TECHNOLOGY FOCUS - ELECTRONICS - Preferred Method: The PVA-based electrolyte solution is dried at a temperature of 40-120 degrees C for at most 48 hours, and the PVA-based solid electrolyte comprises at most 50 wt.% of the solvent. The method further comprises forming two electrode layers respectively on opposite outer surfaces of the PVA-based solid electrolyte layer on the substrate. In the method for preparing the SPEEK-based solid electrolyte, the SPEEK-based electrolyte solution is dried at a temperature of 60-120 degrees C for at most 72 hours, and a solvent content of the SPEEK-based solid electrolyte is smaller than 40 wt.%. The method for preparing the SPEEK-based solid electrolyte further comprises: immersing the SPEEK-based solid electrolyte layer in a liquid solution of a lithium salt for 1-60 seconds after the drying step to reduce the charge transfer resistance between the solid electrolyte and an electrode and increase the mobility of ions in solid electrolyte; and forming two electrode layers respectively on opposite outer surfaces of the SPEEK-based solid electrolyte layer on the substrate. Preferred Components: In the preparation of PVA-based solid electrolyte: a weight ratio of the lithium salt to the PVA is 0.1-5 in the PVA-based electrolyte solution; and the substrate is a flexible substrate. In the preparation of the sulfonated polyether etherketone-based solid electrolyte: molecular weight of the SPEEK is 10000-50000 Daltons; and weight ratio of the lithium salt to the SPEEK is at most 2 in the SPEEK-based electrolyte solution.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: In the preparation of the SPEEK-based solid electrolyte the lithium salt is lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoro arsenide, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonimide), lithium bromide, lithium hydroxide, lithium nitrate or lithium sulfate.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The polar aprotic solvent comprises dimethyl sulfoxide, N-methylpyrrolidinone, dimethylformamide and/or dimethylacetamide. EXAMPLE - No suitable example given.