Solid Acid-Dual Salt Hybrid Electrolyte Unlocks Broad Electrochemical Stability and High Capacitance in Solid-State Devices.

Solid-state electrolytes have garnered significant attention as superior alternatives to liquid electrolytes in energy storage devices, offering enhanced stability and safety. However, the behavior of water molecules in polymer-based solid electrolytes remains a critical determinant of ionic conductivity and electrochemical performance. Conventional polymer electrolyte designs often rely on intricate synthesis routes and expensive materials, posing significant scalability challenges. In this study, we present an innovative and cost-effective strategy for fabricating a fully solid-state electrolyte by incorporating a dual-salt system, where one salt functions as a water retainer while the other acts as a water dehydrator. Lithium bromide (LiBr) was employed to stabilize nonfreezable bound water, thereby enhancing ionic conductivity, while cesium dihydrogen phosphate (CDP), a solid acid introduced here for the first time in energy storage applications, effectively minimized free water molecules, broadening the electrochemical stability window. The resultant CDP-LiBr@PVA(SS) electrolyte was successfully synthesized and integrated into fully solid-state supercapacitor devices, achieving an impressive stable potential window of 2.1 V, compared to 1.8 V for individual CDP@PVA and LiBr@PVA systems. Additionally, the capacitance was significantly enhanced, reaching 90 F/g, in contrast to 34 F/g for the separate systems. Structural and electrochemical enhancements were corroborated through differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and molecular dynamics (MD) simulations, all of which validated the effectiveness of the proposed electrolyte design. This work pioneers a simple yet powerful approach for engineering solid-state electrolytes by precisely modulating the state of water molecules while, for the first time, leveraging a solid acid material like CDP in energy storage applications. This strategy offers a scalable and efficient pathway toward next-generation energy storage devices.