Multifunctional dual Na3V2(PO4) 2F3 cathode for both lithium-ion and sodium-ion batteries

Abstract

Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> with a NASICON-type structure is shown to be synthesised with the particle surface found to be coated with amorphous carbon with its thickness in the range of 25-32 nm. The crystallographic planes (hkl) are labelled according to Density Functional Theory (DFT) calculations towards the as-prepared Na <inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf>. The performances of Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F <inf>3</inf> have been investigated in lithium- and sodium-ion batteries, exhibiting a specific capacity of 147 mA h g^-1 with an average discharge plateau around 4 V vs. Li⁺/Li, and 111.5 mA h g ^-1 with three discharge plateaus in sodium-ion batteries. A predominant Li ion insertion mechanism is verified by comparing the redox potentials from CV and charge/discharge curves. It is found that the main migration from/into the crystallographic sites of Na<inf>3</inf>V <inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> of Li ions is favoured to obtain satisfactory properties by a two-step process, while the Na ions are found to require three steps. The stable and three-dimensional open framework of Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> is considered to be vital for the excellent C-rate and cycling performances, as well as the fast ion diffusion with a magnitude of 10^-11 cm ² s^-1, which could demonstrate that Na<inf>3</inf>V <inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> is a multifunctional dual cathode for both lithium and sodium ion batteries and capable to be a promising candidate in the construction of high-energy batteries. © 2014 The Royal Society of Chemistry.