Solid-state supercapacitors using ionic liquid dispersed Li+-NASICONs as electrolytes

Abstract

Ceramic ionic conductors exhibit inadequate ionic conductivity for device applications. However, when added with a small amount of ionic liquid (IL), exhibit a substantial conductivity rise. This research demonstrates the use of such IL-ceramic composites, with IL content ≤ 13 wt%, for capacitor applications. These supercapacitors are designed using IL dispersed Li+ ion conducting fast ionic ceramics as an electrolyte, viz; LiTi2(PO4)3 (LTP) and Li1.3Al0.3Ti1.7(PO4)3 (LATP). The cells are fabricated in 2032-coin cells using these composites and activated charcoal coated on the copper foil as the electrode. A typical supercapacitor containing LATP-13 wt% EMIM BF4 as electrolyte at ∼ 35 °C exhibits high specific capacitance of ∼181 F-g−1, specific energy ∼ 6.1 Wh-kg−1, and power of ∼140 W-kg−1 at 0.65 mA/cm2 (0.56 A-g−1) and 1 V. Importantly, the choice of IL (size of the ions), as well as the composition of fast ionic ceramic, influences the device performance. For a discharge at 0.56 A-g−1, a supercapacitor with this composite electrolyte exhibit stability at least up to ∼ 13,000 charge/discharge cycles with a fairly stable coulomb efficiency of ∼ 99%. At ∼ 100 °C these cells exhibit a maximum specific capacitance up to ∼ 600 F-g−1. These supercapacitors exhibit appreciable cycling performance at 30–100 °C. At higher discharge currents (≥ 0.34 A-g−1) electric double layer capacitor behavior is witnessed. A stack of two cells is able to glow a white light-emitting diode (3 V) successfully for ∼30 min.