Unlike conventional lithium-ion batteries, which use a liquid electrolyte between two solid electrodes, solid-state batteries replace the liquid with a solid electrolyte. This design offers several potential advantages, including higher energy density, improved safety, and longer battery life. But one stubborn problem has slowed commercial adoption. During charging, tiny tree-like structures called dendrites can grow from the lithium anode, pierce the solid electrolyte, and create internal short circuits.

Now, an interdisciplinary team at the Max Planck Institute for Sustainable Materials (MPI-SusMat) has identified exactly how these dendrites trigger fractures that ultimately lead to battery failure. Their findings were published in the journal Nature.

How Dendrites Crack Solid-State Batteries

Exactly how soft lithium dendrites manage to break through a hard ceramic electrolyte has long puzzled researchers.

"Although the electrodes and the forming dendrites consist of lithium metal, which is soft like a gummy bear, the dendrites are able to penetrate the ceramic electrolyte and lead to a short circuit," says Dr. Yuwei Zhang, first author of the new publication and head of the group "Chemo-Mechanics of Battery Materials" at MPI-SusMat. "How can soft dendrites fracture the stiff solid ceramic? There are two hypotheses: either internal stress is built up inside the dendrites and induces mechanical fracture of the solid electrolyte. Or, electrons leak along the grain boundaries of the solid electrolyte promoting the formation of lithium nuclei that interconnect later."

To determine which explanation was correct, the researchers used an advanced combination of sample preparation and materials characterization techniques. Every step was performed under vacuum and at cryogenic temperatures to eliminate interference from oxygen, water, or even the microscopes' electron beams.

The team examined both the internal stress and the plastic deformation of lithium dendrites trapped inside cracks. Their analysis found no buildup of lithium ahead of the dendrite tip, ruling out one proposed mechanism.

"The soft lithium metal is able to penetrate the stiff ceramic electrolyte, like a continuous waterjet that penetrates a rock. We calculated that hydrostatic stress in the dendrite leads to brittle fracture of the solid electrolyte in the end," says Zhang.

The researchers also confirmed their conclusions using phase field simulations and electron backscatter diffraction measurements.

New Strategies to Prevent Battery Failure

With a better understanding of how dendrites fracture solid electrolytes, the team is now investigating ways to stop or delay the process.

Potential solutions include making the solid electrolyte tougher so it resists cracking for longer, introducing microscopic voids that redirect dendrite growth and steer cracks away from vulnerable areas, or adding protective coatings to lithium electrodes to reduce dendrite formation in the first place.

The researchers say their work demonstrates the importance of understanding how materials behave at the microscopic level. Those insights could help transform solid-state batteries from a promising concept into a practical technology for future smartphones, electric vehicles, and other electronic devices.