An international team of researchers investigating anionic redox activity have found evidence of why lithium-rich cathodes have greater energy storage capacity.
The team used synchrotron radiation to directly observe anionic redox reactions in a lithium-rich battery material.
The researchers performed Compton scattering experiments to observe how the electronic orbital that lies at the heart of the reversible and stable anionic redox activity can be imaged and visualised, and its character and symmetry determined.
The discovery explains the increase of energy density in lithium-rich batteries.
The researchers performed theoretical and experimental studies at SPring-8— the world’s largest third-generation synchrotron radiation facility, which is operated by Japan Synchrotron Radiation Research Institute (JASRI).
The team’s findings were published in the journal Nature.
Collaborating institutions included: Carnegie Mellon University, Northeastern University, Lappeenranta-Lahti University of Technology (LUT) in Finland, and institutions in Japan including Gunma University, JASRI, Yokohama National University, Kyoto University, and Ritsumeikan University.
Venkat Viswanathan (pictured), associate professor of mechanical engineering at Carnegie Mellon, said: “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.”
Researchers attribute lithium-rich cathodes' ability to store much higher storage capacity to the anionic redox mechanism—in this case, oxygen redox.
While previous research has proposed alternative explanations of the anionic redox mechanism, it could not provide a clear image of the quantum mechanical electronic orbitals associated with redox reactions because this cannot be measured by standard experiments.
Bernardo Barbiellini, professor of Computational Material Science at LUT University, said: "How to get more energy in a smaller space is the core in battery development. In order to improve the efficiency, we need to profoundly understand the battery chemistry.
"In this study, we examined advanced battery materials that carry several mechanisms performing chemical reactions.
"We want to see and understand how particles such as Li-ions and electrons move and how they release electrical energy while interacting with oxygen atoms.
"Our group has simulated the working of the positive electrode, and based on our calculations, we generated a model to be verified. With high-energy X-ray measurements, the so-called Compton scattering method, we were able to visualise the electron's state near the oxygen atom in the cathode.
"But the machine and the images alone are not valuable. The model gives us an understanding of what we are seeing. With our joint effort, we are able to contribute to improving existing battery materials and designing new ones.”
The conclusion of the team’s paper states: “It is important to keep in mind that the energy density in a lithium battery pack in an electric car is about 0.4 MJ/liter, which is 100 times smaller than that in gasoline.
“This large difference indicates that there is much room for improving energy densities of rechargeable battery materials and that further work in this direction is needed.
“Since high-energy x-rays can easily penetrate closed electrochemical cells, Compton scattering experiments provide a unique spectroscopic tool for monitoring changes in redox orbitals during charging and discharging processes, and thus facilitate the design and development of high- performance rechargeable batteries.”