In this manuscript are reported the results of an investigation performed on rechargeable, all-solid-state, solvent-free, Li/LiFePO4 polymer batteries incorporating N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide, PYR14TFSI, ionic liquid (IL). The tests show clearly the beneficial effect due to the incorporation of ionic liquids on room temperature battery performance that, conversely, results extremely poor in IL-free lithium polymer batteries. The theoretical capacity is delivered at 30°C whereas about 115mAhg−1 are discharged at 20°C with excellent capacity retention and high coulombic efficiency. At 40°C large capacities (125mAhg−1) are discharged even at medium rates (C/3). Impedance measurements revealed that the overall battery resistance is almost fully located (e.g., above 93%) at the lithium anode/polymer electrolyte interface, which plays a key role in determining the battery performance.