Molecular Dynamics Simulations of Interfacial Lithium-Silicon Interdiffusion in Lithium-Ion-Battery Anodes

Since the initial demonstration that amorphous silicon (a-Si) can serve as a high-energy anode for lithium (Li)-ion batteries, substantial efforts have been made to enhance its fast-charging capability. However, the presence of electrolytes, carbon black and binder, combined with the porous microstructure of the electrode makes it difficult to explicitly capture the kinetics of Li-Si interdiffusion experimentally. In this manuscript we apply a recently developed ReaxFF interatomic potential for Li-Si-C systems to investigate the lithiation of amorphous silicon by performing molecular dynamics (MD) simulations on an amorphous Si@Li interface model at various temperatures. In our simulations we observe the formation and growth of a bulk LixSi interphase between the pure Li and amorphous Si phases. At room temperature the interphase approximates Li5Si2 with x approximate to 2.7; however, lithiation levels as high as x = 3.861 are observed at higher temperatures. The distinct interphase stoichiometry and sharp interfaces suggest a reaction-limited lithiation of Si, with an energy barrier between 15 and 23 kJ mol(-1). The extremely high current density of 4.776 mA mu m(-2) at a voltage of similar to 0.6 V during the intrinsic Li-Si mixing reaction suggests that either effects arising from the microstructure of the active material (i.e., particle geometry, surface roughness, porosity, tortuosity, etc.) or additional battery components (e.g., electrolyte, binder, separator, etc.) are responsible for the significantly lower rate capabilities actually exhibited by Si electrodes in Li-ion batteries.