The effects of room-temperature molten salt addition on the micro-structure and electrochemical properties of composite electrolytes (CEs) based on poly(ethylene oxide) (PEO)/ethylene carbonate (EC)/LiBF4 were studied. Additional salt, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), was found to influence the crystalline structure and heterogeneous morphology, resulting in changes to the ionic conductivity of the CE. The CE containing 0.2mol of EMIBF4 showed a small crystallinity, 27.9%. These CEs showed the highest ion conductivity, 3.1×10−4 S/cm, five times higher than that of the pristine PEO/EC/LiBF4. This enhanced conductivity originated from the decreased crystallinity and improved ion transference due to a Lewis acid–base interaction. The CE containing 0.3mol of EMIBF4 showed decreased conductivity due to the lower mobility, reflecting the high viscosity of the molten salt.