Alternating Silicon and Carbon Multilayer-Structured Anodes Suppress Formation of the c-Li_3.75Si Phase

Abstract

Silicon-based anodes for Li-ion batteries have been gaining a great deal of attention due to their high theoretical gravimetric energy density. Approaches for overcoming the challenge of pulverization associated with Si-based electrodes are required for efficient, reversible, and stable operation of such high energy batteries. This study focuses on addressing the source of pulverization of amorphous silicon films upon cycling, which is typically attributed to the formation of the c-Li3.75Si phase. Cross-sectional samples prepared by focused-ion beam milling revealed fractured sponge-like silicon structures after 150 cycles at a lithiation cutoff voltage of 5 mVLi, at which the c-Li3.75Si phase forms. Cycling at a higher lithiation cutoff voltage, 50 mVLi, however, resulted in a film with a higher degree of integrity, along with the absence of the c-Li3.75Si phase. These results clearly verify and underscore the deleterious effects of the c-Li3.75Si phase. Alternating carbon and silicon layers results in suppression of the formation of the c-Li3.75Si phase to a degree dependent upon the relative thicknesses of both the silicon and carbon layers. Best results were observed for multilayers of 8 nm Si/4 nm C, with which no evidence for the c-Li3.75Si phase up to 149 cycles was observed. Carbon interlayers were also found to beneficially lower the relative irreversible capacity loss due to solid-electrolyte interphase formation and associated electrical disconnection.