Reversible oxide formation during cycling of Si anodes

Highlights: • Unique approach of Si anode investigations by QCM during cyclic voltammetry. • Evolution of SEI thickness is measured by means of QCM and found in good agreement with TEM micrographs. • Reversible Li₂O formation during cycling is demonstrated and quantified in dependence on Si thickness and cycling rate. • In realistic operation conditions, half the storage capacity of Si anodes stems from the reversible formation of Li₂O. • Explanatory model is formulated to explain the dependencies on the Li₂O formation. Silicon is a hot candidate for battery anodes as its theoretical storage capacity is almost ten times larger than that of graphite. Volume expansion and kinetic limitations require the nanostructuring in particles, wires or thin films. One feature that still puzzles researchers is the solid electrolyte interface (SEI). Here we use an electrochemical quartz crystal microbalance (QCM) to inspect in-situ SEI formation on Si films. The measured mass uptake is split into a reversible part representing the battery function and an irreversible part attributed to continuous SEI formation. The evaluation of the latter quantifies the dependence of SEI growth on cycling window, rate and anode thickness. More striking is the reversible part. Advanced QCM mass spectrometry enables identification of the ab/desorbed species. Surprisingly, half of the battery storage is not due to lithiation but due to reversible adsorption of Li₂O to the SEI layer. The dependence on rate and film thickness indicates this Li₂O is formed by a self-limited, field-driven layer growth.