Study On Low-temperature Thermal Management System Of Li-ion Battery Using Phase Change Material

For a long time,li-ion batteries,which are the main power source of green cars,are paid close attention.However,at low temperature,li-ion batteries show poor performance,shortening driving range of green cars and limiting their development severely.Therefore,in order to increase driving range of green cars,low-temperature performance ofli-ion battery should be improved by choosing suitable thermal management material.Besides,composite phase change material(PCM),can store and release heat by changing phase without any additional energy consumption,which could be a good medium of thermal management.Hence,basing on the different low-temperature performance of single li-ion battery and li-ion battery pack and merits of composite PCM,our research put forward low-temperature thermal management of single li-ion battery and that of li-ion battery packs with different composite phase change materials.To performance of low-temperature thermal management system of single li-ion battery,RT28/ fumed silica composite PCMs with low thermal conductivity were used.Scanned by electron micrograph and Fourier transformation infrared(FTIR),the microstructure and components of composite PCMs were characterized which indicated that RT28/hydrophobic fumed silica composite PCMs were just a physical absorption.The thermal physical properties of composite PCMs were measured by Hot Disk thermal constants analyzer and differential scanning calorimeter(DSC).It was found that 70wt% RT28/ fumed silica composite PCM showed the shaped stability,low thermal conductivity,high enthapy of phase change,good insulation and temperature control effect.Charge-discharge tests with different discharge currents at different temperatures were carried out.Increase of temperature,discharge capacity and voltage of single li-ion battery with 70wt% RT28/fumed silica composite PCM were demonstrated,comparing performance of single li-ion battery without any PCMs.To performance of low-temperature thermal management system of li-ion battery pack,our research prepares two kinds of composite PCMs ——60%wt RT44HC/expanded graphite(EG)composite PCM and 60%wt RT44HC/ fumed silica composite PCM,with different thermal conductivity,for thermal management.RT44HC/EG composite PCM with high thermal conductivity,10.27 W/(m ? K),can reinforce thermal diffusion,while RT44HC/fumed silica composite PCM with low thermal conductivity,0.17 W/(m?K),can strengthen thermal insulation.Charge-discharge test by 0.5C,1C,1.5C,2C,at 5? and-10? respectively,and 20 cycles of daily operation test were carried out.Results indicated that with the low conductivity,RT44HC/fumed silica composite PCM was difficult to dissipate heat,leading to the high temperature difference(20.4 ?)and voltage difference(0.1 V)between cells and finally failure,which meant that strengthen insulation only cannot improve low-temperature performance of li-ion battery pack.Yet,with high conductivity,RT44HC/EG composite PCM can narrow temperature gaps as well as voltage gaps between cells,meanwhile decrease internal resistance,boost low-temperature performance of Li-ion battery pack.The generating heat of li-ion battery pack is more than that of single li-ion battery yet with uneven distribution.PCM not only can keep warm,but also can balance heat between cells for the consistency of discharge capacity which is beneficial to improve low-temperature performance of li-ion battery pack.Hence,RT44HC/EG composite PCM with good prospect is the appropriate material of thermal management.