A cross-linked polymer membrane formed by poly(ethylene oxide) (PEO), N-methoxyethyl-N-methylpyrrolidium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (Pyr(12 Omicron 1)FTFSI) ionic liquid and LiFTPSI salt is proposed as the electrolyte for lithium-metal batteries. The ternary membrane has a PEO:Pyr(12 Omicron 1)FTFSI:LiFTFSI composition of 20:6:4 by mole, which ensures thermal stability up to 220 degrees C, overall ionic conductivity of 10(-3) S CM-1 at 40 degrees C and suitable Li+ transport properties. Combined with the LiFePO4 composite electrode, whose pores are filled with the Pyr(12 Omicron 1)FTFSI:LiFTESI electrolyte, and Li-metal anode, it yields Li/LiFePO4 cells delivering at 40 degrees C stable capacity (150 mAh g(-1) or 0.7 mAh cm(-2)) with coulombic efficiency higher than 99.5%. Impedance spectroscopy measurements reveal low resistance of the electrode/electrolyte interface at both the anode and the cathode. Preliminary results at 20 degrees C indicates a capacity of 130 mAh g(-1) at C/10 rate (17 mA g(-1)) with coulombic efficiency higher than 99.5%, thereby suggesting PEO:Pyr(12 Omicron 1)FTFSI:LiFTFSI as suitable electrolyte for lithium-metal polymer batteries for stationary storage applications, coupled for example with PV and wind generation.