Preferential occupation of Na in P3-type layered cathode material for sodium ion batteries

Layered NaxTMO2 (0 < x ≤ 1, TM = transition metal) materials have been extensively investigated as cathodes for sodium ion batteries. However, capacity fading during cycling hinders their practical utilization in sodium ion batteries. The capacity fading of layered NaxTMO2 materials is normally reasoned by unfavorable structural phase transitions accompanied by the slide of TMO2 layers, while the shrinkage and expansion of TMO2 layers is still unclear. P3-type Na0.67Mn0.67Ni0.33O2, synthesized at 750 °C and demonstrating attractive electrochemical behaviour, is investigated by X-ray absorption spectroscopy (XAS). Experimental results explain changes in interatomic distance and bond angle of TMO6 units during cycling of P3-type material. The preferential occupancy of Na around Ni in P3-type material is observed as a result of an ordered arrangement of Mn and Ni, which is responsible for different dimensional changes of various TMO6 units resulting in stress in the overall structure and capacity fading.