Chemical and mechanical degradation and mitigation strategies for Si anodes

Abstract

Atomistic and mesoscopic models are used to analyze cracking and stresses produced during charge of Si nanoparticles covered by a thin SEI film. Mechanical stresses coupled to chemical effects are investigated with classical molecular dynamics simulations and with a mesoscopic model. Rupture of the surface film is the main cause of capacity fade and damage evolution is strongly influenced by the structure of the solid film. For example, high currents can cause rapid amorphization and help preserving the bond integrity. But large damage occurs after the current is above a threshold. In agreement with the atomistic results, mesoscopic modeling reveals that rupture of the surface film is the primary cause of capacity fading for amorphous silicon exhibiting single phase diffusion. It also suggests that conjugated silicon-film fracture in crystalline silicon with two-phase diffusion further exacerbates this deterioration. Fracture damage is slightly diminished by decreasing the Young's modulus of the brittle coating for both amorphous and crystalline silicon; however, controlling the large volumetric expansion induced stress on surface film is crucial towards improving silicon anodes. Mitigation strategies examined by ab initio molecular dynamics and electronic density functional theory simulations show passivation effects of graphene and graphene oxide on lithiated Si surfaces.