Experimental Measurements And Mechanical Characterizations Of Bilayer Electrode During Mechano-Electrochemical Coupling Processes

Lithium-ion batteries are one of the most common energy storage devices,in which electrode materials and structures are core components.Electrochemical processes cause multi-scale mechanical problems in electrodes,meanwhile electrode mechanics affects electrochemical behavior in turn,leading to battery life fade,aging and safety issues.In this thesis,the mechano-electrochemical coupling problems in bilayer electrodes are studied.To solve the problems of modulus evolution and coupled stress “undetectable” during an electrochemical process,in-situ measurement technique and mechanical characterization method are studied,including following aspects.1.An electrochemical stress model with Li-related modulus for a bilayer electrode is established.Starting from a deformation analysis in a bilayer electrode caused by electrochemical process,considering the bending stress from current collector constraint and Li-related modulus of electrode material,the stress model contains three Li-related equations: material modulus evolution equation,electrode material stress equation and current collector stress equation,modifying the commonly used Stoney equation in electrochemistry field.Then,based on the model,the evolution of bilayer stress with three key impact factors is systematically discussed,that is,bilayer structure parameters,material modulus and bending stress.In additional,the optimization of electrode structure is proposed based on reducing stresses in bilayer electrodes.2.Researches on in-situ measurement methods of material modulus and stress of bilayer electrodes are conducted.An in-situ optical acquisition system with an electrochemical cell is built for deformation measurement,where bending deformations of Si-composite and carbon-composite electrodes are collected in real time,showing with the capacity.Combined with the proposed electrochemical stress model,the insitu measurements of modulus evolution for two electrode materials are realized for the first time.The evolutions of material modulus and bilayer stresses vs.specific capacity are quantified.Results show that,as Li concentration(Li volume fraction)increases,Si material and carbon material exhibit a continuous,nonlinear softening and stiffening respectively.The stress values increase nonlinearly with capacity,where electrode material is compressed and current collector undergoes tensile-compressive stress conversion along the thickness direction.Further,experimental analysis reveals that Si material softening reduces and carbon material stiffening increases the bilayer stresses,respectively.And the degree of bilayer stresses affected by variable material modulus is quantified.These data confirms the applicability of proposed electrochemical stress model in experimental measurements of electrode mechanics than Stoney equation.3.An experimental method for visual measurements of electrochemical-coupled stress fields in a biayer electrode is proposed.A visualized electrochemical cell with a concentric ring electrode structure is designed.Integrating DIC deformation measurement technique and Li concentration detective technique in our group,through a dual optical acquisition system,the mechanical strain field,Li concentration and material modulus field of carbon-composite electrode are measured in real time and the spatiotemporal evolution is given.An electrochemical stress model that couples three experimental results is proposed,then coupled stress field of carbon-composite electrode and spatiotemporal evolution is realized for the first time,breaking through the bottleneck of “invisible,undetectable” stress field at present.The visualized results show that the strain field,Li concentration field,material modulus field and stress field are gradiently distributed along diffusion path,displaying increasing values as lithiation process.Based on features of visual method and these results,the electrochemical stress field is decoupled.The experimental analysis indicates that variable material modulus is a key factor that affects the coupled stress,where greater degree of material stiffening(or softening),greater influence on the coupled stress.Strain and Li concentration are both impact factors of electrochemical coupled stress.The two,one positive and one negative,are mutually competitive,in which Li concentration is the dominant factor in electrochemical coupled stress in our experiment.These analyses reflect the misunderstanding of stress characterization in current electrochemical experiments.4.This thesis also discusses mechanical load coupled®ulated electrochemical behavior from an application view.Two kind experiments are done that present new experimental phenomena.One is active stress regulated electrochemical behavior originating from high performance.A battery structure is designed to preset active tensile/compressive stress.And electrochemical experiments of Si-composite electrodes are carried out under seven different active tensile/compressive stress.The experiment finds and confirms that the enhancement(inhibition)effect of active tensile(compressive)stress on the cycle and kinetic performance of Si-composite electrode.And mechanical analysis shows that the enhancement is mainly from relieved coupled compressive stress in the micro-structure.The other is the coupling between microscale deformation and electrochemical parameters originating from high efficiency.The electrochemical experiments of graphene electrodes are carried out at three charging/discharging rates,meanwhile the micro-strain is real-time measured by Raman spectroscopy.The evolution of micro-strain with capacity and cycling is given.The experimental results show that the higher the charging/discharging rate,the larger the tensile strain and the smaller the specific capacity.Based on this,the optimization charge and discharge strategy with first rapid and then slow is proposed.