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 Matrix: The matrix polymer comprises a fluoropolymer with Mw = 300000 to less than 550000, and at least 30 wt.% fluoropolymer with Mw at least 550000. The fluoropolymer is polyvinylidene fluoride and/or vinylidene fluoride hexafluoropropylene copolymer.Preferred Electrodes: The binder of the positive and/or negative electrode is either the same as the matrix polymer of the solid electrolyte, or a polymeric material with a similar structure. The negative electrode comprises a material which can be 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 polymerized at 25 degrees C for 20 hours (conversion ratio = 80%), then 3.0 pts.wt. ethyl acetate added, unreacted monomers were recovered, to give a vinylidene fluoride hexafluoropropylene copolymer (Mw = 600000; logarithmic viscosity = 1.93; hexafluoropropylene content = 7.0 wt.%).    10 pts.wt. this were mixed with 90 pts.wt. pulverized black lead powder and dispersed in N-methyl-2-pyrolidone, to give a slurry. This was coated onto the surface of a 10 micrometers 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 900 degrees C 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 micrometers 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. plasticizer (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 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 bad adhesion and 48% for a comparative example prepared using a vinylidene fluoride hexafluoropropylene copolymer with Mw = 300000 (7.0 wt.% hexafluoropropylene content) for the matrix polymer.