Modelling electrochemical and thermal behaviors of silicon-based electrodes for lithium-ion batteries

Abstract

Silicon (Si) has become an attractive alternative to graphite (Gr) as an anode material for the next generation lithium-ion batteries (LIBs) due to its high theoretical capacity, natural abundance, and reasonable electrode potential. However, Si-based electrodes exhibit distinct electrochemical and thermal behaviors compared to traditionally used carbonaceous electrode, and there have been rare models tailored to the behaviors of Si-based electrodes. Therefore, this thesis aims to develop physics-based models able to describe and predict the electrochemical and thermal behaviors of Si-based electrodes, which can serve as important tools for understanding and designing LIBs with Si-based anodes. A mechanistic model of silicon anodes in LIBs is first developed to describe the unique voltage hysteresis phenomenon of silicon electrodes. The model correlates the voltage hysteresis of Si to its underlying phase transformation, crystallization and amorphization processes. The effects of crystallization rate and surface energy barriers are studied, unveiling the role of surface energy and particle size in determining the performance behaviors of Si. Subsequently, a multi-material model is developed for simulating Si/Gr composite electrodes, considering different behaviors of Gr and Si. Results show that silicon introduces voltage hysteresis to Si/Gr electrodes. The (de)lithiation sequence and competing processes between Si and Gr are comprehensively studied. A dimensionless competing factor is derived to quantify the active operating regions for each material, and demonstrates to be a useful indicator to design cycling protocols for mitigating the degradation of composite electrodes. The multi-material model is further coupled with a thermal submodel. By studying the respective heat contributions of each active material, the model reveals the origin of thermal peaks of Si/Gr composite electrodes, which are highly related to the phase transition processes of Gr. These thermal peaks can be potentially used to detect the ageing of Si-based batteries in service.