Low-Cost “Water-in-Salt” Hydrogel Electrolyte Enabled Flexible Supercapacitors with 2.7 V Voltage and −40 °C Adaptability
Weilin Yi, Shuang Wu, Zixuan Zhou, Fang Xiao, Xiaoyi Sun, Juan Li
Abstract
Aqueous electrolytes endow paramount safety for portable energy storage devices. However, they often suffer from narrow voltage windows and poor low-temperature performance. Here, a carrageenan/polyacrylamide double-network hydrogel electrolyte was synthesized by the one-pot method for high-performance supercapacitors. Instead of using the high-cost LiTFSI-based salt, three low-cost salts (NaNO 3, LiNO 3, and NaClO 4 ) were respectively added to the double-network electrolyte. By exploring the utmost dissoluble concentrations in the double-network hydrogel, we found that the concentration of NaClO 4 in the hydrogel satisfies the water-in-salt (WIS) criterion among the three hydrogel electrolytes, thus showing the best performance. The hydrogel electrolyte containing 15 mol L –1 NaClO 4 (HE-NaClO 4 -15) has an oxygen evolution potential broadened to 2.71 V and a high ionic conductivity of 10.3 mS cm –1 at −40 °C. The corresponding flexible symmetric supercapacitor exhibits a high operating voltage of 2.7 V and a specific energy density of 39.2 Wh kg –1 at a power density of 675 W kg –1 . In addition, the supercapacitor exhibits an impressive cycle life, and the capacitance retention is 90.9% after 20000 cycles at −40 °C. The supercapacitor works stably under mechanical abuse conditions. High voltage, stable electrochemical performance, and low-temperature operation make the supercapacitor adapt to the harsh working environment of portable energy storage devices.