The discovery by the MIT researchers could finally open the door to designing a new type of rechargeable lithium battery that’s lighter, smaller and safer than existing versions, and labs around the world have pursued it for years.
The key to this potential leap in battery technology is to replace the liquid electrolyte between the positive and negative electrodes with a thinner and lighter layer of solid ceramic material, and to replace one of the electrodes with solid lithium metal. This will significantly reduce the overall size and weight of the battery and eliminate safety hazards associated with flammable liquid electrolytes. But this research was beset by one big problem: ramifications.
The dendrites, whose name comes from the Latin for branch, are protrusions of metal that can build up on the lithium surface and permeate the solid electrolyte, eventually crossing from one electrode to the other and shorting the battery cell. Researchers couldn’t agree on why these metal filaments appeared, and there wasn’t much progress on how to prevent them, thus making lightweight solid-state batteries a practical option.
The new research being published today in the journal Joule In a paper by MIT professor Yeet-Ming Chiang, graduate student Cole Venture, and five others at MIT and Brown University, they seem to solve the question of what causes bifurcation to form. It also shows how dendrites can be prevented from crossing through the electrolyte.
In the group’s previous work, they made a “surprising and unexpected” discovery, Qiang says, that the solid electrolyte material used in a solid-state battery can be penetrated by lithium, a very soft metal, during the process. Charging and discharging the battery, as the lithium ions move between the two sides.
Moving ions back and forth changes the size of the electrodes. This inevitably causes stresses in the solid electrolyte, which must remain in full contact with both electrodes sandwiched between them. “To deposit this mineral, there has to be a volume expansion because you’re adding new mass,” Chiang says. “So, there is a volume increase on the side of the cell where the lithium is deposited. And if there are microscopic defects, it will put pressure on those defects that can cause cracking.”
The team has now shown that these stresses cause the cracks that allow dendrites to form. It turns out that the solution to the problem is more pressure, applied in exactly the right direction and with the right amount of force.
While some researchers previously believed that the dendrites formed through a purely electrochemical process, rather than a mechanical one, the team’s experiments show that mechanical stresses are causing the problem.
The process of dendrite formation usually takes place deep within the opaque material of a battery cell and cannot be directly observed, so Fincher developed a method for making thin cells using a transparent electrolyte, allowing the whole process to be viewed and recorded directly. “You can see what happens when you stress the system, and you can see whether the dendrites are behaving in a way that is proportional to the corrosion process or the fracturing process,” he says.
The team showed that they could directly manipulate the growth of dendrites by simply applying and releasing pressure, which causes the squiggle and squiggle in perfect alignment with the direction of the force.
Applying mechanical stresses to the solid electrolyte does not eliminate the formation of dendrites, but controls the direction of their growth. This means that they can be directed to stay parallel to the poles and prevent them from crossing over to the other side, thus rendering them harmless.
In their tests, the researchers used stress created by bending the material, which was shaped into a beam with a weight at one end. In practice, however, they say, there can be many different ways to produce the required pressure. For example, an electrolyte can be made of two layers of material with different amounts of thermal expansion, such that there is an inherent bending of the material, as in some thermostats.
Another method is to “drug” the material with the atoms that will fuse into it, deform it and leave it in a permanent state of stress. Chiang explains that this is the same method used to produce the ultra-hard glass used in smartphone and tablet screens. And the amount of pressure required is not extreme: experiments have shown that pressures of 150 to 200 MPa were sufficient to prevent dendrites from crossing the electrolyte.
Fincher adds that the pressure required is “commensurate with the pressures typically experienced in commercial film growth processes and many other manufacturing processes,” so it shouldn’t be difficult to implement in practice.
In fact, a different type of stress, called stack stress, is often applied to battery cells, by crushing the material in the direction perpendicular to the battery plates — somewhat like compressing a sandwich by placing a weight on top of it. It was thought that this might help prevent the layers from separating. But experiments have now shown that pressure in this direction actually exacerbates dendrite formation. “We’ve shown that this kind of stack compression actually accelerates dendrite-induced failure,” Fincher says.
What we need instead is pressure along the plane of the planks, as if the sandwich was pressed from the sides. “What we’ve shown in this work is that when you apply a compressive force, you can force the dendrites to move in the direction of compression, and if that direction is along the plane of the plates, the dendrites ‘do not reach the other side,'” Fincher says.
This could finally make it practical to produce batteries using solid state electrolytes and lithium metal electrodes. Not only will these pack more power into a given size and weight, but they will eliminate the need for liquid electrolytes, which are flammable materials.
After proving the basic principles involved, the team’s next step will be to try to apply them to creating a functional prototype battery, Chiang says, and then figure out exactly what manufacturing processes are required to mass produce such batteries. Although they have filed for a patent, the researchers do not plan to commercialize the system themselves, he says, as there are already companies developing solid-state batteries. “I would say this is an understanding of the failure patterns in solid-state batteries that we think the industry needs to be aware of and try to use in designing better products,” he says.
The research team included Christos Athanasio 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.