Unveiling the Complex Redox Reactions of SnO2 in Li-Ion Batteries Using Operando X-ray Photoelectron Spectroscopy and In Situ X-ray Absorption Spectroscopy.

We experimentally determine the redox reactions during (de-)lithiation of the SnO2 working electrode cycled in (Li2S)3-P2S5 solid electrolyte by combining operando X-ray photoelectron spectroscopy and in situ X-ray absorption spectroscopy. Specifically, we have accurately determined the composition changes in the SnO2 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 SnOx (x ≤ 1) and Li2SnO3 already at a potential of 1.6 V vs Li+/Li, while Sn 3d/Sn 4d, O 1s, and Li 1s core-level spectra show the formation of Sn0 and Li2O along the first potential plateau at 0.8 V vs Li+/Li and of Li8SnO6 at lower potentials. Below 0.6 V vs Li+/Li, an alloying reaction takes place until the end of the lithiation process at 0.05 V vs Li+/Li, as shown by the formation of LixSn. During delithiation, both the conversion and alloying reactions are found to be partially reversible, starting by the re-formation of Sn0 at 0.3 V vs Li+/Li and followed by the re-formation of Li8SnO6 and SnOx above 0.5 V vs Li+/Li. The conversion and alloying reactions are found to overlap during both lithiation and delithiation. Finally, we validate the theoretical prediction for the SnO2 conversion and alloy (de-)lithiation reactions and clarify the open questions about their reaction mechanism.