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 (<1 μF/cm²), but thermal melting followed by rapid cooling enabled replication of the electrode's smooth morphology, sharply increasing capacitance up to ∼35 μF/cm². Slow cooling induced crystallization in PEO and PLA electrolytes, reducing interfacial conformity and driving capacitance back toward initial values, while PMMA remained amorphous, preserving stable capacitance (∼10.2-10.4 μF/cm²), highlighting the advantage of amorphous polymers for achieving durable electrode-electrolyte contact. These findings confirm that interfacial capacitance stability correlates with morphological stability rather than bulk ionic conductivity, emphasizing the critical role of phase behavior and interfacial engineering as a critical pathway to designing durable, high-performance next-generation SPE-based energy storage devices.