| Under the background of energy transformation,environmental pollution,energy conservation and emission reduction,new energy vehicles have developed rapidly.For power lithium batteries used in new energy vehicles,as their specific energy continues to increase,safety issues are becoming more and more prominent.During the rapid charging and discharging process of lithium batteries,a large amount of heat will be released.If the heat cannot be dissipated in time,it will not only reduce the performance of the lithium battery,but even cause thermal runaway,leading to fire or explosion.In addition,lithium batteries will be damaged by external loads such as vibration,extrusion,and collision during use,causing their deformation,internal short circuit,and sharp rise in temperature,which may lead to thermal runaway accidents.Based on the above problems,this paper conducts research on the thermal safety issues of power lithium batteries,using the phase change heat absorption of composite phase change material(CPCM)to conduct thermal management of lithium batteries,and using flexible composite phase change material(FCPCM)with both phase change and elasticity to conduct thermal management of lithium batteries.Conduct thermal management and mechanical protection,and study their performance for mutual verification through a combination of experiments and numerical simulations.The main research contents and innovations are as follows:(1)A high thermal conductivity ternary CPCM with two different phase change temperatures was prepared.Paraffin(PW)is a high-temperature phase change material,myristanoic acid(MA)and palmitic acid(PA)are low-temperature phase change materials,expanded graphite(EG)is used as an adsorption support material,carbon black(CB)of different particle sizes and short carbon fiber(SCF)cooperates with heat conduction and serves as a composite thermal conductive filler to improve the thermal conductivity of the phase change material(PCM).It is coated with two-component silicone rubber(SR)to further reduce the leakage of the PCM.Through material characterization and charge and discharge experiments,it was found that composite thermally conductive fillers can effectively improve the thermal conductivity of PCM.The thermal conductivity of the CPCM is up to 8.7 W/m-K.Coating with SR film can effectively prevent the leakage of PCM.It also has good thermal stability and can keep the shape of PCM stable at high temperatures.In the charging and discharging process of the single battery,the CPCM without and coated with SR underwent two phase changes,and had good temperature control performance.The highest temperature at the end of discharge was 46.69℃ and 47.03℃,and the temperature difference was 0.89℃ and 1.02℃,respectively.(2)A FCPCM was prepared.PW is a PCM,EG is an adsorbent material,flake copper powder(FCP)is a thermally conductive filler,thermoplastic elastomer(TPE)is a flexible matrix,and when the amount of FCP is 5 wt.%,the thermal conductivity of PW/EG/FCP/TPE FCPCM can be increased to 3.357 W/m·K and has a lower leakage rate.After adding TPE,the CPCM has an obvious compression yield stage and has a certain degree of flexibility.When it is wrapped on the surface of the battery,the battery won’t experience an internal short circuit and the extrusion stroke reaches 13 mm.The temperature control performance of FCPCM during three cycles of charge and discharge of 18650 single lithium battery was analyzed through numerical simulation.The ambient temperature was 20℃,the three-cycle charging and discharging process of 3C,4C and 5C,and the three-cycle charging and discharging process of 3C,4C and the first two discharging process of 5C was at 30℃ of ambient temperature,3C,4C three cycles of charge and discharge was at 40℃ of ambient temperature,FCPCM can control the temperature of the battery surface below 50℃.(3)Apply the developed ternary CPCM to a square battery pack to study its temperature control and uniformity performance through numerical simulation and experiments.Numerical simulation results show that in an environment of 30℃,when the battery pack is discharged at 1C and 1.5C,the heat dissipation requirements can be achieved by using the ternary CPCM alone.When discharging at 2C,liquid cooling needs to be added to assist heat dissipation,through numerical simulation analysis,the liquid cooling start time,liquid cooling flow rate,and the number of liquid cooling tubes were determined and experimental verification was carried out.The highest temperature and maximum temperature difference of the battery pack obtained by numerical simulation are 48.53℃ and 2.7℃ respectively.The highest temperature and maximum temperature difference obtained by the experiment are respectively 48.75℃ and 2.72℃,the numerical simulation is in good agreement with the experimental data.(4)The developed FCPCM was applied to the 18650 lithium battery pack to verify its heat dissipation performance.First,through numerical simulation analysis,it was concluded that the single discharge and the three-cycle charge and discharge of the battery pack required adding liquid cooling working conditions,and the distribution type of the liquid cooling pipe and the liquid cooling flow rate were selected.Finally,the experiment of three-cycle charge and discharge at the ambient temperature of 40℃and 5C is conducted.Comparison between experiments and numerical simulations,the relative error of the maximum temperature is 0.42℃,and the relative error of the maximum temperature difference is 0.02℃.The heat dissipation strategy formulated through numerical simulation can basically reflect the coupled heat dissipation effect of FCPCM and liquid cooling.(5)FCPCM resist drop impact.CPCM with TPE added can withstand a certain degree of bending deformation at room temperature of 20±2±,while CPCM without TPE are highly brittle and fracture directly.Through the numerical simulation of the battery pack 1.5 m and 2 m drop process,at the end of the side drop,the stress on the battery pack wrapped with FCPCM was 34.35%and 41.75%lower than the stress on the battery pack at the end of the bare battery pack drop respectively.At the end of the positive and negative electrode drop,the stress on the battery cells wrapped with FCPCM was significantly reduced compared with the bare battery pack,which was reduced by 70.5%and 74.7%respectively.The 2 m drop test of the battery pack showed that after the drop,the battery did not fall off from the FCPCM,and there was no internal short circuit or electrolyte leakage,and the FCPCM did not appear to be dented,broken or fracture.The prepared FCPCM can effectively resist the impact caused by the battery pack being dropped.In summary,the two kinds of CPCM prepared in this article are excellent heat dissipation materials in battery thermal management systems.The coupled heat dissipation strategy developed through numerical simulation can meet the heat dissipation requirements of the battery pack under different working conditions and has been verified through experiments.The FCPCM was also verified through a combination of experiments and numerical simulations to have good mechanical protection for lithium batteries.This is important since it will enhance the thermal safety of new energy vehicles. |
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