Improved reversibility of phase transformations using electron-rich graphitic carbon matrix in FeF2 cathode for sodium-ion batteries

Sodium-ion batteries (SIBs) have attracted increasing attention as next-generation energy storage devices due to the availability and economic benefits of sodium. Iron difluoride (FeF2) is considered as a promising conversion typed cathode material in SIBs because of its low-cost and high theoretical capacity of 571 mAh g(-1). Additionally, storing larger sodium ions is more suitable in conversion type cathode when compared to other insertion-typed cathodes in SIBs. However, previous studies have reported irreversible redox reactions to cause undesirable phase transformations of FeF2 to Na3FeF6 and FeF3 during the cycling processes. Herein, nitrogen doped graphitic carbon wrapped FeF2 nanoparticles (FeF2@NGC) was synthesized using dopamine and was utilized as conversion-based cathode materials for SIBs. The phase transformations of FeF2 were successfully prevented during cycles because of the electron-rich carbon matrix with adjustable NGC thickness that resulted in excellent electrochemical performance at a high rate over long cycle experiments. A reversible discharge capacity of 214.2 mAh g(-1) and a fading rate of 0.042% per cycle after 500 cycles at a current density of 300 mA g(-1) was observed for electrochemical cells employing a FeF2@NGC based electrode with the optimal thickness of the NGC layer. In addition, a full cell battery that was assembled with C@Fe3C/Fe anode was able to deliver a high energy density of 761.3 Wh kg(-1), indicating that the iron-based conversion-type cathode hold potential for high-performance SIBs.