Novel Phase Change Materials With Segregated-Structure And The Application In Battery Thermal Management

In recent years,humen are facing increasingly serious problems such as energy shortage,environmental pollution and so on,which makes the new energy electric vehicle（EVs）industry develop rapidly.And lithium-ion battery is the most suitable energy storage unit for EVs at present.However,if the heat generated cannot be dissipated in time during the long-term use process of lithium-ion battery,it will lead to attenuate capacity,accelerated aging and even the occurrence of thermal runaway of battery.Paraffin（PA）,as organic phase change material（PCMs）,can absorb/release a large amount of latent heat through the change of physical state of melting/solidification to reduce the surface temperature of the battery.However,PA has the disadvantages of poor thermal conductivity and easy leakage,so it cannot be directly used in battery thermal management（BTMS）.Adding thermal conductive filler and support material is a common method to improve the thermal conductivity and anti-leakage of PCMs,but it often requires a high filling amount.In order to prepare composite phase change materials（CPCMs）which can be effectively applied in BTMS and industrialized.In this study,PA@BN_X or PA-CNTs@BN_Xwere prepared by physical emulsification method,in which PA or multi-wall carbon nanotubes（CNTs）enhanced themal conductivity as the core material,modified hexagonal boron nitride（BN-OH）as the shell material,and bulk PA@BN_X and PA-CNTs@BN_X with segregated-structure were constructed by hot-pressing method to further enhance its themal conductivity.The surface morphology,chemical structure,thermal conductivity and thermophysical properties of CPCMs were characterized by scanning electron microscope（SEM）,Fourier transform infrared spectroscopy（FTIR）,thermal conductivity meter（Hot-disk）,differential scanning calorimeter（DSC）and thermogravimetric analyzer（TGA）.The prepared CPCMs were applied to BTMS,and the change of battery pack surface temperature were monitored in real time by thermocouple,which is compared with natural air cooling to study the application effects of CPCMs in BTMS.The main research results of this paper are as follows:1.The particle PA@BN_X were prepared by physical emulsification method of electrostatic combined,in which PA was used as the oil phase,hexadecyl trimethyl ammonium bromide（DTAB）as the emulsifier and BN-OH as the aqueous phase.Then,the bulk PA@BN_Xwith segregated-structure were prepared by hot-pressing.The effects of the contain of BN-OH and the construct of segregated-structure on the properties of PA@BN_X were investigated.The results show that:BN-OH can combined with cationic emulsifier DTAB under the action of electrostatic force,and coated on the surface of PA.There are continuous heat conduction paths in the bulk PA@BN_X,which is conducive to the transmission of phonons;CPCMs has good chemical stability;The thermal conductivity of PA@BN₁₀ with segregated-structure is higher than that of PA-BN₁₀with random dispersion of filler.With the increase of BN-OH addition,the thermal conductivity of PA@BN_X increases,but the enthalpy of phase transition gradually decreases;The shape stability of PA@BN_X can be maintained,and the leakage of CPCM_S is gradually reduced with the increase of filler content.2.A small amount of CNTs was introduced in PA@BN_X to trengthen the construction of micro-zone heat conduction paths,and the binary filler PA-CNTs@BN_Xwith segregated-structure was prepared.The effects of 1 wt%CNTs and BN-OH with different mass fractions as the compound fillers on the properties of PA-CNTs@BN_Xwere studied.The results found that:BN-OH can still coat on the surface of PA-CNTs,and the introduction of CNTs will reduce the particle size of particle PA@BN_X;There are no chemical reaction among materials,PA-CNTs@BN_X have good chemical stability;The introduction of 1 wt%CNTs can increase the thermal conductivity of PA@BN_X from 0.33 W/m·K to 0.43 W/m·K,it shows that the introduction of CNTs can build heat conduction paths in the micro-region,which helps to further accelerate the transmission of phonons;DSC shows that the thermal conductivity of PA-CNTs@BN₃₀ is 0.69 W/m·K,which is 2.76 times that of pure PA;The enthalpy of phase transition of PA-CNTs@BN₃₀ is 60.67 J/g;In addition,leakage and thermal stability tests show that PA-CNTs@BN_X can also maintain shape stability,and compared with PA@BN₃₀,PA-CNTs@BN₃₀ has better properties of anti-leakage and thermal stability.3.The best thermal conductivity and anti-leakage of PA@BN₃₀ and PA-CNTs@BN₃₀ separate among PA@BN_Xand PA-CNTs@BN_X were applied in BTMS.The results show that:compared with air natural cooling,the maximum temperature of the battery modules with PA-CNTs@BN₃₀ and PA@BN₃₀ during multiple high rate discharge and charging cycles decreased by 2 and 1.3°C,respectively.The maximum temperature during multiple low rate discharge and charging cycles decreased by 3.4and 4.3°C,respectively.Both PA@BN_X and PA-CNTs@BN_Xcan play a certain cooling effect in BTMS.