Stabilized Conductive Agent/Sulfide Solid Electrolyte Interface via a Halide Solid Electrolyte Coating for All‐Solid‐State Batteries

All‐solid‐state batteries (ASSBs) have garnered significant interest as the next‐generation in battery technology, praised for their superior safety and high energy density. However, a conductive agent accelerates the undesirable side reactions of sulfide‐based solid electrolytes (SEs), resulting in poor electrochemical properties with increased interfacial resistance. Here, we propose a wet chemical method rationally designed to achieve a conformal coating of lithium–indium chloride (Li 3 InCl 6 ) onto vapor‐grown carbon fibers (VGCFs) as conductive agents. First, with the advantage of the Li 3 InCl 6 protective layer, use of VGCF@Li 3 InCl 6 leads to enhanced interfacial stability and improved electrochemical properties, including stable cycle performance. These results indicate that the Li 3 InCl 6 protective layer suppresses the unwanted reaction between Li 6 PS 5 Cl (LPSCl) and VGCF. Second, VGCF@Li 3 InCl 6 effectively promotes polytetrafluoroethylene (PTFE) fibrillization, leading to a homogeneous electrode microstructure. The uniform distribution of the cathode active material (CAM) in the electrode results in reduced charge‐transfer resistance ( R ct ) and enhanced Li‐ion kinetics. As a result, a full cell with the LiNi x Mn y Co z O 2 (NCM)/VGCF@Li 3 InCl 6 electrode shows an areal capacity of 7.7 mAh cm −2 at 0.05 C and long‐term cycle stability of 77.9% over 400 cycles at 0.2 C. This study offers a strategy for utilizing stable carbon‐based conductive agents in sulfide‐based ASSBs to enhance their electrochemical performance.