The Role of Electrolyte Composition in Enabling Li Metal-Iron Fluoride Full-Cell Batteries

FeF₃ conversion cathodes, paired with Li metal, are promising for use in next-generation secondary batteries and offer a remarkable theoretical energy density of 1947 Wh kg^-1 compared to 690 Wh kg^-1 for LiNi_0.5 Mn_1.5 O₄ ; however, many successful studies on FeF₃ cathodes are performed in cells with a large (>90-fold) excess of Li that disguises the effects of tested variables on the anode and decreases the practical energy density of the battery. Herein, it is demonstrated that for full-cell compatibility, the electrolyte must produce both a protective solid-electrolyte interphase and cathode-electrolyte interphase and that an electrolyte composed of 1:1.3:3 (m/m) LiFSI, 1,2-dimethoxyethane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether fulfills both these requirements. This work demonstrates the importance of verifying electrode level solutions on the full-cell level when developing new battery chemistries and represents the first full cell demonstration of a Li/FeF₃ cell, with both limited Li and high capacity FeF₃ utilization.