An aqueous magnesium-ion battery working at −50 °C enabled by modulating electrolyte structure

The earth-abundant magnesium resource and aqueous electrolyte make aqueous magnesium-ion batteries (AMIBs) a promising post-lithium-ion battery technology to synchronously address cost and safety concerns. Nonetheless, the freeze issue of aqueous electrolytes is curbing the development of AMIBs working at low-temperature conditions. Here, we report an MgCl2 solution having a great low-temperature property via anion and concentration adjustment, which roots in the variation of the pristine hydrogen bond reticulation. Consequently, a 4 M MgCl2 aqueous electrolyte endows a low freezing point (−62 °C) as well as ultra-high ionic conductivity (2.77 mS/cm at −50 °C). By using 4 M MgCl2 as an electrolyte, the MnO2//VO2 battery is capable of working in a wide temperature range from +25 to −50 °C, achieving a high discharge capacity of 97.9 mAh/g (0.1 A/g) and a high cycling capability of 1000 cycles (∼90 % capacity retention at −20 °C). Furthermore, MgCl2 can be well compatible with polyacrylamide into an outstanding hydrogel electrolyte (in terms of mechanical property, anti-freeze performance, and high ionic conductivity) for a flexible quasi-solid-state MIBs application. The assembled flexible device attains appealing electrochemical properties across a broad temperature region, while the constructed pouch cell demonstrates superior flexibility and reliable safety. Our current exploration depicts an expeditious but productive paradigm for devising the high-performing AMIBs for low-temperature energy storage sectors.