Investigation On Low Temperature Preparation,Embedding Procedure And Temperature Control Performance Of High-stable Phase Change Polymer

Facing the challenges of deteriorating environment and energy shortage,new energy vehicles have gradually attracted people’s attention.Some countries have begun providing policies to encourage the use of new energy vehicles and release plans to forbid the sale of fuel vehicles.Electric vehicles（EVs）are regarded as an effective way to solve the above problems.Therefore,power battery technology has developed rapidly,and pure electric vehicles will become the mainstream.However,in recent years,as the key component of EVs,the smoke and spontaneous combustion of power battery modules,and other safety accidents occur frequently.This is mainly because the heat generated by the batteries can not be dissipated in time during the processes of fast charge-discharge or long-term operation of the battery module.The excessive temperature or temperature difference of the module will result in a series of side reactions inside the battery,reducing the service life and performance of the battery module,and even causes safety accident of thermal runaway.Therefore,in order to strictly control the temperature rise and temperature difference of battery module,maximize the service life of battery and ensure the safety of EVs,it is necessary to design a reasonable battery thermal management system.Phase change material（PCM）cooling technology,is regarded as generation cooling technology of battery thermal management system by researchers because of its passive cooling property and simple installation.However,PCM usually possesses the problems of poor thermal stability and easy leakage during the phase change process.In this context,most researchers adopt the physical mixing strategies,that is,adding various shape-stabilized or adsorption components to limit the flow of phase change components in liquid state.However,after the long-term operation of the material or under extreme working conditions,PCM will still migrate gradually inside the materials,and thus the leakage phenomenon still cannot be solved.Therefore,in order to fundamentally solve this problem and promote the practical application of PCM cooling technology,our group has developed a solid-solid phase change polymer（SSPCP）,which possesses the advantages of low-cost and abundant raw materials,simple preparation process,well thermal stability and anti-leakage performance.However,during the preparation process of SSPCP,the initiator used requires a high initiation temperature to realize rapid polymerization.Thus,the directly contact between the slurry precursor and the batteries should be avoided,and the directly embedding procedure of the battery module can not be realized.It is first necessary to prepare the module with predetermined specifications,followed by secondary processing,and then assemble the batteries modules.In addition to the tedious steps,this strategy will undoubtedly increase the thermal contact resistance between the battery and the PCM module.To address these problems,based on our original free radical polymerization,the SSPCP is successfully prepared under a near-room temperature of 40℃ by introducing a redox initiation system.Besides maintaining the excellent thermal stability and anti-leakage characteristics as the SSPCP prepared by original strategy,this strategy offers us the possibility to directly embed the precursor slurry into the battery module to realize one-step molding of the module.The main research contents and conclusions of this thesis are as follows.（1）The redox initiator system is introduced to reduce the chemical energy required for the original reaction,that is,the two initiators conduct redox reaction to generate a large number of free radical,and thereby realize the polymerization of SSPCP at a near-room temperature of 40℃.By investigating the ratio of peroxide initiator and two kinds of reducing initiator（Benzoyl peroxide BPO:N,N-Dimethyl-p-toluidine DPMT）,the optimal ratio of redox initiator system（2.3%:0.3%）and initiation temperature（40℃）are determined.The prepared SSPCP possesses excellent anti-leakage performance and thermal stability.Even if it is heated at high temperature for a long time,no leakage and deformation phenomenon is observed,which is due to the three-dimensional cross-linked main chain structure and the phase change side chain“bonded”on the main chain through chemical bonds.In addition,the aliphatic hydrocarbon side chain provides SSPCP with a high latent heat.For example,the latent heat and thermal conductivity of SSPCP composite with 6%expanded graphite（EG）reach 99.6 J g^-1 and 2.33 W m^-1 K^-1 respectively,and its phase change temperature is 47.8℃.These properties show that the SSPCP is suitable for battery thermal management.（2）An over-high embedding temperature of the battery module will significantly reduce the capacity of the batteries and increase the internal resistance,thus reducing the service life of the batteries.Therefore,we successfully realize the one-step embedding process of battery module at near-room temperature by introducing the aforementioned redox initiation system.The results of the temperature control performance show that,the SSPCP module demonstrates strong temperature control capability.For example,under the charge-discharge rates of 1C—1.5C,the maximum temperature(T_max)and maximum temperature difference(ΔT_max)of the SSPCP module is 50.7℃ and 2.2℃,respectively,which is 6.5℃ and 1℃ lower than that of the air cooling（NAC）module,respectively.During the 1C—1C charge-discharge for 15 cycles,the SSPCP module shows lower T_max andΔT_max of 45.2℃ and 1.52℃ in all cycles,respectively,2.2℃ and 0.1℃ lower than that of the solid-liquid phase change material（SLPCM）module,respectively.Furthermore,the surface of the SSPCP module after the charge-discharge cycles is smooth and clean,and no obvious reduction on the latent heat of the sample scraped from its surface is observed before and after the cycles,which confirms the excellent anti-leakage performance of the SSPCP.（3）In order to further enhance the latent heat of SSPCP,we subsequently prepare a new SSPCP/PA/EG composite by in-situ constructing an SSPCP skeleton in the mixed slurry of EG and paraffin（PA）.The experimental results show that the maximum amount of PA can reach 50 wt%,and its latent heat value increases with the increase of PA,which is up to 153.1J g^-1.By evaluating the anti-leakage performance and thermal stability of SSPCP/PA composites with different PA contents,the samples with a PA content of 30 wt%are finally selected for battery thermal management.The SSPCP/PA/EG composite’s latent heat and thermal conductivity are 124.6 J g^-1 and 2.36 W m^-1 K^-1 respectively,and it also possesses excellent stability and anti-leakage performance.In battery management applications,the SSPCP/PA/EG battery module shows excellent temperature control and temperature uniform ability.For example,under the charge-discharge rates of 1 C—1 C,1 C—1.5 C,1 C—2 C and1 C—3 C,the T_max of the SSPCP/PA module reaches 41.7,45.8,49.2 and 54.8℃ respectively,which is 2.2,3.1,2.7 and 3.6℃ lower than that of the SLPCM2 module.Moreover,the temperature uniform performance of the SSPCP/PA module is also better than that of the SLCPM2 module.On the other hand,during the cyclic charge-discharge test,the T_max andΔT_max was stable at 52℃ and 4.2℃ in the 15 charge-discharge cycles,respectively,and leakage phenomenon is observed.By stark contrast,after 15 charge-discharge,the SLPCM2 module demonstrates obvious PA precipitation and cooling performance degradation.