Study sheds light on pathways leading to degradation of layered Li-rich oxide cathodes

In recent years, researchers have attempted to develop increasingly advanced battery technologies that can store more energy, charge faster, discharge more slowly, and have a longer lifespan. To achieve this, many have experimented with new cathode materials, as these tend to contribute significantly to a battery’s performance.

Layered lithium-rich transition metal oxides have recently become the focus of many research studies, as they have been shown to be promising cathode materials. As cathode materials, they could theoretically help increase the energy density of rechargeable batteries for electric vehicles and portable devices.

The advantages of layered lithium-rich metal oxide cathodes come from their layered structure and composition. Their structure allows lithium atoms to move through the layers during battery operation, while their richness in lithium allows them to store and release more energy during charge/discharge.

Additionally, these cathodes contain transition metals such as manganese (Mn), cobalt (Co) or nickel (Ni) and oxygen anions, which can facilitate redox (reduction-oxidation) reactions within batteries. It is these reactions that allow batteries to gain and lose electrons, contributing to their energy production.

Despite their advantages, many lithium-rich metal oxide cathodes deteriorate quickly and lose voltage over time. This, along with their instability, has until now prevented their large-scale use in battery development.

Researchers from Sichuan University, Southern University of Science and Technology in China and other institutes in various countries around the world recently conducted a study on the pathways leading to the degradation of these rich oxide cathodes in lithium.

Their article, published in Nature Nanotechnologydescribes some of the structural, chemical, kinetic, and thermodynamic effects that contribute to the short reported lifetimes of batteries containing these cathodes.

“We integrate analyzes of the morphological, structural, and oxidation state evolution of individual atoms with secondary particles,” Zhimeng Liu, Yuqiang Zeng, and colleagues wrote in their paper. “By performing characterizations at the nanoscale and microscale, distinct pathways of structural change associated with heterogeneous intraparticle reactions are identified.”

The researchers took a close look at what was happening in the cathodes at the nanoscale and microscale using a variety of advanced imaging techniques. This included energy-resolved transmission X-ray microscopy (TXM), which allows researchers to visualize materials at remarkably high resolution, while also collecting information about their structural and chemical composition.

Using TMX, the team identified various oxygen defects and distortions at different loading rates during the first operating cycle. These defects were found to cause degradation via a variety of possible pathways.

“The high level of oxygen defects formed in the particle by slow electrochemical activation triggers a gradual phase transformation and the formation of nanovoids,” Liu, Zeng and colleagues wrote.

“The ultrafast (de)intercalation of lithium leads to a lattice shift dominated by oxygen distortion, a dissolution of transition metal ions and a variation of the lithium site. These inhomogeneous and irreversible structural changes are responsible of the low initial Coulombic efficiency, as well as the continued cracking and expansion of the particles in later cycles.

The recent study sheds new light on the structural and chemical factors behind the degradation of Li-rich layered cathodes over time. In the future, the results collected by this research group could inform the development of effective strategies to reduce or mitigate these factors, which could in turn facilitate the use of these cathodes in next-generation batteries.

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