Understanding the Irreversible Lithium Loss in Silicon Anodes Using Multi-edge X-ray Scattering Analysis

During the first charge–discharge cycle, silicon-based batteries show an important capacity loss not only due to the formation of the solid electrolyte interphase (SEI) but also other effects taking place during the expansion-contraction sequence upon (de)alloying, such as electrochemical reduction of native oxide, Li trapping, and loss of active material. To understand this first-cycle irreversibility, quantitative methods are needed to characterize the chemical environment of silicon and lithium in the bulk of the cycled electrodes. Here, a methodology based on multiedge X-ray Raman scattering is reported, as applied to model silicon electrodes prepared in fully lithiated and delithiated states after the first cycle. The spectra are recorded at the C, O, F, and Li K-edges, as well as Si L_2,3 edge, and are analyzed using linear combinations of both experimental and computed reference spectra. Prototypical SEI compounds such as Li₂CO₃, LiF, and LiPF₆, as well as electrode constituents such as binder and conductive carbon, crystalline Si, native SiO₂, and Li_xSi phases (x being the lithiation index) are taken into account to identify the main species, isolate their relative contributions, and quantitatively evaluate the proportions of organic and inorganic products. This analysis shows that 35% of the carbonates formed in the SEI during the lithiation are dissolved upon delithiation and that part of the Li_xSi alloys remains present after delithiation. Moreover, in combination with electrochemical data, it enables the quantification of the lithium lost in the first cycle, 17% of which is trapped in disconnected silicon particles, while 30% forms a fluorine-rich stable SEI and 53% a carbonate-rich partially dissolvable SEI. These results pave the way to systematic, reference data-informed and modeling-assisted studies of SEI characteristics in the bulk of electrodes prepared under controlled state-of-charge and state-of-health conditions.