In Search of a Greener Way to Recycle EV Batteries
Chemical engineering seniors explore particle density separation as a sustainable way to recover precious metals from lithium-ion battery black mass
Electric vehicles (EVs) represent a green revolution in automotive transportation over conventional internal combustion engine vehicles. Poised to reduce the world’s reliance on fossil fuels, EVs are meant to eliminate the carbon dioxide emissions that lead to air pollution, climate change and global warming.
The need for the solution that EVs present is so vital, in fact, that governments around the world have prioritized them as an alternative to improve local and global environmental conditions. California, for example, has moved to ban the sale of emissions-producing vehicles in the state by 2035, while petrol and diesel cars will no longer be sold in the United Kingdom as of 2030. Paris, Athens, Madrid and Mexico City have all announced bans of diesel-powered vehicles in their city centers by 2025.
For all their environmental advantages, however, current options for recycling lithium-ion EV batteries and recovering the precious metals contained inside them are anything but green.
According to Stevens Institute of Technology chemical engineering seniors Caitlin Carroll ’23 M.E. ’23, Louise Gottwald ’23 M.E. ’23, Kaitlyn Rothwell ’23 and Pasquale Vizzoni ’23 M.S. ’24, recycling companies have failed to develop environmentally and economically sustainable methods for recovering the elements of black mass, the e-waste byproduct of EV batteries.
For example, said Vizzoni, one method “rips the battery apart, but you can't recover a significant amount of metals. There's also a method where they burn the battery down to its pure components, which would be the metals that wouldn't melt in the case. But that produces an insane carbon footprint.”
“Electric vehicles are kind of faulty in that sense,” he added. “The major target of manufacturers’ research, time and energy has been on perfecting the vehicle itself, rather than understanding what happens after the vehicle is done.”
The price of precious metals like lithium and nickel, meanwhile, has skyrocketed in recent years with the increased use, but insufficient reuse, of these finite resources in EV batteries.
While conducting research for their senior design project, the students came upon one method for recovering metals from black mass that had not been sufficiently explored: particle density separation, in which a liquid is used to separate different particles according to how dense they are.
“We didn't see any research of people's results of actually trying it,” said Gottwald, who led the team’s lab experiments, “so we decided to do it ourselves.”
In addition to reducing the costs of precious metals by increasing available supply, the team hoped to develop an effective, efficient and sustainable way to separate EV battery components that would ultimately support more sustainable methods of EV development, manufacturing and recycling in general.
“By looking into recovering these precious metals, reducing the need to mine for them as much, and reducing the carbon footprint and making electric vehicles more sustainable than they already are, we are looking to help the general world population from an environmental standpoint,” Carroll said.
The team is advised by Jae Chul Kim, assistant professor and coordinator of graduate studies in the Department of Chemical Engineering and Materials Science.
A pressing problem
Given the popularity of EVs in the last decade, one would think the problem of EV battery recycling would have already been solved. This last decade of popularity, however, actually helps explain why the problem has now become so pressing: because EV batteries generally reach their end-of-life within 10 years.
Rothwell explains this limited shelf life in familiar terms.
“With electric vehicles, I compare it to our [mobile] phones and how the battery starts to have less of a capacity as the phone gets older. My battery now doesn't even last, and it's only a two-year-old phone,” she explained. “It's the same thing with a car. Manufacturers always tell you not to charge it to 100%, but there's only so much range on these EVs, so you have to charge to 100 if you're doing a long trip. That constant charging and use of the battery really wears down on it, and then the battery just doesn't have the capacity anymore.”
Older batteries that are no longer appropriate for use in EVs do sometimes find a second life in other purposes, said Carroll, such as generating electricity for people’s homes or the power grid. But such battery reuse also has its limits.
“Eventually they reach a capacity where they can't be used, and then we're still left with the battery that needs to be recycled,” she said.
A solution in solution
To conduct their particle density difference experiments, the team developed their own synthetic black mass.
Black mass is the e-waste that results when used batteries are shredded and ground down to a fine black powder. This powder contains two of the primary components that make up a battery: an anode, which is primarily composed of carbon, and a cathode, which contains the precious metals like lithium, cobalt, manganese and nickel that recyclers seek to recover for reuse.
“When we make the anode and the cathode in the lab, what goes into the cathode is the precious metals and some carbon, and the anode is two different types of carbon: graphite and Super P. We combine these together, and it mimics what you find in a real battery,” Gottwald said.
To separate the higher density metals of the cathode from the lower density carbon, the students combined their synthetic black mass with an environmentally friendly high-density solution called LST (a standard, off-the-shelf product commonly used for particle density separation). With the help of gravity and time, LST facilitates the black mass’s particles to separate according to their density.
“Water has a density of 1 [g/mL],” explained Gottwald. “LST has a density of 2.8, so it's optimal to be used for our process because the cathode particles are more dense than that, so the precious metals that we want to be able to recover will sink to the bottom and be the bottom layer. The anode particles — our carbon — are less dense than that, so they'll float up to the top. In the middle, we have that liquid.”
With the anode and cathode successfully separated, the students theorized, they could then filter the precious metals from the suspension and recover them for reuse.
The team combined 40 mL of LST with 0.5 grams of black mass and left them to separate in a funnel for 3 days. The process was repeated five times in total, with improvements applied with each new round.
“Our first run-through of the separation trial, we poured our black mass straight into our separatory funnel, and the separation wasn't that good,” explained Carroll. “The next time we decided to mix up the black mass and liquid beforehand, and that had better results. Little things like that, that we just didn't know we should do beforehand, we found out along the way.”
The seniors also found themselves pivoting when their original method of analysis — X-ray diffraction analysis (XRD) — turned out to be insufficient for their purpose.
“X-ray diffraction analysis will tell you exactly what elements are in a sample that you're looking at. But XRD tells you if elements are in a sample, not necessarily how much of that element is in a sample,” explained Gottwald. “So since our separation wasn't exactly perfect — our precious metals are contaminated with a little bit of carbon — XRD picks that up, and none of the graphs were really telling us that much about the success of our process. We ended up scrapping the XRD analysis because it was too broad.”
To remedy the situation, the team switched to scanning electron microscopy (SEM) to analyze their test results. Images from the SEM analysis of the layers from their experiments showed a significant difference in concentration of cathode materials in the two layers.
“Separation was generally successful, but it's not perfect. There's more work that needs to be done. But we were happy with what we got in the amount of time that we had to work on it,” said Gottwald.
Although happy with their iterative improvements, Rothwell expressed frustration that the limited timeframe of the two-semester project constrained the team’s ability to perfect their process.
“Our last separation — the last SEM results — were the best overall. It was fun to see that that last one was the best, but we were like, Dang, if we had more time, how good could we have gotten this here in the lab?” she said.
A business angle
Additionally, the team advanced to compete as one of 45 teams in the semifinal round of the Ansary Entrepreneurship Competition. The competition allows senior design teams to present the viability of their projects as a potential business venture.
“In our Innovation, Design and Entrepreneurship class, we gave a three-minute pitch in front of the class. It's kind of like a Shark Tank pitch about our project where we have to say how we would market our product, what we would do on the business side and present our ask, which is an investment into our company,” said Rothwell.
The team’s angle, she said, was to patent their density separation process and market and sell that process to either electric vehicle or battery manufacturers.
Their process is also applicable for recycling materials from all types of lithium-ion batteries, including consumer-grade AAs and AAAs.
"While lithium-ion battery recycling will be the most needed technology in 10 years, the development is still in its infancy due to low efficiency of element recovery," advisor Kim said. "All the members of this team were motivated to address this technically important problem, and their achievement reflects their teamwork and enthusiasm toward the environment and ensuring a sustainable future."
Powering a sustainable future
The team sees great potential for future improvements and commercialization of the particle separation method they’ve conceived and developed. They’ve already begun documenting their recommendations in the hope of preparing another team of seniors to continue their work next year, Carroll said.
Scalability of the process, said Vizzoni, is of the utmost importance for developing their smaller scale method into a large-scale solution that is feasible for implementation in an industrial setting where hundreds of thousands of batteries are being processed.
“With how that process works and the logistics of it — the speed, what we would need and how often we would need to be filtering and drying and testing — we would need to have a perfect separation of 100%,” he said.
As for the seniors, after graduation each team member will use their background in chemical engineering as a springboard to launch them into new areas.
In the fall Rothwell will begin law school at the University of Miami, while Carroll, Gottwald and Vizzoni will return to Stevens to complete master’s degrees through the Accelerated Master’s Program.
Vizzoni, who has a position lined up with the British banking firm Barclays this summer, will study financial technology and analytics with a concentration in data science through the School of Business. Carroll and Gottwald will both earn degrees in engineering management through the School of Systems and Enterprises. After completing their master’s degrees in December 2023, Carroll will join the water management company Nalco Water (an Ecolab company), while Gottwald is seeking a full-time position in public transportation, ideally with the Port Authority of New York and New Jersey.
This Expo project, she said, aligns well with her desire to develop more sustainable methods of transportation at both a micro and macro level.
“I think public transportation is really cool, and a solution to ‘why use electric vehicles if they're not as sustainable as we think’ could be to put electric vehicles into public transportation,” Gottwald said. “Have people actually use public transportation, and there are fewer vehicles in general.”
On a personal level, the team agreed that the experience was a positive one.
“We all learned a lot from it,” said Rothwell. “We essentially created this entire idea, so it was fun that we got to do whatever we wanted, to be able to do it on our own and create an idea and watch it turn into an actual feasible thing that worked.”