In Situ-Formed Cr2O3 Coating on NaCrO2 with Improved Sodium Storage Performance.

Cr2O3 is generally considered as an impurity phase with negative effects on the electrochemical performance of NaCrO2 because it may cause a certain degree of capacity loss. In this study, however, we have found the bright side of Cr2O3 as a protective coating material, which greatly improves the Na+ storage capability, especially the cycling stability, of NaCrO2. After 1000 cycles at 10C, a capacity of 100.4 mAh g-1 with a high capacity retention of 84.8% can be achieved for a Cr2O3-coated NaCrO2 sample. The optimal sample exhibits a rate performance with 108.0 mAh g-1 at a high rate of 60C. Cyclic voltammetry analysis indicates that such an in situ-formed inactive Cr2O3 layer has little influence on Na+ diffusion in NaCrO2 electrodes, but it prevents the direct contact between the active material and the electrolyte, suppressing the side reactions effectively.