Dual role of divalent cations for enhanced low-temperature ionic conductivity and stability in zwitterionic hydrogel electrolytes

At extremely low temperatures, the incorporation of zwitterionic polymers in constructing high-speed ion channels hold immense potential to exponentially enhance ionic conductivity, which is lethally compromised by solidification and increased viscosity of electrolytes. However, the network structure formed by zwitterionic groups is susceptible to disruption caused by cations and anions in the electrolyte, thereby affecting its structural stability and mechanical properties. In this study, high-concentration divalent cations were successfully employed as both antifreeze agents and cross-linkers to fabricate stretchable and low temperature resistant hydrogel electrolytes. This dual-role mechanism of different high-concentration divalent cations (M 2+ ) in zwitterionic hydrogels was explicitly elucidated at the molecular level through the utilization of ex-situ Raman spectroscopy. This analysis has emphasized their crucial involvement in the formation of [-SO 3 − -M 2+ -SO 3 − -] coordination bonds with negatively charged groups (−SO 3 − ), thereby providing additional cross-linking points, as well as their hydration interactions with free water molecules, which contribute to its exceptional low temperature resistance. With the ion channels constructed from zwitterionic polymers and the low-temperature resistance given by the high salt concentration, the hydrogel electrolyte containing Ca 2+ achieved an impressive low-temperature ionic conductivity of 5.9 mS cm −1 , even at −50 ℃. The assembled supercapacitors (SCs) demonstrated a remarkable areal specific capacitance of 100.8 mF cm −2 at −40 °C, with a capacitance retention rate of 108.8 % after 10,000 cycles. This performance surpassed most reported antifreeze hydrogel electrolytes, offering valuable insights for designing flexible electrolytes with enhanced stability and high performance in low temperature environments.