Batteries have evolved significantly ever since Volta initially stacked zinc and copper discs together 200 years ago.
While this technology has evolved continuously from lead-acid to lithium-ion, several challenges still exist today — such as inhibiting the growth of dendrites and achieving a higher density. Experts are rushing to meet the increasing global demand for safe and energy-efficient batteries.
The electrification of aircraft and heavy-duty vehicles needs batteries that have more energy density. According to a research team, a paradigm shift is required to make a major impact on battery technology for these sectors. This change would leverage the anionic reduction-oxidation mechanism in lithium-rich cathodes.
This is the first-ever study in which this anionic redox reaction was directly observed in a lithium-rich battery material. The study has been published in the Nature journal.
Institutions that collaborated in this study include 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 a class of potential cathode materials because they have been demonstrated to have a relatively higher storage capacity. However, there is an “AND problem” that should be fulfilled by battery materials — the material should be capable of rapid charging, work consistently for thousands of cycles and remain stable at extreme temperatures. To address this problem, researchers need a deeper understanding of how such oxides function at the atomic level and how their fundamental electrochemical mechanisms play a role.
Regular Li-ion batteries function by cationic redox, especially when a metal ion alters its oxidation state as lithium is removed or inserted. Within this insertion framework, only a single lithium-ion can be stored for each metal-ion. However, lithium-rich cathodes have the potential to store much more.
Scientists attribute this aspect to the anionic redox mechanism — in this example, oxygen redox. This mechanism has been credited with the high capacity of the materials, almost doubling the energy storage when compared to traditional cathodes. While this redox mechanism has evolved as the main competitor amongst battery technologies, it implies a pivot in materials chemistry analyses.
The researchers set out to offer decisive proof for the redox mechanism using a phenomenon called Compton scattering, through which a photon deviates from a straight trajectory following its interaction with a particle (generally an electron). The scientists carried out advanced experimental and theoretical studies at the largest third-generation synchrotron radiation facility in the world, SPring-8, which is managed by JASRI.
Synchrotron radiation contains the narrow, strong beams of electromagnetic radiation that are generated when electron beams are expedited to (nearly) the speed of light and are forced to move in a curved path by a magnetic field. This makes Compton scattering visible.
The scientists noted how the electronic orbital, located at the center of the stable and reversible anionic redox activity, can be imaged and observed and its symmetry and character determined. This scientific breakthrough could revolutionize future battery technology.
Although the earlier studies have suggested alternative explanations of the anionic redox mechanism, they could not offer a clear picture of the quantum mechanical electronic orbitals related to redox reactions because this cannot be quantified by regular experiments.
The researchers experienced an “A ha!” moment when they initially observed the agreement in redox character between experimental and theoretical findings.
“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, the lead author of the study who conducted the 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. 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.
Venkat Viswanathan, Associate Professor of Mechanical Engineering, Carnegie Mellon University
Hafiz, H., et al. (2021) Tomographic reconstruction of oxygen orbitals in lithium-rich battery materials. Nature. doi.org/10.1038/s41586-021-03509-z.