How Lithium Battery Salts Shape Dendrites

A lithium battery contains a salt dissolved in liquid, while the metal performs the central role. Yet this unobtrusive salt can also determine whether a lithium battery develops dendrites – metal needles capable of short-circuiting the cell.

The salt is generally treated as an inactive supporting ingredient: essential 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 was, quietly, inviting failure.

Recording lithium’s earliest deposits

Capturing this process required a team headed by Zhiyuan Zeng at City University of Hong Kong (CityU). The researchers created a sealed cell slim enough to sit inside an electron microscope, allowing them to record lithium as it developed on an electrode.

Previous systems had been unable to reveal this stage. Commercial cells relied on windows roughly 100 nanometres thick and a liquid layer close to 1,000 nanometres deep, obscuring lithium’s earliest formation.

Zeng’s cell reduced the window thickness to 35 nanometres and narrowed the gap to a tiny sliver. This improved visibility enabled the team to observe the first metal specks and compare three widely used lithium salts.

All three contain the same lithium, but each has a different negatively charged counterpart – its anion – which the researchers believed was directing the metal’s growth.

Lithium battery dendrites take hold

On camera, the fluorine-free salt performed most poorly. Lithium supplied by this salt grew rapidly and irregularly, sending out dendrites – branching metal spikes that can pierce a cell and short-circuit it.

One spike raced sideways across the field of view in around 30 seconds, continuing to branch throughout. Another deposit began neatly before fragmenting and becoming spiky too. The dendrites appeared to encourage further dendrite growth.

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

The moss-like middle ground

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

Three areas expanded and contracted at different rates, but each remained flat. Here, the film was a hybrid structure: small, hard lithium fluoride crystals distributed through a softer, elastic material.

This combination served two purposes. Hard particles reinforced the film, preventing spikes from forcing their way through, while the softer component bent as the lithium expanded and contracted.

A layered defence develops

The third salt, a fluorine-rich material known as LiTFSI, produced an effect not seen with the others. Rather than forming a single blended film, it created two distinct layers, enabling the team to view lithium growth in a way that had not previously been filmed.

The internal layer was a rigid shell containing abundant lithium fluoride and measured around 20 nanometres thick. It was covered by a soft outer skin just a few nanometres deep: effectively a hard base beneath a flexible covering.

In the footage, lithium did not rise upwards. Instead, small, flat islands emerged, gradually moved together and then joined into a smooth sheet before dissolving. The video captured this lateral growth directly.

The two layers account for that spreading behaviour. The rigid inner shell transports lithium readily and, the team believes, enables newly deposited metal to move sideways and relieve stress – a creep that earlier studies had measured in other batteries.

What anions determine

Computer simulations linked the contrasting behaviour to the anions themselves. Because it had no fluorine available 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 with lithium to form hard crystals.

LiTFSI decomposed in stages. It first split at a weak internal bond, creating the soft outer skin, and then broke down closer to the metal, where it built up the hard layer below.

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

By comparison, films made with fluorine-containing salts distributed that attraction evenly, encouraging the metal to remain flat.

Tests in real batteries

Video evidence is valuable, but enduring operation in a functioning cell is a separate test. In coin cells, batteries containing LiTFSI continued cycling for more than 500 hours, whereas the fluorine-free version short-circuited in less than half that period.

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

Battery researchers were not surprised by this outcome, having long associated fluoride-rich films with longer cell life, as an earlier paper had shown elsewhere. What had not been achieved previously was watching, moment by moment, how that protection develops.

Designing safer cells

The findings provide a straightforward design principle for the film that protects lithium. It requires a hard fluoride inner layer to stop spikes and allow lithium to creep flat, topped by a soft layer that can flex without splitting.

Until this study, the principle was based on residual surfaces and computer simulations. The two-layer film, along with the lateral merging growth it enables, has now been observed directly.

This changes a well-supported theory into something engineers can observe and design for. For battery manufacturers developing lithium-metal cells that withstand fire and premature failure, the goal is now more precisely defined.

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