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 example, may often be used to “abuse” levels. For these critical applications, the research tea sought to create a battery with improved thermal and chemical stability that can be safely pushed to its limits during intense use.
The main challenge to create this battery was finding a way to enhance the cycle life from approximately 80 cycles to 500 to 1000 cycles. Previous efforts to create a high-power aqueous Li-ion battery, including a 3.0 volt battery created by the same research team in 2015, resulted in batteries that either had low energy density or were not intrinsically safe due to solid-electrolyte protected aqueous Li metal cells that presented the risk of fire if the protected layer was broken. The new prototype eliminates both drawbacks.
“We use a solid electrolyte interphase (SEI) to separate a water-in-salt electrolyte from a graphite anode,” Dr. Chunsheng Wang, a professor of chemical and nuclear engineering at the University of Maryland, told Design News. “The SEI can self-heal and the reaction between lithiated graphite with the water-in-salt electrolyte is very slow even if the SEI is broken. The unique approach of inhomogeneous electrolyte additive expels water from the electrode surface, and forms a dense protection that stabilizes water at the extreme potential of four volts.”
Once Xu and Wang, together with UMD assistant research scientist Chongyin Yang, developed the prototypes, extensive abuse testing of the batteries was carried out, including puncturing charged cells with a nail to initiate a short circuit. Under testing, the batteries produced no fire or smoke, unlike their conventional Li-ion cell counterparts which became dangerous due to direct contact between the metal and the electrolyte.
In addition to military uses, the new batteries will likely find significant applications in the aerospace industries or in any place where safety outweighs the concerns of energy density or battery cycle life, such as confined spaces such as submarines. The team’s next steps, according to Drs. Wang and Xu, will be to perfect the interphase chemistry to extend the cycle life of the aqueous batteries, and demonstrate that they are intrinsically nonflammable by making larger cells. With more research and the right funding, the batteries could be ready for commercial markets within five years.
The team’s research was published in the September 6, 2017 issue of Joule.