SEI Formation Vs. Lithium Intercalation: In-Situ AFM Experiments on Graphite and Silicon Electrodes

The formation of the solid-electrolyte interphase (SEI) passivation layer on anode materials in lithium ion batteries (LIB) is not yet fully understood at the micro- and nanometer scale. Therefore, experiments under realistic conditions with high spatial resolution are of primary importance. Hence, atomic force microscopy (AFM) is an ideal technique to study in-situ the mechanisms at the electrode-electrolyte interface down to the (sub-) nanometer level. Within this contribution we present a combined AFM and electrochemical (de-) lithiation study of the electrode-interface for two types of anode materials. Highly oriented pyrolytic graphite (HOPG) as a model system has been investigated in order to observe the topographical and electrochemical properties simultaneously. We find a clear influence of defect structures on the SEI formation process. Furthermore, the structural as well as the mechanical behavior of silicon anodes during (de-) lithiation will be presented. Sputtered silicon thin film electrodes [1] have been used to avoid strain and crack formation [2] on the silicon electrode during lithiation. We will introduce AFM measurements that enable a differentiation between SEI formation and expansion of the silicon electrode upon lithiation. We observed significant variations in the elastic modulus during the (de-) lithiation processes on the silicon electrode that can be directly attributed to the formation of an SEI layer as well as the expansion of the electrode due to the lithium insertion. Additionally, the analysis of the surface roughness will be discussed with respect to (ir-) reversible processes on the electrode surface. References: [1] R. Kumar et al., ACS Energy Lett. , 2016, 1 , 689-697 [2] P. Harks et al., Journal of Power Sources , 2015, 288 , 92-105