In a publication in Nature Communications, researchers at Stanford University and DIFFER study the charging behavior of nanomaterials used for batteries. Nanomaterials are preferred to large bulky structures, as they charge and discharge faster and have a longer lifetime. The team developed a technique to record charging of single nanoparticles in real time and with nanometer spatial resolution. They show how nanoparticles exploit their small size to accomodate strain during the loading process and therefore minimise defect formation. Understanding the loading behavior of nanomaterials can lead to improved battery designs.
In both lithium-ion and metal-hydride batteries, the charging and discharging processes are often associated with large volume changes in the energy storage materials. These systems use relatively large, micrometer-sized particles, where the resulting stresses can damage and degrade their performance over time. On the contrary, nanosized materials degrade less upon charging and discharging and have longer lifetimes. Together with researchers from the Dionne group at Stanford University, tenure track researcher Andrea Baldi at DIFFER investigated the charging process in far-smaller nanoparticles. Their goal: to better understand the underlying mechanisms of energy storage in nanomaterials and help design better energy storage solutions.
Nanoparticles minimise internal stress
Baldi and his colleagues investigated the mechanism of hydrogen absorption in nanosized cubes of palladium, a process very similar to lithium uptake in Li-ion battery electrodes, but easier to study.
"We saw that palladium nanocubes first absorb hydrogen at their corners, where they have more room to expand", explains Baldi. "Hydrogen then spreads out along a front parallel to a side of the cube. This front sweeps across the particle until it fills up entirely in a time span ranging from a few seconds to a few minutes in our experimental conditions. This charging behavior minimises stresses in the nanocube and prevents the formation of defects, contrary to what would happen in larger, bulkier specimens." Interestingly, while during the loading process the presence of both charged and uncharged areas reduces the crystal quality of the nanoparticles, the measurements show that this damage disappears after the charging is complete, which indicates that the nanoparticles can sustain the stresses associated with the charging process without breaking and without forming defects.
For the researchers to follow the charging process in individual nanoparticles, they needed to resolve details on the nanometer scale in a reactive environment and in real-time. The team used an in-situ electron microscope and looked at how the transmitted electron beam intensity varies depending on the amount of hydrogen absorbed in a nanoparticle. In their real-time movies, areas of different brightness indicate regions where the nanoparticle has expanded due to the absorption of hydrogen. Combining such real-time movies with diffraction images, they managed to reconstruct the step-by-step details of the charging process.
The results indicate that nanosized materials can accomodate strain much more effectively than bulky structures, making them very appealing for future battery applications and explaining their observed longer lifetimes. "To our knowledge this is the first time that these processes are visualized in real-time with such spatial resolution. The next step will be to apply these techniques to study more technologically relevant energy storage materials, such as lithium-ion battery electrodes."
Dr. Andrea Baldi
Group leader Nanomaterials for Energy Applications, DIFFER
+31 40 3334 925 | a  baldi  differ  nl (email)
Direct visualisation of hydrogen absorption dynamics in individual palladium nanoparticles
Tarun C. Narayan, Fariah Hayee, Andrea Baldi, Ai Leen Koh, Robert Sinclair & Jennifer A. Dionne
Nature Communications 8, Article number: 14020 (2017)