Among various metal compounds, metal fluorides (MF_x) are one of the promising cathode materials for Li-ion batteries (LIBs) as they exhibit high theoretical conversion potentials as well as large specific capacities. However, only a limited number of synthetic method, generally involving highly toxic or inaccessible reagents, currently exist, which has made it difficult to produce well-designed nanostructures suitable for use as cathodes; consequently, harnessing their potential cathodic properties has proven challenging over the past several decades.

  In this study, in order to address these critical issues and realize potential cathodic performance of MF_x in LIBs, we have developed a new and facile bottom-up approach using ammonium fluoride (NH₄F) to synthesize anhydrous MF_x(CuF₂, FeF₃, and CoF₂)/mesoporous carbon(MSU-F-C) nanocomposites. We discovered that various metal precursor nanoparticles pre-confined in mesoporous carbon can be directly converted to anhydrous MF_x by simple heat treatment with NH₄F under solventless conditions. Furthermore, we also demonstrate the versatility, mild toxicity, and efficiency of this synthetic method, and, using XRD analysis, propose a mechanism for the reaction. As the cathodes of LIBs, all MF_x/MSU-F-C prepared in this study exhibited superior electrochemical performances, through conversion reactions. In the case of FeF₃/MSU-F-C, it maintained a capacity of 650 mAh g⁻¹_,FeF3 across 50 cycles, which is ~90% of its initial capacity; to the best of our knowledge, no such an excellent cycle performance of FeF₃ with a high capacity through conversion reactions has previously been reported. CuF₂/MSU-F-C and CoF₂/MSU-F-C cells also retained 209 mAh g⁻¹_,CuF2 (at the 20^th cycle) and 400 mAh g⁻¹_,CoF2 (at the 30^th cycle) of discharge capacities, respectively. These results show excellent performance in comparison to previous studies; in particular, this is the first result which shows promising cycle properties of CuF₂. We expect that this new synthetic method would open an avenue to application of metal fluorides to cathodes in LIBs and will be adopted by many scientists & engineers for their future research.