Research On Cooling Performance Of Lithium-ion Batteries Based On Hexagonal Boron Nitride / Paraffin Composite Phase Change Materials

The electric vehicles?EVs?are the future development tendency of modern automobile,having the advantages of simple structure,easy maintenance,no noise and so on,compared with the traditional fuel vehicle.Power lithium battery,as the direct power source of pure electric vehicle,is one of the key technologies of new energy vehicle.Its performance directly affects the safety,driving range and span life of the whole vehicle.The battery thermal management system based on phase-change cooling method has been widely concerned by scholars domestic and overseas because of its simple structure,easy maintenance,and excellent temperature controlling performance.However,there are some regular issues of solid-liquid phase change thermal management system,such as leakage and poor thermal conductivity.To solve above problems,this paper firstly prepared paraffin composite phase change materials?C-PCMs?based on h-BN/HDPE/DM,and tested its thermophysical properties,leakage performance and heat conduction enhancement.An experimental platform for the cooling performance of lithium battery based on C-PCM was designed and built,then the C-PCM was applied for lithium-ion battery cooling research.Finally,In order to solve the problem that the single heat dissipation mode of the battery pack with phase change cooling is prone to heat dissipation failure in the cycle of charge and discharge,the heat dissipation structure of the lithium battery based on phase change/liquid cold coupling is constructed.The main contents and conclusions of this paper are exhibited as follows:?1?Heat conduction and electric insulation composite phase change materials were prepared using paraffin as phase change energy storage material,high-density polyethylene/diatomite as shape setting support material,and hexagonal boron nitride as heat conduction reinforcement material.The heat conduction composite phase change materials with 0%,3%,5%,10%,15%and 20%boron nitride mass fraction were prepared by physical mixing,ultrasonic dispersion and vacuum adsorption respectively.In order to further improve the thermal conductivity,the composite phase-change materials was studied under different morphology,particle size and mixing of different particle size.The results show that 76%liquid paraffin can be adsorbed,when the ratio of HDPE/calcined diatomite is 3:1.When 15_wt.%1?m slice of hexagonal boron nitride is added to the composite phase change material,the thermal conductivity reaches 0.8006 W/?m·K?at 40?.When adding the same mass fraction of the granulated hexagonal boron nitride particle size,and set its particle size of 30?m/40?m mixing ratio is 1:1,the thermal conductivity of the composite phase change material at 40?solid-liquid phase change reaches 2.498 W/?m·K?,which is 12.49times of the thermal conductivity of pure paraffin.This mixing ratio is used in the subsequent test of lithium battery cooling performance.?2?Two 10.8 V 9 Ah lithium battery packs were assembled,in order to verify the practical application of the composite phase change materials in the cooling performance of lithium-ion batteries.One was cooled by composite phase change materials,the other was cooled by natural cooling.The thermal performance of the batteries with different discharging rate?1C,2C,3C?at different ambient temperature?10?,20?,30??were studied.The results show that the phase change material cooling mode presents excellent temperature controlling effect compared with the natural cooling mode.The highest discharge temperature decreased 37.91?,45.5?and 46.6?respectively at 10?,20?and 30?ambient temperature.In addition,the composite phase-change material can control the temperature difference between lithium batteries well.The maximum temperature difference in the batteries is only 2.1?even at 10?,when batteries were discharged at 3C.The test of the battery pack charging and discharging cycle shows that the PCM in the battery box can not dissipate the heat absorbed to the outside in time.Therefore,in the absence of no additional cold source,the phase-change heat dissipation of battery pack is easy to fail in the extreme high temperature environment when it is charged and discharged at high rate.?3?In order to solve the problem that the single heat dissipation mode of the battery pack with phase change cooling is prone to heat dissipation failure in the cycle of charge and discharge,the heat dissipation structure of the lithium battery based on phase change/liquid cold coupling is constructed.The liquid cooling structure of a lithium battery with 3C discharge ratio is optimized at 311 K by numerical simulation.The results show that the maximum cell temperature in the lithium battery group decreases by 3.97 K under the phase change/liquid cold coupling compared with phase change cooling.The research on battery pack with different surface control strategy indicates that when the cooling liquid reciprocates every 300s in the liquid cooling channel,the heat dissipation mode of phase change/liquid cold coupling can effectively improve the temperature uniformity of each single cell under the same liquid cooling condition.The maximum temperature difference of the cell in the battery pack can be reduced to 6.91K,which is lower than the maximum temperature difference in the group which only using phase change heat dissipation.