Reversible Cl₂/Cl– Redox for Low-Temperature Aqueous Batteries

Conventional cathodes generally store charges inside solids, and they usually suffer from compromised performance at low temperatures due to the energy barriers for ion transport. Here we report a reversible “plating” reaction of Cl 2 liquid inside nanoporous carbon as a new electrode for low-temperature aqueous batteries. In a 7 m LiCl aqueous electrolyte, the activated carbon electrode delivers a high discharge capacity of 475 mAh/g (based on the carbon active mass in the working electrode) at 100 mA/g at −78 °C with a discharging plateau at ca. 0.95 V vs Ag/AgCl and retains a high capacity of 238 mAh/g at a high current rate of 1 A/g. Furthermore, this electrode system demonstrates stable cycling with ca. 65% capacity retention after 100 cycles at 500 mA/g with an average Coulombic efficiency of above 99%. The results provide a new option for high-energy halogen cathodes at low temperatures.