Multiscale Observations of Inhomogeneous Bilayer SEI Film on a Conversion-Alloying SnO₂ Anode

SnO₂ , storing Li through conversion and alloying reactions, has been regarded as one of the most typical anode materials and has been widely studied for both mechanism exploring and performance tuning. However, the structure of the solid electrolyte interphase (SEI) formed on the SnO₂ electrode and its evolution process are rarely focused and still poorly understood. Herein, time of flight secondary ion mass spectrometry is used to observe the bilayer hybrid structure of SEI formed on a SnO₂ film. Multiscale observations reveal the SEI accumulation after alloying reactions and distinct dissolving of the organic layer at potentials above de-conversion reactions, which results in the inorganic layer being directly exposed to the electrolyte and thus becoming thick and inhomogeneous. The broken and thick SEI causes rapid capacity decay and low Coulombic efficiencies (CEs) of 97.5%. Accordingly, it is demonstrated that, as the SnO₂ is precoated with LiF or Li₂ CO₃ , a robust and thin SEI layer is induced to form and is stabilized in the continuous cycles, resulting in enhanced cycling stability and promoted CEs to 99.5%. This work adds new insights to the SEI evolution mechanisms on SnO₂ -based anodes and suggests an effective strategy to create high performance metal oxide anode for Li-ion batteries.