An all‐solid‐state lithium battery based on a sulfide electrolyte is one of the most promising next‐generation energy storage systems. However, the high interfacial impedance, particularly due to the internal pores in the electrode or electrolyte layers, is the major limiting factor to the development of sheet‐type all‐solid‐state batteries. In this study, a low‐resistance integrated all‐solid composite electrode is developed using a hybrid of a pyrrolidinium‐based ionic liquid and a polyethylene oxide polymer with lithium salt as a multifunctional interphase material, which is engineered to be compatible with the sulfide electrolyte as well as the fabrication process of sheet‐type composite electrode. The interphase material fills the pore in the composite sheet while binding the components together, which effectively increases the interfacial contact area and strengthens the physical network between the components, thereby enabling enhanced ion transport throughout the electrode. The interphase‐engineered sheet‐type LiNi0.8Co0.1Mn0.1O2 /Li10GeP2S12 electrode shows a high reversible capacity of 166 mAh g−1 at 25 °C, corresponding to 92% of the observed capacity in a current liquid‐based cathode system, as well as enhanced cycle and rate performances. This study proposes a novel and practical method for the development of high‐performance sheet‐type all‐solid‐state lithium batteries. An interphase‐engineered sheet‐type LiNi0.8Co0.1Mn0.1O2/Li10GeP2S12 electrode is developed by applying a hybrid of PEO‐Pyr14TFSI‐LiTFSI as a multifunctional interphase material to be compatible with the sulfide electrolyte. It shows a high reversible capacity (92% utilization) with excellent cycling performance at 25 °C. This study provides a practical method for the development of intrinsically safe and high‐performance all‐solid‐state lithium batteries.