Illuminating the Mechanics of Solid-State Batteries

Ed Brown

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, where the electrode particles are in contact with a solid, ceramic-like electrolyte.

Therefore, the importance of mechanics is even higher for solid-state cells since any fracture leads to immediate loss of contact and of the ability of the cell to transfer the ionic current in the electrolyte to electronic current in the electrode.

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 the form of electrolyte fracture.

So, this was pretty much the idea behind this review, to connect the insights of electrochemistry with the insights of mechanics and show the mechanisms of stress reduction that could avoid fracture in solid-state batteries.

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, fully constrained by ceramic electrolyte, can experience very high hydrostatic compressive stress, on the level of gigapascals, thus restricting the diffusion of the lithium.

Tech Briefs: Do you have thoughts about how to reduce the stress concentrations?

Kalnaus: It boils down to quality control of the solid electrolyte. The strength of brittle materials, for example glass and ceramics, depends on the distribution of the surface and volume defects. Minimizing the density of those defects is the key.

Tech Briefs: Is there a solid-state electrolyte that would be less susceptible to damage from mechanical stress?

Kalnaus: So far, lithium phosphorous oxynitride (LiPON) is the only solid electrolyte that has been cycled in a solid-state cell over thousands of cycles with minimal degradation in performance. But here’s the caveat: this has been demonstrated only in a thin-film battery format. Manufacturing of LiPON for now is limited to RF magnetron sputtering. However, we can learn a lot from this material and maybe use it as a blueprint to design an ideal solid electrolyte. It is an inverted glass, meaning there are more ionic bonds with glass modifier (lithium) than covalent bonds corresponding to glass former. This results in its unusual nanoscale and microscale ductility. Surprisingly, we are still at the beginning of understanding its mechanical behavior, despite the fact that it was discovered back in the ‘90s.

Tech Briefs: Do the rates of charging and discharging affect the stresses?

Kalnaus: Definitely. Higher current densities increase the concentration gradients in the cathode and, consequently, the stresses.

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.

I believe the most important applications for solid-state batteries will be in healthcare, health monitoring, wearable technologies, space, and defense industries. These are all applications where the requirements for safety take priority. As the technology progresses and manufacturing scales up and becomes cheaper, 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 whole battery cells. Understanding mechanics, and maybe turning it to our advantage (structural batteries) together with developing manufacturing techniques are the two important barriers remaining. We already have a variety of excellent 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.

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