An α-CrPO4-type NaV3(PO4)3 anode for sodium-ion batteries with excellent cycling stability and the exploration of sodium storage behavior

Abstract

A novel α-CrPO₄-type vanadium-based orthophosphate NaV₃(PO₄)₃ was synthesized as a promising anode material for Na-ion batteries, which exhibits a high reversible capacity of 140 mA h g⁻¹ with the capacity retention about 98% after 100 cycles. Furthermore, the electrochemical reaction mechanism, structure evolution and ionic diffusion of NaV₃(PO₄)₃ during the sodiation process were surveyed by the combination of experiments and first-principles calculations. Two different Wyckoff sites (4b, Na2 and 4c, Na3) which can accommodate more sodium ions, as well as the corresponding Na insertion process were reported to understand the sodium storage mechanism in NaV₃(PO₄)₃. The 3d-electron distribution near the Fermi level for vanadium in different valence states and sites (4a and 8g) in Na_xV₃(PO₄)₃ (x = 1, 2, 3) and the local electron transfer during the sodiation process were described to predict the transition of the electrochemical properties. The cooperative-transport mechanism dominates the Na diffusion in Na_xV₃(PO₄)₃ (1 ≤ x ≤ 3) and the activation barrier for Na⁺ transport in Na_1.125V₃(PO₄)₃ (0.30 eV) is much lower than that in NaV₃(PO₄)₃ (1.28 eV), suggesting the fast-ion transport characteristics during the sodiation process and the strong stability of Na at the 4e site.