Atomistic Mechanisms of Phase Boundary Evolution during Initial Lithiation of Crystalline Silicon

In lithium-ion batteries, the electrochemical reaction between Li and Si causes structural changes in the negative electrode. The dynamics of lithiation of Si can be further complicated by the crystalline-to-amorphous phase transition. In situ TEM experiments show that a sharp interface, known as phase-boundary, is formed in between c-Si and a-LixSi during initial lithiation. Despite intensive study of the mixing mechanism during lithiation of Si negative electrode, the atomistic investigation of the formation and propagation of phase boundary for different orientation of Si remains unclear. We, therefore, performed molecular dynamics simulations to characterize the structural evolution of the phase boundary with a newly developed reactive force field (ReaxFF) potential for Li-Si. Our results confirm the phase boundary formation in between c-Si and a-LixSi. Structure and dynamics of the phase boundary depend on the crystalline phase of the Si. In particular, the location of the (111) plane plays a key role in crystal-toamorphous phase transformation. A relatively thick phase boundary is developed at the (100) surface, while an atomically sharp interface of negligible thickness is formed at the (111) surface. An amorphous phase of lithiated Si is developed beyond the phase boundary, in which the ratio of lithium to silicon atoms is steady at 0.8. Partial RDF studies revealed that the structures of the phase boundary and the lithiated Si region are c-LiSi and a-Li15Si4, respectively.