First Principles Studies of Disordered Lithiated Silicon

A protocol to generate disordered structures of lithiated Si was recently developed to reproduce the experimental potential-composition curve of a-Si lithiation at room temperature. In this paper, the bond angle distributions of these structures were studied, showing results consistent with the literature. Charge transfer studies using Bader charge analysis indicate that Li atoms donate 0.7 electrons to Si atoms, as in crystalline phases. The local environments of Si and Li atoms were shown to have a large impact on charge transfer as evaluated by considering neighbors through the effective coordination model. The variation in the effective Li neighbors with composition provides an insight into the cause of the drop in potential, which is observed in the experimental potential-composition curves of a-Si lithiation near Li2Si. The evolution of the electronic density of states with lithiation was studied, showing behaviors similar to crystalline phases. The limitations of the protocol are discussed in relation to diffusion effects and bond-breaking activation energies. By approximating the lithiation of crystalline Si and the delithiation of Li15Si4 as two-phase reactions, calculated potential plateau values were obtained, having a reasonable agreement with experiment (similar to 0.1 V).