Reversible multielectron redox in NASICON cathode with high energy density for low-temperature sodium-ion batteries

The NASICON-type Na₃V₂(PO₄)₃ cathode based on vanadium multiple redox is particularly attractive for large-scale sodium-ion battery application. In this work, it was found that the reversible redox of V⁴⁺/V⁵⁺ can be activated by replacing a part of V³⁺ with Al³⁺, obtaining Na₃V_1.5Al_0.5(PO₄)₃ with typical NASICON phase structure and offering a reversible specific capacity of 163 mAh g⁻¹ through a three-electron redox reaction with a small volume change (2.62%). The most surprising thing is that in addition to favorable room temperature electrochemical performance, Na₃V_1.5Al_0.5(PO₄)₃ also exhibits a superior low temperature (-20 °C) cycling stability with a capacity retention of 98.9% after 1000 cycles at 5 C. Thin and conformal CEI film as well as stable phase structure across a broad temperature range are responsible for this superior electrochemical property. A Na⁺ storage mechanism consisting of two-phase and solid-solution electrochemical reactions for Na₃V_1.5Al_0.5(PO₄)₃ cathode is elucidated via ex-situ XRD structural analysis. DFT calculations indicate that Al³⁺ substitution could manipulate the localized electronic state and strengthen the oxygen ligand skeleton in the NASICON matrix. Hence, the Na₃V_1.5Al_0.5(PO₄)₃ cathode renders a slight lattice variation upon the repeated desodiation and sodiation and enhanced Na⁺ diffusion rate along with low charge transfer resistance.