[Image above] Credit: Andy Simmons; Flickr CC BY-ND 2.0

A lot of research has gone into developing solid-state batteries recently—which is not surprising, considering the technology has the potential to store more energy and do so more safely than existent batteries, most of which contain a liquid electrolyte rather than a solid one.

As I’ve written before, liquid battery electrolytes have the advantage of reduced resistance for ions shuttling back and forth between the battery’s electrodes. But liquid electrolytes also come with some important limitations—serious safety issues (exploding batteries, anyone?), limited incorporation of other battery materials, and significant thermal drawbacks.

Solid electrolytes, which are often made of ceramic materials, are a much safer alternative and offer the advantage of higher potential energy storage capacity.

Although researchers have played around with a lot of different potential solid electrolyte materials, such as LLZO, and devised various solutions to improving those materials, solid state batteries are still falling short.

Part of the problem is the formation of dendrites, finger-like projections that can stretch from one electrode to another and short-circuit the battery.

Researchers have long known that dendrite formation is a big problem for batteries. But despite a lot of research—including a see-through battery that allows analysis of dendrite formation in real time—dendrites are still a problem.

But according to a new study by a team at MIT, that may be partially because research has been focusing on the wrong key parameter to prevent dendrite formation in solid electrolytes.

To prevent dendrites from infiltrating into solid electrolytes, previous work has focused on firmness of the material—aka its shear modulus—with the thinking that a stiffer material would be more resistant to allowing dendrites to infiltrate within.

But the MIT team’s new work shows that firmness isn’t the most important parameter for developing a solid electrolyte that is effective against dendrite formation—instead, a defect-free surface, which doesn’t provide a place for dendrites to form, is key to a better battery.

The team tested four solid electrolytes: amorphous 70/30 mol% Li2S-P2S5, polycrystalline β-Li3PS4, and polycrystalline and single-crystalline Li6La3ZrTaO12 garnet. Analyzing how those electrolytes performed during charge-discharge cycles revealed important insights into how dendrites form.

New research suggests that achieving smoother surfaces on a solid electrolyte could eliminate or greatly reduce the problem of dendrite formation. Credit: MIT

The results showed the scientists that microscopic defects on the surface of a solid electrolyte provide an opportunity for lithium dendrites to start growing—the lithium migrates from the electrode and latches onto the defect. That deposition seeds growth of a dendrite, growing from its tip to wedge a crack into the electrolyte.

“It’s the crack propagation that leads to failure,” Yet-Ming Chiang, Kyocera Professor of Ceramics at MIT and senior author of the new research, says in an MIT News story. “It tells us that what we should be focusing on more is the quality of the surfaces, on how smooth and defect-free we can make these solid electrolyte films.”

The team thinks that developing solid electrolytes with smoother surfaces could drastically reduce dendrite formation, making batteries that are less prone to short-circuit and that last longer.

And in fact, there’s already previous research to support the idea. Battery grandfather John Goodenough and a team of researchers reported earlier this year that a glass interlayer can prevent dendrite formation in solid-state batteries that the team developed at University of Texas at Austin. It seems plausible that the smooth surface of the glass interlayer could have accounted for the battery benefits. 

In addition to making safer batteries, preventing dendrite formation could also open the door to incorporating solid lithium electrodes into batteries, which would double their capacity—a must for smaller, more powerful batteries to power our future.

The paper, published in Advanced Energy Materials, is “Mechanism of lithium metal penetration through inorganic solid electrolytes” (DOI: 10.1002/aenm.201701003).