Lithium isotope tracing in silicon based electrodes using solid-state MAS NMR: a powerful comprehensive tool for the characterization of lithium battery

International audienceThe introduction of lithiated components with different 7Li/6Li isotopic ratio, also called isotopic tracing, can give access to better understanding of the lithium transport and lithiation processes in lithium-ion batteries. In this work, we propose a simple methodology based on high-resolution solid-state NMR for the determination of 7Li/6Li ratio in silicon electrodes after different strategy of isotopic tracing. The 6Li and 7Li MAS NMR experiments allow obtaining resolved spectra whose spectral components can be assigned to different moieties of the materials. In order to measure the ratio of the 6Li/7Li NMR integrals, a silicon electrode with a natural 7Li/6Li isotope abundance was used as a reference. This calibration can then be used to determine the 7Li/6Li ratio of any similar samples. This method was applied to study the phenomena occurring at the interface between a silicon electrode and a labeled electrolyte, which is an essential step for isotopic tracing experiments in systems after the Solid Electrolyte Interphase (SEI) formation. Beyond the isotopic exchanges between the SEI and the electrolyte already observed in the literature, our results show that the isotopic exchanges also involve the Li-Si alloys in the electrode bulk. Within a 52 hour contact, the electrolyte labeling disappeared: isotopic concentrations of electrolyte and electrode become practically homogenized. However, at the electrode level, different silicides are characterized by rather different isotopic enrichment. In present work the ToF SIMS and liquid state NMR were also used to cross check and discuss the solid-state NMR method we have proposed