JP2003288899A - Positive electrode active material for nonaqueous electrolyte secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode active material for a nonaqueous electrolyte secondary cell, with improved current collecting efficiency, a good rate property and cycle property. <P>SOLUTION: The positive electrode active material for a nonaqueous electrolyte secondary cell is made of a lithium composite cobalt oxide represented as a formula LiCoXO<SB>2</SB>(note, 1.0<X≤1.05), of which, one to 20 primary particles agglutinate and form secondary particles, with a repose angle of not more than 60 degrees, an average particle diameter from 1 to 10 μm, and a specific surface area from 0.5 to 1.0 m<SP>2</SP>/g. Since the number of primary particles to structure the secondary particles is fewer than usual, a dispersion property of a conductive material is better, so that, a rate property and a cycle property are greatly improved. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a positive electrode active material thereof. 2. Description of the Related Art In recent years, portable electronic devices have become portable.
Cordless use is rapidly progressing. As these advances, there is an increasing demand for a secondary battery serving as a driving power source to have a high energy density and a small size and light weight. In recent years, it has been used as a power source for mobile phones, and demand for longer talk time and longer life has been desired along with rapid market expansion, and there has been a great demand for higher energy density and improved cycle life of secondary batteries. It is big. Under such circumstances, a non-aqueous electrolyte secondary battery using a lithium composite transition metal oxide exhibiting a high charge / discharge voltage, for example, LiCoO ₂ as a positive electrode active material and utilizing occlusion and release of lithium ions has been proposed. JP-A-63-59507), which has been improved. [0003] For LiCoO ₂ , for example,
As described in JP-A-304664, JP-A-5-151998, and JP-A-5-54888, the production method and the shape and size of particles have been studied. [0004] LiCo _X O _2, which is a positive electrode active material for a non-aqueous electrolyte secondary battery, has a large number of primary particles aggregated to form secondary particles. Generally, carbon black is mixed as a conductive material for current collection to form a current collection network. However, this carbon black is distributed only on the surface of the secondary particles, and extends to the surface of the primary particles. As a result, there is a problem that a portion having poor current collection efficiency is formed and the rate characteristics and the cycle characteristics are poor. An object of the present invention is to solve such a problem, and an object of the present invention is to provide a positive electrode active material having improved current collection efficiency and excellent rate characteristics and cycle characteristics. [0006] In order to solve the above-mentioned problems, the present invention provides a compound of the formula LiCo _X O ₂ (where 1.0 <X ≦
1.05) is a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium composite cobalt oxide represented by the following formula:
Primary particles aggregate to form secondary particles,
A positive electrode active material for a non-aqueous electrolyte secondary battery characterized by having a repose angle of 60 ° or less, an average particle diameter of 1 to 10 μm and a specific surface area of 0.5 to 1.0 m ² / g, Since the number of primary particles constituting the secondary particles is small, the dispersibility of the conductive material is good, and the rate characteristics and the cycle characteristics are greatly improved. [0007] DETAILED DESCRIPTION OF THE INVENTION The present invention has the formula LiCo _X O ₂ (however, 1.0 <X ≦ 1.05) is a powder of the lithium composite cobalt oxide represented by, 1-20 Primary particles are aggregated to form secondary particles, the angle of repose is 60 ° or less, the average particle diameter is 1 to 10 μm, and the specific surface area is 0.5 to 1.0 m ² / g.
It is a positive electrode active material for non-aqueous electrolyte secondary batteries characterized by the following. [0008] As a method for synthesizing LiCoO _2, a method in which a predetermined amount of a cobalt compound as a starting material and a predetermined amount of a lithium compound are mixed in a fixed ratio and fired at a high temperature is a well-known general method. Also, a method of dividing the heat treatment step of firing into two steps has been studied for various purposes such as temperature and time for various purposes. In the method of synthesizing the lithium composite cobalt oxide of the present invention, the raw material Co ₃ O ₄
By controlling the specific surface area of cobalt oxide so as not to hinder the reaction between cobalt oxide (Co ₃ O ₄ ) and the lithium compound without lowering the synthesis rate of LiCoO ₂ ,
In addition, by controlling the number of primary particles constituting the secondary particles by heat treatment of Co ₃ O ₄ , the dispersibility of the conductive material on LiCoO ₂ is improved, the current collection efficiency of the current collection network is increased, and the rate characteristics are improved. And the cycle characteristics can be improved. Embodiments of the present invention will be described below with reference to the drawings. (Example 1) A method for synthesizing LiCoO ₂ of this example will be described. Cobalt hydroxide (Co (OH) ₃ ) is placed in a ceramic container, heated to a temperature of 400 ° C. in one hour, and maintained at 400 ° C. for two hours after the temperature is raised, thereby obtaining cobalt oxide (Co _3). O ₄ ) A was obtained. Cobalt oxide A and lithium carbonate (Li ₂ CO ₃ ) were mixed such that the atomic molar ratio of Li and Co was 1: 1.01. This mixture was placed in a ceramic container, air was introduced at a rate of 20 l / min from one side, the temperature was raised to 900 ° C. in 3 hours, and the temperature was maintained for 10 hours. Substance A was obtained. A primary particle is a single crystal particle, and its shape and size are defined by an electron micrograph. Secondary particles are 1
The particles were a group of particles in which the next particles were aggregated and sintered. The shape was observed with an electron microscope photograph, and the particle size was measured with a laser analysis type particle size distribution meter. The angle of repose is an index indicating the fluidity of the powder,
The angle of the slope of a cone formed by dropping powder onto a cylinder through a sieve and measured by a Hosokawa Micron powder tester. Specific surface area is determined by gas adsorption (BE
T) was determined by the method. Comparative Example 1 Cobalt hydroxide (Co (O
H) ₃ ) was placed in a ceramic container, heated to a temperature of 250 ° C. in 1 hour, and kept at 250 ° C. for 2 hours after heating to obtain cobalt oxide B. A positive electrode active material B was obtained from the obtained cobalt oxide B in the same manner as in Example 1. Comparative Example 2 Cobalt hydroxide (Co (O
H) ₃ ) was placed in a ceramic container, the temperature was raised to 650 ° C. in 1 hour, and the temperature was maintained at 650 ° C. for 2 hours to obtain cobalt oxide C. A positive electrode active material C was obtained from the obtained cobalt oxide C in the same manner as in Example 1. The specific surface areas of the cobalt oxides A to C are shown in Table 1.
Shown in FIG. 1 shows a schematic view of the positive electrode active materials A to C, and Table 2 shows physical properties of the powder. [Table 1] [Table 2] Battery evaluation was performed using the positive electrode active materials A, B and C obtained in Example 1 and Comparative Examples 1 and 2. FIG. 2 shows a longitudinal sectional view of the cylindrical lithium secondary battery used in this example. In FIG. 2, the positive electrode plate 5 and the negative electrode plate 6
The electrode group 4 is formed by spirally winding a plurality of times through a separator 7 and is housed in a battery case 1 formed by processing a stainless steel plate having resistance to an organic electrolytic solution. A positive electrode aluminum lead 5a is pulled out from the positive electrode plate 5 and connected to the sealing plate 2, and a negative electrode nickel lead 6a is pulled out from the negative electrode plate 6 and connected to the bottom of the battery case 1. An insulating ring 8 is provided on each of the upper and lower portions of the electrode group 4, and an opening of the battery case 1 is closed by a sealing plate 2 provided with a safety valve and an insulating packing 3. The negative electrode plate 6 is composed of a carbon material (in this embodiment, pitch-based spheroidal graphite is used) and an aqueous dispersion of styrene-butadiene rubber in a weight ratio of 100: 3.5.
, And this was suspended in an aqueous solution of carboxymethylcellulose to form a paste. The paste was applied to both surfaces of a copper foil, dried, rolled, cut into a predetermined size, and a negative electrode plate was produced. The mixing ratio of the aqueous dispersion of styrene-butadiene rubber is calculated based on the solid content. The positive electrode plate 5 is composed of LiCoO ₂ of the synthesized positive electrode active materials A, B and C and an aqueous dispersion of acetylene black and polytetrafluoroethylene in a weight ratio of 1%.
The mixture was mixed at a ratio of 00: 2.5: 7.5, and this was suspended in an aqueous solution of carboxymethyl cellulose to form a paste. Then apply this paste on both sides of the aluminum foil,
After drying, the resultant was rolled and cut into a predetermined size to produce a positive electrode plate. The mixing ratio of the aqueous dispersion of polytetrafluoroethylene is calculated based on the solid content. The positive and negative electrode plates 5, 6 produced by the above method
The leads 5a and 6a were respectively attached to the battery case 1 and spirally wound through a polyethylene separator 7 and stored in the battery case 1. For the electrolyte, 1.5 mol of lithium hexafluorophosphate (LiPF ₆ ) was used as a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 3.
1 / l was used. This electrolyte solution was injected into the battery case 1 under reduced pressure and sealed, to obtain batteries A, B and C. In this example, in order to evaluate the characteristics of the positive electrode active material, a negative electrode having a larger capacity was used. Using these batteries A, B and C, a test was conducted under the following conditions. First, a battery voltage of 4.2 V at 20 ° C.
After charging at a constant current of 120 mA, the battery is paused for 1 hour, and then discharged to a battery voltage of 3.0 V at a constant current of 120 mA. Charge / discharge was repeated three times by this method, and the third discharge capacity was used as the initial capacity. Thereafter, the battery voltage was 4.2 V
After charging at a constant current of 120 mA, the battery is paused for 1 hour, and discharged to a battery voltage of 3.0 V at a constant current of 1200 mA. This discharge capacity is defined as a discharge capacity (1200 mA). The discharge capacity (1200 mA) in% with respect to the initial capacity was calculated as a capacity retention rate. Further, the charging / discharging current at 120 ° C. is 120 mA.
A constant current charge / discharge cycle test was performed under the conditions of a charge end voltage of 4.2 V and a discharge end voltage of 3.0 V. The discharge capacity at the time of 300 cycles with respect to the initial capacity expressed in% was calculated as the cycle capacity retention rate. Table 3 shows the capacity retention rates and cycle capacity retention rates of the batteries using the batteries A, B and C. [Table 3] Since the positive electrode active material B has very small primary particles of cobalt oxide as a raw material and has strong agglomeration, the dispersibility of the conductive material of the positive electrode active material B is low, and the rate and cycle characteristics of the battery B are poor. Was something. The positive electrode active material C has a small specific surface area of cobalt oxide as a raw material and has a low reactivity, so that the synthesis rate of LiCoO ₂ is reduced, and the secondary particles are strongly agglomerated by sintering with excess Li. And the cycle characteristics were poor. However, in the positive electrode active material A, the number of primary particles is 1 to 20 and the secondary particles are least formed, and the angle of repose is smaller than that in (Table 2).
Because of the high dispersibility, the dispersion of the conductive material was good, the current collection efficiency was improved, and the rate characteristics and cycle characteristics were improved. In this embodiment, 1 to 20 primary particles are aggregated to form secondary particles and have a repose angle of 55 °, an average particle size of 4.5 μm, and a specific surface area of 0.6 m ² / g. A positive electrode active material for a water electrolyte secondary battery was used.
The secondary particles aggregate to form secondary particles, and the angle of repose is 60
° or less, an average particle size of 1 to 10 μm and a specific surface area of 0.5
Even when a positive electrode active material for a non-aqueous electrolyte secondary battery of about 1.0 m ² / g was used, the effect of similarly improving the rate characteristics and cycle characteristics was obtained. In this embodiment, LiCoO ₂
Used a combination of tricobalt tetroxide (Co ₃ O ₄ ) and lithium carbonate (Li ₂ CO ₃ ) as a starting material, but tricobalt tetroxide (Co ₃ O ₄ ), a lower cobalt oxide and Li
The same effect can be obtained by using a combination of ₂ CO ₃ , LiNO ₃ and LiOH. The same effect can be obtained in a battery using various carbonaceous materials capable of inserting and extracting lithium, a lithium alloy, and an inorganic negative electrode capable of intercalation as the negative electrode. Further, as an electrolyte, a battery using an electrolyte solution in which a lithium salt is dissolved in a solvent of a combination other than a solution in which lithium hexafluorophosphate is dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate used in the present example, and a polymer electrolyte are used. The same effect can be obtained in As described above, according to the present invention, the secondary particles are composed of 1 to 20 primary particles, and the angle of repose is 60 ° or less.
Average particle size of 1 to 10 μm and specific surface area of 0.5 to 1.0
By adjusting to m ² / g, a positive electrode active material for a non-aqueous electrolyte secondary battery having improved dispersibility and excellent rate characteristics and cycle characteristics can be obtained.

[Brief description of the drawings] FIG. 1 is a schematic view of cathode active materials A to C. FIG. 2 is a longitudinal sectional view of a cylindrical lithium secondary battery of the present invention. [Explanation of symbols] 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode group 5 Positive electrode plate 5a Positive electrode lead 6 Negative electrode plate 6a Negative electrode lead 7 Separator 8 Insulation ring

Continuation of front page    F-term (reference) 5H050 AA02 AA07 BA17 CA08 CB07                       CB08 EA10 EA23 EA24 FA05                       FA17 HA00 HA02 HA05 HA07

Claims (1)

Claims: 1. The formula LiCo _X O ₂ (where 1.0 <X ≦
1.05) is a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium composite cobalt oxide represented by the following formula:
Secondary particles are aggregated to form secondary particles and have a repose angle of 60 ° or less, an average particle diameter of 1 to 10 μm, and a specific surface area of 0.5 to 1.0 m ² / g. Positive electrode active material for batteries.