Regulating electron distribution of P2-type layered oxide cathodes for practical sodium-ion batteries

For transition metal oxide materials, high Ni content is an effective method to obtain a high specific capacity. However, the theoretical capacity is determined due to the certain amount of variable charges of transition metal ions. The increased capacity in specific voltage window may attribute to the easier transport of alkali ions, instead of more active elements. Borrowing the theory of Ni-rich materials in LIB, excess Ni elements were added into P2-type layered oxide material to form the Jahn-Teller active Ni3+ ions. About 25%-61% Ni3+ ions can effectively promote de-/intercalation of Na+ ions due to the decreased diffusion energy barrier and increased adsorption energy of Na+. The preferred "Ni-rich" material Na0.67Mn0.45Ni0.22Co0.33O2 (Ni-R1) shows a reversible specific capacity of 114 mA h g−1 in the voltage range of 2.0–4.25 V. In addition, it shows an excellent cycle stability, the capacity retention ration is 80% after 1000 cycles at a current density of 1 A g−1. The in-depth study proves that, Jahn-Teller active Ni3+ ions can effectively regulate the valence electron distribution of surrounding ions in synthesis stage. However, it will promote Jahn-Teller distortion when the Ni3+ content is increased to 74%, which makes the rate performance deteriorate dramatically. The present work provides a simple and efficient way to increase the capacity in suitable voltage range for application.