Evaluation of Polymer Gel Electrolytes for Use in MnO₂ Symmetric Flexible Electrochemical Supercapacitors

Flexible electrochemical supercapacitors (FESCs) are emerging as innovative energy storage systems, characterized by their stable performance, long cycle life, and portability/foldability. Crucial components of FESCs, such as electrodes and efficient electrolytes, have become the focus of extensive research. Herein, we examine deep eutectic solvent (DES)-based polymer gel systems for their cost-effective accessibility, simple synthesis, excellent biocompatibility, and exceptional thermal and electrochemical stability. We used a mixture a DES, LiClO₄-2-Oxazolidinone as the electroactive species, and a polymer, either polyvinyl alcohol (PVA) or polyacrylamide (PAAM) as a redox additive/plasticizer. This combination facilitates a unique ion-transport process, enhancing the overall electrochemical performance of the polymer gel electrolyte. We manufactured and used LiClO₄-2-Oxazolidinone (LO), polyvinyl alcohol-LiClO₄-2-Oxazolidinone (PVA-LO), and polyacrylamide-LiClO₄-2-Oxazolidinone (PAAM-LO) electrolytes to synthesize an MnO₂ symmetric FESC. To evaluate their performance, we analyzed the MnO₂ symmetric FESC using various electrolytes with cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The FESC featuring the PVA-LO electrolyte demonstrated superior electrochemical and mechanical performances. This solid-state MnO₂ symmetric FESC exhibited a specific capacitance of 121.6 F/g within a potential window of 2.4 V. Due to the excellent ionic conductivity and the wide electrochemical operating voltage range of the PVA-LO electrolyte, a high energy density of 97.3 Wh/kg at 1200 W/kg, and a long-lasting energy storage system (89.7% capacitance retention after 5000 cycles of GCD at 2 A/g) are feasibly achieved. For practical applications, we employed the MnO₂ symmetric FESCs with the PVA-LO electrolyte to power a digital watch and a light-emitting diode, further demonstrating their real-world utility.