Solid-state batteries, which utilize solid metal as their charge-carrying electrolyte, offer potential advantages over lithium-ion batteries in terms of safety and energy density. However, these batteries have been hampered by the formation of metallic cracks, known as dendrites, which cause short circuits. This issue has prevented their widespread adoption in energy storage.
Historically, dendrites have been largely attributed to mechanical stress. However, new research from MIT engineers has identified an opposite relationship: faster dendrite growth was linked to lower stress levels in a commonly used battery electrolyte material. Using a novel technique to directly measure stress around growing dendrites, the researchers observed cracks forming at stress levels as low as 25 percent of what would be expected under mechanical stress alone.
Faster dendrite growth in solid-state batteries was unexpectedly linked to lower stress levels, challenging previous assumptions.
Research Findings
The experiments, published in Nature, indicated that chemical reactions caused by high electrical currents weaken the electrolyte, making it more susceptible to dendrite growth. While such reactions had been proposed as a cause for dendrite growth, this study provides the first experimental data on the interplay between chemical and mechanical stress in dendrite formation.
- Material Weakening: Cole Fincher, the paper's first author, stated that the ceramic electrolyte, which is initially as tough as a tooth, becomes significantly weaker during charging, reaching a brittleness comparable to a lollipop.
"The ceramic electrolyte, which is initially as tough as a tooth, becomes significantly weaker during charging, reaching a brittleness comparable to a lollipop."
- Direct Observation: Researchers developed a special solid-state battery cell allowing side-view observation of dendrite growth in the electrolyte. They used birefringence microscopy to precisely measure stress around the dendrite.
- Unexpected Stress Correlation: The study found that as dendrites grew faster, the stress around them was lower, indicating the solid electrolyte was fracturing under reduced stress due to embrittlement.
- Atomic-Scale Evidence: Led by James LeBeau, cryogenic scanning transmission electron microscopy revealed that the passage of ionic current caused chemical reactions around the dendrite, leading to material decomposition and volume contraction, consistent with the observed embrittlement.
Implications for Battery Development
These findings suggest that developing stronger electrolytes alone may not resolve the dendrite problem. The research highlights the importance of creating more chemically stable materials to realize the potential of high-density solid-state batteries. Senior author Yet-Ming Chiang noted that the study provides guidance for efforts to discover and design better solid electrolytes.
"The study provides guidance for efforts to discover and design better solid electrolytes."
The experiment was conducted on a highly stable electrolyte, suggesting the findings are applicable to other electrolyte materials. Future research will focus on identifying the specific electrochemical reactions involved in the electrolyte's weakening. The methodology for directly observing stresses could also aid in improving materials for devices such as fuel cells and electrolyzers.