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

Abstract

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 FeF3@C is aimed to address these issues. The outer carbon shell, derived from an organic carbon source, enhances the electronic conductivity of the embedded FeF3 nanoparticles and protects them from unfavorable dissolution. The hollow design is utilized to provide sufficient space for FeF3 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 FeF3@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 FeF3@C nanocomposites demonstrate air stability, which can simplify the manufacturing process of metal fluoride cathodes. Our study offers a promising direction for designing FeF3 cathodes that achieve both outstanding electrochemical performance and air stability for practical Li-FeF3 batteries.