Effect of vacancy ordering on ionic diffusion in layered Na-ion cathode materials

Sodium-ion batteries are considered a promising post-lithium energy storage technology due to the abundance and low cost of sodium. Among cathode materials, Mn-based P2-type layered NaxTMO2 oxides are attractive for their structural stability and fast Na-ion transport. Sodium vacancies are an inherent feature of the P2 structure. The Na vacancies can order in the structure, but the effect of this ordering on Na-ion diffusion is under debate. To investigate the effect of Na vacancy ordering, we have prepared two samples of P2-type Na0.66(Mn0.54Ni0.13Co0.13)O2 with different degrees of ordering. We hypothesize that Na vacancy ordering leads to improved intrinsic Na-ion diffusion in the material and therefore improved electrochemical perfomance. We propose a muon experiment to test this hypothesis by measuring intrinsic Na-ion diffusion in both samples and to show that the improved performance is not due to extrinsic factors. Understanding the intrinsic Na-ion dynamics in P2-type layered oxides is critical for optimizing performance as cathode materials.