Structural changes during lithiation and delithiation of Si anodes in Li-ion batteries: A large scale molecular dynamics study

Drastic volume change during lithiation and delithiation presents the biggest challenge to the realization of Si anodes in commercial Li-ion batteries. In particular, the anisotropic volume expansion is thought to be the most important feature for electrochemical reactions of Si anodes, since it can lend inspiration in designing advanced battery anodes. In the present study, the initial stage of lithiation and delithiation processes of Si anodes has been investigated using a large scale molecular dynamics simulation based on a recently developed 2NN MEAM interatomic potential for the Li-Si system. The volume expansion during lithiation and pulverization during delithiation is correctly reproduced by the simulation. The volume expansion for a given Li concentration is found to be independent of the crystallographic orientation while the crystal-to-amorphous transition is highly dependent on the crystallographic orientation. The simulation shows that the crystal-to-amorphous transition starts at lower Li concentration along the direction than other directions, which explains the important anisotropic volume expansion in Si wire anodes. The suitability of large scale atomistic simulations for understanding a wider range of atomic level structural processes and designing nanostructures for high capacity Si anodes is discussed.