Microstructure- and concentration-dependence of lithium diffusion in the silicon anode: Kinetic Monte Carlo simulations and complex network analysis

Diffusion of lithium atoms in the silicon anode is a key process for the lithiation and de-lithiation steps in lithium-ion batteries. The relationship between atomic structures of silicon, in forms of crystals and glasses, and the diffusivity of lithium atoms are thus of critical importance to assess the performance of batteries using silicon as the anode. In this work, we probe the microstructure-and concentration-dependence of lithium diffusivity in silicon samples prepared in both crystalline and amorphous phases, by performing molecular dynamics and kinetic Monte Carlo simulations. We find that the diffusivity in the crystalline sample decreases with the concentration due to the blockade effect, while those in the amorphous samples increase first with the concentration as the sites with higher binding energies are occupied, activating long-distance diffusion between sites with lower binding energies, and then decline due to the blockage of diffusion pathways at a high lithium concentration. Complex network analysis of the transport pathway is conducted to measure the underlying microstructure-diffusivity correlation and statistical principles. The methodology and conclusions can be generalized to study the diffusive processes in media with complex microstructures, offering microscopic mechanisms-based understandings. Published by AIP Publishing.