Study On The Heat Generation Characteristics Of High-specific-energy Lithium Ion Batteries With NCA Cathode During Aging

As an ideal energy storage system,lithium-ion batteries(LIBs)play an important role in electronic products,electric vehicles and multi-energy complementary energy systems,and are also a key technology to achieve peaking carbon dioxide emissions and carbon neutrality.The performance,life and thermal stability of LIBs are greatly affected by temperature,and the temperature during its use and storage must be strictly controlled,that is,thermal management.As specific energy of LIBs continues to increase,the problem of thermal management has become increasingly prominent.The temperature of a LIB is determined by its internal heat generation rate and external heat transfer rate.Therefore,mastering the heat generation characteristics of LIBs is a prerequisite for efficient and accurate thermal management.The heat generation characteristics of LIBs are complex,which is not only affected by many factors such as temperature,charging and discharging current,state of charge(SOC),etc.,but also significantly changes with the aging of LIBs.In order to achieve better thermal management throughout the whole life cycle of LIBs,it is necessary to master the evolution of heat generation characteristics during their aging process and the influence mechanism of aging on heat generation.In order to achieve this goal,research has been carried out in this thesis of which the main work contents and conclusions are as follows:(1)Experimental study on the aging and heat generation characteristics during aging process for high-specific-energy cells with NCA cathode.Cycle aging test under different ambient temperatures and charge/discharge rates and heat generation rate measurement test were carried out.Based on experimental data,the cells'capacity loss,change of impedance characteristic,aging mechanism and the evolution of heat generation rate were analyzed.The result shows that cells age fastest when working at low temperature.Lowering the charge/discharge rate can significantly extend cells'life when working at normal temperature.When the charge/discharge rate is large,increasing the ambient temperature can effectively extend cells'life.The charge/discharge rate has little effect on the aging rate of cells at high temperature.The direct-current internal resistance and alternating-current impedance of cells during aging processes significantly increase,of which cells cycled at high temperature shows the fastest increase with loss of capacity.By using differential voltage analysis,it is found that the dominant aging modes are loss of lithium ion inventory(LLI),negative electrode active material loss(LAM)and positive electrode active material loss for cells cycled at low,normal and high ambient temperature,respectively.With aging,cells'heat generation and average heat generation rate during discharging process both increase significantly,and there are differences in the evolution of the heat generation characteristics of cells under different aging conditions.When cells age from before of life to end of life,the ratio of the average heat generation rate to the capacity loss rate(?_k~i)of cells cycled under different aging conditions is between 0.92 and 2.94.In addition,?_k~i at different aging stages of the cell cycled under the same aging condition are also different.(2)Simulation study on the mechanism of influences of aging on cells'heat generation characteristics.Considering LLI and LAM,a coupled electrochemical-aging-thermal model that simulates aging process and change of heat generation characteristics is established for cells.Simulation results show that the total heat generation rate of the aged cell under the same current increases significantly,the main reason of which is that the increase in the thickness of the solid-electrolyte interface(SEI)film and the decrease in anode porosity due to SEI film growth cause the increase of SEI film and anode electrolyte ohmic heat generation rate.In addition,when the level of LAM is greater than that of LLI,the polarization heat also increases.On this basis,the influence of SEI film physical parameters,cells'aging mode and initial anode porosity on changes in heat generation characteristics during aging process was studied.It is found that the decrease in ionic conductivity of SEI film will cause the growth rate of SEI film resistance and its heat generation rate to increase during aging process.The increase in molar volume of SEI film will lead to a decrease in cells'capacity loss rate and increases in the growth rate of SEI film and anode electrolyte ohmic heat generation rate at the same time.Compared with the case that LLI and LAM occur at the same time,the growth rate of SEI film and anode electrolyte ohmic heat generation rate increase more with cells'capacity loss when only LLI occurs,but the polarization generation rate decreases.When only LLI occurs,the influence of SEI film's ionic conductivity and molar volume on heat generation of cells is similar to that when LLI and LAM occur at the same time,but the degree of influence becomes larger.The decrease of the initial anode porosity will increase the growth rate of SEI film and anode electrolyte ohmic heat generation rate during cells'aging process.