Battery researchers have engaged in a lot of work recently to study the formation of lithium dendrites, or needle-like structures and whiskers that form within the battery and cause them to degrade, which can result in fire or an explosion.
New research toward this end comes from a team at Michigan Technological University, where researchers studied tiny defects at the micro scale that form between a lithium anode and a solid electrolyte to find clues to the formation of dendrites.
A team led in part by Erik Herbert, assistant professor of materials science and engineering at Michigan Tech, hopes their work can lead to the development of a safe and long-lasting lithium battery using a solid-state electrolyte, he said.
Researchers focused on studying the unique mechanics of lithium at micro-dimensions to analyze the spot where tiny defects form that can take down the whole battery, Herbert said. “People think of lithium as being soft as butter, so how can it possibly have the strength to penetrate through a ceramic solid electrolyte separator?” he said in a press statement.
By zeroing in on the mechanics of lithium metal at length scales that are on par with those tiny interface defects, Herbert and his team.
discovered that lithium there is much stronger than it is at macroscopic or bulk-length scales, he said.
“Lithium doesn’t like stress any more than you or I like stress, so it’s just trying to figure out how to make the pressure go away,” Herbert explained in the press statement. “What we’re saying is that at small-length scales, where the lithium is not likely to have access to the normal mechanism it would use alleviate pressure, it has to rely on other, less efficient methods to relieve the stress.”
Defect Danger Zone
Researchers identified what they call a defect “danger zone” in lithium-ion batteries where catastrophic problems may occur, said Herbert and his research co-lead, Michigan Tech professor Stephen Hackney.
As the researchers define it, this zone is a window of physical defect dimensions that are defined by the tension that occurs between the opposing diffusion and dislocation motions found in every crystalline metal like lithium. These atomic level defects are needed to relieve significant amounts of stress in the material; however, compared to dislocation motion, diffusion is very inefficient, researchers said. This can lead to an imbalance in the amount of pressure the lithium can support on different scales.
The worst-case scenario, then, is a physical interface defect such as a micro crack, pore, or surface roughness that is too big for efficient stress relief by diffusion, but too small to enable stress relief by dislocation motion, they said. This means that high stresses within the lithium can cause the solid electrolyte and the whole battery to fail spectacularly.
But perhaps is most interesting to researchers in these observations is that the danger zone is the same size as lithium dendrites they also found, Hackney said.
“To make the solid-state technology viable, the power capability and cycle life limitations must be addressed,” he noted in a press statement. “Of course, the first step in solving the problem is to understand the root cause, which is what we are trying to do with this current work.”
Researchers published a paper on their work in the Journal of Materials Research.
While the idea that something smaller is stronger is not new, it’s something that until now has not been applied by researchers studying the problem of dendrites in lithium batteries, Hackney said. However, the team believes they have proven that it is now relevant to the discussion, he said.
Researchers plan to continue their work by examining the effects of temperature and electrochemical cycling on the mechanical behavior of lithium at small-length scales. They think this will help them better understand real-world conditions and strategies to help improve next-generation batteries and prevent dendrite growth, Hackney said.
