In-situ Electrochemical AFM Observations of SEI Film Formation on Graphite Anodes

Solid Electrolyte Interphase (SEI) films play an important role in lithium ion battery. SEI films form ongraphite anode surfaces during early lithium intercalation process. SEI films prevent further solvent decompositionduring charge and discharge in the battery. Morphological study of SEI film formation is very important to understandthe function of SEI film in lithium ion battery system. Simultaneous in-situ electrochemical Atomic Force Microscopy (AFM) and Friction Force Microscopy (FFM) were successfully introduced to monitor SEI film formation in realtime. Topographic and frictional changes on the surface of a Highly Oriented Pyrolytic Graphite (HOPG) electrodein 1 mol/dm3 LiClO4 EC/EMC (1:1) electrolyte were examined during charge and discharge. SEI film formationcommenced at approximately 2V vs. Li/Li+ and stable film formation with an island-like morphology was observedbelow approximately 0.9V vs. Li/Li+. Further experiments on a composite electrode made of synthetic graphite powderand polyvinylidene difluoride (PVDF) binder showed similar phenomena.