Preparation Of Phase Change Microcapsules And Its Application In Thermal Management System Of Lithium-ion Batteries

As energy shortage and environment pollution become increasingly severe, developing renewable energy storage system and new energy vehicles represented by electric cars is an important solution to solve these problems. Power battery is the key component of electric vehicle, its performance directly influences the use and safety of electric vehicle. Lithium-ion batteries have become the predominant batteries in HEV and EV due to its high energy density, high operating voltage and long cycle life. However, the vast heat produced during the operation of lithium ion batteries will rise the internal temperature of batteries, induce the uneven distribution of heat, shorten the cycle life of batteries and even cause thermal runaway. Therefore, it is essential to study the thermal management of power batteries. In the present paper, the thermal management of power batteries based on phase change microcapsule heat dissipation was investigated. the research contents are as follow:In this paper, we first synthesized a kind of phase change microcapsules with paraffin as core and MF resin as shell through in situ polymerization. The effects of type and amount of emulsifier, the dropping speed of prepolymer and the core/shell ratio on the performance of the prepared MPCMs were studied. The heat storage capacity, surface morphology, molecular structure, and the distribution of particle size of the prepared MPCMs were investigated by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and laser-diffraction diameter tester, respectively. The experimental results show that the microcapsule prepared by using SMAHNa as emulsifier has the finest morphology and uniform particle size distribution when the amount of the emulsifier, the stirring speed emulsifier, the feed rate of prepoly and the mass ratio of core material to shell material are lOg (equals the quality of the paraffin),1500rpm,0.5mL/min and4:1, respectively. Its enthalpy and the content of paraffin in phase change microcapsules can reach to120.30J/g and61.59%, respectively.Secondly, in order to improve the thermal conductivity of the MPCMs, the shell material MF resin was modified by doping CNTs. Properties and structure of the modified phase change microcapsule were characterized by DSC, TG, SEM and FTIR. The dispersion experiment proved that the dispersion of the modified CNTs was improved greatly as designed, and the influences of the amount of CNTs on the surface morphology and thermal property of microcapsules were discussed. The results show that compared to un-doped microcapsule, the CNTs-doped microcapsule exhibits smoother surface and better stability. Moreover, the package rate and thermal conductivity of microcapsules increase evidently with the increase in the addition of CNTs. The microcapsule has its best performance when the addition of CNTs is0.15g, its thermal conductivity, enthalpy and package rate can reach to0.356W/(m-K),141.3J/g and72.35%, respectively.Finally, the temperature distribution of single battery and battery pack discharging at different rates and under different environmental temperature were tested by lithium ion battery performance testing platform and temperature measurement system. The results show that the internal temperature in battery pack can be controlled below62℃by using natural cooling when the battery pack discharges at3C under room temperature or even lower temperature. However, when the battery pack discharges at3C under higher temperature, the internal temperature and the maximum temperature difference in battery pack by using natural cooling can reach to72℃and5.6℃, respectively. Under the same circumstance, the internal temperature and the maximum temperature difference in battery pack cooled by using phase change microcapsules is lowered to62.3℃and1.6℃, respectively. Furthermore, the temperature on the surface of single cells in battery pack can also be reduced by using phase change microcapsules as the cooling media, which makes the temperature of single cells more uniform.