Improved Interfacial Stability of an Iron Trifluoride Cathode with a Yolk–Shell Structure in Sulfide-Based All-Solid-State Lithium Batteries

Incorporating conversion-type cathodes such as iron trifluoride (FeF3) into sulfide-based all-solid-state lithium batteries (ASSLBs) can significantly enhance the energy density. However, challenges such as interfacial side reactions and volume-change-induced poor physical contact between sulfide electrolytes and FeF3 limit their application. Herein, a yolk–shell (YS) structure with FeF3 as the yolk and fluorine (F)-doped carbon as the shell was successfully designed. Thanks to this electrode design, the ASSLBs achieve a discharge capacity of 318 mAh·g–1 after 400 cycles at 0.2 C, facilitated by the optimized void space in the shell. Moreover, ASSLBs demonstrate excellent cycling life and capacity retention across a broad temperature range from 0 °C (140 mAh·g–1 after 200 cycles at 0.2 C) to 60 °C (410 mAh·g–1 after 200 cycles at 0.2 C). Postanalysis indicates that the shell effectively not only mitigates interfacial side reactions but also accommodates the volume expansion of FeF3 during cycling while maintaining excellent contact between the shell and the sulfide electrolyte. This chemically and physically stable interface ensures the good cycling reversibility of FeF3. This work underscores the critical importance of YS structural design in stabilizing the interface between conversion-type cathodes and solid electrolytes, thereby accelerating the practical application of FeF3 cathodes with increased energy densities in ASSLBs.