New cells spell lower risk
US researchers have developed a lithium-ion battery with similar characteristics to currently-available cells, but without the same fire and explosive risks.
The team says its lithium-ion battery uses a water-salt solution as its electrolyte and reaches the 4-volt mark desired for household electronics, such as laptop computers, but does not have the same propensity to explode as commercially-available non-aqueous lithium-ion batteries.
Until now, high-energy devices have used non-aqueous lithium-ion batteries, but there is a safety compromise.
When safety is paramount, an aqueous battery such as nickel/metal hydride is preferred, but these come with lower energy capabilities.
Previous studies have described batteries with aqueous electrolytes that were able to hit around three volts before one end of the battery, made from either graphite or lithium metal, was degraded by the aqueous electrolyte.
To solve this problem and make the leap from three volts to four, the team designed a new gel polymer electrolyte coating that can be applied to the graphite or lithium anode.
This hydrophobic coating expels water molecules from around 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 addition of 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.
Aqueous lithium-ion batteries use water-based electrolytes as opposed to the highly flammable organic solvents used in their non-aqueous counterparts.
Unique to this one, however, is that even when the interphase layer is damaged (if the battery casing were punctured, for instance), 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.
Though the power and energy density of the new battery are suitable for commercial applications currently served by more hazardous non-aqueous batteries, certain improvements would make it even more competitive.
In particular, the researchers would like to increase the number of full-performance cycles that the battery can complete and to 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,” senior co-author Dr Chunseng Wang says.
“This is the first time that we are able to stabilize really reactive anodes like graphite and lithium in aqueous media,” says co-senior author Kang Xu.
“This opens a broad window into many different topics in electrochemistry, including sodium-ion batteries, lithium-sulphur batteries, multiple ion chemistries involving zinc and magnesium, or even electroplating and electrochemical synthesis; we just have not fully explored them yet.”