In-situ Characterization And Analysis Of Mechanical-chemical Coupled Mechanism And Fracture Properties In V₂O₅ Electrodes

The structural stability and mechanical integrity of the electrode have a significant impact on the practicability,reliability and safety of the lithium ion batteries.During the electrochemical cycle,the host material will expand or contract to varying degrees due to the reciprocating intercalation/deintercalation of lithium ions.And the periodic volume deformation will cause and accumulate strain and stress in the electrode,which eventually lead to collapse of the crystalline structure of the active material,production of the electrode material cracks,pulverizes and break down,and then those will affect the overall performance of lithium ion batteries.The research and analysis on the mechanical properties of the electrodes are of great significance for optimizing electrodes structure design,improving safety performance and prolong the battery life.In this paper,an in-situ deformation testing system based on digital image correlation technology is designed and assembled.Through the testing system,the strain evolution of the V₂O₅ electrodes are monitored and recorded in real time.The relationship between the strain/stress field and the evolution of electrochemical performance of V₂O₅ electrodes was studied,and the corresponding mechanical-electrochemical coupling mechanism was analyzed and discussed.Based on digital image correlation technology,the real-time fracture strain of the V₂O₅ electrodes were obtained by using uniaxial tensile test.Combined with the shear lag model and the related parameters,the relevant results of the fracture properties of V₂O₅ electrodes were calculated.The main research contents are as follows:In-situ characterization and analysis of mechanical-electrochemical coupling property of V₂O₅ electrodes.V₂O₅ nanomaterials were prepared by solid state reaction method.During the charging and discharging cycle,the first discharge specific capacity is 283 m Ah·g^-1,the first charge specific capacity is 279m Ah·g^-1.And the coulombic efficiency is greater than 98%.During the electrochemical cycle,the strain of the V₂O₅electrode presents a heterogeneous and a sinusoidal-like evolution trend with the increase of the electrochemical processes.During the lithiation processes,the tensile strain of the electrode continuously increases and tends to decrease during the delithiation processes,but it cannot return to the initial state before discharge position.The residual strain in the electrode accumulates continuously with the cycle of charge and discharge.During the electrochemical cycle,the average plane principal strain of V₂O₅ electrode is 1.29%at the end of the first discharge process and 0.95%at the end of charge process.At the end of the eighth cycle,the residual strain value was 1.28%.At the same time,the corresponding average plane stress is calculated by combining constitutive equation.Meanwhile,the mechanical part and the chemical part of stress field under charge and discharge processes are discussed.In-situ characterization and analysis of fracture properties of V₂O₅ electrodes.Taking V₂O₅nanomaterials as the research object,the uniaxial tensile test of the V₂O₅electrodes were carried out by the electronic universal testing machine.The deformation/displacement fields of the active layer of V₂O₅ electrode was dynamically captured by the digital image correlation technology.The surface deformation of the active layer of V₂O₅ electrode was analyzed in real time and the cloud maps were presented.With the increase of PVDF content,the critical fracture strain of the electrode increases continuously,and the fracture strains of S1,S2 and S3 is 6.27%,8.77%and12.36%,respectively.Combined with the shear lag model,the fracture energy,fracture strength and fracture toughness of V₂O₅ electrode were obtained.The electrode surface before and after the tensile test was observed by SEM,and the corresponding microstructure was obtained.It can be seen that the length,width and density of the transverse cracks on the surface of the active layer decreased with the increase of PVDF content.