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

Abstract

Na₃V₂(PO₄)₂F₃ 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 Na₃V₂(PO₄)₂F₃. The performances of Na₃V₂(PO₄)₂F₃ 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 Na₃V₂(PO₄)₂F₃ 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 Na₃V₂(PO₄)₂F₃ 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 cm² s⁻¹, which could demonstrate that Na₃V₂(PO₄)₂F₃ 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.