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/CFx has been proposed, its reversible mechanism has remained unclear. Here, a new fluorinated carbon fiber material with the formula CF0.75 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 19F 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 CFx 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.