Quantitative Elemental Analysis of Silicon Thin-Film Anodes with 100-nm Depth Resolution

The depth distribution of Li as well as other light elements, i.e. C, O, and F, in the electrodes of lithium-ion batteries is significant for understanding mass transport during cycling. In our work, we take advantage of the proton nuclear reactions with Li and F in addition to the elastic scatterings with all elements in electrochemically deposited silicon thin film anodes, and determine the depth distribution of Li, C, O, F, and Si quantitatively at different cycling states of silicon anodes. A high depth resolution of ~20 – 80 nm in nuclear reactions and elastic scatterings is achieved with a glancing exit angle (~80°) of the proton beam, while the resolution of nuclear reactions is higher due to scattered alpha particles with 8-MeV energy. The Li residue is twice of the Si amount in the first ten cycles, indicating rapid accumulation of trapped lithium in the silicon anode. The relative Li atomic concentration compared to Si decreases by ~10% – 50% in the deeper position of the anode. Meanwhile, the amounts of C and O increase by ~100% after the first cycle, but the amounts remain in the later cycling. We also observe larger atomic concentrations of O and F near the surface of the silicon anode, indicative of solid electrolyte interphase (SEI) formation. The quantitative analysis of the heterogeneous element distribution by this non-destructive technique will be useful to further understand the SEI formation and growth and the mechanism of lithium trapping in cycled silicon anodes.