Research Of Li-ion Battery Thermal Management Based On Composite Phase Change Heat Dissipation

With the rapid development of the global economy,countries all around the world are confronted with more and more serious energy crisis problem.As a new generation of non-fossil fuels transportation means,the electric vehicles gradually shows their advantages of energy conservation and environmental protection.However,the power battery thermal management technology has not been made an important breakthrough,which has become the bottleneck restricting their widely spreading application.Li-ion battery is one of the commonly used electric vehicles batteries,so making great effort of the research on the thermal management technology of li-ion power battery is of great significance to make breakthrough of the promotion and use of electric vehicles.The composite phase change materials?PCMs?containing radomly dispersed nano-fillers with high thermal conductivity were prepared and characterized and applied to the thermal management of li-ion batteries.In addition,the molecular dynamics?MDs?method were advocated to explore the mechanism of thermal transport enhancement at microscale.The main contents and conclusions are sumamarized as follows:1.The charging and discharging experiment platform of li-ion battery was set up,the experiment equipments,including charge and discharge cycle instrument,thermostatic water bath were made use of,and in view of different ambient temperature the li-ion battery discharge test experiments were developed,then the surface temperature characteristic curves of li-ion battery at different ambient temperatures were obtained,according to the process of temperature change curves,analysising the highest surface temperature and maximum temperature difference of the battery,the paraffin with melting temperature scope at 43?45?as composite phase change material for battery thermal experiments was selected at last.2.The composite phase change materials with mass contents of boron nitride?BN?of 0.5%?1%and 3%were prepared,the thermal properties of pure paraffin and composite phase change materials were characterized.The experimental results showed that the thermal conductivity?TC?of composite PCMs could increase up to 0.3386W/m?K when the content of fillers was 1%,and the TC enhancemant was about 37%.Then,through the heat dissipation comparison experiment between pure phase change material and composite phase change material at ambient temperature of 37°C,at the 5C discharge rate,the average value of the maximum surface temperature of the li-ion battery when using the composite PCMs was 3.39°C lower than that of the pure paraffin phase change,achieving a cooling effect of about 7.6%,which was 11.43°C lower than the natural air cooling method,achieving a cooling effect of about 21.6%.At the same time,at the 5C discharge rate,the average temperature difference of the surface of the li-ion battery was reduced by 3.18°C and 3.53°C,respectively,when the pure paraffin and the paraffin/BN were used as heat dissipation medium,the cooling effect of the maximum temperature difference was 65.8%and 73.1%compared with the natural cooling of the air respectively,indicating that paraffin/BN composite phase change heat dissipation technology could be more effectively used for li-ion battery thermal management.3.The thermal conductivity and interfacial thermal resistance of multi-layer BN nano-sheets encased in paraffin matrix were calculated through molecular dynamics simulation.It could be concluded that the in-plane thermal conductivity of multi-layer BN encased in paraffin matrix increased with the increase of thickness of BN nano-sheet,and finally converged to 270 W/m·K when the thickness of multi-layer BN layer was 7layers.The interfacial thermal resistance between multi-layer BN and paraffin matrix gradually decreased with the increase of the thickness of BN nano-sheet.When the thickness of BN nano-sheet layer was 20 layers,it converged to 3.56×10^-9m²?K/W.At last,the thermal conductivity of the composite phase change material was predicted by the effective medium theory model,and the relationship between the TC of the composite phase change materials and the dielectric filler content,lateral size and aspect ratio were obtained finally.