Abstract
Polymer electrolytes (PE) have gained attention for energy storage in electronics, offering safer operation, improved mechanical properties, and simplified device manufacturing than liquid electrolytes. Particularly, Solid-state polymer electrolyte-based electric double-layer capacitors (EDLC) offer opportunities for multifunctional energy storage. However, their lower EDL capacitances compared to liquid counterparts have not been extensively studied and explained. Additionally, the presence of solvent molecules introduced during PE processing can strongly impact ionic conductivity and mechanical properties, requiring further investigation. This study employs molecular dynamics simulations to uncover the atomic structure of the polymer electrolyte-electrode interface in an EDLC. We compare a polyethylene oxide/lithium perchlorate polymer electrolyte system between graphene electrodes with an aqueous electrolyte-based system containing the same lithium salt. Moreover, we construct two representative bulk polymer systems: polyethylene oxide with lithium perchlorate and polylactic acid with lithium perchlorate with varying water content to investigate the water impact on ionic conductivity and mechanical properties within and between different polymer systems. In our interface study, novel EDL structures are observed in the polymer-based system, attributed to distinct interaction strengths between ions, polymer/water molecules, and graphene electrodes. Despite these differences, intrinsic interfacial capacitances across different EDLC charge states exhibit similar values. The reported reduced capacitances in polymer electrolytes stem from inadequate electrode-electrolyte interfaces, improvable through thermal treatments. The solvent effect study shows increased ionic conductivity with higher water content (up to 3 water molecules per lithium-ion) in polymer electrolyte systems without forming a continuous liquid phase. Conductivity enhancement varies among polymer systems, correlating with ion/molecule association degree. Trace water diminishes ion-polymer interactions, promoting conductivity by lowering the activation energy. It also highlights the lithium and perchlorate ions' distinct roles in ion conduction and shows weak solvent-polymer interactions preserve the base polymer's mechanical properties. These findings facilitate innovative strategies for designing polymer electrolytes with high ionic conductivity, favorable mechanical properties, and electrochemical stability in energy storage applications.