"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. If you preferred safety, you could use an aqueous battery such as nickel/metal hydride, but you would have to settle for lower energy," said Dr. Kang Xu, ARL fellow who specializes in electrochemistry and materials science. "Now, we are showing that you can simultaneously have access to both high energy and high safety."
Many research teams are looking at solid state batteries to tackle the problem but the liquid cells can be more flexible.
"This is the first time that we are able to stabilize really reactive anodes like graphite and lithium in aqueous media," he said. "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."
Previous research had produced a system that reached 3V. To make the leap from three volts to four, University of Maryland assistant research scientist Chongyin Yang designed a new gel polymer electrolyte coating that can be applied to the graphite or lithium anode.
The hydrophobic coating expels water molecules from the vicinity of the electrode surface and then, upon charging for the first time, decomposes and forms a stable mixture of breakdown products that separates the solid anode from the liquid electrolyte, called the interphase. This protects the anode and allows 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," said Chunsheng Wang, Professor of Chemical & Biomolecular Engineering at the University of Maryland's A. James Clark School of Engineering.