NOVELTY - The polymer comprises a hybrid copolymer solid-solution homogeneous blend of at least two polymers. One polymer is selected from a polar group having pronounced solvent retention properties. The other polymer is selected from at least one of polyester, polypropylene, polyethylene naphthalate, polycarbonate, polyphenylene sulfide and polytetrafluoroethylene.    USE - For high energy density lithium ion secondary batteries, used in portable electronics and electrically powered vehicles.    ADVANTAGE - The base polymer is insoluble in organic solvents currently used in lithium ion batteries, is highly stable with temperature, exhibits little or no swelling and is mechanically strong when in contact with organic solvents compared to PVDF, is predominantly amorphous, and is ionically conductive and more conductive at lower solvents levels. The solvent is immobilized, allowing batteries to be used in any orientation. It can be used in very thin films, provides low resistance and has excellent flexibility. The battery has improved energy density, power density, higher capacity utilization, higher cycle life, greater charge/discharge efficiencies, lower polarization, greater safety and greater reliability, and can be produced at high speed, lower cost and with improved form factors.    DETAILED DESCRIPTION - A solid base polymer for a polymer electrolyte of a lithium ion battery comprises a hybrid copolymer solid-solution homogeneous blend of at least two polymers. One of the polymers is selected from a polar group having pronounced solvent retention properties. The other polymer is selected from at least one of polyester, polypropylene, polyethylene naphthalate, polycarbonate, polyphenylene sulfide and polytetrafluoroethylene. The second polymer has a concentration in the solid-solution blend according to at least one desired property of the base polymer.    INDEPENDENT CLAIMS are also included for:    (I) a method for preparing a solid base polymer as above, which comprises homogeneously mixing the polymers together to form a melt cast film, and then biaxially orienting the film in machine direction orientation and transverse direction orientation by respective stretching of the film to a desired final thickness;    (II) an electrolyte retaining base polymer for a lithium ion battery which comprises a thin film, as above, cast from a solution of polypropylene, polyvinylidene fluoride and acrylate monomer/oligomer, and a liquid electrolyte solution containing a lithium salt absorbed within the film;    (III) a process for forming the base polymer in (II), which comprises:    (i) dissolving the above polymers in a solvent;    (ii) casting the solution to form a thin film, including evaporating the solvent from the solution;    (iii) soaking the thin film in the electrolyte solution to absorb it; and    (iv) curing the acrylate monomer/oligomer;    (IV) a process for forming a base polymer similar to that described above in (II) and (III), except that there is no acrylate monomer/oligomer, but instead 0.1-30 wt.% of a high surface area inorganic filler selected from fumed silica or alumina, is dispersed in a polypropylene and polyvinylidene fluoride blend to create porosity and enhance mechanical stability, before soaking in the liquid solvent electrolyte;    (V) a lithium ion battery which comprises:    (i) a resilient flexible polymer electrolyte film as formed in (IV); and    (ii) a pair of spaced flexible thin film electrodes, each having a polymer substrate with an electrically conductive layer on it, formed around the polymer electrolyte film;    (VI) a lithium ion battery electrode having an ultrathin film metal substrate of thickness 1-10 micron, for a cathode or anode;    (VII) a lithium ion battery polymer substrate comprising a polymer layer with a low resistance metallization layer of conductivity 0.01-1 Ohm/square on one side;    (VIII) a method of fabricating a thin film lithium ion secondary battery, which comprises incorporating the polymer substrate of (VII), where the polymer layer is 0.5-50 micron thick, with the above electrode structures of (VI) having thicknesses less than 5 and 10 micron respectively; and    (IX) a method of coating an ultrathin film metallized polymer substrate with very thin film active anode and cathode materials, by evaporating the respective materials directly onto opposite sides of the substrate.