High-voltage NASICON sodium ion batteries: Merits of fluorine insertion

Abstract

NASICON Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> and Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> have restarted to be investigated electrochemically as promising cathode materials for sodium-ion batteries. Fluorine insertion by replacing partial phosphate groups in Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> allows for new family of host lattice structure, Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf>. Greatly, fluorine is capable to participate in structural construction to change the ions configuration which involves ions occupation, the species and amount of diverse Na sites, leading to distinct modalities for ions extraction. The inductive effects of (PO<inf>4</inf>)^3- polyanion by changing electronic cloud density upon compositional atoms could be enhanced under the effects of the formed F-V bond due to its strong ionicity, which can moderate the energetics of the transition metal redox couple to generate relatively high operating potentials. The substitution of fluorine for negative-charge polyanion or anion could be effective to improve the electrochemical properties, particularly for the purpose to increase performed voltages by changing atomic environments.