Ligand-to-metal charge transfer (LMCT): an experimental validation

While the last GIEC report is stating that electric vehicles offer the greatest potential for decarbonization, researchers are encouraged to increase batteries’ performance and find better materials in lithium-ion batteries. Lithium-ion batteries are for now the best option when it comes to performance, however, their energy density (= energy per unit of mass) could still be improved for driving electric vehicles.

Lithium-rich compounds hold out hope for increasing energy density, but there is still a need to understand how they work by elucidating the underlying science. To achieve this, an international team led by Professor Tarascon has focused on the processes of ligand-metal charge transfer (LMCT) in these materials; a phenomenon that has been proposed but lacks direct experimental evidence. The reason is simple and linked to the fact that this transfer was too fast to be isolated by the various spectroscopic techniques. It was therefore necessary to slow it down in order to observe it. How could this be done?

By using a specific cation-disordered material (Li1.17 Ti0.58 Ni0.25 02), the researchers first identified a redox mechanism with a kinetically activated, long-lived intermediate species (Ni3+/4+) that was able to help them in their quest of validation. Combining electrochemical analysis and spectroscopic techniques, the team was then able to prove that this long-lived intermediate species was reduced through a LMCT process.

Figure: Clear evidence of the LMCT process - Deconvolution of Ni 2p spectra showing the reduction of nickel as a function of heating time (a), and oxygen oxidation increase (b) compared to their reference spectra (c,d).

As a result of their fundamental study, Li, et al were able to prove that LMCT is indeed a crucial process to activate the redox activity of anions, providing a guide for future research on the designing of high-capacity cathode materials. This also provides great chances for chemists and physicists to study electron transfer phenomena in solids.

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Biao Li is a Chinese researcher working as a post-doc at the CSE lab in Collège de France. After doing both his Bachelor and PhD at Peking University (China), he continued his career as a post-doc in the same university with his work focusing on “High-energy-density Li rich cathodes and pseudocapacitance for Li-ion batteries”.

Currently working on the electrode materials of Li-ion batteries, specifically on the fundamental understanding of their redox reaction mechanisms, his purpose is to develop high-energy-density battery electrodes for practical application (e.g. electric vehicles). In the next step Biao Li and his team are trying to seek for strategies to manage with the practical issues in high-energy-density Li-rich electrodes.

Since he joined the RS2E in 2019, Biao Li has written 4 papers and filed 2 patents. He is enjoying being part of the network since “here we can find experts in diverse disciplines, which allows efficient collaborations to explore materials science and couple fundamental and applied. I have participated in several RS2E biannual meetings and I have seen how such this community can help researchers in France, including myself, to communicate and to share science.