Tuning Sodium Occupancy Sites in P2-Layered Cathode Material for Enhancing Electrochemical Performance

Different sodium occupancy sites in P2-layered cathode materials can reorganize Na-ion distribution and modify the Na+/vacancy superstructure, which have a vital impact on the Na-ion transport and Na storage behavior during charge and discharge processes, but have not been investigated specifically and are not yet well understood. Herein, the occupancy ratio of two different Na sites (sites below transition metal ions and sites below oxygen ions along the c direction) in P2-Na-0.67[Mn0.66Ni0.33]O-2 cathode is tuned successfully by inducing Sb5+ ions with strong repulsion toward Na sites right below transition metals. It is found that the decrease of Na occupancy right below transition metal ions is beneficial to the electrochemical performance of P2-layered cathode materials, regarding cycle stability and rate capability. In situ X-ray absorption spectroscopy reveals that the reversible Mn3.3+/Mn4+ and Ni2+/Ni3+ redox couples provide charge compensation in different voltage regions of 1.8-2.3 and 2.3-4.2 V, respectively. The transmission X-ray microscopy confirms the uniform redox reaction over the whole electrode particle. In addition, Sb substitution can suppress the "P2-O2" phase transition in high voltage region by preventing oxygen gliding in a-b planes, thus ensuring robust structure stability during cycling.