Effect and Progress of the Amorphization Process for Microscale Silicon Particles under Partial Lithiation as Active Material in Lithium-Ion Batteries

Abstract

Microscale silicon particles in lithium-ion battery anodes undergo large volume changes during (de)lithiation, resulting in particle pulverization and surface area increase concomitant with a continuous growth of the solid-electrolyte-interphase. One approach to overcome these phenomena is to operate the silicon anode under capacity-limited conditions (i.e., with partial capacity utilization). Since crystalline silicon is irreversibly transformed into amorphous phases upon lithiation, the purpose of the partial capacity utilization is to maintain a crystalline phase and thus prevent particle disintegration. Here, we investigate the amorphization process of micro-sized silicon particles in a silicon-rich anode (70 wt% silicon) over extended charge/discharge cycling in half-cells with a lithium reference electrode, varying the lower cutoff potential of the Si electrode. While the capacity of Si electrodes after formation remain constant for lithiation cutoffs of ≥170 mV vs Li+/Li, their capacity continuously increases over cycling for cutoffs of <170 mV vs Li+/Li, implying an ongoing amorphization of the crystalline phase. To quantify the ratio of the amorphous phase fraction over cycling, we employed an in-situ XRD method, utilizing the copper reflex of the current collector as internal standard. This allowed to determine the extent of amorphization over the course of cycling depending on the lithiation potentials.