A polymer electrolyte comprises a polymer combined with a solution of a salt in a plasticising solvent, and the polymer is a terpolymer of vinylidene fluoride (VdF), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE). The proportion by weight of vinylidene fluoride is at least 85 %. The polymer has a large enough molecular weight that its melt flow index, at 230 DEG C and 21.6 kg, is less than 5.0 g/10 min. The resulting polymer electrolyte may be referred to as a solid electrolyte or a gelled electrolyte, and is suitable for use as the separator/electrolyte in an electrochemical cell, such as a secondary lithium ion cell.