Collaborating with the military, University of Maryland researchers made a 4-volt lithium-ion battery that runs on an aqueous-based electrolyte. With no organic solvents in its composition, the battery can’t possibly ignite or explode like the typical non-aqueous lithium-ion variety. OK, now someone please send Samsung a memo.
“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,” says co-senior author Kang Xu, a lab fellow at the U.S. Army Research Laboratory specializing in electrochemistry and materials science.
“Now, we are showing that you can simultaneously have access to both high energy and high safety.”
Safer batteries for everyone
This video shows how when punctured repeatedly with a nail, a four-volt aqueous lithium-ion battery initially maintains its voltage, and no fire, smoke, or explosion occurs. This contrasts with the instantaneous short-circuit and explosive risk of an analogous non-aqueous battery.
Previously, the same team demonstrated an aqueous battery back in 2015. That version could deliver only 3 volts but, worst of all, also had a very poor cycling ability due to the so-called ‘cathodic challenge’. The problem with using salt-water solutions as electrolyte is it messes up one of the two ends of the battery (anode or cathode electrode).
Not only was the aqueous-based lithium-ion battery upgraded to 4 volts, it also features an innovative coating on the electrode’s surface that avoids its degradation.
The hydrophobic coating is a carefully fashioned gel that expels the water molecules from the electrode’s surface. When the battery is charged for the first time, the coating decomposes and forms an interphase that permanently separates the solid anode from the liquid electrolyte.
Moreover, the interphase greatly enhances water’s rather narrow electrochemical stability window, which is ∼1.23 V under thermodynamic equilibria. With the electrodes kinetically protected, the researchers manage to come up with a battery that operates beyond the above limit.
It’s important to mention at this point that the electrolyte has to also contain salt. The video below shows a lithium metal reacting violently with water, but slowly and undetectably with water-in-salt electrolyte (sWiSE).
Protected from debilitating reactions with water, the battery can now also use anode materials with desirable properties such as graphite or lithium metal that offer 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 co-senior author Chunsheng Wang.
The greatest upside of this kind of battery is safety. It’s basically impossible for it to catch fire or down-right explode, unlike typical lithium-ion batteries based on highly flammable organic solvents. Just take a look at this Stanford study which visually shows how thermal runaway causes battery components to melt at temperatures exceeding 1,085°C.
Even if the battery is punctured, the interphase layer reacts with the lithium or lithiated graphite anode preventing any smoking or fire, the authors reported in the journal Joule.
There are, of course, drawbacks. One would be cycling ability, which severely degrades after 100 charges. Wang and colleagues, however, think they go get to 500 cycles or more which would make the battery competitive with organic electrolyte batteries.
“This is the first time that we are able to stabilize really reactive anodes like graphite and lithium in aqueous media,” says 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.”