Ionic liquids are promising liquid electrolytes because of their wide potential window (high electrochemical stability) and high Li-ion conductivity. However, high electrical resistance at the interface between the electrolyte and electrode hinders their practical application in Li-ion batteries. Here, we report the low interfacial resistance between a solvated ionic liquid, tetraglyme-lithium bis­(trifluoromethanesulfonyl)­amide ([LiG4]­[TFSA]), and positive electrode LiCoO2. We demonstrate stable cycling in a battery using a Li3PO4 buffer layer inserted at the interface of [LiG4]­[TFSA] and a positive electrode LiCoO2(001). Without inserting the buffer layer, the interface resistance drastically increases with repeated charge–discharge cycles, originating from the formation of a solid-electrolyte interphase (SEI) on the LiCoO2(001) surface; the interface resistance was 2.0 × 104 Ω cm2 after the 10th cycle. In contrast, the introduction of Li3PO4 significantly improves the charge and discharge cyclability, suppresses SEI growth, and lowers the [LiG4]­[TFSA]–Li3PO4 interface resistance to 4.5 × 102 Ω cm2. These results highlight the importance of modifying the interface between the ionic liquid electrolyte and the positive electrode to boost battery performance.