Lithium-sulfur batteries using a lithium anode, sulfur-CNT cathode, and gel polymer electrolyte composed of a poly(ionic liquid), an ionic liquid, and lithium salt are reported. Cyclic charge-discharge studies at various discharge and charge rates for 100 cycles and at a fixed rate for 500 cycles show superior performance for the gel polymer electrolyte (GPE) compared to the ionic liquid electrolyte (ILE) containing batteries. Rapid capacity fading for the first few cycles is observed and its causes are studied using an electrochemical impedance spectroscopy (EIS) approach. EIS reveals a depth-of-discharge dependent impedance growth during the first discharge cycle, that is followed by an interfacial resistance growth between the electrodes and the electrolytes in subsequent cycles.