Low-Cost “Water-in-Salt” Hydrogel Electrolyte Enabled Flexible Supercapacitors with 2.7 V Voltage and −40 °C Adaptability

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 (NaNO₃, LiNO₃, and NaClO₄) 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 NaClO₄ 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 NaClO₄ (HE-NaClO₄-15) 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.