A solid state electrochemical cell comprises an anode having lithium as its active material, a polymeric electrolyte, and a cathode comprising a composite of an insertion electrode material or similarly active material and a polymeric material. One of the electrolyte and the cathode possesses an ionically conducting phase at ambient temperature and the other of the electrolyte and the cathode lacks such a phase, lonic conductivity can be induced in said other of the electrolyte and cathode at elevated temperature thereby to render the cell operable and give rise to improvements in comparison with cells where such phases are present ab initio.