Unveiling the intrinsic reaction between silicon-graphite composite anode and ionic liquid electrolyte in lithium-ion battery

Due to the excellent theoretical capacity, silicon-based electrodes have been extensively studied to develop high energy-density lithium-ion batteries (LIBs). Nevertheless, the large volume expansion and unfavorable interface seriously hamper its application, and the underlying physics behind are still unclear. In this work, using in-situ environment scanning electron microscopy (ESEM), the morphological evolution of composite silicon-graphite electrodes with different ionic liquids as electrolytes are compared in the range of 20 degrees C-60 degrees C. Surprisingly, combining the microstructure and surface reaction products from transmission electron microscope (TEM) and Xray photoelectron spectroscopy (XPS), it finds out that the fully reaction of lithium-silicon does not generate huge volume expansion, while the side reaction between graphite and electrolyte decomposition products causes dramatic volume expansion and hinders the further lithiation of silicon. On the other hand, the electrolyte which is capable of rapidly releasing F- to form LiF-rich solid electrolyte interphase (SEI) is favorable to maintain the structure integrity and cycling performance. This work casts new understanding from the perspective of intrinsic reaction between electrode and ionic liquid electrolyte, which suggests that the practical application of silicon based electrodes with appropriate electrolyte is a promising route for high energy-density LIBs.