NOVELTY - Preparing a polyvinyl alcohol (PVA)-based solid electrolyte, comprises: preparing a PVA solution by dissolving polyvinyl alcohol in a solvent containing water; dissolving a lithium salt in the PVA solution to form a PVA-based electrolyte solution; coating the PVA-based electrolyte solution on a substrate; and drying the PVA-based electrolyte solution to form a PVA based solid electrolyte layer on the substrate.    USE - The method is useful: for preparing polyvinyl alcohol (PVA)-based solid electrolyte (claimed) which is used for preparing flexible lithium batteries, solid thin film batteries, textile batteries, textile capacitors; and for preparing sulfonated polyetheretherketone (SPEEK)-based solid electrolyte.    ADVANTAGE - The method: provides polyvinyl alcohol (PVA)-based solid electrolyte that has good tensile strength (1.4-2.5 kgf/mm2) and good ionic conductivity (10-6-10-2 S/cm); provides polymer-based solid electrolyte that has small thermal change rate of conductivity and capacity to provide stable conductivity and capacity over wide temperature ranges; utilizes lithium salt that has lower lattice energy which can increase ionic conductivity of PVA-based solid electrolyte; provides PVA-based solid electrolyte that increases related efficacy of flexible lithium batteries; and provides solid thin film batteries that has high power density, thus can be charged and discharged for many times and in a high-temperature environment.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) the PVA-based solid electrolyte having a tensile strength of 1.4-2.5 kgf/mm2 and an ionic conductivity of 10-6-10-2 S/cm at room temperature, where the PVA-based solid electrolyte comprises the lithium salt comprising lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoro arsenide, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonimide) and/or lithium bromide, the PVA having a molecular weight of 20000-186000 Daltons, a weight ratio of the lithium salt to the PVA is at most 5, a solvent which is present at less than 50 wt.% having water and ethanol, and a weight ratio of the ethanol to the water is at most 2;    (2) preparing a sulfonated polyetheretherketone (SPEEK)-based solid electrolyte, comprising preparing a SPEEK solution by dissolving SPEEK in a polar aprotic solvent, dissolving a lithium salt in the SPEEK solution to form a SPEEK-based electrolyte solution, coating the SPEEK-based electrolyte solution on a substrate, and drying the SPEEK-based electrolyte solution to form a SPEEK-based solid electrolyte layer on the substrate; and    (3) the SPEEK-based solid electrolyte having conductivity's thermal change rate smaller than 80% and capacity's thermal change rate smaller than 60%, comprising the lithium salt comprising lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoro arsenide, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonimide) and/or lithium bromide, SPEEK having a molecular weight of 10000-50000 Daltons, where a weight ratio of the lithium salt to the SPEEK is at most 2, and the polar aprotic solvent, a content of the solvent is less than 40 wt.%.