Insight mechanism of nano iron difluoride cathode material for high-energy lithium-ion batteries: a review

Iron(II) fluoride (FeF 2 ) is a promising candidate as the cathode material for lithium-ion batteries (LIBs) due to its quite high theoretical energy density compared with the commercial cathode materials like LiCoO 2 and its abundance. However, the actual energy density of various FeF 2 materials nowadays is lower than the theoretical one. The actual energy density is largely determined by its conversion process, in which the transportation and migration of lithium ion (Li + ) and electron play a vital role. The completeness of the Li + and electron transport determines the completeness of the conversion process and further determines the capacity, while the ease of Li + migration determines the conversion potential. This review is divided into four parts to introduce the Li + and electron transport, from the aspects of barrier, stability, constructing method, and influence factors, in various existing “FeF 2 materials-electrolyte” systems. The four parts are Li + and electron transport or Li + migration within a FeF 2 particle, electron transport between FeF 2 particles, Li + transport from the electrolyte to FeF 2 , and the amount of Li + that can transport. Finally, several points on optimizing the “FeF 2 materials-electrolyte” systems with superior Li + and electron transport are proposed.