Understanding of the Mechanism and Kinetics of the Fast Solid-State Reaction between NaF and VPO4 to Form Na3V2(PO4)2F3

The solid-state reaction between NaF and VPO4 is widely used to produce Na3V2(PO4)2F3, a promising cathode material for sodium-ion batteries. In the present work, the mechanism and kinetics of the reaction between NaF and VPO4 were investigated, and the effect of preliminary high-energy ball milling (HEBM) was studied using in situ time-resolved synchrotron powder X-ray diffraction, in situ transmission electron microscopy, differential scanning calorimetry, etc. The reaction was attributed to a "dimensional reduction" formalism; it proceeds quickly with the unilateral diffusion of Na+ and F- ions into VPO4 particles as a limiting stage. The use of HEBM leads to the mechanism corresponding to the third-order reaction model and accelerates the interaction. The rate constant k increases from 3.5 × 10-5 to 3.4 × 10-3 s-1, and diffusion coefficient D increases from 2 × 10-14 to 4 × 10-13 cm2 s-1 when HEBM is used. The calculated apparent activation energy is ∼290 kJ mol-1. The electrochemical properties of the as-prepared Na3V2(PO4)2F3 are not inferior to the properties of the materials prepared by conventional solid-state synthesis.