Since their appearance in 1991 as a small, light and large capacity battery, lithium secondary batteries have been widely used as a power supply of mobile devices. Recently, with rapid development of electronics, communication and computer industries, camcorders, mobile phones, laptop PCs have appeared and made remarkable progresses, and demands for lithium secondary batteries as a power source driving these mobile electronic information communication devices have increased each day.
Lithium secondary batteries have a problem in that the lifespan rapidly decreases as charge and discharge are repeated.
Such a lifespan decrease is caused by a side reaction between a positive electrode and an electrolyte, and this phenomenon may become more serious under a high voltage and high temperature condition.
Accordingly, development of secondary batteries for a high voltage is required, and to this end, technologies controlling a side reaction between a positive electrode active material and an electrolyte or electrode interfacial reaction are very important.
In order to solve such a problem, a technology of coating a metal oxide including Mg, Al, Co, K, Na, Ca or the like on the surface of a positive electrode active material has been developed.
Particularly, it is generally known that oxides such as Al2O3, ZrO2, and AlPO4 are capable of being coated on the surface of a positive electrode active material. It is also established that the coating layer enhances stability of the positive electrode active material.
However, in the surface coating using the oxide coating layer, the oxide coating layer adopts a form of being finely dispersed in a nano-sized particle form instead of the oxide coating layer covering an overall surface of a positive electrode active material.
As a result, the surface modification effect of the positive electrode active material by the oxide coating layer is limited. In addition, the oxide coating layer is one type of an ion insulation layer in which lithium ion migration is difficult, and may cause an ion conductivity decrease.
In view of the above, the inventors of the present invention have studied a positive electrode active material capable of exhibiting an excellent lifespan property even under a high voltage condition while having excellent safety, and identified that a surface coated positive electrode active material prepared by forming a nanofilm including polyimide and conductive nanoparticles on the surface of the positive electrode active material may effectively suppress a side reaction between the positive electrode active material and an electrolyte due to the nanofilm, and may exhibit an excellent lifespan property and conductivity even under a high voltage condition while having excellent safety, and completed the present invention.