Tech Briefs Magazine - December 2023 - 9

Q & A
Illuminating the Mechanics of
Solid-State Batteries
Doctor Sergiy Kalnaus and his team at Oak
Ridge National Laboratory have developed a
framework for designing solid-state batteries
that focuses on their underlying mechanics.
Tech Briefs: What got you started on
this project?
Doctor Sergiy Kalnaus: I've long seen a
disconnect between what we call the " battery community " and
mechanics/materials experts when it comes to explaining what
happens inside a battery cell. While pure electrochemists are primarily
focused on the amount of charge that can be stored and
discharged, the same battery cell can be viewed as a mechanical
system that experiences compositional strains and stresses any time
it is charged or discharged.
These effects are recognized as being important in traditional
liquid electrolyte batteries, where the stress build-up leads to fracturing
of electrode particles, and therefore, affects the total surface
area of the electrode. But in liquid electrolyte cells, the fractured
particle still maintains contact with the electrolyte, which can easily
flow into the cracks. This is not the case in solid-state battery cells.
Therefore, the importance of mechanics is even higher for solid-state
cells.
We have often heard about the pesky lithium " dendrites " that
fracture solid electrolytes and short circuit the cell. This is also, to a
great extent, a mechanics problem. As it turns out, lithium metal,
despite being soft in bulk, becomes very hard when confined to the
microscale pores and surface defects of the electrolyte. Without
any plastic flow in the lithium to reduce the stress, the stress
relief usually comes in form of electrolyte fracture.
Tech Briefs: What effects do these forces have on the
battery?
Kalnaus: The effect is mostly in cracking and the formation of
voids. In composite cathodes, cracks between active material and
electrolyte isolate the active material of the cathode, which reduces
battery capacity. Cracks through the solid electrolyte allow lithium
to propagate from one electrode to another and short circuit
the cell. The stress state itself may also influence how the battery
performs by shifting the chemical potential of the lithium and by
affecting diffusion in the cathode. Electrode particles can experience
very high hydrostatic compressive stress, restricting the diffusion
of the lithium.
Tech Briefs: What do you see as the most important
applications for solid state batteries?
Kalnaus: The first true solid-state batteries entered the market a
long time ago - the well-known lithium-iodide cells that are used
for pulse generation in cardiac pacemakers. The most important
Tech Briefs, December 2023
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9
applications for solid state batteries will be in healthcare, wearable
technologies, space, and defense industries. As the technology
progresses and manufacturing scales up, these batteries can then
be considered for automotive applications.
Tech Briefs: What are your next steps?
Kalnaus: With our paper in Science, we hope to provide a framework
to get mechanics into the picture when it comes to designing
a solid-state battery cell. The short-term goal is to come up with
standard protocols for not only electrochemical performance, but
for mechanical testing of battery materials and cells. Understanding
mechanics and developing manufacturing techniques are the
two important barriers remaining. We already have a variety of ionic
conductors and cathode materials capable of high charge-storing
capacity. But we still use an old paradigm for battery design
and manufacturing, the legacy of liquid electrolyte batteries.
To read the full interview, visit www.techbriefs.com.
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Tech Briefs Magazine - December 2023

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