In-Situ Visualization of Solid-Electrolyte Interphase in Lithium-Ion Batteries

Abstract

Solid electrolyte interphase (SEI) is a layer of in-situ formed thin film at the electrolyte-electrode interface in lithium ion batteries. Stable SEI films help to pretect the electrolyte and the electrode, and thus are critically important for the cycle life, stability, and safety of batteries. Ideal SEI shall be electronically insolating, ionic permeable, and mechanically elastic and robust. However, the investigation of SEI strcuture-property relationship has been difficult because of the in-situ formation characteristic of SEI films. This presentation talks about the investigation of SEI morephology, structure and mechanical properties, as well as the control of SEI using SEI-forming additives, by using scanning probe microscopy and spectroscopy. On a MnO thin-film electrode as the model system, in-situ atomic force microscopy (AFM) was applied to visualize the formation of SEI, and force spectroscopy was used to probe the structure and the mechanical modulus of the SEI. Highly inhomogeneous SEI film was found to form on the first cycle, and the SEI was shown to be unstable upon charging. SEI forming additives such as vinyl carbonate and LiBOB lead to SEI films with different properties and a mixture of the two additives was found to give the best SEI with cycle stability as well as good mechanical stability. Cathodic SEI was also investigated on high-voltage LiCoO2. The formation of filamentous SEI was found the be highly dependent of the crystalline plane as well as the exposure of Co ions to the electrolyte. These study establish a new method in the investigation of SEI structure-property relationship. Future developments on SPM based approach on the electronic and ionic conductivity will further enable mapping of SEI characteristics at a nanometer scaled spatial resolution.