Lithium Battery 'Thorns' Are Hard and Brittle, Not Soft - and That Changes Everything

Lithium in bulk is soft and squishy. Press your thumbnail into it and it deforms. So for decades, scientists assumed that lithium dendrites - the microscopic metal thorns that grow inside batteries and cause them to fail - would behave the same way. Strategies to prevent battery failure were designed accordingly: make the barriers stiffer, and the soft dendrites will not push through.

That assumption was wrong. Dendrites are not soft at all.

Measuring what no one had measured

A new study published in Science reports the first direct nanomechanical measurements of lithium dendrites formed under realistic battery conditions. The results reveal structures that fracture at tensile strengths exceeding 150 megapascals (MPa) - roughly 250 times stronger than bulk lithium's approximately 0.6 MPa. Rather than deforming plastically like Play-Doh, these dendrites snap like dry spaghetti.

The work was an international collaboration involving researchers at Rice University, New Jersey Institute of Technology, Georgia Institute of Technology, the University of Houston, and Nanyang Technological University in Singapore. Co-lead author Xing Liu, an assistant professor at NJIT, describes the finding as a fundamental shift in understanding how batteries fail.

Why dendrites form and why they matter

Lithium dendrites - from the Latin word for branch - grow from the anode (negative terminal) of lithium-ion and lithium-metal batteries during charging. About 100 times thinner than a human hair, these metallic filaments extend into the electrolyte. If a dendrite reaches the cathode (positive terminal), it short-circuits the battery. Even when they do not bridge the gap, dendrites can break off and become electrically isolated - creating what is called dead lithium, a gradual drain on battery capacity.

At present, there is no practical method to clear dendrites from a working battery once they form. Prevention is the only viable strategy, which makes understanding their mechanical behavior critically important.

Harvesting thorns from working batteries

Measuring something 100 times thinner than a hair, made of a metal that reacts violently with air, inside a sealed battery cell is exactly as difficult as it sounds. The Rice University team, led by co-corresponding author Jun Lou, harvested individual dendrites from working batteries and transferred them to custom-built mechanical testing platforms.

Because lithium undergoes rapid chemical and structural changes upon air exposure, the entire sample preparation and testing process had to be conducted in airtight environments. Co-lead authors Qing Ai and Boyu Zhang performed the experiments, which they describe as extremely delicate work. High-resolution electron microscopy then captured how individual dendrites deformed and fractured under controlled stress.

The SEI coating makes them rigid

The explanation for dendrites' unexpected strength lies in their structure. As dendrites form inside a battery, they develop a thin coating called the solid electrolyte interphase (SEI). Cryogenic electron microscopy revealed that each dendrite consists of a single-crystal lithium core encased in this thin SEI layer.

The SEI coating transforms the dendrite from a soft metallic whisker into something rigid and needlelike. Teams at NJIT and Georgia Tech conducted scale-bridging simulations - from atomic-level models to continuum mechanics - to explain why this nanoscale structure produces such dramatically different behavior from bulk lithium.

The SEI-coated dendrites are not just strong; they are brittle. They do not bend and deform before breaking. They fracture suddenly, producing fragments of dead lithium that accumulate inside the battery cell and accelerate capacity loss. This brittleness also explains a longstanding puzzle: how supposedly soft lithium dendrites manage to crack through solid electrolyte materials that are much harder than bulk lithium.

New directions for battery design

The findings suggest that battery safety strategies based on the assumption of soft dendrites may be fundamentally misdirected. Rather than simply stiffening barriers, researchers may need to consider approaches that prevent the SEI coating from forming in ways that create brittle structures, or that make dendrites more ductile so they deform rather than snap.

Liu suggests that lithium alloy anodes could be one approach - by changing the composition of the dendrite core, it may be possible to reduce brittleness and the tendency toward fracture.

What remains to be tested

The measurements were performed on dendrites harvested from batteries, not on dendrites growing in real time inside operating cells. Whether the mechanical properties change during active charging and discharging - when the chemical environment is dynamic - is not yet established. The testing conditions, while carefully controlled, may not capture the full range of conditions dendrites experience in commercial batteries.

The study focused on specific electrolyte systems. Different electrolyte chemistries produce different SEI compositions, which could yield dendrites with different mechanical properties. Whether the brittleness finding generalizes across all lithium battery chemistries will require further testing.

Translating this basic understanding into practical battery improvements will also take time. Knowing that dendrites are brittle does not immediately tell engineers how to prevent them, though it does redirect the search toward more promising approaches.