Improving the rate performance of LiCoO2 by Zr doping

Zr-doped LiCoO2 cathode materials for lithium ion batteries were synthesized by an ultrasonic spray pyrolysis method. The synthesized powders with less than 1 mol% Zr had a single phase layered structure while those with 5 mol% Zr had a little secondary phase, Li2ZrO3. The cycle stabilities of Zr-doped and undoped LiCoO2 were compared at various charge-discharge rates. The Zr-doped LiCoO2 showed much improved cycle stability compared to the undoped, especially at a high C-rate of 3C (4.2 mA/cm(2)). To investigate the reasons of the improvement, changes of the lattice parameters and the interatomic distances of Co-Co and Co-O of the doped and the undoped powders were analyzed using XRD and EXAFS. The lattice parameters, a and c, increased in the powders with less than 1 mol% Zr, but decreased in the powder with 5 mol% Zr. On the other hand, the interatomic distances of Co-Co and Co-O did not change with Zr doping. From these results, the improved cycle stability is thought to be due to the expanded inter-slab distance, which enhances Li-ion mobility during charge/discharge processes.