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

Abstract

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 Na_3.5V_1.5Mg_0.5(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 V^5+/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.