Free-standing gel polymer electrolytes (GPE) composed of the polymeric ionic liquid poly[diallyldimethylammonium] bis(trifluoromethane) sulfonimide, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethane) sulfonimide, and lithium salt are reported for their use in lithium batteries. The GPEs are thermally stable and contain ionic liquid and salt contents of up to 80wt.% which results in highly ionically conductive electrolyte films with σ =3.35mScm−1. The affinity of the polymeric ionic liquid with the ionic liquid and salt components of the GPE allows wide electrochemical stability window −0.1–4.9V and increased lithium ion transference number. In addition, the GPEs suppressed dendrite formation compared to ionic liquid electrolytes and in turn enabled the use of these GPEs in batteries with lithium anodes. Galvanostatic charge/discharge cycling of Li/GPE/LiFePO4 cells shows discharge capacity of 169.3mAhg−1 at C/10, 126.8mAhg−1 at 1C, and impressive capacity retention capability for 40 cycles at up to 5C rate, at 22°C.