Battery breakthrough: A look at lithium-rich batteries at the atomic level

Battery technology has come a long way since Volta first stacked copper and zinc disks together 200 years ago. While the technology continues to advance from lead-acid batteries to lithium-ion batteries, many challenges remain, such as achieving higher density and suppressing dendrite growth. Experts are racing to address the growing global demand for energy-efficient and safe batteries.

The electrification of heavy vehicles and aircraft requires batteries with higher energy density. A research team argues that a paradigm shift is necessary to have a significant impact on battery technology for these industries. This shift would exploit anion reduction-oxidation mechanisms in lithium-rich cathodes. The results, published in Nature, mark the first time this anion oxidation reaction has been directly observed in lithium-rich battery materials.

Collaborating institutions include Carnegie Mellon University, Tohoku University, and Lappeenranta-Lahti University of Technology (LUT) in Finland, as well as institutions in Japan, including Gunma University, Japan Synchrotron Radiation Research Institute (JASRI), Yokohama National University, Kyoto University, and Ritsumeikan University.

Lithium-rich oxides are a promising class of cathode materials because they have been shown to have much higher storage capacities. However, there is an "and problem" that battery materials must meet: the materials must be able to charge quickly, be stable to extreme temperatures, and cycle reliably for thousands of times. Scientists need a clear understanding of how these oxides work at the atomic level and how their basic electrochemical mechanisms play out to solve this problem.

Normal lithium-ion batteries work via cation redox reactions, where metal ions change their oxidation state when lithium is inserted or removed. Within this insertion framework, each metal ion can store only one lithium ion. However, a lithium-rich cathode can store more. The researchers attribute this to an anionic redox mechanism - in this case oxygen redox. This is the mechanism responsible for the material's high capacity, which nearly doubles its energy storage compared to conventional cathodes. Although this redox mechanism has been a major contender in battery technology, it marks a pivot in materials chemistry research.

The team set out to provide definitive evidence for this redox mechanism using Compton scattering, the phenomenon whereby photons deviate from a straight trajectory after interacting with a particle, usually an electron. The researchers performed complex theoretical and experimental studies at SPring-8, the world's largest third-generation synchrotron radiation facility, operated by JASRI. Synchrotron radiation consists of narrow, powerful beams of electromagnetic radiation that are produced when electron beams are accelerated to (almost) the speed of light and forced by magnetic fields to travel in curved paths. Compton scattering becomes visible. The researchers observed how the electron orbitals at the heart of reversible and stable anionic redox activity can be imaged and visualized, and their characteristics and symmetries determined. This scientific first could be a game-changer for future battery technology.

While previous studies have proposed alternative explanations for the anion redox mechanism, it has not been able to provide a clear picture of the quantum mechanical electron orbitals associated with redox reactions, as this cannot be measured by standard experiments.

When the team first saw the agreement between theoretical and experimental results on the redox properties, they realized that their analytical work could characterize the oxygen states responsible for the redox mechanism, which is critical for battery research.

"We have strong evidence supporting an anion redox mechanism in lithium-rich battery materials," said Venkat Viswanathan, associate professor in the Department of Mechanical Engineering at Carnegie Mellon University. "Our study provides a clear picture of the operation of lithium-rich batteries at the atomic scale and suggests a path for designing next-generation cathodes to enable electric aviation. The design of high-energy-density cathodes represents the next frontier in batteries."