Study On Preparation And Properties Of High Temperature Resistant PVDF-HFP Based Composite Separator For Lithium-ion Batteries

In addition to the application in daily life,lithium-ion batteries are more and more used in electric vehicles（EV）,but recently the combustion and explosion of new energy vehicles have been reported frequently,which puts forward higher requirements for the safety and high-rate performance of LIBs.Although PP or PE separators for commercial applications have high mechanical properties,their heat resistance,wettability with electrolyte and ionic conductivity are poor,which can not meet the safety and high current charging and discharging requirements of electric vehicles.The main causes of battery safety problems include:（1）thermal shrinkage of internal separators caused by continuous discharge or overcharge;（2）overcharge results in lithium dendrite growth penetrates the separator;（3）external or internal damage caused by external collision.These can cause internal short circuit of batteries,which may cause safety accidents in serious cases.Therefore,the key to solve this problem is to research the composite separator with good thermal stability and high charge-discharge rate.Based on this,the PVDF-HFP/TiO₂-NF and PVDF-HFP/Cellulose/TiO₂ porous separators were prepared by phase inversion.The main research contents and contributions of this paper are as follows:1.PVDF-HFP/TiO₂-NF porous composite separator was prepared by phase inversion method.In this composite separator,TiO₂ nanofibers were used as pore-forming agents and heat-resistant materials,DBP was regard as plasticizer.The addition of TiO₂ fibers could reduce the crystallinity of polymer PVDF-HFP and increase the porosity of the separator,and the performance of composite separator would be reduced,when the amount of TiO2 doping is overabundance.The PVDF-HFP/TiO2-NF composite separator doped with 5 wt%TiO₂ fiber has the best properties.The porosity of the composite separator was over 60%,the electrolyte absorption rate was up to277.9%,and the conductivity could also reach 1.26 mS cm^-1.At room temperature,the battery equipped with PVDF-HFP/TiO₂-NF composite separator has excellent discharge capacity(140 mAh g^-1)and high rate performance,and the battery equipped with PVDF-HFP/TiO₂-NF composite separator can also work normally at 110℃.2.PVDF-HFP/Cellulose/TiO₂ composite separator was prepared by phase inversion method.The cellulose of high temperature resistant were added to the composite separator as the backbone to support the PVDF-HFP substrate,and 5 wt%TiO₂ fibers were added to the PVDF-HFP/cellulose composite substrate.The addition of cellulose particles and TiO₂ fibers could reduce the crystallinity of PVDF-HFP and increase the high temperature resistance of the composite separator.The porosity and electrolyte uptake of PVDF-HFP/Cellulose/TiO₂ composite membrane reached 63.65% and 210.3%,respectively.The ionic conductivity of PVDF-HFP/Cellulose/TiO₂composite separator ran up to 1.68 mS cm^-1.The battery assembled with PVDF-HFP/Cellulose/TiO₂ composite separator has high specific capacity(160 mAh g^-1)and rate performance.The battery assembled with PVDF-HFP/Cellulose/TiO₂ composite separator can also charge or discharge normally at 120℃.