Operando EPR and EPR imaging study on a NaCrO₂Cathode: Electronic property and structural degradation with Cr dissolution

NaCrO2 is a potential cathode material for sodium-ion batteries due to its low cost, safety, and high power. It is necessary to further understand its electronic property during cycling in advance of practical application. In this work, operando EPR is carried out to monitor the evolution of the electronic structure for NaCrO2 cycled between 2.2-3.6 V and 2.2-4.5 V. We discover that electronic delocalization takes place at the early stage of charge, which may account for the excellent rate performance. In addition, via EPR imaging, an EPR signal associated with the irreversible phase transition at 3.8 V is located in the electrolyte, which is then attributed to the Cr5+ ions dissolved with the surface reconstruction. These findings may help researchers to better design and modify the Cr-based cathode materials.