When lithium ions travel in and out of a battery electrode during charge-discharge cycles, a small amount of oxygen leaks out and the battery’s voltage — a measure of the amount of energy it delivers — also reduces in the same corresponding amount. As these losses increase over time, they can ultimately sap the energy storage capacity of the battery by 10% to 15%.
Now scientists have quantified this super-slow process with unparalleled detail, demonstrating how the vacancies or holes, left by escaping oxygen atoms, alter the structure and chemistry of the electrode and slowly reduce the amount of energy it can store.
The latest findings contradict some of the assumptions made by scientists about this process and may provide new ways of designing electrodes to prevent it.
The researchers from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have recently explained their study in the Nature Energy journal.
“We were able to measure a very tiny degree of oxygen trickling out, ever so slowly, over hundreds of cycles. The fact that it’s so slow is also what made it hard to detect,” stated Peter Csernica, a PhD student from Stanford University who worked on the experiments with Associate Professor Will Chueh.
A Two-Way Rocking Chair
Lithium-ion batteries operate similar to a rocking chair, shifting lithium ions to and fro between a pair of electrodes that store charge only for a brief time. Preferably, these ions are the only things that move in and out of an unlimited number of nanoparticles that constitute each electrode.
However, scientists have known for some time that when lithium shifts back and forth, oxygen atoms tend to escape from the particles. These details have been difficult to resolve because the signals from such leaks are too insignificant to be directly quantified.
The total amount of oxygen leakage, over 500 cycles of battery charging and discharging, is 6%. That’s not such a small number, but if you try to measure the amount of oxygen that comes out during each cycle, it’s about one one-hundredth of a percent.
Peter Csernica, PhD Student, Stanford University
In this analysis, the team quantified the leakage indirectly by observing how the loss of oxygen alters the structure and chemistry of the particles. They monitored the process at various length scales — from the smallest nanoparticles to groups of nanoparticles to the entire thickness of an electrode.
Since it is very hard for oxygen atoms to travel around in solid materials at battery-operated temperatures, the traditional wisdom has been that oxygen escapes only from the nanoparticle surfaces, Chueh added, even though this concept has been up for discussion.
To better understand what is exactly taking place, the researchers cycled the batteries for varying amounts of time, then took them apart, and finally cut the electrode nanoparticles for elaborate analysis at Lawrence Berkeley National Laboratory’s Advanced Light Source.
At the laboratory, a dedicated X-ray microscope was used to scan over the samples, making high-resolution images and probing the chemical composition of every tiny spot. This data was integrated with a computational method, known as ptychography, to expose nanoscale details, quantified in billionths of a meter.
In the meantime, at SLAC’s Stanford Synchrotron Light Source, the researchers shot X-rays via complete electrodes to prove that what they were visualizing at the nanoscale level was equally true at a relatively larger scale.
A Burst, Then a Trickle
Matching the experimental findings with computer models of how oxygen loss could have occurred, the researchers surmised that an initial burst of oxygen leaks away from the particle surfaces, followed by a very slow flow from the interior. Wherever nanoparticles clumped together to create bigger clumps, those close to the core of the clump lost less oxygen when compared to those close to the surface.
According to Chueh, another significant question is how the loss of oxygen atoms impacts the material they left behind.
That’s actually a big mystery. Imagine the atoms in the nanoparticles are like close-packed spheres. If you keep taking oxygen atoms out, the whole thing could crash down and densify, because the structure likes to stay closely packed.
Will Chueh, Associate Professor, SLAC National Accelerator Laboratory
Since this feature of the electrode's structure could not be imaged directly, the researchers again made a comparison between other kinds of experimental observations and computer models of different oxygen loss scenarios.
The new findings suggest that the vacancies certainly persist — the material does not densify or crash down — and indicate how they play a role in the gradual decline of the battery.
When oxygen leaves, surrounding manganese, nickel, and cobalt atoms migrate. All the atoms are dancing out of their ideal positions. This rearrangement of metal ions, along with chemical changes caused by the missing oxygen, degrades the voltage and efficiency of the battery over time. People have known aspects of this phenomenon for a long time, but the mechanism was unclear.
Will Chueh, Associate Professor, National Accelerator Laboratory
Currently, Chueh concluded, “we have this scientific, bottom-up understanding” of this significant source of battery degradation, which may result in new ways to reduce oxygen loss and its damaging impacts.
Csernica, P. M., et al. (2021) Persistent and partially mobile oxygen vacancies in Li-rich layered oxides. Nature Energy. doi.org/10.1038/s41560-021-00832-7.