Page 15 - EngineerIT December 2022

ELECTRONICS

Engineers solve a mystery on the

path to smaller, lighter batteries

By David L. Chandler, Massachusetts Institute of Technology

Branchlike metallic filaments can sap the power of solid-state lithium batteries. A new study explains how
they form and how to divert them.

discovery by MIT researchers Chiang says in the group’s earlier work, they made a “surprising and unexpected”
could finally unlock the door finding, which was that the hard, solid electrolyte material used for a solid-state battery
t
A o the design of a new kind of can be penetrated by lithium, which is a very soft metal, during the process of charging and
rechargeable lithium battery that is more discharging the battery, as ions of lithium move between the two sides.
lightweight, compact and safer than This shuttling back and forth of ions causes the volume of the electrodes to change.
current versions, and labs around the That inevitably causes stresses in the solid electrolyte, which has to remain fully in contact
world have pursued that for years. with both of the electrodes that it is sandwiched between. “To deposit this metal, there
The key to this potential leap in has to be an expansion of the volume because you’re adding new mass,” Chiang says. “So,
battery technology is replacing the there’s an increase in volume on the side of the cell where the lithium is being deposited.
liquid electrolyte that sits between the And if there are even microscopic flaws present, this will generate a pressure on those flaws
positive and negative electrodes with that can cause cracking.”
a much thinner, lighter layer of solid Those stresses, the team has now shown, cause the cracks that allow dendrites to form.
ceramic material, and replacing one of The solution to the problem turns out to be more stress, applied in just the right direction
the electrodes with solid lithium metal. and with the right amount of force.
This would greatly reduce the overall size While previously, some researchers thought that dendrites formed by a purely
and weight of the battery and remove electrochemical process, rather than a mechanical one, the team’s experiments
the safety risk associated with flammable demonstrate that it is mechanical stresses that cause the problem.
liquid electrolytes. But that quest has The process of dendrite formation normally takes place deep within the opaque
been beset with one big problem:
dendrites.
Dendrites, whose name comes from
the Latin for branches, are projections
of metal that can build up on the lithium
surface and penetrate into the solid
electrolyte, eventually crossing from one
electrode to the other and shorting out
the battery cell. Researchers haven’t been
able to agree on what gives rise to these
metal filaments, nor has there been much
progress on how to prevent them and
thus make lightweight solid-state batteries
a practical option.
The new research, being published
today in the journal Joule in a paper by
MIT Professor Yet-Ming Chiang, graduate
student Cole Fincher, and five others
at MIT and Brown University, seems Researchers solved a problem facing solid-state lithium batteries, which can be shorted out by metal
to resolve the question of what causes filaments called dendrites that cross the gap between metal electrodes. They found that applying
dendrite formation. It also shows how a compression force across a solid electrolyte material (gray disk) caused the dendrite (dark line
dendrites can be prevented from crossing at left) to stop moving from one electrode toward the other (the round metallic patches at each
side) and instead veer harmlessly sideways, toward the direction of the force. Illustration by the
through the electrolyte. researchers.

EngineerIT | December 2022 | 15