Operando NMR Studies on Si Anode Calendar Aging and the Reactivity of Electrochemically Formed Trapped Li_xSi_y

Replacing graphite anodes with Si anodes can greatly increase the energy of current Li-ion batteries. Detailed characterization of Si lithiation reactions, solid-electrolyte interphase (SEI) formation, and lithium silicide reactivity are therefore active areas of research. Solid-state ⁷Li nuclear magnetic resonance (NMR) spectroscopy is particularly useful for characterizing different lithium local environments within Si anodes. Here, we developed an operando NMR methodology to characterize aging mechanisms in novel pouch cells by tracking the lithium silicides within the Si anodes. Our operando NMR pouch cells improve upon previous in-situ NMR studies by enabling reliable and long-term electrochemical performance, comparable to commercial cells, whilst retaining NMR compatibility. We investigated several Si anode materials, comparing the lithiation mechanisms and the accumulation of trapped lithium silicides before and after cycle and calendar life aging. Using the operando NMR methodology, we were able to observe the chemical reactivity of trapped lithium silicides at rest as well as compare the state of charge effects on chemical reactivity. Furthermore, we investigated electrode degradation with cumulative long-term calendar and cycle aging and the effects on electrochemical performance.