Development Of Hydrated Salt/Aluminum Nitride/Carbon Fiber Composite Phase Change Materials And Cooling Application Of Power Battery

With the aggravation of global environmental pollution and energy crisis,countries all over the world attach great importance to the development of new energy automobile industry with environmental protection and low energy consumption advantages.Among many kinds of new energy vehicles,the development of pure electric vehicles is the most mature.With the advantages of easy maintenance,no noise and environmental friendly,pure electric vehicles have been recognized by consumers.As the direct power source of pure electric vehicles,the performance of lithium battery directly affects the quality and pricing of an electric vehicle.Lithium battery will generate a large amount of heat during the operation of electric vehicles,leading to a sharp rise in temperature.In severe cases,it will even occur spontaneous combustion and explosion,which is very important for its effective thermal management.The battery thermal management system based on phase change cooling method has become a research hotspot of scholars in China and abroad due to its simple structure,easy maintenance and no external energy consumption.Inorganic hydrate phase change materials have gradually replaced conventional phase change materials in the field of heat storage due to their superior thermal storage and thermal conductivity.However,the inherent supercooling and phase separation of hydrate materials have limited their application in the field of battery thermal management.To solve this problem,this paper develops a new type of composite phase change material of disodium hydrogen phosphate dodecahydrate / modified aluminum nitride / carbon fiber combined with melt blending,ultrasonic dispersion and vacuum adsorption.At the same time,the physical and thermal properties are tested and studied.On this basis,a new battery module cooling device based on phase change material / liquid cooling coupling is designed and a battery test bench is built to test and analyze its cooling performance.Finally,the cooling performance of the battery pack under different liquid cooling control strategies is studied for the cooling device.The research contents and main conclusions of this paper are as follows :(1)In view of the inherent defects such as supercooling and phase separation of inorganic hydrated salt material disodium hydrogen phosphate dodecahydrate,the optimal nucleating agent sodium silicate hydrate and thickener carboxymethyl cellulose are selected through investigation and test to modify it.It is determined that 4 wt%sodium silicate hydrate and 4 wt% carboxymethyl cellulose are the optimal additions,which can reduce the supercooling of phase change material to 3.6 ℃ at the cooling temperature of 10 ℃ and there is no phase separation.The results of 100 melting /solidification tests show that the composite has excellent cyclic stability.(2)To further optimize the performance of phase change materials,reduce the supercooling,and improve their thermal conductivity and curing performance,surface modified aluminum nitride and 600 mesh short-cut carbon fiber are added in this paper.The supercooling,shaping effect,thermal performance,thermal conductivity and resistivity of the composite phase change materials are tested,and the preparation scheme of inorganic composite phase change materials and the optimal addition ratio of various materials are determined.The results show that the addition of 12 wt%modified aluminum nitride and 6 wt% carbon fiber further reduces the supercooling to1.9 ℃ and increases the thermal conductivity to 1.86 W /(m·K).At the same time,the composite phase change material has the appropriate phase change temperature of35 ℃,high latent heat of 249 J / g,excellent shaping effect and electrical safety performance.(3)To verify the actual cooling performance of the composite materials used in lithium batteries,four 18650 lithium battery module boxes are constructed,including natural cooling,disodium hydrogen phosphate dodecahydrate cooling,modified composite phase change material cooling and phase change / liquid cooling coupling.The 3C discharge cooling performance of these battery modules is tested at ambient temperature of 30 ℃.The results show that the temperature control effect of the composite phase change material after comprehensive modification is better than that of pure disodium hydrogen phosphate dodecahydrate.The peak value of the average temperature at the end of discharge of the former is reduced by 5.2 ℃ compared with that of the latter,and the maximum temperature difference of the battery group is also reduced by 3.02 ℃.The cooling method based on modified composite phase change material coupled with new ’double helix’ type liquid cooling plate developed in this paper has the most prominent cooling effect on the battery pack.Under the low coolant flow rate of 0.5 L / min,the average temperature of the battery module increases only9.6 ℃,and the maximum temperature difference is controlled at 1.88 ℃.(4)The different operating conditions and control strategies of the coupled cooling system are tested and compared to further improve the cooling performance of the system.Finally,the control strategy of the coolant with low rotation speed and high flow rate is selected.The cooling system of this flow mode makes the average temperature rise of the battery pack only 6.7 ℃ at the 3C discharge rate of 30 ℃,and the maximum temperature difference is 1.41 ℃.The battery module can still maintain good cooling performance and temperature uniformity in multiple charge-discharge cycles.