Aluminum substitution for vanadium in the Na₃V₂(PO₄)₂F₃ and Na₃V₂(PO₄)₂FO₂ type materials

Among the positive electrode materials for Na-ion batteries, Na3V2(PO4)2F3 is considered as one of the most promising and generates high interest. Here, we study the influence of the sol-gel synthesis parameters on the structure and on the electrochemical signature of the partially substituted Na3V2-zAlz(PO4)2(F,O)3 materials. We demonstrate that the acidity of the starting solution influences the vanadium oxidation state of the final product. For the first time we report on the possibility of controlling the double Al/V and O/F substitution that leads to the preparation of the Na3V2-zAlz(PO4)2F1+zO2-z solid solution.