Lithium metal batteries can store a lot of energy while being lightweight and are therefore considered promising for electromobility. But previous models have a problem: During operation, so-called lithium dendrites form, i.e. needle-shaped structures that grow from the anode and, in the worst case, can cause a short circuit. A study now shows that these dendrites are not soft and deformable as previously assumed, but rather hard and brittle. This finding opens up new approaches to counteract the problem.
Today’s electric cars are usually powered by a lithium-ion battery. But for longer ranges with lower weight, lithium metal batteries could offer a promising alternative. While the anode, i.e. the negative pole, in lithium-ion batteries is made of graphite, lithium-metal batteries use pure lithium, which achieves a significantly higher energy density.
However, a crucial problem has so far prevented the commercialization of these batteries: the lithium ions that migrate to the anode when the battery is charged are not deposited there evenly, but rather form needle-shaped structures, so-called dendrites, over time. If these dendrites break off, they accumulate as so-called “dead lithium”, which reduces the battery capacity. It’s even worse if they continue to grow the destructive tips. Because in this case they can reach the cathode and cause a short circuit, which can cause the battery to ignite.
Dry spaghetti instead of dough
In order to prevent the growth of dendrites, previous solutions experimented with solid electrolytes that are twice as stable as the soft light metal lithium. “But recent studies show that even solid electrolytes cannot stop the lithium dendrites,” explains a team led by Qing Ai from Rice University in Houston, Texas. “This begs the question: How can lithium, which is known to be soft and malleable, break through significantly harder electrolytes?”
In search of an answer to this question, the researchers extracted the dendrites, which were only a few nanometers in size, from a lithium metal battery and examined them under an electron microscope. When they applied controlled mechanical stress to the tiny light metal needles, they found that the material behaved very differently than expected. “Lithium dendrites were thought to be soft and pliable, similar to playdough,” says co-author Xing Liu from the Georgia Institute of Technology in Atlanta. “However, our results suggest that they are instead hard and brittle – they break more like dry spaghetti.” The team was able to observe this breaking off in real time under the electron microscope.
New approaches to solutions
But why do lithium dendrites have completely different properties than a lump of lithium? “To explain why lithium dendrites behave differently than previously thought, we carried out cross-scale simulations,” reports Liu. In combination with the electron microscopic data, the computer models showed that when the dendrites form in a battery, they are surrounded by a thin boundary layer of solid electrolyte (Solid Electrolyte Interphase, SEI). The combination of a crystalline lithium core and an SEI coating makes the structure so rigid that it breaks rather than bends under load and can also pierce materials that are significantly stronger than the lithium itself.
“Understanding the underlying physics provides new insights into how dendrites can be made less susceptible to brittle fracture,” explains Liu. For example, lithium alloys could be used for the anode instead of pure lithium to make the dendrites more flexible. “Adjustments to the microstructure of the solid electrolyte could also be a practical approach to reducing battery defects,” explains the team. “Understanding brittle fracture behavior thus provides insights to prevent the formation of inactive lithium and electrolyte breakdown, which could enable safer and more reliable lithium-metal batteries.”
Source: Qing Ai (Rice University, Houston, Texas, USA) et al., Science, doi: 10.1126/science.adu9988