Silicon is a promising high-capacity host material for negative electrodes in lithium-ion batteries with low potential for the lithiation/delithiation reaction that is outside the stability window of organic carbonate electrolytes. Thus, the use of such electrodes critically depends on the formation of a protective solid electrolyte interphase (SEI) from the decomposition products of electrolyte components. Due to the large volume change upon charging, exposure of the electrode material to the electrolyte must be expected, and facile reformation of SEI is a scope for improving the stabilities of such electrodes. Here, we report the formation of incipient SEI layers on monocrystalline silicon by in situ imaging of their passivating properties using scanning electrochemical microscopy after potentiodynamic charging to different final potentials. The images show a local initiation of the SEI growth at potentials of around 1.0 V vs Li/Li+ in 1 M LiClO4 in propylene carbonate.

Silicon is considered as one of the promising alternatives to graphite as negative electrode material in lithium-ion batteries. The electron transfer at uncharged microstructured and planar Si was characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene as redox mediator. Approach curves and images demonstrate that the electron transfer rate constants at pristine Si are relatively small due to the native SiO 2 surface layer. In addition, the electron transfer rate constants show local variations because of the heterogeneous coverage of SiO 2 . The SiO 2 layer is at least partially removed by mechanical contact and abrasion with the microelectrode probe. After SiO 2 removal by the microelectrode or by a hydrofluoric acid dip, the electron transfer rate constants increase strongly and remain heterogeneous. Moreover, the surface of the Si electrodes is at least stable over hours after SiO 2 removal. The consequences for investigating the formation of the solid electrolyte interphase (SEI) on Si are discussed. Silicon is considered as one of the promising alternatives to graphite as negative electrode material in lithium-ion batteries (LIBs). This is due to its wide abundance, low voltage and high theoretical gravimetric capacity of 3579 mAh g −1 .1 During the lithiation, the electrochemical potential of the Si electrode exceeds the stability window of the electrolyte and, consequently, the electrolyte is reductively decomposed.2 The decomposition products form a solid electrolyte interphase (SEI) covering the Si material lying beneath.3 A major challenge of Si as negative battery electrodes is the large volume change of ca. 270% 4 upon lithiation. Recurring lithiation/delithiation causes the electrode material to crack leading, in the worst case, to irreversible loss of active material. During the volume expansion non-passivated Si surfaces are expected to be exposed which are immediately covered by a SEI layer because of ongoing electrolyte decomposition. 5 Thus, similar to metallic Li electrodes the SEI layer is continuously re-formed and remains instable during each lithiation process. 6 The properties of the SEI are important for the performance of Si negative electrodes. 5,7 In order to evaluate the Si electrode performance, reliable in situ characterization of the SEI is definitely a key issue for the progress in this field. In general, scanning probe techniques are frequently applied for various aspects of battery research. 8,9 Especially atomic force microscopy (AFM) has been used to characterize in situ and ex situ the SEI at amorphous Si (a-Si) thin films, [10][11][12][13][14] Si-based thin films, 11,15 a-Si nanopillars 16 and Si nanowires. 17,18 AFM provides information about the morphology evolution and physical properties of the SEI. The present study aims at characterizing the functional properties of Si electrodes in situ by the feedback mode of scanning electrochemical microscopy (SECM), which probes the local electron transfer ra...