Why do lithium batteries degrade over time? US Researchers have collaborated to discover more about the mechanisms behind this degradation, with promises of improved battery life.
Lithium batteries are used today to power many portable electronics devices. But lithium-ion batteries suffer nano-scale structural damage with each charge and discharge cycle.
In two recent Nature Communications papers, scientists from several U.S. Department of Energy national laboratories—Lawrence Berkeley, Brookhaven, SLAC, and the National Renewable Energy Laboratory—collaborated to map these billionths-of-a-meter changes, with the aim of enabling better batteries.
According to Huolin Xin, a materials scientist at Brookhaven Lab's Centre for Functional Nanomaterials, the team discovered surprising and never-before-seen evolution and degradation patterns in two key battery materials.
“Contrary to large-scale observation, the lithium-ion reactions actually erode the materials non-uniformly, seizing upon intrinsic vulnerabilities in atomic structure in the same way that rust creeps unevenly across stainless steel,” Xin said.
The scientists used leading electron microscopy techniques to directly visualize the nanoscale chemical transformations of battery components during each step of the charge-discharge process. This gave them a precise map of the materials' erosion, allowing them to plan new ways to break the patterns and improve performance.
After repeated charge-discharge cycles, Xin and his team extracted and analysed the anodes via electron tomography.
To see the way the lithium-ions reacted with the nickel oxide, the scientists used custom-written software to digitally reconstruct the three-dimensional nanostructures with single-nanometer resolution.
The team found the reactions occurred at isolated points rather than sweeping evenly across the surface. These points were areas of irregularity on the nickel oxide anode which allowed the formation of metallic nickel, which is a contributor to the lithium battery's degradation.
In another study, the scientists wanted to find the voltage sweet-spot for high-performing lithium-nickel-manganese-cobalt-oxide (NMC) cathode.
The amount of power that a NMC-cathode-based lithium battery can store, its intensity, and its cycle life depends on intrinsic material qualities and the structural degradation caused by the charge and recharge cycles.
The researchers found the chemical evolution exhibited sprawling surface asymmetries, though not without profound patterns.
“As the lithium ions race through the reaction layers, they cause clumping crystallization—a kind of rock-salt matrix builds up over time and begins limiting performance,” Xin said.
“We found that these structures tended to form along the lithium-ion reaction channels, which we directly visualized under the TEM. The effect was even more pronounced at higher voltages, explaining the more rapid deterioration.”
Finding these channels means researchers can possibly use atomic deposition to coat the cathodes with elements that resist crystallisation, creating nanoscale boundaries against their degrading effects.
The researchers are also working on ways to watch the complex chemical reactions in the batteries in real-time, allowing better insight into their performance, rather than the "stop-and-go" approach to observation.