Synthesis and Reaction Mechanism of Novel Fluorinated Carbon Fiber as a High-Voltage Cathode Material for Rechargeable Na Batteries

While a rechargeable battery based on Na/CF x has been proposed, its reversible mechanism has remained unclear. Here, a new fluorinated carbon fiber material with the formula CF 0.75 is used as a cathode material for rechargeable sodium batteries, delivering an initial discharge capacity of 705 mA·h g –1 with a high discharge plateau of 2.75 V and a reversible high discharge capacity of 350 mA·h g –1 at 20 mA g –1 . The first discharge plateau of 2.75 V is the highest value reported in this family of materials so far, even slightly higher than that of commercial fluorinated graphite tested in a lithium battery (2.7 V). The origins of the observed high voltage of the material are explored by a combination of theoretical calculations and galvanostatic intermittent titration technique data and determined to be related to the disordered structure of the carbon fiber. Soft X-ray absorption spectroscopy and 19 F magic angle spinning nuclear magnetic resonance characterization results disclose a full view of the conversion reaction mechanism involved in the charge and discharge processes, both on the surface and in the bulk, and show evidence of reversible conversion between CF x and NaF. Although a suitable electrolyte is currently lacking and further research is necessary, the inherent advantages of the compound make it a promising cathode material for future rechargeable sodium batteries.