JP4736943B2 - Positive electrode active material for lithium secondary battery and method for producing the same - Google Patents

Description

  The present invention relates to a positive electrode active material mainly composed of a lithium cobaltate composite oxide used in a lithium secondary battery, and more particularly to a positive electrode active material excellent in thermal stability and cycle characteristics and a method for producing the same.

  The positive electrode active material for a lithium ion secondary battery tends to increase the charging voltage for the purpose of improving the discharge capacity. However, when the charging voltage is increased to around 4.2 V, crystal transition of the positive electrode active material or decomposition of the positive electrode active material occurs, resulting in a decrease in cycle deterioration. Further, along with the crystal transition or decomposition of the positive electrode active material, oxygen from the cobalt acid is released, and this oxygen oxidizes and burns the non-aqueous electrolyte, resulting in a decrease in the thermal stability of the battery itself. This reduction in thermal stability has a significant effect on battery safety, and further improvement in thermal stability of the positive electrode active material is desired.

  In addition, the lithium cobaltate composite oxide of the positive electrode active material has low conductivity, and therefore the conductivity is improved by coating the conductive carbon. However, when the contact with the carbon is poor, cycle deterioration is caused. It was the cause.

  In order to solve such a problem, for example, Patent Document 1 discloses an improvement in the stability of high-temperature storage at full charge by preparing a positive electrode in which a specific low-valent transition metal oxide is added to the positive electrode active material. Is going to be made.

  For the purpose of improving cycle characteristics, Patent Document 2 describes a positive electrode active material in which Al or Fe is dissolved in a lithium cobalt oxide composite oxide.

  However, the thermal stability and cycle characteristics of the positive electrode active material shown in these technologies are still insufficient, and the solid solution amount of the transition metal is as high as 1 to 20% by weight. Absent.

JP-A-9-330719 JP-A-9-190818

  Accordingly, an object of the present invention is to solve the above-described problems, and a positive electrode active material that can improve thermal stability and cycle characteristics during charging without reducing the discharge capacity and charge capacity of the lithium secondary battery. An object of the present invention is to provide a lithium cobalt oxide composite oxide.

The inventors of the present invention have made a number of prototypes of the positive electrode active material for the purpose of improving the cycle characteristics and thermal stability of the lithium ion battery, and as a result of intensive investigations, unexpectedly, specific Al ₂ was added to the particle surface of the positive electrode active material. It was found that there is an effect in coating O ₃ and the present invention was completed.

That is, the method for producing a positive electrode active material for a lithium secondary battery according to the present invention includes a step of producing a lithium cobaltate composite oxide, and a step of mixing the lithium cobaltate composite oxide and Al ₂ O _3. .

It is preferable to heat-treat the mixture of the lithium cobaltate composite oxide and the Al ₂ O ₃ .

The mixing amount of the Al ₂ O ₃ is preferably 0.05 to 10 parts by weight with respect to 100 parts by weight of the lithium cobalt oxide composite oxide.

The average particle diameter of the Al ₂ O ₃ is preferably 0.01 μm to 0.25 μm.

In the step of producing the lithium cobaltate composite oxide, the lithium cobaltate composite oxide has a composition formula represented by Li _x CoO ₂ (where x satisfies 0.98 <x ≦ 1.10.). It is preferable to further contain a transition metal other than Co.

Further, in the positive electrode active material for a lithium secondary battery of the present invention, the particle surface of the lithium cobalt oxide composite oxide is coated with Al ₂ O ₃ , and the lithium cobalt oxide composite oxide has a composition formula of LixCoO ₂ . And the value of x satisfies 0.98 <x ≦ 1.10.

  As shown below, when the positive electrode active material has the configuration of the present invention, the calorific value can be reduced without reducing the single electrode discharge capacity of the positive electrode.

  In addition, the range in which the ratio of Li to Co can be increased can be expanded. As a result, the charging ratio of Li can be increased and the discharge capacity can be increased. Furthermore, even if the Li ratio is increased, a positive electrode active material with little reduction in discharge capacity at the high rate can be obtained.

  Hereinafter, a positive electrode active material for a lithium secondary battery and a method for producing the same according to the present invention will be specifically described. However, the present invention is not limited to this embodiment.

  The thermal stability of the positive electrode active material in the present invention was evaluated by preparing a measurement sample as follows and measuring the heat of reaction by differential thermal analysis.

  (1) First, 90 parts by weight of a positive electrode active material powder as a measurement sample, 2.5 parts by weight of acetylene black powder as a conductive agent, 2.5 parts by weight of carbon, and 5 parts by weight of PVDF (polyvinylidene fluoride) are kneaded. To prepare a paste.

  (2) The obtained paste is applied to a detachable cell positive electrode current collector capable of single electrode evaluation to produce a secondary battery, and charging and discharging with a constant current are performed. The battery that has been conditioned is charged under a constant current until the battery voltage reaches 4.3V.

  (3) When charging is completed, the positive electrode is taken out from the detachable secondary battery, washed and dried, and the positive electrode active material is scraped off from the positive electrode.

  (4) About 2.0 mg of ethylene carbonate used for the electrolytic solution in an Al cell and about 5 mg of the active material scraped from the positive electrode are weighed, and differential thermal analysis is performed in the range of room temperature to 400 ° C.

  In the differential thermal analysis chart thus measured, as shown in FIG. 1, the differential heat does not change even if the temperature rises in the low temperature part, but the differential heat greatly increases above a certain temperature. The temperature A at this time is defined as the heat generation start temperature, and the height at the peak of this differential heat curve is defined as the heat generation amount a (height from the baseline). When the fully charged positive electrode active material is used as a measurement sample and the sample temperature is raised by a differential thermal analyzer, desorption of oxygen from the positive electrode active material occurs as described above, and this oxygen coexists in the sample. It is possible to oxidize ethylene carbonate of the water electrolyte and measure the reaction heat of the oxidation reaction.

In the present invention, coating of Al ₂ O ₃ on the surface of the positive electrode active material particles is effective in improving the thermal stability. FIG. 2 shows the relationship between the coating amount of Al ₂ O _{3 and} the calorific value by differential thermal analysis. Al ₂ O ₃ has an average particle diameter of _Al 2 _{O 3} was used in 0.01 [mu] m. As shown in FIG. 2, when the coating amount of Al ₂ O ₃ is 0.05 parts by weight or more with respect to 100 parts by weight of the positive electrode active material, the heat generation amount is reduced to about 10 mW.

Furthermore, the single-electrode discharge capacity of the positive electrode was charged to a battery voltage of 4.3 V and then discharged to 2.75 V using the above-described detachable cell, and the amount of electricity taken out at that time was measured. The relationship between the coating amount of Al ₂ O ₃ and the discharge capacity is plotted in FIG. When the coating amount of Al ₂ O ₃ is increased, the discharge capacity is not changed up to 10 parts by weight, but when the coating amount is 20 parts by weight, it is greatly reduced. This is presumed to be a result of the resistance of the positive electrode active material being increased by the coating of Al ₂ O ₃ . Therefore, the upper limit of the coating amount of Al ₂ O ₃ is 20 parts by weight. If it exceeds this, the decrease in discharge capacity becomes too large, which is not practical.

Therefore, the positive electrode active material of the present invention is required to have a coating amount of Al ₂ O ₃ in the range of 0.05 to 20 parts by weight based on differential thermal analysis and discharge capacity measurement, and 0.05 to 10 parts by weight. A range of parts is more preferred.

Table 1 shows the results of measurement of reaction heat and discharge capacity by the differential thermal analysis described above for the positive electrode active material in which 0.1 part by weight of Al ₂ O ₃ having various average particle diameters was coated on the positive electrode active material. Show.

From Table 1, when Al ₂ O ₃ of sample 5 is adhered, there is almost no decrease in the amount of generated heat and no effect. Moreover, the discharge capacity tends to decrease as the average particle size increases. Therefore, the average particle diameter of Al ₂ O ₃ is preferably 0.25 μm or less, and preferably 0.10 μm or less.

The positive electrode active material used in the present invention is a lithium-transition metal composite oxide whose composition formula is expressed by Li _x CoO ₂ , and a part of Co is unavoidable transition such as Fe, Mn, Cr, Ni, etc. The present invention is also applicable to a metal containing or mixed metal.

  The x value is the ratio of lithium to cobalt, and it is advantageous for charge / discharge to have a large amount of Li. Therefore, ideally, the x value is usually set larger than the stoichiometric ratio (x = 1). However, if it is too large, the discharge capacity tends to decrease at the high rate (when a large current flows at the time of discharge), and the x value cannot actually be increased too much.

On the other hand, the reduction of the discharge capacity at the high rate due to the increase of the x value can be improved by coating with Al ₂ O ₃ . A positive electrode active material of the present invention that the Al ₂ O ₃ coated 0.1 part by weight in Fig. 4, the positive electrode active material of Comparative Example without coating the Al ₂ O _3, high rate when the case of application to Demantaburuseru described above The relationship between the discharge capacity and the x value was plotted. In this case, the charge / discharge load was 1.5 C, the charge potential was 4.2 V, and the discharge potential was 2.75 V. (1C is a current load that completes charging or discharging in one hour.) From FIG. 4, the discharge capacity of the positive electrode active material of the present invention does not decrease to x = 1.10. On the other hand, the comparative example decreases when the x value is larger than 1.00.

  Further, in the case of discharging at a normal current density (0.25 C), the relationship between the x value and the discharge capacity is plotted in FIG. From this figure, the discharge capacity increases as the x value increases. When the x value is smaller than 1.00, the discharge capacity is greatly reduced. Therefore, the x value needs to exceed 0.98.

  Considering both the discharge rate at the high rate and the discharge capacity at the normal time, x needs to be set in the range of more than 0.98 and not more than 1.1.

When the electrode is immersed in the electrolyte solution and the electrode is electrically connected to an external circuit, an electric double layer is formed in the electrolyte solution near the electrode interface. In this electric double layer, ions exchange electrons. The same phenomenon occurs on the surface of the positive electrode active material of the present invention. Specifically, during discharge, Li ions move in the crystal lattice inside the positive electrode active material, reach the electrode interface, and electrons are transferred to the electrode. Leave it in the electrolyte. At this electrode interface, Li ions diffuse in the electric double layer and migrate into the electrolyte bulk. In this case, during the discharge of a large current, relatively easy electrophoresis in the electric double layer occurs rapidly, but the movement of Li in the crystal lattice having a large movement resistance is delayed. As a result, Li ions are insufficient in the vicinity of the electrode interface, and as a result, the electrical conductivity is lowered due to a decrease in the carrier concentration, which causes a decrease in the discharge capacity. By coating a specific amount of fine Al ₂ O ₃ in the present invention, the moving speed in the electric double layer can be somewhat reduced, and as a result, the moving speed of Li ions in the crystal lattice is balanced with the voltage. It is estimated that the descent could be improved.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

  The positive electrode active material substrate used in the present invention can be applied without limitation as long as it is obtained by a normal production method. For example, the respective oxides of Co and Li, or the respective compounds that generate oxides when heated at a high temperature are mixed in a predetermined ratio, and then the mixture is heated at a temperature of 500 to 1000 ° C. in an air atmosphere. Obtained by firing. A compound that generates an oxide when heated at a high temperature means a carbonate, oxalate, nitrate, sulfate, hydroxide, or the like. Moreover, heating at a high temperature means a temperature at which firing is performed in a later step.

The calcination temperature can be obtained by heating at a temperature in the range of 500 ° C. to 1000 ° C. for 1 to 24 hours, preferably in the temperature range of 700 to 1000 ° C. for 6 to 12 hours.
When the firing temperature is 500 ° C. or lower, unreacted raw materials remain in the positive electrode active material, and the original characteristics of the positive electrode active material cannot be utilized. On the other hand, when the temperature is 1000 ° C. or higher, the particle size of the positive electrode active material becomes too large and the battery characteristics deteriorate.

  When the firing time is less than 1 hour, the diffusion reaction between the raw material particles does not proceed. When 24 hours have elapsed, the diffusion reaction is almost completed, and therefore no further firing is necessary.

The atmosphere during firing is preferably a neutral atmosphere or a weak oxidizing atmosphere in the air. If it is carried out in a reducing atmosphere, it becomes difficult to obtain the target oxide composition, and unnecessary substances such as carbon or sulfur remain, so this must be avoided.
[Example 1]
Cobalt tetroxide (Co ₃ O ₄ ) as a cobalt raw material and lithium carbonate (Li ₂ CO ₃ ) as a lithium raw material were weighed so that x = 1.02, and dry mixed. The obtained mixed powder was baked at 900 ° C. for 10 hours in an air atmosphere to obtain a lithium-transition metal composite oxide represented by Li _1.02 CoO ₂ . Next, this was pulverized using a rough mortar to obtain a positive electrode active material powder having an average particle size of about 5 μm.

A Henschel mixer was charged with 10 kg of the obtained positive electrode active material powder and 200 g of Al ₂ O ₃ (Aron C, manufactured by Degussa) having an average particle diameter of 0.05 μm, and mixed for 6 minutes at a rotation speed of 1000 rpm. Finally, the positive electrode active material of the present invention was obtained through a 200-mesh sieve. As a result of chemical analysis of the obtained positive electrode active material, 2 parts by weight of Al ₂ O ₃ was coated on 100 parts by weight of LiCoO ₂ , and according to the SEM photograph of the particle surface, as shown in FIG. The particle surface is coated with Al ₂ O ₃ .
[Examples 2 and 3]
A positive electrode active material coated with 0.05 part by weight and 10 parts by weight of Al ₂ O ₃ was obtained in the same manner as in Example 1 except that the mixed amounts of Al ₂ O ₃ were 5 g and 1000 g, respectively.
[Comparative Examples 1 and 2]
A positive electrode active material coated with 0 to 20 parts by weight of Al ₂ O ₃ was obtained in the same manner as in Example 1 except that the mixed amount of Al ₂ O ₃ was 0 g and 2000 g, respectively. FIG. 7 shows an SEM photograph of a micrograph of the positive electrode active material obtained in Comparative Example 1. This is shown for comparison with the product of the present invention shown in FIG.
[Comparative Example 3]
The average particle diameter of the Al ₂ O ₃ was the same manner as in Example 1 except for changing the _Al 2 _{O 3} of 0.3μm of Sumitomo Chemical Co., Ltd..
[Comparative Example 4]
The average particle diameter of the Al ₂ O ₃ was the same manner as in Example 1 except for changing the _Al 2 _{O 3} of Sumitomo Chemical Co., Ltd. 0.50 .mu.m.
[Comparative Example 5]
The average particle diameter of the Al ₂ O ₃ was the same manner as in Example 1 except for changing the _Al 2 _{O 3} of Sumitomo Chemical Co., Inc. of 1.25 .mu.m.
[Comparative Example 6]
The average particle diameter of the Al ₂ O ₃ was the same manner as in Example 1 except for changing the _Al 2 _{O 3} of Sumitomo Chemical Co. 2.60Myuemu.
[Comparative Example 7]
The average particle diameter of the Al ₂ O ₃ was the same manner as in Example 1 except for changing the _Al 2 _{O 3} of Sumitomo Chemical Co. 4.90Myuemu.
[Example 4]
The positive electrode active material coated with Al ₂ O _{3 of} Example 1 was subjected to heat treatment at 300 ° C. for 10 hours in the air atmosphere. This heating strengthens the coating of Al ₂ O ₃ . As a result of applying analysis by X-ray diffraction and XPS (X-ray photoelectron spectroscopy) for the purpose of confirming whether Al ₂ O ₃ reacted with LiCoO ₂ by this treatment, Al or Al ₂ O ₃ in LiCoO ₂ The solid solution was not confirmed.
[Comparative Example 8]
The positive electrode active material coated with Al ₂ O _{3 of} Example 1 was subjected to heat treatment at 800 ° C. for 10 hours in the air atmosphere. As a result of analysis by X-ray diffraction and XPS in the same manner as in Example 9, the peak of Al ₂ O _{3 on} the particle surface disappears, and the lattice constant on the surface of the fired product decreases, and Al or Al ₂ O ₃ Solid solution in LiCoO ₂ was confirmed.

FIG. 8 plots the relationship between the heating temperature and the calorific value (peak height) after Al ₂ O ₃ coating. These points are the results of heating at each temperature for 10 hours. The exothermic peak height due to heating is not changed. FIG. 9 plots the relationship between the discharge capacity and the heating temperature. The heating temperature does not affect the calorific value, but tends to lower the discharge capacity when heated above 300 ° C. Accordingly, the heating temperature is preferably 300 ° C. or lower.

Differential thermal analysis chart of positive electrode active material showing values of exothermic start temperature A and exothermic amount a Characteristic diagram showing relationship between Al ₂ O ₃ coating amount and heat generation Characteristic diagram showing the relationship between Al ₂ O ₃ coverage and discharge capacity Characteristic diagram showing the relationship between discharge capacity and x value (at high rate) Characteristic diagram showing the relationship between x value and discharge capacity (normal current density) SEM photograph of the positive electrode active material of the present invention coated with Al ₂ O ₃ SEM photograph of a positive electrode active material of a comparative example of the present invention that does not cover Al ₂ O ₃ Characteristic diagram showing the relationship between the exothermic peak height and the heating temperature of the positive electrode active material of the present invention The characteristic view which shows the relationship between the discharge capacity and heating temperature of the positive electrode active material of this invention

Claims (2)

Producing a lithium cobaltate composite oxide;
Mixing the lithium cobaltate composite oxide and Al ₂ O ₃ with a Henschel mixer;
A step of heat-treating a mixture of the lithium cobaltate composite oxide and the Al ₂ O ₃ at 300 ° C. or less, and a method for producing a positive electrode active material for a lithium secondary battery,
The lithium cobaltate composite oxide is represented by the composition formula Li _x CoO ₂ (where x satisfies 0.98 <x ≦ 1.10.)
The mixing amount of the Al ₂ O ₃ is 0.05 to 10 parts by weight with respect to 100 parts by weight of the lithium cobalt oxide composite oxide,
The average particle diameter of the _Al 2 _{O 3} The manufacturing method of a lithium secondary battery positive electrode active material for which is a 0.01Myuemu～0.25Myuemu. The positive electrode active material for lithium secondary batteries obtained by the manufacturing method of Claim 1.

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