A study in Nature Communications used live electron microscopy to watch lithium metal form inside a working battery cell. The team found that the salt dissolved in the battery liquid can shape lithium’s first moments of growth, including whether it spreads into a smooth layer or erupts into dangerous spikes.
Researchers led by Zhiyuan Zeng at City University of Hong Kong built a thin sealed cell that could survive inside a transmission electron microscope. That window into a live battery let them compare three lithium salts and follow the metal as it appeared, branched, flattened and dissolved.
The discovery matters because lithium metal batteries are widely seen as a route to higher-energy storage. Their biggest weakness is the tendency of lithium to grow unevenly. When needle-like dendrites form, they can pierce internal barriers and trigger short circuits.
The new work points to a hidden design lever. The lithium ion was the same in each salt. The negatively charged partner, called the anion, changed the surface film that formed on the metal. That film then guided the shape of the growing lithium.
Filming Lithium as It Forms
Capturing the first specks of lithium inside a liquid battery is difficult. The metal forms quickly, reacts easily and grows in a space that is far smaller than a human hair. Traditional battery cells are also too thick and opaque for direct viewing at this scale.
Zeng’s team used in situ transmission electron microscopy, a method that allows scientists to watch materials change in real time. They built an electrochemical liquid cell with a very thin viewing window and a narrow liquid gap. According to the study details, the window was about 35 nanometers thick.
That slimmer setup improved the view of lithium’s earliest behavior. Earlier commercial cells used much thicker windows and deeper liquid layers, which blurred the first stages of metal growth. The new design let the researchers observe nucleation, the moment when tiny lithium deposits first appear on an electrode.
Once the cell was operating, the team filmed lithium as it plated onto a surface and later stripped away. In battery language, plating is the buildup of metal during charging. Stripping is the removal of that metal during discharge.
The movies gave the researchers something that surface snapshots could only suggest. They could see where lithium began, how it moved and how one growth pattern changed into another over seconds.
The Hidden Role of Battery Salt
The electrolyte in many lithium batteries contains a lithium salt dissolved in a liquid solvent. When the salt dissolves, it separates into positively charged lithium ions and negatively charged anions. The lithium ions move between electrodes as the battery charges and discharges.
The study focused on the anions because they help build the solid electrolyte interphase. This interphase is a thin film that forms where lithium metal meets the liquid electrolyte. It is a tiny boundary layer, yet it can decide whether the metal grows evenly or breaks into sharp structures.
Three salts were compared in the live-cell experiments. Each supplied lithium, while each carried a different anion. One was fluorine-free. Another was a common fluorine-containing salt used in many battery electrolytes. The third was a fluorine-rich salt known as LiTFSI.
The results showed that the anion can change the chemistry and mechanics of the protective film. A weak or patchy film leaves lithium growth poorly controlled. A stronger film can guide lithium into flatter shapes.
This is where the battery’s quiet chemistry becomes dramatic. A small molecular difference in the salt can alter the architecture of a film only nanometers thick. That film then steers metal growth large enough to damage a battery.
Why One Electrolyte Grew Dangerous Spikes
The clearest failure came from the fluorine-free salt. On camera, lithium grew quickly and unevenly. Branching metal spikes appeared, then continued to extend and split.
One dendrite moved sideways across the microscope view in roughly 30 seconds. Another deposit began in a more orderly shape, then broke apart and turned spiky. The behavior showed a self-reinforcing growth pattern. Once the surface became uneven, the next lithium tended to feed the unevenness.
These spikes are known as lithium dendrites. Their danger comes from shape and reactivity. A dendrite can push through the separator that keeps a battery’s two electrodes apart. If it connects both sides, the cell can short circuit.
Follow-up imaging helped explain the poor behavior. The surface film made by the fluorine-free electrolyte was soft, weak and uneven. It lacked enough hard material to hold the growing lithium flat.
Computer modeling supported the same picture. The fluorine-free anion stayed relatively intact on the lithium surface and failed to provide fluorine for tough lithium fluoride. The model also showed uneven electric-field hotspots on the surface, which would encourage lithium to pile up in narrow regions.
A Fluorine-Rich Film That Keeps Lithium Flat
The fluorine-containing salts produced very different growth. With the salt used in many current batteries, lithium formed low, moss-like deposits. They grew and shrank at different rates, yet they stayed relatively flat and avoided the long spikes seen with the fluorine-free salt.
The protective film in that case had a mixed structure. Tiny hard crystals of lithium fluoride were dispersed through a softer material. That combination gave the surface film two useful properties. It had stiffness from the hard crystals and flexibility from the surrounding material.
The LiTFSI electrolyte went a step further. Instead of one blended film, it formed a two-layer interphase. The inner layer was rich in lithium fluoride and measured about 20 nanometers thick. A softer outer skin sat above it.
On video, lithium grown under this layered film behaved in a striking way. Small flat islands appeared on the surface. They spread sideways, moved toward one another and merged into a smoother sheet. When the lithium dissolved, the deposits disappeared without leaving the same branching structures seen in the weaker electrolyte.
The layered structure gives engineers a useful mental picture. The hard inner layer acts like a floor that resists puncture. The soft outer layer flexes as lithium expands and contracts. Together, they help the metal spread laterally instead of rising into sharp tips.
Real Batteries Put the Discovery to the Test
Microscope movies reveal mechanisms, while battery tests show whether those mechanisms matter in a working device. The researchers therefore tested the electrolytes in coin cells. These small batteries are widely used in laboratory studies because they provide a controlled way to compare chemistries.
The cells using LiTFSI ran for more than 500 hours. The fluorine-free cells short-circuited in less than half that time. That difference matched the live imaging, where the fluorine-free electrolyte produced the most dangerous dendrite growth.
Efficiency also separated the salts. The LiTFSI cells returned about 92% of their lithium during each cycle. The fluorine-free cells returned less than 40%. In simple terms, much more lithium was lost to unwanted reactions and trapped deposits in the weaker chemistry.
The study builds on a long-standing idea in battery research. Fluoride-rich interphases are often linked to better lithium-metal performance. The new contribution is visual proof of how those films form and how they guide lithium second by second.
That distinction is important for design. A final surface image can show what remains after cycling. A live movie shows the path that produced it. For batteries, the path often determines whether a cell ages smoothly or fails early.
A Design Rule for Safer Lithium Metal Cells
The work points to a practical rule for electrolyte design. A strong lithium-metal interface should form a hard fluoride-rich layer close to the metal, with a softer layer above it. That pairing can resist dendrite penetration while still allowing the surface to flex.
For battery makers, the anion becomes a direct target. By choosing or designing salts that break down in the right way, engineers may be able to create better interphases on demand. The study suggests that fluorine-bearing anions can supply the chemistry needed for hard lithium fluoride layers.
The work also gives researchers a screening tool. Instead of waiting until a cell fails after long cycling, scientists can watch early lithium growth inside a thin liquid cell. Smooth lateral growth would be a promising sign. Rapid branching would flag a risky electrolyte.
Several questions remain for future battery development. Laboratory coin cells and microscope cells simplify the messy conditions inside commercial packs. Real batteries face thicker electrodes, higher currents, temperature swings and manufacturing constraints. The interphase design still needs to work under those practical conditions.
Even so, the study sharpens the target for safer lithium metal cells. The salt dissolved in the electrolyte can build the film that controls the metal. When that film forms with the right layers, lithium has a better chance to grow flat, cycle longer and avoid the spikes that can end a battery’s life.
