By spontaneously forming an SEI on the surface of the anode, the ‘water-in-salt’ electrolyte is able to extend the operational voltage window for this novel aqueous battery to approximately 3 Volts. Image: University of Maryland.
By spontaneously forming an SEI on the surface of the anode, the ‘water-in-salt’ electrolyte is able to extend the operational voltage window for this novel aqueous battery to approximately 3 Volts. Image: University of Maryland.

A team of researchers from the University of Maryland (UMD) and the US Army Research Laboratory (ARL) have devised a groundbreaking ‘water-in-salt’ aqueous lithium-ion (Li-ion) battery technology. This novel technology could provide the power, efficiency and longevity of today's Li-ion batteries, but without the fire risk, poisonous chemicals and environmental hazards.

The team of researchers are led by Chunsheng Wang, an associate professor in UMD's Department of Chemical & Biomolecular Engineering, and Kang Xu, senior research chemist at the Sensor and Electron Devices Directorate of ARL. In a paper in Science, the researchers say their work represents a major advance in the long history of water-based (aqueous) batteries, by doubling the voltage, or power, of an aqueous battery.

According to the researchers, this technology holds great promise in applications that involve large energies at kilowatt or megawatt levels, such as electric vehicles or grid-storage devices for energy harvest systems. It also holds promise for applications where battery safety and toxicity are primary concerns, such as safe, non-flammable batteries for airplanes, naval vessels or spaceships, and in medical devices like pacemakers.

"Through this work we were able to increase the electrochemical window of aqueous electrolyte from less than 1.5 Volts to ~ 3.0 Volts and demonstrated high voltage aqueous full lithium-ion cell with 2.3 Volts, showing for the first time that aqueous batteries could seriously compete in terms of power and energy density with the non-aqueous lithium-ion batteries that power our mobile, digital lifestyle" said Wang, who also is affiliated with the UMD Energy Research Center and the Maryland NanoCenter.

According to Lt Col (Retired) Edward Shaffer, who heads the ARL’s Energy and Power Division, this new approach has significant potential advantages over current batteries. "[It] could lead to thermally, chemically and environmentally safer batteries carried and worn by soldiers; safe, reduced-footprint energy storage for confined spaces, particularly submarines; and novel hybrid power solutions for military platforms and systems," he said.

The key to this breakthrough is a type of water-based electrolyte containing ultrahigh concentrations of a carefully selected lithium salt. This approach transformed the battery's chemistry, resulting in the formation of a thin protective film on the anode electrode for the very first time in a water-based battery. Known in battery science as a solid electrolyte interphase (SEI), this protective and stabilizing film is essential to the high performance characteristics of state-of-the-art Li-ion batteries, but has only previously been achieved in non-aqueous electrolytes.

"What's most important about our work is the breakthrough made at the fundamental level," said UMD postdoctoral research associate Liumin Suo, who is a member of Wang's research group and first author of the Science paper. "Prior to this work no one thought it possible to form SEI in water-based [batteries], but we demonstrated that it can happen."

The UMD and ARL team compared the performance of their new ‘water-in-salt’ battery to that of other aqueous battery systems. They showed that in the other aqueous batteries high stability could only be achieved at the expense of voltage and energy density, and vice versa. However, the formation of an anode/electrolyte interphase in their ‘water-in-salt’ electrolyte allowed them to break this inverse relationship between high stability and high voltage, allowing them to achieve both simultaneously.

"Researchers in the Li-ion battery field have recently found that previously ‘useless’ solvents could be made functional in Li-ion cells through the addition of high concentrations of salts, " commented Jeff Dahn, a leading battery researcher at Dalhousie University in Canada, who was not involved in the study. "The work by Suo et al. extends this idea to the case of the solvent water. By extending the operational voltage window to approximately 3 Volts, it is possible that a new generation of safer and possibly less expensive Li-ion cells could result."

"Only further R&D efforts will be able to verify the practicality of this discovery, so prudence is needed in assessing the potential of this, or any basic research advance," he added.

"Our finding opens an entirely new avenue to aqueous electrochemical devices, not only batteries, but also devices like supercapacitors and electroplating devices," said Xu.

This story is adapted from material from the University of Maryland, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.