Researchers at the U.S. Army Research Laboratory (ARL) and the University of Maryland have developed what they claim is the first lithium-ion battery that uses a water-salt solution as the electrolyte.
The battery is able to reach the 4.0-volt market that can power most household electronics such as laptop computers, without the fire or explosive risks associated with widely available non-water-based lithium-ion batteries.
Kang Xu, a fellow that specializes in electrochemistry and materials science at ARL, said the technology will allow for a safe and flexible lithium-ion battery that provides identical energy density to standard lithium-ion batteries but without the risk of something going wrong.
"In the past, if you wanted high energy, you would choose a non-aqueous lithium-ion battery, but you would have to compromise on safety,” Xu explained. “If you preferred safety, you could use an aqueous battery such as nickel/metal hydride, but you would have to settle for lower energy. Now, we are showing that you can simultaneously have access to both high energy and high safety."
A new gel polymer electrolyte coating was developed that is applied to the graphite or lithium anode. This helps make the leap to higher voltage and helps expel water molecules from the vicinity of the electrode surface and then, upon charging for the first time, decomposes and forms a stable interphase — a thin mixture of breakdown products that separates the solid anode from the liquid electrolyte.
This interphase protects the anode from debilitating side reactions, allowing the battery to use desirable anode materials, such as graphite or lithium metal, and achieve better energy density and cycling ability.
"The key innovation here is making the right gel that can block water contact with the anode so that the water doesn't decompose and can also form the right interphase to support high battery performance," says Chunsheng Wang, professor of chemical and biomolecular engineering at the University of Maryland's A. James Clark School of Engineering.
The gel coating also boosts the safety of the new battery because of the inflammability of water, and boosts energy density compared to other aqueous batteries. If the interphase layer is damaged, it reacts slowly with the lithium or lithiated graphite anode, preventing smoking, fire or explosions that would otherwise occur if a damaged battery brought the metal into direct contact with the electrolyte, researchers said.
Before this battery can be ported to commercial applications, researchers added, improvements would need to be made to make the battery more competitive. These improvements include increasing the number of full-performance cycles that the battery can complete. Currently, the battery has about 50 to 100 cycles but the researchers want to increase this to 500 or more. The interphase chemistry also needs to be perfected before it can be commercialized, including scaling up the technology in big cells for testing.
"This is the first time that we are able to stabilize really reactive anodes like graphite and lithium in aqueous media," Xu says. "This opens a broad window into many different topics in electrochemistry, including sodium-ion batteries, lithium-sulfur batteries, multiple ion chemistries involving zinc and magnesium, or even electroplating and electrochemical synthesis; we just have not fully explored them yet.”
The full research can be found in the journal Joule.