Carbon-nanotube-encapsulated FeF₂ nanorods for high-performance lithium-ion cathode materials.

Application of iron fluoride, a promising candidate of cathode materials for lithium ion batteries, is being hindered by its poor electrochemical performance caused by low electronic conductivity and large volume change. Design of carbon-encapsulated transitional metal compounds (including fluoride, oxide, sulfide, etc.) structure is one of the most effective strategies in improving their lithium-ion storage performance. In this work, we successfully synthesize for the first time carbon-nanotube-encapsulated FeF2 nanorods via a facile in situ co-pyrolysis of ferrocene and NH4F. This kind of core/shell carbon nanotube/FeF2 nanorod exhibits better cyclic stability and rate-performance used as cathode materials. Better electrochemical performance of the nanorods should be attributed to the protection of the carbon shell because, experimentally, it is observed that outer carbon shells suffer from high internal stress during Li-ion insertion but efficiently keep the nanorods in the one-dimensional morphology and make nanorods a good electrical contact with the conductive carbon black. This work not only prepares high-performance core/shell carbon/iron fluoride cathode materials, but should also open a facile pathway for design of various novel nanostructures of other metal fluoride/carbon core/shell structures for future lithium-ion batteries.