Every year, many are quick to upgrade when new versions of the latest smartphones, tablets or other electronic devices come out, but what happens to all the batteries from the older, discarded devices?
A Rice University research team believes that a deep eutectic solvent may be the answer to the growing battery-recycling problem. The solvent can extract the valuable elements from the metal oxides used as cathodes in lithium-ion batteries, eliminating the harsh processes used to recycle batteries and keeping them out of landfills.
Rice graduate student and lead author Kimmai Tran explained in an exclusive interview with R&D Magazine that the choline chloride and ethylene glycol solvent, which produces a “blue-green” solution that indicates the presence of cobalt, could transform how these batteries are recycled.
“We chose a couple different deep eutectic solvents to study and, of the ones we tested, we just found that this particular combination of choline chloride and ethylene glycol was the one that not only had the best results and the dissolution, it had a very beautiful color change that I just love to experiment with because you can visually see the solution happening,” Tran said.
A deep eutectic solvent is a mixture of two or more compounds that freezes at temperatures much lower than each of its precursors, enabling researchers to obtain a liquid from a simple combination of solids.
The new solvent is harmless to humans and completely biodegradable. At 180 degrees Celsius,, the solvent extracted nearly 90 percent of lithium ions and up to 99 percent of cobalt ions from powdered compounds when certain conditions were satisfied.
To test the technology, the Rice team built small prototype batteries that were cycled 300 times before exposing the electrodes to the same conditions. The new solvent was able to dissolve the cobalt and lithium, while also separating the metal oxides from other compounds that were present in the electrode.
The researchers also found that the cobalt could be recovered using both precipitation or electroplating to a steel mesh, which could allow for the solvent itself to be recycled and reused.
Another advantage of the solvent is its availability, as the choline chloride is commonly used as a chicken feed additive and the ethylene glycol is widely used as a plastic precursor.
While the new technology showed promise, Tran explained that there were challenges that put some doubt as to whether it could be implemented on a wide-scale.
“One of the biggest problems was really trying to play with the heat components,” she said. “If we have to use this in a mass scale, the energy it takes to dissolve this compound is going to be important. One of the hurdles that we were trying to [overcome] is to modify the procedure such so that we could lower the temperature process part of it.”
With more and more electronic devices being developed annually and electric cars becoming increasingly popular, more and more lithium ion batteries will be thrown out. Without a true recycling plan, this could result in a substantially negative environmental impact.
Tran said that there are currently two recycling strategies commonly used for lithium-ion batteries—pyrometallurgy and hydrometallurgy. These processes can involve high temperatures and caustic chemicals that put the workers at risk.
Tran explained that currently, the U.S. is lagging behind European countries in finding better ways to recycle batteries. To combat that, the U.S. Department of Energy recently launched a competition with $5.5 million in prize money to incentivize innovators and businesses to develop new technology to capture 90 percent of all discarded or spent lithium-ion batteries in the U.S. The researchers indicated that they would be submitting a proposal for the contest.
Tran said that there is a common misconception that lithium-ion batteries are truly a “green” technology, but there are several issues involved in the recycling process of these batteries that prevent them from being anenvironmentally-friendly option.
“When people think of lithium ion batteries, they herald it as this green technology when in fact we have heavy metals that we use in the chemistries of multiple types of lithium ion batteries,” Tran said. “Cobalt, for example, is carcinogenic and it is also the most expensive part of most lithium ion battery chemistries out there.”
Rice research scientist and co-corresponding author Babu Ganguli explained in an interview with R&D Magazine that while lithium-ion batteries are technically greener than other alternatives, the problems arise when they reach the end of their use.
“Lithium ion batteries are greener when you compare them with gasoline engines,” he said. “When you replace that with a battery it is zero emissions that is why a lithium ion battery is a greener technology. The question remains, what do you do after you use your lithium ion battery and want to throw it out?”
Babu said that after experimenting with eutectic solutions as an electrolyte in next-generation high-temperature super capacitors in the lab of materials scientist Pulickel Ajayan, the researchers quickly realized they found a solvent that is able to dissolve metal oxides, giving them the idea that it could be used to recycling lithium-ion batteries.
“We observed that they were reacting with the metal oxides of super capacitor electrodes and we got an idea,” he said.
The researchers now plan to improve their process further so that it can be implemented on a wider scale.
“We know we can get a lot of concentrated cobalt within the solutions, but how to take out that cobalt in greater quantities in that solution is the next biggest challenge,” Tran said.
Babu also said they are doing a cost-benefit analysis to present to industry leaders, showing that ultimately the new technology will save them money.
The study was published in Nature Energy.