Quantifying the Reactivity of Isolated Li _<i>x</i> Si Domains in Si Anodes Using Operando NMR

The use of Si anodes can greatly improve the energy density of Li-ion batteries. However, the understanding and mitigation of calendar aging remain barriers to commercialization. In this short report, we utilize operando nuclear magnetic resonance (NMR) spectroscopy to detect and quantify lithium silicides (Li x Si) as they form and react within Si anodes in pouch cells during calendar aging. We provide direct experimental evidence of complex aging phenomena in the Si anodes including both SEI growth and dissolution during storage. Formation of electrochemically isolated Li x Si is also observed, as indicated by the partial persistence of highly lithiated phases after the cell is discharged. Remarkably, we show that these isolated domains can themselves self-discharge over time, suggesting that their detection can be challenging in post-mortem studies. Finally, we show that aging outcomes depend heavily on the type of silicon particles contained within the electrode and that certain surface coatings can help decrease the reactivity between lithium silicides and the electrolyte.