Introduction: Recent research indicates that the role of oxygen in limiting the performance of lithium-ion batteries has been consistently underestimated. Newly published studies from Japan and the United States attempt to delve deeper into the chemical reactions at the core of lithium-ion storage and better describe the cumulative effects of the small amounts of oxygen released during these reactions on battery performance and safety.
Although lithium-ion batteries power a variety of devices we rely on every day and their use in vehicles and the power grid is rapidly increasing, there are still some shortcomings in the technology in terms of performance and lifespan.
Much of the ongoing research into improving battery technology focuses on new materials, while also considering supply chain and environmental issues associated with several commonly used materials. Regardless of the materials used, however, developing complex new technologies that allow scientists to closely examine the working mechanisms within batteries is crucial for understanding where performance bottlenecks occur and how to address them.
Two independent studies published last month used this technique to investigate the role of oxygen in lithium-ion battery performance. It is well known that small amounts of oxygen are released when batteries are charged and discharged. However, the scale of this process makes it difficult to detect, and the far-reaching effects of oxygen loss are not well understood. Peter Csernica, a scientist at Stanford University involved in one of the studies, explained, “Over 500 charge-discharge cycles, the total amount of oxygen leaked is 6%. That’s a significant number, but if you measure the amount of oxygen released per cycle, it’s only about 1%.”
In this Stanford-led study, the team cut open the battery electrodes after cycling, scanned the samples with X-ray microscopy, and combined this with computational imaging to observe the nanoscale structure. They also pierced the entire electrode with X-rays to confirm that their nanoscale observations could be applied to the entire assembly. The team found that oxygen was initially released from the surface in an “explosive” manner, and then released from deep within the cathode in a slower “dripping” manner.
They discovered that the release of oxygen fundamentally altered the structure of the cathode. As the oxygen left, surrounding manganese, nickel, and cobalt atoms migrated, all jumping out of their ideal positions. Stanford University Associate Professor William Chueh explained, "This rearrangement of metal ions, coupled with the chemical changes caused by oxygen deficiency, reduces the battery's voltage and efficiency over time. This phenomenon has long been known, but the mechanism was unclear."
In another study, scientists led by Tohoku University in Japan found that in cathodes based on equal amounts of nickel, cobalt, and manganese, oxygen release promotes several undesirable reactions that damage the battery structure. The presence of high-valence nickel in the cathode leads to a higher level of oxygen release, and the process as a whole reduces the battery's ability to maintain a balanced charge.
"Our findings will help to further develop next-generation batteries with high energy density and robustness composed of transition metal oxides," said Takashi Nakamura, a researcher at Tohoku University.
These two studies highlight the role of oxygen in battery degradation and confirm that it may be a more significant challenge than previously thought, potentially laying the foundation for future work that considers or even focuses on limiting oxygen loss during cycling and its destructive impact on batteries.
