Cr 2 O 3 is generally considered as an impurity phase with negative effects on the electrochemical performance of NaCrO 2 because it may cause a certain degree of capacity loss. In this study, however, we have found the bright side of Cr 2 O 3 as a protective coating material, which greatly improves the Na + storage capability, especially the cycling stability, of NaCrO 2 . 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 Cr 2 O 3coated NaCrO 2 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 Cr 2 O 3 layer has little influence on Na + diffusion in NaCrO 2 electrodes, but it prevents the direct contact between the active material and the electrolyte, suppressing the side reactions effectively.

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The mixed polyanionic material Na4Fe3(PO4)2(P2O7) combines the advantages of NaFePO4 and Na2FeP2O7 in capacity, stability, and cost. Herein, we synthesized carbon-coated hollow-sphere-structured Na4Fe3(PO4)2(P2O7) powders by a scalable spray drying route. The optimal sample can deliver a high discharge capacity of 107.7 mA h g–1 at 0.2C. It also delivers a capacity of 88 mA h g–1 at 10C and a capacity of retention of 92% after 1500 cycles. Ex situ X-ray diffraction analysis indicates a slight volume change (less than 3%) in the Na4Fe3(PO4)2(P2O7) lattice cell. Therefore, such a spraying-derived carbon-coated Na4Fe3(PO4)2(P2O7) powder is a very attractive cathode electrode for sodium-ion batteries.

Iron-based mixed-polyanionic cathode Na4Fe3(PO4)2(P2O7) (NFPP) has advantages of environmental benignity, easy synthesis, high theoretical capacity, and remarkable stability. From NFPP, a novel Li-replaced material NaLi3Fe3(PO4)2(P2O7) (NLFPP) is synthesized through active Na-site structural engineering by an electrochemical ion exchange approach. The NLFPP cathode can show high reversible capacities of 103.2 and 90.3 mA h g–1 at 0.5 and 5C, respectively. It also displays an impressive discharge capacity of 81.5 mA h g–1 at an ultrahigh rate of 30C. Density functional theory (DFT) calculation demonstrates that the formation energy of NLFPP is the lowest among NLFPP, NFPP, and NaFe3(PO4)2(P2O7), indicating that NLFPP is the easiest to form and the conversion from NFPP to NLFPP is thermodynamically favorable. The Li substitution for Na in the NFPP lattice causes an increase in the unit cell parameter c and decreases in a, b, and V, which are revealed by both DFT calculations and in situ X-ray powder diffraction (XRD) analysis. With hard carbon (HC) as the anode, the NLFPP//HC full cell shows a high reversible capacity of 91.1 mA h g–1 at 2C and retains 82.4% after 200 cycles. The proposed active-site-specific structural tailoring via electrochemical ion exchange will give new insights into the design of high-performance cathodes for lithium-ion batteries.