Facile synthesis of cobalt fluoride (CoF₂)/multi-walled carbon nanotube (MWCNT) nanocomposites and improvement of their electrochemical performance as cathode materials for Li-ion batteries

Conversion-type metal fluoride cathodes are considered promising candidates for electrochemical applications owing to their large specific capacities and high operating voltages. However, harnessing the potentially high energy-density of metal fluorides is challenging because of their inherent chemical and electrochemical properties and synthesis issues. Cobalt fluoride (CoF₂) is an attractive cathode material among metal fluorides, but it has the common limitations of metal fluorides as above. In this study, a facile synthesis method for CoF₂ nanocomposites is developed through the precipitation of cobalt precursors onto multi-walled carbon nanotubes (MWCNTs), followed by heat treatment with ammonium fluoride under an inert atmosphere. This simple and versatile method produces CoF₂/MWCNT nanocomposites with improved reversible capacities (554 mA h g⁻¹ at 0.2C) through conversion reactions and superior rate capabilities (a capacity retention of 84.7% at 2C versus 0.2C) at high current densities. In addition, to prevent capacity fading caused by cobalt dissolution during battery cycling, further in situ and ex situ strategies for surface protection are adopted: in situ formation of cathode-electrolyte interphase layers using a fluoroethylene carbonate-containing electrolyte and ex situ coating of aluminum oxide thin layers on the electrode by atomic layer deposition. Consequently, the CoF₂/MWCNT nanocomposites maintain a capacity of more than 200 mA h g⁻¹ even after 500 cycles. This study is expected to promote the development of advanced metal fluoride cathodes with high energy densities and long cycle lives.