In a megascience-scale collaboration with French researchers from College de France and the University of Montpellier, Skoltech scientists have shown a much-publicized problem with next-generation lithium-ion batteries to have been induced by the very experiments that sought to investigate it. Published in a Russian Science Foundation-backed study in Nature Materials, the team’s findings suggest that the issue of lithium-rich cathode material deterioration be approached from a different angle, giving hope for more efficient lithium-ion batteries that would store some 30% more energy.

Efficient energy storage is critical for the transition to a low-carbon economy, whether in grid-scale applications, electric vehicles, or portable devices. Lithium-ion batteries remain the best-developed electrochemical storage technology and promise further improvements. In particular, next-generation batteries with so-called lithium-rich cathodes could store about one-third more energy as their state-of-the-art counterparts with cathodes made of lithium nickel manganese cobalt oxide, or NMC.

A key challenge hindering the commercialization of lithium-rich batteries is voltage fade and capacity drop. As the battery is repeatedly charged and discharged in the course of normal use, its cathode material undergoes degradation of unclear nature, causing gradual voltage and capacity loss. The problem is known to be associated with the reduction and oxidation of the oxygen atoms in NMC, but the precise nature of this redox process is not understood. This theoretical gap undermines the attempts to overcome voltage fade and bring next-generation batteries to the market.

A leading hypothesis has purported that over the lifetime of a battery, the oxygen atoms originally incorporated into the crystal structure of the cathode, form the familiar O₂ molecules — like those in the air we breathe. In fact, several studies using advanced X-ray spectroscopy have detected the O₂ signature in lithium-rich cathode materials. That form of oxygen is almost electrochemically inactive, degrading the battery’s performance. In a way, this hypothesis spelled disaster for next-generation batteries, because once formed, the O₂ molecules are so stable that this unwanted process would be very hard to reverse.

“Thankfully, our latest study relegates the molecular oxygen hypothesis to history,” said Assistant Professor Dmitry Aksyonov of Skoltech Energy, who co-authored the research. “By examining the data from major X-ray scattering experiments, we have demonstrated that the O₂ molecules trapped in the cathode material and supposedly responsible for its worsening performance are likely the artifact of the experiment. Apparently, their formation was induced by the very X-rays used to discover them.”

By resolving the long-standing uncertainty in the mechanism of oxygen oxidation in NMC cathode materials, the discovery enables further research to focus on ways of stabilizing so-called structural oxygen. This refers to the oxygen atoms that never actually detach from the cathode material’s crystal structure to form separate molecules but merely lose an electron in the course of battery operation. According to the researchers, stabilizing cathode materials with that problem in mind will be easier than if the molecular oxygen hypothesis had proved right.

“This study is an example of great synergy between experiments, theory, and computer modeling,” said Research Scientist Andrey Geondzhian from Skoltech Energy, who modeled the resonant inelastic X-ray scattering spectra, enabling the correct interpretation of the findings of the megascience-class experiment carried out in France. “Without modeling, it would have been impossible to unambiguously determine whether the O₂ molecules are completely detached or still maintain some bonding with the structure. Conversely, the experimental data provided stringent constraints that narrowed down the range of possible scenarios and allowed us to propose the pathway of how X-rays promote the formation of molecular oxygen.”

Study co-author and the director of Skoltech Energy, Distinguished Professor Artem Abakumov commented: “We hope our findings will inspire new optimization strategies to fine-tune the balance between oxygen oxidation, metal dissolution, and nanovoid formation — and their interaction with coating and doping approaches for layered cathodes. A deeper understanding of these factors could significantly enhance the lifespan of future lithium-ion batteries based on NMC materials.”