Understanding the combined effect of Ca doping and phosphate coating on Ni-rich LiNi0.91Co0.06Mn0.03O2 cathode material for Li-ion batteries

Ni-rich cathode materials are one of the promising candidate due to their high discharge capacity and better rate capability for Li-ion batteries. However, these materials retain a large amount of residual lithium compounds (LiOH and Li2CO3) on the surface which are responsible for the gas generation during battery operations. In addition, structural instability of Ni-rich oxides leads to irreversible phase transition during the cycling process. To address these problems, a promising technique has been adopted to reduce the residual lithium and also enhance the structure stability of these materials. In this work, LiNi0.91Co0.06Mn0.03O2 has co-doped and coated with CaHPO4 by a solid-state process. This strategy has successfully achieved the Ca2+ doping and phosphate coating on the surface, thus stabilizing the host structure and reducing the amounts of residual lithium. Among the various amount of CaHPO4, the 0.5 mol% (CaP-0.5) sample exhibits the best electrochemical properties. The CaP-0.5 delivers a discharge capacity of 210.2 mAh g−1 (0.1 C) and capacity retention of 77.9% after 80 cycles at 0.5C and rate capability of 75.5% at 3C. Thus, we can conclude that, the improvements in cyclability and rate performance of CaP-0.5 are mainly due to the enhanced chemical and structural stability.