MIT researchers led by MIT professors and colleagues at Brown University have developed an approach to controlling dendrite propagation in solid-state batteries. An article about her work is published in the journal joules.
MIT Professor Yet-Ming Chiang, the paper’s corresponding author, says that in the group’s previous work, they made a “surprising and unexpected” discovery, namely that the hard solid electrolyte material used for a solid-state battery can be penetrated through Lithium, which is a very soft metal, during the battery charging and discharging process as lithium ions move between the two sides.
This shuttle of ions back and forth causes the volume of the electrodes to change. This inevitably leads to tension in the solid electrolyte, which must remain in full contact with the two electrodes between which it is clamped.
To deposit this metal, the volume must increase because you are adding new mass. So there is an increase in volume on the side of the cell where the lithium is deposited. And if there are even microscopic flaws, this will create pressure on those flaws that can lead to cracking.
– Nor-Ming Chiang
As the team has now shown, these stresses 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 just the right force.
While some researchers previously thought that dendrites are formed by a purely electrochemical process rather than a mechanical process, the team’s experiments show that it is mechanical stress that causes the problem.
The process of dendrite formation usually takes place deep within the opaque materials of the battery cell and cannot be observed directly. MIT student Cole Fincher developed a method for manufacturing thin cells with a transparent electrolyte that allows the entire process to be directly observed and recorded.
You can see what happens when you compress the system and you can see if the dendrites behave in a way that corresponds to a corrosion process or a fracture process.
– Cole Fincher
The team demonstrated that they could directly manipulate the growth of dendrites simply by applying pressure and releasing, causing the dendrites to zig-zag in perfect alignment with the direction of the force.
Although the application of mechanical stress to the solid electrolyte does not prevent the formation of dendrites, it does control their direction of growth. This allows them to be steered to stay parallel to the two electrodes and prevented from ever getting to the other side and thus rendered harmless.
Fincher et al.
In their tests, the researchers used pressure created by flexing the material, which was formed into a beam with a weight on one end. But they say that in practice there could be many different ways to create the stress you need. For example, the electrolyte could be made from two layers of material that have different thermal expansions so that there is inherent flexing of the material, as is the case with 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 create the super-tough glass used in smartphone and tablet screens, Chiang explains. And the pressure required is not extreme: the experiments showed that pressures of 150 to 200 megapascals were sufficient to prevent the dendrites from crossing the electrolyte.
The pressure required corresponds to the stresses commonly induced in commercial film growth processes and many other manufacturing processes, so it shouldn’t be difficult to implement in practice, Fincher adds.
Battery cells are often subjected to another type of stress, referred to as stacking pressure, by crushing the material generally in the direction perpendicular to the plates of the battery. It was thought that this might help prevent the layers from separating. But experiments have now shown that pushing in this direction actually makes dendrite formation worse. 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 move in the direction of the compression.
– Cole Fincher
That could finally make it practical to make batteries with solid electrolytes and metallic lithium electrodes. Not only would these pack more energy into a given volume and weight, but they would also eliminate the need for liquid electrolytes, which are combustible materials.
After demonstrating the basic principles, the team’s next step will be to apply them to making a working prototype battery, says Chiang, and then to figure out exactly what manufacturing processes would be needed to mass-produce such batteries. Although they have a patent pending, the researchers don’t plan to commercialize the system themselves, he says, as there are already companies working on solid-state battery development.
The research team included Brown University’s Christos Athanasiou and Brian Sheldon, and MIT’s Colin Gilgenbach, Michael Wang, and W. Craig Carter. 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.
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” joules doi: 10.1016/j.joule.2022.10.011