Temperature-tolerant solid-state supercapacitors using Li+-garnet-ionic liquid composite electrolyte

Abstract

This investigation uses garnet-structured Li+ conductors added with a nominal amount of ionic liquid (IL) as an electrolyte to develop solid-state supercapacitors (SSCs) that operate across a wide temperature range from 0 to 100 ℃. A composite of LALZO (Li6.75Al0.25La3Zr2O12) with ~ 6 wt% of IL-EMIM BF4, having high conductivity ≥ 10−4 Ω−1 cm−1, is used as an electrolyte by compressing it between high surface area activated carbon (~ 1500 m2-g−1) electrodes and assembled in 2032 type cell geometry. A typical SSC at 35 °C exhibits a specific capacitance value of ~ 562 F-g−1 at 2 V/1 mA (0.57 A-g−1). Such SSCs exhibit stable performance at least up to ~ 4000 galvanostatic charge-discharge (GCD) cycles with a maintained coulombic efficiency of ~ 99%. These SSCs are found to be essentially electric double-layer type for operating voltages of ≤ 1.5 V. However, for 1.5 < V ≤ 2.5 V, pseudo-capacitive effects become significant in charge storage. A stack of four SSCs can effectively power two white LEDs (6 V) in series for about 30 min during direct discharge. The SSCs demonstrate stable GCD and CV cycles at 0 °C and 100 °C, with high performance parameters. The findings indicate that the LALZO-ionic liquid composites can be effectively utilized in high-performance, thermally stable supercapacitors.