A Study On The Low Temperature Performance Of Lithium-ion Battery And Internal Heating Effect

As one of the main components of an electric vehicle,the lithium-ion battery directly affects the performance of the vehicle.The life and performance of the lithium-ion battery are closely related to the working temperature,and the endurance mileage and acceleration performance of electric vehicle are seriously affected by low-temperature environment.Therefore,it is very necessary and urgent to conduct research on improving the temperature performance of automotive lithium-ion battery by battery heating.The commonly used heating methods include heating by circulating liquid,heating by gas,heating by heating film and so on,all of which are heating from outside,thus the heating efficiency is low and the heating is not uniform.In this thesis,in order to solve the problem of poor temperature performance of the lithium-ion battery,a lithium-ion battery cell is taken as the research object and an innovative heating method is proposed,which is an internal heating method by adopting a built-in heating device.Specifically,the research content includes the following aspects.Firstly,low temperature charge and discharge characteristics of lithium-ion battery with graphite as cathode were studied.The experiment results show that under low temperature,the discharge capacity of lithium-ion battery is reduced,the voltage of platform is decreased and the high-rate discharge performance is deteriorated,but low temperature discharge will not cause permanent damage to the battery.Meanwhile,the low temperature charging voltage is high,the proportion of constant current charging stage is smaller,the charging is slow,the charging capacity is greatly reduced,and the low temperature charging is easy to produce lithium precipitation which will cause irretrievable serious damage to the capacity and impedance of lithium-ion battery.Secondly,the heating rate of lithium-ion battery cell was measured by experiment and natural convection heat transfer coefficient was calculated in this thesis.Under 0°C environment,discharge experiments at 1C,2C and 3C rate of lithium-ion battery were carried out,and the voltage temperature coefficients at different discharge levels were calculated.According to the Bernardi equation,the heating power at 2C rate was calculated.The results show that under 0°C environment the calorific value of battery accounts for 16.7% of electric power,which is 19% higher than the calorific value proportion under 25°C environment.According to the dynamic balance equation,the convective heat transfer coefficient was calculated,which provides the necessary parameters for the simulation of heating under low temperature environment.Finally,lithium-ion battery with built-in heating device was produced.NCM lithium-ion battery with capacity of 1800 m Ah was manufactured by formula design of battery and processes of coalescing,coating,rolling,slicing,winding,pouring and sealing and so on.A heating rod with a diameter of 4 millimeters is placed at the hollow core of the battery,and the thermocouple is placed in the heating rod,so that the heating temperature can be monitored in real time.Heating experiment under low temperature was conducted to manufactured lithium-ion battery with built-in heating device.The experiment results show that the heating power of battery is 4.9W under-40oC environment,and the temperature of battery can be rapidly increased to 0oC within 1000 s.The heating power of battery is 3.6W under-30oC environment.The heating power of battery is 2.6W under-20oC environment,and the temperature of battery can reach to target value soon,the heating effect is obvious.The average discharge voltage was increased by 0.2V after heating experiment under-20oC environment and the capacity of battery was almost not damaged by multiple heating experiments.Thermal simulation with the Fluent software is also conducted.The maximum temperature difference between the simulation results of-40oC and the experiment is less than 5oC,and the maximum temperature difference between the simulation and the experiment under-20oCenvironment is within 2oC.The temperature rise curve of simulation is in accordance with the experiment.It is proved that the proposed heating method is a safe,efficient and fast heating method.