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 (EQ/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 0.2 mol of EMIBF4 showed a small crystallinity, 27.9%. These CEs showed the highest ion conductivity, 3.1 x 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 0.3 mol of EMIBF4 showed decreased conductivity due to the lower mobility, reflecting the high viscosity of the molten salt. (C) 2009 Elsevier Ltd. All rights reserved.