Unveiling the Complex Redox Reactions of SnO in Li-Ion Batteries Using X-ray Photoelectron Spectroscopy and X-ray Absorption Spectroscopy.

We experimentally determine the redox reactions during (de-)lithiation of the SnO working electrode cycled in (LiS)-PS solid electrolyte by combining X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Specifically, we have accurately determined the composition changes in the SnO working electrode upon cycling and identified the onset voltage formation of the various phases. Starting from the open-circuit potential, we find that, on lithiation, the Sn M-edge absorption spectra reveal unequivocally the formation of SnO ( ≤ 1) and LiSnO already at a potential of 1.6 V Li/Li, while Sn 3d/Sn 4d, O 1s, and Li 1s core-level spectra show the formation of Sn and LiO along the first potential plateau at 0.8 V Li/Li and of LiSnO at lower potentials. Below 0.6 V Li/Li, an alloying reaction takes place until the end of the lithiation process at 0.05 V Li/Li, as shown by the formation of LiSn. During delithiation, both the conversion and alloying reactions are found to be partially reversible, starting by the re-formation of Sn at 0.3 V Li/Li and followed by the re-formation of LiSnO and SnO above 0.5 V Li/Li. The conversion and alloying reactions are found to overlap during both lithiation and delithiation. Finally, we validate the theoretical prediction for the SnO conversion and alloy (de-)lithiation reactions and clarify the open questions about their reaction mechanism.