Low-Valence Mg2+ Doping Suppresses Irreversible Phase Transition of Sodium-Rich Fluorophosphate upon Additional Na+ Deintercalation

Fluorophosphate Na3V2(PO4)2F3 (NVPF) is considered a promising cathode material for sodium-ion batteries, while its specific capacity is still insufficient compared to that of cathodes of lithium-ion batteries. Activating the third Na+ effectively increases the specific capacity of NVPF. However, the accompanied irreversible phase transition deteriorates the cycling stability. In this study, we synthesized sodium-rich Na3.5V1.5Mg0.5(PO4)2F3 (NVMPF) through doping low-valence Mg2+ with a high content, which introduces an extra 0.5 Na+ in the crystal lattice. The extra 0.5 Na+ remains in the lattice of NVMPF, acting as “pillars” to suppress the irreversible phase transition after the third Na+ is extracted by activating the V5+/4+ redox couple at a high voltage. Thus, NVMPF achieves a specific discharge capacity of 170 mAh g–1 between 1.0 and 4.7 V while maintaining the tetragonal structure of NVMPF. This work offers insightful guidelines to achieve the full utilization of three Na+ with enhanced cycling stability.