Impacts Of Stress And Plastic Deformation On The Performance Of Lithium Ion Batteries

Rechargeable lithium ion battery, due to its advantages such as high energy density, long lifespan and environment-friendly, has been applied in a wide range of regions including communication, transportation and military. It is considered one of the most promising advanced power sources for the demands of energy storage and environmental protection. However, mechanical degradation of electrode materials is one of the key factors that limit the performance of lithium ion batteries. During cycles of charge and discharge, lithium ions move into or out of electrode, resulting in deformation and stress in electrode, and subsequently leading to fracture and failure of Li-ion battery. Therefore, it is important to study mechanics related problems of lithium ion battery.This thesis aims to study the impacts of stress and plastic deformation of electrode on the performance of lithium ion batteries. Specifically, the work is divided into three parts.Part One,the impacts of plastic yield of current collector on the diffusion induced stress in a symmetric layered electrode are studied. Based on analytical formulations of the stress in whole electrode, three types of elastoplastic behaviors of current collector, i.e. pure elastic deformation, plastic shakedown and cyclic plasticity, are identified. Criterions separating the three cases are proposed. It is found applying a thin current collector and allowing it plastically yield in the charge/discharge cycles is beneficial not only to capacity as more space can be provided for active materials but also to electrochemical stability because the stress in active layer is significantly reduced. Structural design corresponding to plastic shakedown shows good balance between the said improvements and structural safety, whereas the case of cyclic plasticity further enhances the improvements but with the sacrifice that the cycle life is significantly shortened. Therefore, structural designing scheme is provided for the former case according to the criterion of plastic shakedown but for the latter one based on the Coffine-Manson relation with expected cycle life.Part Two, a mathematical model is established to describe the effect of plastic yield of current collect on deformation of an asymmetric bilayer electrode and develop analytic expressions for the evolution of curvature. Three stages of elastoplastic behaviors of current collector, i.e. pure elastic deformation, elastic-plastic coexistence, plastic flow, are identified during insertion of lithium ions into active layer. Based on elastic-plastic constitutive model, criterions separating the three stages are proposed. It is found that during lithium insertion, plastic yield of current collector would lead to dissipation of elastic energy and slow curvature growth of electrode. In addition, an in situ experimental observation of electrode bending upon lithium intercalation is carried out. The simulated curvature evolution agrees well with experimental measurement.Finally, the impact of stress coupling on electrochemical reaction of insertion/de-insertion in a lithium ion battery is investigated. Based on the traditional Butler-Volmer model which describes the kinetics of electrode reaction and ignores stress coupling, elastic energy, which arises from the deformation and stress of an electrode, is firstly introduced into the free energy system. After that, the impacts of variation of free energy under electro-mechanical coupling condition on the forward and reverse reaction are comprehensively evaluated. Finally, the electrochemical reaction rate and current density are obtained and further employed to establish the modified electrochemical model with consideration of stress coupling. It is found that compressive stress raises the energy barrier in the direction of lithiation by improving the free energy, and therefore hinders the intercalation of lithium ions from electrolyte into active material. At last, according to the modified Butler-Volmer model, lithiation processions of a Si thin film electrode and a spherical particle were simulated. The results verify that stress effects the intercalation process significantly, both rate dependent and size dependent.