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The lithiation of crystalline silicon was studied over several cycles using operando neutron reflectometry over six cycles. A thin layer of aluminum oxide was employed as an artificial coating on the silicon to suppress the solid electrolyte interphase (SEI) layer-related aging effects. Initially, the artificial SEI prevented side effects but led to increased lithium trapping. This layer degraded after two cycles, followed by side reactions, which decrease the coulombic efficiency. No hint for electrode fracturization was found even though the lithiation depth exceeded 1 μm. Two distinct zones with high and low lithium concentrations were found, initially separated by a sharp interface, which broadens with cycling. The correlation of the reflectometry results with the electrochemical current showed the lithium fraction that is lithiated in the silicon and the lithium consumed in side reactions. Also, neutron reflectometry was used to quantify the amount of lithium that remained inside of the silicon. Additional electrochemical impedance spectroscopy was used to gain insights into the electrical properties of the sample via fitting to an equivalent circuit.

This work enables highly "uniform" and "reversible" deposition of Li metal in carbonate electrolytes through a one-time rapid oxidation and reduction (ROAR) treatment. Over the years, Li metal has been plagued with irreversible dendritic growths that create isolated and unusable structures called "dead Li". Accumulation of dead Li negatively impacts the ion transport, performance, and safety of Li metal batteries. To address this long-standing problem, we have developed an in situ process to uniformly create reversible Li deposits. Our results demonstrate that a combination of high-voltage pulses, which rapidly oxidize and reduce Li in both directions (ROAR treatment), leads to strikingly more homogeneous morphology and eliminates reaction pathway transitions. We validate that ROAR treatments eliminate traditional "mossy dendrites" under extended cycling (<250 cycles) in standard carbonate-based electrolytes. Moreover, ROAR treatments create a 500% reduction in overpotential for electrodissolution/deposition and eliminate "peaking" voltage behavior.