Just add water: lithium-ion batteries without explosive risks

US researchers have developed a lithium-ion battery that uses a water-salt solution as its electrolyte, with their results published in the journal Joule. Not only does the battery reach the 4 V mark desired for household electronics, such as laptop computers, it does so without the fire and explosive risks associated with some commercially available non-aqueous lithium-ion batteries.

“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,” said Dr Kang Xu, a lab fellow at the US Army Research Laboratory and co-senior author on the study. “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.”

The research follows a 2015 study in the journal Science that produced a similar 3 V battery with an aqueous electrolyte but was stymied from achieving higher voltages by the so-called ‘cathodic challenge’, in which one end of the battery, made from either graphite or lithium metal, is degraded by the aqueous electrolyte. To solve this problem, 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.

This 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 interphase — a thin mixture of breakdown products that separates the solid anode from the liquid electrolyte. This interphase, inspired by a layer generated within non-aqueous batteries, 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,” said co-senior author Chunsheng Wang, a professor of chemical & biomolecular engineering at the University of Maryland. The gel coating also boosts the safety advantages of the new battery when compared to standard non-aqueous lithium-ion batteries and boosts the energy density when compared to any other proposed aqueous lithium-ion batteries, according to the researchers.

All aqueous lithium-ion batteries benefit from the inflammability of water-based electrolytes, as opposed to the highly flammable organic solvents used in their non-aqueous counterparts. The new battery is unique, however, in that even when the interphase layer is damaged (eg, if the battery casing was punctured), it reacts slowly with the lithium or lithiated graphite anode. This prevents the smoking, fire or explosion that could otherwise occur if a damaged battery brought the metal into direct contact with the electrolyte.

This technology will thus bring the soldiers a “completely safe and flexible Li-ion battery” that provides identical energy density to state-of-the-art lithium-ion batteries, according to Dr Xu, who said the batteries “will remain safe — without fire and explosion — even under severe mechanical abuses”.

4 V Li-ion batteries assembled with a water-in-salt gel electrolyte. Image credit: Jhi Scott, ARL photographer.

According to the researchers, the power and energy density of the new battery are suitable for commercial applications currently served by more hazardous non-aqueous batteries. The team also noted that the electrochemical manipulations behind the jump to four volts have importance within battery technology and beyond.

“This is the first time that we are able to stabilise really reactive anodes like graphite and lithium in aqueous media,” said Xu. “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 researchers acknowledged that they would also like to implement certain improvements in order to make their battery even more competitive, with Xu saying the interphase chemistry needs to be perfected before the product can be commercialised — hopefully in about five years. He and his fellow researchers say more work needs to be done on scaling up the technology in big cells for testing, and that they are looking to increase the number of full-performance cycles that the battery can complete.

“Right now, we are talking about 50–100 cycles, but to compare with organic electrolyte batteries, we want to get to 500 or more,” said Wang.

Top image credit: ©stock.adobe.com/au/alphaspirit