SnO₂ Model Electrode Cycled in Li-Ion Battery Reveals the Formation of Li₂SnO₃ and Li₈SnO₆ Phases through Conversion Reactions

SnO₂ is an attractive negative electrode for Li-ion battery owing to its high specific charge compared to commercial graphite. However, the various intermediate conversion and alloy reactions taking place during lithiation/delithiation, as well as the electrolyte stability, have not been fully elucidated, and many ambiguities remain. An amorphous SnO₂ thin film was investigated for use as a model electrode by a combination of postmortem X-ray photoelectron spectroscopy supported by density functional theory calculations and scanning electron microscopy to shed light on these different processes. The early stages of lithiation reveal the presence of multiple overlapping reactions leading to the formation of Li₂SnO₃ and Sn⁰ phases between 2 and 0.8 V vs Li⁺/Li. Between 0.45 V and 5 mV vs Li⁺/Li Li₈SnO₆, Li₂O and Li _xSn phases are formed. Electrolyte reduction occurs simultaneously in two steps, at 1.4 and 1 V vs Li⁺/Li, corresponding to the decomposition of the LiPF₆ salt and ethylene carbonate/dimethyl carbonate solvents, respectively. Most of the reactions during delithiation are reversible up to 1.5 V vs Li⁺/Li, with the reappearance of Sn⁰ accompanied by the decomposition of Li₂O. Above 1.5 V vs Li⁺/Li, Sn⁰ is partially reoxidized to SnO _x. This process tends to limit the conversion reactions in favor of the alloy reaction, as also confirmed by the long-term cycling samples.