Delithiation-Driven Topotactic Reaction Endows Superior Cycling Performances for High-Energy-Density FeSx (1 ≤ x ≤ 1.14) Cathodes

Abstract Conversion-type materials are promising high-energy-density lithium metal battery cathodes. However, they suffer from severe capacity decay and poor cycling performance due to phase transformations. In this study, we report a topotactic reaction to overcome this issue. A series of transition metal monosulfides (MSs = TiS, VS, CrS, MnS, FeS, CoS, NiS, and CuS) are synthesized. FeS is found to be fully reversible between Fe and Li2S. Delithiation on the Li2S|Fe heterojunction creates a new FeS phase via a topotatic reaction, enabling FeS to undergo a complete reaction. FeS demonstrates a capacity retention of 82% after 700 cycles at 1C in the voltage window of 1.0−3.0 V, remaining 407 mAh g−1. A pouch cell using Fe7S8 (FeSx, x=1.14) as the cathode is found to exhibit a superior gravimetric energy density of 389 Wh kg−1. These findings highlight the importance of structural continuity to guide good cycling stabilities for conversion-type cathodes.