Bottom-up assembly of yolk-shell FeF3@C nanocomposites as high-rate, long-term and air-stable cathodes

Lithium-metal fluoride batteries with higher energy density than commercial lithium-ion batteries are promising candidates for future energy storage. However, challenges such as poor electronic and Li + conductivity, active material dissolution, and volume expansion during cycling hinder their practical application. Here, a well-designed yolk-shell FeF 3 @C is aimed to address these issues. The outer carbon shell, derived from an organic carbon source, enhances the electronic conductivity of the embedded FeF 3 nanoparticles and protects them from unfavorable dissolution. The hollow design is utilized to provide sufficient space for FeF 3 nanoparticles to expand during cycling, preserving the carbon shell to not be destroyed. Post-mortem studies reveal the in-situ formation of Fe/O shell during cycling, which further prevents the loss and shuttle of Fe. Consequently, the FeF 3 @C nanocomposites deliver a capacity of 400 mAh g −1 at 1.0 C over 1500 cycles with a high capacity retention close to 95 %, and achieve super-rate capability up to 40 C. Moreover, the produced yolk-shell FeF 3 @C nanocomposites demonstrate air stability, which can simplify the manufacturing process of metal fluoride cathodes. Our study offers a promising direction for designing FeF 3 cathodes that achieve both outstanding electrochemical performance and air stability for practical Li-FeF 3 batteries.