Safer non-explosive chemistry offers 4V output, key for commercial electronic devices
Researchers at the US Army Research Laboratory (ARL) and the University of Maryland (UMD) have developed a new form of lithium-ion battery which is both safe and flexible.
The battery, which uses a water-salt solution as its electrolyte, is capable of the 4V output desired for household electronics, such as laptop computers – but without the potential risk of fire and explosion associated with some commercially available non-aqueous lithium-ion batteries.
The results are described in a new paper in Joule, an interdisciplinary energy journal.
According to co-senior author Dr. Kang Xu, an ARL fellow specialising in electrochemistry and materials science, this brings a “completely safe and flexible Li-ion battery that provides identical energy density as the SOA Li-ion batteries. The batteries will remain safe – without fire and explosion – even under severe mechanical abuses.”
“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,” Xu said. “Now, we are showing that you can simultaneously have access to both high energy and high safety.”
Previous efforts had produced a 3V battery with an aqueous electrolyte, but the team was prevented 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 lead author 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 when charged for the first time decomposes and forms a stable interphase.
This 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 UMD Professor of Chemical & Biomolecular Engineering Chunsheng Wang.
The gel coating increases overall energy density compared with any other proposed aqueous lithium-ion batteries so far.
While all aqueous li-ion batteries benefit from the non-flammability of water-based electrolytes, this approach has an even greater advantage. Even when the interphase layer is damaged (e.g. if the battery casing were punctured), it reacts slowly with the lithium or lithiated graphite anode, preventing the smoking, fire, or explosion that could otherwise occur if a damaged battery brought the metal into direct contact with the electrolyte.
While this system could be adapted for commercial use as it is, it is not yet commercially competitive. The researchers say they would also like to increase the number of charging cycles and reduce material expenses where possible. “Right now, we are talking about 50–100 cycles, but to compare with organic electrolyte batteries, we want to get to 500 or more,” Wang said.
However, the achievement of the 4V output is a major step forward. “This is the first time that we are able to stabilise really reactive anodes like graphite and lithium in aqueous media,” Xu 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.”
Although more work must be done on scaling up the technology in larger cells, with sufficient funding, the 4-volt chemistry could be ready for commercialisation in about five years, he said.