A dynamic polyanion framework with anion/cation co-doping for robust Na/Zn-ion batteries

Among various cathode materials for sodium-ion batteries (SIBs), Na₃V₂(PO₄)₃ has an open and stable three-dimensional framework to reversibly (de)intercalate sodium ions (Na⁺), but its inherently insulated and stiff features originated from VO₆ octahedron and PO₄ tetrahedron result in limited capacity and poor rate performance. Herein, this work resolves these issues by implementing anion (Cl⁻) and cation (Mg²⁺) to respectively substitute a small fraction of O and V elements to expand Na⁺ migration channel and boost electronic conductivity, which is verified by both experiment and theory calculations. Even with carbon coating less than 1 wt% in content, which is one of the least carbon coating as reported, a dramatically improved capacity (120 mA h g⁻¹ at 0.1 C) and rate performance (65 mA h g⁻¹ at 30 C) are realized for Na₃V_1.95Mg_0.05(PO₄)_2.9Cl_0.1 in contrast to Na₃V₂(PO₄)₃ that merely releases 66.7 mA h g⁻¹ at 0.1 C and decrease to 0 mA h g⁻¹ at 30 C. Meanwhile, stable full SIBs and zinc-ion batteries are also demonstrated with this dynamic polyanion framework.