the positive electrode active material was added to 100 ml of ultrapure water, stirred for 1 minute, and the excess lithium (lithium hydroxide and lithium carbonate) dissolved during dispersion was measured by neutralization titration (sequential titration using the R.B. Warder method) with 1 mol / L hydrochloric acid. The measurement device was an automatic titrator COM-1750 (manufactured by Hiranuma Sangyo Co., Ltd.), and the endpoint (potential difference) was determined using a pH composite electrode.

[0072] (moisture content) The water content was evaluated by analysis at 300°C by Karl Fischer titration using a coulometric Karl Fischer moisture meter MKC-710D (manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

[0073] (Viscosity of positive electrode mixture slurry) A positive electrode mixture slurry for viscosity evaluation was prepared by mixing 20 g of positive electrode active material, 2.2 g of acetylene black (conductive material), 2.2 g of polyvinylidene fluoride (KF polymer #1120, binder), and N-methyl-2-pyrrolidone (organic solvent) using a planetary kneader. The positive electrode active material for viscosity evaluation was stored in air for 90 days after production. N-methyl-2-pyrrolidone (organic solvent) was mixed in an amount such that the solid content (g) / organic solvent (g) ratio was 1.875. The solid content refers to the lithium metal composite oxide (excluding the coating layer containing the polymer compound) and the conductive material in the positive electrode active material. The binder and the coating layer containing the polymer compound are dissolved in the organic solvent. Note that the above ratio "1.875" is an example of a solid-liquid ratio for a positive electrode mixture slurry that can produce a positive electrode that provides good performance in lithium-ion secondary batteries.

[0074] More than 10 g of the positive electrode composite slurry for viscosity evaluation was placed in a designated container and the fluid temperature was adjusted to 20°C in a water bath to measure the viscosity. A vibration viscometer, Viscomate VM-100A (manufactured by Sekonic Corporation), was used to evaluate the viscosity of the positive electrode composite slurry. Two-point calibration of the vibration viscometer was performed using JS-200 (viscosity at 20°C: 170 mPa·s) and JS-2000 (viscosity at 20°C: 1800 mPa·s) (manufactured by Nippon Grease Co., Ltd.), which are standard fluids for calibration of viscometers specified in JIS-Z-8809.

[0075] (Method for evaluating battery characteristics) [Initial discharge capacity] The initial discharge capacity was evaluated by leaving the evaluation battery (coin-type battery CBA) shown in Figure 3 for about 24 hours after fabrication, and after the open circuit voltage OCV (open circuit voltage) had stabilized, applying a current density of 0.1 mA / cm to the positive electrode. 2 The initial charge capacity was calculated by charging the battery to a cutoff voltage of 4.3 V, and after a one-hour rest, the battery was discharged to a cutoff voltage of 3.0 V. The measurement was performed using a multi-channel voltage / current generator, R6741A (manufactured by Advantest Corporation).

[0076] [Method for producing evaluation battery (coin-type battery CBA)] The coin-type battery CBA for evaluation was fabricated by the following method.

[0077] 52.5 mg of positive electrode active material, 15 mg of conductive material (acetylene black), and 7.5 mg of binder (polytetrafluoroethylene) were weighed and mixed, and then press-molded at 100 MPa to a diameter of 11 mm and a thickness of 100 μm to prepare a positive electrode (PE) (electrode for evaluation). The prepared positive electrode (PE) was dried in a vacuum dryer at 120°C for 12 hours. A 2032-type coin-type battery (CBA) was then fabricated using this positive electrode (PE) in a glove box with an argon gas atmosphere and a dew point controlled at -80°C. The negative electrode (NE) was a 17 mm diameter, 1 mm thick lithium metal. The electrolyte was a mixture of equal parts ethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M lithium perchlorate (LiClO4) as the supporting electrolyte (manufactured by Toyama Pharmaceutical Co., Ltd.). The separator (SE) was a 25 μm thick polyethylene porous membrane. The coin-type battery CBA was assembled by arranging the gasket GA and wave washer WW, and then assembling the positive electrode can PC and negative electrode can NC into a coin-type battery.

[0078] In the following examples and comparative examples, unless otherwise specified, reagents manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. were used. Furthermore, the present invention is not limited in any way by the following examples and comparative examples.

[0079] Example 1 [Crystallization process] First, 14 L of water was placed in a 60 L reaction tank, and the temperature inside the tank was set to 40°C while stirring. Nitrogen gas was introduced into the tank, and the gas phase inside the tank was controlled to a non-oxidizing atmosphere with an oxygen concentration of 0.1% by volume.

[0080] Next, an appropriate amount of alkaline solution and ammonium ion donor were added to the water in the tank to prepare a reaction solution so that the pH (based on a liquid temperature of 25°C, the same applies below) was 12.8 and the ammonium ion concentration was 10 g / L.

[0081] On the other hand, nickel sulfate hexahydrate, manganese sulfate monohydrate, and cobalt chloride hexahydrate were weighed out so that the composition ratio of nickel, manganese, and cobalt was Ni:Mn:Co=35:30:35, and dissolved in water so that the total concentration of nickel, manganese, and cobalt was 2 mol / L to prepare a raw material solution.

[0082] Then, the raw material solution was added to the reaction solution in the tank at a rate of 100 ml / min, and at the same time, an alkaline solution and an ammonium ion donor were also added to the reaction solution at a constant rate.Nucleation was carried out by carrying out crystallization for 1 minute while maintaining the pH of the reaction solution at 12.8 (pH for the nucleation step) and the ammonium ion concentration at 10 g / L.

[0083] After temporarily stopping the supply of the raw material solution, alkaline solution, and ammonium ion donor, sulfuric acid was added until the pH of the reaction solution reached 11.6 (particle growth step pH). After reaching 11.6, the supply of the raw material solution, alkaline solution, and ammonium ion donor was resumed. Crystallization A (in a non-oxidizing atmosphere) was continued for 1.2 hours while maintaining the pH at 11.6 and the ammonium ion concentration at 10 g / L, respectively, to perform particle growth. The supply of the raw material solution, alkaline solution, and ammonium ion donor was then stopped again, and the gas phase in the reaction vessel was switched to an oxidizing atmosphere (air atmosphere with an oxygen concentration of 21% by volume). After confirming that the oxygen concentration reached 21% by volume, the supply of the raw material solution, alkaline solution, and ammonium ion donor was resumed. Crystallization B (in an oxidizing atmosphere) was then performed under the same conditions as Crystallization A for 0.2 hours, except that the atmosphere was air. Then, the above-mentioned switching between Crystallization A and Crystallization B was carried out three times (a total of four switching times, i.e., non-oxidizing → oxidizing → non-oxidizing → oxidizing → non-oxidizing), and the crystallization was continued for four hours, resulting in the formation of Ni, a porous nickel-manganese-cobalt composite hydroxide (metal composite hydroxide). 0.35 Mn 0.30 Co 0.35 A slurry containing (OH)2 was obtained.

[0084] [Cleaning process] The obtained slurry containing the metal composite hydroxide (before washing) was charged into a filter press and filtered under pressure to recover the metal composite hydroxide (before washing). This metal composite hydroxide (before washing) was returned to the reaction vessel, and the vessel was filled with an alkaline washing solution (a 5% by mass aqueous solution of sodium hydroxide) and stirred for 30 minutes for alkali washing. After that, pressure filtration was again performed using the filter press to recover the alkali-washed cake. This alkali-washed cake was returned to the reaction vessel, and the vessel was filled with water and stirred for 30 minutes for finish washing. After that, pressure filtration was again performed using the filter press to recover the washed cake (water-washed cake).

[0085] [Drying process] The collected washed cake was dried at 150° C. for 5 hours using an electric heating dryer, thereby obtaining a metal composite hydroxide (precursor).

[0086] [Oxidation roasting process] This metal composite hydroxide (precursor) was calcined by heating it in an air stream (oxygen concentration: 21% by volume) at 450°C for 2 hours using an electric furnace, and then oxidized and roasted by heating it in an air stream at 600°C for 5 hours to produce a porous nickel-manganese-cobalt composite oxide (metal composite oxide), Ni 0.35 Mn 0.30 Co 0.35 I got O.

[0087] [Lithium mixing process] The obtained metal composite oxide (intermediate) and lithium hydroxide, a lithium compound, were weighed out so that the ratio (Li / Me), which is the ratio of the number of lithium atoms (Li) to the number of atoms of metals other than lithium (Me), was 1.05, and the mixture was thoroughly mixed to obtain a lithium mixture.

[0088] [Firing process] This lithium mixture was calcined by heating it at 450°C for 10 hours in an oxygen (oxygen concentration: 100% by volume) flow, and then calcined by heating it at 860°C for 5 hours in an oxygen flow, to produce lithium nickel manganese cobalt metal composite oxide particles having a porous structure (lithium calcined product (base material): Li 1.05 Ni 0.35 Mn 0.3 Co 0.35 O2) was obtained.

[0089] [Washing process] The lithium sintered product (base material) was placed in a washing tank and the mass ratio of the lithium sintered product to water was controlled to be 1:1, i.e., the concentration of the lithium sintered product in the slurry was controlled to be 1000 g / L. The slurry was stirred for 30 minutes, and then pressure filtered using a filter press until the moisture content was 10 mass% or less, and the lithium water-washed product was recovered.

[0090] [Mixing process] Hydroxyethyl cellulose (HEC) particles, a polymer compound, were added to the recovered lithium-washed product so that the content of the coated lithium metal composite oxide, i.e., the positive electrode active material, was 0.18 mass %, and the mixture was then placed in a mixer and mixed for 3 hours while maintaining the temperature at 30°C.

[0091] [Heat treatment process: Step S30] The resulting aqueous mixture was then placed in a vacuum dryer and heat-treated for 10 hours at a pressure of 0.1 kPa or less and a temperature of 105° C. Thereafter, the aggregates contained in the coated lithium metal composite oxide were crushed to obtain a positive electrode active material.

[0092] The obtained positive electrode active material (Li 1.01 Ni 0.35 Mn 0.30 Co 0.35 O2) had a porous structure, and a coating layer containing a polymer compound was formed on at least a portion of the surface of the secondary particles. The evaluation results of the obtained positive electrode active material are shown in Table 1.

[0093] Example 2 A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the mixing step, hydroxyethyl cellulose was added so that the content in the positive electrode active material was 0.018 mass %.

[0094] Example 3 A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the mixing step, carboxymethyl cellulose (CMC) was used instead of hydroxyethyl cellulose (HEC) as the polymer compound.

[0095] Example 4 A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was carried out for 0.8 hours / cycle and the crystallization B step was carried out for 0.8 hours / cycle, that cobalt sulfate heptahydrate was used instead of cobalt chloride hexahydrate in preparing the raw material solution, and that the polymer compound was added so that its content in the positive electrode active material was 0.10 mass%.

[0096] Example 5 A positive electrode active material was obtained in the same manner as in Example 1, except that in the crystallization step, the crystallization A step was carried out for 0.8 hours / time and the crystallization B step was carried out for 0.8 hours / time.

[0097] Example 6 A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was carried out for 0.8 hours / cycle and the crystallization B step was carried out for 0.8 hours / cycle, that cobalt sulfate heptahydrate was used instead of cobalt chloride hexahydrate in preparing the raw material solution, that carboxymethyl cellulose was used instead of hydroxyethyl cellulose as the polymer compound in the mixing step, and that the polymer compound was added so that its content in the positive electrode active material was 0.10 mass%.

[0098] Example 7 A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was carried out for 0.8 hours / cycle and the crystallization B step was carried out for 0.8 hours / cycle, and in the mixing step, carboxymethyl cellulose was used as the polymer compound instead of hydroxyethyl cellulose.

[0099] Example 8 A positive electrode active material was obtained in the same manner as in Example 1, except that in the crystallization step, the composition ratio of the raw material solution was controlled to Ni:Mn:Co=60:20:20.

[0100] Example 9 A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the crystallization step, the composition ratio of the raw material solution was controlled to Ni:Mn:Co=60:20:20, and in the mixing step, carboxymethyl cellulose was used as the polymer compound instead of hydroxyethyl cellulose.

[0101] Example 10 A positive electrode active material was obtained by the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was performed for 0.8 hours / time and the crystallization B step was performed for 0.8 hours / time, and the composition ratio of the raw material solution was controlled to be Ni:Mn:Co=60:20:20.

[0102] Example 11 A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was carried out for 0.8 hours / cycle and the crystallization B step was carried out for 0.8 hours / cycle, the composition ratio of the raw material solution was controlled to be Ni:Mn:Co=60:20:20, and carboxymethyl cellulose was used as the polymer compound in place of hydroxyethyl cellulose in the mixing step.

[0103] (Comparative Example 1) A positive electrode active material was obtained in the same manner as in Example 1, except that the mixing step (that is, the addition of the polymer compound) was not carried out and a lithium metal composite oxide without a coating layer was obtained.

[0104] (Comparative Example 2) A positive electrode active material was obtained by the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was performed for 0.8 hours / time and the crystallization B step was performed for 0.8 hours / time, and the mixing step was not performed to obtain a lithium metal composite oxide.

[0105] (Comparative Example 3) A positive electrode active material was obtained by carrying out the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was carried out for 0.8 hours / cycle and the crystallization B step was carried out for 0.8 hours / cycle, and that in preparing the raw material solution, cobalt sulfate heptahydrate was used instead of cobalt chloride hexahydrate, and the mixing step was not carried out, thereby obtaining a lithium metal composite oxide.

[0106] Comparative Example 4 A positive electrode active material was obtained by the same procedure as in Example 1, except that in the crystallization step, the composition ratio of the raw material solution was controlled to Ni:Mn:Co=60:20:20, and the mixing step was not performed to obtain a lithium metal composite oxide.

[0107] (Comparative Example 5) A positive electrode active material was obtained by the same procedure as in Example 1, except that in the crystallization step, the crystallization A step was performed for 0.8 hours / time and the crystallization B step was performed for 0.8 hours / time, the composition ratio of the raw material solution was controlled to be Ni:Mn:Co=60:20:20, and no mixing step was performed to obtain a lithium metal composite oxide.

[0108] [Table 1]

[0109] [Table 2]

[0110] [Evaluation results] The coated lithium metal composite oxides (positive electrode active materials) of Examples 1 to 11 all had a moisture content of 0.1 mass% or less, and when used to produce positive electrode mixture slurries, all had a viscosity of 6000 mPa·s or less, even when containing a relatively large amount of chlorine.

[0111] Furthermore, when incorporated into a lithium ion secondary battery, as shown in Tables 1 and 2, when Examples 1 to 3, which have the same composition formula of the coated lithium metal composite oxide and approximately the same porosity, are compared with Comparative Example 1, Examples 1 to 3 have higher initial discharge capacities than Comparative Example 1. Similarly, when Examples 4 to 7, which have the same composition formula of the coated lithium metal composite oxide and approximately the same porosity, are compared with Comparative Examples 2 and 3, Examples 4 to 7 have higher initial discharge capacities than Comparative Examples 2 and 3. As shown in Table 2, when Examples 8 and 9, which have the same composition formula of the coated lithium metal composite oxide and approximately the same porosity, are compared with Comparative Example 4, Examples 8 and 9 have higher initial discharge capacities than Comparative Example 4. Similarly, when Examples 10 and 11, which have the same composition formula of the coated lithium metal composite oxide and approximately the same porosity, are compared with Comparative Example 5, Examples 10 and 11 have higher initial discharge capacities than Comparative Example 5.

[0112] When Example 1 and Example 5 are compared, which have the same composition formula of the coated lithium metal composite oxide, the same polymer compound and content of the coating layer, but different porosities, the viscosity of the positive electrode composite slurry in Example 5, which has a porosity of 55%, is the same as the viscosity of the positive electrode composite slurry in Example 1, which has a porosity of 20%. That is, it was confirmed that the coating layer effectively suppresses gelation of the positive electrode composite slurry, even when the porosity is increased, the contact area with the nonaqueous electrolyte is increased, and the initial discharge capacity is increased.

[0113] As described above, it is clear that the coated lithium metal composite oxides of the examples can impart excellent weather resistance to the positive electrode active material, can suppress an increase in viscosity when used to produce a positive electrode mixture slurry, and can maintain a sufficiently high initial discharge capacity when incorporated into a lithium ion secondary battery.

[0114] Furthermore, the technical scope of the present invention is not limited to the aspects described in the above embodiment, etc. One or more of the requirements described in the above embodiment, etc. may be omitted. The requirements described in the above embodiment, etc. may be combined as appropriate. Furthermore, to the extent permitted by law, the contents of all documents cited in this specification are incorporated by reference and are incorporated by reference into the present specification. [Explanation of symbols]

[0115] 10 Coated lithium metal composite oxide 1. Lithium metal composite oxide particles 1a primary particle 1b Secondary particles 1c void 2 Covering layer CBA Coin Cell Battery (for evaluation) PE positive electrode (electrode for evaluation) NE negative electrode SE Separator GA Gasket WW Wave Washer PC positive electrode can NC negative electrode can

Claims

1. The present invention relates to a coated lithium metal composite oxide, the coated lithium metal composite oxide comprising lithium metal composite oxide particles containing lithium, nickel, manganese, and cobalt, and a coating layer that coats at least a portion of the surface of the particles. the lithium metal composite oxide particles include secondary particles formed by aggregation of a plurality of primary particles, the secondary particles have a porous structure and a porosity of more than 15% but not more than 70%, the coating layer contains a polymer compound, the coated lithium metal composite oxide contains chlorine; Positive electrode active material for lithium-ion secondary batteries.

2. The present invention relates to a coated lithium metal composite oxide, the coated lithium metal composite oxide comprising lithium metal composite oxide particles containing lithium, nickel, manganese, and cobalt, and a coating layer that coats at least a portion of the surface of the particles. the lithium metal composite oxide particles include secondary particles formed by aggregation of a plurality of primary particles, the secondary particles have a porous structure and a porosity of more than 15% but not more than 70%, the coating layer contains a polymer compound, The amount of lithium carbonate in the coated lithium metal composite oxide is 0.05% by mass or less. Positive electrode active material for lithium-ion secondary batteries.

3. The lithium metal composite oxide particles have the general formula: Li ａ Ni １－ｘ－ｙ－ｚ Mn ｘ Co ｙ M ｚ O ２＋α 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein M is one or more elements selected from W, Mo, V, Ca, Mg, Sr, Ba, Ti, Cr, Zr, Al, Nb, Ta, Si, P, B, S, Na, and K, satisfying 0.95≦a≦1.5, 0.01≦x≦0.5, 0.01≦y≦0.5, 0≦z≦0.2, 0<(1−x−y−z), and −0.1≦α≦0.

2.

4. 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the polymer compound comprises one or more selected from the group consisting of ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxyethyl cellulose, carboxymethyl cellulose, carboxymethyl ethyl cellulose, polyvinyl alcohol, and polyvinylpyrrolidone.

5. 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the polymer compound has an average molecular weight of 3,000,000 or less.

6. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the chlorine content is 0.1 mass % or less.

7. 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the content of the polymer compound contained in the coated lithium metal composite oxide is 0.01% by mass or more.

8. The positive electrode active material includes a conductive material, a binder, a polymer compound, and an organic solvent, the positive electrode active material has a porous structure and a porosity of more than 15% but not more than 70%, The positive electrode mixture slurry for a lithium ion secondary battery includes one or more polymer compounds selected from ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxyethyl cellulose, carboxymethyl cellulose, carboxymethyl ethyl cellulose, polyvinyl alcohol, and polyvinylpyrrolidone.

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