Operando NMR characterization of cycled and calendar aged nanoparticulate silicon anodes for Li-ion batteries

Replacing graphite anodes with Si anodes can greatly increase the energy of current Li-ion batteries. Detailed characterization of Si lithiation reactions, SEI formation, and reversibility are therefore active areas of research. Solid-state 7Li nuclear magnetic resonance (NMR) spectroscopy is useful for characterizing different lithium local environments within Si anodes. Here, we developed an operando NMR methodology to characterize aging of carbon-coated nanoparticulate Si anodes in pouch cells paired with Ni-rich cathodes. We observed a new lithiation mechanism in the Si nanoparticles: direct formation of over-lithiated Li15+xSi4 (x < 0.6) phase. Furthermore, our novel operando cells maintained good performance with long-term cycle and calendar aging. We identified trapped lithium silicides as a major contributor to capacity fade with aging. Finally, we determined that the addition of Mg (TFSI)2 to the electrolyte decreased the amount of trapped lithium silicides and therefore increased the capacity and capacity retention for the nanoparticulate Si used.