Structure and evolution of disordered deep-charge phases in NaxCrO2 Na-ion battery electrodes

Layered sodium chromium oxide, O3-NaCrO2, is a promising electrode material for Na-ion batteries due to the high thermal stability, high rate-capability, and good coulombic efficiency of the material. However, it is well-known that the material loses crystallinity and the ability to store Na-ions reversibly when taken to high states of charge. While the structural identity of the disordered state has been investigated, a bulk structural model has so far been lacking, as has the mechanism for the order-disorder transition from the P ' 3-NaxCrO2 (the last crystalline phase observed during charge) to the disordered state. In this work, we combine pair distribution function analysis from total X-ray scattering and powder X-ray diffraction to obtain a complete structural model for the disordered O3-like NaxCrO2 forming at high states-of-charge, and from operando pair distribution function analysis and X-ray absorption spectroscopy, we elucidate the structural evolution of the order-disorder transition and shed new light on the Cr-migration in the disordered state, which is the cause of the irreversibility of the Na -ion storage.