Understanding sodium-ion diffusion in layered P2 and P3 oxides via experiments and first-principles calculations: a bridge between crystal structure and electrochemical performance

Abstract Layered Na x MeO 2 (Me=transition metal) oxides, the most common electrode materials for sodium-ion batteries, fall into different phases according to their stacking sequences. Although the crystalline phase is well known to largely influence the electrochemical performance of these materials, the structure–property relationship is still not fully experimentally and theoretically understood. Herein, a couple consisting of P2-Na 0.62 Ti 0.37 Cr 0.63 O 2 and P3-Na 0.63 Ti 0.37 Cr 0.63 O 2 materials having nearly the same compositions is reported. The atomic crystal structures and charge compensation mechanism are confirmed by atomic-scale characterizations in the layered P2 and P3 structures, respectively, and notably, the relationship of the crystal structure–electrochemical performance is well defined in the layered P-type structures for the first time in this paper. The electrochemical results suggest that the P2 phase exhibits a better rate capability and cycling stability than the P3 phase. Density functional theory calculations combined with a galvanostatic intermittent titration technique indicates that the P2 phase shows a lower Na diffusion barrier in the presence of multi-Na vacancies, accounting for the better rate capability of the P2 phase. Our results reveal the relationship between the crystal structure and the electrochemical properties in P-type layered sodium oxides, demonstrating the potential for future electrode advancements for applications in sodium-ion batteries.