Engineers solve short-circuit puzzle, paving way for fast charging EVs

Finding ways to stop lithium dendrite growth hasn't seen much success, holding up further development of lightweight solid-state batteries, which replace one of the electrodes with solid lithium metal.

Fundamentally, the cause of these metal filaments has yet to be well understood by researchers - until now. 

A recent study published in the journal Joule seems to finally resolve the question of what causes dendrite formation. Even better, it demonstrates how to stop dendrites from piercing the electrolyte.

Interesting Engineering (IE) spoke with MIT professor Yet-Ming Chiang and graduate student Cole Fincher to gain a more in-depth understanding of the development's role in future batteries.

What causes dendrites in lithium batteries?

"For a number of years, we have known that dendrites create short circuits in solid-state batteries, but we didn't understand why," said professor Yet-Ming Chiang.

He explained that in this work, there are two main findings. The first was a "surprising and unexpected" discovery made in one of the team's earlier studies which revealed that dendrites grow because of mechanical stresses accompanying charging. 

Essentially, during the process of charging and discharging the battery, the shuttling back and forth of ions causes the volume of the electrodes to change. That causes stress in the solid electrolyte, which leads to cracking and allows dendrites to form.

"To deposit this metal, there has to be an expansion of the volume because you're adding new mass," Chiang explained earlier to MIT News.

"So, there's an increase in volume on the side of the cell where the lithium is being deposited. And if there are even microscopic flaws present, this will generate a pressure on those flaws that can cause cracking."

Remarkably, this finding debunks a widely-held belief that dendrites are formed solely by electrochemical rather than mechanical processes. 

Simply put, these mechanical stresses cause the initial cracks that allow dendrites to form. 

"Secondly, mechanical stresses – deliberately engineered – can be designed to "steer" dendrites and avert short circuits," Dr. Chiang told IE. "Still, we wanted to know how the scientists came to this discovery, given how many others had tried and failed before them?"

Can we now prevent dendrite formation in batteries?

"A key step was rendering the cells translucent by extensive mechanical polishing and thinning of the samples. This allowed us to optically observe dendrites as they grew through the cell," explained graduate student Cole Fincher. This makes sense, given that the dendrite formation process frequently occurs inside the battery cell's opaque components, where it would otherwise go undetected.

He highlighted that by viewing dendrites' response to the team's "prodding" and "poking," they could gain insight into how dendrites behave. "[We] then generalize that insight to more common cell formats," Fincher added. 

The team's work suggests that while dendrite formation can't be prevented (at least not yet), it can be rendered harmless. That is, dendrites can be directed — by pressure — to remain parallel to the two electrodes and kept from ever crossing to the opposite side. This direction would be like you were squeezing a sandwich from the sides. 

Oddly enough, a type of stress known as stack pressure is frequently applied to battery cells by compressing the material perpendicular to the battery's plates, similar to squeezing a sandwich by placing a weight on top of it. While this was intended to assist in keeping the layers from separating,  Chiang and Fincher's research has now shown that pressure in that direction actually makes dendritic formation worse.

Fincher emphasized that the team specifically described how enough engineered residual stresses should prevent short-circuiting. "While the stress we prescribe (150 MPa) may seem high, we encounter similar stresses in everyday life," he stated. 

"In your tempered glass car window, cellphone screen, and engineering structures such as composites for automotive and aerospace applications."

In their tests, they used pressure induced by bending the material, which was formed into a beam with a weight at one end. However, they asserted that various techniques could generate the required stress.

For instance, in certain thermostats, the electrolyte could be created using two layers of material with varying heat expansion rates, resulting in an inherent bending of the material. 

Another strategy would be to "dope" the material with implanted atoms, which would cause irreversible distortion and tension. The ultra-hard glass used in the screens of smartphones and tablets uses this same process.

How does the dendrite problem hold back electric cars?

The team's new solution could finally make it practical to produce batteries using solid electrolytes and metallic lithium electrodes. Not only would these pack more energy into a given volume and weight, but they would also eliminate the need for liquid electrolytes, which are flammable materials. 

This could serve as a holy grail, particularly for developing more efficient future electric vehicles (EVs).  

"Dendrite growth prevents us from "refueling" our electric vehicles as quickly as we could fuel an internal combustion engine," said Cole. "The strategies we outline in this work guide the way towards higher-performing batteries of the future."

After all, most of us are used to pulling up to a gas station and quickly refueling before moving on to the next part of our journey. Electric vehicles (EVs) may be more accessible for some customers once we can achieve something comparable.

The future: batteries using solid electrolytes and metallic lithium electrodes

Chiang explained that one limitation might be whether this effect occurs in all solid electrolytes. "Through future work, we will have to see," he said.

Another limitation may be whether the team can really engineer residual or retained stresses into batteries in a practical way to prevent such failures. "That remains to be demonstrated," Chiang added. 

Ultimately, this is an understanding of failure modes in solid-state batteries that the team believes the industry needs to be aware of and try to use in designing better products. 

The next steps will attempt to apply these concepts to produce a functional prototype battery. After this, Chiang and his team will determine precisely what manufacturing techniques would be required to build such batteries in commercial quantities.

Even though they have applied for a patent, the researchers don't intend to market the technology themselves since other businesses are currently developing solid-state batteries. 

Then who knows, their results could lead to a time when fast-charging, long-distance electric automobiles are the rule rather than the exception if replicated and scaled up.