The present invention discloses methods and materials to add a polymer material to a liquified gas electrolyte solution for use in an electrochemical energy storage device such as a lithium-ion battery or related technology to further improve the battery cell's safety properties. An example device includes an ionically conducting electrolyte comprised of a liquefied gas solvent, a salt, and a polymer. The liquefied gas solvent has a vapor pressure above 100 kPa at a temperature of 293.15 K, and the polymer is at low enough concentration that it is fully dissolved in the liquefied gas solvent. The device may include an anode, a cathode, and a separator layer in contact with the ionically conducting electrolyte. A housing may enclose the ionically conducting electrolyte, the anode, the cathode and the separator layer.