Modulating ionic transport in PEO-based composite polymer electrolytes using a sulfonate-functionalized 3D metal–organic framework for solid-state supercapacitor devices

Abstract

The growing demand for safe, high-power, and long-cycle energy storage devices has accelerated the development of all-solid-state supercapacitors. However, the limited ionic conductivity of polymer electrolytes remains a key challenge. Herein, we report the fabrication of poly(ethylene oxide) (PEO)-LiClO₄ composite polymer electrolytes (CPEs) using a sulfonate-functionalized three-dimensional metal–organic framework (MOF) (BITSP) as an active additive. Out of all the synthesized CPEs, the CPE, PEO-10% BITSP-LiClO₄, exhibits a high ionic conductivity of 6.8 × 10⁻⁴ S cm⁻¹ at room temperature and 1.4 × 10⁻³ S cm⁻¹ at 50 °C, a near-unity ionic transference number of 0.996 at room temperature, and an electrochemical stability window of 2.02 V. The CPE, PEO-10% BITSP-LiClO₄, is further assembled into an all-solid-state supercapacitor with a configuration of Cu|AC|PEO-10%-BITSP-LiClO₄|AC|Cu, which delivers a high specific capacitance (185 F g⁻¹ at 2 V) with 99% coulombic efficiency at 1 V and a capacitance retention of 75% after 10 000 galvanostatic charge–discharge (GCD) cycles at 1 V. The role of BITSP in CPEs in ionic conduction is validated using DSC, XPS and DFT studies where it is inferred that the presence of heteroatoms such as free sulfonate oxygen and imine nitrogen sites in BITSP provides interaction sites for the Li⁺ ions to facilitate ionic transport while the Cd(II) centre in BITSP plays a dual role by capturing the counter anion of the Li salt and interacting with PEO thereby creating amorphous domains. Further, the conductivity and capacitive nature of PEO-10% BITSP-LiClO₄ are compared with those of PEO-x% BITSP-LiTFSI to verify the cooperative effect of the chemical features present in BITSP, which enhances the electrochemical performance of CPE, PEO-10%-BITSP-LiClO₄.