Octahedral FeF3·0.33H2O nanocrystalline fixed on carbon fibers as the cathode of lithium-ion battery based on the “gravel and glue” strategy

•The surface of kapok carbon fiber provides a vast amount of active sites to fix the Fe-MOF precursor effectively.•During the calcining, oligo-graphene and the spongy-like porous carbon (SPC) matrix are formed on the intermediate material.•Using solid NH4HF2 instead of HF solution, as in-situ fluorination reagent has cultivated the FeF3·0.33H2O single crystal. Due to its high theoretical specific capacity and low cost, iron trifluoride (FeF3) is a potential cathode material for the next generation of lithium-ion batteries. However, the problems, such as poor electronic conductivity and volume change during cycling of metal fluoride, seriously hinder its practical application. To solve these problems, the FeF3·0.33H2O nanocrystalline @ Spongy Porous Carbon @ Carbon Fiber (FF@SPC@CF) composite is prepared based on the "gravel and glue" strategy. In the composite, the octahedral FeF3·0.33H2O nanocrystals (∼100 nm) coated with oligo-graphene are first uniformly embedded in the porous spongy carbon to form a mixed "mortar", which is then firmly bonded to the kapok carbon fiber. The prepared FF@SPC@CF composite offers the initial discharge capacity of 407 mAh·g−1 and remains at 108 mAh·g−1 after 400 cycles when it was used as the cathode with the voltage in the range of 1.5–4.5 V at 0.2 C (1C=237 mAh·g−1). The capacities are much better than 256 mAh·g−1 (first discharge capacity) and 8 mAh·g−1 (400 cycles later) of pure FeF3·0.33H2O electrode. Therefore, the "gravel and glue" design strategy for the carbon-supported composite material in this study offers an effective way to improve the electrochemical performance of FeF3·0.33H2O cathode material. [Display omitted]