Synergistic Cu‐Fe Interactions Enhance Phase Transformation Kinetics Toward High‐Performance CuFeS₂‐Based All‐Solid‐State Batteries

Abstract

Chalcopyrite (CuFeS₂), composed of earth-abundant and environmentally benign elements, was synthesized via solid-state sintering of an equimolar CuS/FeS mixture and evaluated as a cathode material for lithium all-solid-state batteries (ASSBs). CuFeS₂ was compared with a mixture of the CuS and FeS parent materials, which are theoretically expected to show similar conversion reaction products during lithiation. Galvanostatic cycling, XRD, XAS, SEM, and EDX analyses revealed that both samples undergo displacement reactions with lithium, leading to phase separation into Cu⁰, Fe⁰, and Li₂S. However, CuFeS₂ exhibits superior reversibility due to the formation of intermediate phases (LiCuFeS₂, Li₃CuS₂, Li₂FeS₂), where Cu-related phases may promote uniform Fe reactivation during charging. The synergistic Cu-Fe interactions improve reaction kinetics and reversibility, thereby enhancing overall electrochemical performance. In contrast, the CuS-FeS composite exhibits rapid capacity decay due to independent phase segregation and irreversible Fe⁰ passivation. Electrochemically, CuFeS₂ delivers 207 mAh g⁻¹ with 61% retention after 100 cycles, outperforming CuS-FeS (132 mAh g⁻¹, 40% retention). This work provides the most detailed mechanistic insight to date into the function of CuFeS₂ in rechargeable Li cells and shows that controlled intermediate distribution and dynamically evolved conductive networks during phase transformation can improve conversion-type electrodes for ASSBs.