By improving the stability of internal interfaces between different solid materials, the team will give SSBs greater reliability and higher energy density.
Solid-state batteries (SSBs) are thus called because their electrolyte—the substance through which ions flow from the positive cathode to the negative anode—is a solid material, not a liquid or gel as in conventional batteries. Relative to other batteries, SSBs may offer important advantages in safety, energy density, and long-term stability, and lately they’ve attracted particular attention for possible use in electric vehicles. However, unstable and poorly controlled interfaces between the different solids within large-format SSBs have limited their usefulness to date.
Now, under a new Defense Advanced Research Projects Agency (DARPA) award of up to $10 million, a team of researchers from seven institutions are joining together to enable high-performance SSBs by overcoming those challenges.
The project's PI is Paul Braun, Professor and Grainger Distinguished Chair in Engineering in UIUC’s Department of Materials Science & Engineering (MatSE), as well as the Director of the Materials Research Laboratory (MRL). He notes that it is critical to improve the interfaces in SSBs. “In an SSB, the interfaces between the cathode and solid electrolyte, and between the anode and solid electrolyte, are the strongest drivers of battery performance. If these interfaces fail, the battery will immediately also fail.”
Project co-PI Ben Zahiri, who is a Research Assistant Professor in MRL and MatSE, explains that with a solid electrolyte, all the interfaces within the battery are solid-against-solid. “The interfaces between these solids are extremely important, because now we’re talking about two pieces of rigid bodies in direct contact. This contact must remain stable for the battery to perform as it should,” says Zahiri.
Unfortunately, those interfaces have been plagued by multiple problems.
For example, the interface between the anode and the electrolyte can undergo mechanical failure because ions, instead of going into the anode, may get deposited in the form of local structures. “They start forming what we call dendrites, needle-shaped features that grow and go all the way to the top towards the cathode. And then the cell short-circuits and... fails,” explains Zahiri. The good news is that it doesn’t catch fire, as conventional lithium-ion batteries sometimes do because of their liquid electrolyte. Even so, such failures have been a barrier to SSB adoption.
The interface between the cathode and the electrolyte has a different set of challenges, which, according to the team’s preliminary calculations, should be solvable through use of previously developed “dense cathode” technology from UIUC and the Xerion Advanced Battery Corp.
In conventional batteries, “a cathode is usually a mixture of powder, which is the actual cathode (active) material, and some conductive material and binder, forming a composite,” says Zahiri. Unfortunately, the non-active components contribute to the battery’s mass without contributing to its capacity: in other words, they lower the cell’s energy density. “A dense cathode,” on the other hand, “doesn’t have any of those additives.” Because of the dense cathodes, the research team will be able to meet DARPA’s stringent requirements for energy density as they manipulate the cathode/electrolyte interface to improve stability.
Xerion will produce the extremely dense cathodes needed for success. Project co-PI John Cook, who is the VP of Technology at Xerion, notes that it’s important to show that the new technology is “manufacturable” so it can mature beyond the lab level. The project “has great potential to advance solid-state batteries,” he says, “while assuring we have a manufacturing pathway that will enable us to deploy this technology in a meaningful way.”
Extensive prior research efforts have addressed individual pieces of SSB batteries, but a unified effort that considers the entire battery—anode, cathode, electrolyte, casing, and all the interfaces between them—hasn’t been done before. “A universal approach to look at the whole assembly hasn’t been, I think, fully implemented,” says Zahiri. “I think that’s... the uniqueness of [our project].”
The project, entitled “GRadient EnhAnced Transformative Solid-State Batteries (GREAT SSB),” will run for up to 4 years. It is being funded by DARPA’s Morphogenic Interfaces (MINT) program, whose goal is to extend the lifetimes of high-performance electrochemical systems by supporting research to develop “self-regulating” interfaces within such systems to preserve function by limiting degradation at the interfaces. The participating institutions are UIUC, Caltech, Georgia Tech, the University of Michigan, Princeton, Purdue, and Xerion. Braun is the CTO and a co-founder of Xerion. Elif Ertekin, an associate professor in Mechanical Science & Engineering, will help lead the UIUC effort as another co-PI.