Mechanical Analysis In Lithium Ion Batteries:Delamination,Electrochemical Reactions And Charge Methods

Lithium ion(Li-ion)batteries have been widely used in many applications nowadays due to its advantages such as high energy density,high operating voltage and low self-discharge rate.It is also one of the most promising technologies that meet the demands of applications in wider fields,e.g.electric vehicles.The electromechanical coupling problem in Li-ion batteries has attracted much attention.In this study,in order to take insight into the mechanical-electrochemical coupling problem in Li-ion batteries,we focus on mechanical analysis in interfacial delamination between active layer and current collotor,electrochemical reactions on the surface of active materials,as well as selections of charge methods by considering diffusion-induced stress and charge time.There is a critical problem affecting electrode safety,i.e.delamination between active material and current collector.The delamination may lead to change of electrical current flow as well as peeling off of active materials,and therefore,should be avoided.The state of charge(SOC)or the depth of discharge(DOD)to and the time to delamination onset and the critical size for delamination,which are potentially important in Li-ion battery design for preventing the patterned thin film electrodes from delamination,are investigated in this dissertation.A theoretical approach based on the cohesive model is proposed to formulate the time to and the SOC(or DOD)to delamination onset,as well as the critical size for patterned disk-like electrodes.It is found that for negative electrode the delamination initiation is dominated by mode-? during cell charging and by mode-? during cell discharging.For positive electrode both delamination modes are theoretically possible during cell charging.When the electrochemical load factor is very high,the mode-? delamination is possible and an optimal size of electrodes exists for a given maximum electrochemical load factor.The SOC to mode-? delamination onset decreases significantly with increasing dimensionless radius,being independent of electrochemical load factor.When the mode-? is the dominated mode the critical size for delamination exists,and a formula for the critical size for delamination has been deduced,which is proportional to interface ductility and inversely proportional to maximum volumetric strain.Since common materials of current collectors such as copper and aluminum are high-modulus comparing to the lithiated silicon and are usually assumed as rigid,this dissertation aims to develop an analytical approach based on the cohesive model to investigate the progressive interfacial delamination of the patterned disk-like electrodes where the substrate is rigid.Different from the semi-analytical method and the finite element methods in which the governing equations are discretized,the proposed methodology aims at solving the prescribed differential equations directly.The assumption of rigid substrate has been proved acceptable for high-modulus substrates such as copper and aluminum.For the case where the weak interface is assumed and the radial concentration gradient is neglected,an extremely simplified solution has been obtained.The simplified solution which has acceptable accuracy provides a good guidance for understanding and predicting the interfacial debonding.The voltage hysteresis of Li-ion batteries in charge–discharge cycles leads to low charge–discharge efficiency,energy dissipation,heat generation,difficulty in battery controls,etc.The crucial role of mechanical stress in voltage hysteresis of Li-ion batteries in charge–discharge cycles is investigated theoretically and experimentally.A modified Butler–Volmer equation of electrochemical kinetics is proposed to account for the influence of mechanical stresses on electrochemical reactions in Li-ion battery electrodes.It is found that the compressive stress in the surface layer of active materials impedes lithium intercalation,and therefore,an extra electrical overpotential is needed to overcome the reaction barrier induced by the stress.The theoretical formulation has produced a linear dependence of the height of voltage hysteresis on the hydrostatic stress difference between lithiation and delithiation,under both open-circuit conditions and galvanostatic operation.Predictions of the electrical overpotential from theoretical equations agree well with the experimental data for thin film silicon electrodes.Besides,evolution of diffusion-induced stress in electrodes depends significantly on charge and discharge operations.This dissertation demonstrates the design of charging strategies for Li-ion batteries with considering the balance between diffusion-induced stress and total charge time for two-and three-stage charge methods.For the two-stage galvanostatic-potentiostatic charge method the low mechanical stress can be achieved without increasing total charge time by switching the galvanostatic to the potentiostatic at the time moment when the lithium concentration at the surface of particles reaches the limit of concentration.A three-stage method,which consists of an initial galvanostatic stage of high current,a galvanostatic stage of low current and a potentiostatic ending stage,is suggested.Employing the initial galvanostatic stage of high current is helpful not only in accelerating the charge process,but also in controlling the mechanical stress once the electrical current and time duration of the initial galvanostatic stage are properly designed.