Li2Fe(SO4)2 as a 3.83 V positive electrode material

We report on the preparation and electrochemical characterization of Li2M(SO4)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+/Li0 for the Fe3+/Fe2+ redox potential. This corresponds to the highest voltage ever obtained for the Fe3+/Fe2+ redox couple in an iron-based, fluorine-free compound and is only matched by the triplite phase of LiFeSO4F. While the theoretical capacity of 102 mA h g− 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.

► We prepared a previously unknown compound Li2Fe(SO4)2.
► This phase shows a redox activity at 3.83 V.
► Gives insight into the structural origin of the elevated potentials.