Dendrites in lithium batteries: the nanometre experiment that overturned a long-held battery myth

From smartphones and e-bikes to electric cars, lithium batteries are now everywhere. Yet many cells lose capacity far earlier than the brochures promise - and in the worst cases, they can short-circuit and catch fire. A research team in the United States has now observed a crucial mechanism directly, exposing a decades-old misunderstanding in battery science.

Tiny metal needles can disable entire lithium batteries

Inside every lithium-ion battery lies an invisible troublemaker: so-called dendrites. These are extremely fine metal structures that form on the lithium anode during charging. They grow into the electrolyte like tiny needles or branches - delicate as cobwebs, but with enormous consequences.

Dendrites are about 100 times thinner than a human hair. Even so, they can do serious damage: once they grow far enough, they pierce the separator, the thin layer that keeps the positive and negative electrodes apart inside the battery.

When that happens, an internal short circuit is created. Electrons then no longer travel through the intended device, but move directly from one electrode to the other. The consequences range from noticeable capacity loss and severe heating to thermal runaway - the feared scenario in which a cell catches fire.

Researchers describe them as “metallic micro-needles” that silently bore through the battery and, in the worst case, puncture it from the inside.

For years, many specialists assumed these dendrites were relatively soft - as soft as the bulk lithium they are made from. On that basis, the idea was to push them back or cushion them. That basic assumption is now being challenged.

Even outside the laboratory, charging habits can play a role here. Frequent rapid charging, very high state-of-charge levels, and low temperatures can all place additional stress on a cell, making the conditions more favourable for dendrite growth.

Lithium battery dendrites turn out to be as hard as glass

A team from the New Jersey Institute of Technology and Rice University has, for the first time, deliberately mechanically loaded dendrites under an electron microscope, under high vacuum so that they would not immediately react or oxidise. The goal was to see not only their shape, but their actual behaviour under pressure.

The result is startling: the dendrites do not bend, and they do not deform elastically - they snap suddenly. The researchers compare the behaviour with dry spaghetti: a little pressure, then a clean break, with no visible deformation beforehand.

Measurements show that these tiny needles can withstand mechanical stresses of around 150 megapascals. By contrast, bulk lithium reaches only about 0.6 megapascals. In other words, the dendrites are roughly 250 times more resistant than the same material in its “normal” form.

The explanation lies in an inconspicuous layer: within fractions of a second, a few-nanometre-thin oxide or reaction layer forms on the surface of the dendrites. This shell turns the otherwise soft metal into a brittle, hard structure - effectively a micro-needle with ceramic armour.

What starts as soft battery metal becomes, through an ultrathin reaction layer, a brittle micro-harpoon spear that cannot simply be pushed aside and instead punches straight through the separator.

That finding calls many existing safety strategies into question. If you expect a soft target, you design differently than you would for a stiff, brittle material that shatters into sharp fragments under pressure.

“Dead” lithium eats away at capacity cycle after cycle

The brittleness of dendrites creates a second problem that is less dramatic at first glance, but quietly destroys every battery over time. When one of these needles breaks under stress, tiny pieces of lithium are left behind in the electrolyte. These fragments are electrically isolated and no longer take part in the charge and discharge process.

In technical terms, this material is known as “dead lithium”. With each charging cycle, more of these dead islands are created. For the user, the effect looks like this:

• a phone only lasts half a day after a year,
• an electric car loses range even though it appears perfectly fine,
• home storage systems for solar power deliver less energy than originally stated.

The amount of active lithium in the battery shrinks, even though nothing outwardly seems to have changed. As a result, the cell reaches its practical end of life much sooner than it should in theory.

Why lithium-metal batteries have struggled for so long

The new study is especially important for the next generation of batteries: lithium-metal batteries. In laboratories and development departments, they are seen as a major hope. Instead of a graphite anode, they use almost pure lithium. That promises a far higher energy density.

Put simply: where today’s electric cars may achieve 300 to 400 kilometres per charge, lithium-metal cells could one day make 800 to 900 kilometres possible - without making the battery enormous. That is the goal behind billions of pounds in investment from carmakers and battery start-ups.

The problem is that dendrites form especially easily in these cells. And here their brittleness has its full destructive effect. Even supposedly robust solid electrolytes, often presented as a miracle solution, are perforated by the hard needles because their mechanical strength has been underestimated.

The new measurements show that many concepts for safe solid-state batteries simply underestimate how strong dendrites really are.

That helps explain why prototypes can perform well in the laboratory yet fail during long-term testing. The issue is less chemistry than mechanics on the nanometre scale.

Three material strategies to tame lithium battery dendrites

The research team now proposes three concrete material approaches to reduce dendrite formation at least to some extent:

• New lithium alloys: Pure lithium reacts extremely quickly at the surface. Adding other metals could influence the formation of the brittle surface layer. The aim would be dendrites that are less hard and therefore less destructive.
• Smarter separators: Instead of relying only on thin plastic films, batteries need mechanically adaptable layers that absorb stress before a dendrite tip can punch all the way through. Multi-layer separators with soft and hard zones are one possible approach.
• Targeted electrolyte additives: Special additives in the electrolyte could alter the crystal structure of the dendrites as they form. This might make it possible to encourage them to spread more broadly rather than growing as sharp needles.

Together, these routes offer a realistic chance of significantly increasing the range of future electric vehicles without constantly having to worry about fire risk or rapid capacity loss.

Another important point is manufacturing quality. At mass-production scale, even small changes in surface chemistry, pressure, or temperature can affect how dendrites begin and grow. That means battery design will increasingly depend on controlling not just the materials themselves, but also the conditions under which cells are built and operated.

What this means for electric cars and the energy transition

For the automotive industry, the finding is both awkward and promising. Awkward, because many development programmes were based on assumptions that now turn out to be wrong. Promising, because a clear analysis of the fault is often the first step towards practical solutions.

If dendrites can be controlled mechanically, high-energy-density batteries could solve several problems at once: smaller battery packs, lower raw material demand, cheaper vehicles and longer service life. That would make electric cars more attractive not only to high-mileage drivers, but also help scale up large storage systems for wind and solar power.

For consumers, the message is very direct: battery durability depends not only on chemistry, but also heavily on internal mechanics. Anyone who charges devices or a car very quickly on a regular basis places greater stress on the cell - and encourages dendrite growth. Conservative charging habits, such as slower overnight charging and avoiding a permanent 100% charge, can slow the process, even if they cannot stop it entirely.

A mistaken assumption held battery research back for decades

The new work also shows how risky long-unexamined assumptions can be in science. The similarity between dendrites and bulk lithium seemed so obvious that hardly anyone bothered to measure their true strength directly. Only precise observation at the nanoscale has now cleared up that error.

Studies like this will become even more important in the years ahead. As battery technology and electromobility move forward, entire industries become more sensitive to small errors in the models. A mechanism misunderstood at the micro- or nanoscale can ultimately decide the range, price and safety of entire generations of vehicles.