In-situ Characterization And Analysis Of Mechanical Properties Of The Active Layers In Silicon/carbon And Graphite Anode

The structural stability and integrity of the electrode material have a direct impact on the overall performance of the battery.During the charging and discharging process,lithium ions are extracted/intercalated from/into the active material,which will cause the electrode to produce different levels of diffusion stress,and further lead the electrode material to crack or even break.The fracture phenomenon of the electrode is closely related to the degradation of the battery performance,which is the core problem of the electrode mechanics research.All in all,the evaluation of electrode mechanical properties is of great significance for the design of electrode structure and the extension of battery cycle life.This paper designs and assembles an electrode in-situ tensile test system,and the relevant fracture properties of different electrode active layers under different cycling conditions are obtained based on the simplified shear lag model.The main research contents are as follows:First,Taking four kinds of silicon/carbon materials with different silicon contents as the research object,the morphology,structure,particle size and other characterization tests were carried out,and then the electrochemical performance including impedance,cyclic voltammetry curve and cyclic stability were tested.Last the mechanical parameters of the electrode active layer such as fracture strain,fracture strength,fracture toughness and other mechanical parameters were measured and analyzed by digital image correlation testing technology combined with a simplified shear lag model.The electrochemical test results show that as the silicon content of the electrode material increases,the specific capacity of the battery continues to increase,but the cycle stability decreases.The mechanical test results show that the fracture strain of the electrode increases as the silicon content increases.However,the fracture strength and fracture toughness of the electrode active layer shows a slight decrease.The fracture strengths of S420、S600、S950 and S1100 are 143.9 MPa,140.2 MPa,135.4 MPa and 134.6 MPa,respectively.These parameters can not only directly reflect the mechanical strength of the active layer of the electrode so as to optimize the structural design of the electrode,but also contribute to the establishment of the mechano-electrochemical coupling model.Second,the graphite anode of Li Mn₂O₄/Graphite commercial pocket batteries was taken as the research object and then the batteries were cycled at 0.5 C for 0 times,100times,200 times,300 times,400 times and 500 times,respectively.After that,the morphological characterization and tensile test of the electrode material before and after cycling were carried out.The test results show that as the number of cycles increases,the capacity of the battery gradually decreases,and there will be different degrees of expansion.After 500 cycles,the overall thickness of the battery increases by 0.6-1.3mm,and the edge area expands most seriously.After analyzing the tensile data,it is found that as the number of cycles increases,the fracture strain,the fracture strength and the fracture energy gradually decreases.Among them,the fracture strengths of the active layer of the battery electrode without cycling and cycling for 500 times are 311MPa and 143 MPa,respectively.These test results truly reflect the degradation of the electrochemical and mechanical properties suffered by the electrode during normal use,which have an important guiding significance for the design of the battery.