Abstract
Silicon-based composite electrodes in lithium ion batteries attract increasing attention because of their high theoretical capacity. Here, numerical simulations are used to better understand the interplay between electrochemical and morphological behavior of the silicon-graphite (1:2.7) composite electrode during galvanostatic cycling. Finite element methodology is used to solve a one-dimensional model based on the porous electrode and concentrated solution theory. Porosity changes in the silicon electrode and solid electrolyte interphase layer growth are also included in the model. The simulation results show that at lower rates, the electrode with high initial porosity is being fully utilized before the lower cut-off potential is reached. When comparing the computational results with experimental observations, it can be seen that the main reason for capacity fade is the increase in tortuosity in the diffusion pathway of lithium ions due to cracking of the silicon composite electrode upon electrochemical cycling.
Highlights
The increasing demand for electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles has spurred interest in using silicon as a component in lithium ion battery anodes
Simulations were conducted at high initial porosity of the composite anode to provide enough space for expansion of the silicon particles without changing the dimension of electrode
It is considered that there is no significant expansion of the graphite particles, and the volume fraction of graphite in the electrode is fixed while the silicon volume fraction changes according to Eq (2)
Summary
The increasing demand for electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles has spurred interest in using silicon as a component in lithium ion battery anodes. Computational modelling of lithium ion batteries has been growing extensively in recent years and is becoming a necessary tool to analyze a vast range of degradation phenomena in these electrochemical cells [17,18,19,20,21] In such studies, a theoretical analysis of the porosity changes has been shown to be important since the effective transport properties of the electrolyte, such as the diffusion coefficient of the lithium ions in the electrolyte and the ionic conductivity decrease with the decline in porosity [22]. A detailed computational model of silicon-graphite composite anodes is still largely missing Thereby, it is still poorly understood how changes in the porous structure, both during lithiation and over repeated cycling, affect its cycling performance and capacity retention. The most reasonable degradation mechanism in terms of microcracks is identified based on comparison to simulation results and incorporated into the model description
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