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/cm2). 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.