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In recent years mobile devices have become increasingly dependent on energy storage components such as batteries. Multifunctional structural supercapacitors, which can serve as structural components and store electrical energy at the same time, offer a solution to reducing the total weight of all kinds of mobile devices. Previous work on structural supercapacitors always point to devices with good mechanical strength or good electrical properties but rarely both, as those solid polymer electrolytes with good ionic conductivities use a liquid phase for the transfer of ions which can diminish the mechanical properties of the base polymer. We present here a strong and conductive all-solid-state electrolyte composed of polyethylene terephthalate (PET) and lithium perchlorate (LiClO4) with a single continuous phase. We find that the electrolyte shows extraordinary ionic conductivity while maintaining its all-solid state when exposed to controlled humidity. The hygroscopic electrolyte will absorb controlled amount of water molecules, all bound to LiClO4 without forming a liquid phase, when the humidity is below 70% at room temperature. Such bound water facilitates ion transport, improves electrode-electrolyte interface, and has minimum impact on the mechanical properties of the electrolyte. Through both X-Ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis of numerous solid electrolyte compositions that have undergone different treatments it has been shown that the electrolyte has an amorphous phase. The electrochemical tests conducted were electrochemical impedance spectroscopy (EIS) to determine the ionic conductivity, linear sweep voltammetry (LSV) to determine the operating voltage range, and cyclic voltammetry (CV) to determine the interfacial capacitance. These tests were conducted on a range of electrolytes varying in the ratio of PET and LiClO4, as well as varying in the relative humidity that the samples were exposed to. These results demonstrate comparable ionic conductivity to gel electrolytes with operating ranges of at least +2V. The preliminary mechanical testing shows that the solid electrolyte has a high stiffness and good interfacial bonding strength to carbon electrode surface. The use of hydrated lithium salts as opposed to an ionic liquid allows for the electrolyte to perform similarly to the base polymer. In conclusion, we believe that this solid electrolyte shows great promise for application in composite supercapacitors for structural applications. This solid electrolyte when applied to structural energy storage can be used to deliver a better power to weight ratio than a conventional battery, solving the problem of large and heavy power banks that increase overall weight and take up space.