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.
Keywords: chromium oxide; long cycle life; rate performance; sodium chromium oxide; sodium-ion battery; surface modification.