Lithium dendrites in batteries are harder and more brittle than assumed

Researchers in the United States have, for the first time, directly measured the mechanical behaviour of so-called lithium dendrites. These unassuming structures form inside batteries and have long been viewed as a major cause of capacity fade, internal short circuits and, in rare but headline-grabbing cases, battery fires. The latest results suggest the issue is not soft and easily shaped, but hard and brittle.

What really goes wrong inside lithium batteries

Each time a lithium-ion battery charges and discharges, a web of processes runs in the background. Lithium ions shuttle back and forth between anode and cathode, current flows through the device, and everything appears to be under control. Yet, out of sight on the anode surface, a kind of metallic undergrowth can start to sprout from the “floor” of the cell: dendrites.

These dendrites are ultra-fine metal needles-around 100 times thinner than a human hair. They form during charging when lithium plates unevenly onto the anode. With every charge–discharge cycle, the needles can extend further towards the separator, the thin film that sits between the electrodes.

Once a needle punctures the separator, an internal short circuit becomes only a matter of time. Electrons then take a shortcut inside the cell. The consequences include:

• intense heating up to overheating
• rapid loss of capacity
• in extreme cases, fire or explosion of the cell

As a result, millions of batteries worldwide age noticeably faster than intended. Until now, many countermeasures were built on an assumption that sounded reasonable-but was wrong: dendrites were treated as soft and malleable, much like the bulk lithium metal they are made from.

Dendrites are not soft - they snap like dry spaghetti

A team from the New Jersey Institute of Technology (NJIT) and Rice University set out to test that assumption directly. Under a high-resolution electron microscope, and in an extreme vacuum to prevent oxidation, the researchers subjected individual dendrites to mechanical stress.

"The surprising result: the fine needles do not bend - they fracture abruptly like dry spaghetti."

That finding overturns a decades-old mental model. The measured yield strength of the dendrites is about 150 megapascals. By comparison, solid lithium begins to give way at roughly 0.6 megapascals. In other words, these growing structures are around 250 times more resistant to deformation than the metal from which they originate.

The explanation sits on their surface: within fractions of a second, an ultra-thin oxide layer forms around a dendrite. This shell, only a few nanometres thick, turns otherwise butter-soft lithium into a brittle composite. As a result, dendrites behave less like gummy metal filaments and more like tiny glass spikes or micro-harpoons.

Why this undermines previous strategies

Many safety concepts have tried to “push” dendrites away or smooth them out using pressure, flexible separators or hard electrolytes. That makes sense if you picture soft needles. But stiff, brittle structures with very high strength cannot simply be flattened.

Instead, they can drill through separators even when those separators are relatively robust. And rather than bending out of the way, they prefer to splinter. Industrial models have largely failed to account for that particular mechanical behaviour.

The lithium-metal battery dream starts to wobble

The new insight is especially awkward for a battery technology the automotive industry has pinned major hopes on: lithium-metal batteries. In place of a graphite anode, these use near-pure lithium. The potential upside is huge: energy density could increase by about a factor of three.

In practical terms, that would mean:

• electric cars with 900 kilometres of range rather than 300
• lighter battery packs at the same range
• less raw material required per vehicle

However, dendrites are particularly aggressive in exactly these cells. The more pure lithium is available, the easier it is for new needles to grow. The study now indicates these dendrites are not only more numerous, but also form exceptionally robust spikes.

When such needles break off, they leave behind electrically isolated lithium fragments. The researchers describe this figuratively as "dead lithium". These remnants drift somewhere in the electrolyte and can no longer take part in charging. Cycle by cycle, usable capacity shrinks.

"Every broken dendrite consumes a piece of usable lithium - and with it a piece of battery life."

Why solid-state electrolytes are not an automatic fix

Solid-state electrolytes are often promoted as a cure-all: no liquid electrolyte, lower fire risk, improved safety. Many approaches rely on them specifically to halt dendrites. The revised picture shows that being solid is not enough.

If dendrites have such high strength, they can pierce even rigid solid-state electrolytes rather than being pressed flat at the surface. So a simple barrier strategy only works to a limited extent. The material must be not only hard, but engineered to match this specific mechanical reality.

Three new material strategies against rigid needles

The teams involved therefore argue for a change in direction. Instead of making today’s solutions merely thicker, stiffer or harder, they propose more targeted material “tricks”. Three approaches are highlighted:

• Modified lithium alloys: By adding other elements, pure lithium could be altered so that the rigid oxide layer forms less readily at the surface, or becomes less brittle.
• Smarter separators: New separator materials should absorb and redirect mechanical stresses, acting like a shock absorber. The aim is to stop dendrites travelling straight through.
• Targeted electrolyte additives: Specific additives in the electrolyte should change dendrite crystal structure as they form. This could steer growth into less harmful shapes that are less sharp and less penetrating.

These ideas are meant to work together: less brittle dendrites, tougher separators, and an electrolyte that suppresses critical growth modes. The objective is not necessarily to prevent every single needle, but to make their behaviour controllable.

What this means for electric cars and the energy transition

Car makers around the world are waiting for a breakthrough in high-energy batteries. Range anxiety still deters many buyers, particularly where rapid-charging infrastructure remains only moderately developed. If the lifetime of batteries with a lithium-metal anode cannot yet be predicted reliably, manufacturers hesitate to commit to large-scale production.

At the same time, large-scale storage of solar and wind power depends on dependable, affordable, long-lived storage. Here, batteries with three- or four-times the energy density would be a major lever: they could make entire container farms smaller or cheaper, and smooth peak demand more effectively.

This work nudges the door open a little further. Researchers can now calibrate models and simulations using real mechanical measurements. Materials developers gain a clearer view of what a separator or solid-state electrolyte truly needs to withstand. And investors can better judge which approaches rest on marketing claims versus those built on a physically sound foundation.

How one misconception held back research for decades

The story of lithium dendrites shows how strongly an untested assumption can steer an entire field. The image of a “soft metallic filament” seemed consistent with lithium’s familiar behaviour and felt plausible to many. Countless papers, patents and products were built on that premise.

Only direct observation at the nanometre scale has dismantled the old picture. For a long time, limits in advanced microscopy made it difficult to capture the true mechanical response of these microstructures cleanly. With newer methods, it is now possible to measure not just images but forces at the scale of tiny nanostructures.

This should also serve as a warning signal for other areas of battery research. Models need to be grounded more firmly in direct measurements, especially where interphases, surface films or other transition regions are involved. In such places, just a few nanometres can decide between a working cell and a total failure.

What users can take away today

It will take time before these findings show up in mass-produced batteries. Even so, end users can benefit from paying closer attention to charging habits. Extremely fast charging at very high or very low temperatures, keeping a battery permanently at 100%, or repeatedly deep-discharging all place noticeable strain on cells and encourage uneven lithium plating-meaning more dendrite growth.

Anyone who keeps an electric car or smartphone more often between 20% and 80%, uses moderate charging power, and avoids high heat reduces the mechanical stress inside the cell. That will not eliminate dendrites entirely, but it can slow their growth substantially.

At the same time, the study points to where research is heading: away from blunt “harder and thicker” fixes, towards finely tuned material systems that consider chemistry and mechanics together at the nanoscale. If that approach succeeds, the next generation of batteries could not only travel much further-they could also last longer and operate more safely.