Iron Fluoride–Carbon Nanocomposite Nanofibers as Free‐Standing Cathodes for High‐Energy Lithium Batteries

Abstract The development of low‐cost, high‐energy cathodes from nontoxic, broadly available resources is a big challenge for the next‐generation rechargeable lithium or lithium‐ion batteries. As a promising alternative to traditional intercalation‐type chemistries, conversion‐type metal fluorides offer much higher theoretical capacity and energy density than conventional cathodes. Unfortunately, these still suffer from irreversible structural degradation and rapid capacity fading upon cycling. To address these challenges, here a versatile and effective strategy is harnessed for the development of metal fluoride–carbon (C) nanocomposite nanofibers as flexible, free‐standing cathodes. By taking iron trifluoride (FeF 3 ) as a successful example, assembled FeF 3 –C/Li cells with a high reversible FeF 3 capacity of 550 mAh g −1 at 100 mA g −1 (three times that of traditional cathodes, such as lithium cobalt oxide, lithium nickel cobalt aluminum oxide, and lithium nickel cobalt manganese oxide) and excellent stability (400+ cycles with little‐to‐no degradation) are demonstrated. The promising characteristics can be attributed to the nanoconfinement of FeF 3 nanoparticles, which minimizes the segregation of Fe and LiF upon cycling, the robustness of the electrically conductive C network and the prevention of undesirable reactions between the active material and the liquid electrolyte using the composite design and electrolyte selection.