All-solid-state batteries (ASSBs) are attracting more attention in recent years due to their safety, use of Li metal anode, higher energy density, and so on. However, high interfacial resistance, especially on the cathode side, is one of the major challenges to commercialize ASSBs. Catholyte, both liquid or solid, is added to lower the cathode/electrolyte interfacial resistance. In this study, we found residual solvent remaining in a solid polymer catholyte after incomplete drying can dramatically lower the interfacial resistance between LLZO electrolyte and NMC cathode active material. Cells with different amounts of residual solvent are compared to optimize the solvent loading. With residual solvent, the capacity can reach 135 mA h/g when cycled at 25 °C and 0.5 C. Capacity retention is more than 60% after 125 cycles. The cell performs even better at low temperatures than a cell with liquid catholyte. This study demonstrates the possibility of widely differing results for ASSBs when the cathode is not completely dried. Also, it provides a hint for a potential method to lower the cathode/electrolyte interfacial resistance.