Preparation And Performance Of Lithiumion Battery Separator With Heat-resistant Flame Retardant Function

In recent years,frequent safety accidents occurred during the use of lithium-ion batteries,making people question the safety of this clean energy.Lithium batteries have safety issues due to the separator shrinking under high temperatures,which results in battery disconnection and fire.One of the most efficient approaches to deal with the issue of flammability and explosion in lithium batteries is to design a new kind of separator with dual functions of heat resistance and flame retardance.In this study,we build multiple microstructures of"bone-meat-armor"to create composite fiber separators with triple coaxial structure,to have good electrophilicity and heat resistance.By doing so,we hope to take advantage of the benefits of high-temperature-resistant polymers,polymers with good electrolyte affinity and heat-resistant,and heat-insulating inorganic materials.First,the polyetherimide/polyvinylidene fluoride（PEI/PVDF）fiber membrane was prepared by the coaxial electrospinning method.PEI was used as the fiber core layer skeleton because of its good high-temperature resistance.Because of its high electrolyte affinity,PVDF was used as a skin layer to ensure the electrochemical performance of the battery.Then by the method of st?ber,SiO₂ nano-microspheres are grown in situ on the surface of the coaxial fiber structure,and the microspheres are aggregated and coated on the surface of the fiber to form a layer of inorganic armor,and a PEI/PVDF/SiO₂ triple coaxial composite fiber membrane is obtained.By controlling the concentration of TEOS in the SiO₂ precursor solution,the coating effect of SiO₂ on the fiber surface is changed.When the concentration of TEOS increases from 0.4mol/L,0.8mol/L to 1.2mol/L,the density of the armor layer increases gradually.When the concentration of TEOS is 1.2mol/L,SiO₂ nano-microspheres are completely covered on the fiber surface,forming a dense inorganic armor shell.The heat-resistant and flame-retardant performance,electrolyte affinity,mechanical properties,and cycle rate performance of the battery were investigated for the prepared separator.The results show that the triple coaxial structure composite fiber separator（PPS3）with a TEOS concentration of 1.2mol/L has the best overall performance,the shrinkage rate is only 8.6%at 200°C,and the separator has no obvious change after burning with an igniter for 2 seconds,indicating that it has an excellent heat resistance and flame retardancy.The breaking strength is as high as 9 MPa;the initial discharge capacity is 160.2 m Ah/g,and the capacity retention rate after 100 charge-discharge cycles at 0.5C is 96.0%,and at a discharge rate of 4C,the capacity retention rate is 74.8%.Considering that the flame retardancy of the membrane can be further improved by adding flame retardants,an attempt was made to grow Al（OH）₃ in situ on the surface of the flame retardant particle polyammonium phosphate to obtain APP-Al（OH）₃ flame retardant particles,which were coated on the surface of PEI/PVDF/SiO₂ triple coaxial composite fiber membrane by dip coating method to obtain PEI/PVDF/SiO₂/APP-Al（OH）₃ composite membrane（PPSAA for short）.It was found that the APP-Al（OH）₃flame retardant particles and SiO₂ nano-microspheres played a synergistic flame retardant effect so that the separator showed excellent flame retardant performance and could not shrink,crack and burn after 60s continuous burning by an igniter.In this paper,PPSAA composite fiber membranes with 5wt%,10wt%,15wt%,and 20wt%dip coating concentrations were prepared by changing the additional number of flame-retardant particles,among which PPSAA2（10wt%）had the best comprehensive performance,with shrinkage of only 2.5%at 200°C,bending resistance and fracture strength of 7.9MPa,initial discharge capacity of 158.8 m Ah/g and the capacity retention rate is 94.5%after100 charge-discharge cycles at 0.5C,and 71.3%at a discharge rate of 4C.In summary,the PEI/PVDF/SiO₂ three-layer core-shell structure fiber membrane and PEI/PVDF/SiO₂/APP–Al（OH）₃ composite fiber membrane prepared in this study provide a fresh approach to enhancing the safety of lithium batteries possibly.