Batteries have come a long way since Volta first stacked copper and zinc disks together 200 years ago. While the technology has advanced from lead acid to lithium ion, many challenges remain – such as achieving higher density and suppressing dendrite growth. Experts are struggling to meet the growing global demand for energy-efficient and safe batteries.
The electrification of heavy commercial vehicles and aircraft requires batteries with a higher energy density. One team of researchers believes a paradigm shift is needed to significantly impact battery technology for these industries. This shift would take advantage of the anionic reduction-oxidation mechanism in lithium-rich cathodes. Results published in Nature mark the first direct observation of this anionic redox reaction in a lithium-rich battery material.
The cooperating institutions included Carnegie Mellon University, Northeastern University, Lappeenranta-Lahti University of Technology (LUT) in Finland and institutions in Japan such as Gunma University, Japan Synchrotron Radiation Research Institute (JASRI), Yokohama National University, Kyoto University and Ritsumeikan University.
Lithium-rich oxides are promising classes of cathode materials because they have been shown to have much higher storage capacity. But there is an “AND problem” that battery materials must meet – the material must be able to charge quickly, be stable to extreme temperatures, and reliably run through thousands of cycles. Scientists need a clear understanding of how these oxides work at the atomic level and how their underlying electrochemical mechanisms play a role in this.
Normal Li-ion batteries operate by cationic redox when a metal ion changes its oxidation state when lithium is introduced or removed. Only one lithium ion can be stored per metal ion within this insertion framework. However, lithium-rich cathodes can store much more. Researchers attribute this to the anionic redox mechanism – in this case oxygen redox. This mechanism is attributed to the high capacity of the materials, which almost doubles the energy storage compared to conventional cathodes. Although this redox mechanism has turned out to be a leading competitor among battery technologies, it is a pivotal point in materials chemistry research.
The team wanted to provide conclusive evidence of the redox mechanism that uses Compton scattering, the phenomenon in which a photon deviates from a straight path after interacting with a particle (usually an electron). Researchers conducted sophisticated theoretical and experimental studies on the SPring-8, the world’s largest third-generation synchrotron radiation facility operated by JASRI.
Synchrotron radiation consists of the narrow, strong rays of electromagnetic radiation that are generated when electron beams are accelerated to (almost) the speed of light and forced onto a curved path by a magnetic field. Compton scattering becomes visible.
The researchers observed how the electronic orbital, which forms the heart of the reversible and stable anionic redox activity, can be mapped and visualized and its character and symmetry determined. This scientific premiere could be groundbreaking for future battery technology.
While previous research has suggested alternative explanations for the anionic redox mechanism, it has not been able to provide a clear picture of the quantum mechanical electronic orbitals associated with redox reactions, as this cannot be measured by standard experiments.
The research team had an “A ha!” Moment when they first saw the correspondence in the redox character between theory and experimental results. “We found that our analysis can map the oxygen states that are responsible for the redox mechanism, which is fundamentally important for battery research,” explains Hasnain Hafiz, first author of the study, who carried out this work during his time as a postdoctoral fellow at Carnegie Mellon .
“We have conclusive evidence of the anionic redox mechanism in a lithium-rich battery material,” said Venkat Viswanathan, associate professor of mechanical engineering at Carnegie Mellon. “Our study provides a clear picture of how a lithium-rich battery works at the atomic level and suggests ways to develop next-generation cathodes to enable electric aviation. The design of high energy density cathodes represents the next frontier for batteries. “
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Materials provided by College of Engineering, Carnegie Mellon University. Originally written by Lisa Kulick. Note: The content can be edited in terms of style and length.