Synergistic Cu‐Fe Interactions Enhance Phase Transformation Kinetics Toward High‐Performance CuFeS 2 ‐Based All‐Solid‐State Batteries

ABSTRACT Chalcopyrite (CuFeS 2 ), 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). CuFeS 2 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 Cu 0 , Fe 0 , and Li 2 S. However, CuFeS 2 exhibits superior reversibility due to the formation of intermediate phases (LiCuFeS 2 , Li 3 CuS 2 , Li 2 FeS 2 ), 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 Fe 0 passivation. Electrochemically, CuFeS 2 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 CuFeS 2 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.