MIT researchers led by MIT Professor and colleagues at Brown University have developed an approach to controlling dendrite propagation in solid-state batteries. A paper on their work is published in the journal Joule.

MIT Professor Yet-Ming Chiang, the corresponding author of the paper, says in the group’s earlier work, they made a “surprising and unexpected” finding, which was that the hard, solid electrolyte material used for a solid-state battery can be penetrated by lithium, which is a very soft metal, during the process of charging and discharging the battery, as ions of lithium move between the two sides.

This shuttling back and forth of ions causes the volume of the electrodes to change. That inevitably causes stresses in the solid electrolyte, which has to remain fully in contact with both of the electrodes that it is sandwiched between.

To deposit this metal, there has to be an expansion of the volume because you’re adding new mass. So, there’s an increase in volume on the side of the cell where the lithium is being deposited. And if there are even microscopic flaws present, this will generate a pressure on those flaws that can cause cracking.

—Yet-Ming Chiang

Those stresses, the team has now shown, cause the cracks that allow dendrites to form. The solution to the problem turns out to be more stress, applied in just the right direction and with the right amount of force.

While some researchers previously thought that dendrites formed by a purely electrochemical process, rather than a mechanical one, the team’s experiments demonstrate that it is mechanical stresses that cause the problem.

The process of dendrite formation normally takes place deep within the opaque materials of the battery cell and cannot be observed directly. MIT graduate student Cole Fincher developed a way of making thin cells using a transparent electrolyte, allowing the whole process to be directly seen and recorded.

You can see what happens when you put a compression on the system, and you can see whether or not the dendrites behave in a way that's commensurate with a corrosion process or a fracture process.

—Cole Fincher

The team demonstrated that they could directly manipulate the growth of dendrites simply by applying and releasing pressure, causing the dendrites to zig and zag in perfect alignment with the direction of the force.

Applying mechanical stresses to the solid electrolyte doesn’t eliminate the formation of dendrites, but it does control the direction of their growth. This means they can be directed to remain parallel to the two electrodes and prevented from ever crossing to the other side, and thus rendered harmless.

Fincher et al.

In their tests, the researchers used pressure induced by bending the material, which was formed into a beam with a weight at one end. But they say that in practice, there could be many different ways of producing the needed stress. For example, the electrolyte could be made with two layers of material that have different amounts of thermal expansion, so that there is an inherent bending of the material, as is done in some thermostats.

Another approach would be to “dope” the material with atoms that would become embedded in it, distorting it and leaving it in a permanently stressed state. This is the same method used to produce the super-hard glass used in the screens of smart phones and tablets, Chiang explains. And the amount of pressure needed is not extreme: The experiments showed that pressures of 150 to 200 megapascals were sufficient to stop the dendrites from crossing the electrolyte.

The required pressure is commensurate with stresses that are commonly induced in commercial film growth processes and many other manufacturing processes, so should not be difficult to implement in practice, Fincher adds.

A different kind of stress, called stack pressure, is often applied to battery cells, by essentially squishing the material in the direction perpendicular to the battery’s plates. It was thought that this might help prevent the layers from separating. But the experiments have now demonstrated that pressure in that direction actually exacerbates dendrite formation. What is needed instead is pressure along the plane of the plates.

What we have shown in this work is that when you apply a compressive force you can force the dendrites to travel in the direction of the compression.

—Cole Fincher

That could finally make it practical to produce batteries using solid electrolyte and metallic lithium electrodes. Not only would these pack more energy into a given volume and weight, but they would eliminate the need for liquid electrolytes, which are flammable materials.

Having demonstrated the basic principles involved, the team’s next step will be to try to apply these to the creation of a functional prototype battery, Chiang says, and then to figure out exactly what manufacturing processes would be needed to produce such batteries in quantity. Though they have filed for a patent, the researchers don’t plan to commercialize the system themselves, he says, as there are already companies working on the development of solid-state batteries.

The research team included Christos Athanasiou and Brian Sheldon at Brown University, and Colin Gilgenbach, Michael Wang, and W. Craig Carter at MIT. The work was supported by the US National Science Foundation, the US Department of Defense, the US Defense Advanced Research Projects Agency, and the US Department of Energy.

Resources

• Cole D. Fincher, Christos E. Athanasiou, Colin Gilgenbach, Michael Wang, Brian W. Sheldon, W. Craig Carter, Yet-Ming Chiang (2022) “Controlling dendrite propagation in solid-state batteries with engineered stress” Joule doi: 10.1016/j.joule.2022.10.011