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Cr2O3is generally considered as an impurity phase with negative effects on the electrochemical performance of NaCrO2because it may cause a certain degree of capacity loss. In this study, however, we have found the bright side of Cr2O3as 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–1with a high capacity retention of 84.8% can be achieved for a Cr2O3-coated NaCrO2sample. The optimal sample exhibits a rate performance with 108.0 mAh g–1at a high rate of 60C. Cyclic voltammetry analysis indicates that such an in situ-formed inactive Cr2O3layer has little influence on Na+diffusion in NaCrO2electrodes, but it prevents the direct contact between the active material and the electrolyte, suppressing the side reactions effectively.

Surface coating and lattice doping are widely used to enhance the interfacial and structural stabilities of Li1.2Ni0.13Co0.13Mn0.54O2(LNCM). In this paper, KF is used to modify LNCM for the first time. A Li+/K+exchange in the Li slabs is realized via a high-temperature treatment. Consequently, subsurface K+gradient doping and surface K1–xLixF gradient coating are obtained simultaneously on LNCM. Such an Li+/K+exchange mechanism and double-gradient modification are clarified by X-ray diffraction, energy-dispersive spectrometry line scans, and high-resolution transmission electron microscopy analyses. As a result, the optimal 0.5 wt % KF-modified LNCM material shows markedly alleviated voltage degradation (0.0031 V@1 cycle), improved cycling stability (88%@100 cycles@0.5 C), and rate capability (108 mA h g–1@10 C), revealing large application potential in high-energy materials.