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.