Thermodynamic and Interdiffusion Properties of Lithiation in Silicon-Based Anode Materials Obtained from Reactive Molecular Dynamics Simulations

To obtain physicochemical insights into the lithiation and delithiation rates of Si-based anodes, reactive molecular dynamics simulations were performed on amorphous LiXSi1–X phases forming in Li-ion batteries. Enthalpies, Gibbs free energies, specific volumes and tracer diffusivities were extracted from analytical functions fit to the simulation results, which account for both temperature and concentration effects and potential solid–liquid phase transitions. The Gibbs free energy predicts a two-phase domain below X = 0.542 (∼LiSi), in agreement with previous phase diagrams. Assuming a chemically mixed initial state, the interdiffusivity would exhibit an inverse hyperbolic-cosine-like dependence on the Li concentration up to 1.655 × 10–7 cm2 s–1 at X = 0.277, leading to rapid phase segregation. In the stable lithiation domain with spontaneous Li–Si mixing, the interdiffusivity exhibits a hyperbolic-sine-like dependence on Li mole fraction with 8.67 × 10–7 cm2 s–1 at the inflection point, X = 0.731, after which the diffusion rate increases rapidly. Due to higher vibrational attempt frequencies, Li diffuses by a factor of 1.137 × 100 to 3.828 × 104 faster than Si, at X = 0.662 (∼Li7Si3 or ∼Li5Si2) and X = 0 (Si), respectively. Furthermore, the higher activation energy of Li increases its diffusivity much faster than Si atoms at higher temperatures and would facilitate Li surface deposition, which in turn triggers solid electrolyte interphase growth and dendritic plating. Therefore, a proper kinetic balance between Li and Si should be considered to prevent early capacity annihilation.