The lithium-ion batteries that power most of our devices today are little powerhouses of energy. It’s estimated that they’re contained in about nine billion portable electronic devices globally.
The batteries are relatively simple, consisting of two electrodes (anode and cathode) separated by electrolyte. Unfortunately, they’re also flammable, toxic and sensitive to ambient atmosphere, which has made them unsuitable for large-scale applications or extreme environments.
Significant efforts in engineering have reduced the risk of fire and explosion in portable devices, but a few well-publicized cases occur every year. This has inspired extensive research to mitigate or eliminate the risk. In some cases, it has led to attempts to create an aqueous lithium-ion battery that uses water as a natural replacement for typical flammable non-aqueous solvents.
While aqueous lithium-ion batteries that won’t explode have been developed in the past, they’ve typically not been able to compete with their non-aqueous competitors in terms of energy density because of the narrow electrochemical stability window of water: essentially, what makes it safe limits its power. Previous efforts have run into what’s known as a “cathodic challenge” because the aqueous batteries were inherently unable to use the most ideal anode materials such as graphite and Li metal.
Now, researchers from the University of Maryland and the U.S. Army Research Laboratory have developed a new water-based lithium-ion battery that can reach the critical 4.0 volt threshold without the danger of explosion and fire inherent in non-aqueous lithium-ion batteries. The research is built on a new class of aqueous electrolyte, “water-in-salt” electrolytes (WiSE), named for their high salt concentration.
|Researchers from the University of Maryland and the U.S. Army Research Laboratory have developed a new water-based lithium-ion battery that can reach the critical 4.0 volt threshold without the danger of explosion and fire inherent in non-aqueous lithium-ion batteries.|
The prototype UMD/USARML battery uses an aqueous solid-electrolyte-interphase (SEI) that stabilizes graphite and lithium-metal anodes in the aqueous electrolyte. The research team was able to get around the “cathodic challenge” with what they call an “inhomogeneous additive” approach: a fluorinated additive immiscible with aqueous electrolyte in the form of a gel was applied on anode surfaces to act as an interphase precursor coating to reduce the competitive water reduction during interphase formation (essentially minimizing water molecules at the anode surface before the SEI forms, creating a favorable environment for interphase.) The protective coating permitted high-capacity/low-potential anode materials to couple with different cathode materials to produce batteries with higher efficiency.
Co-senior author of the research Dr. Kang Xu, an Army Research Laboratory fellow who specializes in materials science and electrochemistry, told Design News that what the team did was eliminate the fuel in a lithium-ion battery.
“The risk of explosion comes from thermal run-away due to abuse, which results in the catching fire of non-aqueous electrolytes (the risk comes from the combination of high energy electrode and flammable electrolytes),” he said. “By making electrolytes aqueous, we removed ‘fuel’ from the combination.”
While most of us put down our devices and rest them on occasion, devices used in a military setting, for