Revolutionary 'water battery' has almost double the energy capacity of lithium-ion cells
Aqueous batteries (also known as water batteries), long recognized for their safety due to water-based electrolytes, have struggled to compete with traditional lithium-ion batteries in terms of energy density.
This new study, published in Nature Energy by a research group led by Prof. LI Xianfeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), presents a significant leap forward.
The Challenge: Safety vs. Energy Density
Lithium-ion batteries power our portable electronics and are increasingly being explored for electric vehicles. However, their flammable organic electrolytes pose safety risks.
Aqueous batteries, with their non-flammable water-based electrolytes, offer a safer alternative. But they come with a trade-off: lower energy density, meaning they store less energy per unit volume.
The Innovation: Multi-Electron Transfer Cathode
Prof. LI's team addressed this limitation by developing a novel cathode material that utilizes a mixed-halogen solution (iodide and bromide ions) and undergoes a multi-electron transfer reaction.
During charging, iodide ions (I-) are oxidized to iodate (IO3-) on the positive electrode, while generated hydrogen ions (H+) travel to the negative electrode. Discharging reverses the process, with IO3- being reduced back to I-.
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This multi-electron transfer plays a key role. The researchers achieved a specific capacity exceeding 840 Ah/L for the cathode alone.
When combined with a metallic cadmium anode in a full battery configuration, the energy density reached an impressive 1200 Wh/L based on the newly developed catholyte (electrolyte solution for the cathode).
Bromide's Supporting Role
The presence of bromide ions (Br-) in the electrolyte proved crucial. During charging, Br- interacts with I- to form polar iodine bromide (IBr), which readily reacts with water to generate IO3-.
During discharge, IO3- oxidizes Br- to bromine (Br2), which then participates in the electrochemical reaction, promoting a reversible and rapid discharge process. Essentially, the bromide intermediate acts as a catalyst, optimizing the overall reaction and improving both reaction kinetics and reversibility.
Validation and Future Implications
Prof. FU Qiang's group, collaborators on the project, employed techniques like in-situ optical microscopy and Raman spectroscopy to confirm the multi-electron transfer process.
Prof. LI emphasizes the significance of this research: "This study offers a new direction for designing high-energy-density aqueous batteries, potentially expanding their applications in the realm of power batteries."
This study paves the way for a new generation of aqueous batteries with energy densities approaching those of lithium-ion batteries, while maintaining their inherent safety advantages.
The multi-electron transfer cathode design with mixed-halogen electrolytes presents a promising path forward for aqueous battery technology.
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