Li[subscript 2]Fe(SO[subscript 4])[subscript 2] as a 3.83 V positive electrode material
We report on the preparation and electrochemical characterization of Li{sub 2}M(SO{sub 4}){sub 2} (M = Co, Fe), where the Fe-based phase was previously unknown. Both compounds can be made at low temperatures (< 320 C) and the Fe-based phase displays an open circuit voltage of 3.83 V vs. Li{sup +}/Li{sup 0} for the Fe{sup 3}+/Fe{sup 2+} redox potential. This corresponds to the highest voltage ever obtained for the Fe{sup 3+}/Fe{sup 2+} redox couple in an iron-based, fluorine-free compound and is only matched by the triplite phase of LiFeSO{sub 4}F. While the theoretical capacity of 102 mA h g{sup -1} will likely limit the commercial prospects for this phase, knowledge of the structure will provide useful insight into the structural origin of these increased potentials.