(Left) These diagrams illustrate the two different configurations the researchers used to minimize dendrite formation: (top) a liquid layer between the solid electrode and the solid electrolyte; (bottom) a semi-solid electrode. (Right) This photograph shows a metal electrode (the textured inner circle) on a grey disc of solid electrolyte. After being tested through many charging-discharging cycles, the electrolyte shows the beginnings of dendrite formation on its surface. Image courtesy of the researchers.
(Left) These diagrams illustrate the two different configurations the researchers used to minimize dendrite formation: (top) a liquid layer between the solid electrode and the solid electrolyte; (bottom) a semi-solid electrode. (Right) This photograph shows a metal electrode (the textured inner circle) on a grey disc of solid electrolyte. After being tested through many charging-discharging cycles, the electrolyte shows the beginnings of dendrite formation on its surface. Image courtesy of the researchers.

As researchers push the boundaries of battery design, seeking to pack ever greater amounts of power and energy into a given amount of space or weight, one of the more promising technologies being studied is lithium-ion batteries that use a solid electrolyte material between the two electrodes, rather than the typical liquid.

But such solid-state batteries have been plagued by a tendency for branch-like projections of metal called dendrites to form on one of the electrodes, eventually bridging the electrolyte and shorting out the battery cell. Now, researchers at Massachusetts Institute of Technology (MIT) and elsewhere have found a way to prevent such dendrite formation, which promises to unleash the potential of this new type of high-powered battery.

The findings are reported in a paper in Nature Energy by MIT graduate student Richard Park, professors Yet-Ming Chiang and Craig Carter, and seven others at MIT, Texas A&M University, Brown University and Carnegie Mellon University.

Solid-state batteries, Chiang explains, have been a long-sought technology for two reasons: safety and energy density. But, he says, "the only way you can reach the energy densities that are interesting is if you use a metal electrode". And while it's possible to couple a metal electrode with a liquid electrolyte and still get good energy density, it does not provide the same safety advantage as a solid electrolyte.

Solid-state batteries only make sense with metal electrodes, Chiang says, but attempts to develop such batteries have been hampered by the growth of dendrites. These eventually bridge the gap between the two electrode plates and short out the circuit, weakening or inactivating that cell in a battery.

It's well 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, Chiang says, 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.

Chiang and his colleagues were able to solve 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, unlike the brittle surface of a solid that can lead to the tiny cracks that provide the initial seeds for dendrite formation.

They were inspired by experimental high-temperature batteries, in which one or both electrodes consist of molten metal. According to 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 permit 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," Park says.

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 with solid lithium, without forming any dendrites. The next step was to replicate that performance with an actual lithium-containing electrode.

In a second version of their solid battery, 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.

According to Chiang, the new approaches could easily be adapted to the 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," says co-author Venkatasubramanian Viswanathan, professor of mechanical engineering at Carnegie Mellon University. "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."

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.