Lithiation Products of a Silicon Anode Based on Soft X‑ray Emission Spectroscopy: A Theoretical Study

Because of its exceptional lithium storage capacity, silicon is considered as a promising candidate for anode material in lithium-ion batteries (LIBs). In the present work, we demonstrate that methods of soft X-ray emission spectroscopy can be used as a powerful tool for the comprehensive analysis of the electronic and structural properties of lithium silicides Li x Si forming in LIB’s anode upon Si lithiation. On the basis of density functional theory and molecular dynamics simulations, it is shown that the coordination number of Si atoms in Li x Si decreases with an increase in Li concentration both for the crystalline and amorphous phases. In amorphous a-Li x Si alloys, Si tends to cluster, forming Si–Si covalent bonds even at the high lithium concentration. It is demonstrated that the Si-L2,3 emission bands of the crystalline and amorphous Li x Si alloys show different spectral dependencies, reflecting the process of disintegration of Si–Si network into Si clusters and chains of the different sizes upon Si lithiation. The Si-L2,3 emission bands of Li x Si alloys become narrower and shift toward higher energies with an increase in Li concentration. The shape of the emission band depends on the relative contribution of the X-ray radiation from the Si atoms having different coordination numbers. This feature of the Si-L2,3 spectra of Li x Si alloys can be used for the detailed analysis of the Si lithiation process and LIB’s anode structure identification.