Interest in higher energy-density batteries that pair alkali metal electrodes with solid electrolytes is high; however, such batteries have been plagued by a tendency for dendrites to form on one of the electrodes, eventually bridging the electrolyte and shorting out the battery cell. N
A team of researchers from MIT, Texas A&M University, Brown University, and Carnegie Mellon University, has now found a way to prevent such dendrite formation through the use of electrode architectures in which the presence of a liquid phase enables high current densities while it preserves the shape retention and packaging advantages of solid electrodes. A paper on the work is published in the journal Nature Energy.
It’s been known that dendrites form more rapidly when the current flow is higher—which is generally desirable in order to allow rapid charging. So far, the current densities that have been achieved in experimental solid-state batteries have been far short of what would be needed for a practical commercial rechargeable battery. But the promise is worth pursuing, says MIT Professor Yet-Ming Chiang, because the amount of energy that can be stored in experimental versions of such cells is already nearly double that of conventional lithium-ion batteries.
The team solved the dendrite problem by adopting a compromise between solid and liquid states. They made a semisolid electrode, in contact with a solid electrolyte material. The semisolid electrode provided a kind of self-healing surface at the interface, rather than the brittle surface of a solid that could lead to tiny cracks that provide the initial seeds for dendrite formation.
The idea was inspired by experimental high-temperature batteries, in which one or both electrodes consist of molten metal. According to MIT graduate student Richard Park, the first author of the paper, the hundreds-of-degrees temperatures of molten-metal batteries would never be practical for a portable device, but the work did demonstrate that a liquid interface can enable high current densities with no dendrite formation.
The motivation here was to develop electrodes that are based on carefully selected alloys in order to introduce a liquid phase that can serve as a self-healing component of the metal electrode.—Richard Park
The material is more solid than liquid, he explains, but resembles the amalgam dentists use to fill a cavity—solid metal, but still able to flow and be shaped. At the ordinary temperatures that the battery operates in, “it stays in a regime where you have both a solid phase and a liquid phase,” in this case made of a mixture of sodium and potassium.
The team demonstrated that it was possible to run the system at 20 times greater current than using solid lithium, without forming any dendrites, Chiang says. The next step was to replicate that performance with an actual lithium-containing electrode.
In a second version, the team introduced a very thin layer of liquid sodium potassium alloy in between a solid lithium electrode and a solid electrolyte. They showed that this approach could also overcome the dendrite problem, providing an alternative approach for further research.
The new approaches, Chiang says, could easily be adapted to many different versions of solid-state lithium batteries that are being investigated by researchers around the world. He says the team’s next step will be to demonstrate this system’s applicability to a variety of battery architectures.
We think we can translate this approach to really any solid-state lithium-ion battery. We think it could be used immediately in cell development for a wide range of applications, from handheld devices to electric vehicles to electric aviation.—Co-author Venkatasubramanian Viswanathan, professor of mechanical engineering at Carnegie Mellon University
The work was supported by the US Department of Energy, the National Science Foundation, and the MIT-Skoltech Next Generation Program.
Park, R.J-Y., Eschler, C.M., Fincher, C.D. et al. (2021) “Semi-solid alkali metal electrodes enabling high critical current densities in solid electrolyte batteries.” Nat Energy doi: 10.1038/s41560-021-00786-w