All-solid-state batteries (ASSBs) offer a fundamental solution to mitigate the safety and reliability issues of conventional lithium-ion batteries utilizing flammable liquid electrolytes. However, interfacial resistances between the cathode materials and solid electrolytes, which originate from poor solid-solid contacts and detrimental interfacial reactions, limit the electrochemical performances of ASSBs. Recently, various approaches have been developed to realize high-capacity composite cathodes with improved cycling stability. For example, single-crystal LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) has been reported to mitigate the capacity fading resulting from cracking of commonly adopted multi-crystal NCM811. Also, halide-type solid electrolytes are reported to exhibit high Li-ion conductivity and electrochemical stability with NCM. In this work, we systematically study the interfacial and charge-transport impedances of NCM composite cathodes with halide solid electrolytes. The composite cathodes comprising single- or multi-crystal NCM811 and Li₆PS₅Cl or Li₃InCl₆ (and conductive agents) are fabricated to characterize their electrochemical impedance behaviors. In particular, we analyze the evolution of the interfacial and charge-transport impedances during charge–discharge cycling based on a multi-rail transmission line model combined with interfacial reactions. In addition, we investigate the chemical, electrochemical, and mechanical degradations of composite cathodes using various analytical tools to elucidate performance-determining factors of ASSBs.