Study On Mechanical Behavior Characteristics Of Lithium Ion Battery

With the deterioration of energy shortage and environmental pollution,renewable energy technologies attract intensive research efforts.In virtue of high energy density,long cycling life and environment-friendly,lithium ion battery is used widely in diverse fields,such as vehicles,home energy storage system and consumer electronics..The safety issues involved with over-charging,over-discharging and internal short circuit has also become the focus of current research.Pressure is one important factor to the electrochemical performance of lithium ion battery.In worse case,it could induce thermal runaway,which endangers the personal safety and property security of the consumers.In addition to the binding force introduced by the component structure fixing the battery packs,the expansion force comes from the lithium ion battery cells themselves during the charging and discharging procedure.In this dissertation,the common NCM811/Si C and LFP/graphite systems for power lithium ion batteries are taken as the research objects.The influence of pressure on lithium batteries,the underlying mechanism of the change of expansion force of lithium ion batteries are experimentally investigated.And the force model of lithium ion battery cell is studied through finite element simulations.The research contents include:(1)The battery charging and discharging test platform with pressure control/measure system was designed.It can exert pre-tightening force to 4500 N.The accuracy of the measurement of the expansion force of the battery is verified.(2)The effects of different pressures on the capacity and internal resistance of the two systems of lithium-ion batteries were studied.The most suitable pre-tightening force for the two systems and the variation of the internal resistance of the battery with the pre-tightening force were explored.(3)It is found that the expansion force of NCM811 system and LFP system change with different trend during charge and discharge.In addition,the XRD characterization analysis of the change of the particle size of the electrode material during charge and discharge is carried out to further reveal the underlying mechanisms for the change of the expansion force of the lithium battery.Finally,similar researches are performed on the battery pack with several lithium battery cells.(4)The 117 Ah hard-shell power battery model of NCM811 system was established by COMSOL to simulate the expansion force change of the battery during charging and the influence of battery expansion on the battery shell,so as to realize the expansion simulation of the single cell.The actual 117 Ah battery external pressure data was tested and compared with the simulation model.It was found that the simulation model can accurately simulate the change of battery expansion force and the distribution of pressure on battery shell.The experimental results show that applying an appropriate preload to the lithium ion battery can promote the capacity of the lithium ion battery,reduce the internal resistance of the battery,and improve the performance of the lithium ion battery.In addition,the pressure shows different effects on different lithium battery systems as well as different compaction density.Through the analysis of the particle size of the electrode material of the lithium ion battery in different SOC states,it is found that the change of the expansion force of the lithium battery during charging and discharging is the result of the coupling effect of the change of the positive and negative electrode materials.