How Lithium Battery Salts Shape Dendrite Growth

Within a lithium battery, a salt is dissolved in liquid while the metal carries out the crucial task. Yet this unobtrusive salt can also determine whether the lithium battery develops dendrites: metal spikes capable of causing a short circuit.

The salt is supposed to be an inactive supporting component, the sort of plumbing that attracts little attention. However, by observing lithium accumulate inside an operating cell, researchers discovered that changing one component of the salt altered the way the metal formed.

One formulation appeared to be heading quietly towards failure.

Capturing the first lithium deposits

A group led by Zhiyuan Zeng at City University of Hong Kong (CityU) had to develop a way to record that process. They made a sealed cell sufficiently thin to sit inside an electron microscope, allowing them to film lithium forming on an electrode.

Previous arrangements had been unable to see this stage. Commercial cells had windows around 100 nanometres thick and liquid layers approaching 1,000 nanometres deep, obscuring the earliest stages of lithium deposition.

Zeng’s cell reduced the window thickness to 35 nanometres and narrowed the gap to a thin sliver. With that improved view, the researchers could record the first metal specks and compare three widely used lithium salts.

All three contain lithium, but each has a different negatively charged counterpart - its anion - which the team believed could control how the metal developed.

Lithium battery dendrites take hold

The fluorine-free salt gave the poorest result on film. Lithium supplied by this salt deposited rapidly and irregularly, producing dendrites - branching metal spikes that can pierce a cell and short-circuit it.

One spike travelled sideways through the field of view in roughly 30 seconds and continued branching throughout. Another deposit began neatly before breaking apart and also becoming spiky. The dendrites effectively encouraged further dendrite formation.

Chilled images of the remaining surface revealed the reason. The thin film produced where lithium contacts the liquid was soft and fragile in this case, leaving it too weak and uneven to maintain a flat metal surface.

A moss-like middle ground

Lithium performed much better with the salt found in most current batteries. It formed shallow, uniform mounds described by the team as moss-like, and no spikes appeared after the metal dissolved.

Although three patches expanded and contracted at different rates, each remained flat. In this case, the film was a hybrid structure, combining tiny hard lithium fluoride crystals with a softer, elastic material.

That combination served two purposes. The hard particles reinforced the film to prevent spikes breaking through, while the softer material could bend as the lithium expanded and contracted.

LiTFSI creates a layered defence

The third salt, a fluorine-rich material known as LiTFSI, behaved unlike either of the others. Rather than forming one blended film, it created two distinct layers, enabling the team to observe lithium growth in a way that had not previously been filmed.

The inner layer was a rigid shell containing abundant lithium fluoride and measured about 20 nanometres thick. It was topped by a soft outer skin only a few nanometres thick: effectively a flexible cover over a hard base.

In the footage, lithium did not rise upwards. Instead, small flat islands emerged, moved gradually towards one another and then merged into a single smooth layer before dissolving. The video captured this lateral growth directly.

The two layers account for that spreading behaviour. The rigid inner shell allows lithium to move readily and, the researchers believe, enables fresh metal to flow sideways and relieve stress - a creeping process that earlier work had measured in other batteries.

What anions control

Computer simulations linked these contrasting outcomes to the anions. Lacking fluorine to release, the fluorine-free salt remained intact on lithium and did not provide the hard fluoride needed to strengthen a film.

In the simulations, both salts containing fluorine broke down much more easily, releasing fluorine that bonded to lithium to form hard crystals.

LiTFSI broke down in separate stages. It first split at a weak internal bond, producing the soft outer skin, then decomposed closer to the metal, where it assembled the hard underlying layer.

The models also charted the electrical force across each surface. The fluorine-free film showed intense, irregular hotspots that could direct lithium into tall spikes.

By comparison, the fluorine-containing films distributed that force more evenly, encouraging the metal to remain flat.

Tests in real batteries

Video evidence is valuable, but durability in an active cell is another matter. In coin cells, batteries containing LiTFSI operated for more than 500 hours, whereas the fluorine-free version short-circuited in less than half that time.

Efficiency produced the same contrast. LiTFSI cells recovered about 92% of their lithium during each cycle, while fluorine-free cells achieved under 40%, losing most of their metal.

Battery researchers were not surprised by this result, as fluoride-rich films had long been associated with longer cell life, as an earlier paper had shown elsewhere. What had not been achieved before was observing, second by second, how that protective structure develops.

Designing safer cells

The findings offer a straightforward design principle for the film protecting lithium. It requires a hard fluoride-rich inner layer to stop spikes and allow lithium to creep sideways, beneath a soft layer that can flex without splitting.

Previously, this principle was based on residual surface evidence and computer modelling. The two-layer film, along with the lateral merging growth it permits, has now been observed directly.

This changes a well-supported idea into something engineers can monitor and design for. For battery manufacturers pursuing lithium-metal cells that avoid fire and premature failure, the objective is now more clearly defined.

They can adjust a salt’s anion to produce the appropriate two-layer film, then assess new salts by observing how the lithium deposits.