High‐Temperature Supercapacitors Enabled by Fluorine‐Free Ionic Liquid and Synergistic Alkali‐Doped Graphene Oxides

ABSTRACT This study explores the synthesis and electrochemical performance of graphene oxide co‐doped with sodium and potassium (Na–K–GO) as electrode materials for supercapacitors (SCs) designed to operate at 60°C over an extended voltage window. The Na–K–GO is employed as the electrode material, while a fluorine‐free ionic liquid (IL), [P 4444 ][MEEA]—comprising a tetrabutylphosphonium cation and a 2‐2‐(2‐methoxyethoxy)ethoxy anion—served as the electrolyte, enabling stable operation over a wide voltage window at elevated temperatures. Using this combination, three coin‐cell SCs are fabricated: two symmetric devices (SC‐1 and SC‐2) and one asymmetric device (SC‐3). All the three exhibited remarkable charge storage abilities, a retaining performance over 10 000 charge–discharge cycles at 60°C. Among the three devices, SC‐3 exhibited the best overall electrochemical performance, delivering a high specific capacitance of 47.01 F g −1 and an energy density of 27.77 Wh kg −1 at 0.5 A g −1 . Even at a higher current density of 1 A g −1 , SC‐3 maintained a maximum power density of 1000 W kg −1 while sustaining an energy density of 14.21 Wh kg −1 , reflecting its strong rate capability. Moreover, the long‐term cycling tests at 2 A g −1 demonstrated an outstanding durability of SC‐3, which retained 99% coulombic efficiency after 10 000 cycles, significantly outperforming the SC‐2 (90%) and SC‐1 (79%).