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Sulfide electrolyte all-solid-state lithium batteries (ASLBs) with uncoated Li-Ni_xMn_yCo_1−x−yO₂(NMC) cathodes suffer from a large capacity loss during initial cycling and an increase in cell impedance. Decomposition reactions are known to occur at the Li₆PS₅Cl-NMC111 interface due to incompatibility between the two materials. If a stabilizing coating is applied to the NMC, it delivers full capacity during initial charge. However, the loss in capacity during discharge still occurs. The interface was studied byμXANES and through EIS analysis. A chemically-formed interphase was detected byμXANES, evident from reduction of Co at an uncoated NMC particle surface. This interphase was produced by decomposition at rest. To study the effect of the interphase on electrochemically active surface area, piecewise in situ EIS was performed and the data was modeled using a transmission line model (TLM). The charge transfer resistance R_CTwas used to estimate the volume specific active surface area (a_act). The median value for a_actwas 296 cm⁻¹, a factor of 7.5 lower than the theoretical value of 2216 cm⁻¹. This provided evidence of a lower electrochemically active surface area in the ASLB.