The invention relates to a method for preparing an alkali metal and transition metal fluorosulfate that is useful as an electrode material in an electrochemical battery that operates by alkali ion exchange. The method includes a first step that involves: preparing a mixture of precursors of elements making up fluorosulfate, placing said mixture in contact with a solid polymer at ambient temperature, subjecting the reaction medium to a heat treatment that aims to melt the polymer, and subsequently reacting the precursors with one another. Said polymer is one that is solid at ambient temperature, has a melting point lower than the reaction temperature of the precursors, is stable at least up to the reaction temperature of the precursors, and is soluble in a nonpolar aprotic organic solvent.