Today, lithium (sodium) battery cathodes based on the intercalation/deintercalation of lithium (sodium) ions are approaching their instinctive limits, which remains a big challenge for the development of next-generation batteries. Iron fluorides (FeF₂, FeF₃), a potential alternative to current intercalation cathodes, are believed to offer much higher capacity and higher energy due to their reversible conversion reactions during charging/discharging. However, their structural degradation and capacity fading upon cycling could block the road to future applications.

To address these issues, we developed a stable iron fluoride cathode based on nanoconfined FeF₃ in carbon nanofibers for lithium and sodium batteries. The produced FeF₃^/C nanocomposite is free-standing and can be produced using an electrospinning-based strategy with post carbonization and fluorination process. When tested in lithium cells, the nanocomposite can exhibit a high specific capacity up to 550 mAh g^–1 and a cycle stability over 400 cycles [1]. When tested in sodium cells, the nanocomposite can deliver a high specific capacity of 230 mAh g^–1 in sodium-difluoro(oxalato)borate (NaDFOB) electrolyte [2]. The performance especially cycle stability was further improved by atomic layer deposition (ALD) of Al₂O₃ coating on nanofiber surface [3]. We find the coating can reduce the direct contact between active material and liquid electrolyte, minimize active material dissolution and significantly improve the overall performance.

[1] W. Fu, E. Zhao, Z. Sun, X. Ren, A. Magasinski, G. Yushin, Iron Fluoride–Carbon Nanocomposite Nanofibers as Free-Standing Cathodes for High-Energy Lithium Batteries, Advanced Functional Materials 28 (2018) 1801711.

[2] Z. Sun, W. Fu, M.Z. Liu, P. Lu, E. Zhao, A. Magasinski, M. Liu, S. Luo, J. McDaniel, G. Yushin, A nanoconfined iron(iii) fluoride cathode in a NaDFOB electrolyte: towards high-performance sodium-ion batteries, Journal of Materials Chemistry A 8 (2020) 4091-4098.

[3] Z. Sun, M. Boebinger, M. Liu, P. Lu, W. Fu, B. Wang, A. Magasinski, Y. Zhang, Y. Huang, A.Y. Song, M.T. McDowell, G. Yushin, The roles of atomic layer deposition (ALD) coatings on the stability of FeF3 Na-ion cathodes, Journal of Power Sources 507 (2021) 230281.