The journey begins on a winter morning in Los Angeles where I was attending the March meeting of the American Physical Society. I had re-scheduled my return flight to catch my student, Greg Houchins present his work on discovering low-Co cathode materials. The talk right before that was given by Hasnain Hafiz, a student in Arun Bansil's group at Northeastern University. He presented beautiful work showing how Compton Scattering could be used to visualize the redox states in cathode materials used today in Li-ion batteries. I was blown away and that afternoon from the airport, I made an offer for a postdoctoral position, which Hasnain accepted.
Compton Scattering is a powerful technique that probes both the energy and momentum characteristics. Prof. Bansil, along with collaborators at SPring-8 have been using Compton Scattering to extract fine-scale atomic features for decades. They had pioneered the use of X-ray Compton Scattering technique for battery materials and had used it to analyze how much lithium had been inserted, popularly known as the state-of-charge.
At the time, my group was focussed on identifying the next leap in cathode materials. Li-ion cathode materials, used in electric vehicles, are typically layered oxides with the lithium occupying the sites in between the layers through what is known as the insertion mechanism. When the lithium-ion is inserted into a cathode material, the metal-ion changes its oxidation state (redox), to ensure charge balance. The amount of lithium stored is one-to-one for every metal-ion present. It would be great indeed if we could store more than one Li-ion per metal center as this would increase the capacity of the cathode, and thus the energy density of the battery. But this raises the question, how would the charge be compensated?
A new class of cathode materials, known as Li-rich cathodes achieve exactly that by storing more than 1 Li per metal center. The prevailing explanation for this charge compensation is that oxygen is participating in the redox process. However, such a proposal is indeed radical. As Max Radin and co-authors write, "Although the oxygen redox interpretation has been widely embraced as explaining the anomalous capacity of Li-excess manganese oxides, it represents a radical departure from the corpus of materials chemistry. Being an extraordinary claim, the oxygen redox interpretation of Li-excess manganese oxides requires extraordinary evidence.'' We set out to provide this extraordinary evidence with X-ray Compton Scattering.
Through a global collaboration, we set out to understand the nature of redox processes in a Li-rich cathode material consisting of titanium and manganese. Using X-ray Compton measurements together with first-principles modelling, we showed how the electronic orbital that lies at the heart of the reversible and stable oxygen redox activity can be imaged and visualized, and its character and symmetry determined, providing conclusive evidence in support of the oxygen redox mechanism.
Now comes the question of why understanding the precise redox mechanism matters? Batteries suffer from what I like to call the "AND problem". A cathode material must charge and discharge for thousands of cycles, charge fast, work reliably at cold and hot temperatures and satisfy a variety of other constraints. Satisfying all of them simultaneously is a massive challenge and understanding the precise mechanism of redox processes is a necessary step towards solving the AND problem.
Li-rich cathodes paired with lithium metal represents a stepping stone towards achieving the needs of electric aviation. Electric aviation requires batteries that need a significant boost from today's Li-ion batteries. New mechanistic paradigms like oxygen redox are required to unlock electric aviation. A truly remarkable innovation journey, one which may not have happened at all had I not re-scheduled my flight.