Fabrication And Application Of The Poly-m-phenyleneisophthalamide Modified Separators For Lithium-ion Batteries

Owing to high energy density,long service life and other advantages,lithium-ion batteries have been used as one kind of mature energy device.Nowadays,lithium-ion batteries not only have already been applied in the fields of mobile phones,laptops,electric vehicles and so on,but also keep developing at a fast pace.Separator is the key component of lithium-ion batteries,guaranteeing the safety of batteries and relating to the electrochemical performances.The current separators in the market are made of polyethylene or polypropylene,which belongs to polyolefin.These kinds of separators have great chemical stability and mechanical property with mature manufacturing process,but on the one hand they have poor affinity with the electrolyte,too difficult to absorb it and thus influencing the performances of cells,on the other hand they suffer severe thermal shrinkage because of low inherent melting points,causing short circuits and other safety accidents.Based on the above disadvantages,we chose PMIA,which is both thermal stable and hydrophilic,to coat the PE separator by two different phase inversion method,aiming to improve the thermal stability and electrolyte wetting ability.The specific details are as follows:1.By immersion precipitation phase separation,PMIA blended with PVDF were coated on one side of the PE separator.The formation of porous structure was controlled by investigating the coagulation bath(DMAc/H₂O),followed by researching the effect of PVDF on reducing the resistance of the membrane.SEM and EDX were used to analyze the morphology and element,and the characterization of electrolyte wetting behavior,thermal behavior and mechanical property was also conducted afterwards.The results of EIS and Gurley values manifested the effect of PVDF.The ionic conductivity rose from 0.012 m S/cm to 0.513 m S/cm and the Gurley values reduced by 79.7%.The results indicated that the separator had an electrolyte contact angle reduced from 48.3°to 0°,remained stable at 140?and exhibited enhanced mechanical property.With the improved wetting ability,thermal stability and mechanical property,the separator also showed good cycling performances and rate performances.2.In order to further improve the effect of the coating layer,we used another method,vapor induced phase inversion to achieve the introduction of PMIA.The key factors,humidity,temperature and polymer concentration were investigated to optimize the structure of PMIA on both sides of PE separator for better thermal stability and great transportation of lithium ions.The characterization results indicated that the membrane had better affinity with electrolyte based on the contact angle of 0°and the immersion height rose from 5 mm to 18mm.The membrane kept stable at 150?and displayed enhanced mechanical elongation from 68%to 96%.Due to the tortuous structure and improved electrolyte affinity on both sides,the separator suppressed the growth of lithium dendrites and thus delayed the short circuit of the cell.The cell kept working for 250 h.With the modified separator,Li Fe PO₄/Li cell showed greater capacity retention(98.2%)and better rate performances.This work introduced a method to coat PMIA on the polyolefin separator and achieved great results,which was important for the application of lithium-ion batteries.