All-climate and air-stable NASICON-Na₂TiV(PO₄)₃ cathode with three-electron reaction toward high-performance sodium-ion batteries

Developing all-climate and air-stable cathode with multi-electron redox reaction is intriguing and challenging for sodium-ion batteries (SIBs). Herein, we synthesize a NASICON (Na super-ionic conductor)-type Na₂TiV(PO₄)₃ with an elaborately-engineered architecture, which shows ultrastable (1000 cycles at 40C) and ultrafast (up to 40C) three-sodium storage performances with an ultrahigh capacity of 187 mA h g⁻¹ (equals to theoretical value) at 0.1C. Also, it displays good air-stability and high/low-temperature properties. A series of stepwise solid-solution, two-phase, solid-solution and two-phase mechanisms are involved and a small volume change of 4.9% is identified according to the operando X-ray diffraction results. Ex situ X-ray photoelectron spectroscopy reveals that V⁴⁺/V³⁺, Ti⁴⁺/Ti³⁺ and V³⁺/V²⁺ redox reactions are accompanied with three-sodium storage processes. The small band gap of 1.48 eV and fast 3D Na⁺ diffusion pathways with low energy barrier of 0.2 eV calculated through first-principles calculations are responsible for the superior electrochemical performances. Our work prepares Na₂TiV(PO₄)₃ with three-electron redox and high performance for future practical SIBs applications.