Investigation On Low-thermal-conductive And Flame Retardant Phase Change Composite Material For The Application In Battery Thermal Protection

Energy storage is of great strategic significance due to the large-scale consumption of non-renewable energy and environmental pollution.As the most popular energy storage unit,lithium-ion batteries（LIBs）are widely used in the fields of new energy vehicles and electrochemical energy storage with the advantages of environmental protection,high energy and power density,long cycle life,no memory effect and so on.Although manufacturing technology and production quality has had great progress,there are still many frequent hot security incidents occurred in the application of lithium-ion battery products,including not limited to,fire,the release of toxic gas fire,even explosion,etc.,which is not only the hinder of the wide application of battery,and significant damage to train personnel security,social security.In order to further improve the thermal safety of batteries,a robust thermal safety system is necessary.In addition to its own manufacturing defects,lithium battery thermal safety problems are usually caused by various abusive working conditions.Once the thermal runaway of the battery is triggered,it is bound to increase the impact of the disaster if the thermal runaway cannot be prevented from spreading to the adjacent battery in a short time.In this paper,a low thermal conductivity flame retardant phase change material（LTC-PCM）was developed to mitigate thermal runaway（TR）propagation of batteries in extreme conditions.Furthermore,the thermal management performance with LTC-PCM in conventional conditions was explored.A cold plate structure was designed to homogenise the cell temperature within the module,and the thermal safety performance of material for battery thermal management is further explored through simulation.First,considering the requirement of low thermal conductivity of materials,a low thermal conductivity phase change material of 0.051 W/（m·K）was obtained by using composite silica aerogel（SA）,and paraffin（PA）,an organic phase change material readily available from raw materials.By comparing different ways of adding flame retardants,including direct mixing and surface coating,it is found that the surface coating method can not only achieve V-0 flame retardant effect with less flame retardants,but also improve the mechanical properties of materials.By means of thermal conductivity and enthalpy value,mechanical strength,vertical burning test,the paraffin and silica aerogel proportion of 40%-60%was selected,and flame retardants（APP:DEPR=1:1）was coated on the surface.Low thermal conductivity flame retardant phase change materials with good mechanical properties,flame retardant high performance was attained.A study of low thermal conductivity flame retardant phase change materials（LTC-PCM）for mitigating battery thermal runaway by building a battery thermal runaway propagation test.The battery was heated to induce the thermal runaway（TR）,compared with the battery module without LTC-PCM.The experimental results show that the material can effectively restrain the thermal runaway propagation on the first battery.DSC test showed that 3 mm LTC-PCM which successfully mitigate thermal runaway propagation,still had an enthalpy value of 53.92 J/g,indicating that the material still had a strong heat absorption/insulation performance after the thermal runaway experiment.The direction of battery expansion after thermal runaway was predicted by analyzing the expansion deformation of the battery,which can be used to guide the analysis and judgement of the first thermal runaway battery within a failed battery module in the market.The thermal safety design of the battery not only needs to make countermeasures of thermal runaway,but also needs to consider the normal working condition of the battery.Low thermal conductivity flame retardant phase change material（LTC-PCM）is more advantageous than HTC-S in thermal runaway,and more advantageous than LTC-F in normal working conditions.In order to comprehensively consider the performance of LTC-PCM in the thermal safety design of batteries,the performance of LTC-PCM,HTC-S and LTC-F in the normal operating conditions of the battery were compared through experiments.The results show that the temperature rise of the battery module containing LTC-PCM is 3.64℃lower than that of the battery module containing LTC-F at 2C discharge rate,and the temperature uniformity of the battery module is basically the same.LTC-PCM has better temperature uniformity(σ_Tave=2.23℃)than HTC-S(σ_Tave=4.13℃),although it has a higher temperature rise than HTC-S.Finally,a cold plate to homogenise the module temperature was designed and the effects of LTC-PCM,HTC-S and LTC-F on battery thermal management performance by comparing simulations were analysed.Futhermore,operating conditions without and with heat transfer fluid（HTF）flow were considered.Without HTF flow in cooling plate,battery module with LTC-PCM shows the lowest average temperature(T_ave)at the end of 1 C discharge process.The maximum difference of the uniformity forΔT_ave,σ_Taveandσ_ΔTave between module with LTC-PCM and the other two materials are 0.0611°C,0.00725°C,and 0.00595°C,respectively,indicating that the uniformity of batteries in BTMS is not significantly affected when applying LTC-PCM between batteries.Under low HTF flow rate of 1 L/min,T_ave of batteries in module with 3 mm LTC-PCM has already decreased to 40.847°C,revealing excellent cooling and average temperature uniformity of BTMS coupling with LTC-PCM and liquid cooling.Moreover,increasing the width of LTC-PCM（within 6 mm）when flowing HTF is applied,is found to have a little effect on BTMS performance.When the thickness of the LTC-PCM is increased from 3 mm to 6 mm,T_ave of the battery module decreases by only 0.174°C andσ_ΔTaveincreases slightly by 0.10℃.In this paper,LTC-PCM is found not only to mitigate the battery thermal runaway propagation in extreme conditions but also a superior thermal management performance than LTC-F in normal working conditions.It indicates that LTC-PCM has a good prospect in the field of battery thermal protection which can cooperate with the heat insulation requirements of thermal runaway and the dissipation requirements of thermal management protection.