JP7371364B2 - Positive electrode active material for lithium ion secondary batteries and its manufacturing method, positive electrode for lithium ion secondary batteries, and lithium ion secondary batteries - Google Patents

Description

The present invention relates to a positive electrode active material for a lithium ion secondary battery, a method for manufacturing the same, a positive electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

In recent years, with the spread of portable electronic devices such as smartphones, tablet terminals, and notebook computers, there has been a strong desire to develop small and lightweight secondary batteries with high energy density. Furthermore, there is a strong desire to develop secondary batteries with excellent output characteristics and charge/discharge cycle characteristics as batteries for electric vehicles such as hybrid cars.

A lithium ion secondary battery is a secondary battery that meets such requirements. This lithium ion secondary battery is composed of a negative electrode, a positive electrode, an electrolyte, etc., and a material capable of desorbing and inserting lithium is used as the active material of the negative electrode and the positive electrode. As the electrolyte, a non-aqueous electrolyte such as a non-aqueous electrolytic solution prepared by dissolving a lithium salt serving as a supporting salt in an organic solvent, or a non-flammable solid electrolyte having ion conductivity is used.

Such lithium ion secondary batteries are currently being actively researched and developed, and among them, lithium ion batteries using layered rock salt or spinel type lithium transition metal-containing composite oxides as the positive electrode active material are currently being actively researched and developed. Since ion secondary batteries can obtain a high voltage of 4V class, they are being put into practical use as batteries with high energy density.

The main lithium-transition metal-containing composite oxides that have been proposed so far include lithium-cobalt composite oxide (LiCoO 2 ), which is relatively easy to synthesize, and lithium-nickel composite oxide (LiCoO ₂ ), which uses nickel, which is cheaper than cobalt. LiNiO ₂ ), lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), and lithium manganese composite oxide using manganese (LiMn ₂ O ₄ ).

Among the above positive electrode active materials, lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), which has excellent thermal stability and high capacity, has attracted attention in recent years. Lithium nickel cobalt manganese composite oxide is a layered compound like lithium cobalt composite oxide and lithium nickel composite oxide, and basically has a composition ratio of nickel, cobalt, and manganese at the transition metal site of 1:1: Contains at a ratio of 1.

On the other hand, in order to obtain a positive electrode with high performance (high cycle characteristics, high capacity, and high output) as a lithium ion secondary battery, lithium and Various techniques have been proposed for compounding complex oxides containing niobium.

For example, Patent Document 1 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery made of a lithium nickel manganese composite oxide composed of secondary particles in which a plurality of primary particles aggregate, the lithium nickel manganese composite oxide is the general formula (1): Li _d Ni _1-a-b-c Mn _a M _b Nb _c O _2+γ (In the general formula (1), M is Co, W, Mo, V, Mg, Ca, At least one element selected from Al, Ti, Cr, Zr and Ta, 0.05≦a≦0.60, 0≦b≦0.60, 0.0003≦c≦0.03, 0 .95≦d≦1.20, 0≦γ≦0.5), and at least a part of the niobium in the lithium nickel manganese composite oxide is solid dissolved in the primary particles. Active materials for electrolyte secondary batteries have been proposed. Further, it is described that in the positive electrode active material, a compound containing lithium and niobium may be present on at least a portion of the surface of the primary particles. According to Patent Document 1, the lithium niobium compound formed on the surface of the primary particles reduces the positive electrode resistance of the obtained secondary battery and improves the output characteristics.

For example, in Patent Document 2, on the particle surface of a first composite oxide containing Li, Ni, Co, Mn, and W, a second composite oxide composed of a composite oxide of Li and Al or Nb is added. A positive electrode active material for lithium ion batteries has been proposed in which a composite oxide of Li and Al or Nb is applied to the particle surface of a first composite oxide containing Li, Ni, Co, Mn, and W. It has been shown that excellent battery characteristics can be obtained due to the presence of the second composite oxide composed of a compound.

International Publication No. 2018/043669 Japanese Patent Application Publication No. 2017-084674

However, the mere presence of a composite oxide containing lithium and niobium on the surface of a composite oxide containing Li, Ni, Co, and Mn as shown in Patent Documents 1 and 2 does not improve the output characteristics of the secondary battery. not enough to improve

In view of the above problems, an object of the present invention is to provide a positive electrode active material that can significantly reduce positive electrode resistance (reaction resistance) and have higher output characteristics when used as a positive electrode active material for lithium ion secondary batteries. The purpose is to provide substances and methods for their production.

In order to solve the above problems, the inventors have conducted intensive research and found that the presence of a specific amount of first particles having lithium ion conductivity on the surface of a lithium metal composite oxide significantly reduces the positive electrode resistance. The present invention was completed based on the discovery that it is possible to reduce the

According to the first aspect of the present invention, a positive electrode includes a first lithium metal composite oxide composed of secondary particles in which a plurality of primary particles aggregate, and first particles having lithium ion conductivity. The positive electrode active material contains lithium (Li), nickel (Ni), cobalt (Co), and element M, and the material amount (mole) ratio of these elements is Li:Ni:Co. :M=d1:(1-a-b):a:b (however, 0≦a<0.40, 0≦b<0.35, a+b<0.40, 0.95≦d1≦1.5 , M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta), and the first particles are lithium , oxygen, and an element other than lithium and oxygen, and the amount of the element other than lithium and oxygen in the first particle is 0.01% by mass or more and 4% by mass or less based on the entire positive electrode active material. A positive electrode active material for a lithium ion secondary battery is provided, in which at least a portion of the first particles exist bonded to the surface of the first lithium metal composite oxide.

Further, it is preferable that at least a portion of the first particles exist separated from the first lithium metal composite oxide. Further, it is preferable that the first particles include at least one selected from the group consisting of lithium niobium composite oxide, lithium tungsten composite oxide, lithium phosphorus composite oxide, and lithium silicon composite oxide. Moreover, it is preferable that the first particles contain at least one of LiNbO ₃ and Li ₃ NbO ₄ . Moreover, it is preferable that at least some of the first particles have a longest diameter of 0.5 μm or more.

According to the second aspect of the present invention, the positive electrode includes a first lithium metal composite oxide composed of secondary particles in which a plurality of primary particles aggregate, and first particles having lithium ion conductivity. A method for producing an active material, the method comprising: mixing a second lithium metal composite oxide and second particles capable of reacting with lithium to form the first particles; and by the mixing. heat-treating the obtained mixture at a temperature of 400°C or more and 900°C or less, and the second lithium metal composite oxide contains lithium (Li), nickel (Ni), cobalt (Co), and the elements M is included, and the substance amount (mole) ratio of these elements is Li:Ni:Co:M=d1:(1-ab):a:b (however, 0≦a<0.40, 0≦ b<0.35, a+b<0.40, 0.95≦d2≦1.5, M is Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta the first particle contains lithium, oxygen, and an element other than lithium and oxygen, and the second particle contains the element other than lithium and oxygen. , and the amount of the elements other than lithium and oxygen in the mixture is 0.01% by mass or more and 4% by mass or less based on the entire positive electrode active material, and at least a part of the first particles are Provided is a method for producing a positive electrode active material for a lithium ion secondary battery, which is bonded to the surface of a lithium metal composite oxide.

Moreover, it is preferable that the second particles contain at least one element selected from the group consisting of niobium, tungsten, phosphorus, and silicon. Moreover, it is preferable that the second particles contain niobic acid. Further, the second lithium metal composite oxide is formed by firing a compound containing nickel, cobalt, and manganese obtained by crystallization and a lithium compound at a temperature of 700°C or higher and 1000°C or lower. It is preferable.

According to a third aspect of the present invention, there is provided a positive electrode for a lithium ion secondary battery, comprising a first lithium metal composite oxide and first particles having lithium ion conductivity. The lithium metal composite oxide contains lithium (Li), nickel (Ni), cobalt (Co), and element M, and the substance amount (mole) ratio of these elements is Li:Ni:Co:M=d3 :(1-a-b):a:b(However, 0≦a<0.40, 0≦b<0.35, a+b<0.40, 0.95≦d3≦1.5, M is (at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta), and the first particles include lithium, oxygen, and , an element other than lithium and oxygen, a portion of the first particles exist bonded to the surface of the first lithium metal composite oxide, and a portion of the first particles: A positive electrode for a lithium ion secondary battery is provided that exists separately from a first lithium metal composite oxide.

Moreover, it is preferable that the first particles contain at least one of LiNbO ₃ and Li ₃ NbO ₄ .

According to a fourth aspect of the present invention, a lithium ion secondary battery is provided that includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and an electrolyte.

By containing a lithium metal composite oxide and first particles having lithium ion conductivity, the positive electrode active material of the present invention can greatly reduce the positive electrode resistance of the resulting lithium ion secondary battery. Therefore, a lithium ion secondary battery using the positive electrode active material of the present invention as a positive electrode can further improve output characteristics. Further, the method for producing a positive electrode active material of the present invention allows the above-mentioned positive electrode active material to be easily produced with high productivity.

FIG. 1(A) is a diagram showing an example of the positive electrode active material according to the present embodiment, and FIG. 1(B) is a diagram showing an example of the structure of a lithium metal composite oxide. FIG. 2 is a diagram illustrating an example of a method for manufacturing a positive electrode active material according to this embodiment. FIG. 3 is a diagram showing an example of a method for manufacturing a positive electrode active material according to this embodiment. FIG. 4 is a schematic cross-sectional view of a coin-type battery used for battery evaluation. FIG. 5 is a schematic explanatory diagram of an example of measurement for impedance evaluation and an equivalent circuit used for analysis. FIG. 6 is a graph showing the relationship between the positive electrode resistance and the amount of niobium of the positive electrode active materials obtained in Examples 1 to 5 and Comparative Examples 1 to 4. FIG. 7 is a graph showing the relationship between the positive electrode resistance and the amount of niobium of the positive electrode active materials obtained in Examples 7 to 9 and Comparative Examples 5 to 7. FIG. 8 is a diagram showing a backscattered electron image (COMPO image) by SEM of the positive electrode active material obtained in Example 1. FIG. 9 is a diagram showing a backscattered electron image (COMPO image) by SEM of the positive electrode active material obtained in Comparative Example 4.

Hereinafter, with reference to the drawings, a positive electrode active material for a lithium ion secondary battery, a method for manufacturing the same, a positive electrode mixture paste for a lithium ion secondary battery, and a lithium ion secondary battery of the present embodiment will be described. Further, in the drawings, in order to make each configuration easier to understand, some parts are emphasized or some parts are simplified, and the actual structure, shape, scale, etc. may be different.

1. Positive electrode active material for lithium ion secondary battery FIG. 1(A) is a schematic diagram showing an example of a positive electrode active material for a lithium ion secondary battery according to the present embodiment. As shown in FIG. 1(A), a positive electrode active material 100 according to this embodiment includes a first lithium metal composite oxide 10 and first particles 20 having lithium ion conductivity.

(1) Composition of positive electrode active material (overall) The positive electrode active material 100 contains lithium (Li), nickel (Ni), cobalt (Co), and element M, and the material amount (mole) ratio of these elements is , Li:Ni:Co:M=d1:(1-ab):a:b:c (however, 0≦a<0.40, 0≦b<0.35, a+b<0.40, 0 .95≦d1≦1.5, M is at least one element selected from W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta). Note that, as described later, the positive electrode active material 100 also contains niobium derived from the first particles 20.

Nickel (Ni) is an element that contributes to increasing the potential and capacity of secondary batteries. In the above molar ratio, the value of (1-a-b) indicating the Ni content is preferably greater than 0.6, more preferably 0.65 or more and 0.99 or less, and even more preferably 0.8 or more and 0. .95 or less. When the value of (1-ab) exceeds 0.6, the battery can have a high capacity.

Cobalt (Co) is an element that contributes to improving charge-discharge cycle characteristics. In the above molar ratio, the value of a indicating the Co content is 0 or more and less than 0.4, preferably 0.05 or more and less than 0.4, preferably 0.05 or more and less than 0.4, more preferably 0. It is 1 or more and less than 0.4. If the value of a is less than 0.05, the effect of improving charge/discharge cycle characteristics obtained by containing Co cannot be sufficiently obtained.

M indicating an additive element is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta. In the above molar ratio, the value of b indicating the content of M is 0≦b<0.35, and in particular, when b is larger than 0, thermal stability, output characteristics, cycle characteristics, etc. can be improved. can. Moreover, it is preferable that M contains Al. When M is Al, the value of b is 0.01 or more and 0.1 or less.

In the above molar ratio, d1 indicating the lithium content is preferably 0.95 or more and 1.5 or less, more preferably 1.0 or more and 1.10 or less. When the molar ratio (d1) of lithium is less than 0.95, the site that should be occupied by lithium in the crystal of the lithium metal composite oxide is occupied by other elements, and the charge/discharge capacity may decrease. On the other hand, if the molar ratio of lithium exceeds 1.5, there will be an excess of lithium compounds that do not contribute to charging and discharging together with the lithium-nickel composite oxide, increasing battery resistance and decreasing charging and discharging capacity. Sometimes.

In addition, the positive electrode active material 100 has a composition excluding niobium of the general formula (1): Li _d1 Ni _1-ab Co _a M _b O _2+α (in the general formula (1), M is Mn, W, At least one element selected from Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta, 0≦a≦0.40, 0≦b<0.35, a+b <0.40, 0.95≦d1≦1.5, 0≦α≦0.5). In addition, in the said general formula (1), the preferable range of each element can be made into the same range as the molar ratio mentioned above.

(2) First lithium metal composite oxide FIG. 1(B) is a schematic diagram showing an example of the first lithium metal composite oxide 10. The first lithium metal composite oxide 10 is composed of secondary particles 2 formed by agglomerating a plurality of primary particles 1. The shape of the secondary particles 2 is not particularly limited, and may have, for example, a solid structure, a hollow structure, a porous structure, or the like. Here, the solid structure refers to a structure in which the entire secondary particle 2 is composed of an aggregate of a plurality of primary particles 1, and the hollow structure refers to a structure in which one or more spaces exist inside the secondary particle 2. It refers to the structure that Further, the porous structure refers to a structure in which a large number of voids exist inside the secondary particles 2. Note that the first lithium metal composite oxide 10 may include not only the secondary particles 2 but also a small number of individual primary particles 1.

Although the size of the first lithium metal composite oxide 10 is not particularly limited, it is preferable that the volume average particle diameter MV is about 1 μm or more and 30 μm or less, and more preferably 3 μm or more and 10 μm or less. The volume average particle diameter MV can be determined, for example, from a volume integrated value measured with a laser beam diffraction scattering particle size distribution analyzer.

(3) First Particles The first particles 20 are not particularly limited as long as they have lithium ion conductivity and contain lithium, oxygen, and an element other than lithium and oxygen. Elements other than lithium and oxygen are not particularly limited, and may be, for example, at least one element from the group consisting of Nb, W, P, and Si. Note that the first particle may be of one type or may be of multiple types.

It is preferable that the first particles 20 include, for example, at least one member selected from the group consisting of lithium niobium composite oxide, lithium tungsten composite oxide, lithium phosphorus composite oxide, and lithium silicon composite oxide. For example, the first particles 20 include lithium niobium composite oxides such as Li ₃ NbO ₄ and LiNbO ₃ , lithium tungsten composite oxides such as Li ₂ WO ₄ , Li ₂ W ₂ O ₇ , and Li ₄ WO ₅ , and Li. Examples include lithium phosphorus composite oxides such as ₃ PO ₄ and lithium silicon composite oxides such as Li ₄ SiO ₄ . Among these, from the viewpoint of increasing lithium ion conductivity, it is more preferable to contain at least one of LiNbO ₃ and Li ₃ NbO ₄ , and even more preferably to consist of LiNbO ₃ . If the first particles 20 include LiNbO ₃ , they can have higher lithium ion conductivity.

The composition and species of the compound in the first particles 20 can be evaluated and identified by analysis using ICP emission spectrometry and X-ray diffraction (XRD), which will be described later. Note that the first particles 20 may include an amorphous phase. The amorphous phase has excellent lithium ion conductivity and may further improve battery characteristics.

The amount of elements other than lithium and oxygen in the first particles 20 is 0.02 mass% or more and 5.2 mass% or less, preferably 0.1 mass% or more and 5 mass% or less, based on the entire positive electrode active material 100. The content is more preferably 0.15% by mass or more and 4.5% by mass or less. When the amount of elements other than lithium and oxygen is within the above range, the reaction resistance (positive electrode resistance) of the secondary battery can be reduced and the output characteristics can be improved.

Further, when the first particles 20 are a lithium niobium composite oxide, the amount of niobium in the lithium niobium composite oxide is, for example, 2% by mass or more and 4% by mass or less with respect to the entire positive electrode active material. Good too. When the amount of niobium is within the above range, the positive electrode resistance (reaction resistance) of the secondary battery can be further reduced and the output characteristics can be further improved.

In the positive electrode active material 100, at least a portion of the first particles 20 are bonded (or attached) to the surface of the first lithium metal composite oxide 10 (see 20a in FIG. 1(A)). . The first particles 20 only need to exist bonded to at least a portion of the surface of the first lithium metal composite oxide 10, and may cover the entire surface of the first lithium metal composite oxide 10. It may be bonded to a part of the surface of 10. The first particles 20 can be bonded (adhered) to the surface of the first lithium metal composite oxide 10 by heat treatment (step S20) described below.

Further, in the positive electrode active material 100, it is preferable that at least a portion of the first particles 20 exist separated from the first lithium metal composite oxide 10 (see 20b in FIG. 1(A)). Due to the presence of the first particles 20a that exist in combination with the first lithium metal composite oxide 10 and the first particles 20b that exist separately, when forming the positive electrode of the secondary battery, It is estimated that a lithium ion conduction path is formed between the particles of the first lithium metal composite oxide 10, and the ionic conductivity between the particles of the first lithium metal composite oxide 10 is improved.

The first particles 20b existing separately from the first lithium metal composite oxide 10 contain the same element as the first particles 20a existing bonded to the surface of the first lithium metal composite oxide 10. It is preferable that it contains the same composition, and more preferably that it has the same composition.

Further, the size of the first particles 20 is not particularly limited, but the longest diameter of at least a portion of the first particles 20 is preferably 0.5 μm or more, and may be 1 μm or more. Moreover, the average particle diameter of the first particles 20 may be 0.5 μm or more. When the first particles 20 have an amorphous shape or the like, the average particle size may be the average longest diameter when a plurality of first particles 20 are observed with a scanning electron microscope (SEM).

2. Method for Manufacturing a Positive Electrode Active Material FIG. 2 is a diagram showing an example of a method for manufacturing a positive electrode active material according to the present embodiment. The method for producing a positive electrode active material includes mixing a second lithium metal composite oxide and second particles capable of reacting with lithium to form first particles (step S10); and heat-treating the obtained mixture (step S20). By this method for manufacturing a positive electrode active material, the above-described positive electrode active material 100 can be easily manufactured with high productivity.

Further, as shown in FIG. 3, the second lithium metal composite oxide is obtained by crystallizing a compound containing nickel, cobalt, and optionally element M (hereinafter also simply referred to as a "nickel compound"). (Step S1), mixing the obtained nickel compound and lithium compound (Step S2), and firing the obtained mixture (Step S3). When a nickel compound obtained by crystallization is used as a precursor of a positive electrode active material, a positive electrode active material having a more uniform composition can be obtained. Note that the nickel compound includes one or both of hydroxide and oxide forms. An example of a method for manufacturing a positive electrode active material according to this embodiment will be described below.

[Crystallization process (step S1)]
The compound containing nickel (precursor of the positive electrode active material) can be obtained by a known production method, but it is preferably obtained by crystallization, and in the crystallization, two processes are performed: a nucleation step and a particle growth step. It is more preferable to separate the steps into two steps. By separating the crystallization into the two steps described above, a nickel compound having a more uniform particle size and a narrow particle size distribution can be obtained. Since the form of the nickel compound (precursor of the positive electrode active material) is carried over to the form of the second lithium metal composite oxide after firing, the finally obtained positive electrode active material must also have a uniform particle size. It is possible to improve battery characteristics such as cycle characteristics. Hereinafter, as an example of the crystallization process, a case where the crystallization process is separated into a nucleation stage and a particle growth stage will be described.

(Nucleation process)
First, a compound containing nickel, cobalt, and optionally element M is dissolved in water at a predetermined ratio to prepare a raw material aqueous solution. When element M is aluminum, an aluminum supply liquid is separately prepared using sodium aluminate or the like. In addition, an alkaline aqueous solution and an aqueous solution containing an ammonium ion donor are supplied and mixed into the reaction tank, and the pH value measured at a liquid temperature of 25°C is 12.0 or more and 14.0 or less, and the ammonium ion concentration is 3 g. /L or more and 25 g/L or less is prepared. Note that the pH value of the pre-reaction aqueous solution can be measured with a pH meter, and the ammonium ion concentration can be measured with an ion meter.

Next, a raw material aqueous solution is supplied while stirring this pre-reaction aqueous solution. Moreover, when the element M is aluminum, an aluminum supply liquid is separately supplied. As a result, a nucleation aqueous solution, which is a reaction aqueous solution in the nucleation step, is formed in the reaction tank. Since the pH value of this aqueous solution for nucleation is within the above-mentioned range, in the nucleation step, nucleation occurs preferentially, with almost no nuclei growing. In addition, in the nucleation process, the pH value and ammonium ion concentration of the nucleation aqueous solution change as the nucleation occurs, so the alkaline aqueous solution and the ammonia aqueous solution are supplied in a timely manner so that the pH value of the solution in the reaction tank reaches 25% of the liquid temperature. The pH is controlled to be maintained in the range of 12.0 to 14.0 and the ammonium ion concentration is maintained in the range of 3 g/L to 25 g/L on a C basis.

(Particle growth process)
After the nucleation process is completed, the pH value of the nucleation aqueous solution in the reaction tank is adjusted to 10.5 or more and 12.0 or less based on the liquid temperature of 25°C, and the particle growth aqueous solution that is the reaction aqueous solution in the particle growth process is Form. Note that the pH value can be adjusted by stopping the supply of the alkaline aqueous solution, but in order to obtain a nickel compound with a narrow particle size distribution, it is necessary to temporarily stop the supply of all aqueous solutions and adjust the pH value. is preferred. Specifically, after stopping the supply of all aqueous solutions, it is preferable to adjust the pH value by supplying the same type of inorganic acid as the acid constituting the metal compound serving as the raw material to the aqueous nucleation solution.

Next, a raw material aqueous solution is supplied while stirring the particle growth aqueous solution. At this time, since the pH value of the aqueous solution for particle growth is within the above-mentioned range, new nuclei are hardly generated, and nucleus (particle) growth progresses to form nickel compound particles having a predetermined particle size. . In addition, in the particle growth process, the pH value and ammonium ion concentration of the particle growth aqueous solution change as the particles grow, so the alkaline aqueous solution and ammonia aqueous solution are supplied in a timely manner to maintain the pH value and ammonium ion concentration within the above range. It is necessary to do so.

Note that the reaction atmosphere in the nucleation step and the particle growth step is adjusted as appropriate depending on the required structure of the secondary particles 2. For example, when a nickel compound obtained when the reaction atmosphere is a non-oxidizing atmosphere with an oxygen concentration of 5% by volume or less throughout the nucleation process and the particle growth process is fired, a lithium transition metal composite oxide with a solid structure is formed. is obtained.

On the other hand, if the reaction atmosphere at the beginning of the nucleation step and the particle growth step is an oxidizing atmosphere with an oxygen concentration exceeding 5% by volume, and the reaction atmosphere is switched from the oxidizing atmosphere to the non-oxidizing atmosphere in the particle growth step, The resulting nickel compound is composed of a low-density center made of fine primary particles and a high-density outer shell made of plate-shaped primary particles larger than the fine primary particles. When such a nickel compound is fired, a first lithium metal composite oxide 10 having a hollow structure is obtained.

In addition, the reaction atmosphere in the nucleation process is made into a non-oxidizing atmosphere, and in the particle growth process, the reaction atmosphere is controlled by switching to an oxidizing atmosphere and then switching to a non-oxidizing atmosphere again, once or multiple times. , a nickel compound having a laminated structure in which low-density layers and high-density layers are alternately laminated can be obtained. When such a nickel compound is fired, a first lithium metal composite oxide 10 having a porous structure in which a plurality of voids are dispersed inside the particles is obtained.

The nickel composite hydroxide (nickel compound) obtained after crystallization (step S1) may be oxidized and roasted to convert at least a part of the hydroxide into a nickel composite oxide, or all of the nickel composite hydroxide may be converted into a nickel composite oxide. It may also be a nickel compound.

[Mixing process before firing (step S2)]
Next, the obtained nickel compound and lithium compound are mixed (step S2). The type of lithium compound to be mixed before firing is not particularly limited, but lithium carbonate, lithium hydroxide, lithium nitrate, etc. can be suitably used. Among these, in consideration of ease of handling and quality stability, it is preferable to use lithium carbonate or lithium hydroxide, and it is more preferable to use lithium carbonate.

In the mixing step (step S2), the sum of the numbers of metal atoms other than lithium, nickel, cobalt, and other elements M (Me) in the mixture obtained by mixing, the number of atoms of lithium (Li), The ratio (Li/Me) is 0.95 or more and 1.5 or less, preferably 1.0 or more and 1.5 or less, more preferably 1.0 or more and 1.35 or less, and still more preferably 1.0 or more and 1.35 or less. The nickel compound and the lithium compound are mixed so that the concentration is 2 or less. That is, since Li/Me does not change before and after the firing step (step S3) described later, the Li/Me in the mixing step (step S2) becomes the target cathode active material 100 (or the second lithium metal composite). A nickel compound and a lithium compound are mixed so that the ratio is Li/Me (oxide).

It is preferable that the nickel compound and the lithium compound are sufficiently mixed to such an extent that no fine powder is generated. If mixing is insufficient, Li/Me may vary between individual particles, and sufficient battery characteristics may not be obtained. Note that a general mixer can be used for mixing. For example, a shaker mixer, Loedige mixer, Julia mixer, V blender, etc. can be used.

[Baking process (step S3)]
Next, the obtained mixture is fired to obtain a second lithium metal composite oxide (Step S3). The furnace used in the firing process is not particularly limited, and may be any furnace that can be heated in the atmosphere or in an oxygen stream. Similarly, from the viewpoint of maintaining a uniform atmosphere in the furnace, it is preferable to use a batch-type or continuous-type electric furnace that does not generate gas.

The firing temperature in the firing step (step S3) is preferably 700°C or more and 1000°C or less, more preferably 730°C or more and 950°C or less. If the firing temperature is less than 700°C, the crystallinity of the obtained second lithium metal composite oxide may be insufficient. On the other hand, when the firing temperature exceeds 1000° C., intense sintering occurs between particles of the second lithium metal composite oxide, causing abnormal grain growth and increasing the proportion of irregularly shaped coarse particles.

The holding time at the firing temperature in the firing step (step S3) is preferably 1 hour or more and 10 hours or less, more preferably 2 hours or more and 6 hours or less. If the holding time at the firing temperature in the firing step (step S3) is less than 1 hour, the resulting second lithium metal composite oxide may have insufficient crystallinity.

The atmosphere during firing is preferably an oxidizing atmosphere, more preferably an atmosphere with an oxygen concentration of 18% by volume or more and 100% by volume or less, and a mixed atmosphere of oxygen at the above oxygen concentration and an inert gas. is particularly preferred. That is, the firing is preferably performed in the atmosphere or in an oxygen stream. If the oxygen concentration is less than 18% by volume, the crystallinity of the lithium metal composite oxide may be insufficient.

The secondary particles constituting the obtained second lithium metal composite oxide may be agglomerated or slightly sintered. In such cases, it is preferable to crush the aggregates or sintered bodies of secondary particles. Thereby, the average particle size and particle size distribution of the obtained second lithium metal composite oxide can be adjusted to a suitable range.

Note that crushing is a process in which mechanical energy is applied to an aggregate made up of multiple secondary particles that is generated due to sinter necking between secondary particles during firing, without destroying most of the secondary particles themselves. It means the operation of separating and loosening aggregates.

Known means can be used for crushing, for example, a pin mill, a hammer mill, etc. can be used. In addition, at this time, it is preferable to adjust the crushing force to an appropriate range so as not to destroy the secondary particles.

The second lithium metal composite oxide obtained after the calcination step (step S3) contains lithium (Li), nickel (Ni), cobalt (Co), and element M, and the substance amount (mol) of these elements is ) ratio is Li:Ni:Co:M=d2:(1-ab):a:b (however, 0≦a<0.40, 0≦b<0.35, a+b<0.40, 0.95≦d2≦1.5, M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta) may be done. The preferable range of the molar ratio of each element is the same as that of the first lithium metal composite oxide described above. Note that, as described later, in the heat treatment step (step S20), some of the lithium in the second lithium metal composite oxide may react with the second particles. Therefore, the molar ratio (d2) of lithium in the second lithium metal composite oxide is higher than the molar ratio (d3) of lithium in the finally obtained first lithium metal composite oxide. The amount may be increased by the amount that reacts with.

[Mixing process (step S10)]
Next, the second lithium metal composite oxide and second particles capable of reacting with lithium to form the first particles 20 are mixed (step S10). Note that the second lithium metal composite oxide may be obtained by a method other than the methods of steps S1 to S3 described above.

The second particles contain the same elements as those contained in the target first particles 20 other than lithium and oxygen. For example, the second particles preferably contain at least one element selected from the group consisting of niobium, tungsten, phosphorus, and silicon, and are more preferably a compound containing niobium (niobium compound). Further, the second particles may or may not contain lithium, but it is preferable that the second particles do not contain lithium. When the second particles do not contain lithium, they can efficiently react with surplus lithium or the like in the second lithium metal composite oxide in the heat treatment step (step S20) described below. In this specification, "not containing lithium" means not containing substantially lithium. For example, it may contain a trace amount of lithium as an impurity, for example, it may contain 1% by mass or less of lithium. It may contain up to 0.1% by mass of lithium.

As the niobium compound, a known compound containing niobium can be used, and for example, niobic acid, niobium oxide, niobium nitrate, niobium pentachloride, niobium nitrate, etc. can be used. Among these, niobic acid, niobium oxide, or a mixture thereof is preferable from the viewpoint of easy availability and prevention of contamination of impurities into the first lithium metal composite oxide, and niobic acid (Nb ₂ O ₅ ·nH ₂ O) is more preferable. Note that if impurities are mixed into the first lithium metal composite oxide, the thermal stability, battery capacity, and cycle characteristics of the resulting secondary battery may be reduced.

The amount of the second particles mixed is such that the amount of the above elements other than lithium and oxygen in the mixture is 0.02% by mass or more and 5.2% by mass or less, preferably 0.02% by mass or less, based on the entire positive electrode active material 100 obtained. They can be mixed in an amount of 0.05% by mass or more and 5% by mass or less, more preferably 0.1% by mass or more and 4.5% by mass or less.

Further, it is preferable that the second particles are mixed in the form of particles (solid phase). The average particle diameter of the second particles is preferably 0.01 μm or more and 10 μm or less, more preferably 0.03 μm or more and 3.0 μm or less, and even more preferably 0.05 μm or more and 1.5 μm or less. When the average particle size is within the above range, the reactivity with the second lithium metal composite oxide will be good. Further, the average particle diameter of the second particles may be, for example, 0.1 μm or more, or 0.5 μm or more.

If the average particle size of the second particles is smaller than 0.01 μm, a problem may arise in that the powder becomes very difficult to handle. On the other hand, if the average particle size is larger than 10 μm, elements other than lithium and oxygen (e.g., Nb) contained in the second particles are not uniformly distributed in the positive electrode active material 100, and battery characteristics are not improved. There is. Note that the average particle diameter of the second particles is the volume average particle diameter MV, and can be determined from, for example, a volume integrated value measured by a laser beam diffraction scattering particle size distribution meter.

The second particles may be pulverized in advance to a particle size within the above range using various pulverizers such as a ball mill, a planetary ball mill, a jet mill/nano jet mill, a bead mill, and a pin mill. Further, the niobium compound may be classified using a dry classifier or a sieve, if necessary. For example, sieving can be performed to obtain particles close to 0.01 μm.

It is preferable that the second lithium metal composite oxide and the second particles are sufficiently mixed to such an extent that no fine powder is generated. If the mixing is insufficient, there will be variations in the reactivity between the lithium derived from the second lithium metal composite oxide and the second particles among individual particles, making it impossible to obtain sufficient battery characteristics. There are cases. Note that a general mixer can be used for mixing. For example, a shaker mixer, Loedige mixer, Julia mixer, V blender, etc. can be used.

Note that in addition to the second lithium metal composite oxide and the second particles, a lithium compound may be further mixed. The second particles can react with lithium (excess lithium or lithium in the crystal) in the second lithium metal composite oxide in the heat treatment step (step S20), which will be described later. Excessive lithium in the lithium metal composite oxide crystals may be extracted, resulting in deterioration of battery characteristics. In the mixing step (step S10), when a lithium compound is further mixed, the lithium compound becomes a lithium source that reacts with the second particles, and lithium in the crystals of the second lithium metal composite oxide is extracted excessively. can be suppressed. Note that the lithium compound is not particularly limited, but for example, the lithium compound described in the mixing step before firing (step S2) can be used.

[Heat treatment process (step S20)]
Next, the mixture obtained by mixing is heat-treated at a temperature of 400° C. or more and 900° C. or less (Step S20). By heat treatment (step S20), lithium derived from the second lithium metal composite oxide or arbitrarily mixed lithium compound reacts with the second particles to form the above-mentioned first particles 20. I can do it.

The heat treatment temperature is 400°C or more and 900°C or less, preferably 500°C or more and 800°C or less, more preferably 550°C or more and 700°C or less. When the heat treatment temperature is within the above range, lithium and the second particles react, and the first particles 20 can be easily and uniformly formed within the positive electrode active material.

There are no particular restrictions on the furnace used during the heat treatment, as long as it can heat in the atmosphere or in an oxygen stream. Similarly, from the viewpoint of maintaining a uniform atmosphere in the furnace, it is preferable to use a batch-type or continuous-type electric furnace that does not generate gas.

The holding time at the above heat treatment temperature is preferably 1 hour or more and 10 hours or less, and preferably 2 hours or more and 6 hours or less. If the holding time at the heat treatment temperature in this step is less than 1 hour, the resulting lithium metal composite oxide may have insufficient crystallinity.

The atmosphere during the heat treatment (step S20) is preferably an oxidizing atmosphere, more preferably an atmosphere with an oxygen concentration of 18% to 100% by volume, and a mixed atmosphere of oxygen and inert gas with the above oxygen concentration. It is particularly preferable that That is, the firing is preferably performed in the atmosphere or in an oxygen stream. If the oxygen concentration is less than 18% by volume, the crystallinity of the lithium metal composite oxide may deteriorate.

3. Positive Electrode The positive electrode according to the present embodiment includes the first lithium metal composite oxide 10 and the first particles 20 having lithium ion conductivity, and can be suitably used in a lithium ion secondary battery.

The first lithium metal composite oxide 10 includes, for example, lithium (Li), nickel (Ni), cobalt (Co), and element M, and the substance amount (mole) ratio of these elements is Li:Ni :Co:M=d3:(1-a-b):a:b (however, 0≦a<0.40, 0≦b<0.35, a+b<0.40, 0.95≦d3≦1 .5, M may be represented by at least one element selected from W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta). The preferable range of the molar ratio of each element is the same as that of the first lithium metal composite oxide described above. Note that, as described above, in the heat treatment step (step S20), some of the lithium in the second lithium metal composite oxide may react with the second particles. Therefore, the molar ratio (d3) of lithium in the first lithium metal composite oxide finally obtained is lower than the molar ratio (d2) of lithium in the first lithium metal composite oxide. It may be reduced by the amount of reaction.

Further, the preferable compositions and characteristics of the first lithium metal composite oxide 10 and the first particles 20 are the same as those described in the section of the positive electrode active material above.

In the positive electrode according to the present embodiment, a part of the first particles 20 exists bonded to the surface of the first lithium metal composite oxide 10, and a part of the first particles 20 exists in the first lithium metal composite oxide 10. Preferably, the lithium metal composite oxide 10 exists separately from the lithium metal composite oxide 10. In the positive electrode, it is presumed that by distributing the first particles 20 as described above, a suitable lithium ion conduction path is formed between the particles of the first lithium metal composite oxide 10.

The method for producing the positive electrode is not particularly limited, but for example, using the positive electrode active material 100 including the first lithium metal composite oxide 10 and the first particles 20 having lithium ion conductivity, the method is as follows. manufactured as follows.

First, the above positive electrode active material 100, a conductive material, and a binder are mixed, and if necessary, activated carbon and a solvent for purposes such as viscosity adjustment are added, and the mixture is kneaded to prepare a positive electrode composite paste. . At that time, the respective mixing ratios in the positive electrode composite material paste can be adjusted as appropriate depending on the performance of the intended secondary battery. For example, if the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, the content of the positive electrode active material is 60 parts by mass or more and 95 parts by mass or less, and the content of the conductive material is 1 part by mass or more and 20 parts by mass or less. The content of the binder may be 1 part by mass or more and 20 parts by mass or less.

The conductive material is not particularly limited, and for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black-based materials such as acetylene black and Ketjen black can be used.

The binder plays a role in binding the active material particles, and is not particularly limited. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, and cellulose. based resins and polyacrylic acids can be used.

If necessary, a solvent for dispersing the positive electrode active material, conductive material, and activated carbon and dissolving the binder may be added to the positive electrode mixture. Specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent. Furthermore, activated carbon may be added to the positive electrode mixture in order to increase the electric double layer capacity.

The obtained positive electrode mixture paste is applied to the surface of a current collector made of aluminum foil, for example, and dried to scatter the solvent. If necessary, pressure may be applied using a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode can be produced. The sheet-like positive electrode can be cut into an appropriate size depending on the intended battery and used for battery production. Note that the method for producing the positive electrode is not limited to the above method, and other methods may be used.

4. Lithium Ion Secondary Battery The lithium ion secondary battery (hereinafter also referred to as "secondary battery") according to the present embodiment includes a positive electrode containing the above-described positive electrode active material, a negative electrode, and an electrolyte. A lithium ion secondary battery can be constructed from the same components as conventionally known lithium ion secondary batteries, and includes, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte. Further, the secondary battery may be, for example, an all-solid secondary battery including a positive electrode, a negative electrode, and a solid electrolyte. Each component other than the positive electrode will be explained below.

Note that the embodiment described below is merely an example, and the lithium ion secondary battery according to this embodiment can be modified in various ways based on the embodiment described in this specification and based on the knowledge of those skilled in the art. , can be implemented in an improved form. Moreover, the lithium ion secondary battery according to this embodiment is not particularly limited in its use.

(Negative electrode)
The negative electrode may be made of metallic lithium or a lithium alloy, or a negative electrode composite made by mixing a binder with a negative electrode active material that can absorb and desorb lithium ions and adding an appropriate solvent to form a paste. A material may be formed by applying the material to the surface of a metal foil current collector such as copper, drying it, and compressing it to increase the electrode density if necessary.

As the negative electrode active material, for example, fired organic compounds such as natural graphite, artificial graphite, and phenol resin, and powdered carbon materials such as coke can be used. In this case, as the negative electrode binder, a fluororesin such as PVDF can be used as in the positive electrode, and as the solvent for dispersing these active materials and the binder, an organic resin such as N-methyl-2-pyrrolidone can be used. Solvents can be used. Furthermore, as the binder, in addition to the above-mentioned organic binder such as PVDF, an aqueous binder such as styrene butane diene rubber may be used. Further, the aqueous binder may use carboxymethyl cellulose (CMC) as a viscosity modifier. For example, in a water-based binder that uses styrene-butadiene latex as a main additive and CMC as a viscosity modifier, the binding strength can be increased even with a small amount.

(Separator)
A separator is placed between the positive electrode and the negative electrode, if necessary. The separator separates the positive electrode and the negative electrode and holds the electrolyte, and can be a thin film made of polyethylene, polypropylene, or the like, and has many minute pores.

(Non-aqueous electrolyte)
As the non-aqueous electrolyte, a non-aqueous electrolyte can be used. As the non-aqueous electrolyte, for example, a solution in which a lithium salt as a supporting salt is dissolved in an organic solvent may be used. Further, as the non-aqueous electrolyte, an ionic liquid in which a lithium salt is dissolved may be used. Note that the ionic liquid refers to a salt that is composed of cations and anions other than lithium ions and remains in a liquid state even at room temperature.

Examples of organic solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and trifluoropropylene carbonate, linear carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate and dipropyl carbonate, and also tetrahydrofuran, 2- One type selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate may be used alone, or two or more types may be used in combination. It can be used as

As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ , complex salts thereof, and the like can be used. Furthermore, the nonaqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

Further, when the lithium ion secondary battery is an all-solid secondary battery, a solid electrolyte may be used as the nonaqueous electrolyte. A solid electrolyte has the property of being able to withstand high voltage. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes.

As the inorganic solid electrolyte, oxide-based solid electrolytes, sulfide-based solid electrolytes, etc. are used.

The oxide-based solid electrolyte is not particularly limited, and any material that contains oxygen (O) and has lithium ion conductivity and electronic insulation can be used. Examples of oxide- _based solid electrolytes include lithium _{phosphate ( Li 3} PO _{4 ) , Li 3} PO ₄ N _X , LiBO _{2 N} PO ₄ , Li ₄ SiO ₄ -Li ₃ VO ₄ , Li ₂ O-B ₂ O ₃ -P ₂ O ₅ , Li ₂ O-SiO ₂ , Li ₂ O-B ₂ O ₃ -ZnO, Li _1+X Al _X Ti _2-X (PO ₄ ) ₃ (0≦X≦1), Li _1+X Al _X Ge _2-X (PO _{4 ) 3} (0≦X≦1), LiTi ₂ (PO ₄ ) ₃ , Li _{3 3-X} TiO ₃ (0≦X≦2/3), Li ₅ La ₃ Ta ₂ O ₁₂ , Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₆ BaLa ₂ Ta ₂ O ₁₂ , Li _3.6 Si _0.6 Examples include P _0.4 O ₄ and the like.

The sulfide-based solid electrolyte is not particularly limited, and any material can be used as long as it contains sulfur (S) and has lithium ion conductivity and electronic insulation. Examples of the sulfide solid electrolyte include Li ₂ SP ₂ S ₅ , Li ₂ S-SiS ₂ , LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ S ₅ , LiI-Li ₂ S-B ₂ S ₃ , Li ₃ PO ₄ -Li ₂ S-Si ₂ S, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , LiPO ₄ -Li ₂ S-SiS, LiI-Li ₂ S-P ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , and the like.

Note that as the inorganic solid electrolyte, other than those mentioned above may be used, for example, Li ₃ N, LiI, Li ₃ N-LiI-LiOH, etc. may be used.

The organic solid electrolyte is not particularly limited as long as it is a polymer compound that exhibits ionic conductivity, and for example, polyethylene oxide, polypropylene oxide, copolymers thereof, etc. can be used. Moreover, the organic solid electrolyte may contain a supporting salt (lithium salt). Note that when a solid electrolyte is used, the solid electrolyte may also be mixed in the positive electrode material in order to ensure contact between the electrolyte and the positive electrode active material.

(Battery shape and configuration)
The lithium ion secondary battery according to this embodiment, which is composed of the positive electrode, negative electrode, separator, and non-aqueous electrolyte described above, and the positive electrode, negative electrode, and solid electrolyte, can be of various shapes such as a cylindrical shape or a stacked type. can be made into the shape of

When using a non-aqueous electrolyte as the non-aqueous electrolyte, a positive electrode and a negative electrode are laminated with a separator in between to form an electrode body, the obtained electrode body is impregnated with the non-aqueous electrolyte, and the positive electrode current collector and external The lithium ion secondary battery is completed by connecting the connected positive electrode terminal and between the negative electrode current collector and the negative electrode terminal connected to the outside using current collection leads, etc., and sealing it in the battery case. .

Next, a positive electrode active material according to an embodiment of the present invention will be explained in detail using Examples. Note that the present invention is not limited to these examples. In addition, the analysis method of the metal contained in the positive electrode active material and the various evaluation methods of the positive electrode active material in Examples and Comparative Examples are as follows.

[composition]
After the positive electrode active material was dissolved in acid, it was measured using an ICP emission spectrometer (manufactured by Shimadzu Corporation, ICPS-8100).

[Compound species identification]
The positive electrode active material was evaluated using a powder X-ray diffractometer (X'Pert, PROMRD, manufactured by Spectris).

[Evaluation of battery characteristics of positive electrode active material]
A 2032 type coin battery CBA (hereinafter also referred to as "coin battery CBA") having the structure shown in FIG. 6 was used. The method for manufacturing the 2032 type coin battery CBA is as follows.

First, 52.5 mg of positive electrode active material, 15 mg of acetylene black, and 7.5 mg of PTEE were mixed and press-molded at a pressure of 100 MPa into a diameter of 11 mm and a thickness of 100 μm, and then heated at 120° C. in a vacuum dryer. By drying for 12 hours, a positive electrode PE was produced.

Next, a 2032 type coin battery CBA was manufactured using this positive electrode PE in a glove box with an Ar atmosphere whose dew point was controlled at -80°C. The negative electrode NE of this 2032 type coin battery _CBA is made of lithium metal with a diameter of 17 mm and a thickness of 1 mm. An equal volume mixed solution (manufactured by Toyama Pharmaceutical Co., Ltd.) was used. Moreover, a polyethylene porous film with a film thickness of 25 μm was used as the separator SE. The 2032 type coin battery CBA has a gasket GA and is assembled into a coin-shaped battery with a positive electrode can PC and a negative electrode can NC.

The coin battery CBA was left for about 24 hours after being produced, and after the open circuit voltage OCV (Open Circuit Voltage) was stabilized, the current density for the positive electrode was set to 0.1 mA/cm ² and the cutoff voltage was set to 4.3 V. After a 1-hour rest, the battery was discharged until the cutoff voltage reached 3.0V. After that, the coin-type battery CBA was charged to a potential corresponding to 20% SOC and measured by the AC impedance method using a frequency response analyzer and a potentiogalvanostat (Solartron, 1255B), and the Nyquist plot shown in Figure 5 was obtained. It will be done. This Nyquist plot is expressed as the sum of characteristic curves showing the solution resistance, the negative electrode resistance and its capacity, and the positive electrode resistance and its capacity, so based on this Nyquist plot, a fitting calculation is performed using an equivalent circuit, and the positive electrode resistance is (reaction resistance) was calculated.

(Example 1)
(A) Crystallization process [Nucleation process]
First, water was placed in a reaction tank and the temperature inside the tank was set at 40° C. while stirring. At this time, nitrogen gas was passed through the reaction tank for about 30 minutes to make the reaction atmosphere a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. Next, an appropriate amount of 25% by mass sodium hydroxide aqueous solution and 25% by mass aqueous ammonia was supplied into the reaction tank, and the pH value was adjusted to 12.8 and the ammonium ion concentration to 10g/L. An aqueous solution was formed.

Next, nickel sulfate and cobalt sulfate were dissolved in water to prepare a 2 mol/L raw material aqueous solution. Separately, element M is aluminum, sodium aluminate as a raw material is dissolved in water, and a 25% aqueous sodium hydroxide solution is added to the solution so that the molar ratio of sodium to aluminum is 1.7. /L of aluminum feed solution was prepared. Furthermore, an alkaline aqueous solution was prepared by dissolving sodium hydroxide and sodium carbonate in water such that [CO ₃ ²⁻ ]/[OH ⁻ ] was 0.025.

The raw material aqueous solution, aluminum supply solution, alkaline aqueous solution, and 25% by mass aqueous ammonia are added to the pre-reaction aqueous solution at a constant rate to form an aqueous solution for the nucleation process, with the ammonium ion concentration maintained at 10 g/L, and the pH Nucleation was performed while controlling the pH at 12.8 (nucleation step pH). The rate of addition of the raw material aqueous solution and the aluminum feeder was adjusted so that the metal element molar ratio of the slurry was Ni:Co:Al=82:15:3.

[Particle growth process]
After the completion of nucleation, the supply of all aqueous solutions was temporarily stopped, and sulfuric acid was added to adjust the pH value to 11.6, thereby forming an aqueous solution for particle growth. After confirming that the pH value has reached the predetermined value, restart the supply of all aqueous solutions and continue crystallization while controlling the pH to 11.6 to remove the nuclei (particles) generated in the nucleation process. grew.

Thereafter, the obtained product was washed with water, filtered, and dried to obtain powdery nickel composite hydroxide (nickel compound) particles. Note that when the element M is aluminum, it is preferable to use a solution containing sodium aluminate and sodium hydroxide as the aluminum supply liquid used in the crystallization step. When other compounds such as aluminum sulfate are used, aluminum hydroxide precipitates at a lower pH than nickel hydroxide or cobalt hydroxide, so aluminum hydroxide tends to precipitate alone and the particle size It is not possible to obtain a nickel composite hydroxide (nickel compound) with a narrow distribution and uniform particle size.

(B) Mixing and firing process The nickel compound obtained as described above was heat-treated at 700°C for 6 hours in an air stream (oxygen: 21% by volume) to form a nickel composite oxide (nickel compound). Recovered. Furthermore, lithium hydroxide, which is a lithium compound, was weighed so that Li/Me=1.025, and mixed with the recovered nickel composite oxide to prepare a lithium mixture. For the mixing operation, a shaker mixer device (TURBULA-Type T2C manufactured by Willy & Backofen (WAB)) was used.

Next, the prepared lithium mixture was calcined at 500°C for 4 hours in an oxygen (oxygen: 100% by volume) gas stream, further calcined at 730°C for 24 hours, and crushed after cooling to produce a lithium metal composite oxide. I got something.

The lithium metal composite oxide thus obtained was agglomerated or slightly sintered. Therefore, this lithium metal composite oxide was crushed using a continuous mill (manufactured by IKA, MF10 Basic).

(C) Mixing step of second particles (niobium compound) The obtained lithium metal composite oxide powder and niobic acid powder (D50 particle size: 1.0 μm) were mixed and heated in an electric furnace (manufactured by Toyo Seisakusho, electric A positive electrode active material was produced by holding the material at a temperature of 650° C. for 3 hours in a flow of air (oxygen concentration: 21% by volume) using a muffle furnace.

(D) Evaluation of positive electrode active material [composition]
Analysis using an ICP emission spectrometer revealed that the positive electrode active material had a molar ratio of metal elements of Li:Ni:Co:Al=1.1:0.82:0.15:0.03. It was found that the content of Nb was 5.2 wt%.
[Crystal structure]
The obtained positive electrode active material was evaluated by powder X-ray diffraction. As a result, peaks derived from the layered rock salt type of LiNi _0.82 Co _0.15 Al _0.03 O ₂ and peaks derived from LiNbO ₃ and Li ₃ NbO ₄ were confirmed. Table 1 shows the evaluation results of the obtained positive electrode active material.
[Shape observation]
When the obtained positive electrode active material was confirmed by observing a backscattered electron image (COMPO image) using SEM, it was found that the surface of the first lithium metal composite oxide 10 and the first lithium metal High brightness particles were observed in a state separated from the composite oxide 10. In backscattered electron images, elements with higher atomic numbers are observed with higher brightness contrast, so when combined with the powder X-ray diffraction results mentioned above, particles observed with high brightness contrast are LiNbO ₃ and / or Li ₃ NbO ₄ (first particles 20).

(Example 2)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 2.70 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. observed, and it is considered that LiNbO ₃ and/or Li ₃ NbO ₄ (first particles) are present as in Example 1.

(Example 3)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 1.00 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. Furthermore, the obtained positive electrode active material was evaluated by powder X-ray diffraction. As a result, a peak derived from the layered rock salt type of LiNi _0.82 Co _0.15 Al _0.03 O ₂ and a peak derived from LiNbO ₃ were confirmed. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. It is believed that LiNbO ₃ (first particle) is present.

(Example 4)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 0.60 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. observed, and it is thought that LiNbO ₃ (first particles) exist similarly to Example 3.

(Example 5)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 0.10 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. observed, and it is thought that LiNbO ₃ (first particles) exist similarly to Example 3.

(Example 6)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 0.02 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. observed, and it is thought that LiNbO ₃ (first particles) exist similarly to Example 3.

(Example 7)
The positive electrode was prepared in the same manner as in Example 1, except that the addition rate of the raw material aqueous solution and the aluminum supplier was adjusted so that the metal element molar ratio of the slurry was Ni:Co:Al=92.5:2:5.5. The active material was prepared and evaluated. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. It is thought that LiNbO ₃ and/or Li ₃ NbO ₄ (first particles) are present as in Example 1.

(Example 8)
A positive electrode active material was produced and evaluated in the same manner as in Example 7, except that niobic acid was mixed so that the amount of the niobium compound added was 2.70 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. It is thought that LiNbO ₃ and/or Li ₃ NbO ₄ (first particles) are present as in Example 1.

(Example 9)
A positive electrode active material was produced and evaluated in the same manner as in Example 7, except that niobic acid was mixed so that the amount of the niobium compound added was 0.02 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. observed, and it is considered that LiNbO ₃ and/or Li ₃ NbO ₄ (first particles) are present as in Example 1.

(Comparative example 1)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 6.4 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. Observed.

(Comparative example 2)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 5.5 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. Observed.

(Comparative example 3)
A positive electrode active material was produced and evaluated in the same manner as in Example 1, except that niobic acid was mixed so that the amount of the niobium compound added was 0.01 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. Observed.

(Comparative example 4)
In Example 1, a positive electrode active material was produced and evaluated in the same manner as in Example 1, except that the calcination step was performed, and the niobium compound was not mixed and the heat treatment was not performed. Table 1 shows the evaluation results of the obtained positive electrode active material. Furthermore, when it was confirmed by observing a backscattered electron image (COMPO image) using SEM, no high-luminance particles were observed, as shown in FIG.

(Comparative example 5)
A positive electrode active material was produced and evaluated in the same manner as in Example 7, except that niobic acid was mixed so that the amount of the niobium compound added was 6.4 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. Observed.

(Comparative example 6)
A positive electrode active material was produced and evaluated in the same manner as in Example 7, except that niobic acid was mixed so that the amount of the niobium compound added was 0.01 wt%. Table 1 shows the evaluation results of the obtained positive electrode active material. In addition, it was confirmed by observing a backscattered electron image (COMPO image) by SEM that high brightness particles were observed on the surface of the lithium metal composite oxide and in a state separated from the lithium metal composite oxide, as in Example 1. Observed.

(Comparative example 7)
In Example 7, a positive electrode active material was produced and evaluated in the same manner as in Example 7, except that the calcination step was performed and the niobium compound was not mixed and the heat treatment was not performed. Table 1 shows the evaluation results of the obtained positive electrode active material. Furthermore, when it was confirmed by observing a backscattered electron image (COMPO image) using a SEM, no high-luminance particles were observed.

[Evaluation results]
Graphs in which the reaction resistance of the Examples and Comparative Examples shown in Table 1 are plotted against the amount of niobium are shown in FIGS. 6 and 7.

As shown in the graphs of FIGS. 6 and 7, the examples in which a specific amount of niobium compound is added reduce the reaction resistance and are effective in improving the output characteristics of the secondary battery. In addition, as in Comparative Example 4, with the positive electrode active material mixed with a niobium compound before firing, high brightness particles (first particles) with a clear particle shape were not observed, and the reaction rate was lower than in Example 4. Resistance was high. In addition, in Comparative Example 4, niobium is estimated to mainly exist inside the secondary particles. From this, the reaction resistance is reduced by the presence of niobium compound particles (first particles) having lithium ion conductivity in the form of distinct particles on the surface of the lithium metal composite oxide. it is conceivable that.

100...Cathode active material 10...First lithium metal composite oxide 1...Primary particle 2...Secondary particle 20...First particle 20a...First particle (present on the surface of the first lithium metal composite oxide)
20b...First particles (existed separately from the first lithium metal composite oxide)
CBA...Coin battery PE...Positive electrode NE...Negative electrode GA...Gasket PE...Positive electrode NE...Negative electrode SE...Separator

Claims (11)

A positive electrode active material comprising a first lithium metal composite oxide composed of secondary particles in which a plurality of primary particles aggregate, and first particles having lithium ion conductivity,
The positive electrode active material contains lithium (Li), nickel (Ni), cobalt (Co), and element M, and the material amount (mole) ratio of these elements is Li:Ni:Co:M=d1: (1-a-b): a: b (0≦a<0.40, 0≦b<0.35, a+b<0.40, 0.95≦d1≦1.5, M is Mn , W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta),
The first particles include at least one selected from the group consisting of lithium niobium composite oxide, lithium phosphorus composite oxide, and lithium silicon composite oxide,
The lithium niobium composite oxide consists of lithium, oxygen, and niobium,
The amount of elements other than lithium and oxygen in the first particles is 0.02% by mass or more and 5.2% by mass or less based on the entire positive electrode active material,
At least a portion of the first particles are present bonded to the surface of the first lithium metal composite oxide,
Positive electrode active material for lithium ion secondary batteries. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein at least a portion of the first particles are present separately from the first lithium metal composite oxide. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the first particles contain at least one of LiNbO ₃ and Li ₃ NbO ₄ . The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein at least some of the first particles have a longest diameter of 0.5 μm or more. A method for producing a positive electrode active material comprising a first lithium metal composite oxide composed of secondary particles in which a plurality of primary particles are aggregated, and first particles having lithium ion conductivity, the method comprising:
mixing a second lithium metal composite oxide and second particles capable of reacting with lithium to form the first particles;
heat-treating the mixture obtained by the mixing at a temperature of 400°C or more and 900°C or less,
The second lithium metal composite oxide contains lithium (Li), nickel (Ni), cobalt (Co), and element M, and the substance amount (mole) ratio of these elements is Li:Ni:Co. :M= d2 :(1-a-b):a:b(However, 0≦a<0.40, 0≦b<0.35, a+b<0.40, 0.95≦d2≦1.5 , M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta),
The first particles include at least one selected from the group consisting of lithium niobium composite oxide, lithium phosphorus composite oxide, and lithium silicon composite oxide,
The second particle contains the element other than lithium and oxygen,
The amount of the elements other than lithium and oxygen in the mixture is 0.02% by mass or more and 5.2% by mass or less based on the entire positive electrode active material,
At least a portion of the first particles are present bonded to the surface of the first lithium metal composite oxide,
A method for producing a positive electrode active material for a lithium ion secondary battery. 6. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 5, wherein the second particles contain at least one element selected from the group consisting of niobium, tungsten, phosphorus, and silicon. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 5 or 6, wherein the second particles contain niobic acid. The second lithium metal composite oxide is formed by firing a compound containing nickel, cobalt, and manganese obtained by crystallization and a lithium compound at a temperature of 700° C. or higher and 1000° C. or lower. A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 5 to 7. A positive electrode for a lithium ion secondary battery, comprising a first lithium metal composite oxide and first particles having lithium ion conductivity,
The first lithium metal composite oxide contains lithium (Li), nickel (Ni), cobalt (Co), and element M, and the substance amount (mole) ratio of these elements is Li:Ni:Co. :M=d3:(1-a-b):a:b (however, 0≦a<0.40, 0≦b<0.35, a+b<0.40, 0.95≦ d3 ≦1.5 , M is at least one element selected from Mn, W, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, and Ta),
The first particles include at least one selected from the group consisting of lithium niobium composite oxide, lithium phosphorus composite oxide, and lithium silicon composite oxide,
The amount of elements other than lithium and oxygen in the first particles is 0.02% by mass or more and 5.2% by mass or less based on the entire positive electrode active material,
The lithium niobium composite oxide consists of lithium, oxygen, and niobium,
A portion of the first particles are present bonded to the surface of the first lithium metal composite oxide, and a portion of the first particles are bonded to the surface of the first lithium metal composite oxide. is a separate positive electrode for lithium-ion secondary batteries. The positive electrode _for a lithium ion secondary battery according to claim 9, wherein the first particles contain at least one of _LiNbO3 and _Li3NbO4 . A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material according to any one of claims 1 to 4, a negative electrode, and an electrolyte.

Images