JP7347062B2 - Positive electrode active material 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 comprising a lithium transition metal-containing composite oxide, and a lithium ion secondary battery using the positive electrode active material for a lithium ion secondary battery as a positive electrode material.

In recent years, with the spread of mobile electronic devices such as smartphones, tablet terminals, digital cameras, and notebook computers, there is a strong desire to develop small and lightweight secondary batteries with high energy density. Furthermore, there is a strong desire to develop high-capacity, high-output secondary batteries as power sources for electric vehicles such as hybrid electric vehicles, plug-in hybrid electric vehicles, and battery-powered electric vehicles.

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, a non-aqueous electrolyte, a solid electrolyte, etc. The active material used for the negative and positive electrodes includes a material that can desorb and insert lithium. used. As a non-aqueous electrolyte, there is a non-aqueous electrolyte made by dissolving a lithium salt as a supporting salt in an organic solvent, and as a solid electrolyte, there is an inorganic or organic solid electrolyte that is nonflammable and has lithium ion conductivity. be.

Among lithium ion secondary batteries, lithium ion secondary batteries that use a lithium transition metal-containing composite oxide with a layered rock salt type or spinel type crystal structure as the positive electrode material can obtain a voltage of 4V class, so they have high energy. Research and development and practical application of batteries with high density are progressing.

Positive electrode materials for lithium ion secondary batteries include lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, lithium nickel composite oxide (LiNiO ₂ ), which uses nickel, which is cheaper than cobalt, and lithium nickel manganese cobalt. Composite oxide (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), lithium manganese composite oxide using manganese (LiMn ₂ O ₄ ), lithium nickel manganese composite oxide (LiNi _0.5 Mn _0. A positive electrode active material made of a lithium transition metal-containing composite oxide such as ₅ O ₂ ) has been proposed.

In recent years, among these lithium transition metal-containing composite oxides, at least nickel, manganese, and transition metals, including lithium nickel manganese cobalt composite oxide (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), have been developed. A ternary positive electrode active material made of a lithium nickel manganese cobalt composite oxide (NMC) containing cobalt has excellent thermal stability, high capacity, good battery capacity cycle characteristics, and low resistance. It is attracting attention as a material that can provide high output. Lithium nickel manganese cobalt composite oxide, like lithium cobalt composite oxide and lithium nickel composite oxide, is a compound having a layered rock salt type crystal structure.

Regarding lithium-transition metal-containing composite oxides, the focus of development is on increasing output by reducing their internal resistance. In particular, in power supply applications for electric vehicles, there is a high level of demand for higher output by further reducing internal resistance.

In order to improve the output characteristics and cycle characteristics of positive electrode active materials made of lithium-transition metal-containing composite oxides such as lithium-nickel-containing composite oxides and lithium-nickel-manganese-cobalt-containing composite oxides, it is necessary to use materials with small particle size and narrow particle size distribution. It is important that it is composed of particles. Particles with a small particle size have a large specific surface area, and when used as a positive electrode active material, they can secure a sufficient reaction area with the electrolyte. The travel distance can be shortened. Therefore, by using particles with a small particle size, it is possible to reduce the positive electrode resistance. Furthermore, by using particles with a narrow particle size distribution, the voltage applied to the particles within the electrode can be made uniform, so it is possible to suppress a decrease in battery capacity due to selective deterioration of the fine particles.

In order to further improve the output characteristics, research and development is also underway to improve the particle structure of lithium-transition metal-containing composite oxides. For example, in order to improve the output characteristics, it is considered effective to form a space inside the positive electrode active material into which the electrolyte can enter. By adopting such a structure, the reaction area with the electrolyte can be increased compared to a positive electrode active material with a solid structure with a similar particle size, which can significantly reduce the positive electrode resistance. It becomes possible. It is known that the positive electrode active material inherits the particle properties of the transition metal-containing composite hydroxide that is its precursor. In other words, in order to obtain a positive electrode active material with spaces, it is necessary to appropriately control the particle size, particle size distribution, particle structure, etc. of the secondary particles of the transition metal-containing composite hydroxide, which is the precursor thereof. It becomes necessary.

For example, in JP2012-246199A and WO2012/131881A, a crystallization reaction that is clearly separated into two stages: a nucleation process in which nucleation is mainly performed, and a particle growth process in which grains are mainly grown. discloses a method for producing transition metal-containing composite hydroxide particles that serve as a precursor of a positive electrode active material. In these methods, by appropriately adjusting the pH value and reaction atmosphere in the nucleation process and particle growth process, a low-density core consisting of small particles with a narrow particle size distribution and fine primary particles and a plate-like Alternatively, we have obtained transition metal-containing composite hydroxide particles consisting of a high-density outer shell made of acicular primary particles, and the positive electrode active material obtained from such composite hydroxide particles has a hollow structure. This increases the contact area with the electrolyte, making it possible to improve output characteristics.

WO 2014/181891 and JP 2018-104276 disclose that the pH value of an aqueous nucleation solution containing at least a metal compound containing a transition metal and an ammonium ion donor is set to 12.0 to 14.0. A nucleation step in which the nuclei are generated by controlling the pH value of the particle growth aqueous solution containing the generated nuclei to be lower than the pH value of the nucleation step and 10.5 to 12.0. The nucleation step and the initial stage of the particle growth step are made into a non-oxidizing atmosphere, and at a predetermined timing in the particle growth step, after switching to an oxidizing atmosphere, the particle growth step is controlled to grow. A method for producing transition metal-containing composite hydroxide particles is disclosed, which comprises controlling the atmosphere to switch to an oxidizing atmosphere at least once. According to this method, the particle size is small and the particle size distribution is narrow, and the core is formed by agglomeration of plate-shaped or needle-shaped primary particles, and the fine primary particles are agglomerated outside the center. It is said that it is possible to obtain transition metal-containing composite hydroxide particles having two laminated structures in which low-density layers formed by agglomeration and high-density layers formed by agglomeration of plate-shaped primary particles are alternately laminated.

A positive electrode active material using these transition metal-containing composite hydroxide particles as a precursor has a small particle size, a narrow particle size distribution, and a multilayer structure having a hollow structure or a space. Therefore, in secondary batteries using these positive electrode active materials, it is possible to simultaneously improve battery capacity, output characteristics, and cycle characteristics.

Japanese Patent Application Publication No. 2012-246199 WO2012/131881 publication WO2014/181891 publication Japanese Patent Application Publication No. 2018-104276

However, since there is room for improvement in the output characteristics of the positive electrode active materials described in these documents, further improvement in the output characteristics is required.

In view of the above problems, an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that can exhibit excellent output characteristics when used in a lithium ion secondary battery. do.

The positive electrode active material for lithium ion secondary batteries of the present invention is
General formula (A): Li _1+u Ni _x Co _y Mn _z M _t O ₂ (however, -0.05≦u≦0.5, x+y+z+t=1, 0.3≦x≦0.9, 0≦y≦ 0.5, 0≦z≦0.5, 0<t≦0.05, M is Mg, Al, Si, Ca, Ti, Si, V, Cr, Zr, Nb, Mo, Hf, Ta, and consisting of lithium transition metal-containing composite oxide particles having a composition represented by (one or more additive elements selected from W) and a layered rock salt type crystal structure,
The lithium transition metal-containing composite oxide particles are composed of secondary particles that are aggregated primary particles,
The secondary particles have a 50% cumulative diameter d50 determined from a particle size distribution measurement value of 3.0 μm or more and 7.0 μm or less, a BET specific surface area of 1.8 m ² /g or more and 5.3 m ² /g or less, and 0. Pore peak diameter in the pore distribution obtained by a nitrogen gas adsorption method of .01 μm or more and 0.20 μm or less, log differential pore in the range of the pore peak diameter of 0.008 cc/g or more and 0.03 cc/g or less has a volume [dV/d (logD)], and
Among the primary particles, for each of the plurality of primary particles having a primary particle diameter in the range of 0.1 μm or more and 1.0 μm or less, the concentration of the additive element M is measured at multiple locations by cross-sectional STEM or TEM-EDX analysis. However, when the coefficient of variation of the concentration of the additive element M is calculated, which is the standard deviation of the concentration of the additive element M divided by the average concentration of the additive element M, the coefficient of variation is 1.5 or less. .

The 50% cumulative diameter d50 of the secondary particles is preferably 4.0 μm or more and 6.0 μm or less.

It is an index indicating the spread of the particle size distribution of the secondary particles [(d90-d10)/d50] (where d10 is the 10% cumulative diameter obtained from the particle size distribution measurement value, and d90 is the 10% cumulative diameter obtained from the particle size distribution measurement value. 90% cumulative diameter) is preferably 1.0 or less.

It is preferable that the value of d90/d10 of the secondary particles is 1.0 or more and 2.0 or less.

The secondary particles have an agglomerated part, one or more communication holes that communicate with the outer peripheral part of the agglomerated part, and two or more spaces that exist inside the agglomerated part and communicate with the communication holes. It is preferable. It is preferable that the number of communicating holes is two or more. It is preferable that the two or more spaces communicate with the communication hole or another space.

The additive element M is preferably one or more selected from Mg, Al, Si, Ti, Zr, Nb, Mo, and W. The additional element M is preferably W.

The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte (nonaqueous electrolyte secondary battery), or a positive electrode, a negative electrode, and a solid electrolyte (solid electrolyte secondary battery), The positive electrode active material used in the positive electrode is characterized in that the positive electrode active material for a lithium ion secondary battery of the present invention is used.

Since the positive electrode active material for lithium ion secondary batteries of the present invention has a sufficiently large contact area with the nonaqueous electrolyte or solid electrolyte, insertion and desorption of lithium ions at the particle interface is promoted, improving its output characteristics. It is possible to do so. Therefore, a lithium ion secondary battery to which the positive electrode active material for lithium ion secondary batteries of the present invention is applied can provide high output characteristics, and therefore has great industrial significance.

FIG. 1 is an SEM photograph of a cross section of secondary particles constituting a positive electrode active material for a lithium ion secondary battery according to Example 1 of the present invention. FIG. 2 is an SEM photograph of a cross section of secondary particles constituting the positive electrode active material for a lithium ion secondary battery of Comparative Example 1. FIG. 3 is a schematic cross-sectional view of a 2032 type coin-shaped battery used for battery evaluation. FIG. 4 is a graph showing a measurement example of impedance evaluation. FIG. 5 is a schematic explanatory diagram of the equivalent circuit used in the analysis.

1. Positive electrode active material for lithium ion secondary batteries The positive electrode active material for lithium ion secondary batteries (hereinafter referred to as "positive electrode active material") of the present invention is a lithium transition metal-containing composite oxide particle (hereinafter referred to as "composite oxide particle"). ), and is characterized by meeting the following requirements (1) to (7).

(1) Composition The composite oxide particles have the general formula (A): Li _1+u Ni _x Co _y Mn _z M _t O ₂ (however, -0.05≦u≦0.5, x+y+z+t=1, 0.3≦ x≦0.9, 0≦y≦0.5, 0≦z≦0.5, 0<t≦0.05, M is Mg, Al, Si, Ca, Ti, Si, V, Cr, Zr , Nb, Mo, Hf, Ta, and W).

In the positive electrode active material of the present invention, the value of u, which indicates the excess amount of lithium (Li), is in the range of -0.05 to 0.5. It is preferably in the range of 0 to 0.50, more preferably in the range of 0 to 0.35. By regulating the value of u within the above range, it is possible to improve the output characteristics and battery capacity of a lithium ion secondary battery using this positive electrode active material as a positive electrode material. On the other hand, if the value of u is less than -0.05, the positive electrode resistance of the secondary battery will increase, so it may not be possible to improve the output characteristics. If it exceeds 0.5, not only the initial discharge capacity will decrease, but also the positive electrode resistance may increase.

Nickel (Ni) is an element that contributes to increasing the potential and capacity of a secondary battery, and the value of x indicating its content is in the range of 0.3 to 0.9. It is preferably in the range of 0.4 to 0.7, more preferably in the range of 0.4 to 0.6. If the value of x is less than 0.3, it may not be possible to sufficiently improve the energy density of a secondary battery using this positive electrode active material. When the value of x exceeds 0.9, the content of other elements that improve output characteristics and durability decreases, and there is a possibility that sufficient characteristics as a positive electrode active material cannot be obtained.

Manganese (Mn) is an element that contributes to improving thermal stability, and the value of y indicating its content is in the range of 0 to 0.5. It is preferably in the range of 0.1 to 0.4. If the value of y exceeds 0.5, Mn may be eluted from the positive electrode active material during high-temperature operation, and charge-discharge cycle characteristics may deteriorate.

Cobalt (Co) is an element that contributes to improving charge/discharge cycle characteristics and output characteristics, and the value of z indicating its content is in the range of 0 to 0.5. It is preferably in the range of 0.1 to 0.4. When the value of z exceeds 0.5, the initial discharge capacity of a secondary battery using this positive electrode active material may decrease.

The positive electrode active material of the present invention can contain the following additive element M. Additional elements M include magnesium (Mg), aluminum (Al), silicon (Si), calcium (Ca), titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), and niobium (Nb). , molybdenum (Mo), hafnium (Hf), tantalum (Ta), and tungsten (W).

The additive element M is preferably at least one selected from the group consisting of Mg, Al, Si, Ti, Zr, Nb, Mo, and W, and more preferably W.

The value of t, which indicates the content of the additive element M, is in the range of 0 to 0.05. It is preferably in the range of 0.001 to 0.03. When the value of t exceeds 0.05, the amount of metal elements contributing to the Redox reaction decreases, resulting in a decrease in the battery capacity of the secondary battery.

The additive element M may be dispersed inside the composite oxide particles, or may be coated on the surface of the composite oxide particles. In either case, it is preferable to control the content of the additive element M to be within the above range.

The composite oxide particles of the present invention are preferably composed of a lithium nickel manganese cobalt-containing composite oxide (NMC) containing Ni, Mn, and Co as main transition metals. In this case, the composite oxide particles have the general formula (B): Li _1+u Ni _{x Mny} Co _z W _s M _t O ₂ (-0.05≦u≦0.5, x+y+z+s+t=1, 0.3≦x ≦0.7, 0.15≦y≦0.4, 0.15≦z≦0.4, 0.001≦s≦0.03, 0≦t≦0.05, M is Mg, Al, It is preferable to have a composition of one or more additive elements selected from Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.

The composition of the positive electrode active material can be confirmed by Rietveld analysis of parameters obtained by inductively coupled plasma emission spectroscopy and powder X-ray diffraction.

When the positive electrode active material of the present invention has a composition represented by the above general formula (A) or general formula (B), it is a layered rock salt type and has a hexagonal crystal structure.

(2) Particle structure In the positive electrode active material of the present invention, the composite oxide particles are composed of secondary particles formed by agglomeration of a plurality of primary particles.

The secondary particles have an agglomerated part, one or more communication holes that communicate with the outer peripheral part of the agglomerated part, and two or more spaces that exist inside the agglomerated part and communicate with the communication holes. It is preferable.

It is preferable that the two or more spaces communicate with the communication hole or another space. It is also preferable that the number of communicating holes is two or more.

When the composite oxide particles have such a secondary particle structure, it becomes possible to easily realize the particle properties of the positive electrode active material of the present invention, which will be described later.

If the communication holes on the outer periphery do not communicate with the internal space, the space inside the particle will become isolated, and the electrolyte will not be able to enter the inside of the composite oxide particle (secondary particle) sufficiently. Since a reaction field cannot be secured, desired battery performance may not be fully expressed in secondary batteries using this positive electrode active material. The two or more internal spaces can each directly communicate with the communication hole in the outer circumference. Alternatively, it can also communicate with the communication hole in the outer peripheral portion through another communication hole.

Due to this structure, the electrolyte sufficiently penetrates into the space inside the particle through the communication holes on the outer periphery, so lithium can be desorbed and inserted not only from the surface of the particle but also inside the particle. Therefore, a sufficient reaction field is secured, and the internal resistance of the battery can be significantly reduced.

Therefore, when a lithium ion secondary battery is constructed using a positive electrode active material having such a structure as a positive electrode material, the output characteristics can be further improved.

(3) Particle Size In the positive electrode active material of the present invention, the secondary particles have a 50% cumulative diameter d50 determined from a particle size distribution measurement value of 3.0 μm or more and 7.0 μm or less. The 50% cumulative diameter d50 of the secondary particles is preferably 4.0 μm or more and 6.5 μm or less, more preferably 4.0 μm or more and 6.0 μm or less.

More specifically, the 50% cumulative diameter d50 is the particle size at which the cumulative volume calculated from the integrated volume value measured with a laser beam diffraction scattering particle size analyzer is 50% of the total volume of all particles (the total volume is 100%). When calculating the cumulative curve of the particle size distribution in %, this is the particle size at the point where the cumulative curve becomes 50%.

If the d50 of the particles constituting the positive electrode active material is within the above range, it is possible not only to increase the battery capacity per unit volume of a secondary battery using this positive electrode active material, but also to improve safety and output characteristics. can do.

On the other hand, if the 50% cumulative diameter d50 is less than 3.0 μm, the filling properties of the positive electrode active material may deteriorate, and the battery capacity per unit volume may not be increased. When the 50% cumulative diameter d50 exceeds 7.0 μm, the reaction area of the positive electrode active material decreases and the interface with the electrolyte decreases, so it may be difficult to improve the output characteristics.

(4) BET specific surface area In the positive electrode active material of the present invention, the secondary particles have a BET specific surface area of 1.8 m ² /g or more and 5.3 m ² /g or less. The BET specific surface area is preferably 2.0 m ² /g or more and 5.0 m ² /g or less, more preferably 2.5 m ² /g or more and 4.5 m ² /g or less.

The BET specific surface area of the secondary particles is an index representing the particle properties (contact area with the electrolyte) of the positive electrode active material. The secondary particles have an agglomerated part, one or more communication holes that communicate with the outer periphery of the agglomerated part, and two or more spaces that are present inside the agglomerated part and communicate with the communication holes. In this case, this BET specific surface area represents a surface area including communication pores (openings) existing on the outer periphery of the secondary particles and micro cavities (spaces) inside the secondary particles.

The BET specific surface area is measured by the BET method using nitrogen gas adsorption.

If the BET specific surface area of the secondary particles is less than 1.8 m ² /g, when a secondary battery is constructed using this positive electrode active material, a sufficient reaction area with the electrolyte cannot be secured, There is a possibility that the output characteristics will not be improved sufficiently. When the BET specific surface area of the secondary particles exceeds 5.3 m ² /g, the reaction area between the positive electrode active material and the electrolyte becomes too large, which may reduce the durability of the secondary battery.

(5) Pore peak diameter In the positive electrode active material of the present invention, the secondary particles have a pore peak diameter in a pore distribution obtained by a nitrogen gas adsorption method of 0.01 μm or more and 0.20 μm or less. The pore peak diameter is
It is preferably 0.02 μm or more and 0.18 μm or less, more preferably 0.03 μm or more and 0.15 μm or less.

Specifically, the pore peak diameter is defined as the diameter at which the pore peak exists in the pore distribution obtained by obtaining the pore distribution from the adsorption side isotherm of the secondary particles using a nitrogen gas adsorption method. Ru.

The pore peak diameter of the secondary particles is an index representing the particle properties (average pore diameter) of the positive electrode active material. More specifically, when the pore peak diameter of the secondary particles constituting the positive electrode active material is within the above range, the particles have the characteristic of having a porous shape with many fine pores. It means that.

If the pore peak diameter of the secondary particles is less than 0.01 μm, the pore diameter is so small that particle strength cannot be maintained and the particles are crushed, potentially impairing battery performance. If the pore peak diameter of the secondary particles exceeds 0.20 μm, when a secondary battery is constructed using this positive electrode active material, a sufficient reaction area with the electrolyte cannot be secured, and the output characteristics cannot be fully demonstrated. There is a possibility that it cannot be done.

(6) Log differential pore volume In the positive electrode active material of the present invention, the secondary particles have a log differential pore volume [dV/d (logD)]. The log differential pore volume is preferably from 0.01 cc/g to 0.02 cc/g, more preferably from 0.015 cc/g to 0.02 cc/g.

The log differential pore volume [dV/d (logD)] at the pore peak diameter is an index representing the particle properties (volume of total cavities) of the positive electrode active material. More specifically, the log differential pore volume [dV/d (logD)] at the pore peak diameter is determined for each pore diameter based on the pore distribution determined from the adsorption side isotherm by the gas adsorption method. The value obtained by dividing the obtained differential pore volume dV by the difference value d (logD) treated as a logarithm of the pore diameter is the value at the pore peak diameter in the log differential pore volume distribution. When the log differential pore volume [dV/d (logD)] of the secondary particles constituting the positive electrode active material is within the above range, an appropriate amount of space is formed inside the particles, and a sufficient amount of space is formed inside the particles. This means that the BET specific surface area can be obtained.

If the log differential pore volume [dV/d (logD)] at the pore peak diameter is less than 0.008 cc/g, sufficient space will not be formed inside the secondary particles, resulting in insufficient BET specific surface area. may not be obtained. If the log differential pore volume [dV/d (logD)] at the pore peak diameter exceeds 0.03 cc/g, there will be many structural spaces inside the secondary particles, and the bulk density will decrease. As a result, when a secondary battery is constructed using the positive electrode active material, a sufficient battery capacity per unit volume may not be obtained.

(7) Coefficient of variation in the concentration of the additive element M In the positive electrode active material of the present invention, the coefficient of variation (CV) indicating the variation in the concentration of the additive element M in the primary particles constituting the secondary particles is 1.5. It is as follows. The coefficient of variation (CV) is preferably 1.3 or less, more preferably 1.2 or less.

The coefficient of variation in the concentration of the additive element M is an index indicating the variation in the concentration of the additive element M in the entire primary particle including the inside of the grain and the grain boundary within the primary particle.

The coefficient of variation in the concentration of the additive element M is determined as follows. First, the positive electrode active material is processed into thin pieces using a focused ion beam device (FIB), an ion milling device, or the like to expose a cross section of the secondary particles. Using a scanning transmission electron microscope (STEM) or a transmission electron microscope (TEM), two connected primary particles having a primary particle diameter in the range of 0.1 μm or more and 1.0 μm or less among the cross sections of the secondary particles With the aim of Find the concentration of the additive element M at the location.

From the obtained concentration data of the additive element M, the average concentration (μ) of the additive element M and the standard deviation (σ) of the concentration of the additive element M are determined.

The coefficient of variation (CV) of the concentration of the additive element M is determined by the formula: CV=σ/μ.

When the coefficient of variation (CV) of the concentration of the additive element M exceeds 1.5, the distribution of the additive element M in the primary particles and/or the secondary particles becomes biased, and secondary particles using this positive electrode active material In secondary batteries, battery performance such as battery capacity and output characteristics may be impaired.

(8) Particle size distribution Although optional, in the positive electrode active material of the present invention, [(d90-d10)/d50], which is an index indicating the spread of the particle size distribution of the secondary particles, is preferably 1.0 or less, More preferably it is 0.7 or less, still more preferably 0.6 or less. A positive electrode active material made of secondary particles with such a narrow particle size distribution has a small proportion of fine particles and coarse particles, so a secondary battery using this material has poor safety, cycle characteristics, and output characteristics. It will be excellent.

When [(d90-d10)/d50] exceeds 1.0, the proportion of fine particles and coarse particles in the positive electrode active material increases. For example, if the proportion of fine particles is high, the secondary battery will easily generate heat due to local reactions of the fine particles, which will not only reduce safety, but also cause selective deterioration of the fine particles. Characteristics may be inferior. Furthermore, if the proportion of coarse particles is high, a sufficient reaction area between the electrolyte and the positive electrode active material cannot be ensured, resulting in poor output characteristics.

Assuming industrial scale production, it is not practical to use a positive electrode active material with an excessively small [(d90-d10)/d50]. Therefore, in consideration of cost and productivity, the lower limit of [(d90-d10)/d50] is preferably about 0.3.

In addition, the meaning and calculation method of d10 and d90 in the index showing the spread of particle size distribution [(d90-d10)/d50] are the same as in the case of d50 described above, and d90 is the volume of each particle as the particle size. The particle size, d10, is the particle size whose cumulative volume is 90% of the total volume of all particles, and the cumulative volume is the sum of all particles. It means the particle size that accounts for 10% of the volume.

(9) d90/d10
Optionally, in the positive electrode active material of the present invention, the ratio of the d90 value to the d10 value (d90/d10) determined from the particle size distribution measurement value of the secondary particles is 1.0 or more and 2.0 It is preferably the following, more preferably 1.2 or more and 1.8 or less.

If the value of d90/d10 of the secondary particles constituting the positive electrode active material is within this range, it is possible not only to increase the battery capacity per unit volume of the secondary battery using this positive electrode active material, but also to ensure safety. It is also possible to improve performance and output characteristics. If the value of d90/d10 of the secondary particles exceeds 2.0, the reaction area of the positive electrode active material decreases and the interface with the electrolyte decreases, so it may be difficult to improve the output characteristics. .

(10) Crystallite diameter determined from the X-ray diffraction pattern of the (003) plane Although it is optional, half of the peak of the (003) plane by X-ray diffraction of the composite oxide particles constituting the positive electrode active material of the present invention When the crystallite diameter of the primary particles is determined from the value range using the Scherrer equation, the crystallite diameter is preferably in the range of 300 Å to 1500 Å, more preferably in the range of 400 Å to 1300 Å, and still more preferably in the range of 700 Å to 1250 Å. in range. A positive electrode active material having a crystallite diameter in such a range has extremely high crystallinity, and can reduce the positive electrode resistance of a secondary battery and improve its output characteristics.

If the crystallite size of the (003) plane is less than 300 Å, the primary particles will be so fine that the pores existing between the primary particles in the positive electrode active material will become too fine, making it difficult for electrolyte to penetrate into the positive electrode active material. As a result, the reaction area with the electrolyte decreases, and the output characteristics of the secondary battery deteriorate. When the crystallite diameter of the (003) plane exceeds 1500 Å, the primary particles become too coarse and the proportion of pores in the secondary particles is extremely reduced, reducing the number of entry routes for the electrolyte. The reaction area of the secondary battery decreases, and the output characteristics of the secondary battery deteriorate.

2. Manufacturing method of positive electrode active material for lithium ion secondary battery The positive electrode active material of the present invention is not limited by its manufacturing method as long as it has the above-mentioned composition, crystal structure, particle structure, and particle properties. .

However, by manufacturing the positive electrode active material of the present invention using the manufacturing method described below, it becomes possible to easily manufacture a positive electrode active material having the above-mentioned composition, crystal structure, particle structure, and particle properties. .

Specifically, in the positive electrode active material of the present invention, a transition metal-containing composite hydroxide such as a nickel-manganese-cobalt-containing composite hydroxide is prepared by switching between an oxidizing atmosphere and a non-oxidizing atmosphere during the crystallization process. It can be obtained by mixing the composite hydroxide or heat-treated particles obtained by heat-treating the composite hydroxide with a lithium compound, and firing the resulting lithium mixture.

Each step will be described in detail below, taking as an example the case where the composite oxide particles are lithium nickel manganese cobalt-containing composite oxide particles.

(1) Crystallization process: Supply appropriate amounts of water, sodium hydroxide aqueous solution, potassium carbonate aqueous solution, and ammonia water into the reaction tank, and the pH value is 11 or more and 13 or less based on the liquid temperature of 25°C, and the ammonium ion concentration is 9 g/L or more. A pre-reaction aqueous solution adjusted to have a concentration of 15 g/L or less is prepared. On the other hand, a compound of nickel sulfate, manganese sulfate, cobalt sulfate, and additional element M (sulfate, sodium salt, etc.) is prepared such that the molar ratio of each metal element (Ni, Co, Mn, M) satisfies the above formula (A). It is desirable to prepare an aqueous raw material solution having a concentration of 1.0 mol/L or more and 3.0 mol/L or less by dissolving it in water so as to satisfy the above conditions.

In order to suppress the coefficient of variation in the concentration of the additive element M that is dissolved inside the particles, manganese sulfate and cobalt sulfate are added to prevent local coprecipitation of the additive element M around the transition metal element. Dissolve each metal element (Ni, Co, Mn) in water so that the molar ratio satisfies the above formula (A), prepare a raw material aqueous solution with a concentration of 1.0 mol/L or more and 3.0 mol/L or less, and add the added elements. It is more preferable to separately prepare an aqueous solution containing the compound M and add it to the raw material aqueous solution so that the amount of the additive element M satisfies the above formula (A).

Next, nucleation is performed by supplying the raw material aqueous solution to the pre-reaction aqueous solution while blowing in a non-oxidizing atmosphere gas such as argon gas or nitrogen having an oxygen concentration of 2% by volume or less. At this time, it is desirable to supply the aqueous sodium hydroxide solution, the aqueous potassium carbonate solution, and the aqueous ammonia at appropriate times to maintain the pH value and ammonium ion concentration of the aqueous nucleation solution within the above-mentioned ranges.

After the nucleation is completed, an acid such as sulfuric acid is added to adjust the pH value to 10 or more and 12 or less based on the liquid temperature of 25°C and smaller than the pH value in the nucleation step. , forming an aqueous solution for particle growth. After confirming that the pH value has reached a predetermined value, the raw material aqueous solution is supplied to grow the nuclei (particles) generated in the nucleation step. Note that when an aqueous solution containing a compound of the additive element M is prepared separately, the aqueous solution containing the compound of the additive element M is used only in the particle growth process or at a specific stage of the particle growth process (for example, in the third stage). It can also be added only in stages (stage and stage 4).

As the first stage of particle growth, crystallization in a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less is continued from the start of the particle growth process while stirring using a stirrer for a certain period of time.

As a second step, while continuing to supply the raw material aqueous solution, air or oxygen is circulated in the reaction tank using a diffuser tube with a pore size in the range of 100 μm to 1 cm, and the reaction atmosphere is changed to an oxygen concentration of 21% by volume or more. Perform switching operation 1 to adjust the oxidizing atmosphere. After the start of switching operation 1, crystallization is performed while maintaining an oxidizing atmosphere and stirring using a stirrer for a certain period of time.

As the third step, while continuing to supply the raw material aqueous solution, a non-oxidizing atmosphere gas is circulated in the reaction tank using an aeration pipe to adjust the reaction atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. , perform switching operation 2. After the start of switching operation 2, crystallization is performed while maintaining a non-oxidizing atmosphere and stirring using a stirrer for a certain period of time.

As a fourth step, while continuing to supply the raw material aqueous solution, an oxidizing atmosphere gas is circulated in the reaction tank using an aeration pipe, and the reaction atmosphere is adjusted to an oxidizing atmosphere with an oxygen concentration of 21% by volume or more. Perform switching operation 3. After the start of switching operation 3, crystallization is performed while maintaining an oxidizing atmosphere and stirring using a stirrer for a certain period of time.

As the fifth step, while continuing to supply the raw material aqueous solution, a gas in a non-oxidizing atmosphere is circulated in the reaction tank using an aeration pipe, and the reaction atmosphere is adjusted to a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. , perform switching operation 4. After the start of switching operation 4, crystallization is performed while maintaining a non-oxidizing atmosphere and stirring using a stirrer for a certain period of time.

The time of each stage in the particle growth process is set as 2nd stage ≦ 4th stage < 1st stage ≦ 3rd stage ≦ 5th stage. It is desirable that the crystallization time for the entire grain growth process be within 8 hours.

The particle growth step can also similarly include switching operations 5 and 6 to provide a sixth stage in an oxidizing atmosphere and a seventh stage in a non-oxidizing atmosphere. Furthermore, switching operations 7 and 8 can be carried out to provide an eighth stage in an oxidizing atmosphere and a ninth stage in a non-oxidizing atmosphere. Additionally, switching operations 9 and 10 can also be carried out to provide a ninth stage in an oxidizing atmosphere and an eleventh stage in a non-oxidizing atmosphere.

In these cases, the time for each stage in the particle growth process is as follows: 2nd stage ≦ 4th stage ≦ 6th stage ≦ 8th stage ≦ 10th stage < 1st stage ≦ 3rd stage ≦ 5th stage ≦ 7th stage Stage≦9th stage≦11th stage.

After completion of crystallization, the obtained product is washed with water, filtered, and dried to obtain predetermined nickel-cobalt-manganese-containing composite hydroxide particles.

In the particle growth step, it is desirable to maintain the pH value and ammonium ion concentration of the particle growth aqueous solution within the above range by supplying an aqueous sodium hydroxide solution and aqueous ammonia at appropriate times throughout this step.

The composite hydroxide particles obtained by such a crystallization step have a particle structure in which a low density layer and a high density layer are stacked on each other around a high density center. By using composite hydroxide particles having such a particle structure as a precursor, an agglomerated part and one or more communication holes that communicate with the outer peripheral part of the agglomerated part, and one or more communication holes that exist inside the agglomerated part and that communicate with each other. Composite oxide particles having a particle structure having two or more spaces communicating with pores can be easily obtained.

(2) Mixing step The mixing step is a step in which a lithium compound is mixed with the above-described composite hydroxide particles or heat-treated particles obtained by heat-treating the composite hydroxide particles to obtain a lithium mixture.

In the mixing step, the ratio of the sum of the numbers of metal atoms other than Li in the lithium mixture, specifically Ni, Mn, Co, and the additional element M (Me), to the number of Li atoms (Li) (Li/Me) is 0.95 to 1.5, preferably 1.0 to 1.5, more preferably 1.0 to 1.35, and even more preferably 1.0 to 1.2. , mix the composite hydroxide particles and the lithium compound. That is, since Li/Me does not change before and after the firing process, the composite hydroxide and the lithium compound should be mixed so that the Li/Me in the mixing process becomes the desired Li/Me of the positive electrode active material. Is required.

The lithium compound used in the mixing step is not particularly limited, but it is preferable to use lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture thereof from the viewpoint of easy availability. In particular, in consideration of ease of handling and stability of quality, it is preferable to use lithium hydroxide or lithium carbonate.

A common mixer can be used for mixing. For example, a shaker mixer, Loedige mixer, Julia mixer, V-blender, etc. can be used.

(3) Calcination process When using lithium hydroxide or lithium carbonate as the lithium compound, after the mixing process and before the firing process, the lithium mixture is heated at a temperature lower than the firing temperature described below and from 350°C to A calcination step of calcination at 800°C, preferably 450°C to 750°C can also be performed. Thereby, Li can be sufficiently diffused into the composite hydroxide particles, and more uniform composite oxide particles can be obtained.

Note that the holding time at the above temperature is preferably 1 hour to 10 hours, more preferably 3 hours to 6 hours. Further, the atmosphere in the calcination step is preferably an oxidizing atmosphere, similar to the firing step described later, and more preferably an atmosphere with an oxygen concentration of 18% by volume to 100% by volume.

(4) Firing process The firing process is a process in which the lithium mixture obtained in the mixing process is fired under predetermined conditions, and Li is diffused and reacted in the composite hydroxide to obtain composite oxide particles.

The furnace used in the firing process is not particularly limited, and may be any furnace that can heat the lithium mixture in the atmosphere or in an oxygen stream. However, from the viewpoint of maintaining a uniform atmosphere in the furnace, an electric furnace that does not generate gas is preferable, and either a batch type or a continuous type electric furnace can be suitably used. Regarding this point, the same applies to the furnace used in the heat treatment step and the calcination step.

a) Firing temperature The calcination temperature of the lithium mixture is set at 850 °C to 980°C. If the firing temperature is less than 850 °C, the composite hydroxide particles and Li will not react sufficiently, and excess Li or unreacted composite hydroxide may remain, or the resulting positive electrode active material may have insufficient crystallinity. It becomes something. On the other hand, when the firing temperature exceeds 980°C, the communicating pores and spaces within the composite oxide particles are collapsed, resulting in a decrease in the BET specific surface area of the final positive electrode active material (secondary particles) and pores. This may lead to a decrease in the peak diameter and the log differential pore volume at the pore peak diameter. In addition, the particles of the positive electrode active material are intensely sintered, causing abnormal grain growth and increasing the proportion of irregularly shaped coarse particles.

By controlling the firing temperature, the BET specific surface area of the positive electrode active material (secondary particles), the pore peak diameter, the log differential pore volume at the pore peak diameter, and the 50% cumulative value determined from the particle size distribution measurement value can be improved. It is possible to control the diameter d50 .

The temperature increase rate in the firing step is preferably set to 1°C/min to 10°C/min, more preferably 3°C/min to 8°C/min. Furthermore, during the firing process, at a temperature near the melting point of the lithium compound, it is more preferable to reduce the temperature increase rate and set it to 0.3° C./min to 6° C./min. This allows the composite hydroxide particles and the lithium compound to react more uniformly.

b) Firing time Among the firing times, the holding time at the above-mentioned firing temperature is preferably at least 2 hours, more preferably from 3 hours to 20 hours. If the holding time at the firing temperature is less than 2 hours, Li will not diffuse sufficiently into the composite oxide particles, and excess Li or unreacted composite oxide particles may remain, or the crystallinity of the resulting positive electrode active material may deteriorate. There is a risk that it may become insufficient.

c) Firing atmosphere The atmosphere during firing is preferably an oxidizing atmosphere, more preferably an atmosphere with an oxygen concentration of 18% to 100% by volume, and an atmosphere with an oxygen concentration of 50% to 100% by volume. It is more preferable that It is particularly preferable to use a mixed atmosphere of oxygen at the above oxygen concentration and an inert gas. 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 positive electrode active material may be insufficient.

(5) Crushing step The secondary particles constituting the positive electrode active material obtained by the firing step may be agglomerated or slightly sintered. In such a case, it is preferable to crush the aggregate or sintered body. Thereby, the average particle size and particle size distribution of the obtained positive electrode active material can be adjusted to a suitable range. Note that crushing is a process in which mechanical energy is applied to an aggregate of multiple secondary particles, which is generated due to sinter necking between particles during firing, to separate the secondary particles without destroying them. means an operation to loosen aggregates.

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

As described above, by adjusting the firing conditions, the BET specific surface area, pore peak diameter, log differential pore volume at the pore peak diameter, and particle size distribution measurement value of the positive electrode active material (secondary particles) can be adjusted. It becomes possible to control the 50% cumulative diameter d50 determined from the above, and finally, it is possible to easily obtain the positive electrode active material of the present invention having predetermined particle properties.

3. Lithium Ion Secondary Battery The lithium ion secondary battery of the present invention can have the same configuration as a general non-aqueous electrolyte secondary battery, including constituent members such as a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. . Alternatively, the lithium ion secondary battery of the present invention can have the same configuration as a general solid electrolyte secondary battery, including constituent members such as a positive electrode, a negative electrode, and a solid electrolyte. That is, the present invention is widely applicable to a secondary battery that is charged and discharged by desorption and insertion of lithium ions, from non-aqueous electrolyte secondary batteries to all-solid lithium ion secondary batteries. Note that the embodiments described below are merely illustrative, and the present invention is applied to lithium ion secondary batteries with various changes and improvements based on the embodiments described in this specification. Is possible.

(1) Components a) Positive electrode Using the positive electrode active material of the present invention, a positive electrode for a lithium ion secondary battery is produced, for example, in the following manner.

First, a conductive material and a binder are mixed with the positive electrode active material of the present invention, and if necessary, activated carbon and a solvent for adjusting viscosity are added, and the mixture is kneaded to prepare a positive electrode composite paste. At this time, the mixing ratio of each material in the positive electrode composite material paste is also an important factor that determines the performance of the lithium ion secondary battery. For example, when 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 to 95 parts by mass, similar to the positive electrode of a general lithium ion secondary battery. The content of the conductive material can be 1 part by mass to 20 parts by mass, and the content of the binder can be 1 part by mass to 20 parts by mass.

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-mentioned example, and other methods may be used.

As the conductive material, 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 includes, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, or polyvinylidene. Acrylic acid can be used.

In addition, a solvent that disperses the positive electrode active material, the conductive material, and the activated carbon and dissolves the binder can be added to the positive electrode mixture, if necessary. Specifically, organic solvents such as N-methyl-2-pyrrolidone can be used as the solvent. Moreover, activated carbon can also be added to the positive electrode composite material in order to increase the electric double layer capacity.

b) Negative electrode Metallic lithium, lithium alloy, etc. can be used for the negative electrode. In addition, a negative electrode active material that can absorb and deintercalate lithium ions is mixed with a binder and an appropriate solvent is added to form a paste, which is then applied to the surface of a metal foil current collector such as copper. , dried and compressed to increase the electrode density if necessary.

Examples of negative electrode active materials include materials containing lithium such as metallic lithium and lithium alloys, fired organic compounds such as natural graphite and artificial graphite, and phenol resins that can absorb and desorb lithium ions, and carbon materials such as coke. powder, etc. 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 a solvent for dispersing these negative electrode active materials and binder, N-methyl-2-pyrrolidone or the like can be used. Organic solvents can be used.

c) Separator A separator is placed between a positive electrode and a negative electrode in a lithium ion secondary battery, and has the function of separating the positive electrode and negative electrode and retaining the electrolyte. As such a separator, for example, a thin film made of polyethylene or polypropylene having many fine pores can be used, but there is no particular limitation as long as it has the above-mentioned functions. It is also possible to use a solid electrolyte.

d) Electrolyte The nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery includes a nonaqueous electrolyte prepared by dissolving a lithium salt serving as a supporting salt in an organic solvent.

Organic solvents used in the nonaqueous electrolyte include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate; tetrahydrofuran. , ether compounds such as 2-methyltetrahydrofuran and dimethoxyethane; sulfur compounds such as ethylmethylsulfone and butane sultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate, either singly or in combination of two or more. 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.

Note that the nonaqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

Solid electrolytes used in solid electrolyte secondary batteries such as all-solid lithium ion secondary batteries have the property of being able to withstand high voltage. Solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

As the oxide solid electrolyte, an oxide containing oxygen (O) and having lithium ion conductivity and electronic insulation, such as Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) _{3 , Li3PO4 , Li3PO4NX} , _{LiBO2NX , LiNbO3 , LiTaO3 , Li2SiO3 , Li4SiO4 - Li3PO4} , _{Li4SiO4 - Li3VO4 ,} 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 _ _ _ _} , 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 P _0.4 O ₄ and the like can be used.

Sulfide solid electrolytes include sulfides that contain sulfur (S) and have lithium ion conductivity and electronic insulation, such as Li ₂ S-SiS ₂ , Li ₂ S-P ₂ S ₅ , LiI -Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ S ₅ , LiI-Li ₂ SB 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 SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , etc. can be used.

As the inorganic solid electrolyte other than the oxide solid electrolyte and the sulfide solid electrolyte, for example, Li ₃ N, LiI, Li ₃ N-LiI-LiOH, etc. can be used.

As the organic solid electrolyte, a polymer compound exhibiting ionic conductivity can be used. For example, polyethylene oxide, polypropylene oxide, copolymers thereof, etc. can be used. Further, the organic solid electrolyte can include a supporting salt (lithium salt).

Note that when a solid electrolyte is used, the solid electrolyte can also be mixed in the positive electrode material in order to ensure contact between the electrolyte and the positive electrode active material.

(2) Structure of lithium ion secondary battery The structure of a lithium ion secondary battery is not particularly limited, and may be a structure consisting of a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte, etc. in a nonaqueous electrolyte secondary battery, or a structure consisting of a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte, etc. in a nonaqueous electrolyte secondary battery, A configuration consisting of a positive electrode, a negative electrode, a solid electrolyte, etc. in a secondary battery can be adopted. Further, the shape of the secondary battery is not particularly limited, and may be in various shapes such as a cylindrical shape or a stacked shape.

In the case of a non-aqueous electrolyte secondary battery, for example, a positive electrode and a negative electrode are laminated with a separator interposed therebetween to form an electrode body, the obtained electrode body is impregnated with a non-aqueous electrolyte, and a positive electrode that communicates with a positive electrode current collector and the outside is formed. The lithium ion secondary battery is completed by connecting the battery to the terminal and between the negative electrode current collector and the negative terminal communicating with the outside using a current collecting lead or the like, and sealing it in a battery case.

(3) Characteristics of lithium ion secondary battery As described above, the lithium ion secondary battery of the present invention uses the positive electrode active material of the present invention as a positive electrode material, and therefore has excellent output characteristics and durability characteristics.

(4) Applications of lithium ion secondary batteries As mentioned above, the lithium ion secondary batteries of the present invention have excellent output characteristics and durability characteristics, and are used in power tools and environmentally friendly vehicles that require high levels of these characteristics. It can be suitably used as a power source.

Hereinafter, the present invention will be explained in detail using Examples and Comparative Examples. Throughout the nucleation process and particle growth process, the pH value of the reaction aqueous solution is measured by a pH controller, and the pH value of the reaction aqueous solution in each process is adjusted based on this measurement value by adjusting the supply amount of the sodium hydroxide aqueous solution. The variation width was controlled within the range of ±0.2.

(Example 1)
a) Production of composite hydroxide [Nucleation process]
First, 17 L of 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 1 hour to make the reaction atmosphere a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. Subsequently, appropriate amounts of 25% by mass sodium hydroxide aqueous solution, 25% by mass potassium carbonate aqueous solution, and 25% by mass aqueous ammonia were supplied into the reaction tank, and the pH value was 12.6 based on the liquid temperature of 25°C and ammonium ion. A pre-reaction aqueous solution was formed by adjusting the concentration to 10 g/L.

At the same time, nickel sulfate, manganese sulfate, and cobalt sulfate were dissolved in water so that the molar ratio of each metal element was Ni:Co:Mn=5:2:3 to prepare a 2 mol/L raw material aqueous solution. .

W was used as the additive element M. In the composite hydroxide particles obtained, the molar ratio of each metal element including the additive element M is set to Ni:Mn:Co:W=5.0:2.0:3.0:0.1. , sodium tungstate dihydrate was dissolved in water to prepare a sodium tungstate aqueous solution.

Next, the raw material aqueous solution was supplied to the pre-reaction aqueous solution at a rate of 115 ml/min to form an aqueous solution for the nucleation step, and nucleation was performed for 1 minute. At this time, 25 mass% sodium hydroxide aqueous solution, 25 mass% potassium carbonate aqueous solution, and 25 mass% aqueous ammonia were supplied at appropriate times to maintain the pH value and ammonium ion concentration of the nucleation aqueous solution within the above range.

[Particle growth process]
After the nucleation was completed, the supply of all aqueous solutions was temporarily stopped, and sulfuric acid was added to adjust the pH value to 11.0 based on the liquid temperature of 25° C., thereby forming an aqueous solution for particle growth. After confirming that the pH value had reached the predetermined value, the raw material aqueous solution was supplied at a constant rate of 100 ml/min, the same as in the nucleation process, and the nuclei (particles) generated in the nucleation process were grown. . Note that the sodium tungstate aqueous solution was supplied at a constant rate of 10 ml/min at the same time as the raw material aqueous solution in the third and fourth stages.

As a first step, crystallization was continued for 35 minutes in a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less from the start of the particle growth process while stirring using a stirrer.

In the second step, while continuing to supply the raw material aqueous solution, air was circulated in the reaction tank using an SUS aeration tube with a pore diameter of 1.0 mm, and the reaction atmosphere was changed to an oxidizing atmosphere with an oxygen concentration of 21% by volume. The atmosphere was adjusted (switching operation 1). After starting switching operation 1, crystallization was performed for 20 minutes while stirring using a stirrer.

As the third step, while continuing to supply the raw material aqueous solution and supplying the sodium tungstate aqueous solution, nitrogen is passed into the reaction tank using an aeration pipe to change the reaction atmosphere to an oxygen concentration of 2% by volume or less. The atmosphere was adjusted to a non-oxidizing atmosphere (switching operation 2). After starting switching operation 2, crystallization was performed for 65 minutes while stirring using a stirrer.

As the fourth step, while continuing to supply the raw material aqueous solution and the sodium tungstate aqueous solution, air was circulated in the reaction tank using an aeration pipe to adjust the oxidizing atmosphere to an oxygen concentration of 21% by volume (switching operation 3). ). After starting switching operation 3, crystallization was performed for 40 minutes while stirring using a stirrer.

As the fifth step, while stopping the supply of the sodium tungstate aqueous solution, while continuing the supply of the raw material aqueous solution, nitrogen is circulated in the reaction tank using an aeration pipe to change the reaction atmosphere to an oxygen concentration of 2% by volume or less. The atmosphere was adjusted to a non-oxidizing atmosphere (switching operation 4). After starting switching operation 4, crystallization was performed for 110 minutes while stirring using a stirrer.

Thereafter, the supply of all aqueous solutions including the raw material aqueous solution was stopped, thereby completing the particle growth process. The obtained product was washed with water, filtered and dried to obtain composite hydroxide particles.

In the particle growth step, 25% by mass aqueous sodium hydroxide solution, 25% by mass potassium carbonate aqueous solution, and 25% by mass aqueous ammonia are appropriately supplied throughout this step, and the pH value and ammonium ion concentration of the aqueous solution for particle growth are controlled. , maintained within the ranges mentioned above.

b) Preparation of positive electrode active material The obtained composite hydroxide was sufficiently mixed with lithium hydroxide using a shaker mixer device so that Li/Me was 1.10, to obtain a lithium mixture (mixed process).

This lithium mixture was heated to 965°C at a heating rate of 1.3°C/min in an oxygen (oxygen concentration: 100% by volume) atmosphere, held at this temperature for 3 hours, and then cooled to room temperature. (firing process).

In order to eliminate agglomeration or slight sintering of the obtained lithium transition metal-containing composite oxide particles, the positive electrode active material was crushed (pulverization step).

c) Evaluation of positive electrode active material [Composition]
The composition of this positive electrode active material was analyzed using an ICP emission spectrometer (ICPE-9000, manufactured by Shimadzu Corporation).

[Particle size distribution]
50% cumulative diameter d50, 10% cumulative diameter determined from the particle size distribution measurement value of the positive electrode active material (secondary particles) using a laser light diffraction scattering particle size analyzer (Microtrac MT3300EXII, manufactured by Microtrac Bell Co., Ltd.) The diameter d10 and the 90% cumulative diameter d90 were measured.

[BET specific surface area, pore peak diameter, log differential pore volume at the pore peak diameter]
Using an independent 4-station type specific surface area/pore distribution measuring device (manufactured by Quantachrome Instruments, QUADRASORB SI), the BET specific surface area of the positive electrode active material (secondary particles), the pore peak diameter in the pore distribution, and , the log differential pore volume at the pore peak diameter was determined.

[Particle structure]
A part of the positive electrode active material was embedded in a resin, and the cross section was made observable by cross-section polisher (IB-19530CP, manufactured by JEOL Ltd.), and then SEM (FE-SEM: JSM, manufactured by JEOL Ltd.) was applied. -6360LA). As shown in FIG. 1, the positive electrode active material of the present invention is formed from secondary particles in which a plurality of primary particles are aggregated, and has about six communicating holes on the outer periphery of the secondary particles. It was confirmed that there were about eight spaces inside, and that the communicating holes on the outer periphery were connected to the internal voids.

[Coefficient of variation]
The cross section of the positive electrode active material (secondary particles) was processed into a thin section using an ion milling device (Cryoion Slicer, IB-09060CIS, manufactured by JEOL Ltd.). Using a TEM (manufactured by JEOL Ltd., JEM-ARM200F), in the cross section of the secondary particles, aim at primary particles (two connected with a grain boundary in between) of approximately 0.5 μm, and measure each primary particle. Electron beam irradiation is performed on a total of 10 points randomly selected from the entire primary particle, including inside the grain and grain boundaries, and the energy of the generated characteristic X-rays is analyzed using EDX (Thermo Fisher Scientific, NSS). The average W concentration (μ), the standard deviation (σ) of the W concentration, and the coefficient of variation (CV) of the W concentration in each primary particle were analyzed. Table 1 shows the measurement results of the W concentration by TEM-EDX analysis of the obtained primary particles.

As a result, the average W concentration (μ) was 1.0, the standard deviation (σ) of W concentration was 1.1, and the coefficient of variation (CV) of W concentration was 1.1.

d) Preparation of secondary battery A 2032 type coin-shaped battery 10 as shown in FIG. 3 was prepared. Specifically, 52.5 mg of the positive electrode active material obtained as described above, 15 mg of acetylene black, and 7.5 mg of PTFE were mixed and pressed at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm. It was molded to produce positive electrode 1.

Next, using this positive electrode 12, a 2032 type coin-shaped battery was fabricated in a glove box with an Ar atmosphere whose dew point was controlled at -60°C. The negative electrode 2 of the 2032 type coin-shaped battery uses lithium metal with a diameter of 14 mm and a thickness of 1 mm, and the non-aqueous electrolyte contains ethylene carbonate (EC) and diethyl carbonate (DEC) with 1M _LiClO4 as the supporting electrolyte. An equal volume mixture of (manufactured by Ube Industries, Ltd.) was used. Further, as the separator 3, a porous polyethylene film having a film thickness of 25 μm was used. In this way, a 2032-type coin-shaped battery 10 was assembled, which included a gasket 4, a wave washer 5, a positive electrode can 6, and a negative electrode can 7.

e) Battery evaluation [Positive electrode resistance]
The positive electrode resistance was measured using the impedance measurement method. A 2032 type coin-type battery was charged at 3.8V, and the positive electrode resistance was measured using the AC impedance method using a frequency response analyzer and a potentiogalvanostat (manufactured by Solartron, 1255B). The impedance spectrum shown in 4 was obtained. In the obtained impedance spectrum, two semicircles are observed in the high frequency region and the intermediate frequency region, and a straight line is observed in the low frequency region, so we constructed the equivalent circuit shown in Figure 5 to analyze the positive electrode interface resistance. did. Here, Rs is bulk resistance, _R1 is positive electrode film resistance, _Rct is electrolyte/positive electrode interface resistance, W is Warburg component, and CPE1 and CPE2 are constant phase elements. Regarding the positive electrode interfacial resistance, the resistance reduction rate with respect to the positive electrode active material of Comparative Example 1, which will be described later, is shown as a reference.

Table 2 shows the overall composition and properties of the obtained positive electrode active material, as well as the properties of the obtained lithium ion secondary battery. Note that Examples 2 to 5, Comparative Example 1, and Comparative Example 2 are also shown in Table 1.

(Example 2)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the lithium mixture was heated to 915° C. and fired by holding at this temperature for 3 hours.

(Example 3)
Except that the sodium tungstate aqueous solution was prepared so that the molar ratio of each metal element was Ni:Mn:Co:W=5.0:2.0:3.0:0.07. A positive electrode active material was obtained and evaluated in the same manner as in Example 2.

(Example 4)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the lithium mixture was fired at 980°C.

(Example 5)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the lithium mixture was fired at 880°C.

(Comparative example 1)
In the crystallization process, all steps up to switching operation 1 in the nucleation process and particle growth process are performed in an oxidizing atmosphere with an oxygen concentration of 21% by volume, and thereafter in a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the composite hydroxide was obtained by adjusting the atmosphere to a neutral atmosphere. FIG. 2 shows the particle structure of the positive electrode active material obtained. It is formed from a secondary particle in which a plurality of primary particles are agglomerated, and has one communicating hole on the outer periphery of the secondary particle, and has one space inside the secondary particle, and has a communicating hole on the outer periphery. It was confirmed that it consists of a hollow structure that is connected to the internal cavity.

(Comparative example 2)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the lithium mixture was fired at 810°C.

When the positive electrode active materials of Examples 1 to 5, which are within the scope of the present invention, are used in a lithium ion secondary battery, the positive electrode resistance decreases in comparison with Comparative Example 1 and Comparative Example 2. It was confirmed that the output characteristics were improved.

1 Positive electrode (evaluation electrode)
2 Negative electrode 3 Separator 4 Gasket 5 Wave washer 6 Positive electrode can 7 Negative electrode can 10 Coin-shaped battery

Claims (8)

General formula (A): Li _1+u Ni _x Co _y Mn _z M _t O ₂ (however, -0.05≦u≦0.50, x+y+z+t=1, 0.3≦x≦0.90, 0≦y≦ 0.5, 0.0≦z≦0.5, 0<t≦0.05, M is Mg, Al, Si, Ca, Ti, Si, V, Cr, Zr, Nb, Mo, Hf, Ta , and one or more additional elements selected from W), and lithium transition metal-containing composite oxide particles having a layered rock salt crystal structure,
The lithium transition metal-containing composite oxide particles are composed of secondary particles that are aggregated primary particles,
The secondary particles have a 50% cumulative diameter d50 determined from a particle size distribution measurement value of 3.0 μm or more and 7.0 μm or less, a BET specific surface area of 1.8 m ² /g or more and 5.3 m ² /g or less, and 0. Pore peak diameter in the pore distribution obtained by a nitrogen gas adsorption method of .01 μm or more and 0.20 μm or less, log differential pore in the range of the pore peak diameter of 0.008 cc/g or more and 0.03 cc/g or less has a volume (dV/d(logD)), and
Among the primary particles, for each of the plurality of primary particles having a primary particle diameter in the range of 0.1 μm or more and 1.0 μm or less, the concentration of the additive element M is measured at multiple locations by cross-sectional STEM or TEM-EDX analysis. However, when the coefficient of variation is calculated by dividing the standard deviation of the average concentration of the additive element M by the average concentration, the coefficient of variation is 1.5 or less.
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 the 50% cumulative diameter d50 of the secondary particles is 4.0 μm or more and 6.0 μm or less. It is an index indicating the spread of the particle size distribution of the secondary particles [(d90-d10)/d50] (where d10 is the 10% cumulative diameter obtained from the particle size distribution measurement value, and d90 is the 10% cumulative diameter obtained from the particle size distribution measurement value. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode active material for a lithium ion secondary battery has a 90% cumulative diameter) of 1.0 or less. The value of d90/d10 of the secondary particles (where d10 is the 10% cumulative diameter determined from the particle size distribution measurement value, and d90 is the 90% cumulative diameter determined from the particle size distribution measurement value) is 1.0 The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, which has a value of 2.0 or more. The secondary particles have an agglomerated part, one or more communication holes that communicate with the outer peripheral part of the agglomerated part, and two or more spaces that exist inside the agglomerated part and communicate with the communication holes. , a positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4. The lithium ion secondary battery according to any one of claims 1 to 5, wherein the additive element M is at least one selected from the group consisting of Mg, Al, Si, Ti, Zr, Nb, Mo, and W. Cathode active material for use. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the additive element M consists of W. The lithium ion secondary battery according to any one of claims 1 to 7, comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte, or a positive electrode, a negative electrode, and a solid electrolyte, and as a positive electrode active material used in the positive electrode. A lithium-ion secondary battery that uses a positive electrode active material.

Images