Traditional non-aqueous lithium-ion batteries have a high energy density, but their safety is compromised due to the flammable organic electrolytes they utilize. 

Aqueous batteries use water as the solvent for electrolytes, significantly enhancing battery safety. However, due to the limited solubility of the electrolyte and low battery voltage, aqueous batteries typically have lower energy density. This means that the amount of electricity stored per unit volume of aqueous battery is relatively low. 

In a new study published in Nature Energy, a research group led by Prof. LI Xianfeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. FU Qiang’s group, also from DICP, developed a multielectron transfer cathode based on bromine and iodine, realizing a specific capacity of more than 840 Ah/L and achieving an energy density of up to 1200 Wh/L based on catholyte in full battery testing. 

To improve the energy density of aqueous batteries, researchers used a mixed halogen solution of iodide ions (I^-) and bromide ions (Br^-) as the electrolyte. They developed a multielectron transfer reaction, transferring electrons from iodide ions (I^-) to elemental iodine (I₂) and then to iodate (IO₃^-). During the charging process, I^- was oxidized to IO₃^- on the positive side and the H⁺ generated was conducted to the negative side in the form of supporting electrolyte. During the discharge process, H⁺ was conducted from the positive side and IO₃^- was reduced to I^-. 

The multielectron transfer cathode developed by the researchers had a specific capacity of 840 Ah/L. Combining the cathode with metallic Cd to form a full battery, the researchers achieved an energy density of up to 1200 Wh/L based on the catholyte developed. 

In addition, researchers confirmed that Br^- added to the electrolyte generated polar iodine bromide (IBr) during the charging process, which facilitated the reaction with H₂O to form IO₃^-. During the discharge, IO₃^- oxidized Br^- to Br₂ and participated in the electrochemical reaction to realize reversible and rapid discharge of IO₃^-. Therefore, the bromide intermediate formed during the charging and discharging process optimized the reaction, effectively improving the kinetics and reversibility of the electrochemical reaction. 

Prof. FU’s group proved the multielectron transfer process through in-situ optical microscopy, Raman spectroscopy, etc. 

“This study provides a new idea for the design of high-energy-density aqueous batteries, and may expand aqueous battery applications in the power battery field,” said Prof. LI.