BATTERY CATHODES New discovery could provide a boost to battery cathodes

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A collaborative team of researchers from the University of Oxford, Henry Royce and Faraday Institutions, and Diamond Light Source say that they’ve been able to identify the nature of oxidized oxygen in lithium-rich nickel manganese cobalt oxide, an important battery material.

This is according to a new paper published in the journal Nature Energy on September 21. It describes how the research team has been able to identify the nature of oxidized oxygen in the lithium-rich material using resonant inelastic X-ray scattering (RIXS) at Diamond Light Source, the UK’s national synchrotron light source facility in Oxfordshire.

The compound, lithium-rich nickel manganese cobalt oxide (Li-rich NMC), as it’s known in the battery industry, has attracted lots of attention from researchers who are considering it for use in next-generation lithium-ion (Li-ion) batteries because it can deliver higher energy density than even current state-of-the-art battery materials. This could see applications using batteries based on the compound benefitting from, for example, longer driving ranges in the case of electric vehicles or more power capacity in the case of robotics.

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What’s the study about?

In their paper, the researchers explain how they expect that their work will enable scientists and engineers to overcome issues such as voltage fade and battery longevity.

Their work focuses very much on understanding the first cycle voltage hysteresis where the O-redox process cannot be fully recovered. This results in a voltage loss and thus a lower energy density.

"Our current work focuses on the Li-rich material Li1.2Ni0.13Co0.13Mn0.54O2. The key findings as before show the formation of free O₂ molecules inside the materials, which has not been appreciated before in the community. This is a very important discovery as the material has higher TM-O covalency which was thought to suppress formation of molecular O₂ ,” says Kejin Zhou, a scientist at Diamond Light Source.

Why is this important?

According to the study, this work could have a huge impact on battery cathode designs in minimizing the unstable honeycomb structure. In addition, it may also have important consequences for solving other issues associated with the Li-rich NMC compound, such as voltage fade which hinders its commercialization. The team also hopes that their work will enable the discovery of new materials that might be able to harness O-redox more reversibly.

What’s more, lithium-rich cathode materials are one of only a few known options that could increase the energy density of Li-ion batteries. Almost all of the lithium in these structures can be removed and compensated by oxidation of the transition metal ions first and then the oxide ions. However, as mentioned earlier, the high voltage of this O-redox process when charging is not recovered when discharging, and this leads to a loss of energy density and a substantial challenge standing in the way of making full use of these lithium-rich materials.

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Although the understanding of why this happens remains incomplete, this study represents a major leap forward as it provides a new mechanism for explaining the O-redox process.

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