Visualization of a lithium-rich cathode. Image: Carnegie Mellon University and Northeastern University.
Visualization of a lithium-rich cathode. Image: Carnegie Mellon University and Northeastern University.

Batteries have come a long way since Volta first stacked copper and zinc discs together 200 years ago. While the technology has continued to evolve, from lead-acid to lithium-ion, many challenges still exist – like achieving higher energy density and suppressing dendrite growth. Experts are racing to address the growing, global need for energy-efficient and safe batteries.

The electrification of heavy-duty vehicles and aircraft requires batteries with more energy density. A team of researchers from various institutions, including Carnegie Mellon University, Northeastern University and Lappeenranta-Lahti University of Technology (LUT) in Finland, believes a paradigm shift is necessary to make a significant impact in battery technology for these industries. This shift would take advantage of the anionic reduction-oxidation mechanism in lithium-rich cathodes.

Now, for the first time, this team of researchers has directly observed the anionic redox reaction in a lithium-rich battery material. The researchers report their findings in a paper in Nature.

Lithium-rich oxides are promising materials for battery cathodes because they have been shown to have a high storage capacity. But there are other requirements that cathode materials must satisfy – they must be capable of fast charging, be stable to extreme temperatures and cycle reliably for thousands of cycles. To address this, scientists need a clear understanding of how lithium-rich oxides work at the atomic level and how their underlying electrochemical mechanisms play a role.

Normal lithium-ion batteries work by cationic redox, where a metal ion changes its oxidation state as lithium is inserted or removed. Within this insertion framework, only one lithium-ion can be stored per metal-ion. Lithium-rich cathodes, however, can store much more, giving them nearly double the energy storage of conventional cathodes.

Researchers attribute this high storage capacity to the anionic redox mechanism – in this case, oxygen redox. Although this redox mechanism has emerged as the leading contender among battery technologies, it signifies a pivot in materials chemistry research.

The team set out to provide conclusive evidence for the anionic redox mechanism by utilizing Compton scattering, the phenomenon by which a photon deviates from a straight trajectory after interacting with a particle (usually an electron). To do this, the researchers performed sophisticated theoretical and experimental studies at SPring-8, the world's largest third-generation synchrotron radiation facility, which is operated by the Japan Synchrotron Radiation Research Institute.

Synchrotron radiation is the narrow, powerful beams of electromagnetic radiation produced when electron beams are accelerated to (almost) the speed of light and forced to travel in a curved path by a magnetic field. This radiation allows Compton scattering to become visible.

The researchers observed how the electronic orbital that lies at the heart of the reversible and stable anionic redox activity can be imaged and visualized, and its character and symmetry determined. This scientific first could be game-changing for future battery technology.

While previous studies have proposed alternative explanations for the anionic redox mechanism, they could not provide a clear image of the quantum mechanical electronic orbitals associated with redox reactions because they cannot be measured by standard experiments.

The research team had an 'A ha!' moment when it first saw the agreement in redox character between theory and experimental results. "We realized that our analysis could image the oxygen states that are responsible for the redox mechanism, which is something fundamentally important for battery research," explained Hasnain Hafiz, lead author of the paper, who carried out this work during his time as a postdoctoral research associate at Carnegie Mellon.

"We have conclusive evidence in support 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 the workings of a lithium-rich battery at the atomic scale and suggests pathways for designing next-generation cathodes to enable electric aviation. The design for high-energy density cathodes represents the next-frontier for batteries."

This story is adapted from material from Carnegie Mellon University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.