In recent years, rechargeable secondary batteries are widely used as an energy source for wireless mobile equipment. Among other things, there has been an increasing demand for lithium secondary batteries due to high-energy density, high-discharge voltage and superior power output stability.
The lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode material and a carbonaceous material as an anode material, and is fabricated by disposition of a porous polyolefin separator between the anode and the cathode and impregnation of the resulting electrode assembly with a non-aqueous electrolyte containing a lithium salt such as LiPF6. Even though it is widely used as a cathode material for the lithium secondary battery, LiCoO2 suffers from various problems such as relative expensiveness, low charge/discharge capacity of about 150 mAh/g, instability of the crystal structure at a voltage of more than 4.3 V, and the risk of ignition by reaction with an electrolyte. Furthermore, LiCoO2 also suffers from the disadvantage of a significant fluctuation in properties in response to a slight change of some parameters during a fabricating process. In particular, some changes in process parameters lead to significant fluctuations in cycle properties and high-temperature storage properties at high potentials.
In this regard, in order to ensure operation of the battery at high potentials there have been suggested a method of coating the outer surface of LiCoO2 with a metal (for example, aluminum), and a method of subjecting LiCoO2 to heat treatment or mixing LiCoO2 with other materials, and the like. However, the secondary battery fabricated using such a cathode material exhibits poor safety at high potentials or suffers from limitations in application thereof to large scale production of batteries.
Recently, as secondary batteries have also drawn a great deal of attention as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) which are presented as countermeasures capable of solving problems of air pollution caused by fossil-fuel driven vehicles such as gasoline vehicles and diesel vehicles, it is expected that the demand for secondary batteries will further increase in the future. Therefore, there are increased concerns on the aforementioned problems, as well as the safety and high-temperature properties of batteries at high potentials.
As one of various schemes to cope with such problems of LiCoO2, a technique was proposed which uses a mixture of more than two different lithium transition metal oxides as a cathode material to compensate disadvantages that may occur upon preparation of the cathode material via a single use of each lithium transition metal oxide. As disclosed in Korean Patent No. 424638, the cathode material in the form of such a mixture is prepared generally by separately preparing each lithium transition metal oxide and then mixing the resulting oxides. Each lithium transition metal oxide is commonly prepared by mixing a certain lithium compound and a transition metal compound and heat-treating the mixture.
On the other hand, there are some examples involving a heat treatment during preparation of the cathode material. Such a heat treatment is intended for surface treatment of the cathode material or the modification of the physical properties of the mixture as the cathode material.
For example, Korean Patent No. 315227 discloses a technique for increasing a concentration of cobalt ions up to a certain depth from the particle surface so as to prevent the dissolution of manganese ions, in which a mixture of a lithium salt, a cobalt salt, an alcohol and a chelating agent is heated to prepare a sol- or gel-like material and the resulting material is mixed with Li2MnO4, followed by heat treatment.
However, the aforementioned techniques have inevitable shortcomings such as complicated processes for achieving desired physical properties and the consequent increased production costs of the secondary batteries. Furthermore, it was confirmed that high potential and high temperature properties of the battery are not improved to a desired level only by a simple heat treatment as in the above-mentioned conventional arts, because such a simple heat treatment shows no beneficial effects on the physical properties of crystalline particles.
In addition, the conventional cathode materials, in the form of a mixture, suffer from limitations in achieving synergistic effects far surpassing the effects obtained by simply combining two components.