| The polyimide?PI?fiber membrane prepared by electrospinning exhibits the excellent properties of high porosity,large specific surface area and unique porous structure,along with high thermal stability,chemical resistance and other advantages of polyimide.Therefore,the polyimide fiber membrane using as lithium-ion battery separator can not only improve the liquid absorption rate of the separator and the ionic conductivity,enhance the compatibility between the separator and the electrode,but also solve the shortcomings of the poor thermal dimensional stability existing in the current commercial polyolefin separator,thus it can meet the need of new generation of high-performance lithium-ion battery.It has great potentials for developmen and has become the focus of research.However,the fiber membranes prepared by electrospinning generally have the disadvantage of poor mechanical properties and couldn't satisfy the requirements of winding and battery packaging during the production of lithium ion battery separators,consequently,improving the mechanical properties of polyimide fiber membrane is the key to the study of lithium ion battery separator.In this paper,firstly,3,3',4,4'-biphenyl tetracarboxylic dianhydride and 4,4-diaminodiphenyl ether were used as raw materials,in-situ polymerization were adopted to introduce nano-Al2O3 or Ti O2 into polyamic acid?PAA?,thus biphenyl polyamic acid spinning solution containing Al2O3 or TiO2 was obtained;Then two kinds of composite fiber membranes,Al2O3/PAA and TiO2/PAA,were both prepared by high-pressure electrospinning;Finally,the mixture of Al2O3/PI and Ti O2/PI were obtained by thermal imidization with gradient heating and uniaxial stretching fiber membrane,the related properties of Al2O3/PI and TiO2/PI composite membranes were tested as lithium ion battery separators.The infrared spectroscopy and micro-morphology analysis confirmed the presence of the corresponding inorganic nanoparticles in the membrane and the two composite membranes had good three-dimensional network microstructure.Refering to the their porosity of the separator,the infiltration performance,mechanical properties,thermal properties and flame retardance,it can be concluded that the optimal content of nano-Al2O3 and TiO2 in the two composite membranes were 3 wt%and 4 wt%,respectively.Moreover,the porosities of the two composite membranes were 75.2%and 77.5%,and the liquid absorption rates were 493%and 550%,the static contact angles were close to 0°and the tensile strengths were 18.63 MPa and 16.74 MPa.The decomposition temperatures were 566?and 591?,and both had excellent thermal shrinkage rates and flame retardance.The button-test batteries were assembled by using Al2O3/PI composite membrane with nano-Al2O3 content of 3 wt%and Ti O2/PI composite membrane with nano-Ti O2 content of 4 wt%as well as PI and PP separator,respectively,for electrochemical performance and battery performance testing,analysis of the impact of the addition of Al2O3 and TiO2 on the performance of PI membranes.The results showed that the electrochemical stability windows of Al2O3/PI composite membrane and TiO2/PI composite membrane were 5.5 V and 5.7 V,and the interfacial impedances were 346.72?and 292.85?,and the ionic conductivities were 1.961×10-33 S/cm and 2.429×10-33 S/cm,and initial discharge capacities were 154.75 m Ah·g-11 and 157.68 mAh·g-1,and their discharge capacity retentions at 10 C rate were 69.5%and 69.9%,and their capacity retentions rates were 95.2%and 96.7%after 100 cycles at 25?and 1 C rate,their capacity retentions rates were 95.2%and 96.7%after 100 cycles at 120?and 1C rate,those had differfnt degrees of increase than PI which were 5.2 V,451.23?,1.214×10-33 S/cm,149.38 mAh·g-1,66.1%,86.6%and 80.3%,so it had good application prospects,better than those of PP membrane which were 4.5 V,553.36?,0.480×10-33 S/cm,141.69 m Ah·g-1,47.6%,75.1%and could not be used at high temperatures,and had a good application prospect. |
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