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 Sn-0 phases between 2 and 0.8 V vs Li+/Li. Between 0.45 V and 5 mV vs Li+/Li Li8SnO6, Li2O and LixSn 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 with the reappearance of Sn accompanied by the decomposition of Li2O. Above 1.5 V vs Li+/Li, Sn-0 is partially reoxidized to SnOx. This process tends to limit the conversion reactions in favor of the alloy reaction, as also confirmed by the long-term cycling samples.