The improvement of the cycle life of Li2.6Co0.4N as an anode of Li-ion secondary battery

Graphite has been widely used as the anode material of commercial Li-ion secondary battery. With the growing demands of high-capacity secondary battery, the low capacity of graphite (theoretical capacity: 372 mA h/g) has been thought to be the limiting factor in wide applications and a new anode material with a high capacity has been sought. Of the many materials, Li2.6Co0.4N showed the best anode performance. It showed very high capacity of 1024 mA h/g, good rate capability (1C/0.2C=94.94%) and extraordinary initial coulometric efficiency (96%). In addition to the excellent capacity of this material, its rate capability was much superior to that of graphite (82–88%). However, it cannot be commercialized because its capacity loss after 30 cycles is around 40%. As a result of various analyses, it was confirmed that the formation of passivating surface film caused by the decomposition reaction between electrolyte and Li2.6Co0.4N is the main reason of its cyclic degradation. As a method of improving the cyclic degradation, iron doping in Li2.6Co0.4N was suggested and we attempted to restrain the formation of passivating film on the surface of Li2.6Co0.4N. Li2.6Co0.35Fe0.05N had a bit lower capacity (about 900 mA h/g) than Li2.6Co0.4N (1024 mA h/g) but showed much better cycle life than Li2.6Co0.4N (35%→60% after 50 cycles).