The influence of adding the room-temperature ionic liquid 1-ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) to poly(ethylene oxide) (PEO) lithium difluoro(oxalato)borate (LiDFOB) solid polymer electrolyte and the use of these electrolytes in solid-state Li/LiFePO4 batteries has been investigated. Different structural, thermal, electrical and electrochemical studies exhibit promising characteristics of these polymer electrolyte membranes, suitable as electrolytes in rechargeable lithium-ion batteries. The crystallinity decreased significantly due to the incorporation of ionic liquid, investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The ion polymer interaction, particularly the interaction of cations in LiDFOB and ionic liquid with ether oxygen atom of PEO chains, has been evidenced by FT-IR studies. The polymer electrolyte with similar to 40 wt% of ionic liquid offers a maximum ionic conductivity of similar to 1.85 x 10(-4) S/cm at 30 degrees C with improved electrochemical stabilities. The Li/PEO-LiDFOB-40 wt% EMImTFSI/LiFePO4 coin-typed cell cycled at 0.1 C shows the 1st discharge capacity about 155 mAh g(-1) and remains 134.2 mAh g(-1) on the 50th cycle. The addition of the ionic liquid to PEO20-LiDFOB polymer electrolyte has resulted in a very promising improvement in performance of the lithium polymer batteries. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.