Development of rechargeable high-energy hybrid zinc-iodine aqueous batteries exploiting reversible chlorine-based redox reaction

The chlorine-based redox reaction (ClRR) could be exploited to produce secondary high-energy aqueous batteries. However, efficient and reversible ClRR is challenging, and it is affected by parasitic reactions such as Cl-2 gas evolution and electrolyte decomposition. Here, to circumvent these issues, we use iodine as positive electrode active material in a battery system comprising a Zn metal negative electrode and a concentrated (e.g., 30 molal) ZnCl2 aqueous electrolyte solution. During cell discharge, the iodine at the positive electrode interacts with the chloride ions from the electrolyte to enable interhalogen coordinating chemistry and forming ICl3-. In this way, the redox-active halogen atoms allow a reversible three-electrons transfer reaction which, at the lab-scale cell level, translates into an initial specific discharge capacity of 612.5 mAh g(I2)(-1) at 0.5 A g(I2)(-1) and 25 degrees C (corresponding to a calculated specific energy of 905 Wh kg(I2)(-1)). We also report the assembly and testing of a Zn | |Cl-I pouch cell prototype demonstrating a discharge capacity retention of about 74% after 300 cycles at 200 mA and 25 degrees C (final discharge capacity of about 92 mAh). Cl-redox reactions cannot be fully exploited in batteries because of the Cl2 gas evolution. Here, reversible high-energy interhalogen reactions are demonstrated by using a iodine-based cathode in combination with a Zn anode and a Cl-containing aqueous electrolyte solution.