JP2022052817A - Positive electrode active material for lithium ion secondary battery and method for producing the same, and lithium ion secondary battery - Google Patents

Abstract

To provide a positive electrode active material for a lithium ion secondary battery, which has a Ni ratio of 80% or more and is good in discharge capacity, rate characteristics and cycle characteristics.SOLUTION: A positive electrode active material for a lithium ion secondary battery includes a lithium transition metal composite oxide represented by the following composition formula (1): Li1+aNibCocMndM1eAlfO2+α (1) [M1 represents one or more elements selected from among Ti, Ga, Mg, Zr and Zn, and a, b, c, d, e, f and α satisfy -0.04≤a≤0.08, 0.80≤b<1.0, 0≤c<0.2, 0≤d<0.2, 0<e<0.08, 0<f<0.04, 0<e+f<0.08, b+c+d+e+f=1 and -0.2<α<0.2.]. An atomic concentration D1 of M1 and an atomic concentration D2 of Al at the interface between primary particles at the inside of a secondary particle, and an atomic concentration D3 of M1 and an atomic concentration D4 of Al in the central part of each primary particle satisfy D1>D3 and D1/D3>D2/D4.SELECTED DRAWING: Figure 3

Description

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

Lithium-ion secondary batteries are widely used as lightweight secondary batteries with high energy density. Lithium-ion secondary batteries are required to have even higher capacities as their applications expand. Further, excellent charge / discharge cycle characteristics and the like are also required.

Under such circumstances, regarding the positive electrode active material that greatly affects the battery characteristics, in addition to establishing high capacity and mass productivity, studies on reduction of lithium ion insertion / desorption resistance and diffusion resistance, stabilization of crystal structure, etc. Has been made. As a positive electrode active material for a lithium ion secondary battery, a lithium transition metal composite oxide having an α-NaFeO type ₂ crystal structure (hereinafter, may be referred to as a layered structure) is widely known. Conventionally, LiCoO ₂ has been used as an oxide having a layered structure, but due to the demand for high capacity and mass productivity, a ternary system represented by Li (Ni, Co, Mn) O ₂ or a ternary system is used. Nickel-based materials in which LiNiO ₂ is substituted with different elements have been developed.

Among the lithium transition metal composite oxides having a layered structure, nickel-based ones have a disadvantage that the life characteristics are not always good. However, the nickel-based material is composed of nickel, which is cheaper than cobalt and the like, and exhibits a relatively high capacity, so that it is expected to be applied to various applications. In particular, expectations are rising for a chemical composition in which the proportion of nickel per metal (Ni, Co, Mn, etc.) excluding lithium is increased.

For example, Patent Document 1 describes a fine and uniform mixture of a main component raw material of a lithium transition metal compound and a compound (additive) composed of a metal element having a valence of 5 or hexavalent. Lithium is calcined and has a continuous composition gradient structure in which the additive element exists in the depth direction from the particle surface, specifically, in a range of about 10 nm in depth with a concentration gradient. A lithium transition metallic compound powder for a secondary battery positive electrode material is described.

Further, in Patent Document 2, Li _{1 + a} Ni _b Mn _{c Cod Tie} M _f O _{2 + α} ... (1)
(However, in the above formula (1), M is at least one element selected from the group consisting of Mg, Al, Zr, Mo and Nb, and is a, b, c, d, e, f and α. Is −0.1 ≦ a ≦ 0.2, 0.7 <b ≦ 0.9, 0 ≦ c <0.3, 0 ≦ d <0.3, 0 <e ≦ 0.25, 0 ≦ f It is a number that satisfies <0.3, b + c + d + e + f = 1, and −0.2 ≦ α ≦ 0.2), and Ti ³⁺ and Ti ⁴⁺ atoms based on X-ray photoelectron spectroscopic analysis. A positive electrode material for a lithium ion secondary battery having a ratio of Ti ³⁺ / Ti ⁴⁺ of 1.5 or more and 20 or less is described.

Further, Patent Document 3 comprises lithium nickel cobalt manganese oxide, which is composed of lithium titanium oxide, lithium tantalum oxide, lithium zirconium oxide, and one or more lithium metal oxides of lithium tungsten oxide. A positive electrode material for a lithium ion secondary battery having a first coating layer and a second coating layer composed of an aluminum oxide on the first coating layer is described.

Japanese Patent No. 5428251 Japanese Patent No. 6199981 Japanese Patent No. 65333733

Nickel with a high nickel content of 70% or more per metal excluding lithium has low crystal structure stability, making it difficult to achieve good charge / discharge cycle characteristics. It has the drawback of. In general, a lithium transition metal composite oxide having a high nickel content has a property that the crystal structure tends to be unstable during charging and discharging. In the crystal structure, Ni forms a layer composed of MeO ₂ (Me represents a metal element such as Ni). At the time of discharge, lithium ions are inserted between these layers to occupy the lithium sites, and at the time of discharge, the lithium ions are desorbed. Lattice distortion or crystal structure changes that occur with the insertion and desorption of lithium ions affect the charge / discharge cycle characteristics and the like. Therefore, in order to suppress lattice strain and crystal structure changes, there is a method of adding stable additive elements that do not contribute to charge / discharge.

The above-mentioned Patent Documents 1 and 2 suppress lattice strain and crystal structure change by forming a concentration gradient and a surface-concentrated layer due to added elements on the surface of a Ni—Co—Mn-based positive electrode active material, and have a certain effect. Is getting. Further, Patent Document 3 obtains a certain effect on improving the cycle characteristics by further forming a coating layer. However, if the proportion of nickel is increased to 80% or more for the purpose of increasing the capacity or the like, it becomes more difficult to maintain stability. Further, if the coating layer is provided, the resistance becomes high, and there is a concern that the discharge capacity and the output may decrease. Furthermore, since stability is maintained with a relatively large amount of cobalt, productivity including raw material costs is inferior. Therefore, in a positive electrode active material having a higher Ni ratio and a lower Co ratio, or containing no Co, a high discharge capacity and good charge / discharge cycle characteristics are required.

Therefore, the present invention relates to a positive electrode active material having a high Ni ratio of 80% or more of Ni, and has a high discharge capacity and good charge / discharge cycle characteristics. (Sometimes referred to simply as "positive electrode active material"), and further, a positive electrode active material having a lower Co content or having similar characteristics even in a positive electrode active material containing no Co, and a method for producing the same. Another object of the present invention is to provide a lithium ion secondary battery using this positive electrode active material.

The positive electrode active material for a lithium ion secondary battery according to the present invention is
The following composition formula (1);
Li _{1 + a} Ni _b Co _c Mn _d M1 _e Al _f O _{2 + α} ... (1)
[However, in the composition formula (1), M1 is one or more elements selected from the group consisting of Ti, Ga, Mg, Zr, and Zn, and a, b, c, d, e, f, and α are , -0.04 ≤ a ≤ 0.08, 0.80 ≤ b <1.0, 0 ≤ c <0.2, 0 ≤ d <0.2, 0 <e <0.08, 0 <, respectively. It is a number satisfying f <0.04, 0 <e + f <0.08, b + c + d + e + f = 1, and −0.2 <α <0.2. ], Which is a positive electrode active material for a lithium ion secondary battery containing a lithium transition metal composite oxide represented by the above, wherein the positive electrode active material contains secondary particles formed by aggregating a plurality of primary particles. In the primary particles inside the primary particles, the atomic concentration D1 of M1 and the atomic concentration D2 of Al at the interface between the primary particles, and the atomic concentration D3 of M1 and the atomic concentration D4 of Al in the central part of the primary particles. Is a positive electrode active material for a lithium ion secondary battery in which D1> D3 and D1 / D3> D2 / D4.

The method for producing a positive electrode active material for a lithium ion secondary battery according to the present invention is as follows.
The following composition formula (1);
Li _{1 + a} Ni _b Co _c Mn _d M1 _e Al _f O _{2 + α} ... (1)
[However, in the composition formula (1), M1 is one or more elements selected from the group consisting of Ti, Ga, Mg, Zr, and Zn, and a, b, c, d, e, f, and α are , -0.04 ≤ a ≤ 0.08, 0.80 ≤ b <1.0, 0 ≤ c <0.2, 0 ≤ d <0.2, 0 <e <0.08, 0 <, respectively. It is a number satisfying f <0.04, 0 <e + f <0.08, b + c + d + e + f = 1, and −0.2 <α <0.2. ], Which is a method for producing a positive electrode active material for a lithium ion secondary battery containing a lithium transition metal composite oxide represented by In the primary particles having the chemical composition represented by the composition formula (1) and inside the secondary particles, the atomic concentrations of M1 and Al at the interface between the primary particles are the atomic concentrations of D1 and Al. A firing step of obtaining a lithium composite compound in which D2 and the atomic concentration D3 of M1 and the atomic concentration D4 of Al in the central portion of the primary particles are D1> D3 and D1 / D3> D2 / D4. It is a method for producing a positive electrode active material for a lithium ion secondary battery.

According to the present invention, in a positive electrode active material having a high Ni ratio of 80% or more of Ni, a positive electrode active material for a lithium ion secondary battery having a high discharge capacity and good charge / discharge cycle characteristics is provided. Can be done. In particular, even in a positive electrode active material having a Ni ratio of 80% or more and a Co ratio of 6% or less, high discharge capacity and good charge / discharge cycle characteristics can be exhibited.

It is a partial cross-sectional view schematically showing an example of a lithium ion secondary battery. It is a figure which shows typically an example of the secondary particle and the primary particle of the positive electrode active material of this invention. It is a figure which shows the STEM image of the primary particle in Example 1 of this invention, and the measurement point of each element by EDX analysis.

Hereinafter, a positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention, a method for producing the same, and a lithium ion secondary battery using the same will be described in detail.

<Positive electrode active material>
The positive electrode active material according to the present embodiment has an α-NaFeO type ₂ crystal structure exhibiting a layered structure, and contains a lithium transition metal composite oxide composed of lithium and a transition metal. This positive electrode active material is mainly composed of primary particles of a lithium transition metal composite oxide and secondary particles composed of a plurality of primary particles aggregated together. Further, the lithium transition metal composite oxide has a layered structure capable of inserting and removing lithium ions as a main phase.

The positive electrode active material according to the present embodiment includes a lithium transition metal composite oxide as a main component, unavoidable impurities derived from raw materials and manufacturing processes, and other components that coat particles of the lithium transition metal composite oxide, for example. It may contain a boron component, a phosphorus component, a sulfur component, a fluorine component, an organic substance, and other components mixed with particles of a lithium transition metal composite oxide.

The lithium transition metal composite oxide according to this embodiment is represented by the following composition formula (1).
Li _{1 + a} Ni _b Co _c Mn _d M1 _e Al _f O _{2 + α} ... (1)
[However, in the composition formula (1), M1 is one or more elements selected from the group consisting of Ti, Ga, Mg, Zr, and Zn, and a, b, c, d, e, f, and α are , -0.04 ≤ a ≤ 0.08, 0.80 ≤ b <1.0, 0 ≤ c <0.2, 0 ≤ d <0.2, 0 <e <0.08, 0 <, respectively. It is a number satisfying f <0.04, 0 <e + f <0.08, b + c + d + e + f = 1, and −0.2 <α <0.2. ]

The lithium transition metal composite oxide represented by the composition formula (1) has a nickel ratio of 80% or more per metal excluding lithium. That is, Ni is contained in an amount of 80 at% or more in terms of atomic number fraction with respect to the total of Ni, Co, Mn, M1 and Al. Since the nickel content is high, it is a nickel-based oxide that can realize a high discharge capacity. Further, since the nickel content is high, the raw material cost is lower than that of LiCoO ₂ and the like, and the productivity including the raw material cost is also excellent.

In general, a lithium transition metal composite oxide having a high nickel content has a property that the crystal structure tends to become unstable during charging and discharging. In the crystal structure, Ni forms a layer composed of MeO ₂ (Me represents a metal element such as Ni). At the time of discharge, lithium ions are inserted between these layers to occupy the lithium sites, and at the time of discharge, the lithium ions are desorbed. Since a large amount of nickel is present at the transition metal sites of the layer composed of MeO ₂ , when the valence of nickel changes for charge compensation due to the insertion and desorption of lithium ions, there is no large lattice distortion. Causes a change in crystal structure. Lattice distortion or crystal structure changes that occur with the insertion and desorption of lithium ions affect the discharge capacity characteristics and charge / discharge cycle characteristics.

Therefore, as described above, there is a method of adding a stable additive element that does not contribute to charge / discharge in order to suppress lattice strain and crystal structure change. The above-mentioned Patent Documents 1 and 2 are also examples, but in the past, only the primary particles on the surface of the secondary particles, in other words, the surface constituting the secondary particles, were modified to form the secondary particles. There was room for improvement in the internal primary particles. Therefore, in this embodiment, attention is paid to each primary particle located inside the secondary particle, and the influence of the added element is examined.

As a result, in each primary particle (hereinafter, may be simply referred to as a primary particle) located inside the secondary particle, the atomic concentration on the surface of M1 in the composition formula is increased to increase the atomic concentration of Ni. The lattice strain and crystal structure change near the surface of the primary particles due to the valence change can be reduced, and the lattice strain and crystal structure change can be further suppressed for the secondary particles. As a result, good charge / discharge cycle characteristics can be obtained. Further, by adding M1 and Al in the above composition formula at the same time, Al promotes the formation of the layered structure and the temperature at which the layered structure is formed decreases. The difference between the reaction temperature of Li and M1 that occurs at a high temperature becomes large, and the surface concentration of M1 is promoted. As a result, it has been found that the atomic concentration on the surface of M1 is increased, and the charge / discharge cycle characteristics are further improved while maintaining the discharge capacity. The definition of the atomic concentration of M1, the interface in the primary particles, the surface and the central portion will be described later.

(Chemical composition)
Here, the significance of the chemical composition represented by the composition formula (1) will be described.

A in the composition formula (1) is −0.04 or more and 0.08 or less. a represents the excess or deficiency of lithium with respect to Li (Ni, Co, Mn, M1, Al) _O2 in the stoichiometric ratio. a is not the value charged at the time of raw material synthesis, but the value in the lithium transition metal composite oxide obtained by firing. When the excess or deficiency of lithium in the composition formula (1) is excessive, that is, the composition has an excessively small amount of lithium or an excessively large amount of lithium with respect to the total of Ni, Co, Mn, M1 and Al. At the time of firing, the synthesis reaction does not proceed properly, and cation mixing in which nickel is mixed in lithium sites is likely to occur, and crystallinity is likely to be lowered. In particular, when the proportion of nickel is increased to 80% or more, such cation mixing is likely to occur and the crystallinity is likely to be significantly reduced, and the discharge capacity and charge / discharge cycle characteristics are likely to be impaired. On the other hand, when a is in the above numerical range, cation mixing is reduced and various battery performances can be improved. Therefore, even in a composition having a high nickel content, high discharge capacity and good charge / discharge cycle characteristics can be obtained.

a is preferably −0.03 or more and 0.06 or less. When a is −0.03 or more and 0.06 or less, the excess or deficiency of lithium with respect to the stoichiometric ratio is smaller, so that the synthetic reaction proceeds appropriately during firing, and cation mixing is less likely to occur. Therefore, a layered structure with fewer defects is formed, and high discharge capacity and good charge / discharge cycle characteristics can be obtained. Regarding the positive electrode active material containing the lithium transition metal composite oxide represented by the composition formula (1) as a main component, the atomic concentration (number of moles) of lithium contained in the positive electrode active material and the metal elements other than lithium are also used. The ratio to the total atomic concentration (number of moles) is preferably 0.96 or more and 1.08 or less, and more preferably 0.97 or more and 1.06 or less. Other components may be mixed in the calcination precursor fired by the heat treatment, and the reaction ratio at the time of calcination may deviate from the stoichiometric ratio. However, with such an atomic concentration ratio, it is highly possible that cation mixing and deterioration of crystallinity are suppressed based on the chemical composition represented by the composition formula (1) at the time of firing. Therefore, a positive electrode active material that improves the performance of various batteries can be obtained.

The above describes the preferable range of a in the state of the powder produced as the positive electrode active material, but when the positive electrode active material represented by the composition formula (1) is incorporated in the positive electrode of the lithium ion secondary battery. In, a is preferably in the range of −0.9 to 0.06 because charging / discharging accompanied by insertion / removal of Li is carried out.

The coefficient b of nickel in the composition formula (1) is 0.80 or more and less than 1.00. When b is 0.80 or more, it is higher than other nickel-based oxides having a low nickel content and ternary oxides represented by Li (Ni, Co, M1) O ₂ . The discharge capacity can be obtained. In addition, since the amount of transition metal, which is rarer than nickel, can be reduced, the raw material cost can be reduced.

The nickel coefficient b may be 0.85 or more, 0.90 or more, or 0.92 or more. The larger the b, the higher the discharge capacity tends to be obtained. Further, the coefficient b of nickel may be 0.95 or less, 0.90 or less, or 0.85 or less. The smaller b is, the smaller the lattice distortion or crystal structure change due to the insertion or desorption of lithium ions is, and the less likely it is that cation mixing in which nickel is mixed in lithium sites or the deterioration of crystallinity occurs during firing, so a good rate is obtained. Characteristics and charge / discharge cycle characteristics tend to be obtained.

The coefficient c of cobalt in the composition formula (1) is 0 or more and less than 0.2. Cobalt may be positively added or may have a composition ratio corresponding to unavoidable impurities. When cobalt is in the above range, the crystal structure becomes more stable, and effects such as suppression of cation mixing in which nickel is mixed in lithium sites can be obtained. Therefore, high discharge capacity and good charge / discharge cycle characteristics can be obtained. On the other hand, if the amount of cobalt is excessive, the raw material cost of the positive electrode active material becomes high. Further, the ratio of other transition metals such as nickel becomes low, and there is a possibility that the discharge capacity becomes low or the effect of the metal element represented by M1 becomes low. On the other hand, if c is in the above numerical range, the raw material cost of the lithium transition metal composite oxide exhibiting high discharge capacity, good rate characteristics and charge / discharge cycle characteristics can be reduced.

The cobalt coefficient c may be 0.01 or more, 0.02 or more, 0.03 or more, or 0.04 or more. The larger c is, the more effective the effect of elemental substitution of cobalt can be obtained, so that better charge / discharge cycle characteristics and the like tend to be obtained. The cobalt coefficient c may be 0.06 or less, 0.03 or less, or 0.01 or less. The smaller c is, the more the raw material cost can be reduced. In the present embodiment, when the nickel coefficient b is 0.90 or more, the cobalt coefficient c can be 0 or more and 0.03 or less. The larger the amount of cobalt added, the more stable the crystal structure becomes, and the effect of suppressing cation mixing in which nickel is mixed in lithium sites can be obtained, whereas when the nickel coefficient b is 0.90 or more, the effect is obtained. It is considered that the amount of cobalt can be reduced due to the effect that the room for adding cobalt is originally small and the crystal structure near the surface of the primary particles is stabilized by the concentration of M1 on the surface of the primary particles.

Although manganese in the composition formula (1) is a transition metal, it is considered that manganese remains stable at +4 valence even during charging and discharging. Therefore, the use of manganese has the effect of stabilizing the crystal structure during charging and discharging.

The coefficient d of manganese in the composition formula (1) is 0 or more and less than 0.2. If the manganese is excessive, the proportion of other transition metals such as nickel is low, and the discharge capacity of the positive electrode active material may be low. On the other hand, when d is in the above numerical range, higher discharge capacity and charge / discharge cycle characteristics tend to be obtained.

The manganese coefficient d may be 0.01 or more, 0.03 or more, 0.05 or more, or 0.10 or more. The larger d is, the more effective the effect of elemental substitution of manganese can be obtained. The manganese coefficient d may be 0.15 or less, 0.10 or less, or 0.05 or less. The smaller d, the higher the proportion of other transition metals such as nickel, and the higher the discharge capacity and the like.

M1 in the composition formula (1) is at least one element selected from the group consisting of Ti, Ga, Mg, Zr, and Zn. (Hereafter referred to as M1). These elements react with Li to form a concentrated layer after they start to form an α-NaFeO type ₂ crystal structure that exhibits a layered structure by reacting with Li. In this case, M1 is present on the surface of the primary particles of the positive electrode active material in the relatively low temperature firing step where the reaction between Li and Ni starts, and M1 tends to form a concentrated layer on the surface of the primary particles in the subsequent high temperature firing step. .. In particular, M1 is primary by premixing and pulverizing all the elements including M1 except Li using the solid phase method, or by premixing and pulverizing all the elements including Li and M1. It can be distributed on the surface of particles. Further, it is more preferable to contain at least Ti. Since Ti can be tetravalent, it has a strong bond with O and has a great effect of stabilizing the crystal structure. This is also because the molecular weight is relatively small and the decrease in the theoretical capacity of the positive electrode active material when added is small. When a metal element having such properties is used, it is possible to concentrate and distribute it on the surface of the primary particles by selecting appropriate synthetic conditions. Therefore, when these metal elements M1 are used, the effect of suppressing the deterioration of the crystal structure from the surface of the positive electrode active material during charging / discharging can be obtained.

The coefficient e of M1 in the composition formula (1) is more than 0 and less than 0.08. When M1 is added, the crystal structure on the surface of the positive electrode active material becomes more stable as described above, and good charge / discharge cycle characteristics can be obtained. On the other hand, when M1 is excessive, the ratio of other transition metals such as nickel becomes low, the discharge capacity becomes low, and the effect of stabilizing the crystal structure by manganese becomes low, and the charge / discharge cycle characteristics deteriorate. There is a risk of Further, in the case of Ti or the like in which M1 can be tetravalent, the proportion of divalent nickel relatively increases in the vicinity of the surface of the primary particles, and cation mixing is likely to occur. On the other hand, when e is in the above numerical range, a lithium transition metal composite oxide exhibiting high discharge capacity and good charge / discharge cycle characteristics can be obtained. The coefficient e of M1 is preferably 0.01 or more and 0.05 or less, and more preferably 0.01 or more and 0.03 or less. When e is 0.01 or more, the proportion of nickel on the surface of the primary particles is low, and the change in the crystal structure on the surface of the primary particles is reduced. On the other hand, when e is 0.05 or less, the ratio of other transition metals such as nickel can be sufficiently maintained, and a high discharge capacity can be obtained. Therefore, a layered structure with fewer defects is formed, and high discharge capacity and good charge / discharge cycle characteristics can be obtained.

Aluminum in the composition formula (1) does not form a concentrated layer on the surface of the primary particles, and is uniformly solid-solved inside the primary particles. When aluminum is used, the surface concentration of M1 is rather promoted, and the proportion of nickel on the surface of the primary particles is lowered. Therefore, changes in the crystal structure near the surface of the primary particles are reduced.

The coefficient f of aluminum in the composition formula (1) is more than 0 and less than 0.04. When aluminum is added, the crystal structure near the surface of the positive electrode active material becomes more stable as described above, and good charge / discharge cycle characteristics can be obtained. On the other hand, if the amount of aluminum is excessive, the ratio of other transition metals such as nickel becomes low, the discharge capacity becomes low, and the effect of stabilizing the crystal structure by manganese becomes low, and the charge / discharge cycle characteristics deteriorate. There is a risk of The coefficient f of aluminum is preferably 0.01 or more and less than 0.04, and more preferably 0.01 or more and 0.02 or less. When f is 0.01 or more, the proportion of M1 on the surface of the primary particle is high, and as a result, the proportion of nickel on the surface of the primary particle is low, and the change in crystal structure near the surface of the primary particle is reduced. On the other hand, when f is less than 0.04, the ratio of other transition metals such as nickel can be sufficiently maintained, and a high discharge capacity can be obtained. Therefore, a layered structure with fewer defects is formed, and high discharge capacity and charge / discharge cycle characteristics can be obtained.

The sum e + f of the coefficient e of M1 and the coefficient f of aluminum in the composition formula (1) is more than 0 and less than 0.08. If M1 and aluminum are excessive, the ratio of other transition metals such as nickel becomes low, the discharge capacity becomes low, and the effect of stabilizing the crystal structure by manganese becomes low, and the charge / discharge cycle characteristics may deteriorate. There is. Further, the ratio e / f of the coefficient e of M1 and the coefficient f of aluminum is preferably 0.5 or more and 5 or less. When e / f is 0.5 or more, the coefficient e of M1 becomes relatively high, and the ratio of M1 on the surface of the primary particles can be increased, which is preferable. When e / f is 5 or less, the coefficient f of aluminum becomes relatively high, and the effect of promoting the surface concentration of M1 due to the addition of aluminum can be obtained, which is preferable. Further, the e / f is more preferably 1.0 or more and 3 or less. When e / f is 1.0 or more and 3 or less, the atomic concentration D1 of M1 at the interface between the primary particles becomes high, and as a result, the proportion of nickel on the surface of the primary particles becomes low, and the charge / discharge cycle characteristics are good. Become.

Α in the composition formula (1) is more than −0.2 and less than 0.2. α represents the excess or deficiency of oxygen with respect to Li (Ni, Co, Mn, M1, Al) O ₂ in the stoichiometric ratio. When α is in the above numerical range, there are few defects in the crystal structure, and a high discharge capacity and good charge / discharge cycle characteristics can be obtained by an appropriate crystal structure. The value of α can be measured by the Infrared gas melting-infrared absorption method.

(Secondary particles)
The average particle size of the primary particles of the positive electrode active material is preferably 0.05 μm or more and 2 μm or less. By setting the average particle size of the primary particles of the positive electrode active material to 2 μm or less, the reaction field of the positive electrode active material can be secured and a high discharge capacity can be obtained. It is more preferably 1.5 μm or less, still more preferably 1.0 μm or less. Further, the average particle size of the secondary particles of the positive electrode active material is preferably, for example, 3 μm or more and 50 μm or less.

The secondary particles (granulators) of the positive electrode active material can be made into secondary particles by granulating the primary particles produced by the method for producing the positive electrode active material described later by dry granulation or wet granulation. can. As the granulation means, for example, a granulator such as a spray dryer or a rolling fluidized bed device can be used.

The lithium transition metal composite oxide represented by the composition formula (1) has a BET specific surface area of preferably 0.1 m ² / g or more, more preferably 0.2 m ² / g or more, still more preferably 0.3 m ² . / G or more. The BET specific surface area is preferably 1.5 m ² / g or less, more preferably 1.2 m ² / g or less. When the BET specific surface area is 0.1 m ² / g or more, a positive electrode having a sufficiently high molding density and filling rate of the positive electrode active material can be obtained. Further, when the BET specific surface area is 1.5 m ² / g or less, fracture, deformation, falling off of particles, etc. are less likely to occur at the time of pressure molding of the lithium transition metal composite oxide or at the time of volume change due to charge / discharge. , It is possible to suppress the suction of the binder by the pores. Therefore, the coatability and adhesion of the positive electrode active material are improved, and high discharge capacity and good charge / discharge cycle characteristics can be obtained.

(Primary particles that make up secondary particles)
The positive electrode active material of the embodiment of the present invention contains secondary particles in which a plurality of primary particles of a lithium transition metal composite oxide are aggregated, and inside the secondary particles, a plurality of primary particles are interposed at an interface (surface). It is configured to be adjacent to each other. However, not all the primary particles form an interface, but many primary particles may form an interface. In the primary particle inside the secondary particle, the atomic concentration D1 (at%) of M1 and the atomic concentration D2 (at%) of Al at the interface (which may be said to be the surface of the primary particle) and the center of the primary particle. It is preferable that the atomic concentration D3 (at%) of M1 and the atomic concentration D4 (at%) of Al in the part satisfy the relationship of D1> D3 and D1 / D3> D2 / D4. That is, it can be said that M1 is concentrated on the surface and aluminum is uniformly dissolved inside the particles. Here, the atomic concentrations D1 and D3 of M1 are represented by (M1 / (Ni + Co + Mn + M1 + Al)), and the atomic concentrations D2 and D4 of aluminum are represented by (Al / (Ni + Co + Mn + M1 + Al)), which can be confirmed by EDX or the like. It is important that the primary particles whose surface is enriched and whose aluminum is uniformly solidified are the primary particles excluding the primary particles located on the surface of the secondary particles, and at least among the number of internal primary particles. It is preferable that the primary particles have 50% or more, preferably 70% or more, more preferably 100% M1 surface-concentrated, and aluminum is uniformly and solidly dissolved.

It is effective to have a surface-concentrated layer not only on the surface of the secondary particles but also on the individual primary particles inside the secondary particles. Even if the layer in which M1 is concentrated is extremely thin, the proportion of Ni is relatively low, so that the crystal structure near the surface becomes more stable, which leads to obtaining good charge / discharge cycle characteristics. Further, when the concentrated layer of M1 is present only on the surface of the secondary particles, when the electrolytic solution permeating the inside of the secondary particles comes into contact, the crystal structure starts to deteriorate from the surface of the primary particles, and the charge / discharge cycle characteristics deteriorate. Because it becomes. As an index indicating the existence of a layer in which M1 is concentrated on the surface layer of the primary particles, the concentration D1 of M1 on the surface of the primary particles is used. The central portion of the primary particle is a range having a depth of 0.2 r or more from the surface of the primary particle toward the central portion of the primary particle when the average particle diameter of the primary particle is r.

It is preferable that the atomic concentrations D1 and D3 of M1 are D1> (100 × e) ≧ D3. When D1> (100 × e), the concentration of M1 in the vicinity of the surface of the primary particles is higher than that when M1 is uniformly distributed in the primary particles, so that the Ni ratio on the surface of the primary particles is relatively low. .. Therefore, the crystal structure near the surface becomes more stable, and good charge / discharge cycle characteristics can be obtained. Further, it is more preferable that D1> (3.5 × (100 × e). The Ni ratio on the surface is further reduced. On the other hand, when (100 × e) ≧ D3, in the central portion of the primary particles. , M1 is equal to or lower than the concentration when it is uniformly distributed in the primary particles, so it is considered that M1 forms a concentrated layer other than the central portion, for example, near the surface. > (100 × e / 4), M1 is diffused not only near the surface of the primary particles but also to the central part, so that the surface expands and contracts due to the insertion and desorption of Li in the charge / discharge cycle. Good charge / discharge cycle characteristics can be obtained without a large difference between the vicinity and the center.

Further, it is preferable that the atomic concentrations D1 and D3 of M1 are 3.5 times or more that of D1. When D1 is 3.5 times or more that of D3, M1 is sufficiently concentrated in the vicinity of the surface of the primary particles, and the proportion of Ni in the vicinity of the surface of the primary particles is relatively reduced. Therefore, the crystal structure near the surface becomes more stable, and good charge / discharge cycle characteristics can be obtained. More preferably, D1 is 3.7 times or more that of D3. Further, it is more preferable that D1 is 30 times or less of D3. If the difference between D1 and 3 is too large, a relatively large difference in concentration of Ni, Co, Mn, and Al occurs between the surface of the primary particle and the central portion of the primary particle. Therefore, there is a large difference in the expansion and contraction of the crystal structure due to the insertion and desorption of Li in the charge / discharge cycle between the vicinity of the surface and the central portion, and the concentrated layer of M1 may be isolated due to the difference in expansion and contraction.

Further, for Co, Mn, and Al, it is preferable that the concentration difference between the surface of the primary particle and the central portion of the primary particle is smaller than the concentration difference of M1. If the concentration difference of Co, Mn, and Al between the surface of the primary particle and the central part of the primary particle is small compared to the concentration difference of M1, the expansion and contraction of the crystal structure due to the insertion and desorption of Li will cause the expansion and contraction of the crystal structure near the surface and the central part. Does not make a big difference. Therefore, good charge / discharge cycle characteristics can be obtained.

The average composition of the particles of the positive electrode active material can be confirmed by high frequency inductively coupled plasma (ICP), atomic absorption spectroscopy (AAS) and the like. The atomic concentrations of M1 and aluminum in the primary particles of the positive electrode active material are determined by using a cross-sectional processed positive electrode active material, using a scanning transmission electron microscope (STEM) and energy dispersive X-ray. Atomism; EDX) can be used to quantitatively analyze the concentrations of each element of Ni, Co, Mn, M1, and Al.

<Manufacturing method of positive electrode active material>
The positive electrode active material according to the present embodiment includes lithium, nickel, cobalt, etc. under appropriate firing conditions under a raw material ratio such that the lithium transition metal composite oxide has a chemical composition represented by the composition formula (1). It can be produced by surely advancing the synthetic reaction with. A method for producing a positive electrode active material according to an embodiment of the present invention will be described below.

The method for producing the positive electrode active material for a lithium ion secondary battery according to the present embodiment includes a step of obtaining a granulated body by either a granulation step or a co-precipitation step, and firing in which the granulated body is fired in an oxidizing atmosphere. Including the process. In addition, steps other than these steps may be added. For example, a mixture of the granulated product and lithium carbonate or lithium hydroxide may be added. In addition, when a large amount of lithium hydroxide or lithium carbonate remains in the positive electrode active material obtained in the firing step, the slurry-like electrode mixture gels in the mixture coating step for producing the positive electrode. Therefore, it is possible to reduce residual lithium hydroxide and lithium carbonate by adding a washing step and a drying step following the firing step. When the washing and drying steps are added, the positive electrode active material obtained after the washing and drying steps shall satisfy the composition formula (1). When the process is completed in the firing step, the positive electrode active material obtained after the firing step shall satisfy the composition formula (1). This is because, with respect to a, the value of the positive electrode active material in the state of being used for the battery is important.

When the granulated material is obtained by the granulation step, the raw material mixing step is performed before the granulation step. The compound containing lithium and the compound containing a metal element other than Li in the composition formula (1) may be mixed, or only the compound containing a metal element other than Li in the composition formula (1) may be mixed. good. From the viewpoint of more uniformly dispersing Li, it is preferable to mix the compound containing lithium with the compound containing a metal element other than Li in the composition formula (1). For example, by weighing each of these raw materials, pulverizing and mixing them, a powdery mixture in which the raw materials are uniformly mixed can be obtained. As the crusher for crushing the raw material, for example, a general precision crusher such as a ball mill, a bead mill, a jet mill, a rod mill, or a sand mill can be used. The raw material may be pulverized by dry pulverization or wet pulverization. After dry pulverization, a solvent such as water may be added to form a slurry composed of a raw material and a solvent, or a solvent such as water may be added to the raw material in advance to form a slurry and then wet pulverization may be performed. From the viewpoint of obtaining a uniform and fine powder having an average particle size of 0.3 μm or less, it is more preferable to perform wet pulverization using a medium such as water. Further, it is preferable to uniformly disperse the raw materials, and for example, in wet mixing, it is preferable to use a dispersant to improve the dispersibility of the raw materials in the slurry. When the dispersibility of the raw material is improved, the thickness of the concentrated layer of M1 becomes uneven, which is preferable. As the dispersant, a polycarboxylic acid-based, urethane-based, or acrylic resin-based dispersant can be used, and an acrylic resin-based dispersant is preferable. The amount of the dispersant added can be arbitrarily added in order to adjust the viscosity of the slurry.

In the granulation step, the mixture obtained in the raw material mixing step is granulated to obtain secondary particles (granulated bodies) in which the particles are aggregated with each other. Granulation of the mixture may be carried out by using either dry granulation or wet granulation. For granulation of the mixture, for example, an appropriate granulation method such as a rolling granulation method, a fluidized bed granulation method, a compression granulation method, or a spray granulation method can be used.
As the granulation method for granulating the mixture, the spray granulation method is particularly preferable. As the spray granulator, various methods such as a two-fluid nozzle type, a four-fluid nozzle type, and a disk type can be used. If it is a spray granulation method, a slurry that has been precisely mixed and pulverized by wet pulverization can be granulated while being dried. In addition, it is possible to precisely control the particle size of secondary particles within a predetermined range by adjusting the concentration of slurry, spray pressure, disk rotation speed, etc., and it is a granulated body that is close to a true sphere and has a uniform chemical composition. Can be obtained efficiently. In the granulation step, it is preferable to granulate the mixture obtained in the mixing step so that the average particle size (D50) is 3 μm or more and 50 μm or less. In the present embodiment, the secondary particles of the granulated body having an average particle size (D50) of 5 μm or more and 20 μm or less are more preferable.

When the granulated body is obtained by the coprecipitation step, the coprecipitated body may be obtained by adjusting the pH of the aqueous solution containing Ni, Co, Mn, M1 and Al. It should be noted that Ni, Co, Mn, M1 and Al may all be coprecipitated at once, or one or more of Ni, Co, Mn, M1 and Al may be coprecipitated (precipitated) and then other elements. May be simultaneously or individually settled (coprecipitated) on the surface of the coprecipitate.

When the granulated product is obtained by mixing only the compound containing a metal element other than Li in the composition formula (1) in the granulation step, or when the granulated product is obtained in the co-precipitation step, the granulated product is obtained. And the lithium compound are mixed. The granulation body and the lithium compound can be mixed by drywall mixing. As a mixer for mixing the granulated product and the lithium compound, for example, a V-type mixer, an attritor, or the like can be used. Further, the granulated product may be heat-treated before mixing the granulated product and the lithium compound. The particle strength of the granulated body is increased, and the lithium compound can be mixed without damaging the granulated body.

Examples of the lithium-containing compound include lithium carbonate, lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, lithium sulfate and the like. Further, it is preferable to use lithium carbonate or lithium hydroxide, and the gas generated in the firing step is steam or carbon dioxide, which causes less damage to the manufacturing apparatus and is excellent in industrial usability and practicality. In particular, it is more preferable to use at least lithium carbonate, and it is more preferable to use lithium carbonate in a proportion of 80% by mass or more in the raw material containing lithium. Lithium carbonate is easily available because it has excellent supply stability and is inexpensive as compared with other compounds containing lithium. Further, since lithium carbonate is weakly alkaline, there is little damage to the manufacturing equipment, and it is excellent in industrial usability and practicality.

As the compound containing a metal element other than Li, a compound composed of C, H, O, N such as carbonate, hydroxide, oxyhydroxide, acetate, citrate, and oxide is preferably used. .. Carbonates, hydroxides, or oxides are particularly preferable from the viewpoint of ease of pulverization and the amount of gas released by thermal decomposition. Further, a sulfate may be used. It dissolves easily in a solvent such as water and is preferable.

The atomic concentration ratio (molar ratio) between the atomic concentration (number of moles) of lithium contained in the firing precursor and the total atomic concentration (number of moles) of metal elements other than lithium is approximately 1 according to the chemical quantity theory ratio. It is desirable to react at a ratio of 1. A lithium transition metal composite oxide can be obtained in which the proportion of divalent nickel occupying the transition metal site is high and the lattice distortion or crystal structure change due to the insertion or desorption of lithium ions is reduced. Further, the atomic concentration ratio (molar ratio) between the atomic concentration (number of moles) of lithium contained in the firing precursor and the total atomic concentration (number of moles) of metal elements other than lithium is more preferably 0.98 or more. It is 1.07 or less. However, at the time of firing, lithium contained in the firing precursor may react with the firing container or volatilize. Considering that a part of lithium is lost due to the reaction with the firing container and evaporation during firing, it is not hindered to add excessive lithium at the time of charging.

In the firing step, the granulated body is heat-treated to fire the lithium transition metal composite oxide represented by the composition formula (1). The firing step may be performed by a one-stage heat treatment in which the heat treatment temperature is controlled within a certain range, or may be performed by a plurality of stages of heat treatment in which the heat treatment temperatures are controlled in different ranges. However, from the viewpoint of obtaining a lithium transition metal composite oxide having high crystal purity, high discharge capacity, good rate characteristics and charge / discharge cycle characteristics, the first heat treatment step and the second heat treatment step may be included. Preferably, it is important to satisfy the conditions of the first heat treatment step and the second heat treatment step.

In the first heat treatment step, a first precursor is obtained by heat treatment in an oxidizing atmosphere at a heat treatment temperature of 400 ° C. or higher and lower than 750 ° C. for 2 hours or more and 50 hours or less. The main purpose of the first heat treatment step is to remove water or carbon dioxide components by reacting a lithium compound with a nickel compound or the like, and to form crystals of a lithium transition metal composite oxide. Nickel in the calcination precursor is sufficiently oxidized to suppress cation mixing in which nickel is mixed in lithium sites, and to suppress the formation of cubic domains by nickel. Further, in order to reduce the lattice distortion or crystal structure change due to the insertion and desorption of lithium ions, manganese is sufficiently oxidized to increase the uniformity of the composition of the layer composed of MeO2, and the ionic radius is large. Increase the proportion of divalent nickel.

In the second heat treatment step, the first precursor obtained in the first heat treatment step is heat-treated at a heat treatment temperature of 700 ° C. or higher and 900 ° C. or lower for 2 hours or more and 100 hours or less to obtain a lithium transition metal composite oxide. The main purpose of the second heat treatment step is to grow the crystal grains of the lithium transition metal composite oxide having a layered structure to an appropriate particle size and specific surface area.

In the second heat treatment step, if the heat treatment temperature is 700 ° C. or higher, nickel is sufficiently oxidized to suppress cation mixing, and the crystal grains of the lithium transition metal composite oxide are grown to an appropriate particle size and specific surface area. be able to. In addition, manganese can be sufficiently oxidized to increase the proportion of divalent nickel. A main phase is formed in which the lattice constant of the a-axis is large and the lattice distortion or crystal structure change due to the insertion and desorption of lithium ions is reduced, resulting in high discharge capacity, good charge / discharge cycle characteristics and output characteristics. Obtainable. Further, when the heat treatment temperature is 900 ° C. or lower, lithium is hard to volatilize and the layered structure is hard to be decomposed. Therefore, a lithium transition metal composite oxide having high crystal purity and good discharge capacity and charge / discharge cycle characteristics can be obtained. Obtainable.

In the firing step, as a means for heat treatment, an appropriate heat treatment device such as a rotary furnace such as a rotary kiln, a continuous furnace such as a roller harsher knives, a tunnel furnace, a pusher furnace, or a batch furnace can be used. The first heat treatment step and the second heat treatment step may be performed using the same heat treatment apparatus, or may be performed using different heat treatment apparatus.

By going through the above granulation step, coprecipitation step, and firing step, a positive electrode active material composed of the lithium transition metal composite oxide represented by the composition formula (1) can be produced. The distribution of M1, the amount of cation mixing, and the specific surface area are mainly determined by the method for producing the precursor before heat treatment, the composition ratio of metal elements such as nickel, and the unreacted lithium hydroxide remaining in the first precursor. It can be controlled by adjusting the residual amount of lithium carbonate, the heat treatment temperature in the second heat treatment step, and the heat treatment time. In the chemical composition represented by the composition formula (1), when M1 is concentrated on the surface of the primary particles, aluminum is uniformly dissolved in the primary particles, and the cation mixing is sufficiently reduced, a high discharge capacity is obtained. , An excellent positive electrode active material showing good charge / discharge cycle characteristics can be obtained.

The synthesized lithium transition metal composite oxide is subjected to a washing step of washing with deionized water or the like after the firing step for the purpose of removing impurities, and drying of the washed lithium transition metal composite oxide. It may be used in a process or the like. Further, it may be subjected to a crushing step of crushing the synthesized lithium transition metal composite oxide, a classification step of classifying the lithium transition metal composite oxide into a predetermined particle size, or the like.

In the water washing step, the lithium transition metal composite oxide obtained in the firing step is washed with water. The washing of the lithium transition metal composite oxide with water can be carried out by an appropriate method such as a method of immersing the lithium transition metal composite oxide in water or a method of passing water through the lithium transition metal composite oxide. By washing the lithium transition metal composite oxide with water, residual alkaline components such as lithium carbonate and lithium hydroxide remaining on the surface and the vicinity of the surface layer of the lithium transition metal composite oxide can be removed. The water in which the lithium transition metal composite oxide is immersed may be still water or may be agitated. As the water, deionized water, pure water such as distilled water, ultrapure water and the like can be used.
In the washing step, when the lithium transition metal composite oxide is immersed in water, the solid content ratio of the lithium transition metal composite oxide to the immersed water is preferably 33% by mass or more and 77% by mass or less. When the solid content ratio is 33% by mass or more, the amount of lithium eluted from the lithium transition metal composite oxide into water can be suppressed to a small amount. Therefore, a positive electrode active material showing a high discharge capacity and a good rate can be obtained. Further, when the solid content ratio is 77% by mass or less, the powder can be uniformly washed with water, so that impurities can be reliably removed.
The time for washing the lithium transition metal composite oxide with water is preferably 20 minutes or less, more preferably 10 minutes or less. If the washing time with water is 20 minutes or less, the amount of lithium eluted from the lithium transition metal composite oxide into water can be suppressed to a small amount. Therefore, it is possible to obtain a positive electrode active material having a high discharge capacity and good charge / discharge cycle characteristics.

The lithium transition metal composite oxide immersed in water can be recovered by an appropriate solid-liquid separation operation. Examples of the solid-liquid separation method include decompression filtration, pressurization filtration, filter press, roller press, and centrifugation. The water content of the lithium transition metal composite oxide separated from water in solid and liquid is preferably 20% by mass or less, and more preferably 10% by mass or less. When the water content is low as described above, a large amount of the lithium compound eluted in the water does not reprecipitate, so that it is possible to prevent the performance of the positive electrode active material from deteriorating. The moisture content of the lithium transition metal composite oxide after solid-liquid separation can be measured using, for example, an infrared moisture meter.

In the drying step, the lithium transition metal composite oxide washed with water in the washing step is dried. By drying the lithium transition metal composite oxide, water that reacts with the components of the electrolytic solution to deteriorate the battery or deteriorate the binder to cause poor coating is removed. Further, since the surface of the lithium transition metal composite oxide is modified through the washing step and the drying step, the effect of improving the compressibility of the positive electrode active material as a powder can be obtained. As the drying method, for example, vacuum drying, heat drying, vacuum heat drying and the like can be used.
The atmosphere in the drying step is an inert gas atmosphere that does not contain carbon dioxide or a reduced pressure atmosphere with a high degree of vacuum. In such an atmosphere, it is possible to prevent lithium carbonate and lithium hydroxide from being mixed due to the reaction with carbon dioxide and moisture in the atmosphere.
The drying temperature in the drying step is preferably 300 ° C. or lower, more preferably 80 ° C. or higher, and more preferably 300 ° C. or lower. When the drying temperature is 300 ° C. or lower, side reactions can be suppressed and drying can be performed, so that deterioration of the performance of the positive electrode active material can be avoided. Further, when the drying temperature is 80 ° C. or higher, the water content can be sufficiently removed in a short time. The moisture content of the lithium transition metal composite oxide after drying is preferably 500 ppm or less, more preferably 300 ppm or less, and even more preferably 250 ppm or less. The moisture content of the lithium transition metal composite oxide after drying can be measured by the Karl Fischer method.

In the drying step, it is preferable to perform two or more stages of drying treatment in which the drying conditions are changed. Specifically, the drying step preferably includes a first drying step and a second drying step. By performing such a plurality of stages of drying treatment, it is possible to avoid deterioration of the powder surface of the lithium transition metal composite oxide due to rapid drying. Therefore, it is possible to prevent the drying speed from being lowered due to the deterioration of the powder surface.

In the first drying step, the lithium transition metal composite oxide washed with water in the water washing step is dried at a drying temperature of 80 ° C. or higher and 100 ° C. or lower. In the first drying step, most of the water present on the particle surface of the lithium transition metal composite oxide is removed mainly at the drying rate during the constant drying period.
In the first drying step, if the drying temperature is 80 ° C. or higher, a large amount of water can be removed in a short time. Further, when the drying temperature is 100 ° C. or lower, deterioration of the powder surface of the lithium transition metal composite oxide, which tends to occur at high temperatures, can be suppressed.
The drying time in the first drying step is preferably 10 hours or more and 20 hours or less. When the drying time is in this range, most of the particles present on the particle surface of the lithium transition metal composite oxide are present even at a relatively low drying temperature at which deterioration of the powder surface of the lithium transition metal composite oxide is suppressed. Moisture can be removed.

In the second drying step, the lithium transition metal composite oxide dried in the first drying step is dried at a drying temperature of 190 ° C. or higher and 300 ° C. or lower. In the second drying step S42, the moisture existing in the vicinity of the particle surface layer of the lithium transition metal composite oxide is reduced while suppressing the side reaction that deteriorates the performance of the positive electrode active material, and the lithium is dried to an appropriate moisture content. Obtain a transition metal composite oxide.
In the second drying step, if the drying temperature is 190 ° C. or higher, the water permeating the vicinity of the particle surface layer of the lithium transition metal composite oxide can be sufficiently removed. Further, when the drying temperature is 300 ° C. or lower, side reactions that deteriorate the performance of the positive electrode active material can be suppressed and the drying can be performed.
The drying time in the second drying step is preferably 10 hours or more and 20 hours or less. When the drying time is within this range, side reactions that deteriorate the performance of the positive electrode active material can be suppressed, and the lithium transition metal composite oxide can be dried to a sufficiently low moisture content.

<Lithium-ion secondary battery>
Next, a lithium ion secondary battery using the positive electrode active material (positive electrode active material for a lithium ion secondary battery) containing the lithium transition metal composite oxide as the positive electrode will be described.
FIG. 1 is a partial cross-sectional view schematically showing an example of a lithium ion secondary battery.
As shown in FIG. 1, the lithium ion secondary battery 100 includes a bottomed cylindrical battery can 101 containing a non-aqueous electrolyte solution, a winding electrode group 110 housed inside the battery can 101, and a battery can. It includes a disk-shaped battery lid 102 that seals the opening at the top of the 101.
The battery can 101 and the battery lid 102 are made of a metal material such as stainless steel or aluminum. The positive electrode 111 includes a positive electrode current collector 111a and a positive electrode mixture layer 111b formed on the surface of the positive electrode current collector 111a. Further, the negative electrode 112 includes a negative electrode current collector 112a and a negative electrode mixture layer 112b formed on the surface of the negative electrode current collector 112a.

The positive electrode current collector 111a is formed of, for example, a metal foil such as aluminum or an aluminum alloy, an expanded metal, a punching metal, or the like. The positive electrode mixture layer 111b contains a positive electrode active material containing the above-mentioned lithium transition metal composite oxide. The positive electrode mixture layer 111b is formed of, for example, a positive electrode mixture in which a positive electrode active material, a conductive material, a binder, and the like are mixed.
The negative electrode current collector 112a is formed of a metal foil such as copper, a copper alloy, nickel, or a nickel alloy, an expanded metal, a punching metal, or the like. The negative electrode mixture layer 112b contains a negative electrode active material for a lithium ion secondary battery. The negative electrode mixture layer 112b is formed of, for example, a negative electrode mixture in which a negative electrode active material, a conductive material, a binder, and the like are mixed.
As the negative electrode active material, the conductive material, the binder, and the like, an appropriate type used for a general lithium ion secondary battery can be used.

As shown in FIG. 1, the positive electrode current collector 111a is electrically connected to the battery lid 102 via the positive electrode lead piece 103. On the other hand, the negative electrode current collector 112a is electrically connected to the bottom of the battery can 101 via the negative electrode lead piece 104. An insulating plate 105 for preventing a short circuit is arranged between the winding electrode group 110 and the battery lid 102, and between the winding electrode group 110 and the bottom of the battery can 101. A non-aqueous electrolytic solution is injected into the battery can 101.

The lithium ion secondary battery 100 has a cylindrical shape, but the shape and battery structure of the lithium ion secondary battery are not particularly limited, and for example, an appropriate shape such as a square shape, a button shape, or a laminated sheet shape. And other battery structures.
Further, the lithium ion secondary battery according to this embodiment can be used for various purposes. Applications include small power supplies such as portable electronic devices and household electric devices, stationary power supplies such as power storage devices, uninterruptible power supplies, and power leveling devices, ships, railroad vehicles, hybrid railroad vehicles, and hybrids. Examples include, but are not limited to, drive power sources for automobiles, electric vehicles, and the like. The lithium transition metal composite oxide has a high nickel content and exhibits a high discharge capacity, and also exhibits good charge / discharge cycle characteristics, so that it is particularly suitable for automobiles and the like where a long life is required. Can be used.

The chemical composition of the positive electrode active material used in the lithium ion secondary battery is determined by disassembling the battery to collect the positive electrode active material that constitutes the positive electrode, and performing high-frequency induction coupled plasma emission spectroscopic analysis, atomic absorption analysis, etc. You can check. Since the composition ratio of lithium (1 + a in the composition formula (1)) depends on the state of charge, the chemistry of the positive electrode active material is based on whether the coefficient a of lithium satisfies −0.9 ≦ a ≦ 0.08. The composition can also be determined.

Hereinafter, the present invention will be specifically described with reference to examples, but the technical scope of the present invention is not limited thereto.

The positive electrode active material according to the embodiment of the present invention was synthesized, and the element distribution, the discharge capacity, and the charge / discharge cycle characteristics (capacity retention rate) were evaluated. In addition, as a control of the examples, the positive electrode active material according to the comparative example in which the chemical composition was changed was synthesized and evaluated in the same manner.

[Example 1]
First, lithium carbonate, nickel hydroxide, cobalt carbonate, manganese carbonate, titanium oxide, and aluminum oxide are prepared as raw materials, and Li: Ni: Co: Mn: Ti: Al is 1 for each raw material in terms of the molar ratio of metal elements. Weighed so as to be 0.03: 0.85: 0.03: 0.09: 0.01: 0.02, and pure water was added so that the solid content ratio was 30% by mass. Then, wet pulverization (wet mixing) was performed with a pulverizer to prepare a raw material slurry so that the average particle size was less than 0.2 μm (raw material mixing step).

Subsequently, the obtained raw material slurry was spray-dried with a nozzle-type spray dryer (manufactured by Okawara Kakoki Co., Ltd., ODL-20 type) to obtain a granulated material having a _D50 of 12 μm (granulation step). The spray pressure is 0.13 MPa and the spray amount is 260 g / min. Then, the dried granules were heat-treated to calcin the lithium transition metal composite oxide (calcination step). Specifically, the granulated material was dehydrated in a continuous transfer furnace at 400 ° C. for 5 hours in an air atmosphere. Then, in a firing furnace replaced with an oxygen gas atmosphere, a first precursor was obtained by heat treatment at 700 ° C. for 6 hours in an oxygen stream (first heat treatment step). Then, the first precursor was heat-treated at 820 ° C. for 10 hours in a calcining furnace in which the atmosphere was replaced with an oxygen gas atmosphere (second heat treatment step) to obtain a lithium transition metal composite oxide (second heat treatment). Process). The calcined powder obtained by calcining was classified using a sieve having an opening of 53 μm, and the powder under the sieve was used as the positive electrode active material of the sample.

[Example 2]
Except for the fact that the molar ratio of the raw materials was weighed so that Li: Ni: Co: Mn: Ti: Al was 1.03: 0.85: 0.03: 0.09: 0.02: 0.01. A positive electrode active material was obtained in the same manner as in Example 1.

[Example 3]
Except for the fact that the molar ratio of the raw materials was weighed so that Li: Ni: Co: Mn: Ti: Al was 1.03: 0.85: 0.03: 0.08: 0.02: 0.02. A positive electrode active material was obtained in the same manner as in Example 1.

[Example 4]
Except for the fact that the molar ratio of the raw materials was weighed so that Li: Ni: Co: Mn: Ti: Al was 1.03: 0.85: 0.03: 0.08: 0.03: 0.01. A positive electrode active material was obtained in the same manner as in Example 1.

[Example 5]
Except for the fact that the molar ratio of the raw materials was weighed so that Li: Ni: Co: Mn: Ti: Al was 1.03: 0.85: 0.03: 0.05: 0.03: 0.03. A positive electrode active material was obtained in the same manner as in Example 1.

[Example 6]
Except for the fact that the molar ratio of the raw materials was weighed so that Li: Ni: Co: Mn: Ti: Al was 1.03: 0.88: 0.03: 0.05: 0.03: 0.01. A positive electrode active material was obtained in the same manner as in Example 1.

[Example 7]
Except for the fact that the molar ratio of the raw materials was weighed so that Li: Ni: Co: Mn: Ti: Al was 1.03: 0.90: 0.03: 0.03: 0.03: 0.01. A positive electrode active material was obtained in the same manner as in Example 1.

[Comparative Example 1]
The molar ratio of the raw materials was the same as in Example 1 except that the molar ratio of Li: Ni: Co: Mn: Ti was 1.03: 0.85: 0.03: 0.10: 0.02. Obtained a positive electrode active material.

[Comparative Example 2]
The molar ratio of the raw materials was the same as in Example 1 except that the molar ratio of Li: Ni: Co: Mn: Ti was 1.03: 0.85: 0.03: 0.09: 0.03. Obtained a positive electrode active material.

[Comparative Example 3]
The molar ratio of the raw materials was the same as in Example 1 except that the molar ratio of Li: Ni: Co: Mn: Ti was 1.03: 0.90: 0.03: 0.05: 0.02. Obtained a positive electrode active material.

[Example 8]
First, nickel hydroxide, cobalt carbonate, manganese carbonate, titanium oxide, and aluminum oxide are prepared as raw materials, and each raw material has a molar ratio of metal elements of Ni: Co: Mn: Ti: Al, 0.88: 0. Weighed so as to be 03: 0.05: 0.03: 0.01, and pure water was added so that the solid content ratio was 20% by mass. Then, wet pulverization (wet mixing) was performed with a pulverizer to prepare a raw material slurry so that the average particle size was less than 0.2 μm (raw material mixing step).

Subsequently, the obtained raw material slurry was spray-dried with a nozzle-type spray dryer (manufactured by Okawara Kakoki Co., Ltd., ODL-20 type) to obtain a granulated material having a _D50 of 12 μm (granulation step). The spray pressure is 0.13 MPa and the spray amount is 260 g / min. Then, the dried granules were heat-treated at 650 ° C. to calcin the transition metal composite oxide (calcination step). Lithium hydroxide and the obtained transition metal composite oxide were weighed so that the molar ratio of Li: transition metal was 1.03: 1.00, and the mixture was mixed using a V-type mixer. Then, in a firing furnace replaced with an oxygen gas atmosphere, a first precursor was obtained by heat treatment at 500 ° C. for 10 hours in an oxygen stream (first heat treatment step). Then, the first precursor was heat-treated at 820 ° C. for 32 hours (second heat treatment step) in a calcining furnace in which the first precursor was replaced with an oxygen gas atmosphere to obtain a lithium transition metal composite oxide (second heat treatment). Process). The calcined powder obtained by calcining was classified using a sieve having an opening of 53 μm, and the powder under the sieve was used as the positive electrode active material of the sample.

[Comparative Example 4]
The molar ratio of the raw materials was the same as in Example 8 except that the molar ratio of Li: Ni: Co: Mn: Ti was 1.03: 0.85: 0.03: 0.09: 0.03. Obtained a positive electrode active material.

(Measurement of chemical composition of positive electrode active material)
The chemical composition of the synthesized positive electrode active material was analyzed by high frequency inductively coupled plasma emission spectroscopy using an ICP-AES emission spectroscopic analyzer "OPTIMA8300" (manufactured by PerkinElmer). In addition, the amount of oxygen in the positive electrode active material (α in the composition formula (1)) was analyzed by the inert gas melting-infrared absorption method. As a result, the positive electrode active materials according to Examples 1 to 8 and the positive electrode active materials according to Comparative Examples 1 to 4 each had a chemical composition as shown in Table 1, in which only lithium was different from the charged material.

(Elemental concentration distribution)
The concentration distribution of M1 and Al of the synthesized positive electrode active material was measured by the following procedure. First, the prepared positive electrode active material powder is processed by FIB under the conditions of acceleration voltage: 30 kV (sampling) and 10 kV (finishing) using the focused ion / electron beam processing observation device "nano DUET NB5000" (manufactured by Hitachi High-Technologies Corporation). And thinned. Next, the primary particle interface (surface) inside the secondary particles was identified by STEM observation using a scanning transmission electron microscope "JEM-ARM200F" (manufactured by JEOL Ltd.). Then, using the energy dispersive X-ray analyzer "JED-2300T" (manufactured by JEOL Ltd.), the concentration D1 of M1 at the interface of the primary particles inside the secondary particles, the concentration D2 of Al, and the concentration M1 of the central part of the primary particles. Concentration D3 and Al concentration D4 were measured. Two measurement points were used, and the average value was used. The results are shown in Table 1.

FIG. 2A schematically shows an example of secondary particles of a positive electrode active material for a lithium ion secondary battery, and FIG. 2B schematically shows an example of primary particles. As shown in FIG. 2A, the positive electrode active material contains secondary particles formed by aggregating a plurality of primary particles, and the surface of the primary particles also exists inside the secondary particles. The surfaces of adjacent primary particles are in contact with each other to form an interface (see FIG. 3). As shown in FIG. 2B, the atomic concentrations D1 and D2 of M1 and Al at the interface of each primary particle and the atomic concentrations D3 and D4 of M1 and Al at a depth of 0.2r or more from the surface are measured. It is something to do. It should be noted that the interface and the surface are substantially synonymous, and the term "interface of primary particles" in the present specification may be paraphrased as "surface of primary particles".

FIG. 3 shows an STEM image of the primary particles inside the secondary particles of Example 1, an EDX analysis point at the interface (surface) and the central portion.
Further, for each of the concentrations of Ni, Co, Mn, M1 (Ti), and Al in Examples and Comparative Examples, the concentrations (at%) at the interface of the primary particles and the central portion of the primary particles were measured. The results are shown in Table 2.

(Positive electrode)
A lithium ion secondary battery was produced using the synthesized positive electrode active material as the material for the positive electrode, and the discharge capacity and capacity retention rate of the lithium ion secondary battery were determined. First, the mass ratio of the prepared positive electrode active material, the carbon-based conductive material, and the binder previously dissolved in N-methyl-2-pyrrolidone (NMP) is 94: 4.5: 1.5. It was mixed as follows. Then, the uniformly mixed positive electrode mixture slurry was applied onto a positive electrode current collector of an aluminum foil having a thickness of 20 μm so that the coating amount was 10 mg / cm ² . Next, the positive electrode mixture slurry applied to the positive electrode current collector was heat-treated at 120 ° C., and the solvent was distilled off to form a positive electrode mixture layer. Then, the positive electrode mixture layer was pressure-molded by a hot press and punched into a circular shape having a diameter of 15 mm to obtain a positive electrode.

(Discharge capacity)
Subsequently, a lithium ion secondary battery was manufactured using the prepared positive electrode, negative electrode, and separator. As the negative electrode, metallic lithium punched into a circular shape having a diameter of 16 mm was used. As the separator, a polypropylene porous separator having a thickness of 30 μm was used. The positive electrode and the negative electrode were opposed to each other in the non-aqueous electrolytic solution via the separator, and the lithium ion secondary battery was assembled. As the non-aqueous electrolytic solution, a solution in which LiPF6 was dissolved in a solvent in which ethylene carbonate and dimethyl carbonate were mixed so as to have a volume ratio of 3: 7 and LiPF6 was dissolved so as to be 1.0 mol / L was used.

The prepared lithium ion secondary battery was charged in an environment of 25 ° C. with a constant current / constant voltage of 40 A / kg and an upper limit potential of 4.3 V based on the mass of the positive electrode mixture. Then, the battery was discharged to a lower limit potential of 2.5 V at a constant current of 40 A / kg based on the mass of the positive electrode mixture, and the discharge capacity (initial capacity) was measured.

(Charge / discharge cycle characteristics (capacity retention rate))
A lithium ion secondary battery was manufactured using the prepared positive electrode, negative electrode, and separator. As the negative electrode, a negative electrode coated with graphite on a negative electrode final body of copper foil was used by punching it into a circular shape having a diameter of 16 mm. As the separator, a polypropylene porous separator having a thickness of 30 μm was used. The positive electrode and the negative electrode were opposed to each other in the non-aqueous electrolytic solution via the separator, and the lithium ion secondary battery was assembled. As the non-aqueous electrolytic solution, LiPF6 was dissolved in a solvent in which ethylene carbonate and dimethyl carbonate were mixed so as to have a volume ratio of 3: 7 so as to be 1.0 mol / L, and further 1.5% by mass was added. A solution in which vinylene carbonate was dissolved was used.

The prepared lithium ion secondary battery was charged in an environment of 50 ° C. at a constant current / constant voltage of 200 A / kg based on the mass of the positive electrode mixture and an upper limit potential of 4.2 V. Then, a total of 100 cycles of discharging to the lower limit potential of 2.5 V at a constant current of 200 A / kg based on the mass of the positive electrode mixture were performed, and the discharge capacity after 100 cycles was measured. The fraction of the discharge capacity after 100 cycles with respect to the initial capacity was calculated as the capacity retention rate. The results are shown in Table 1.

As shown in Table 1, Examples 1 to 8 were satisfied with the chemical composition represented by the composition formula (1), D1> D3, and D1 / D3> D2 / D4. Further, the e / f of Examples 1 to 8 was in the range of 0.5 ≦ e / f ≦ 5. In particular, in Example 1, although the e / f was 0.5 and the Al content was higher than that of M1, D1> D3, and it was found that Ti was concentrated on the surface. From this, it was found that Al is uniformly solid-dissolved and Ti is surface-concentrated regardless of the ratio of Ti and Al. Further, D1 / D3 is 3.3 or less in Comparative Examples 1 to 4 to which Al is not added, whereas it is as large as 3.5 or more in Examples 1 to 8 to which Al is added, and Al is added. It was found that this promoted the surface thickening of Ti. Al is known to reduce the energy for forming LiNiO ₂ , and by adding Al, the temperature for forming LiNiO ₂ is lowered, and the difference between the temperature at which LiNiO ₂ is formed and the temperature at which Ti starts to dissolve in LiNiO ₂ . It is considered that the surface concentration of Ti was promoted by increasing the temperature. Further, since this event can occur even if the Al content is higher than the Ti content, Al is uniformly solid-solved and Ti is surface-enriched even if the Al content is higher than the Ti content. It is probable that it was done. As a result, it was confirmed that the examples of the present invention obtained a high discharge capacity exceeding 190 Ah / kg, showed a capacity retention rate of 79% or more, and were excellent in charge / discharge cycle characteristics. That is, when the chemical composition represented by the composition formula (1) is satisfied, D1> D3, and D1 / D3> D2 / D4, the charge / discharge cycle characteristics are maintained while maintaining a high discharge capacity. It was confirmed that it was possible to improve. In particular, in Examples 1 to 2, 4 to 8 in which D1 / D3 was 3.7 or more, a capacity retention rate of 80% or more was obtained. Further, in Examples 4 to 6 in which the interfacial Ni ratio was less than 70%, a capacity retention rate of 90% or more was obtained.

100 Lithium ion secondary battery 101 Battery can 102 Battery lid 103 Positive electrode lead piece 104 Negative electrode lead piece 105 Insulation plate 106 Sealing material 110 Winding electrode group 111 Positive electrode 111a Positive electrode current collector 111b Positive electrode mixture layer 112 Negative electrode 112a Negative electrode current collector 112b Negative electrode mixture layer 113 Separator

Claims (5)

The following composition formula (1);
Li _{1 + a} Ni _b Co _c Mn _d M1 _e Al _f O _{2 + α} ... (1)
[However, in the composition formula (1), M1 is one or more elements selected from the group consisting of Ti, Ga, Mg, Zr, and Zn, and a, b, c, d, e, f, and α are , -0.04 ≤ a ≤ 0.08, 0.80 ≤ b <1.0, 0 ≤ c <0.2, 0 ≤ d <0.2, 0 <e <0.08, 0 <, respectively. It is a number satisfying f <0.04, 0 <e + f <0.08, b + c + d + e + f = 1, and −0.2 <α <0.2. ] Is a positive electrode active material for a lithium ion secondary battery containing a lithium transition metal composite oxide represented by.
The positive electrode active material contains secondary particles formed by aggregating a plurality of primary particles.
In the primary particles inside the secondary particles, the atomic concentration D1 of M1 and the atomic concentration D2 of Al at the interface between the primary particles, and the atomic concentration D3 of M1 and the atom of Al in the central part of the primary particles. A positive electrode active material for a lithium ion secondary battery, wherein the concentration D4 is D1> D3 and D1 / D3> D2 / D4. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the coefficient e of M1 and the coefficient f of Al are 0.5 ≦ e / f ≦ 5. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein D1 is 3.7 times or more that of D3. A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3. The following composition formula (1);
Li _{1 + a} Ni _b Co _c Mn _d M1 _e Al _f O _{2 + α} ... (1)
[However, in the composition formula (1), M1 is one or more elements selected from the group consisting of Ti, Ga, Mg, Zr, and Zn, and a, b, c, d, e, f, and α are , -0.04 ≤ a ≤ 0.08, 0.80 ≤ b <1.0, 0 ≤ c <0.2, 0 ≤ d <0.2, 0 <e <0.08, 0 <, respectively. It is a number satisfying f <0.04, 0 <e + f <0.08, b + c + d + e + f = 1, and −0.2 <α <0.2. ], Which is a method for producing a positive electrode active material for a lithium ion secondary battery containing a lithium transition metal composite oxide represented by In the primary particles having the chemical composition represented by the composition formula (1) and inside the secondary particles, the atomic concentrations of M1 and Al at the interface between the primary particles are the atomic concentrations of D1 and Al. A firing step of obtaining a lithium composite compound in which D2 and the atomic concentration D3 of M1 and the atomic concentration D4 of Al in the central portion of the primary particles are D1> D3 and D1 / D3> D2 / D4. A method for producing a positive electrode active material for a lithium ion secondary battery.

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