Thermally Induced Interfacial Changes of Solid Polymer Electrolytes in Electric Double-Layer Supercapacitors

All-solid-state supercapacitors are promising candidates for energy storage in flexible electronics and multifunctional structural components. However, when solid polymer electrolytes (SPEs) replace liquid electrolytes, capacitance drops dramatically, limiting the achievable energy density of these devices. Prior research has largely focused on enhancing ionic conductivity, while the role of the electrode–electrolyte interface remains underexplored and may represent the dominant performance bottleneck. We investigated PEO, PLA, and PMMA-based SPEs (each with 20 wt % lithium salt) under identical controlled thermal histories to assess the impact of morphology and interfacial contact on electrochemical performance. Temperature-dependent ionic conductivity measurements show that PLA exhibits the lowest conductivity, PEO the highest, and PMMA intermediate, following Arrhenius-type behavior. Despite its lower conductivity relative to PEO, PMMA maintains a smooth, amorphous interface, resulting in stable interfacial capacitance, whereas crystalline PEO and PLA undergo crystallization, producing ordered structure and capacitance decay. All as-cast films showed rough surfaces and poor contact with gold electrodes (