Mechanistic investigation of ion migration in Na3V2(PO4)2F3 hybrid-ion batteries

Abstract

The ion-migration mechanism of Na₃V₂(PO₄)₂F₃ is investigated in Na₃V₂(PO₄)₂F₃–Li hybrid-ion batteries for the first time through a combined computational and experimental study. There are two Na sites namely Na(1) and Na(2) in Na₃V₂(PO₄)₂F₃, and the Na ions at Na(2) sites with 0.5 occupation likely extract earlier to form Na₂V₂(PO₄)₂F₃. The structural reorganisation is suggested to make a stable configuration of the remaining ions at the centre of Na(1) sites. After the extraction of the second Na ion, the last ion prefers to change occupation from 1 to 0.5 to occupy two Na(2) sites. The insertion of predominant Li ions also should undergo structural reorganization when the first Li ion inserts into the centre of Na(1) site theoretically forming NaLiV₂(PO₄)₂F₃, and the second ion inserts into two Na(2) sites to form NaLi₂V₂(PO₄)₂F₃. More than a 0.3 Li ion insertion would take place in the applied voltage range by increasing the number of sites occupied rather than occupy the vacancy in triangular prismatic sites. An improved solution-based carbothermal reduction methodology makes Na₃V₂(PO₄)₂F₃ exhibit excellent C-rate and cycling performances, of which the Li-inserted voltage is evaluated by first principles calculations.