Adhesion and Surface Layers on Silicon Anodes Suppress Formation of c-Li3.75Si and Solid Electrolyte Interphase

Abstract

<div>The formation of c-Li3.75Si is known to be detrimental to silicon anodes in lithium-ion batteries. To suppress the formation of this crystalline phase and improve the electrochemical performance of Sibased anodes, three approaches were amalgamated: addition of a nickel adhesion sublayer, alloying of the silicon with titanium, and the addition of either carbon or TiO2 as a capping layer. The silicon-based films were analyzed by a suite of methods, including scanning electron microscopy (SEM) and a variety of electrochemical methods, as well as X-ray photoelectron spectroscopy (XPS) to provide insights into the composition of the resulting solid electrolyte interphase (SEI). A nickel adhesion layer decreased the extent of delamination of the silicon from the underlying copper substrate, compared to Si deposited directly on Cu, which resulted in less capacity loss. Alloying of silicon with titanium (85% silicon, 15% titanium) further increased the stability. Finally, capping these multilayer electrodes with either a thin 10 nm layer of carbon or TiO2 resulted in the best electrode behavior, and lowest cumulative relative irreversible capacity. TiO2 is slightly more effective in enhancing the capacity retention, most likely due to differences in the resulting solid electrolyte interphase (SEI). The combination of an adhesion layer, alloying, and surface coatings shows a cumulative suppression of the formation of c-Li3.75Si and SEI, resulting in the greatest improvement of capacity retention when all three are incorporated together. However, these strategies appear to only delay the onset of the c-Li3.75Si phase; eventually, the c-Li3.75Si phase will form, and at that point, the rate of capacity degradation of all the electrodes becomes similar.</div>