A TEM study of cycled nano-crystalline HT-LiCoO2 cathodes for rechargeable lithium batteries

LiCoO2 has α-NaFeO2 structure type and it has been reported that layered cation ordering is preserved during repeated insertion and removal of Li+. We have observed, at a nano-particle scale, cation disorder induced in LiCoO2 after prolonged cycling. LiCoO2 cathode powders with nano-grain sized of 70–300 nm were synthesized by a mechano-chemical method. Transmission electron microscopy study of LiCoO2 showed that the initial O3 crystal structure partially transformed to a cubic spinel phase. This spinel phase formation may be responsible for capacity degradation after prolonged cycling of LiCoO2-based rechargeable lithium batteries. Cycle life of small size (70 nm) LiCoO2 powder until 200 cycles is better than that of large size (300 nm) LiCoO2 powder due to shorter diffusion distance.