Lithium Batteries: Why Rigid Dendrites Threaten Safety and Range

For years, developers have been frustrated by lithium batteries that lose performance too soon or, in the worst cases, catch fire. A new nanoscale study now suggests that the real culprit lies in inconspicuous structures inside the cells - and that they behave mechanically in a way that textbooks and models have not previously anticipated.

What actually goes wrong inside a lithium battery

Smartphones, laptops and electric cars all rely, in most cases, on lithium-ion batteries. During charging, lithium atoms are deposited on the anode. Ideally, this happens evenly, creating a smooth metallic surface. In reality, however, thin needle-like formations known as dendrites often grow there.

These metallic needles are exceptionally fine - roughly 100 times thinner than a human hair. They can extend further into the cell with every charging cycle. Eventually, they may pierce the separator, the thin membrane intended to keep the anode and cathode apart.

That is when the situation becomes serious: dendrites form a kind of short-circuit bridge between the electrodes. Instead of travelling in a controlled manner through the intended circuit, current flows directly inside the cell. The consequences can include overheating, rapid capacity loss and, in extreme cases, thermal runaway and fire.

“New measurements show that these dendrites are not soft and deformable, but rigid, brittle and remarkably resilient.”

Until now, many safety concepts were based on the belief that the needles were as soft as the surrounding lithium metal and could be “pushed aside” or plastically deformed. This fundamental assumption has now been overturned.

Dendrites like dry spaghetti: what researchers actually observed

A team from the New Jersey Institute of Technology and Rice University has, for the first time, deliberately subjected lithium dendrites to mechanical stress under an electron microscope. The experiments were carried out in a high vacuum so that the sensitive structures would not react with oxygen in the air.

The outcome surprised even the specialists: when pressure is applied, dendrites do not bend but snap suddenly, much like dry spaghetti. Measurements indicate a yield strength of around 150 megapascals. By comparison, bulk lithium metal gives way at only about 0.6 megapascals.

The fine needles are therefore around 250 times more resilient than the original “lump” of metal from which they form. The reason is an extremely thin oxide layer that develops immediately on the dendrites’ surface. Although it is only a few nanometres thick, it dramatically alters their mechanical behaviour.

A soft metal is thus transformed into a rigid, brittle structure. Inside a battery, these needles act like tiny harpoons: they force their way through separators and even comparatively hard electrolyte layers without yielding to any meaningful extent.

“Dendrites behave more like glass fibres than a soft metal - and perforate separators rather than moving aside for them.”

“Dead lithium” - the invisible capacity killer

This brittleness brings a second, more insidious consequence. If a metal needle breaks inside the cell, a small electrically isolated fragment remains behind. That piece of lithium can no longer take part in charging and discharging.

Over successive cycles, increasing amounts of “dead lithium” can therefore accumulate. The quantity of lithium that remains actively usable falls, and capacity declines far earlier than theory would suggest. For users, this appears as rapidly diminishing range or significantly shorter battery life.

Why the great hope of the lithium-metal battery has so far failed

These findings are particularly significant for a technology in which the industry has placed major hopes: batteries with a pure lithium-metal anode. They are regarded as the next major advance beyond today’s lithium-ion cells.

The potential benefit would be substantial: using pure lithium as the anode could roughly triple energy density. An electric car that currently manages 300 kilometres only with difficulty could theoretically travel 800 to 900 kilometres. This is precisely why car manufacturers and suppliers around the world are investing billions in related research programmes.

Yet it is in these high-end systems that the dendrite issue is cornering developers. The new study clarifies why many prototypes have so far survived only a few hundred charging cycles.

• Rigid dendrites readily bore through separators and solid electrolytes.
• Broken fragments create large quantities of “dead lithium”.
• Capacity and safety deteriorate substantially earlier than planned.

This makes one point clear: without targeted dendrite management, the lithium-metal battery will remain a laboratory promise - appealing on paper but scarcely ready for mass production.

Three material strategies researchers want to use to control dendrites

The new understanding of dendrite mechanics forces the sector to change course. A particularly rigid solid-state electrolyte on its own is not enough. If the metal needles themselves are harder, they will simply push through it.

The research team therefore outlines three approaches, some of which could be combined:

1. Tailored lithium alloys

Rather than relying on pure lithium, alloys containing other metals could alter the spontaneous formation of the brittle oxide layer. The aim would be to create a surface less likely to produce harpoon-like dendrites, or one that forms blunter shapes that are more difficult to penetrate with as they grow.

Such alloys must meet several requirements at once: high storage capacity, good conductivity, low density and, above all, stability across many cycles. Considerable fundamental work remains, including research into the precise crystal structure and phase behaviour.

2. Smarter separators

Instead of simply becoming “thicker and stronger”, future separators should respond more intelligently from a mechanical perspective. One possibility is multilayer films in which individual layers yield in different ways. Dendrites would lose energy locally, break and, ideally, be prevented from spreading further.

Microscopic voids or flexible polymer inserts could also be used to absorb stress around the metal needles. The separator would then serve not merely as a barrier, but as an active buffer zone against mechanical penetration.

3. Electrolyte additives

The third route addresses dendrite growth directly. Certain additives in liquid or solid electrolytes can influence how lithium is deposited on the anode. With some luck, compact, rounded structures form instead of long, slender needles.

These additives control what is known as interfacial chemistry and the formation of the passivation layer (SEI). Even small quantities can change the crystal structure and, in turn, shape the dendrites’ later mechanical properties.

What this research means for electric cars and the energy transition

For car manufacturers, the study is a wake-up call. Anyone developing future generations of high-energy batteries must treat dendrites as a mechanical challenge, not solely an electrochemical one. Test protocols, safety standards and lifetime models need to account for this new perspective.

Longer range is only part of the story. Service life is at least as important. Batteries that still retain 80 per cent of their capacity after several thousand cycles significantly reduce total costs per kilometre and make electric cars more attractive to second and third owners.

Large-scale storage systems for solar and wind energy also depend on reliable cells. In this context, low failure rates and stability over many years matter especially. Every degradation mechanism that is better understood improves the predictability of such installations.

How persistent assumptions can hold back battery research

The work also demonstrates how long an inaccurate idea can persist. For decades, many groups simply assumed that dendrites behaved mechanically like ordinary lithium. The assumption fitted established models well - yet hardly anyone tested it directly.

Only nanoscale observation has now corrected that mistake. Such high-tech measurement techniques are consequently becoming a strategic tool: they make it possible to compare widely accepted model assumptions with reality on a regular basis.

Readers may find it useful to remember two terms:

• Dendrite: a tree-like or needle-shaped metallic structure that grows on the anode during charging.
• Separator: a porous barrier film within a battery that allows ions through but is designed to prevent short circuits.

Anyone who drives an electric car or is considering buying one need not be alarmed by these findings. Production vehicles have extensive safety systems, ranging from temperature monitoring to sophisticated battery-management systems. The study concerns primarily the next and subsequent generations of batteries.

The longer-term effect is what is particularly intriguing: if developers plan for dendrites from the outset as rigid, brittle structures with high strength, they can design materials, layer configurations and charging protocols far more precisely. That would improve range and safety - and could genuinely bring the breakthrough for lithium-metal technology a step closer.