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

Abstract

The ion-migration mechanism of Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> is investigated in Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf>-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 Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf>, and the Na ions at Na(2) sites with 0.5 occupation likely extract earlier to form Na<inf>2</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf>. 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 NaLiV<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf>, and the second ion inserts into two Na(2) sites to form NaLi<inf>2</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf>. 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 Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> exhibit excellent C-rate and cycling performances, of which the Li-inserted voltage is evaluated by first principles calculations. This journal is