JP7532880B2 - Positive electrode active material for lithium ion secondary battery, manufacturing method thereof, and lithium ion secondary battery - Google Patents

Description

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

Lithium-ion secondary batteries are widely used as lightweight secondary batteries with high energy density. Compared to other secondary batteries such as nickel-metal hydride batteries and nickel-cadmium batteries, lithium-ion secondary batteries have features such as high energy density and small memory effect. As a result, their applications are expanding from small power sources for portable electronic devices, power tools, household electrical appliances, etc., to stationary power sources such as power storage devices, uninterruptible power supplies, and power leveling devices, to medium- and large-sized power sources such as drive power sources for ships, railway vehicles, hybrid railway vehicles, hybrid automobiles, electric automobiles, etc.

As the range of applications for lithium-ion secondary batteries expands, there is a demand for even higher capacity. There is also a need for excellent charge/discharge cycle characteristics and output characteristics suited to the application. For various applications such as stationary power sources and drive power sources, there is a high demand for high output, and for in-vehicle use, output stability is required to enable EVs to travel longer distances. There is a demand for output characteristics that provide stable output throughout discharge and enable continuous high-output operation regardless of the state of charge (SOC).

Under these circumstances, in addition to establishing high capacity and mass production, studies are being conducted on the positive electrode active material, which greatly affects the battery characteristics, to reduce the resistance of lithium ions and stabilize the crystal structure. As a positive electrode active material for lithium ion secondary batteries, lithium transition metal composite oxides having an α-NaFeO ₂ type crystal structure (hereinafter sometimes referred to as a layered structure) are widely known. As an oxide having a layered structure, LiCoO ₂ has been used conventionally, but due to demands for high capacity and mass production, ternary systems represented by Li(Ni,Co,Mn)O ₂ and nickel systems in which LiNiO ₂ is substituted with different elements have been developed.
Among lithium transition metal composite oxides having a layered structure, nickel-based oxides have the disadvantage that their life characteristics are not necessarily good. However, nickel-based oxides are composed of nickel, which is cheaper than cobalt and the like, and have a relatively high capacity, so they are expected to be used in various applications. In particular, there is growing expectation for chemical compositions with a high ratio of nickel per metal other than lithium (Ni, Co, Mn, etc.).

For example, Patent Document 1 describes a lithium transition metal compound powder for use as a positive electrode material in lithium secondary batteries, which is prepared by firing a fine and uniform mixture of the main component raw material of a lithium transition metal compound and a compound (additive) composed of a metal element that can have a valence of five or six, and is characterized by having a continuous composition gradient structure in which the additive element exists with a concentration gradient in the depth direction from the particle surface, specifically within a range of about 10 nm deep.

In addition, Patent Document 2 describes Li _1+a Ni _b Mn _c Co _d Ti _e M _f O _2+α (1)
(wherein, in formula (1), M is at least one element selected from the group consisting of Mg, Al, Zr, Mo and Nb, and a, b, c, d, e, f and α are numbers that satisfy -0.1≦a≦0.2, 0.7<b≦0.9, 0≦c<0.3, 0≦d<0.3, 0<e≦0.25, 0≦f<0.3, b+c+d+e+f=1, and -0.2≦α≦0.2), and the atomic ratio of Ti3 ⁺ to Ti4 ⁺ , Ti3 ⁺ /Ti4 ⁺ , based on X-ray photoelectron spectroscopy is 1.5 or more and 20 or less.

Furthermore, Patent Document 3 describes a positive _electrode active material for non-aqueous _electrolyte secondary _batteries , which is represented by the general formula _{LitNi1 -xyzCoxAlyTizO2 (} 0.98≦t≦1.10, 0<x≦0.30, 0.03≦y≦0.15, 0.001≦z≦0.03), in which a titanium-enriched layer is formed on the surfaces of primary particles and/or on grain boundaries between primary particles.

Patent No. 5428251 Patent No. 6197981 Patent No. 4807467

Nickel-based materials with a high nickel content and a ratio of nickel per metal excluding lithium of 80% or more have the disadvantage that the stability of the crystal structure is low, and therefore the capacity is likely to decrease and the resistance is likely to increase, especially during charge-discharge cycles at high temperatures. In general, lithium transition metal composite oxides with a high nickel content have a tendency to have unstable crystal structures during charge-discharge. In the crystal structure of LiMeO ₂ (Me represents a metal element such as Ni), there are many trivalent Ni, but in the charged state in which Li is desorbed, Ni forms a layer composed of MeO ₂ , and there are many tetravalent Ni. Tetravalent Ni is likely to become more stable divalent Ni. Divalent Ni has an ionic radius close to Li ⁺ , so it is likely to occupy Li sites (this is called cation mixing). In addition, oxygen is likely to be desorbed near the surface, and a different phase that transitions from NiO ₂ to a NiO-like crystal structure is likely to be formed. Since this heterophase formation is irreversible, it does not return to _LiMeO2 even with charging and discharging, resulting in a decrease in capacity and an increase in resistance. Therefore, in order to stabilize the surface structure, there is a method of placing an additive element that stabilizes the crystal structure on the crystal surface.

The above-mentioned Patent Documents 1 to 3 suppress changes in the crystal structure by forming a concentration gradient or surface enrichment layer due to the added element on the surface of the Ni-Co-Mn-based positive electrode active material, and achieve a certain level of effect. However, if the proportion of nickel is increased to 80% or more for the purpose of increasing capacity, etc., it becomes even more difficult to maintain stability. Furthermore, if the amount of added element is increased too much in order to increase stability, capacity reduction and rate characteristics will occur. Therefore, in positive electrode active materials with a higher Ni ratio, there is a demand for improved durability (charge-discharge cycle characteristics) in addition to high discharge capacity and good output characteristics (rate characteristics).

The present invention relates to a high Ni ratio positive electrode active material with a Ni ratio of 80% or more, and aims to provide a positive electrode active material for lithium ion secondary batteries (hereinafter sometimes simply referred to as "positive electrode active material") that has a high discharge capacity, good rate characteristics, and good charge/discharge cycle characteristics, a method for producing the same, and a lithium ion secondary battery that uses this positive electrode active material.

The positive electrode active material for a lithium ion secondary battery according to the present invention has the following composition formula (1):
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[In the composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number satisfying −0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, and −0.2<α<0.2. a lithium-ion secondary battery positive electrode active material containing a lithium transition metal composite oxide represented by the formula (1), wherein the positive electrode active material contains secondary particles formed by agglomeration of a plurality of primary particles, and an atomic concentration A0 of X at an interface between the primary particles inside the secondary particles, an atomic concentration A1 of X at a depth of 1 nm from the interface between the primary particles inside the secondary particles, and an atomic concentration A2 of X at a center of the primary particle inside the secondary particles satisfy A0>A1>A2, and in an element distribution of Ni and X in a region spanning the interface of the primary particles, a distance |β1-β2|=L(β) between positions β1 and β2 corresponding to an inflection point of a concentration gradient of Ni is regarded as a grain boundary, and relative to this, positions α1 and α2 corresponding to an inflection point of a concentration gradient of X are each located inside the primary particles, and the distance therebetween is |α1-α2|=L(α), where 1 nm≦L(α)-L(β)≦3 nm.

Another positive electrode active material for a lithium ion secondary battery according to the present invention has the following composition formula (1):
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[wherein, in composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number that satisfies −0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, −0.2<α<0.2.], the positive electrode active material for a lithium ion secondary battery includes a lithium transition metal composite oxide represented by the following formula: [wherein, in composition formula (1), M represents at least one selected from Al and Mn, X represents Ti, and is a number that satisfies −0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, −0.2<α<0.2.], the positive electrode active material for a lithium ion secondary battery includes secondary particles formed by agglomeration of a plurality of primary particles, and an atomic concentration A0 of X at an interface between primary particles inside the secondary particles, an atomic concentration A1 of X at a depth of 1 nm from the interface between primary particles inside the secondary particles, and an atomic concentration A2 of X at a center of a primary particle inside the secondary particles satisfy the following relationships: A1×3.5>A0>A1>A2.

The positive electrode active material for lithium ion secondary batteries according to the present invention is a positive electrode active material for lithium ion secondary batteries containing the above-mentioned lithium transition metal composite oxide, and it is preferable that the concentration B0 of X on the surface of the primary particles on the surface of the secondary particles, the atomic concentration B1 of X at a depth of 1 nm from the surface of the primary particles on the surface of the secondary particles, and the atomic concentration B2 of X in the center of the primary particles on the surface of the secondary particles are in the range of B1 x 2.5 > B0 > B1 > B2.

Furthermore, in the positive electrode active material for a lithium ion secondary battery of the present invention, it is preferable that 0≦c≦0.05 in the composition formula (1).

The present invention is a lithium ion secondary battery having a positive electrode containing the positive electrode active material for lithium ion secondary batteries represented by the above composition formula (1).
In the lithium ion secondary battery, the positive electrode active material for lithium ion secondary batteries may satisfy the formula (1) of -0.9<a<0.1.

The method for producing a positive electrode active material for a lithium ion secondary battery according to the present invention comprises the steps of:
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[In the composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number satisfying −0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, and −0.2<α<0.2. The method for producing a positive electrode active material for a lithium ion secondary battery comprising a lithium transition metal composite oxide represented by the formula (1) includes a mixing step of mixing compounds containing the metal elements Li, Ni, Co, M, and X in the formula (1) to obtain a mixture, and a firing step of firing the mixture to obtain the lithium transition metal composite oxide represented by the formula (1), the firing step includes a stage of maintaining the mixture at 750° C. or higher and 900° C. or lower, and then includes a stage of annealing treatment in which the temperature is lowered from the maximum temperature and the temperature is maintained in a temperature range of 700° C. or higher and 800° C. or lower for 1.5 hours or more, and the positive electrode active material for a lithium ion secondary battery comprising the lithium transition metal composite oxide comprises secondary particles constituted by agglomeration of a plurality of primary particles, and a metal oxide is present inside the secondary particles. the atomic concentration A0 of X at an interface between primary particles, the atomic concentration A1 of X at a depth of 1 nm from the interface between primary particles inside the secondary particle, and the atomic concentration A2 of X at a center of the primary particle inside the secondary particle satisfy A0>A1>A2, and in the element distribution of Ni and X in a region spanning the interface of the primary particles, a distance |β1-β2|=L(β) between positions β1 and β2 corresponding to an inflection point of the Ni concentration gradient is regarded as a grain boundary, and positions α1 and α2 corresponding to an inflection point of the X concentration gradient are each located inside the primary particle, and the distance therebetween is |α1-α2|=L(α), where 1 nm≦L(α)-L(β)≦3 nm.

According to the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery having a high discharge capacity, good rate characteristics, and good charge/discharge cycle characteristics, in a positive electrode active material with a high Ni ratio of 80% or more.

FIG. 2 is a flow diagram showing an example of a method for producing a positive electrode active material of the present invention. FIG. 1 is a partial cross-sectional view illustrating an example of a lithium ion secondary battery. FIG. 2 is a diagram illustrating an example of a secondary particle and a primary particle of a positive electrode active material. 1 is an STEM image of primary particles in Comparative Example 1. 1 is a STEM image of a primary particle in Example 2. FIG. 13 is a diagram showing the intensity distribution of each element by STEM-EELS analysis of primary particles in Comparative Example 1. FIG. 13 is a diagram showing the intensity distribution of each element by STEM-EELS analysis of primary particles in Example 2. FIG. 1 is a diagram showing a smoothed intensity distribution of each element by STEM-EELS analysis of primary particles in Comparative Example 1. FIG. 11 is a diagram showing a smoothed intensity distribution of each element by STEM-EELS analysis of primary particles in Example 2. FIG. 13 is a diagram showing the absolute value of the STEM-EELS intensity change of primary particles in Comparative Example 1. FIG. 13 is a diagram showing the absolute value of the STEM-EELS intensity change of primary particles in Example 2.

The following provides a detailed description of a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention, and a lithium ion secondary battery using the same.

<Positive electrode active material>
The positive electrode active material according to the present embodiment includes a lithium transition metal composite oxide having a layered α- _NaFeO2 type crystal structure and containing lithium and a transition metal. This positive electrode active material is mainly composed of primary particles of the lithium transition metal composite oxide and secondary particles formed by agglomeration of a plurality of primary particles. The lithium transition metal composite oxide has a layered structure as a main phase that allows insertion and desorption of lithium ions.

The positive electrode active material according to this embodiment may contain, in addition to the lithium transition metal composite oxide which is the main component, unavoidable impurities derived from the raw materials or the manufacturing process, other components that coat the lithium transition metal composite oxide particles, such as boron components, phosphorus components, sulfur components, fluorine components, organic substances, etc., and other components that are mixed with the lithium transition metal composite oxide particles.

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 M _d X _e O _2+α ...(1)
[In the composition formula (1), M represents at least one selected from Al and Mn, and X represents one or more metal elements other than Li, Ni, Co, Al, and Mn, and is a number that satisfies -0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=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 at 80 at% or more in terms of atomic fraction with respect to the total of Ni, Co, M and X. Since the nickel content is high, it is a nickel-based oxide that can realize a high discharge capacity. In addition, since the nickel content is high, the raw material cost is cheaper than _LiCoO2 etc., and it is also excellent from the viewpoint of productivity including raw material cost.

In general, lithium transition metal composite oxides with a high nickel content have a tendency to have unstable crystal structures during charging and discharging. In the crystal structure, Ni forms a layer composed of MeO ₂ (Me represents a metal element such as Ni), and there is a lot of tetravalent Ni. Tetravalent Ni becomes stable divalent Ni and tends to occupy Li sites (this state is called cation mixing), so it transitions from the surface of the crystal structure to a NiO-like crystal structure, causing a decrease in capacity and an increase in resistance.

As described above, there is a method of arranging a stable additive element that can suppress cation mixing on the crystal surface to stabilize the surface structure. The above-mentioned Patent Documents 1 to 3 are examples of this method, but in the past, it was limited to protecting the surface from the outside of the crystal or preventing the movement of main component atoms between primary particles to suppress grain growth, and there was room for improvement in the interior of the crystal near the surface. Therefore, in this embodiment, we focus on the solid solution behavior of the additive element inside the crystal and consider the effect of its concentration distribution.

That is, in this embodiment, in each primary particle (hereinafter sometimes simply referred to as a primary particle) located inside a secondary particle, the concentrated region of the metal element X in the above composition formula is expanded to the inside of the crystal of the primary particle, thereby reducing the change in the crystal structure near the surface of the primary particle due to the change in the valence of Ni. Therefore, it is possible to further suppress lattice distortion and crystal structure change in the secondary particle. In addition, since the concentrated region inside the crystal is very thin, at 3 nm or less, it does not hinder charging and discharging. As a result, it has been found that good rate characteristics and good charge and discharge cycle characteristics can be obtained.

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

In the composition formula (1), a is preferably -0.1 or more and 0.1 or less, more preferably -0.04 or more and 0.04 or less, and even more preferably -0.02 or more and 0.02 or less. a represents the excess or deficiency of lithium with respect to the stoichiometric ratio Li(Ni,Co,M,X) _O2 . a is not the charge value at the time of raw material synthesis, but the value in the lithium transition metal composite oxide obtained by firing. The same applies to the following elements. When the excess or deficiency of lithium in the composition formula (1) is excessive, that is, when the composition has too little lithium or too much lithium relative to the total of Ni, Co, and M, the synthesis reaction does not proceed properly during firing, and cation mixing in which nickel is mixed into the lithium site is likely to occur, or the crystallinity is likely to decrease. In particular, when the proportion of nickel is increased to 80% or more, the occurrence of such cation mixing and the decrease in crystallinity are likely to become significant, and the discharge capacity and charge/discharge cycle characteristics are likely to be impaired. In contrast, when a is in the above-mentioned range, cation mixing is likely to be reduced, and various battery performances can be improved, which contributes to obtaining a high discharge capacity, good rate characteristics, and good charge/discharge cycle characteristics even in a composition with a high nickel content.

The above describes the preferred range of a in the powder state produced as a positive electrode active material, but when the positive electrode active material represented by composition formula (1) is incorporated into the positive electrode of a lithium ion secondary battery, charging and discharging are performed with insertion and removal of Li, so a is preferably in the range of -0.9 to 0.1. When a in the initial state is -0.1 or more and 0.1 or less, the amount of Li is 0.9 to 1.1 in atomic ratio, so Li changes in the range of 0 to 1.1 when charging and discharging are involved, and a range of -0.9<a<0.1 is appropriate for the range of a.

The nickel coefficient b in the composition formula (1) is 0.80 or more and less than 1.00. When b is 0.80 or more, a higher discharge capacity can be obtained compared to other nickel-based oxides with a low nickel content and ternary oxides represented by Li(Ni,Co,M) _O2 . In addition, the amount of transition metals that are rarer than nickel can be reduced, thereby reducing raw material costs.

The nickel coefficient b may be 0.85 or more, 0.90 or more, or 0.92 or more. The larger b is, the higher the discharge capacity tends to be. The nickel coefficient b 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 associated with the insertion and removal of lithium ions becomes, and the less likely cation mixing, in which nickel is mixed into the lithium site, or the less likely a decrease in crystallinity occurs during firing, so that good rate characteristics and charge/discharge cycle characteristics tend to be obtained.

The coefficient c of cobalt in the composition formula (1) is preferably 0 or more and less than 0.2. Cobalt may be actively added, or may have a composition ratio equivalent to unavoidable impurities. When the cobalt is in the above range, the crystal structure becomes more stable, and effects such as suppressing cation mixing in which nickel is mixed into the lithium site can be obtained. Therefore, high discharge capacity and good charge/discharge cycle characteristics can be obtained. On the other hand, if there is an excess of cobalt, the raw material cost of the positive electrode active material increases. In addition, the ratio of other transition metals such as nickel decreases, and there is a risk that the discharge capacity will be low or the effect of the metal element represented by M will be reduced. On the other hand, if c is in the above numerical range, the raw material cost of the lithium transition metal composite oxide that shows high discharge capacity, good rate characteristics and charge/discharge cycle characteristics can be reduced.

The cobalt coefficient c is preferably 0 or more and 0.3 or less. Although cobalt has the effect of suppressing cation mixing, it is preferable that the content is small in order to reduce raw material costs. It is more preferable that the content is 0 or more and 0.1 or less, and even more preferable that the content is 0 or more and 0.05 or less. When the added element X described below is added in an amount exceeding 0, the crystal structure can be stabilized without making c exceed 0.05.

In the composition formula (1), M is at least one metal element selected from Al and Mn. These elements can substitute for the Ni site, and since Al is a typical element, it exists stably without changing its valence even during charging and discharging, and although Mn is a transition metal, it is thought to exist stably with a valence of +4 even during charging and discharging. Therefore, the use of these metal elements has the effect of stabilizing the crystal structure during charging and discharging.

The coefficient d of M in the composition formula (1) is preferably 0 or more and less than 0.2. If the metal element represented by M is in excess, the ratio of other transition metals such as nickel will be low, and the discharge capacity of the positive electrode active material may be low. In contrast, if d is within the above-mentioned numerical range, a higher discharge capacity, good rate characteristics, and charge/discharge cycle characteristics tend to be obtained.

X in the composition formula (1) is one or more metal elements other than Li, Ni, Co, Al, and Mn (hereinafter, sometimes referred to as X element or element X). These elements are metal elements that react with Li to form a concentrated layer after Li and Ni react to form an α-NaFeO ₂ type crystal structure that exhibits a layered structure. In this case, the X element is present on the primary particle surface of the positive electrode active material in a relatively low-temperature baking process where the reaction between Li and Ni begins, and the X element is likely to form a concentrated layer on the primary particle surface in the subsequent high-temperature baking process. In particular, by premixing all elements including the X element using a solid-phase method and finely pulverizing, the X element can be distributed on the primary particle surface. The X element is preferably at least one element selected from the group consisting of Ti, Ga, Mg, Zr, and Zn. Of these, it is more preferable to contain at least Ti. Ti can be tetravalent, so it has a strong bond with O and has a large effect of stabilizing the crystal structure. In addition, the molecular weight is relatively small, and the theoretical capacity of the positive electrode active material is reduced only slightly when the metal element is added. By using a metal element with such a property, it is possible to concentrate and distribute the metal element on the surface of the primary particle by selecting appropriate synthesis conditions. Therefore, by using these metal elements X, the effect of suppressing the deterioration of the crystal structure from the vicinity of the surface of the positive electrode active material during charging and discharging can be obtained.

(Method of Measuring Concentration Distribution of X Element)
The concentration distribution of the X element in the synthesized positive electrode active material is preferably measured using a scanning transmission electron microscope (STEM). To analyze the concentration of a specific element at the interface of the primary particles inside the secondary particles or at the surface of the secondary particles, electron energy loss spectroscopy (EELS) analysis or energy dispersive X-ray spectroscopy (EDS) analysis is used. The concentration distribution of the target element can be obtained by linear analysis of the interface or the vicinity of the surface. In the present invention, attention is paid to the distribution of Ni and X element, and in particular, when scanning perpendicularly across the interface, Ni, which is the main component element, shows a peak that is minimal at the interface. On the other hand, X element shows a peak that is maximal at the interface, and it can be seen that it is concentrated at the interface of the primary particles.

In order to quantitatively evaluate whether the X element is dissolved from the interface to the inside of the primary particle, the concentration distribution was analyzed by the method shown below. The Ni intensity profile I obtained by line analysis using EELS is differentiated at the position x to calculate dI/dx. Next, a graph is created with the absolute value of dI/dx on the vertical axis and x on the horizontal axis. Since the Ni intensity profile across the interface is a single minimum peak, two peaks of |dI/dx| appear at the position corresponding to the inflection point. The interval (hereinafter, L(β)) between these two peaks (hereinafter, referred to as double peaks) is regarded as the grain boundary. Similarly, a double peak of |dI/dx| is obtained from the intensity profile of the X element. This peak interval (hereinafter, referred to as L(α)) is the concentrated region of the X element. If L(α)>L(β), it indicates that the X element is dissolved inside the primary particle.
L(α)-L(β) is preferably 1.0 nm or more and 3.0 nm or less. If L(α)-L(β) is less than 1.0 nm, the effect of the X element being dissolved in the inside of the primary particles cannot be obtained. If L(α)-L(β) exceeds 3.0 nm, the X element cannot be concentrated at a depth of 1 nm from the surface of the primary particles, which is undesirable because it reduces the discharge capacity and rate characteristics.

(Primary particles that make up secondary particles)
The positive electrode active material of the embodiment of the present invention includes secondary particles in which a plurality of primary particles of a lithium transition metal composite oxide are aggregated, and the inside of the secondary particle is configured such that a plurality of primary particles are adjacent to each other via an interface. However, not all primary particles form an interface, but many primary particles may form an interface. In the primary particles inside the secondary particle, the atomic concentration A0 (at%) of the X element at the interface (which may be called the surface of the primary particle), the atomic concentration A1 (at%) of the X element at a depth of 1 nm from the interface, and the atomic concentration A2 (at%) of the X element at the center of the primary particle are A0>A1>A2. Here, the atomic concentrations A0, A1, and A2 of the X element are expressed as (X/(Ni+Co+M+X)) and can be confirmed by EDS or the like. In the EDS analysis, by appropriately selecting the analysis conditions such as the device performance and the beam current, it is possible to achieve both a spatial resolution of 0.2 nm or less and an analysis accuracy of 0.1 at% or less, and analysis at 1 nm intervals is possible. Furthermore, the primary particles having thickened surfaces should account for at least 50% by number of the primary particles inside the secondary particles, preferably 70% by number or more, and more preferably 100%.

Here, it is effective that the surface enrichment layer is present not only on the surface of the secondary particle but also on each primary particle inside the secondary particle, and that the X element is enriched in an extremely thin region several nm deep from the surface. In general, it is known that even an insulator has electronic conductivity due to the tunnel effect if it is 10 nm or less thick. Therefore, it can be said that if the enrichment layer is thinner than 10 nm, the charge/discharge reaction proceeds even if the material is insulating. In this embodiment, each primary particle has an extremely thin enrichment layer several nm deep, so that the charge/discharge reaction between the primary particles occurs uniformly.
By dissolving the X element from the primary particle interface to the inside of the particle by several nm, the ratio of Ni is relatively decreased even if the concentrated layer is very thin, so that the crystal structure near the surface becomes more stable and leads to obtaining good charge-discharge cycle characteristics. In order to maintain high discharge capacity and rate characteristics, a thinner layer is preferable, but since a thickness of 5 nm or more may cause a decrease in discharge capacity, a thickness of less than 5 nm is preferable, and a thickness of 3 nm or less is more preferable.

On the other hand, if the layer of concentrated X element exceeds 10 nm, the concentrated layer of X element becomes a resistance component and inhibits the insertion and desorption of Li, resulting in a decrease in rate characteristics. In addition, if the concentrated layer of X element is only on the surface of the secondary particles, when the electrolyte that has penetrated into the inside of the secondary particles comes into contact with the secondary particles, the deterioration of the crystal structure begins from the interface of the primary particles, which causes the charge-discharge cycle characteristics to decrease. If the state in which the concentration of X element is higher than that of the center of the primary particles is within a depth of 10 nm from the surface of the primary particles, the deterioration of the crystal structure can be suppressed, and a high discharge capacity, good rate characteristics, and charge-discharge cycle characteristics can be obtained. In addition, in the present invention, the concentration A0 of the X element at the interface between the primary particles and the concentration A1 of the X element at a depth of 1 nm from the interface are used as indicators indicating the presence of a layer of concentrated X element on the surface layer of the primary particles. In addition, the center of the primary particles refers to a range that is 0.2r or more deep from the surface of the primary particles toward the center of the primary particles, where r is the average particle diameter of the primary particles and the diameter of the primary particles in a specified direction is selected to be within the range of r±10%.

The atomic concentrations A0, A1, and A2 of the X element are preferably A0>A1>A2. When A0>A1, it indicates that the X element is more concentrated at the outermost surface of the primary particles than at a position 1 nm deep inside the grain, and the Ni concentration at the outermost surface is lower than that inside the grain, so that the effect of maintaining the stability of the crystal structure can be sufficiently obtained. When A1>A2, the concentration of the X element near the surface up to a depth of 1 nm inside the primary particles is higher than that when it is uniformly distributed in the primary particles, so the Ni ratio near 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.
Furthermore, it is preferable that 3.5×A1>A0. The present invention realizes the concentration of the X element at a depth of 1 nm from the surface of the primary particles, and aims to stabilize the crystal structure while maintaining the capacity without reducing the lithium transition metal composite oxide having the R-3m structure. If the interface A0 between the primary particles is 3.5×A1 or more, the X element itself dissolved in the primary particles is small, and the effect of stabilizing the crystal structure is not sufficiently obtained. In addition, there is no large difference between the surface and a depth of 1 nm inside in the expansion and contraction of the crystal structure accompanying the insertion and desorption of Li during the charge and discharge cycle, and good charge and discharge cycle characteristics can be obtained.

It is preferable that the X element is concentrated on the surface of the primary particle located at the outermost surface of the secondary particle as well as on the interface of the primary particle inside the secondary particle. When the atomic concentration of the X element on the surface of the primary particle located at the outermost surface of the secondary particle is expressed as B0 (at%), the atomic concentration of the X element at a depth of 1 nm from the surface as B1 (at%), and the atomic concentration of the X element at the center of the primary particle as B2 (at%), it is preferable that B0>B1>B2. Here, the atomic concentrations B0, B1, and B2 of the X element are expressed as (X/(Ni+Co+M+X)).
Furthermore, it is preferable that B1×2.5>B0. When B0 is less than B1×2.5, the amount of X element itself dissolved in the primary particles is not reduced, and the effect of stabilizing the crystal structure is obtained. Since the insertion and desorption of Li is more prominent at the outermost surface of the secondary particles than at the internal interface, it is preferable that the amount of X element dissolved in the primary particles is greater than that at the interface between the primary particles inside the secondary particles. In addition, there is no significant difference between the surface and the 1 nm depth inside in the expansion and contraction of the crystal structure accompanying the insertion and desorption of Li during the charge and discharge cycle, and good charge and discharge cycle characteristics can be obtained.

(Composition Analysis)
The average composition of the particles of the positive electrode active material can be confirmed by high-frequency inductively coupled plasma (ICP), atomic absorption spectrometry (AAS), or the like.

(Powder properties)
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, a reaction field of the positive electrode active material can be secured, and high discharge capacity and good rate characteristics can be obtained. It is more preferably 1.5 μm or less, and even more preferably 1.0 μm or less. The primary particle size can be measured from an image of the positive electrode active material observed with an SEM. That is, the diameters of the inscribed circle and the circumscribed circle of the primary particle are averaged to obtain the particle size of the primary particle. This is measured for 50 particles and averaged to obtain the average particle size.
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 of the positive electrode active material can be produced by granulating the primary particles produced by the method for producing the positive electrode active material described below, using dry granulation or wet granulation. As the granulation means, for example, a granulator such as a spray dryer or a tumbling 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, and even 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. If the BET specific surface area is 0.1 m ² /g or more, a positive electrode having a sufficiently high molding density and a sufficiently high packing rate of the positive electrode active material can be obtained. If the BET specific surface area is 1.5 m ² /g or less, destruction, deformation, particle detachment, etc. are unlikely to occur during pressure molding of the lithium transition metal composite oxide or during volume change accompanying charge and discharge, and the suction of the binder by the pores can be suppressed. Therefore, the coating property and adhesion of the positive electrode active material are improved, and a high discharge capacity, good rate characteristics, and charge and discharge cycle characteristics can be obtained.

<Method of manufacturing positive electrode active material>
The positive electrode active material according to the present embodiment can be produced by reliably promoting a synthesis reaction between lithium and nickel, etc. under appropriate firing conditions under a raw material ratio such that the lithium transition metal composite oxide has a chemical composition represented by composition formula (1). The method for producing the positive electrode active material according to the present embodiment uses the production method described below, but is not limited thereto.

[First manufacturing method]
FIG. 1(A) is a flow diagram of a first method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention.
As shown in FIG. 1A, the method for producing a positive electrode active material for a lithium ion secondary battery according to this embodiment includes a mixing step S10, a granulation step S20, and a firing step S30 in this order. Note that other steps may be added. For example, when a large amount of lithium carbonate remains in the positive electrode active material obtained in the firing step S30, the electrode mixture in the form of a slurry is gelled in the mixture coating step for producing the positive electrode 111 shown in FIG. 2, so that the remaining lithium carbonate can be reduced by adding a water washing step and a drying step following the firing step S30. Note that, when the water washing and drying steps are added, the positive electrode active material obtained after the water washing and drying steps satisfies the composition formula (1). When the firing step is completed, the positive electrode active material obtained after the firing step satisfies the composition formula (1). This is because the value of a in the positive electrode active material in the state in which it is used in the battery is important for a. In addition, even after the positive electrode active material is incorporated into a battery and is charged and discharged, the characteristics of the present invention are maintained, but it is considered that the amount of Li is lost and the value of a changes to approximately −0.99 to 0.

In the mixing step S10, a compound containing lithium and a compound containing a metal element other than Li in the composition formula (1) are mixed. For example, by weighing each of these raw materials, grinding and mixing them, a powdery mixture in which the raw materials are uniformly mixed can be obtained. As a grinder for grinding the raw materials, for example, a general precision grinder such as a ball mill, a jet mill, a rod mill, a sand mill, etc. can be used. The grinding of the raw materials may be dry grinding or wet grinding. After the dry grinding, a solvent such as water may be added to form a slurry composed of the raw materials and the solvent, or a solvent such as water may be added to the raw materials in advance to form a slurry and then wet grinding. From the viewpoint of obtaining a uniform and fine powder with an average particle size of 0.3 μm or less, it is more preferable to perform wet grinding using a medium such as water.

In this embodiment, in order to concentrate the X element near the surface of the primary particles, a solid-phase method is used to crush and mix the raw materials, rather than a coprecipitation method in which the raw materials are dissolved and precipitated in a solvent. In the process of mixing the raw materials, it is important to crush the raw material of the X element at the same time as the other raw materials and mix them into a uniform and fine powder with an average particle size of 0.3 μm or less, preferably 0.1 μm to 0.3 μm. Furthermore, it is important to uniformly disperse the raw materials. For example, in wet mixing, it is good to use a dispersant to improve the dispersibility of the raw materials in the slurry. When the dispersibility of the raw materials is improved, the thickness of the concentrated layer of the X element becomes uniform, which is preferable. The dispersant can be a polycarboxylic acid type, a urethane type, or an acrylic resin type, and an acrylic resin type is preferable. The amount of dispersant added can be arbitrarily added to adjust the viscosity of the slurry.

In this embodiment, as described above, the average particle size of the raw material is 0.3 μm or less, preferably 0.1 μm to 0.3 μm, and the raw material is uniformly and finely ground, and further uniformly dispersed. Therefore, the raw material is pulverized finer than the primary particles of the positive electrode active material, which are several hundred nm to several μm. From this raw material, individual primary particles can be uniformly nucleated and grown. In the process of this nucleation and crystal growth reaction, Li and Ni react at a relatively low temperature to form a layered structure, α-NaFeO ₂ type crystal structure, as the primary particles. Then, the X element reacts with Li to form a concentrated layer of the primary particles. The dispersibility of the X element at this time is important, and by dispersing it finely and uniformly, the concentrated layer can be realized to a very thin and uniform thickness of a few nm deep from the surface of the primary particles.

Examples of lithium-containing compounds include lithium carbonate, lithium acetate, lithium nitrate, lithium hydroxide, lithium chloride, and lithium sulfate. As shown in FIG. 1(A), it is preferable to use lithium carbonate, and it is more preferable to use lithium carbonate in a proportion of 80 mass% or more of the lithium-containing raw material. Lithium carbonate is easily available because it is inexpensive and has excellent supply stability compared to other lithium-containing compounds. In addition, lithium carbonate is weakly alkaline, so it causes less damage to manufacturing equipment and has excellent industrial usability and practicality.

As compounds containing metal elements other than Li, a compound containing nickel, a compound containing cobalt, a compound containing a metal element represented by M, and a compound containing a metal element represented by X are mixed according to the composition of the lithium transition metal composite oxide. As compounds containing metal elements other than Li, compounds composed of C, H, O, and N, such as carbonates, hydroxides, oxyhydroxides, acetates, citrates, and oxides, are preferably used. From the viewpoint of ease of pulverization and the amount of gas released by thermal decomposition, carbonates, hydroxides, and oxides are particularly preferred.

In the mixing step S10, it is preferable to mix the raw materials so that the calcination precursor to be subjected to the calcination step S30 has a chemical composition represented by the composition formula (1). Specifically, it is preferable to adjust the atomic concentration ratio (molar ratio) between the atomic concentration (molar number) of lithium contained in the calcination precursor and the total atomic concentration (molar number) of metal elements other than lithium contained in the calcination precursor to 0.96 or more and 1.08 or less, and more preferably 0.96 or more and 1.04 or less. If the atomic concentration ratio is less than 0.96, there is a high possibility that an appropriate main phase with few heterophases cannot be calcined due to a shortage of lithium. On the other hand, if the atomic concentration ratio exceeds 1.08, the synthesis reaction does not proceed properly, and there is a risk that the crystallinity of the layered structure will be low.

The nickel occupying the transition metal site is a mixture of divalent and trivalent, and when b<0.8, other transition metal elements cobalt and manganese become tetravalent and compensate for the valence, so the proportion of divalent nickel is high. In order to obtain a lithium transition metal composite oxide in which the lattice distortion or crystal structure change associated with the insertion and removal of lithium ions is reduced, it is preferable to increase the proportion of divalent nickel occupying the transition metal site as much as possible. However, in a high Ni composition of b≧0.8, divalent nickel is prone to occupying the lithium site, so-called cation mixing, and this must be sufficiently suppressed. From the viewpoint of sufficiently suppressing cation mixing during firing, it is necessary to ensure that the synthesis reaction between lithium and nickel, etc. proceeds, so it is desirable to react lithium and nickel, etc. in a stoichiometric ratio of approximately 1:1.

Therefore, it is preferable to adjust these atomic concentration ratios in advance at the mixing step S10 where fine grinding and mixing can be performed. When adjusted in advance, the atomic concentration ratio (molar ratio) of the atomic concentration (number of moles) of lithium contained in the calcination precursor to the total atomic concentration (number of moles) of metal elements other than lithium is more preferably 0.98 or more and 1.02 or less. However, during calcination, the lithium contained in the calcination precursor may react with the calcination container or volatilize. Considering that part of the lithium may be lost due to reaction with the container used for calcination or evaporation during calcination, it is not prohibited to add an excess of lithium at the time of charging.

In addition, to obtain a lithium transition metal composite oxide in which the proportion of divalent nickel occupying the transition metal sites is high and the lattice distortion or crystal structure change associated with the insertion and desorption of lithium ions is reduced, it is preferable not to mix in metal elements other than lithium, such as unavoidable impurities resulting from the manufacturing process, other components that coat the lithium transition metal composite oxide particles, or other components that are mixed with the lithium transition metal composite oxide particles, after the mixing step S10 and before the firing step S30.

In the granulation step S20, the mixture obtained in the mixing step S10 is granulated to obtain secondary particles (granules) in which the particles aggregate. The mixture may be granulated by either dry granulation or wet granulation. For granulation of the mixture, an appropriate granulation method such as rolling granulation, fluidized bed granulation, compression granulation, spray granulation, etc. may be used.

As a granulation method for granulating the mixture, the spray granulation method is particularly preferred. 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. With the spray granulation method, the slurry precisely mixed and pulverized by wet pulverization can be granulated while drying. In addition, by adjusting the concentration of the slurry, the spray pressure, the disk rotation speed, etc., it is possible to precisely control the particle size of the secondary particles within a predetermined range, and granules that are close to true spheres and have a uniform chemical composition can be efficiently obtained. In the granulation step S20, it is preferable to granulate the mixture obtained in the mixing step S10 so that the average particle size (D ₅₀ ) is 3 μm or more and 50 μm or less. In this embodiment, the secondary particles of the more preferred granules have an average particle size (D ₅₀ ) of 5 μm or more and 20 μm or less.

In the calcination step S30, the granules produced in the granulation step S20 are heat-treated to calcinate the lithium transition metal composite oxide represented by the composition formula (1). The calcination step S30 may be performed in a single heat treatment step in which the heat treatment temperature is controlled within a certain range, or in multiple heat treatment steps in which the heat treatment temperature is controlled within different ranges. However, from the viewpoint of obtaining a lithium transition metal composite oxide having high crystal purity, high discharge capacity, and good rate characteristics and charge/discharge cycle characteristics, it is preferable to include the first heat treatment step S31, the second heat treatment step S32, and the third heat treatment step S33 as shown in FIG. 1, and it is particularly preferable to satisfy the conditions of the second heat treatment step S32 and the third heat treatment step S33.

In the first heat treatment step S31, the granules produced in the granulation step S20 are heat-treated at a heat treatment temperature of 200°C or higher and lower than 600°C for 0.5 hours or more and 5 hours or less to obtain a first precursor. The main purpose of the first heat treatment step S31 is to remove moisture and other substances that interfere with the synthesis reaction of the lithium transition metal composite oxide from the fired precursor (the granules produced in the granulation step S20).

In the first heat treatment step S31, if the heat treatment temperature is 200°C or higher, the impurity combustion reaction and the thermal decomposition of the raw materials will proceed sufficiently, and it is possible to suppress the formation of inactive heterogeneous phases, deposits, etc. in the subsequent heat treatments. In addition, if the heat treatment temperature is less than 600°C, the crystallization of the lithium transition metal composite oxide will be almost never completed in this step, and it is possible to prevent low-purity crystal phases from remaining in the presence of moisture, impurities, etc.

The heat treatment temperature in the first heat treatment step S31 is preferably 250°C or higher and 550°C or lower, and more preferably 300°C or higher and 500°C or lower. If the heat treatment temperature is within this range, moisture, impurities, etc. can be efficiently removed while reliably preventing the lithium transition metal composite oxide crystallization from being completed in this step. The heat treatment time in the first heat treatment step S31 can be appropriately set depending on, for example, the heat treatment temperature, the amount of moisture, impurities, etc. contained in the mixture, the target for removing moisture, impurities, etc., the target degree of crystallization, etc.

The first heat treatment step S31 is preferably carried out under a flow of atmospheric gas or under exhaust by a pump. When heat treatment is carried out under such an atmosphere, gas containing moisture, impurities, etc. can be efficiently removed from the reaction site. The flow rate of the atmospheric gas flow or the amount of exhaust per hour by the pump is preferably greater than the volume of gas generated from the calcination precursor. The volume of gas generated from the calcination precursor can be calculated based on, for example, the amount of raw material used or the molar ratio per raw material of the components gasified by combustion or pyrolysis.

The first heat treatment step S31 may be performed under an oxidizing gas atmosphere, a non-oxidizing gas atmosphere, or a reduced pressure atmosphere. The oxidizing gas atmosphere may be either an oxygen gas atmosphere or an air atmosphere. The reduced pressure atmosphere may be a reduced pressure condition with an appropriate degree of vacuum, for example, below atmospheric pressure.

In the second heat treatment step S32, the first precursor obtained in the first heat treatment step S31 is heat-treated at a heat treatment temperature of 600°C or more and less than 750°C for 2 hours or more and 50 hours or less to obtain a second precursor. The main purpose of the second heat treatment step S32 is to remove carbonate components and generate crystals of lithium transition metal composite oxide by reacting lithium carbonate with a nickel compound or the like. The nickel in the fired precursor is sufficiently oxidized to suppress cation mixing in which nickel is mixed into the lithium site, and to suppress the generation of cubic domains by nickel. In addition, in order to reduce lattice distortion or crystal structure changes associated with the insertion and desorption of lithium ions, the metal element represented by M is sufficiently oxidized to increase the uniformity of the composition of the layer composed of _MeO2 and increase the proportion of divalent nickel with a large ionic radius.

In the second heat treatment step S32, the amount of unreacted lithium carbonate remaining in the second precursor is preferably reduced to 0.3% by mass or more and 3% by mass or less, more preferably 0.3% by mass or more and 2% by mass or less, based on the total mass of the first precursor introduced. If the amount of lithium carbonate remaining in the second precursor is too large, the lithium carbonate may melt and form a liquid phase in the third heat treatment step S33. When the lithium transition metal composite oxide is fired in the liquid phase, the primary particles having a layered structure may become excessively oriented due to oversintering, or the specific surface area may decrease. As a result, there is a risk that the discharge capacity, rate characteristics, etc. may deteriorate. Furthermore, the X element penetrates into the inside of the primary particles, and a concentrated layer of the X element cannot be obtained near the surface of the primary particles. Furthermore, if the amount of lithium carbonate remaining in the second precursor is too small, the specific surface area of the calcined lithium transition metal composite oxide becomes too large, the contact area with the electrolyte increases, and the X element does not concentrate near the surface of the primary particles, which may result in poor charge-discharge cycle characteristics. On the other hand, if the amount of unreacted lithium carbonate remaining is within the above range, a high discharge capacity, good rate characteristics, and charge-discharge cycle characteristics can be obtained.

In addition, if the reaction of lithium carbonate is insufficient in the second heat treatment step S32 and a large amount of lithium carbonate remains, there is a risk that the lithium carbonate will melt in the third heat treatment step S33 and form a liquid phase. If the lithium transition metal composite oxide is fired in the liquid phase, the crystal grains are likely to become coarse, which may deteriorate the output characteristics. In contrast, if most of the lithium carbonate is allowed to react in the second heat treatment step S32, a liquid phase is unlikely to be generated in the third heat treatment step S33, so that the crystal grains are unlikely to become coarse even if the heat treatment temperature is increased. Therefore, it is possible to fire a lithium transition metal composite oxide with high crystal purity at a high temperature while suppressing the coarsening of the crystal grains.

In addition, if the reaction of lithium carbonate is insufficient and a large amount of lithium carbonate remains in the second heat treatment step S32, oxygen is difficult to reach the precursor. If oxygen is not reached in the second precursor in the third heat treatment step S33, nickel is not sufficiently oxidized, and cation mixing is likely to occur due to divalent nickel. In contrast, if most of the lithium carbonate is reacted in the second heat treatment step S32, oxygen is easily reached in the powdered second precursor. Therefore, it is possible to sufficiently oxidize manganese, etc., to increase the proportion of divalent nickel in the layer composed of _MeO2 , while suppressing the excess remaining of divalent nickel, which is likely to cause cation mixing.

In the second heat treatment step S32, if the heat treatment temperature is 600° C. or higher, the reaction between lithium carbonate and nickel compounds etc. promotes the generation of crystals, and it is possible to avoid a large amount of unreacted lithium carbonate remaining. Therefore, lithium carbonate is less likely to form a liquid phase in the subsequent heat treatment, the coarsening of crystal grains is suppressed, good output characteristics etc. are obtained, oxygen is more likely to be distributed in the second precursor, and cation mixing is more likely to be suppressed. In addition, if the heat treatment temperature is less than 750° C., grain growth does not proceed excessively in the second heat treatment step S32, and manganese etc. can be sufficiently oxidized to increase the uniformity of the composition of the layer composed of MeO ₂ .

The heat treatment temperature in the second heat treatment step S32 is preferably 650° C. or higher, and more preferably 680° C. or higher. The higher the heat treatment temperature, the more the synthesis reaction is promoted, and the more reliably lithium carbonate is prevented from remaining. Furthermore, if the heat treatment temperature is 700° C. or higher, the uniformity of the composition of the layer composed of MeO ₂ can be increased, and the X element can be easily diffused to the surface of the primary particles. Therefore, in order to form a concentrated layer of the X element near the surface of the primary particles, a relatively high temperature of about 700° C. is more preferable.

The heat treatment temperature in the second heat treatment step S32 is preferably less than 750°C. If the heat treatment temperature exceeds 750°C, the lithium carbonate that was unreacted in the first heat treatment step S31 will form a liquid phase, and the crystal grains will become coarse. In addition, element X will diffuse not only to the primary particle interfaces but also to the interior of the primary particles, making it difficult to form a concentrated layer near the interfaces.

The heat treatment time in the second heat treatment step S32 is preferably 4 hours or more. Also, the heat treatment time is preferably 15 hours or less. When the heat treatment time is within this range, the reaction of lithium carbonate proceeds sufficiently, so that the carbonate component can be reliably removed. Also, the time required for the heat treatment is shortened, improving the productivity of the lithium transition metal composite oxide.

The second heat treatment step S32 is preferably performed in an oxidizing atmosphere. The oxygen concentration of the atmosphere is preferably 50% or more, more preferably 60% or more, and even more preferably 80% or more. In the case of a high Ni ratio positive electrode active material in which the Ni ratio is 80% or more, in an atmosphere with a high carbon dioxide concentration, Li of the positive electrode active material reacts with carbon dioxide to form lithium carbonate. When Li is extracted from the positive electrode active material to form lithium carbonate, the crystallinity decreases, which causes a decrease in discharge capacity and a decrease in charge-discharge cycle characteristics. Therefore, the carbon dioxide concentration of the atmosphere is preferably 5% or less, more preferably 1% or less. In addition, the second heat treatment step S32 is preferably performed under a flow of oxidizing gas. By performing heat treatment under a flow of oxidizing gas, nickel can be reliably oxidized and carbon dioxide released into the atmosphere can be reliably removed.

In the third heat treatment step S33, the second precursor obtained in the second heat treatment step S32 is heat-treated at a heat treatment temperature of 750°C to 900°C for 2 hours to 50 hours to obtain a lithium transition metal composite oxide. The main purpose of the third heat treatment step S33 is to grow the crystal grains of the lithium transition metal composite oxide having a layered structure to an appropriate grain size and specific surface area.

In the third heat treatment step S33, if the heat treatment temperature is 750°C or higher, nickel can be sufficiently oxidized to suppress cation mixing, while the crystal grains of the lithium transition metal composite oxide can be grown to an appropriate grain size and specific surface area. In addition, the metal element represented by M can be sufficiently oxidized to increase the proportion of divalent nickel. Since a main phase is formed in which the lattice constant of the a-axis is large and the lattice distortion or crystal structure change associated with the insertion and desorption of lithium ions is reduced, high discharge capacity, good charge/discharge cycle characteristics, and output characteristics can be obtained. In addition, if the heat treatment temperature is 900°C or lower, lithium is unlikely to volatilize and the layered structure is unlikely to decompose, so that a lithium transition metal composite oxide with high crystal purity and good discharge capacity, rate characteristics, etc. can be obtained.

The heat treatment temperature in the third heat treatment step S33 is preferably 780°C or higher, more preferably 800°C or higher, and even more preferably 820°C or higher. The higher the heat treatment temperature, the more sufficiently nickel and the metal element represented by M can be oxidized, and the more the grain growth of the lithium transition metal composite oxide can be promoted.

The heat treatment temperature in the third heat treatment step S33 is preferably 880°C or less, and more preferably 860°C or less. The lower the heat treatment temperature, the less likely lithium is to volatilize, so that the decomposition of the lithium transition metal composite oxide can be reliably prevented, and a lithium transition metal composite oxide with good discharge capacity, rate characteristics, etc. can be obtained.

The heat treatment time in the third heat treatment step S33 is preferably 2 hours or more. The heat treatment time is preferably 15 hours or less. When the heat treatment time is within this range, nickel and the like can be sufficiently oxidized to obtain a lithium transition metal composite oxide in which the lattice distortion or crystal structure change associated with the insertion and desorption of lithium ions is reduced. In addition, the time required for the heat treatment is shortened, thereby improving the productivity of the lithium transition metal composite oxide.

The third heat treatment step S33 includes an annealing step in which the temperature is kept at 700°C or more and 800°C or less for 1.5 hours or more in the process of decreasing the temperature from the maximum temperature after heat treatment at a predetermined temperature. If the temperature is less than 700°C, the X element cannot be sufficiently diffused and is insufficiently dissolved inside the primary particles. If the temperature exceeds 800°C, the X element remains concentrated at the interface of the primary particles and A0 becomes excessively large. The X element is preferably at least one element selected from the group consisting of Ti, Ga, Mg, Zr, and Zn, and as shown in Table 1, since the ion radius is larger than that of the other component metal elements (Ni, Co, Mn, Al), only a small amount can be substituted and dissolved in the α-NaFeO ₂ type crystal structure. As a result, it is difficult to uniformly dissolve the entire amount of the added X element, and it is concentrated on the surface of the secondary particles or on the interface between the primary particles inside the secondary particles. In the present invention, it has been found that an effective means for promoting the dissolution of the X element into the primary particles as much as possible is to carry out an annealing treatment in which the temperature is maintained in a temperature range of 700° C. or more and 800° C. or less for 1.5 hours or more during the temperature decreasing process of the third heat treatment.

The third heat treatment step S33 is preferably performed in an oxidizing atmosphere. The oxygen concentration of the atmosphere is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The carbon dioxide concentration of the atmosphere is preferably 2% or less, and more preferably 0.5% or less. In the third heat treatment step, the second precursor that has been subjected to the second heat treatment step is used, so that the precursor contains a low amount of carbonate, and it is possible to perform heat treatment in an atmosphere with a lower carbon dioxide concentration than the second heat treatment step. Therefore, compared to the second precursor, the lithium transition metal composite oxide after the third heat treatment step has a higher crystallinity. In addition, the third heat treatment step S33 is preferably performed under a flow of oxidizing gas. When heat treatment is performed under a flow of oxidizing gas, nickel and the like can be reliably oxidized and carbon dioxide released into the atmosphere can be reliably removed.

In the firing step S20, a suitable heat treatment device such as a rotary kiln or other rotary furnace, a roller hearth kiln, a tunnel furnace, a pusher furnace or other continuous furnace, or a batch furnace can be used as a heat treatment means. The first heat treatment step S31, the second heat treatment step S32, and the third heat treatment step S33 may each be performed using the same heat treatment device, or different heat treatment devices. Each heat treatment step may be performed intermittently by changing the atmosphere, or may be performed continuously when heat treatment is performed while exhausting gas in the atmosphere. Note that the first heat treatment step S31 has the main purpose of removing moisture, etc., so that when it is not necessary to dehydrate the moisture derived from the raw material, such as when an oxide rather than a hydroxide is used as the raw material, the first heat treatment step S31 may be omitted and the second heat treatment step S32 may be started. By reducing the amount of unreacted lithium carbonate remaining in the second precursor to 0.3% by mass or more and 2% by mass or less based on the total mass of the first precursor added, and then baking in the third heat treatment step S33 in an atmosphere with a lower carbon dioxide concentration, element X is concentrated near the primary particle interface, and cation mixing is sufficiently reduced, resulting in a positive electrode active material of the present invention that exhibits high discharge capacity and good charge/discharge cycle characteristics.

By going through the above mixing step S10, granulation step S20, and firing step S30, a positive electrode active material composed of a lithium transition metal composite oxide represented by composition formula (1) can be manufactured. The distribution of element X, the amount of cation mixing, and the specific surface area can be controlled mainly by adjusting the method of preparing the precursor before heat treatment, the composition ratio of metal elements such as nickel, the amount of unreacted lithium carbonate remaining in the second precursor, and the heat treatment temperature and heat treatment time of the second heat treatment step S32 and the third heat treatment step S33. In the chemical composition represented by composition formula (1), if element X is concentrated near the primary particle interface and cation mixing is sufficiently reduced, an excellent positive electrode active material exhibiting high discharge capacity and good charge/discharge cycle characteristics can be obtained.

The synthesized lithium transition metal composite oxide may be subjected to a washing process in which it is washed with deionized water or the like, and a drying process in which the washed lithium transition metal composite oxide is dried, etc., after the firing process S30, for the purpose of removing impurities, etc. Also, the synthesized lithium transition metal composite oxide may be subjected to a crushing process in which it is crushed, and a classification process in which the lithium transition metal composite oxide is classified into a predetermined particle size, etc.

[Second manufacturing method]
FIG. 1B is a flow diagram of a second method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention.
As shown in FIG. 1(B) , the raw materials are mixed, granulated, dried, and subjected to a first heat treatment in the same manner as in Production Method A, except that the starting raw materials are metal raw materials other than Li (mixing step B10, granulation step B20, and first heat treatment step B31 in FIG. 1(B) ).
In the dry mixing step B11, for example, lithium hydroxide is weighed as a lithium source to the first precursor that has been subjected to B31 so as to have a chemical composition represented by the composition formula (1), and the mixture is dry-mixed to obtain a raw material mixed powder. In this step, a ball mill, an attritor, a V-type mixer, a Henschel mixer, a rocking mixer, or the like can be used. However, it is necessary to adjust the crushing force so as not to destroy the granulated dry powder during mixing.

In the second heat treatment step B32, for example, the raw material mixed powder is heat-treated for 2 hours to 50 hours to obtain a second precursor at a heat treatment temperature of 460° C. or more and less than 550° C. The main purpose of the second heat treatment step B32 is to remove hydrates and hydroxyl groups by a reaction between lithium hydroxide and a nickel compound, etc., and to generate crystals of a lithium transition metal composite oxide.
The heat treatment time in the second heat treatment step B32 is preferably 4 hours or more. If it is less than 4 hours, the production of the lithium transition metal composite oxide will be insufficient. The atmosphere is preferably an oxidizing atmosphere, with an oxygen concentration of 50% or more, more preferably 80% or more, and even more preferably 90% or more. The heat treatment means is the same as the second heat treatment step S32 in the manufacturing method A.
The next third heat treatment step B33 is preferably carried out in the same manner as in the manufacturing method A.

The present invention will be specifically described below with reference to examples, but the technical scope of the present invention is not limited to these examples.
Positive electrode active materials according to the examples of the present invention were synthesized and evaluated for element distribution, X-ray diffraction profile, discharge capacity, discharge rate characteristics, and charge/discharge cycle characteristics (capacity retention rate). As a control for the examples, positive electrode active materials according to comparative examples having different chemical compositions and annealing conditions were synthesized and similarly evaluated.

[Example 1]
First, lithium carbonate, nickel hydroxide, cobalt carbonate, manganese carbonate, and titanium oxide were prepared as raw materials, and each raw material was weighed so that the molar ratio of metal elements was Li:Ni:Co:Mn:Ti was 1.03:0.85:0.03:0.10:0.02, and pure water was added so that the solid content ratio was 50 mass%. Then, the raw materials were wet-pulverized (wet-mixed) in a pulverizer to prepare a raw material slurry with an average particle size of less than 0.2 μm (mixing step S10).
The obtained raw material slurry was then spray-dried using a nozzle-type spray dryer (Okawahara Chemical Engineering Co., Ltd., ODL-20 type) to obtain granules with an average particle size of 3 μm to 50 μm (granulation step S20). The spray pressure was 0.13 MPa, and the spray amount was 260 g/min. The dried granules were then heat-treated to calcinate the lithium transition metal composite oxide (calcination step S30). Specifically, the granules were heat-treated in a continuous conveying furnace under an air atmosphere at 400° C. for 5 hours to obtain a first precursor (first heat treatment step S31). The first precursor was then heat-treated in a calcination furnace with an oxygen gas atmosphere replaced in an oxygen stream at 700° C. for 20 hours to obtain a second precursor (second heat treatment step S32). Thereafter, the second precursor was heat-treated (main calcination) at 820°C for 10 hours in an oxygen gas flow in a calcination furnace replaced with an oxygen gas atmosphere (third heat treatment step S33). Then, the temperature was lowered to 740°C at 5°C/min and held at 740°C for 4 hours, and the temperature drop rate from 740°C to 700°C was adjusted to 5°C/min, and the holding time in the temperature range from 800°C to 700°C was set to 4.3 hours to obtain a lithium transition metal composite oxide (annealing treatment). The calcined powder obtained by calcination was classified using a sieve with an opening of 45 μm, and the powder under the sieve was used as the positive electrode active material of the sample.

[Example 2]
A positive electrode active material was obtained in the same manner as in Example 1, except that the holding time at 740° C. in the annealing treatment was 20 hours and the holding time in the temperature range from 800° C. to 700° C. was 20.3 hours.

[Example 3]
A positive electrode active material was obtained in the same manner as in Example 1, except that the raw materials were weighed out so that the molar ratio was Li:Ni:Co:Mn:Ti=1.03:0.80:0.15:0.04:0.01 and the firing temperature was 800° C.

[Example 4]
A positive electrode active material was obtained in the same manner as in Example 1, except that the raw materials were weighed out so that the molar ratio was Li:Ni:Co:Mn:Ti=1.03:0.90:0.03:0.04:0.03 and the firing temperature was 840° C.

[Example 5]
A positive electrode active material was obtained in the same manner as in Example 1, except that in the annealing treatment, the temperature was decreased to 710° C. at 5° C./min and then held at 710° C. for 4 hours, the temperature decrease rate from 710° C. to 700° C. was adjusted to 5° C./min, and the holding time in the temperature range from 800° C. to 700° C. was set to 4.3 hours.

[Comparative Example 1]
A positive electrode active material was obtained in the same manner as in Example 1, except that in the annealing treatment, the temperature was not maintained at 740° C., but was decreased to 700° C. at a rate of 5° C./min, and the holding time in the temperature range from 800° C. to 700° C. was 0.3 hours.

[Comparative Example 2]
A positive electrode active material was obtained in the same manner as in Example 1, except that in the annealing treatment, the temperature was held at 740° C. for 0.7 hour and the holding time in the temperature range from 800° C. to 700° C. was set to 1.0 hour.

[Comparative Example 3]
A positive electrode active material was obtained in the same manner as in Example 3, except that in the annealing treatment, the temperature was not maintained at 740° C., but was decreased to 700° C. at a rate of 5° C./min, and the holding time in the temperature range from 800° C. to 700° C. was 0.3 hours.

[Comparative Example 4]
A positive electrode active material was obtained in the same manner as in Example 4, except that in the annealing treatment, the temperature was not maintained at 740° C., but was decreased to 700° C. at a rate of 5° C./min, and the holding time in the temperature range from 800° C. to 700° C. was 0.3 hours.

[Comparative Example 5]
A positive electrode active material was obtained in the same manner as in Comparative Example 1, except that no titanium oxide was used as a raw material, the raw materials were weighed out so that the molar ratio of Li:Ni:Co:Mn was 1.03:0.60:0.20:0.20, and the firing temperature was 900° C.

(Measurement of chemical composition and specific surface area of positive electrode active material)
The chemical composition of the synthesized positive electrode active material was analyzed by high-frequency inductively coupled plasma atomic emission spectrometry using an ICP-AES optical emission spectrometer "OPTIMA 8300" (manufactured by PerkinElmer). As a result, the positive electrode active materials according to Examples 1 to 6 and the positive electrode active materials according to Comparative Examples 1 to 5 all had chemical compositions as shown in Table 2, in which only lithium was different from the charged amount. In addition, the specific surface area of the positive electrode active material was determined by the BET method using an automatic specific surface area measuring device "BELCAT" (manufactured by BEL JAPAN). The results are shown in Table 2.

(Elemental concentration distribution)
The concentration distribution of element X in the synthesized positive electrode active material was measured by the following procedure. First, the powder of the prepared positive electrode active material was processed by FIB processing under the conditions of acceleration voltage: 30 kV (sampling) and 10 kV (finishing) using a focused ion/electron beam processing and observation device "nano DUET NB5000" (manufactured by Hitachi High-Technologies Corporation) to form a thin slice. Next, a scanning transmission electron microscope "JEM-ARM200F" (manufactured by JEOL Ltd.) was used for STEM observation to identify the primary particle interface inside the secondary particle. Then, an energy filter "GIF-Quantum" (manufactured by Gatan) was used to measure the EELS spectrum from the vicinity of the interface of the primary particle to a depth of 50 nm toward the center of the primary particle to obtain the concentration distribution of each element including X. Furthermore, using an energy dispersive X-ray analyzer "JED-2300T" (manufactured by JEOL), the concentration A0 of X at the interface of the primary particles inside the secondary particles, the concentration A1 of X at a depth of 1 nm from the interface of the primary particles inside the secondary particles, and the concentration A2 of X at the center of the primary particles were measured. The measurement points were three, and the average value was used. As a result, it was confirmed that the positive electrode active materials according to Examples 1 to 6 and the positive electrode active materials according to Comparative Examples 1 to 4 all had the A0, A1, and A2 shown in Table 3. In addition, the same measurement was performed on the primary particles on the secondary particle surface, and the X concentration B0 on the secondary particle surface, the X concentration B1 at a depth of 1 nm from the secondary particle surface, and the X concentration B2 at the center of the primary particles on the secondary particle surface were measured. As a result, it was confirmed that the positive electrode active materials according to Examples 1 to 6 and the positive electrode active materials according to Comparative Examples 1 to 4 all had the B0, B1, and B2 shown in Table 3.

STEM-EELS analysis was carried out at the primary particle interface inside the secondary particles. FIG. 4 shows the STEM image of Comparative Example 1, and FIG. 5 shows the STEM image of the analysis position of Example 2. FIG. 6 shows the intensity profile of Ni and Ti in the STEM-EELS analysis measured across the primary particle interface inside the secondary particles for Comparative Example 1 and Example 2, respectively. In Comparative Example 1 of FIG. 6, the intensity of Ni, which is the main component of the positive electrode active material, drops in a spike-like manner at the grain boundary between the primary particles in the STEM image, and the intensity of Ti increases in a spike-like manner at approximately the same position. This shows that Ti is segregated at the grain boundary between the primary particles. On the other hand, in Example 2 of FIG. 7, the intensity of Ti increases in a rectangular shape in a range about 1 to 2 nm wider than the grain boundary between the primary particles where the intensity of Ni drops in a spike-like manner. This shows that in Example 2, Ti is dissolved in the crystal of the positive electrode active material by about 1 to 2 nm due to the annealing treatment.
As described above, the annealing treatment changes the concentration distribution of element X. In Examples 1 to 6, in which the holding time in the temperature range of 800° C. to 700° C. was 4.3 to 20.3 hours under the annealing treatment conditions, the A0/A1 values were 1.1 to 3.0, whereas in Comparative Examples 1 to 4, in which the holding time in the temperature range of 800° C. to 700° C. was 0.3 to 1.0 hour under the annealing treatment conditions, the A0/A1 values were 3.7 to 4.0.

The STEM-EELS intensity profile was smoothed (Ni: thin line, Ti: dashed line) as shown in FIG. 8 (Comparative Example 1) and FIG. 9 (Example 2), and the absolute value |dI/dx| of the differential value (intensity change) of the smoothed profile intensity I with respect to position x was calculated. As a result, the absolute value of the intensity change for each position is shown in FIG. 10 (Comparative Example 1) and FIG. 11 (Example 2). In the figure, Ni and Ti have two peaks at positions corresponding to the inflection points of the intensity change, and the peak positions for Ni are β1 and β2, respectively, and the peak positions for Ti are α1 and α2, respectively. The area between β1 and β2 is considered to be the grain boundary between primary particles, and the interval is described as |β1-β2|=L(β). The area between the Ti peaks indicates a high concentration region, and the interval is described as |α1-α2|=L(α). In Comparative Example 1, L(α)=2.4 nm and L(β)=1.9 nm, and L(α)-L(β) is 0.5 nm, which is smaller than 1.0 nm, and it can be seen that the Ti-enriched region almost coincides with the grain boundary between the primary particles. Note that when the peak top is split and the peak position is unclear, such as the Ni peak at 31-33 nm in Figure 10, the peak position is the center of the entire peak including the split part. In contrast, in Example 2 in Figure 11, L(α)=4.3 nm and L(β)=2.4 nm, and L(α)-L(β) is 1.9 nm, so it can be seen that the Ti-enriched region is not only at the grain boundary but also extends to the inside of the primary particles on both sides of the grain boundary by about 1.9 nm. In other words, Ti is dissolved inside the crystal.

Comparing the electrode performance of Comparative Example 1 and Example 2 (see Table 3), Example 2 has a high capacity retention rate and excellent cycle resistance. Since the compositions of Comparative Example 1 and Example 2 are the same, it is believed that the good cycle resistance of Example 2 is due to the above-mentioned L (α) - L (β) being 1.0 nm or more. It is believed that the Ti concentrated on the primary particle surface reduces the amount of Ni exposed on the outermost surface, thereby suppressing the decomposition reaction of the electrolyte accompanying the charge-discharge cycle and suppressing Ni ions eluted into the electrolyte. As a result, not only the generation of decomposition by-products inside the battery is suppressed, but also the deterioration of the positive electrode active material is suppressed, improving the cycle resistance (effect A). In Example 2, by performing annealing treatment, Ti segregated at the grain boundary can be dissolved into the inside of the primary particles on both sides of the grain boundary to about 1.9 nm, and it is presumed that the effect A can be enhanced by sufficiently forming a Ti-concentrated layer on the surface layer of each primary particle.

(Discharge capacity, rate characteristics, capacity retention rate, resistance increase rate)
A lithium ion secondary battery was prepared using the synthesized positive electrode active material as a positive electrode material, and the discharge capacity, capacity retention rate, rate characteristics, and resistance increase rate of the lithium ion secondary battery were obtained. First, the prepared positive electrode active material, a carbon-based conductive material, and a binder dissolved in advance in N-methyl-2-pyrrolidone (NMP) were mixed in a mass ratio of 94:4.5:1.5. Then, the uniformly mixed positive electrode mixture slurry was applied to a positive electrode current collector of aluminum foil having a thickness of 20 μm so that the coating amount was 16 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. Thereafter, the positive electrode mixture layer was pressurized by a hot press and punched into 2.5 cm x 4.1 cm to form a positive electrode.

Next, a lithium ion secondary battery was produced using the produced positive electrode, negative electrode and separator. The negative electrode was produced as follows. A graphite-based negative electrode active material, carboxyl methyl cellulose and styrene butadiene rubber were mixed in a mass ratio of 98:1:1 to produce a slurry using pure water as a solvent, and the slurry was applied to a negative electrode collector of copper foil having a thickness of 10 μm in an amount of 10 mg/cm ^2. The positive electrode mixture slurry applied to the negative electrode collector was heat-treated at 100° C., and the solvent was distilled off to form a negative electrode mixture layer. The negative electrode mixture layer was then pressure-molded by a hot press and punched into a size of 2.7 cm×4.2 cm to form a positive electrode.
The separator used was a 30 μm thick porous polypropylene separator. The positive electrode and the negative electrode were opposed to each other in the non-aqueous electrolyte via the separator, and a lithium ion secondary battery was assembled.
The nonaqueous electrolyte used was a solution in which _LiPF6 was dissolved at 1.0 mol/L in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 3:7, and further vinylene carbonate was dissolved at 1 mass %.

The positive electrode material, separator, and negative electrode material were layered, inserted into an aluminum laminate bag, and a non-aqueous electrolyte was injected, followed by heat sealing in a vacuum to complete a lithium ion secondary battery.
The fabricated lithium ion secondary battery was charged in an environment of 25° C. at a constant current/constant voltage of 40 A/kg based on the weight of the positive electrode mixture and an upper limit voltage of 4.2 V. Then, the battery was discharged at a constant current of 40 A/kg based on the weight of the positive electrode mixture to a lower limit voltage of 2.5 V, and the discharge capacity (initial capacity) was measured.

The lithium secondary battery whose initial capacity had been measured was then charged in an environment of 25°C at a constant current/constant voltage of 40 A/kg based on the weight of the positive electrode mixture and an upper limit voltage of 4.2 V, and then discharged to a lower limit voltage of 2.5 V at a constant current of 40 or 600 A/kg based on the weight of the positive electrode mixture. The ratio of the discharge capacity of 600 A/kg to that of 40 A/kg was calculated as the discharge rate characteristic.

Next, the battery was charged at a constant current of 200 A/kg based on the weight of the positive electrode mixture and an upper limit voltage of 4.2 V in an environment of 50°C. Then, the battery was discharged at a constant current of 200 A/kg based on the weight of the positive electrode mixture to a lower limit potential of 2.5 V for a total of 100 cycles, and the discharge capacity after 100 cycles was measured. The ratio of the discharge capacity after 100 cycles to the initial capacity was calculated as the capacity retention rate. The results are also shown in Table 3.

As shown in Table 3, in Examples 1 to 5, the holding time in the temperature range of 800°C to 700°C was 4.3 hours to 20.3 hours, so solid solution progressed from the primary particle surface to the inside, and in this case A0/A1 was 1.1 to 3.0, or 3.5 or less. Also, B0/B1 was 1.1 to 2.0, or 2.5 or less. And in these Examples 1 to 5, electrode characteristics were obtained that combined a high capacity of 180 Ah/kg or more, a capacity retention rate of 85% or more, and a discharge rate characteristic of 85% or more.

On the other hand, in Comparative Examples 1 to 4, the holding time in the temperature range from 800°C to 700°C during the temperature drop process was short, from 0.3 hours to 1.0 hours, so solid dissolution from the primary particle surface to the inside was insufficient, and A0/A1 was 3.7 to 4.0, which was 3.5 or more. In addition, B0/B1 was 2.7 to 3.1, which was 2.5 or more. As a result, the capacity retention rate was low, less than 80%.
In Comparative Example 5, since the Ni ratio was as low as 60%, a capacity retention rate of 85% or more and a discharge rate characteristic of 85% or more were obtained even without the addition of element X. However, since the Ni ratio was low, a high capacity of 180 Ah/kg or more could not be obtained.

It is known that in a positive electrode active material with a high Ni ratio, the transition from crystals to NiO-like crystals progresses from near the surface to the inside, causing a decrease in capacity and an increase in resistance. In the present invention, it is believed that the element X is dissolved in the crystal interior, lowering the Ni concentration of the crystal near the surface, and the high capacity retention rate at 50°C is obtained due to the effect of stabilizing the crystal of the element X. In addition, it is believed that the solid solution of the element X increases the adhesion between the concentrated layer of element X present on the secondary particle surface or the grain boundary between primary particles and the positive electrode material crystal, making it difficult to peel off due to stress caused by temperature change or volume change accompanying charge and discharge, and suppressing the reaction with the electrolyte.
In addition, it is believed that the high-concentration region that is dissolved inside the crystal is very thin, about 1 to 2 nm, which is why high rate characteristics and high capacity were maintained.

REFERENCE SIGNS LIST 100 Lithium ion secondary battery 101 Battery can 102 Battery cover 103 Positive electrode lead piece 104 Negative electrode lead piece 105 Insulating plate 106 Sealing material 110 Wound 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 (8)

The following composition formula (1):
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[wherein, in composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number that satisfies −0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, and −0.2<α<0.2],
The positive electrode active material includes secondary particles formed by agglomeration of a plurality of primary particles,
an atomic concentration A0 of X at an interface between primary particles inside the secondary particle; and
an atomic concentration A1 of X at a depth of 1 nm from an interface between primary particles inside the secondary particle;
the atomic concentration A2 of X at the center of the primary particle inside the secondary particle;
A0>A1>A2, and
In the element distribution of Ni and X in a region spanning the interface of the primary particle, the distance |β1-β2|=L(β) between positions β1 and β2 corresponding to the inflection point of the Ni concentration gradient is regarded as a grain boundary, and positions α1 and α2 corresponding to the inflection point of the X concentration gradient are located inside the primary particle, respectively, and the distance therebetween is |α1-α2|=L(α). This positive electrode active material for a lithium ion secondary battery is characterized in that 1 nm≦L(α)-L(β)≦3 nm. The following composition formula (1):
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[wherein, in composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number that satisfies −0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, and −0.2<α<0.2],
The positive electrode active material includes secondary particles formed by agglomeration of a plurality of primary particles,
an atomic concentration A0 of X at an interface between primary particles inside the secondary particle; and
an atomic concentration A1 of X at a depth of 1 nm from an interface between primary particles inside the secondary particle;
the atomic concentration A2 of X at the center of the primary particle inside the secondary particle;
A positive electrode active material for a lithium ion secondary battery, characterized in that A1×3.5>A0>A1>A2. A concentration B0 of X on the surface of the primary particle on the surface of the secondary particle;
an atomic concentration B1 of X at a depth of 1 nm from the surface of the primary particle on the surface of the secondary particle;
the atomic concentration B2 of X at the center of the primary particle on the surface of the secondary particle;
3. The positive electrode active material for a lithium ion secondary battery according to claim 2, wherein B1×2.5>B0>B1>B2. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, characterized in that in the composition formula (1), 0 ≦ c ≦ 0.05. A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material for lithium ion secondary batteries according to claim 1 . The following composition formula (1):
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[wherein, in composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number that satisfies −0.9<a<0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, −0.2<α<0.2],
The positive electrode active material includes secondary particles formed by agglomeration of a plurality of primary particles,
an atomic concentration A0 of X at an interface between primary particles inside the secondary particle; and
an atomic concentration A1 of X at a depth of 1 nm from an interface between primary particles inside the secondary particle;
the atomic concentration A2 of X at the center of the primary particle inside the secondary particle;
A0>A1>A2, and
In the element distribution of Ni and X in a region spanning the interface of the primary particle, the distance |β1-β2|=L(β) between positions β1 and β2 corresponding to the inflection point of the Ni concentration gradient is regarded as a grain boundary, and positions α1 and α2 corresponding to the inflection point of the X concentration gradient are located inside the primary particle, respectively, and the distance therebetween is |α1-α2|=L(α). This lithium ion secondary battery is characterized in that 1 nm≦L(α)-L(β)≦3 nm. The following composition formula (1):
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[wherein, in composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number that satisfies −0.9<a<0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, −0.2<α<0.2],
The positive electrode active material includes secondary particles formed by agglomeration of a plurality of primary particles,
an atomic concentration A0 of X at an interface between primary particles inside the secondary particle; and
an atomic concentration A1 of X at a depth of 1 nm from an interface between primary particles inside the secondary particle;
the atomic concentration A2 of X at the center of the primary particle inside the secondary particle;
A lithium ion secondary battery, characterized in that A1×3.5>A0>A1>A2. The following composition formula (1):
Li _1+a Ni _b Co _c M _d X _e O _2+α ...(1)
[wherein, in composition formula (1), M represents at least one selected from Al and Mn, X represents Ti , and is a number that satisfies −0.1≦a≦0.1, 0.8≦b<1.0, 0≦c≦0.3, 0≦d<0.2, 0<e<0.05, b+c+d+e=1, and −0.2<α<0.2],
A mixing step of mixing compounds containing the metal elements Li, Ni, Co, M, and X in the composition formula (1) to obtain a mixture;
A calcination step of calcining the mixture to obtain a lithium transition metal composite oxide represented by composition formula (1),
The firing step includes a step of maintaining the temperature at 750° C. or more and 900° C. or less,
Then, the temperature is lowered from the maximum temperature and the annealing step is held in a temperature range of 700° C. to 800° C. for 1.5 hours or more.
The positive electrode active material for a lithium ion secondary battery containing the lithium transition metal composite oxide contains secondary particles formed by agglomeration of a plurality of primary particles,
an atomic concentration A0 of X at an interface between primary particles inside the secondary particle; and
an atomic concentration A1 of X at a depth of 1 nm from an interface between primary particles inside the secondary particle;
the atomic concentration A2 of X at the center of the primary particle inside the secondary particle;
A0>A1>A2, and
In the element distribution of Ni and X in a region spanning the interface of the primary particle, a distance |β1-β2|=L(β) between positions β1 and β2 corresponding to an inflection point of the concentration gradient of Ni is regarded as a grain boundary, and positions α1 and α2 corresponding to an inflection point of the concentration gradient of X are located inside the primary particle, respectively, and the distance therebetween is |α1-α2|=L(α). When this distance is expressed as 1 nm≦L(α)-L(β)≦3 nm, the method for producing a positive electrode active material for a lithium ion secondary battery is characterized in that: