New Research Reveals How Oxygen Reduces a Battery’s Energy Storage Capacity Over Time

July 31, 2021 by Stephanie Leonida

Scientists investigate how the escape of oxygen from electrode nanoparticles contributes to battery degradation.

Researchers from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University conducted research concerning what happens when oxygen leaks out of lithium-ion batteries (LiBs) during charging and discharging.

The team observed and pierced apart the intricate nature of the process and observed how escaping oxygen atoms changed electrode structure, chemistry, and the amount of energy it could store. With further research, new ways of creating electrodes could be developed to prevent the reduction in energy storage capacity within the LiBs we use today. The research work was originally published in the journal Nature Energy.

Oxygen leaks out from nanoparticles that constitute the electrode of a LiB. Image used courtesy of the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University

Loss of Storage Capacity  

The SLAC and Stanford researchers describe LiBs as functioning like rocking chairs; moving lithium ions to and throw between two electrodes that temporarily store charge. It is known that oxygen atoms leak out of the many nanoparticles that constitute the electrode as lithium ions move back and forth. Until now, the signals produced from such leaks have been too small to measure.

In a news release from last month, a Ph.D. student from Stanford who worked on the project commented: “The total amount of oxygen leakage, over 500 cycles of battery charging and discharging, is 6%.” Csernica added: “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.”

The research team cycled LiBs for different lengths of time, deconstructed them, and then cut through the nanoparticles to examine what was going on. The team worked in the Lawrence Berkeley National Laboratory’s Advanced Light Source and used a specialized X-ray microscope to scan the samples. High-res images were produced and the vacancies left (or holes) by escaping oxygen atoms were probed. With the information from this examination and the use of a computational technique known as ptychography, the researchers planned to uncover nanoscale details (measured in billionths of a meter) of the oxygen-leaking process.

The team also pulsed X-rays through electrodes at SLAC’s Stanford Synchrotron Light Source. This was done to help confirm that what was happening at the nanoscale level was also true at a much larger scale.

Getting to the Source of Battery Degradation

In addition to experimental results, the team used computer modeling to uncover how oxygen might be escaping. Where nanoparticles were found more closely packed together in clumps, less oxygen appeared to be lost than nanoparticles closer to the surface.

The researchers also found that oxygen atoms escape the nanoparticles in a way not unlike a volcanic eruption. An initial burst of oxygen atoms escape from the nanoparticle surface, which would be like the volcano erupting, and then the oxygen atoms trickle out slowly from the interior like a lava flow. As oxygen trickles out of these nanoparticles, the voltage of the battery also fades. Although this process may be occurring at a small scale, over time, a battery can lose 10 to 15% of its storage capacity.

The research was also co-led by the Berkeley Lab and at its Advanced Light Source, used X-ray measurements to discover how oxygen leaks out of the nanoparticles. Image used courtesy of the Berkeley Lab

In the same news release, Associate Professor Will Chueh commented: “When oxygen leaves, surrounding manganese, nickel, and cobalt atoms migrate. All the atoms are dancing out of their ideal positions,” Chueh said. “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.” Chueh is an investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC.

From the research, methods for tackling battery degradation could be developed to create LiBs that are more safe and durable than existing models.