K-Mg Co-doping enabling fast Na+ diffusion kinetics and structural stability in Na3V2(PO4)2F3 cathodes

Large K + ions act as structural pillars to enhance crystal stability. • Mg doping introduces additional vacancies to accelerate Na + diffusion. • K Mg co-doping significantly enhances rate performance and cycling stability at 4.5 V. The polyanionic compound Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) cathode material has attracted extensive attention due to its high theoretical specific capacity (128 mAh g −1 ) and energy density (500 Wh kg −1 ). The primary bottleneck for NVPF is the low intrinsic electronic conductivity, which results in poor long cycling and rate performance. A K Mg co-doped Na 2.92 K 0.08 V 1.95 Mg 0.05 (PO 4 ) 2 F 3 (NKVMPF) cathode material is designed in this work to address the above problem. K Mg co-doping improves the Na + diffusion kinetics and crystal structure stability of NVPF due to the increased lattice volume, introduced additional vacancies, and the pillar supporting role of K + with larger radius. At a potential window of 2.0 V–4.5 V, the NKVMPF delivers superior rate performance and long-term cycle ability compared to NVPF, with a specific discharge capacity of 66.9 mAh g −1 at the high current rate of 20C and a capacity retention of 81.1% after 250 cycles at 0.2C. This work provides a binary doping strategy suitable for polyanion cathode materials and offers a new perspective for the development of high-performance cathode materials for sodium-ion batteries (SIBs).