W doping enhances rate and cycle performance of Na4VMn(PO4)3 in sodium-ion batteries

Abstract

NASICON-type (Na⁺ superionic conductor) Na₃V₂(PO₄)₃ cathode is a promising cathode material for sodium-ion batteries because of its high operating voltage, stable structure, and fast Na⁺ diffusion rate. However, due to the high cost and toxicity of vanadium, finding a low-cost and non-toxic element to replace it has become urgent. In this context, Na₄VMn(PO₄)₃ has emerged as a widely recognized superior alternative. However, its practical application is severely limited by its low electronic conductivity and the unavoidable Jahn-Teller effect of Mn³⁺ during cycling. This work prepared a tungsten-doped Na_3.6VMn_0.9W_0.1(PO₄)₃/C (VMW-0.1) cathode material by a facile sol-gel method, VMW-0.1 showed an excellent capacity of 74.7 mAh g⁻¹ at a discharge rate of 100C (1C = 117 mA g⁻¹) and a capacity retention of 74.9 % after 10,000 cycles at 20C. Meanwhile, the electrochemical performance of VMW-0.1 is significantly better than that of Na₄VMn(PO₄)₃ without W doping at extreme temperatures. Cyclic voltammetry and in situ X-ray diffraction revealed the sodium storage mechanism and kinetic behavior of VMW-0.1. Theoretical calculations showed that W doping significantly narrowed the bandgap of Na₄VMn(PO₄)₃ and increased the local electron density. Meanwhile, four probe method showed that VMW-0.1 has very high electronic conductivity (4.27 × 10⁻³ S cm⁻¹). It will provide new insights into the application of manganese-rich phosphate cathodes.