Impact of the Native SiO2 Surface Layer on the Electron Transfer at Amorphous Si Electrodes
Characterization of the SEI is challenging, because of the variety of chemically similar components and enclosed electrolyte species. Furthermore, ex situ analysis of the SEI requires separation and isolation of the SEI, which may change the content and the structure of the SEI. Recently we used the feedback mode of scanning electrochemical microscopy (SECM) to investigate in situ the electron transport at the lithiated graphite (Figure).
2,5-di-tert-butyl-1,4-dimethoxy benzene was identified as an useful SECM mediator providing sufficient stability and sensitivity to study the passivation properties of the SEI. Our results by SECM show gradual and significant short-term spatiotemporal changes of the SEI properties and demonstrate the dynamic and spontaneous behavior of SEI formation, damage and reformation under open circuit conditions above lithiated graphite anodes. The results emphasize that spatiotemporal changes of the passivating SEI properties are highly localized and occur preferentially in between the gaps of graphite particles. A HOPG model electrode shows significant differences to a graphite composite electrode, indicating the impact of stressed graphite particles on the SEI stability. In addition, imaging experiments show the formation, detachment and reformation of gas bubbles.
The outcome of significant short-term spatiotemporal changes of the SEI properties clarifies that electrolyte reduction still occurs after SEI formation at localized spots. All measurements were conducted under open circuit conditions and thus the charging state remained constant. Consequently, the SEI is unstable even under constant charging state. The detection of gas bubbles is in line with the electrolyte reduction events nearby, since electrolyte reduction is a requirement for gas formation. Further research using this methodology focuses on Li and Si electrodes.
H.B. thanks the Lower Saxony Ministry of Science and Art for funding within the Graduate Program GEENI - Graduate school of electromobility and energy storage in Lower Saxony. M.S. and M. W. gratefully acknowledge the provision of anode substrates by Infineon Technologies Austria AG.
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