Enhancement in the performance of Nb₁₄W₃O₄₄ high power anodes through P doping in the tetrahedral sites

Niobium-based anodes are emerging as promising candidates for high-power lithium-ion batteries due to their high theoretical capacities, robust safety at high current densities, and excellent long-term cycling stability. In this respect Nb₁₄W₃O₄₄ (NWO) has been attracting significant interest, and in this work, we investigate for the first time phosphorous-doping into this phase. Structural characterisation confirmed the substitution of phosphorous into the tetrahedral sites of the NWO framework, with a resulting contraction in the unit cell dimensions. Electrochemical testing demonstrated superior capacities for the P-doped materials, attributed to the replacement of W in the tetrahedral sites by non-reducible P, with a corresponding increase in W content in the octahedral sites. The P doped samples also showed excellent rate capability, with x = 0.5 and x = 1 retaining high capacities of 180(5) and 190(10) mAh g^-1 at 2 A g^-1, respectively and 163(15) and 166(12) mAh g^-1 at 4 A g^-1 respectively, which could be additionally related to the P doping reducing particle size, which shortens lithium-ion diffusion paths. These results suggest further studies are warranted on the effect of such site-selective doping on the structure and morphology control, and the resulting influence on the electrochemical properties, in other Wadsley Roth niobate anodes with tetrahedral sites.