Research On The Mechanism Of Mechanical Degradation Of Lithium Ion Battery Induced By Overdischarge

With the explosive growth of new energy vehicles in recent years,the safety hazards and capacity degradation of lithium ion batteries have attracted high attention.As a typical case of electrical abuse,overdischarge can lead to thermal runaway of lithium ion batteries and pose safety hazards,while working at low temperatures can cause battery capacity degradation.Therefore,it is of great significance to explore the mechanism of mechanical property degradation of battery materials caused by over discharge and establish low-temperature battery testing instruments for the promotion of lithium ion batteries for vehicles.By means of scanning electron microscopy(SEM),X-ray diffraction(XRD)and energy dispersive spectroscopy(EDS)characterization of battery materials with different discharge depths,the side effects of over-discharge and the structural deterioration of battery materials caused by high temperature were found.By the indentation,scratch,tensile and thermal shrinkage tests of the battery materials with different discharge depths and temperatures,it was found that the mechanical properties of the battery materials continued to decline with the increase of the discharge depth,and the mechanical properties of the battery materials showed a precipitous decline at the stage of deep over-discharge(discharge depth≥120%),and finally proposed the decline mechanism of the mechanical properties of the over-discharge battery materials.The root cause of the mechanical property degradation of the battery induced by overdischarge was determined.In addition,this paper designed two sets of low-temperature test platforms for lithium ion battery monomer,which could be integrated with the electrochemical test equipment and mechanical test equipment of the battery respectively to realize the performance test and observation of lithium ion battery monomer in low-temperature environment.The main research content of the full text is divided into three parts,which are summarized as follows:(1)SEM,XRD,and EDS were used to characterize the cathode and anode of the battery at different discharge depths.It was found that over discharge induced mixing of lithium and nickel in the cathode and the shedding of active particles from the cathode and anode.The relationship between the discharge depth and the mechanical properties of the cathode was established by indentation testing of the battery cathode at different discharge depths.The indentation test results showed that the mechanical properties of the cathode decrease to varying degrees as the discharge depth increases.When the discharge depth was 100%,110%,120%,and 150%,the average elastic modulus was 20.12 GPa,16.82 GPa,6.06 GPa,and 3.26 GPa,respectively,and the maximum indentation depthes were 1.88 μm,2.27 μm,3.06 μm and 3.73 μm.This indicateed that the irreversible damage of copper dendrites to the cathode during the deep overdischarge stage led to a significant decrease in the mechanical properties of the cathode.The research results revealed the fundamental reason for the significant decline in the mechanical properties of the cathode:the destruction of active particles and adhesives by copper dendrites.(2)A series of characterization analyses and mechanical performance tests were conducted on battery separators that experienced different discharge depths and temperatures.The phenomenon of detachment of the membrane coating and the penetrating damage of copper dendrites to the membrane were proposed and confirmed,and the relationship between the discharge depth and the mechanical properties of the membrane was established.The tensile test results of the over discharge membrane showed that when the discharge depth was 100%,110%,120%,and 150%,the elongation after fracture was 67.21%,62.65%,48.68%,and 46.05%,respectively.The results of the constant load scratch and thermal shrinkage tests of the overdischarge membrane showed that the average friction coefficient of the membrane coating continuously decreased with the increase of the discharge depth,which was 0.6324,0.5211,0.4493,and 0.3217,respectively.The thermal shrinkage rates in the mechanical direction of the membrane were 0.39%,0.78%,0.83%,and 1.59%,respectively.The mechanical test results after high temperature showed that the influence of temperature on the mechanical properties of the membrane is limited.Therefore,the deterioration of the mechanical properties of the overdischarge membrane could be attributed to the effect of the overdischarge side reactions,where copper dendrites penetrated and destoryed the base membrane of the membrane,leading to a sharp decline in the overall mechanical properties of the membrane.(3)Two low-temperature testing platforms for different lithium-ion battery monomers were designed and built based on Partier semiconductor refrigeration chips.The low-temperature testing platform for cylindrical battery cells can achieve preset testing temperature through temperature closed-loop control.In addition,the platform can be integrated with the electrochemical workstation or battery divider cabinet of the battery,enriching the electrochemical testing conditions such as over discharge of the battery.The paper designed and optimized a low-temperature testing platform for square shell battery cells,which can be combined with the battery extrusion and needle punching integrated machine.In addition,this article integrates an optical microscope observation module for the extrusion needle punching integrated machine,achieving real-time observation during the extrusion/needle punching loading process of square shell battery cells at low temperatures.