1. Field of the Invention
The present invention relates to a method of producing an active material powder for a positive electrode or a negative electrode of a lithium secondary battery using non-aqueous electrolyte.
2. Description of the Related Arts
Among various kinds of secondary batteries, the lithium secondary battery is known as supplying high voltage as well as less electrical discharge, resulting in excellent storage capability. Therefore, the lithium secondary battery is highly expected as being applicable to a wide variety of fields.
One of prior arts has disclosed that the lithium secondary battery is obtained by using metallic oxide such as LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2 O.sub.4 or the like as an active material for the positive electrode and using lithium metal, lithium alloy or carbon that can occlude and release lithium ions or the like as an active material for the negative electrode (JPA Publication No. 114065/1988).
The above-described active materials are conventionally prepared by mixing lithium related material powder such as Li metal, Li oxide, Li hydroxide, Li carbonate or the like with a metal powder such as Mn, Ni and Co or oxide, hydroxide, carbonate thereof. The resultant mixture is heated at a high temperature for an extended period, which is called as a solid state reaction method.
The conventional method of producing the active material through the solid phase reaction method requires the process for heating the substance at a high temperature for an extended period. Conventionally as the active material reacts at a solid phase, the ingredient of the active material does not react sufficiently, thus failing to form the active material exhibiting uniform composition. The resultant active material tends to have a relatively large particle size ranging from 10 .mu.m to 20 .mu.m.
When the lithium secondary battery formed from the above-described active material is subjected to charge/discharge cycle under high current density, its capacity may be deteriorated to a greater extent.
Another prior art has disclosed the method of producing active material powder having uniform composition and small particle size for the lithium secondary battery that hardly deteriorates its capacity resulted from the charge/discharge cycle under high current density. More specifically, the active material powder is prepared by spraying the ingredient of the active material that has been dissolved in a solvent in the form of droplet. Then the droplet is heated to evaporate the solvent, thus providing the active material powder. This process is called as a spray pyrolysis method (Publication of JPA No. 9722/1990).
In the conventional spray pyrolysis method, temperature distribution occurs around a spot heated either from inside or outside. The resultant product, thus, is likely to fail to form uniform composition.
Especially when producing composite oxide using substances each having different vapor pressure, the resultant product is likely to have an irregular composition. The temperature distribution may cause increase or decrease in the temperature dependent on a certain spot even though the temperature condition has been set to be optimum for producing the oxide. As a result, it is difficult to control production of the composite oxide using the substance containing volatile component.
The third prior art does not relate to a method of producing the active material powder for the lithium secondary battery. It discloses the method of producing the above-described composite oxide powder exhibiting uniform composition with narrow temperature distribution at a heated spot.
In the aforementioned producing process called as a spray combustion method, a suspension prepared by suspending a material convertible into oxide by oxidation or an emulsion prepared by emulsifying a solution of the material in the combustible liquid is sprayed and the resultant mist is fired in an oxidizing atmosphere (Publication of JPA No. 81905/1995).
As the above spray combustion method consumes a large amount of oxygen for burning the combustible liquid, synthesizing the composite oxide exhibiting a spinel structure through this method may provide the material that has a large amount of oxygen defects.
Especially a spinel oxide containing lithium such as LiMn.sub.2 O.sub.4 tends to deform its crystalline structure owing to lack of oxygen. Being used as the active material for the lithium secondary battery, this type of oxide may cause capacity deterioration resulted from repetitive charge/discharge cycles.