Mechano-chemical Failure Analysis And Technology Design Optimization Of V₂O₅ And Silicon-graphite Electrodes

The structural stability and mechanical integrity of electrode materials seriously affect the performance of lithium ion battery.In charge-discharge process,the phase changes of active material cause a significant variation of its size and volume,which in turn causes deformation and stress in the electrode.The periodic deformation/stress variation will lead to cracking,pulverization and disconnection of the electrode material,and accelerate the degradation of the electrode performance,which is one of the main reasons for the failure of lithium ion battery.Therefore,it is urgent to develop advanced in-situ strain/stress measurement technology,establish appropriate mechano-electrochemical coupling constitutive model,reveal the performance degradation mechanism of the electrode material,and optimize the structure design and technology parameters of the electrode.In this thesis,a set of in-situ test system of electrode deformation based on digital image correlation technology was developed.The relationship between strain/stress field and electrochemical performance of free-standing V2O5 electrodes was studied,the effect of multi-wall carbon nanotubes(MWCNTs)doping on the performance of the V2O5 electrodes was also researched.The mechano-electrochemical coupling mechanism in V2O5 electrodes was summarized.Based on the composite mechanics,the lithiation induced stress model of layered composite electrodes was deduced.The effect of technology parameters on the stress in silicon-graphite based electrodes was systematically analyzed.The main contents can be summarized as follows:In-situ characterization of mechano-electrochemical performance of the free-standing V2O5 electrodes.Firstly,V2O5 nanobelts was synthesized through hydrothermal method,and free-standing V2O5 electrodes were obtained by using vacuum filtration deposition technique.When cycled at 0.2C,the discharge specific capacity of the electrodes is 133 mA h g-1 for the first discharge-charge cycle,and it remains at 114 mA h g-1 after 150 cycles.Whereafter,an in-situ experimental system for electrode deformation characterization in charge-discharge process was developed based on the digital image correlation technology.The evolution of the strain field in the free-standing V2O5 electrodes was studied for the first time.The effect of mechanical stress and electrochemically induced stress were discussed through a mechano-chemical constitutive model.In-situ characterization of mechano-electrochemical property of the free-standing MWCNTs/V2O5 electrodes.MWCNTs/V2O5 composite was prepared through an electrostatic self-assembly process,and the free-standing MWCNTs/V2O5 electrodes were obtained by vacuum filtration deposition technique.MWCNTs improve the electrochemical kinetics of the V2O5 electrodes.When cycled at 0.2C,the discharge specific capacity of the 20 wt.%MWCNTs/V2O5 electrodes for the second cycle reaches at 206 mA h g-1.The evolution of strain/stress field in the free-standing MWCNTs/V2O5 electrodes was studied for the first time.And a general Arrhenius relationship between strain and voltage was established based on the mechano-electrochemical analysis.Dimensionless analysis of the lithiation induced stress in layered electrodes.Based on the composite mechanics,the relationship between equivalent mechanical parameters of porous composite active layer and the state of charge(SOC)was improved,the lithiation induced strain model for active layer was corrected.The model of lithiation induced stress in layered electrodes was developed.Based on the stress model,dimensionless qualitative analysis for the effect of SOC,biaxial modulus ratio and thickness ratio of current collector to composite active layer on the distribution of stress were performed.The analysis shows that increasing the thickness ratio and/or decreasing the biaxial modulus ratio can relieve the stress in the electrodes.Theoretical analysis of lithiation induced stress in layered silicon-graphite(SG)based electrodes.The effect of SOC,thickness ratio of current collector to composite active layer,mass ratio of silicon to graphite and porosity of composite active layer on the lithiated induced stress in the SG based electrodes were analyzed quantitatively for the first time.Analysis shows that stress in the SG based electrodes can be reduced by lowering the mass ratio and/or increasing the porosity,and stress gradient at the interface between active layer and current collector can be alleviated by reducing the thickness ratio.A series of critical parameters for architectural design of SG based electrodes were obtained based on the tensile fracture strength of current collector.