Bifunctional electrolyte regulation towards low-temperature and high-stability Zn-ion hybrid capacitor

Aqueous Zinc-based energy storage devices are considered as one of the potential candidates in future power technologies. Nevertheless, poor low temperature performance and uncontrollable Zn dendrite growth lead to the limited energy storage capability. Herein, an anti-hydrolysis, cold-resistant, economical, safe, and environmentally friendly electrolyte is developed by utilizing water, ethylene glycol (EG), and ZnCl2 with high ionic conductivity (7.9 mS cm−1 in glass fiber membrane at −20 °C). The spectra data and DFT calculations show the competitive coordination of EG and Cl- to induce a unique solvation configuration of Zn2+, conducive to effectively inhibiting the hydrolysis of Zn2+, suppressing the dendrite growth, and broadening the working voltage range and temperature range of ZnCl2 electrolyte. The isotope tracing data confirm that Cl- could effectively destroy the ZnO passivation film, promoting the formation of Zn nuclei and improving its reaction activity. Compared to the corresponding ZnSO4 electrolyte, the Cu/Zn half-cell with the ZnCl2 electrolyte exhibits a stable cycle life of more than 1600 h at −20 °C, even at the current density of 5 mA cm−2. The assembled Zn-ion hybrid capacitor possesses an average capacity of 42.68 mA h g−1 under −20 °C at a current density of 5 A g−1, 3.5 times than that of the modified ZnSO4 electrolyte. Our work proposes a new approach for optimizing aqueous electrolytes to meet low temperature energy storage applications.