Aqueous Battery’s Energy Storage Capacity Blows Li-ion Cells Out of the Water

Traditional lithium-ion batteries, despite their high energy density, pose significant safety risks due to their use of flammable organic electrolytes. In contrast, aqueous batteries, which use water-based electrolytes, are much safer. However, these batteries have historically suffered from lower energy density, limiting their ability to store electricity efficiently.

In a groundbreaking study published in Nature Energy, a research team 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, has made significant strides in addressing this challenge. They developed a multi-electron transfer cathode based on bromine and iodine, achieving a remarkable specific capacity of over 840 Ah/L and an energy density of up to 1200 Wh/L in full battery testing.

To give an idea of just what a breakthrough that energy density is: The energy storage capacity of lithium-ion battery cells typically ranges from about 200 Wh/L to 700 Wh/L, depending on the specific chemistry and design of the battery. High-performance lithium-ion batteries used in consumer electronics and electric vehicles often fall within the upper part of this range, around 500-700 Wh/L. This aqueous battery, at 1200 Wh/L is around double that figure.

A halogen solution

The researchers tackled the energy density issue by using a mixed halogen solution of iodide (I-) and bromide (Br-) ions as the electrolyte. This innovation enabled a multi-electron transfer reaction: iodide ions are converted to iodine (I2) and then to iodate (IO3-) during charging. In this process, iodide ions are oxidized to iodate on the positive electrode, with hydrogen ions (H+) moving to the negative electrode as part of the supporting electrolyte. During discharge, the reverse occurs: hydrogen ions move back to the positive electrode, and iodate is reduced to iodide.

This multi-electron transfer cathode demonstrated a specific capacity of 840 Ah/L. When combined with metallic cadmium to form a complete battery, the researchers achieved an energy density of up to 1200 Wh/L based on the new catholyte.

Moreover, the inclusion of bromide in the electrolyte was found to generate iodine bromide (IBr) during charging, which reacts with water to form iodate. During discharge, iodate can oxidize bromide to bromine (Br2), facilitating a reversible and rapid discharge of iodate. This bromide intermediate optimized the reaction process, significantly enhancing the kinetic and reversibility of the electrochemical reaction.

Prof. FU's team validated the multi-electron transfer process using in-situ optical microscopy and Raman spectroscopy, among other techniques.

"This study opens up new possibilities for the design of high-energy-density aqueous batteries and may broaden their application in power batteries," said Prof. LI.

This advancement in battery technology not only promises safer and more efficient energy storage but also positions aqueous batteries as a viable option for a wider range of power applications.