JP3489391B2 - Positive active material for lithium ion secondary battery and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode active material for a secondary battery, and more particularly to a non-aqueous positive electrode active material for a lithium ion secondary battery which can improve cycle characteristics and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, new cordless electronic devices such as VTRs with built-in cameras, audio / video devices, notebook computers, and mobile phones have appeared one after another, and have spread rapidly and widely in a short period of time. In order to reduce the size and weight of these devices, it is essential to improve the performance of secondary batteries, which are portable power sources.

The non-aqueous lithium ion secondary battery has a high battery voltage, a high discharge capacity, and excellent cycle characteristics.
It has been actively researched recently to meet such applications. The positive electrode active material for this battery is lithium cobalt oxide (LiCoO 2).
2) is used.

Conventionally, a non-aqueous secondary battery using LiCoO 2 as a positive electrode active material has a problem of deterioration in cycle characteristics that the discharge capacity of the battery is gradually reduced by repeating charge and discharge cycles. The cause is LiC
It was believed that the crystals of oO2 collapsed. In particular, by repeating charge and discharge, the fine particles constituting the positive electrode active material expand and contract in the c-axis direction, and in the case of a polycrystalline body or the like, there are many crystallite interfaces, and the crystal collapses from there. It has been considered that the peeling of the positive electrode active material from the current collector of the positive electrode causes deterioration of cycle characteristics. On the other hand, in order to improve the cycle characteristics, Japanese Patent Laid-Open No. 9-22693 discloses a method in which a crystal is made into a single crystal and flat particles oriented in a direction ((003) plane) perpendicular to the c-axis are grown. Proposed.

Further, it is possible to improve cycle characteristics by limiting the peak intensity ratio of the (003) plane and the (104) plane of the X-ray diffraction line of LiCoO 2 as the positive electrode active material to a specific range. No. 258751, Japanese Patent Laid-Open No. 9-2
2692, JP-A-9-22693, and JP-A-8-55624.

However, even if the peak intensity ratios of the (003) plane and the (104) plane of the X-ray diffraction line exist in a specific range, there are some that deteriorate rapidly at a high cycle of 100 cycles or more. Therefore, a positive electrode active material having excellent cycle characteristics even with such a high cycle is desired.

[0007]

The present invention has been made in view of the above circumstances, and an object thereof is to provide a positive electrode active material for a lithium ion secondary battery having good charge / discharge cycle characteristics.
In particular, it is an object of the present invention to provide a positive electrode active material capable of improving cycle characteristics (capacity retention rate) at high cycles of 100 cycles or more.

[0008]

Means for Solving the Problems The inventors of the present invention have made an intensive study on the crystal structure of the positive electrode active material, and as a result, have found that the crystal structure itself is closely related to the charge / discharge cycle characteristics of a lithium ion battery. We have succeeded in improving charge / discharge characteristics by controlling the crystal structure.

That is, the positive electrode active material for a lithium ion secondary battery of the present invention is a positive electrode active material for a lithium ion secondary battery represented by the general formula LiCoO2 and having a hexagonal crystal structure, and its lattice constant is C-axis length is 14.05
It is characterized in that it is 1 angstrom or less.

The crystallite diameter of the crystallite of the positive electrode active material in the (110) vector direction is preferably in the range of 450 to 1000 angstroms. For a positive electrode active material having a crystallite size in this range, it is easier to set the c-axis length in the target range.

In order to produce the positive electrode active material for a lithium ion secondary battery of the present invention, the heavy metal oxide and the lithium salt as raw materials are mixed so that the Li / Co ratio is in the range of 0.98 to 1.01. The raw material heavy metal oxide having a crystallite diameter in the (222) vector direction in the range of 50 to 400 angstroms is used.

The center particle diameter of the heavy metal oxide as a raw material is a value measured by using a particle size distribution measuring device of electric resistance method,
Here, the median particle diameter is measured using a Coulter Multisizer 2. It can be said that this is a particle size that includes the knowledge of whether it is in a dispersed state or an aggregated state from the measurement principle.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION LiCoO2, which is a positive electrode active material for a lithium-ion battery, belongs to the hexagonal system shown in FIG. 1, and when charged, Li is released from the crystal in the positive electrode active material, and the c-axis of the lattice constant. It is known that the length increases linearly. Charging progresses and reaches a certain size (14.4
It is described that when it reaches angstrom), the lattice constant does not increase further, the crystal structure changes from the hexagonal system to the monoclinic system, and the crystal collapses. (JNReimers and
JRDahn: J.Electrochem.Soc., 139, No.8,2091 (1992))

FIG. 2 is a plot of the present invention (a in the figure) and the conventional product (b in the figure) with respect to the relationship between the x value and the c-axis length of LixCoO2 as the positive electrode active material. The product of the present invention has a c-axis length of 14.048 in the state of x = 1.0 where Li is not desorbed.
It is Angstrom and that of the conventional product 1 is 14.10.
Angstrom. Further, with respect to these positive electrode active materials, X-ray diffraction was performed again on the positive electrode active materials charged to x = 0.5 to determine the c-axis length. For this, the positive electrode active material paste is used as the positive electrode plate, the Li metal is used as the negative electrode,
A positive electrode active material was separated by applying a fixed amount of current load, and then dissolving and cleaning the paste electrode in a solvent.

When x = 0.5 in the conventional product (straight line b), c
While the axial length is 14.40 angstrom, the product of the present invention (straight line a) has x = 0.425 and the c-axis length is 14.4.
It will be 0 angstrom. That is, the embodiment is
A conversion point of a crystal structure transition of 14.40 angstrom is achieved with a composition containing less Li. In other words, in the product of the present invention, the crystal does not collapse even if it is charged more. This is also advantageous for cycle characteristics.

<Refinement of Lattice Constant> The difference in the c-axis length between the present invention product and the conventional product is a slight difference of 0.1 angstrom or less. It is possible to identify a difference of 01 angstrom or less. Here, the refinement of the lattice constant is Cu
In X-ray diffraction using Kα1 as the radiation source, the surface index (00
3), (102), (104), (105), (11
0), (113), (204), (208), (001
In 5), the surface spacing d was obtained by the half-width method, and the calculation was performed by the least squares method. NIST S when determining the surface spacing
The angle was corrected by i. In addition to this, the c-axis length is R
Similar values can be obtained by the ietveld method and the WPPD method.

<Capacity Retention Ratio> FIG. 3 shows the relationship between the cycle characteristics (capacity retention ratio (%) of 200 cycles) and the c-axis length of the positive electrode active material of the present invention and a comparative product. As shown in this figure, when the c-axis length becomes shorter, the capacity retention rate is improved to about 95%. The c-axis length of the comparative example shown in the figure is 14.0576 (5) angstrom, and the product of the present invention is 14.0474 (4) angstrom. This difference is only about 0.01 angstrom, but this difference in crystal structure is identified as a significant difference by performing refinement processing of the lattice constant.

Regarding the crystal of the positive electrode active material, it is necessary that the crystallite diameter in the (110) vector direction does not grow excessively to 1000 angstroms or more. This is (1
10) When the crystal grows too much in the vector direction, when Li is inserted (discharged) in the positive electrode active material, Li ions can be inserted only in a direction parallel to the layer, so that a surface on which Li ions can be relatively inserted is formed. This is because the amount of the Li ions decreases, and the diffusion of Li ions at the grain interface deteriorates. This tendency becomes particularly noticeable when discharged at a high current density.

The charge / discharge characteristics do not substantially change in the crystallite diameter range of 500 to about 1000 Å, but the charge / discharge characteristics deteriorate when the crystallite diameter exceeds about 1000 Å.

<Measurement of Crystallite Diameter> A crystallite indicates the maximum set considered to be a single crystal,
It can be calculated from the XRD (X-ray diffraction) measurement by using the following Scherrer's formula. Crystallite size D (Angstrom) = Kλ / (βco
sθ) K: Scherrer constant (when β is calculated from the integration width, K =
1.05) λ: wavelength of X-ray tube used (CuKα1 = 1.54056)
2 angstrom) β: Width of spread of diffraction line due to crystallite size (rad
ian) θ: Diffraction angle 2θ / 2 (degree)

The term "particle" as used herein refers to the smallest particle imaged by SEM, and when the particle is composed of one single crystal, the crystallite diameter and the particle diameter are the same. When a plurality of single crystals are included in one particle, the sizes do not match.

The center particle size of the heavy metal oxide is 1.0 to
The range of 10.0 μm is preferable. Here, the central particle size is Co
It is the value measured by ultra Multisizer2. If the heavy metal oxide is smaller than this range, the particles agglomerate and become difficult to handle, and if it is larger than this range, the degree of crystallite growth is large and the c-axis length tends to be large.
Not preferable.

<Heavy Metal Oxide Raw Material> The positive electrode active material for a lithium ion secondary battery of the present invention is characterized by a shorter c-axis length than conventional products. Cobalt (C
The key is to use a crystal having a small degree of crystal growth of o3O4). That is, Co3O4 having a small crystal growth rate is required to increase the reactivity with the lithium salt. A crystal with a low crystallinity is a crystal with a small crystallite size, and the crystallite size of the raw material Co3O4 has a C-axis length of 14.051 angstroms or less.
0-400 Angstroms is preferred.

When the crystallite length of the raw material Co3O4 in the (222) vector direction is smaller than 50 angstroms, there occurs a problem that crystals grow abnormally in the (110) vector direction of LiCoO2. The reactivity with Li is lowered, and it becomes difficult for the crystal to grow. As a result, the C-axis length becomes larger than 14.051 angstrom.

<Lithium Raw Material> Li as a raw material used for the lithium secondary battery in the present invention.
As a salt, as a result of various studies, Li having a relatively high melting point was used.
2CO3, Li2 (COO) 2 or LiOH can be preferably used.

<Mixing of Heavy Metal Raw Material and Lithium> In the present invention, Co3O4 and lithium salt are mixed so that the Li / Co ratio is in the range of 0.98 to 1.01. This is because if the Li / Co ratio deviates from this range, the excess acts as a flux to cause abnormal growth of particles, making it difficult to control the particle diameter and particle shape.

<Firing> The obtained mixed raw material is placed in an atmosphere of 750 to 1100.
Bake at ° C. When an alkali metal salt, B, further Bi, Pb, or the like is added as a flux, or when granulating, the growth of the c-axis length is easily promoted and the cycle characteristics are deteriorated, which is not preferable.

[0028]

[Examples] [Example 1] <Preparation of raw materials> -Cobalt trioxide (Co3O4) ... 3.11
0kg ・ Lithium carbonate (Li2CO3) ・ ・ ・ ・ ・ 1.38
0 kg of cobalt trioxide is a polycrystalline particle having a crystallite diameter in the (222) vector direction of 200 angstroms and a secondary particle having a substantially spherical shape. Regarding the secondary particle size, C
The median particle size was 5.0 μm as measured with an ulter Multisizer 2. L of the above raw materials
The charging ratio of i / Co is 1.00. These raw materials are placed in a ceramic pot and ball-milled to obtain a mixed raw material for the positive electrode active material.

The obtained mixed raw material was heated in air at 900 ° C. for 10 minutes.
It was calcined for hours and pulverized to synthesize the desired LiCoO2.

The obtained LiCoO 2 was measured by powder X-ray diffraction using CuKα as a radiation source to refine the lattice constant. The c-axis length was 14.474 (4) angstrom and the a-axis length was 2. 81447 (3) angstroms. Also, the crystallite system in the (110) vector direction is 8
It was 15 Å. 200 by the method described above
The capacity retention of the cycle was measured and found to be 95%.

Comparative Example 1 Cobalt tetraoxide particles having a crystallite size in the (222) vector direction of 578 angstroms were used under the same conditions as in Example 1 except that the raw materials were mixed and fired to obtain LiCoO2. Was synthesized. When the obtained LiCoO2 was refined in lattice constant in the same manner as in Example 1, a-axis length = 2.8143 (2) angstrom, c-axis length = 1.
It was 4.0576 (5) angstroms. Also,
The crystallite diameter in the (110) vector direction was 520 Å. A secondary battery was made in the same manner as in Example 1 except that the obtained positive electrode active material was used, and the capacity retention rate after 200 cycles of charging and discharging was 87.8%.

[0032]

As described above, LiCoO2 (lithium heavy metal oxide) of the positive electrode material of the present invention has a lattice constant c
By making the axial length shorter than that of the conventional product, it is possible to keep the crystal collapse that occurs when Li ions are desorbed from the positive electrode active material crystal or enter the crystal by charge / discharge, and as a result, the crystal structure is improved. It is possible to stabilize, improve cycle characteristics under high cycle, and improve battery life.

In the present invention, attention is paid to the property that the c-axis length increases in a linear function, and crystal collapse that occurs when Li is desorbed from the positive electrode active material crystal or enters the crystal by charge / discharge is kept small. For this purpose, by decreasing the c-axis length of the positive electrode active material in which uncharged Li is not desorbed, the crystal collapse is less likely to occur and the cycle characteristics are improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a hexagonal basic lattice.

FIG. 2 shows Lix of a positive electrode active material of the present invention and a comparative product.
Characteristic diagram plotting the relationship between x-value of CoO2 and c-axis length

FIG. 3 is a characteristic diagram showing the relationship between the capacity retention rate (200 cycles) and the c-axis length of the product of the present invention and the comparative product.

[Explanation of symbols]

a ... The present invention b ・ ・ ・ ・ Comparison product

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-181062 (JP, A) JP-A-7-192719 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 4/00-4/04 H01M 4/36-4/62 H01M 10/40

Claims (2)

(57) [Claims] 1. A crystallite diameter of Co3O4 as a raw material is (2
22) 50 to 400 angstroms in the vector direction
A positive electrode active material for a lithium-ion secondary battery, which is represented by the general formula LixCoO2 and has a hexagonal crystal structure, and has a c-axis length of a lattice constant of 14 when x = 1.0. .0
48 angstroms or less, and in the state of x = 0.425 or less, the c-axis length of the lattice constant is 14.40 angstroms, with the conversion point of the crystal structure transition, and the crystallite diameter of the crystallite in the (110) vector direction. Is 500 ~
A positive electrode active material for a lithium ion secondary battery in the range of 1000 Å. 2. Co / O4 as a raw material and a lithium salt are used as Li /
A method for producing a positive electrode active material for a lithium ion secondary battery, which comprises a lithium cobalt oxide having a hexagonal crystal structure and is mixed and fired so that the Co ratio is in the range of 0.98 to 1.01. The crystallite diameter of the Co3O4 is 5 in the (222) vector direction.
A method for producing a positive electrode active material for a lithium ion secondary battery, which is in the range of 0 to 400 angstroms.