High-Safety And High Temperature Resistant Lithium Ion Battery Separator Materials

In recent years,lithium ion batteries have been widely used in portable electronics,electric vehicles and clean energy,due to their advantages of high energy density,environmental friendliness and long cycle life,and so forth.However,with increasing usage of lithium ion batteries in a wide range of market applications,the safety issues of lithium ion batteries mainly originate from short circuits that are becoming increasingly serious,such as ignition of tesla electric cars and explosion of Samsung Note 7 phones.The main causes of batteries short circuits include:（1）high current charging and discharge of batteries causes overheating,resulting in thermal shrinkage of internal separators;（2）lithium dendrite growth penetrates the separators during battery cycling.Therefore,separators play a critical role in determining the safety and electrochemical performance of cells.However,as the most common used commercial separators,the polyolefin-based separators are hard to meet the safety requirements of lithium ion batteries due to its poor thermal stability.Furthermore,commercial polyolefin-based separators have poor wettability and low liquid absorption of liquid electrolytes,which are hard to improve the electrochemical performances of cells.Therefore,it is highly desirable to fabricate high-performance separators with reliable safety assurance.Herein,two new kinds of composite separators with high safety assurance and high temperature resistance are developed via simple methods.The main contents are as follows:（1）A new kind of poly（ionic liquid）/P（VDF-HFP）（PIL/P（VDF-HFP））composite separators with high safety assurance and high temperature resistance is fabricated via a simple and scalable phase inversion method.The ion-dipole interactions between the PIL and P（VDF-HFP）endow the PIL/P（VDF-HFP）separator with significantly enhanced mechanical performances and thermal stability.Flexible gel polymer electrolytes（GPEs）can be obtained after absorption of the liquid electrolytes into the PIL/P（VDF-HFP）separator.The PIL/P（VDF-HFP）GPE exhibits a much higher ionic conductivity(1.78×10^-3 S cm^-1)compared to the liquid electrolytes absorbed in the commercial Celgard 2325 separator(3.5×10^-4 S cm^-1).The Li/LiFePO₄ battery assembled with the GPE exhibits a high reversible capacity of 99.2 mAh g^-1 at 12 C and high discharge capacities of 138.4 and 125.4mAh g^-1 after 200 charge-discharge cycles at 1 C and 4 C,respectively.These performances are all superior to the Li/LiFePO₄ battery using the commercial polyolefin-based Celgard 2325 separator saturated with liquid electrolytes.More importantly,the GPE derived from the PIL/P（VDF-HFP）separator is fire-resistant,benefiting from the intrinsic nonflammable property of the PIL.Therefore,the as-developed PIL/P（VDF-HFP）separator shows great potential for the practical application in lithium ion batteries with superior electrochemical performances and high safety.（2）A novel kind of halloysite nanotube（HNT）coated polypropylene（PP）composite separators with high safety assurance and high temperature resistance is facilely prepared by blade-coating HNT onto two sides of commercial PP separators.The HNT coated PP composite separator has outstanding wettability of liquid electrolytes,resulting in a high ionic conductivity of 8.4×10^-4 S cm^-1 at 25oC.Moreover,the HNT coated PP composite separator exhibits good thermal stability,the shape and dimension of the composite separator can be well kept up to 150oC.The Li/LiFePO₄ battery assembled with the composite separator exhibits a high reversible capacity of 77.7 mAh g^-1 at 16 C and high discharge capacities of 96.4mAh g^-1 after 350 charge-discharge cycles at 4 C.Therefore,the HNT coated PP composite separator shows great potential for the practical application in high-performance and high-safety lithium ion batteries.