Understanding of the Mechanism and Kinetics of the Fast Solid-State Reaction between NaF and VPO 4 to Form Na 3 V 2 (PO 4 ) 2 F 3 .

The solid-state reaction between NaF and VPO 4 is widely used to produce Na 3 V 2 (PO 4 ) 2 F 3 , a promising cathode material for sodium-ion batteries. In the present work, the mechanism and kinetics of the reaction between NaF and VPO 4 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 VPO 4 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 cm 2 s -1 when HEBM is used. The calculated apparent activation energy is ∼290 kJ mol -1 . The electrochemical properties of the as-prepared Na 3 V 2 (PO 4 ) 2 F 3 are not inferior to the properties of the materials prepared by conventional solid-state synthesis.