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.