As part of the transition to net zero, the Faraday Institution’s CATMAT (Lithium Ion Cathode Materials) project is focusing on improving lithium-ion battery energy density and electric vehicle (EV) range. Its scope includes adding to our understanding of lithium-rich (Li-rich) oxygen-redox cathodes and novel anion-chemistry cathodes, as well as developing scalable synthesis routes for these materials. As part of this project, researchers from the University of Oxford are working with Diamond’s I21 beamline to explore the cause of voltage fade in Li-rich cathodes, using high-resolution resonant inelastic X-ray scattering (RIXS) spectroscopy. In work recently published in Nature Materials, they followed the oxygen redox reaction in Li-rich cathodes over cycling and quantitatively measured the O2 trapped within the material. Their results show that a gradual increase in electrochemically inactive O2 and the loss of O2 from voids near the cathode surface lead to a reduction in the O redox capacity and the observed voltage fade. These important insights could lead to innovations in cathode chemistry and aid the transition to low-carbon energy sources.
The net zero transition necessary to limit the effects of climate change requires dramatic cuts to carbon emissions. One of the cornerstones of the UK’s transition will be switching to fossil-free transport, with electric vehicles (EV) one of the most developed options. However, the cathode is a critical limiting factor in efforts to increase the energy density of lithium-ion (Li-ion) batteries for EV applications. As changes to the chemistry of the cathode are likely to lead to improvements in battery performance, such as boosting battery life, storing greater energy to improve range, reducing battery cost and increasing the power available to the EV during acceleration, developing next-generation lithium-ion cathodes is a major priority.
“Lithium-ion batteries are very critical to the net zero transition, enabling electric vehicles and grid storage, says Dr Robert House, at the University of Oxford.
In order to get better batteries, we need new materials which are able to store more energy in the same volume and the same mass - an increase the energy density. One of the biggest limitations is the cathode material. These typically have layered structures with alternating layers of transition metal oxide and lithium ions. The best-known cathode materials are NMCs, named for the combination of nickel, cobalt and manganese within the transition metal layer.
Dr Robert House
Lithium-rich cathodes are next-generation materials which have higher concentrations of lithium within the cathode structure, replacing some of the transition metals. They have a higher capacity because they store energy via oxidation of the oxygen in the structure as well as the transition metal. However, although these materials were first discovered over twenty years ago, a long-standing question has been how the oxygen undergoes charge storage. Dr House says:
Over the past few years, working on the CATMAT project, we’ve uncovered that the oxideoxygen anion converts to O2 molecules, which are trapped within the crystal structure of the cathode. And this discovery of the exact nature of the oxygen was first made possible by using high resolution RIXS on the I21 beamline.
One of the challenges with Li-rich cathodes is that the material evolves when the battery is used, as it is charged and discharged, causing a gradual loss of voltage. As this voltage fade is associated with the oxygen redox – the oxidation and reduction of the ligand oxygen – the team came back to I21 to track the oxygen evolution during cycling. “RIXS allows us to track the oxygen formation, and the amount of oxygen trapped in the material,” Dr House says:
It shows us that there is less and less oxygen redox as the battery cycles, so the battery becomes less and less effective. We’ve found two causes for this drop. The first is that some of O2 isn’t fully reduced back to oxide on the discharge cycle, which leaves residual O2 trapped in the material. The second is that O2 is escaping from the cathode.
Dr Robert House
By correlating the RIXS data with microscopy studies, the research team has been able to show that large (nanometre-sized) pores form in the Li-rich material as it evolves. As these pores get larger, it becomes increasingly difficult to reduce the O2 trapped inside them. Pores can also open at the surface of the material, releasing O2 so that it is irreversibly lost. Dr House says:
High resolution RIXS at I21 has proved to be a tremendously powerful tool for investigating oxygen redox in Li-rich materials. There are very few techniques that allow you to detect O2 when it’s trapped inside a material, and that’s why RIXS is critical to understanding next-gen battery materials.
To find out more about the I21 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Kejin Zhou: [email protected]
Marie J-J et al. Trapped O2 and the origin of voltage fade in layered Li-rich cathodes. Nature Materials 23, 818–825 (2024). DOI:10.1038/s41563-024-01833-z
McColl K et al. Phase segregation and nanoconfined fluid O2 in a lithium-rich oxide cathode. Nature Materials23, 826–833 (2024). DOI:10.1038/s41563-024-01873-5
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