Investigation of Electron Transfer Properties of the SEI on Metallic Lithium Electrodes by SECM at Open Circuit

In our society Li ion batteries are widely used. Mainly during the first charging process the solid electrolyte interphase (SEI) between the electrode and electrolyte is formed by the decomposition of electrolyte components at the lithiated graphite. This layer is critical for the performance and safety of the Li ion batteries.[1] The aim of this study is to characterize the electron transport across the SEI, which determines the ongoing reduction of electrolyte components. Thus, understanding the electron transport across the SEI helps to improve Lithium ion batteries.

Characterization of the SEI is a challenge, 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.[2] Recently we used the feedback mode of scanning electrochemical microscopy (SECM) to investigate in situthe electron transport at the lithiated graphite (Figure).[3]

2,5-di-tert-butyl-1,4-dimethoxy benzene was identified as an useful SECM mediator providing sufficient stability and sensitivity to study 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 spatiotemporalchanges 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 spatiotemporalchanges 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. 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.

References

[1] S. Krueger, R. Kloepsch, J. Li, S. Nowak, S. Passerini, M. Winter, J. Electrochem. Soc. 160, 2013, A542.

[2] P. Verma, P. Maire, P. Novák, Electrochim. Acta 55, 2010, 6332.

[3] H. Bülter, F. Peters, J. Schwenzel, G. Wittstock, Angew. Chem. Int. Ed. 53, 2014, 10531.