A novel approach to an energy storage device run on an aqueous electrolyte can go from flat to fully-charged in just 20 seconds, making it perfect for portable electronics that frequently need a quick boost.
While the concept isn’t new, previous attempts have resulted in devices that suffer from low power and short working lives.
We ask for a lot of power storage tech these days. Not only must it be compact, powerful, long-lasting, and quick to recharge, it also has to be environmentally friendly. Oh, and preferably not blow up if you happen to chew on it.
For those and other reasons, aqueous storage devices – those that contain water-based solutions rather than a mush of toxic or flammable organic paste – have gained some serious attention as safe and reliable options.
Lithium-ion batteries are commonly used in small portable electronics. However, their large-scale application still causes safety, environmental, and cost concerns. These result from the use of highly inflammable and toxic non-aqueous electrolytes. Aqueous electrolytes as safe and green alternatives are only stable at small voltage outputs, as the water is split into hydrogen and oxygen at higher voltages.
Although less flammable than modern lithium batteries and potentially a whole lot cheaper, the way the solution carries electrons introduces a serious problem.
The cells that make up a battery work by transferring electrons between two materials. Aqueous solutions limit the voltage range between the two points more than other solutions, resulting in the anode being eaten away faster.
That makes for a poor life span and low amounts of power – not exactly great for reliably pushing your latest smart device through the day.
Chunsheng Wang, University of Maryland, College Park, USA, Kang Xu, U.S. Army Research Laboratory Electrochemistry Branch, Adelphi, MD, USA, and colleagues built on “water-in-salt” electrolytes that they had previously developed and found that by incorporating a second salt, lithium trifluoromethane sulfonate (LiOTf), the stability window could be further expanded. This new class of “water-in-bisalt” aqueous electrolytes allows the use of a more energy-intense anode (anatase titanium dioxide).
The team assembled a full lithium-ion battery delivering over 100 Wh/Kg at an average of 2.1 V, which is unprecedented for aqueous Li-ion batteries. Combined with its environmentally safe nature, the researchers believe that this new electrolyte would have a significant impact on battery technology in large applications such as automotive and grid-storage batteries.
Researchers at Korea Advanced Institute of Science and Technology (KAIST) have put a new spin on the system, modifying the way a device called an aqueous hybrid capacitor (AHC) is constructed. Hybrid capacitors such as these are essentially a mix of battery and capacitor – with electrodes that store their power electrochemically as an electrostatic charge. Adding an aqueous solution of ions inbetween can help carry the current.
By using graphene-based polymers instead of more traditional metallic conductors on the anode turns out to be far more efficient at transferring electrons into the aqueous solution, allowing for batteries with more than 100 times the power density than previous devices while still sustaining capacity for over 100,000 charges. Better yet, the new anodes coupled with liquid electrolytes mean the whole thing can go from zero to 100 perfect with just 20 seconds of charging. All of this is at no cost to its safety or economics.
Since the power source doesn’t need to be a strong one, its rapid rate of charging might see it couple up neatly with photovoltaic cells or other micro-generating power sources.
It’ll be a while before we see these kinds of devices outcompete the likes of lithium ion batteries, but cheap cells that can handle extreme conditions without catching fire will no doubt find a place in future portable technology.
Who knows where quick-charge aqueous batteries will fit among large storage technologies. But given the rate at which cheap, power-hungry smart devices are spreading, we’re going to need more like them.
