Study On Thermal Characteristics And System Optimization Design Of Power Battery Based On Polyethylene Glycol-based Flexible Phase Change Materials

The accumulation of carbon dioxide caused by the abuse of fossil fuels has induced a series of negative effects.Under the background,the theme of carbon neutrality promoted all domestic parties to respond positively to the transformation of commercial automobile industry to new energy vehicles with low-carbon even zero-carbon.The rapidly expanding new energy automobile industry drived the industrialization development of downstream lithium batteries and a series of thermal safety issues.In the present study,polyethylene glycol 1500(PEG1500)was used as phase change material(PCM),and expanded graphite(EG)and styrene-butadiene-styrene(SBS)were blended to improve the thermal conductivity and flexibility,respectively.The optimal proportion of 30% SBS was taken out and used to explore the temperature rise characteristics of lithium-ion batteries during cyclic charging and discharging,which provided guidelines for the structural optimization of thermal management system through systematic and simulation comparison.The main results are as follows:1.The microstructure,thermal properties and mechanical properties of the composite PCMs with different SBS ratios were tested.The results of SEM and FT-IR showed that the flexible composite PCM sythesized was uniform and stable without chemical reaction and functional group generation.PEG1500 was effectively adsorbed by EG with the layered porous structure and coated by SBS,which reduced the possibility of melting leakage of PEG1500.Anti-leakage and flexibility tests showed that the absorption and flexibility increased significantly with the increase of SBS addition ratio up to 30% without leakage after heat treatment at 50 ℃ and no fracture after bending at 180°,respectively.DSC tests showed that the latent heat of the composite PCMs decreased in proportion with the addition of SBS while the phase change temperature did not change obviously.Moreover,the latent heat of PES30 decreased by less than 6.3% after 100 times of melting because SBS prevented PCM from leakake.TGA tests showed that the composite PCMs began to decompose at 200 ℃,and the weight loss accelerated at 325～430 ℃,indicated the excellent thermal stability.2.The thermal conductivity evolutions of flexible composite PCM at different temperatures and phases were tested by a self-designed steady-state test rig.The addition of SBS decreased the thermal conductivities of the composite PCMs,and the decreasing value increased with the increase of the addition ratio while the decreasing trend slowed down.The thermal conductivities of the flexible PCMs with different SBS contents were obviously different due to different setting effects.The thermal conductivities of PES0 and PES10,which were most easily leaked,increased with the increase of temperature in solid state,but suddenly decreased after reaching the phase change temperature,and continued to decrease with the increase of temperature.With the temperature rising,the thermal conductivities of PES20,PES40 and PES50 increased from 0.725W/(m·k),0.541 W/(m·k),and 0.362 W/(m·k),to 1.290 W/(m·k),respectively.3.Based on PEG1500,PES0 and PES30,the cyclic charging and discharging tests of four series lithium batteries were carried out,and the optimal working times of the batteries under natural convection was 3815 s,8305 s and 5865 s respectively.Flexible PCMs improved the cyclic stability and flexibility,but at the same time led to the decrease of latent heat and thermal conductivity,which weakened the thermal storage capacity to some extent with a relatively excellent thermal management effect.In this way,the system temperature difference of PES30 was slightly larger than that of PES0,while the average and maximum temperature difference of single battery were obviously improved due to the small thermal contact resistance,especially at the initial stage of charge and discharge.Taking PES30 as an example,the maximum temperature difference of single cell in the initial stage of charge and discharge decreased by 1.23 ℃,0.17 ℃and 2.39 ℃ respectively compared to PEG1500 under the conditions of natural convection,coupled heat pipe and coupled air cooling.The best working time of the battery was prolonged after coupling heat pipe and air cooling,which was 13.2% and 39.1% for PEG1500 system,5.7%and 11.6% for PES0 system and 17.8% and 16.3% for PES30 system.The extension effect of air cooling was relatively better than that of heat pipe,but PES30 has better local heat dissipation and heat diffusion with heat pipe,and the effect of coupling heat pipe was better.Finally,the thermal conductivity of PES30 was strengthened and the experiment was repeated.It was found that the optimal working hours under natural convection,heat pipe and air cooling were extended to 8785 s,11685 s and 11555 s respectively,indicating that the significance of large thermal conductivity of the composite PCM in battery thermal management.4.The multi-heat source battery model based on internal resistance test and PCM thermal storage model based on enthalpy method were used in numerical simulation and compared with the experiment.The maximum deviation of the optimal working time was about 10% while the maximum temperature difference of the battery pack in the simulation was relatively small due to the simplification of the complex structure of the battery core and the heat generation deviation of the tab.By comparing the temperature nephograms,the temperature difference of PCM modules was not obvious in the vertical direction.The upper PCM was slightly higher due to the heat released from the battery poles.In the horizontal direction,the PCM in the battery interlayer firstly completely changes phase and loses its heat storage capacity,while the outer PCM content is sufficient,and the temperature diffusion decreases step by step and the PCM area above 40 ℃ is within 1.5 cm away from the battery.