Atomistic study on the origins of the anisotropic lithiation behaviors of the silicon anode using the reactive force field based molecular dynamics simulations

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We employed the Reactive-Force-field (ReaxFF) based molecular dynamics simulations to investigate the lithiation mechanisms and anisotropic lithiation behaviors of the Si-based anode during the lithiation processes. In order to accurately predict the relative lithiation kinetics on different surface facets of c-Si, we have developed new ReaxFF parameters for the Li-Si system based on first-principles calculations. Our results demonstrated that the lithiation kinetics on the Si(110) and (112) surfaces manifestly outperformed those occurred on the Si(100) and (111) surfaces. The lithiation reactions on the Si(110) and (112) surfaces tend to proceed through a ledge mechanism involving the layer-by-layer peeling of the {111} atomic facets. However, the lithiation reaction on the Si(100) surface is more likely to occur via the layer-by-layer etching of the {100} atomic facets. The discrepancy in the lithiation kinetics among the c-Si(100), (110) and (112) substrates was mainly ascribed to the variation in the compressive stresses that developed within the phase boundary upon lithiation. Regarding the Si(111) surface, the significantly slower lithiation kinetics than other surface facets were primarily attributed to the presence of a considerably larger barrier for Li insertion into the subsurface region to initiate the lithiation reaction. Furthermore, the order of the energy barriers for Li insertion into different c-Si surfaces was determined as follows: Si(111)>>(100)>(110)>(112), indicating that Li atoms tend to migrate into c-Si predominantly through the (110) and (112) surfaces upon lithiation. Consequently, lithiation reactions would occur majorly on the (110) and (112) surfaces due to the much lower compressive stresses that developed within the phase boundary and smaller Li insertion barriers than other surface facets, resulting in the observed anisotropic lithiation behaviors of the c-Si nanomaterials.