A Study on the Kinetics of the Phase Transformation in Silicon Anodes in Lithium Ion Batteries - in-Situ Experiments and a Computational Model

Abstract

Being one of the major incentives in advancing technologies, characterizing the phase transformation in materials is yet far from reaching a fully comprehended subject. Understanding the mechanics, thermodynamics, and kinetics of the phase transformation serves a significant role in modeling the performance of materials. In this talk, we introduce a non-destructive in situ method for monitoring the advancement of the reaction front with a resolution of ~10 nm. We combine Picosecond Ultrasonics with Atomic Force Microscopy to study the velocity of the phase boundary propagation and the corresponding volumetric strain. Picosecond Ultrasonics is a pump and probe technique that can measure the thickness of the internal layers of material using ultrasounds that are generated and detected optically. Crystalline silicon is chosen as our model system due to its characteristic phase transformation behavior during its reaction with lithium. Using this technique, we have examined the diffusion of lithium atoms through different crystallographic orientations under various galvanostatic and potentiostatic conditions. Based on these experimental results, we have calibrated a modified Cahn-Hilliard type of phase field model of a moving phase boundary problem. In contrast to the classical formulation, the mobility of the interface can be determined using this modified Cahn-Hilliard model. The relevant thermodynamic and kinetic parameters for the phase transformation due to diffusion of Li in crystalline Si for the two crystallographic orientations are extracted. The extracted model parameters can facilitate the simulation of phase transformation in more complex structures with multiple crystallographic facets.