New fluorine-free triazine-anion-based ionic liquids as supercapacitors electrolytes

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 (P 4444 )(DET) than in (N 4444 )(DET). (P 4444 )(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 −1 and the power density of 1873 W kg −1 while maintaining ∼94% capacitance retention and ∼98% coulombic efficiency after 1000 cycles. SCs. Fluorine-free electron deficient triazine-anion-based ionic liquids (ILs) with phosphonium and ammonium cations are introduced. Phosphonium IL have weaker ion-pairing and thus higher thermal, electrochemical and ion-transport, compare to its ammonium analogue, and as supercapacitor electrolyte delivers high capacitance, energy and power density, and excellent cycling stability at variable temperatures. • Introduce new fluorine-free anion and ionic materials. • (P 4444 )(DET) displays the higher conductivity and wider ESW compared to (N 4444 )(DET). • (P 4444 )(DET)-based SC offers high capacitance and energy density.