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Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy studies of silicon anodes for Lithium-ion batteries

Final Report Summary - NMRSILION (Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy studies of silicon anodes for Lithium-ion batteries)

In the field of Li-ion batteries, silicon attracts considerable interest as an alternative to carbon for the negative electrode due to its high capacity for a reasonable mass. Silicon is amorphised in the first lithiation and is therefore difficult to characterise by conventional techniques. The challenges with silicon come from its inherent high capacity. Large volume changes occur upon cycling that impact on the reversibility and charge-rate of silicon. The objectives of this project were to study the structural changes occurring in the bulk and at the surface of silicon anodes upon charge and discharge by solid-state nuclear magnetic resonance (NMR), and to extend the researcher's field of knowledge towards the study of inorganic and composite materials by solid-state NMR.

Work performed and main results

We prepared silicon electrodes suitable for electrochemical cycling and NMR in collaboration with a partner laboratory from the Alistore European Research Institute: the LRCS laboratory (Université de Picardie Jules Verne & CNRS, Amiens, France).

These electrodes were studied by ex-situ NMR, using the one-dimensional 7Li and the 29Si NMR signals. On cycled electrodes, we showed that the particle size had an effect on the 7Li spectra only for particles smaller than 15 nm. We also demonstrated that 29Si NMR, combined with the CPMG acquisition method, was a powerful tool to follow the amorphisation of the silicon network upon lithiation. We successfully implemented Si-Si and Si-Li 2D correlation methods on a model compound, Li12Si7 and are now working on transferring these to cycled electrodes.

We established that the calculations on lithium silicides could not be performed using the existing methods and that further development in the field of chemical shift calculations would be needed.

Finally, we studied silicon nanowires by in-situ NMR and we could observe for the first time several cycles under reasonable electrochemical cycling conditions. In-situ NMR is especially powerful to track transient phases that are hard to observe by ex-situ NMR. Particularly, we could prove that the transient overlithiated Li15Si4 phase becomes more pronounced at deep lithiation in the second cycle, probably owing to its more open framework. We also showed for the first time the electrochemical signature of the formation of this phase.

In parallel, the researcher got trained in the field of NMR of solid-state materials. She could develop and apply NMR techniques that she had not been familiar with before : REDOR, RFDR, TEDOR, CPMG. She learned how to handle air-sensitive samples and she got familiar with the electrochemical methods for the study of solid-state battery materials. She got a broader knowledge of the solid-state chemistry field by attending the seminars in the Department of Chemistry. Her training was completed by the supervision of one PhD student, and a second one starting three months before she left. This fellowship was instrumental for the researcher in getting a permanent position.

Impact:

This study, terminated after 10 months, already provided promising tools for the study of silicon as a negative electrode by solid-state NMR. 7Li in-situ NMR was pushed forward and applied on silicon nanowires to study transient phases in multiple cycles. Ex-situ methods were developed, especially using 29Si NMR. we demonstrated their utility on model compounds and already applied the 1D methods to cycled electrodes. We expect these studies to improve the understanding of the phenomena occurring in silicon anodes and to trigger progress in the use of silicon as a negative electrode in Li-ion batteries.