TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Battery: At least one opposite facing surface of the positive and negative electrodes is immersed in a solution of solid electrolyte, and then the solvent is removed, to form a solid electrolyte layer.Preferred Electrolyte: Copolymer (C) includes 3-7.5 wt.% hexafluoropropylene, and has units with Mw at least 5.5 x 105 and with Mw = 3 x 105 to less than 5.5 x 105. The solid electrolyte comprises the matrix polymer and at least 80 wt.% electrolytic liquid.Preferred Electrodes: The negative electrode comprises a material which is lithium doped and undoped (especially a carbon material). The positive electrode comprises a composite oxide of lithium and a transition metal. EXAMPLE - 93 pts.wt. vinylidene fluoride, 7 pts.wt. hexafluoropropylene, 300 pts.wt. pure water, 0.2 pt.wt. sodium pyrophosphate, 0.61 pt.wt. di-n-propyl peroxycarbonate and 0.1 pt.wt. methylcellulose were polymerised at 25degreesC for 20 hours (conversion ratio = = 80%), then 3.0 pts.wt. ethyl acetate added, unreacted monomers were recovered, and the obtained polymer slurry dehydrated, washed with water and dried, to give a vinylidene fluoride hexafluoropropylene block copolymer (Mw = 6 x 105; logarithmic viscosity = 1.93; hexafluoropropylene content = 7.0 wt.%).    10 pts.wt. of this was mixed with 90 pts.wt. pulverised black lead powder and dispersed in N-methyl-2-pyrolidone, to give a slurry. This was coated onto the surface of a 10 microm thick belt shaped Cu foil, dried and compression moulded, to give a negative electrode. 10 pts.wt. vinylidene fluoride hexafluoropropylene copolymer, 6 pts.wt. black lead and 91 pts.wt. LiCoO2 (prepared by mixing 0.5 mole lithium carbonate and 1 mole cobalt carbonate, and baking in air at 900degreesC for 5 hours) were mixed and dispersed in N-methyl-2-pyrolidone, to give a slurry. This was coated onto the surface of a 20 microm thick belt shaped Al foil, dried and compression moulded, to give a positive electrode.    The positive and negative electrodes were coated by immersing in a mixed solution of 30 pts.wt. plasticiser (comprising 42.5 pts.wt. ethylene carbonate, 42.5 pts.wt. propylene carbonate and 15 pts.wt. LiPF6), 10 pts.wt. vinylidene fluoride hexafluoropropylene copolymer and 60 pts.wt. diethyl carbonate, for 8 hours and evaporating off the diethyl carbonate, to give a 25 microm thick gel type electrolyte layer. The coated positive and negative electrodes were superimposed oppositely and pressed together, to give a gel type electrolyte battery (2.5 by 4.0 cm surface area; 0.3 mm thick).    The electrolyte had excellent peeling strength and the battery had 92% retention of discharge output after 500 cycles at 0.5 C, compared to poor adhesion an d 80% for a comparative example prepared using a vinylidene fluoride hexafluoropropylene block copolymer with Mw = 3 x 105 (7.0 wt.% hexafluoropropylene content) for the matrix polymer.