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

Abstract

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⁻¹ and the power density of 1873 W kg⁻¹ while maintaining ∼94% capacitance retention and ∼98% coulombic efficiency after 1000 cycles. SCs.