Exploring Li pathways in Si-C/Gr electrodes and their SEI in Li batteries using Li isotope tracing by MAS-NMR and FIB-SIMS

Abstract

One promising way for enhancing Li-ion battery performance is the use of silicon-graphite-composite anodes. Even though this complex system has been extensively studied, the lithium dynamics in silicon/graphite-composite electrode material, as well as at the interfaces, is still not well understood. To investigate lithium mobility and trapping effects in the solid electrolyte interphase (SEI) during lithiation and at rest state in Si-C/Gr composites, we use here a methodology based on combined MAS NMR and FIB-SIMS, both sensitive to lithium isotopic labeling. Labeling electrolyte and Li metal counter electrode with ⁶Li, and SEI with ⁷Li allows distinguishing the origin of lithium ions within graphite and silicon active materials. Following electrochemical lithiation, the SEI shows the lowest ⁷Li fraction while it is higher in both Si and graphite, providing a direct observation of the so-called knock-off mechanism for the movement of lithium ions through the SEI. However, the lower ⁷Li fraction of silicides points out that another mechanism of Li transport, possibly through vehicular mechanism (solvated or desolvated Li), is also at play, enabling a more direct transfer of lithium ions from the electrolyte to silicon particles. In addition, in Open Circuit Voltage (OCV) experiments, Li movement seem to be different in Li-rich and Li-poor silicides, as the Li-rich phase exchanges faster with ⁶Li enriched electrolyte. The innovative ^6/7Li isotope tracing approach described here paves the way for further understanding of the transport properties between electrodes and electrolyte, controlling these phenomena and developing knowledge for the development of high-performance battery technology.