Team overcomes challenge of volumetric change as a major step toward practical solid-state batteries

The researchers shed light on the impact of electrodes’ size changes during use and created solid-state batteries that work at unprecedentedly low stack pressures.

Solid-state batteries (SSBs) have long been pursued as a superior alternative to today’s lithium-ion batteries. SSBs offer multiple advantages, ranging from higher energy density and faster charging times to greatly improved safety, as their solid electrolytes are believed to present much less of a fire hazard than the liquid electrolytes used in lithium-ion batteries. Several major technical barriers, however, mean that SSBs are not yet practical or affordable for widespread use. 

Now, a new Nature Communications paper by Saeed Moradi, Ben Zahiri and Paul V. Braun has announced a breakthrough in overcoming one big barrier: how to deal with the expansion and contraction of electrodes during battery cycles if the electrolyte between them is a solid, so can’t accommodate the changing shapes simply by flowing around them.

The cathode and anode electrodes in batteries are made from solid materials, and their volumes change as lithium is transferred back and forth between them during the charge and discharge cycles. In the batteries widely available today, the electrolyte that conducts lithium ions between the two electrodes is a liquid, so the electrodes’ changing sizes generally aren’t a problem. In an SSB, however, the cathode, the anode and the electrolyte are all solids. 

“And as you can imagine, when you transition everything to a stack of three pieces of solid all together, keeping contact becomes a crucial issue,” said Zahiri, who is a research assistant professor in Materials Science and Engineering and the Materials Research Laboratory. 

Zahiri explained that for years, people have been trying to handle that “volumetric misalignment” in SSBs by stacking the components and then applying an enormous amount of pressure to keep everything in contact. Unfortunately, although that approach can work in a small cell, it cannot scale up to something like a car battery. For example, Zahari said, his group used to apply 80 megapascals (MPa) of pressure to get such a battery to work, whereas “in a real application such as an electric vehicle, you can’t get more than probably 1 or 2 MPa of pressure.” 

That’s only about 10 to 20 times the atmospheric pressure of Earth at sea level.

One of the contributions of the paper is the novel observation that the chemomechanical nature of components’ expansion and contraction – meaning the ways in which changes in chemistry affect mechanical forces – can be designed such that the changing components operate in concert. It was already known that the crystallographic lattice of the cathodes during a typical charge cycle expands in one direction and contracts in other directions, but the impact of the phenomenon in SSBs was not well understood. 

“In one crystal orientation, we ended up having the cathode reducing the stack pressure, because it’s shrinking,” Zahiri said. “In the other crystal orientation, we have the cathode expanding, and we see the response in the force that is increasing the stack pressure. This is the first time that the anisotropic nature of cathode chemomechanics has been captured experimentally in a solid-state battery.”

But that wasn’t all. The researchers used their new insights to pair cathodes in different configurations with a lithium metal anode to see how the anode’s behavior changes in response to the cathode’s behavior. “Under a low stack pressure of around 1 to 2 MPa, we discovered the cathode’s chemomechanics strongly impacts the cycling performance of the anode – something that no one has seen before,” said Braun, who is Moradi’s advisor and a professor and Grainger Distinguished Chair in Engineering in Materials Science and Engineering, as well as the director of the Materials Research Laboratory. He added, “This provides a new tuning knob to improve SSB cycling performance.”

In particular, they showed that seemingly small changes to the design of the cathode contribute significantly to the overall battery stack pressure in a commercially relevant low-pressure setting.

“Through this understanding, we were able to design solid-state batteries that operate with only a 1 MPa stack pressure, for a thousand cycles. And this is the other significant outcome of this study,” said Zahiri.

The lead author of the paper is Saeed Moradi, a Ph.D. student in Chemical and Biomolecular Engineering. He noted that the work demonstrates “electrode crosstalk” in SSBs. Relative to today’s commercially produced batteries, “solid-state batteries are more sensitive to mechanical changes,” he said. “And I think overcoming this may remove one of the biggest challenges that hinders solid-state batteries from reaching commercialization.”

Moradi added that further understanding of the relationships among the battery components will “help us find even better ways to match different electrodes together to have the best performance.” 

Zahiri said that the next big step for the team will be to scale up the batteries to demonstrate that they can be practical for real-world applications.

Grainger Engineering Affiliations:

Paul V. Braun is a professor and Grainger Distinguished Chair in Engineering in Materials Science and Engineering, as well as the director of the Materials Research Laboratory.

Saeed Moradi is a Ph.D. student in Chemical and Biomolecular Engineering.

Ben Zahiri is a research assistant professor in Materials Science and Engineering and the Materials Research Laboratory.