Using Cold Sintering to Develop Recyclable, Rechargeable Batteries

Andrew Corselli

Rechargeable solid-state lithium batteries are an emerging technology that could someday power cell phones and laptops for days with a single charge. Offering significantly enhanced energy density, they are a safer alternative to the flammable Li-ion batteries currently used in consumer electronics — but they are not environmentally friendly. Current recycling methods focus on the limited recovery of metals contained within the cathodes, while everything else goes to waste.

A team of Penn State researchers may have solved this issue. Led by Enrique Gomez, interim Associate Dean for Equity and Inclusion and Professor of Chemical Engineering in the Penn State College of Engineering, the team reconfigured the design of these solid-state lithium batteries so that all their components can be easily recycled. They published their findings in ACS Energy Letters.

Traditionally, most of the core battery components have gone to waste because they mix during the recycling process, forming a “black mass,” according to the researchers. This black mass is rich in materials needed for batteries but separating them out remains a challenge. In solid-state batteries, the use of solid electrolytes compounds this problem, as they become intermixed with the black mass.

To more easily separate these components from the other metal components in a coin cell battery, researchers inserted two polymer layers at the interfaces between the electrode and the electrolyte prior to the start of the recycling process.

“We proposed that by dissolving the polymer layer during the recycling process, you can easily separate the electrode from the electrolyte,” said First Author Yi-Chen Lan, doctoral student in chemical engineering. “Without the polymer layer separating them, you would have the electrode and electrolyte mixed together, which makes them hard to recycle.”

Once the researchers successfully separated out the components, they made a composite with the recovered metals and electrodes using cold sintering — the process of combining powder-based materials into dense forms at low temperatures through applied pressure using solvents. Cold sintering was developed in 2016 by a team of researchers led by Clive Randall, Director of Penn State’s Materials Research Institute and Distinguished Professor of Materials Science and Engineering. Gomez and his team recently demonstrated the recycling of solid-state electrolytes using cold sintering.

“We used cold sintering to combine the recovered electrodes with recovered composite solid electrolyte powders, then reconstructed the battery with the polymer layers added,” said Co-Author Po-Hao Lai, doctoral student in chemical engineering. “This enables us to recycle the whole battery, which we are then able to recycle again after its use.”

After testing its performance, they found that the reconstructed battery achieved between 92.5 percent and 93.8 percent of its original discharge capacity.

Here is an exclusive Tech Briefs interview, edited for length and clarity, with Gomez.

Tech Briefs: What was the biggest technical challenge you faced while developing this recyclable, rechargeable battery?

Gomez: The biggest challenge if you take a solid-state battery and try to recycle it is that you're trying to either A) prevent the creation of black mass, so prevent the creation of a mixed material that is then very challenging to reuse. Or B), alternatively, how do you then do separations to then reseparate all of the different components?

That, to me, is the biggest challenge, but one of the interesting concepts is: When I try to design the next battery or flexible cell phone or something like that, I don't always think about what's going to happen when I'm done with it.

A lot of times when we do research, we're trying to innovate for the next product without thinking about what's going to happen at the end of life. So, we don't necessarily always design with end of life in mind. I think what we try to do in this case is address that specific question that you asked, ‘What are the biggest challenges in recycling,’ by incorporating it into the design. We use, for example, interfacial layers that are designed to fall apart, to be able to enable easy separations of components. Plus, we leverage this cold-centering technology to recycle solid electrolytes. That allows us to recycle the solid-state electrolytes in a way that hasn't been previously demonstrated beyond our own prior paper to then allow the reprocessing of that key battery component.

Tech Briefs: Can you explain in simple terms how it works?

Gomez: We designed the battery to have breakaway layers. So, we put some polymer layers in between these cathode and electrolyte components to enable easy separation of the individual layers. Now, our approach, to be honest, is rather crude; it's really rudimentary. We just put a nice polymer electrolyte, something that could conduct ions, almost acts like an interfacial binder, but then could be attacked via solvents. I'm not saying that that's the ideal case of how you want to eventually upscale this, because you could think about, now, well if that's the key, that's the key concept to design a breakaway layer. We use all types of different chemistries and approaches to be able to do that, but we wanted to demonstrate that concept. Even if you do that, that means now instead of having one bucket of waste, now you have three buckets: electrolyte, anode, cathode.

How do you recycle each of those? There are different challenges associated with recycling some of those. For metal cathodes, like lithium, to some extent is more straightforward. We know how to recycle metals relatively well. Cathodes could be a little bit challenging in terms of composites as well as a composite electrolyte. So, what we really leveraged to recycle the composites is called cold sintering; that turned out to be the key enabling technology. Otherwise, we could get good separations as raw materials, but we couldn't reconstitute the battery.

Tech Briefs: How did this project and the prior one come about? What was the catalyst for your work?

Gomez: We’re always innovating without thinking about what happens at the end of life. We wanted to begin to think about examples of how you might be able to begin to change that paradigm. What if you designed a new innovation, but thinking about how are you going to deal with the waste at the end of life? So, we've been pursuing different directions on how we might demonstrate this.

Yi-Chen was very passionate about e-waste. She saw in the news how much e-waste is becoming more of a problem. We're kind of stuck in this interesting conundrum, right? We need to develop these technologies to help reduce CO₂ emissions. Things like batteries and even composite materials, those are very challenging to recycle.

And as far as we know, this was the first demonstration of being able to actually recycle a solid-state battery fully. And composites in general are supposed to be the next-generation material that's going to help us with lightweighting to be able to enable electrification of vehicles. But all of these things are going to go to waste. We're going to go from 80 percent recyclable cars right to maybe 50 percent, at best. We're going to somehow reduce CO₂ by creating this waste problem.

Tech Briefs: My next question is in two parts. One, do you have any set plans for further research, work, etc.? Two, how soon do you think they'll become more prevalent?

Gomez: We do want to continue the work in exploring other materials and continue to push, to what extent can we continue to develop recyclable composite electrical materials. We want to continue to demonstrate higher energy densities while being able to get to 100 percent recyclable but also to demonstrate higher-performing cathodes.

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