In Situ Strain Measurements And Mechanical-Chemical Coupled Mechanism Analysis Of Silicon/Carbon Composite Electrode During Electrochemical Cycling

Lithium ion batteries?LIBs?have been widely used in consumer 3C,electric vehicles and advanced energy-storage systems due to their higher capacity,longer cycle lives and environmental protection.However,the composite electrode is inevitable to be subjected to periodic expansion and contraction deformation during electrochemical cycling,destroying the structural integrity of the electrode,and rapidly degrading the cycling performance of LIBs.Therefore,it is important to in-situ measure the strain of composite electrode,revealing the corresponding stress variation features.It is helpful for optimizing the structure of electrode and battery safety assessment.In this work,an in-situ deformation testing system was designed and assembled to measure the strain fields of silicon/carbon composite electrode.According to the structural characteristics and service environment of the silicon/carbon composite electrode,the corresponding mechanical-chemical constitutive equation was deduced.Combined with the in-situ electrochemical data,the stress evolution of silicon/carbon composite electrode was obtained.The main conclusions are summarized as follows:Firstly,the effects of current density and electrochemical cycling on the mechanical-electrochemical performance of silicon/carbon 600 bilayer electrode were investigated.The silicon/carbon 600 bilayer electrode exhibits anisotropy and sinusoidal variation during cycling.The tensile strain increases continuously during each discharge process and decreases gradually during the charging phase,but the residual strain gradually grows with the increase of electrochemical cycles.The current density plays a vital role in the diffusion and concentration gradient distribution of lithium ions.The greater the applied current density is,the higher the lithium ion concentration on the surface of the active layer.After the 5^th cycle,the maximum of residual strain on the surface of the active layer achieves to be about2.57%for 100 mA/g,4.31%for 150 mA/g and 6.32%for 200 mA/g,respectively.When the current density is increased from 100 mA/g to 150 mA/g,the biggest stress of the electrode is increased from-32.3 MPa to-44.6 MPa.However,when the current density increases to 200 mA/g,the biggest stress decreases to be-22.1 MPa.And the compressive stress caused by the electrochemical part is much larger than the tensile stress of the mechanical part.Secondly,the mechanical-electrochemical performance of silicon/carbon 950symmetrical electrode during electrochemical cycling was analyzed.During the 2^nd–4^th cycles,the increment of the average principal strain in each lithiation process is about 0.71%,and it decreases about 0.65%in each delithiation for I=250 mA/g.After the 4^th cycle,the average residual principal strain achieves the maximum of1.73%,which was much smaller than that of the silicon/carbon 600 bilayer electrode for I=200 mA/g.There is a typical gradient distribuiton of lithium ion concentration with different thickness of the active layer,and the concentration gradient distribution increases with electrochemical cycling.After the 9^th cycle,the maximum of residual lithium ion concentration on the surface of active layer achieves to be about 49890.4mol/m³,which is 2.8 times larger than the lithium ion concentration after the 1^st cycle.The stress of silicon/carbon 950 symmetrical electrode exhibits sinusoidal variation during electrochemical cycling.As the number of cycles increases,the residual stress keeps increasing,and achieves the maximum of-43.0 MPa after the 9^th cycle.And the biggest residual stress in the the current collector is 37.9 MPa.It is found that the compressive stress caused by the chemical part was much larger than the tensile stress of the mechanical part.