Research On The Lithium Ion Battery Degradation Mechanism Among The Whole Life Cycle

Lithium-ion batteries(LIBs)will have certain failure phenomena during commercial applications,which will lead to shorter cycle life and even safety issues[1].Understanding the decay mechanism of lithium-ion batteries can help to provide theoretical guidance for improving the safety design of the batteries and ensure the safety performance of the batteries.Aiming at the research of the lithium ion battery degradation mechanism among the whole life cycle,this paper mainly does the following work:By testing the long-cycle electrochemical performance of lithium-ion batteries at low temperatures of-10?,and analyzing the changes in the properties,morphology,and structural composition of electrode materials,the effects of low temperatures on the lifetime degradation of Li Co O2/MCMB batteries were studied.Research findings:(1)After low temperature,the battery charging voltage platform rises,the discharge voltage platform drops,and the constant voltage charging time becomes longer,indicating that the polarization of the battery has increased significantly.At the same time,there is a clear discharge plateau in the initial part of the discharge curve,and the corresponding IC curve also shows the dissolution peak of MCMB anode metal lithium at about 4.0V.After recovering to room temperature,the capacity recovery rate of low-temperature decayed to 85% SOC is 11.34%,and the capacity recovery rate of the low-temperature decayed to 75% SOC is 6.15%.As the degree of attenuation increases at low temperatures,the impedance of the battery increases gradually,which causes the polarization of the battery to increase,leading to the loss of the reversible capacity of the full battery.(2)Through analysis of the reversible capacity and residual capacity of the positive and negative electrode materials,it is found that with the aging of the battery,the active materials of the anode and cathode are all lost,but the low temperature has a greater impact on the polarization of the negative electrode,leading to the loss of reversible capacity.Through the same area of the residual capacity of the positive and the negative of the ICP test results,the amount of lithium lost from the positive electrode material was quantitatively increased from 0.112 mg to 0.514 mg,while the remaining lithium amount of the negative electrode was increased from the initial 0.117 mg to 0.596 mg,indicating that the decay to the end of life the positive materials structure may decay,leading to a decrease in its remaining active lithium insertion sites,which is also an important reason for the attenuation of the full battery after the end of life(capacity retention rate is less than 80%).(3)Through macroscopic morphology,micromorphology observation and EDS detection of positive and negative electrode materials,it is found that before the end of life(capacity retention rate is greater than 80%),a large amount of lithium deposits are formed on the surface of the low temperature negative electrode,resulting in increased battery resistance and active lithium decreased,causing the battery capacity to decay,while the morphology of the positive electrode remained unchanged.After the end of life,cracks began to appear between the layers of positive electrode particles,the structure began to change,and even the surface structure fell off.This exacerbates the irreversible capacity loss of the battery.(4)Analysis of the surface composition of the positive and negative electrode materials by XPS found that Co element is detected on the negative electrode surface after low temperature,and the content of Co element on the positive electrode surface is reduced.The relative content of P element is increasing,indicating that after the low temperature decays to the end of life,the electrolyte will also have side reactions on the surface of the positive and negative electrodes.XRD and Raman analysis further proved that the low-temperature battery decays to the end of the anode layer structure will change after the end of life,which is an important reason for the battery irreversible capacity loss after the end of the battery.By overcharging the battery at different voltages(4.4V,4.5V,4.6V,and 4.8V),and then cycle normally,the mechanism of the influence of charging on the battery capacity attenuation was studied.Research findings:(1)Battery capacity loss of only 0.71% and 0.94% after 4.4V and 4.5V mild overcharge,battery capacity loss increased to 8.17% and 28.36% after 4.6V and 4.8V severe overcharge,and long cycle after the capacity loss was increased from 6.75% to 19.92%.The ratio of battery capacity loss caused by overcharge to the total capacity loss rate of long-cycle batteries after overcharge increased from 10.52% to 41.01%,shows the effects of mild of charging the battery capacity attenuation almost negligible.But after severe overcharging,the impact of overcharging on the battery will increase,and the battery capacity decay will speed up.(2)Analysis of the reversible capacity of the positive and negative electrode materials and their corresponding IC curves and residual capacity,it is found that overcharging and long cycles after overcharging have little effect on the reversible capacity and polarization of the negative electrode,but can increase the positive polarization,loss of active substances and decrease the reversible capacity.At the same time,after slight overcharge(4.4V and 4.5V)and the corresponding long cycle,the residual capacity of the positive and negative electrodes increased.After severe overcharge(4.6V and 4.8V),the residual capacity of the negative electrode suddenly increased,while the positive electrode,on the contrary.It further shows that mild overcharge does not greatly reduce battery capacity,but severe overcharge will degrade the structure of the positive electrode and reduce the number of lithium vacancies,which will cause the negative active lithium to not be effectively used,and the remaining capacity of the negative electrode will increase sharply.(3)Through observation of macro morphology and micro morphology,and EDS detection of positive and negative electrode,it is found that overcharging has little effect on the morphology of the positive electrode material,and there are a large number of lithium deposits on the surface of the negative electrode after severe overcharging,and found the presence of Co element.Combined with XPS surface composition analysis,it is found that after severe overcharging,the surface film of the negative electrode thickened and Li Cx increased,and the long-cycle process would cause the lithium to precipitate out of the negative electrode,while the surface composition of the positive electrode changed little.(4)By XRD structure analysis,it is found that the structure of the negative electrode did not change much,and the characteristic peaks(003)and(104)of the positive electrode shifted to a low angle,indicating that lithium vacancies appeared in the Li Co O2 positive electrode,shows that the full battery was fully discharged there is still some lithium that is not embedded back to the positive electrode.After severe overcharging,its peak intensity is weakened significantly,indicating that the positive electrode material is attenuated and defects are increased,resulting in irreversible capacity loss.