Lithium-ion batteries work like a rocking chair, moving lithium ions back and forth between two electrodes that temporarily store charge. Ideally, those ions are the only things moving in and out of the billions of nanoparticles that make up each electrode.
But researchers have known for some time that oxygen atoms leak out of the particles as lithium moves back and forth. The details have been hard to pin down because the signals from these leaks are too small to measure directly.
Now, in a study published in Nature Energy, a research team co-led by SLAC National Accelerator Laboratory, Stanford University, and Berkeley Lab has measured this process with unprecedented detail, showing how the vacancies left by escaping oxygen atoms change the electrode’s structure and chemistry and gradually reduce how much energy it can store.
At the Advanced Light Source (ALS), the researchers used scanning transmission x-ray microscopy (STXM) at Beamlines 7.0.1 (COSMIC) and 11.0.2 to scan electrode nanoparticles. This information was combined with a computational technique called ptychography to reveal nanoscale details. At SLAC’s Stanford Synchrotron Radiation Lightsource, the team shot x-rays through entire electrodes to confirm that what they were seeing at the nanoscale was also true at a much larger scale.
Comparing the experimental results with computer models of how oxygen loss might occur, the team concluded that an initial burst of oxygen escapes from the surfaces of particles, followed by a very slow trickle from the interior. Where nanoparticles glommed together to form larger clumps, those near the center of the clump lost less oxygen than those near the surface.
The results contradict some of the assumptions scientists had made about this process and could suggest new ways of engineering electrodes to prevent it.
“Previously, researchers were not able to access the length scales needed to study oxygen release in batteries from the primary particle to the electrode level. COSMIC’s ability to achieve few-nanometer spatial resolution with chemical specificity across a wide field allowed us, for the first time, to study the influence of microstructure on this phenomenon,” said co-senior author David Shapiro, who is the lead scientist for COSMIC’s microscopy experiments. Shapiro also leads the ALS Microscopy Program.
P.M. Csernica, S.S. Kalirai, W.E. Gent, K. Lim, Y.-S. Yu, Y. Liu , S.-J. Ahn , E. Kaeli, X. Xu, K.H. Stone, A.F. Marshall, R. Sinclair, D.A. Shapiro, M.F. Toney, and W.C. Chueh, “Persistent and Partially Mobile Oxygen Vacancies in Li-Rich Layered Oxides,” Nat. Energy 6, 642 (2021), doi: 10.1038/s41560-021-00832-7.
Adapted from the SLAC press release, “Scientists discover how oxygen loss saps a lithium-ion battery’s voltage.”