Novel P2-type concentration-gradient Na0.67Ni0.167Co0.167Mn0.67O2 modified by Mn-rich surface as cathode material for sodium ion batteries

Developing rechargeable sodium ion batteries with high capacity and long cycle life is still a big challenge. Herein, we present P2-type concentration-gradient material with average composition of Na0.67Ni0.167Co0.167Mn0.67O2, which exhibits relatively high discharge capacity and excellent stability. The material delivers higher discharge capacity and better rate performance than that of the concentration-constant Na0.67Ni0.167Co0.167Mn0.67O2 during cycling. Also, the material exhibits excellent capacity retention of ∼87% after 100 cycles. The high discharge capacity is attributed to the Ni-rich core and the improvement in cycling stability is due to a gradual and continuous increase of Mn4+ in the concentration-gradient spherical particles. The primary particles on the Mn-rich surface have smaller size than that of the constant-concentration samples, leading to a preferential crystalline state, therefore they can facilitate excellent transport properties for Na ions insertion/extraction process. The P2-type concentration-gradient material provides a new way for the development of advanced sodium ion batteries with high capacity and long cycle life.