Intercalation of graphite in Li-ion batteries: In situ microscopic characterization of the solid-electrolyte interface (SEI)

Energy storage technologies are crucial in the next green-energy transition. In particular, Li-ion batteries (LIBs) are nowadays the reference technology for many applications including, in particular, the forthcoming electrical mobility. In the last years, the deep investigation of the plethora of complex phenomena controlling the charge storage mechanism in LIBs allowed a considerable improvement in LIBs performance. In this regard, in situ analysis played a fundamental role in the comprehension of the degradation processes occurring at the electrode/electrolyte interface. In particular, in situ electrochemical atomic force microscopy (EC-AFM), consisting in high-resolution microscopy under electrochemical control, uniquely allowed to monitor the evolution in morphology and mechanical properties of the solid-electrolyte interface (SEI) forming at low potential on the graphitic anodic electrodes of LIBs. In this work, we employed in situ EC-AFM measurements and ex situ ToF-SIMS analysis as representative techniques to provide a comprehensive morphological and compositional overview of the SEI film formed under cyclic voltammetry experiments. The work aims to provide guidelines in the SEI evolution when highly oriented pyrolytic graphite (HOPG), which can be considered a model electrode, is employed. The reported in situ data are in agreement with current interpretative models. The ex situ ToF-SIMS analysis discloses a film composition where different Li compounds are uniformly distributed on the electrode surface.