Towards enhanced structural stability by investigation of the mechanism of K ion doping in Na3V2(PO4)3/C for sodium ion batteries

Na3V2(PO4)3 is particularly suitable as cathode materials for sodium ion batteries (SIBs). Its NASICON structure is not only conducive to the rapid migration of Na+, but also has less volume deformation during Na+ deintercalation, and the main frame mechanism remains unchanged. However, owing to its own structure, the electronic conductivity is limited, which limits its practical application. Here, on the basis of carbon coating to enhance the electronic conductivity, the effect of sodium site doping of K+ on the structure and properties of Na3V2(PO4)3/C is deeply explored by comparing series Na3-xKxV2(PO4)3/C (x = 0, 0.05, 0.1, 0.15, 0.2). The initial discharge capacity of Na2.9K0.1V2(PO4)3/C is 107.7 mAh·g−1 in the potential range of 2.5–3.8 V at 0.2C, and the capacity remains 95 % after 300 cycles. The excellent performance of Na2.9K0.1V2(PO4)3/C benefits from the large radius of K+ as functional support ions, which lightly relieves the deformation pressure of Na+ during the deintercalation process, thereby stabilizing the crystal structure. Moreover, the doping of K+ plays a critical role in improving the diffusion coefficient of Na+. Density functional theory (DFT) calculations demonstrate that the Na-site doping of K+ can enhance the electronic conductivity of the material, which in turn brings excellent electrochemical performance. On the basis of carbon coating, Na2.9K0.1V2(PO4)3/C cathode materials with excellent electrochemical performance are obtained by doping K at theNa-site. The effect of Na-site doping of K+ on the structure and properties of Na3V2(PO4)3/C is deeply explored by comparing series Na3-xKxV2(PO4)3/C (x = 0, 0.05, 0.1, 0.15, 0.2). The mechanism of excellent electrochemical performance and structural stability caused by Na-site doping of K+ is revealed theoretically by DFT calculation.