Fluorolytic Sol-Gel Route and Electrochemical Properties of Polyanionic Transition-Metal Phosphate Fluorides

Fluorine-containing polyanionic compounds have attracted much attention in the last few years as potential positive electrode materials for rechargeable batteries. With their formula A_a M_b X_c O₄ Y_d (A=Li, Na…; M=Ti, V, Mn, Fe, Co, Ni…; X=P or S, and Y=F, OH, O), they offer a very rich chemistry and their electrochemical properties can be tuned by carefully choosing the different constituting elements. However, synthesis approaches that allow these materials to be obtained at low temperature are almost nonexistent. In this paper, the use of a nonaqueous fluorolytic sol-gel approach is reported to synthesize a tavorite-type LiFePO₄ F material and its electrochemical characterization was performed. The obtained material displays an electrochemical performance that positively compares with the literature with an excellent cycling stability (115 mA h^-1  g^-1 after 100 cycles at C/2 rate). A slight change in the synthesis parameters allowed Li₂ CoPO₄ F to be successfully obtained, demonstrating the versatility of the reported route, which can be adapted to synthesize other fluorine-containing polyanionic compounds, which are of great interest for energy storage applications.