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

Abstract

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 Na₃V_2−zAl_z(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 Na₃V_2−zAl_z(PO₄)₂F_1+zO_2−z solid solution.