Understanding of the Mechanism and Kinetics of the Fast Solid-State Reaction between NaF and VPO₄ to Form...

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.