Abstract: A secondary electrochemical cell is made by(i) assembling a solid state electrochemical cell having an anode containing an alkali metal such as lithium as active anode material; a solid alkali metal ion conducting electrolyte, for example a polymer electrolyte comprising a complex of a solid polymer (e.g. poly(ethylene oxide)) and an alkali metal salt (e.g. LiClO.sub.4), which polymer is capable of forming donor-acceptor type bonds with alkali metal ions; and a cathode containing manganese (IV) oxide as active cathode material; and(ii) discharging the cell under conditions, e.g. of elevated temperature, such that the cathode material is transformed in situ into a rechargeable cathode material which may, for example be LiMn.sub.2 O.sub.4.