Expanded solid-solution behavior and charge-discharge asymmetry in NaxCrO2 Na-ion battery electrodes

Sodium chromium oxide, NaCrO2, has received significant attention as positive electrode (cathode) for Na-ion batteries mainly due the high capacity retention of the material. In this paper, we use operando powder X-ray diffraction to investigate the structural evolution of O3–NaCrO2 during Na-ion extraction and insertion with use of both liquid and solid-state electrolytes. This reveals a previously overlooked phase transition, causing formation of an unexplored O′3-NaxCrO2 intermediate during Na-ion extraction (charge). The phase transitions within the electrode are investigated by sequential Rietveld refinement and distortion analysis, which shows that the discovered O′3-NaxCrO2 intermediate accommodates the increasing interlayer repulsion and in-plane contraction with less distortion of the [NaO6]-polyhedra as compared to the other observed phases. Our structural findings are corroborated by analysis of the Na-ion diffusion coefficients through the galvanostatic intermittent titration technique. Furthermore, the operando powder X-ray diffraction reveals substantial charge-discharge asymmetry with a significant preference for solid-solution and two-phase behavior during charge (Na-extraction) and discharge (Na-insertion), respectively.