Research On Heat Resistance Modification Of UHMWPE Separator For Lithium Ion Battery

Separator plays a vital role in lithium ion batteries.However,the large degree of shrinkage deformation phenomenon will occur after being heated in the current commercial polyethylene separator,which inevitably makes the short circuit in lithium ion battery.Therefore,solving the heat resistance problem of separator applied in lithium ion battery has become a research hotspot of lithium ion battery materials and is also the purpose of this chapter.In this paper,the UHMWPE/PP blend composite separator is prepared using ultra-high molecular weight polyethylene(UHMWPE)and polypropylene(PP)as matrix resins and paraffin oil(LP)as diluent.Then,the technological conditions,rheological properties,experiment formulas and so on,are also explored.Adopting the method of combining granulation process and wet process to manufacture the UHMWPE/PP blend composite separator in this chapter.Some preparation process factors,the dosage of PP that impact on rheological properties of system,the effects of stretching ratio,heat setting time and extraction time on the performance of the separator are also investigated.Its rheological properties,microscopic morphology,thermal properties,and other properties are measured by rotary rheometer,scanning electron microscope(SEM),differential scanning calorimetry(DSC)and other methods,respectively,and are analyzed.The research results demonstrate that the blend system with different proportions of UHMWPE/PP have the characteristics of pseudoplastic fluid(“shear thinning”),which means the viscosity decrease with the increase of the shear rate.The processing properties of the system is outstanding,such as excellent mixing uniformity,low viscous flow activation energy and the significantly improved flow performance,when the dosage of PP is 5wt%.At this time,the current value of the twin-screw extruder is 12.86 A during the system is extruded smoothly.Subsequently,the influence of the stretching ratios on the performance of the separator is studied,and it is found that the separator with the stretching ratio of 7(unheated)has the best performance at the stretching temperature of 120℃: up to55.34% porosity,air permeability as low as 325.76s/100 cc,high tensile strength(126.8MPa),while the thermal contraction of the separator increases(MD(machine direction)=23.24%,TD(transverse direction)=23.96%).Then try to reduce the degree of heat shrinkage of the separator by changing the heat setting time at the heat setting temperature of 130 ℃.And considering the processing efficiency and separator performance,it is finally concluded that the separator prepared in the thermoforming time is 90 s has the best performance,high crystallinity(77.02%),high tensile strength(180.7MPa),and preferable dimensional stability of the separator(low the degree of thermal shrinkage).Finally,it is found that the effect of extraction time on the performance of the separator can be ignored through extraction experiments.The blend composite separator prepared by the UHMWPE/PP dosage ratio of95/5 has perfect performance,excellent porosity(44.02%),good air permeability(380.30s/100cc),low shrinkage after extraction(21.77%),and low thermal shrinkage rate(MD=12.12%,TD=12.37%).Compared with the pure UHMWPE separator,the initial decomposition temperature is increased by 55.1 ℃,and can quickly achieve the obturator foramen in 138 ℃,and the heat resistance has been further improved.