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This study introduces two new fluorine-free ionic liquids (ILs) produced by coupling biomass-derived heterocyclic anions, i.e., tetrahydro-2H-pyran-4-carboxylate (THP) and furan-3-carboxylate (3-FuA), and tetrahydroxyphosphonium cation (P4444). The (P4444)(3-FuA) IL exhibits slightly higher thermal stability, displays a lower glass-transition temperature and significantly higher ionic conductivity than (P4444)(THP). This improvement arises from π-electron delocalization in the (3-FuA) anion, by dispersing the negative charge over the ring, weakening the cation–anion attractions, and thus enhancing the ion mobility. Owing to the favorable ion transport characteristics, (P4444)(3-FuA) performs exceptionally well as a supercapacitor electrolyte. When paired with multiwalled carbon nanotubes (MWCNT)-based electrodes, (P4444)(3-FuA) delivers an areal capacitance of 430 mF cm−2 at 2 mV s−1, an energy density of 86 µWh cm−2 at 0.298 mA cm−2, and a power density of 1492 µW cm−2 at 0.995 mA cm−2, while maintaining 97% Coulombic efficiency after 6 000 cycles at 60°C. In comparison, the (P4444)(THP) IL demonstrate a lower capacitance performance, albeit with robust long-term stability. Overall, both the ILs display enhanced capacitance with increasing temperature, underscoring their potential as fluorine-free electrolytes for supercapacitors operating under elevated thermal conditions.

Fluorine-free supercapacitor (SCs) electrolytes are desirable to minimize environmental impact and toxicity while maintaining high electrochemical performance and long-term sustainability. Here, we introduce the new class of fluorine-free ionic liquids (ILs) engineered around the unique electron-deficient triazine-derived anion, 4,6-diethoxy-2-oxo-2H-1,3,5-triazin-5-ide (DET), coupled with n-tetrabutyl- phosphonium and ammonium cations. Both the ILs exhibit distinct and well-defined thermal behaviors, the former behaves as a glass-forming liquid, that is, have glass transition at −63 °C, while the latter exists as a supercooled liquid with complex thermal events and is ca. 90 K less thermally stable. We find relatively weaker cation–anion interactions – well supported by the FTIR data – and, thus higher ionic conductivity and higher electrochemical stability, with supporting voltage range up to 4.4 V, for (P4444)(DET) than in (N4444)(DET). (P4444)(DET) as SCs electrolyte delivers excellent capacitive performance within the 2.0 V window at 30 °C and 60 °C. The device achieved areal capacitance of 78 mF cm−2(at 0.146 mA cm−2) and gravimetric capacitance of 60.6 F g−1(at 0.15 A g−1) at 60 °C and delivered an energy density of 34.6 Wh kg−1and the power density of 1873 W kg−1while maintaining ∼94% capacitance retention and ∼98% coulombic efficiency after 1000 cycles. SCs.

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), [P4444][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%).