| Lithium-ion batteries are widely used in various fields due to their excellent performance,such as high energy density,light weight,stable cycle performance,no memory effect and so on.The separator is an important part of the lithium-ion battery.Its main function is to isolate the positive and negative electrodes from short circuit,and absorb the electrolyte to provide channels for ion transportation.Therefore,the performance of the separator affects the safety and electrochemical performance of the battery.Commercial polyolefin separators are widely used at present,which are low in production cost,high in strength,and excellent in chemical stability,but have poor wettability and thermal stability.Electrospinning is a new method for the preparation of lithium-ion battery separator,which can flexibly choose polymer,and the prepared nanofiber membrane has high porosity.In this paper,electrospinning was taken and used polyvinylidene fluoride(PVDF)as the matrix material.The additions of cellulose acetate(CA)and triphenyl phosphate(TPP),which used two different combinations,was to improve the performance of PVDF-based fiber membranes.These as-prepared membranes have flame retardant.The physical and electrochemical performance of electrospinning composite membranes were tested and characterized,which were also compared with commercial polyethylene(PE)membranes.The main work is given as follows:First,with the addition of CA,it was determined that the blending modification of CA can improve the lyophilicity,porosity and electrolyte absorption rate of PVDF-based separators.However,the excessive addition of CA will sacrifice the mechanical property of the separators,so adding 5%CA was the most suitable.Comparing the porosity(36%)and electrolyte absorption rate(85%)of PE separator,those of PVDF/CA separator reached 86%and 394%,respectively.And the amorphous region introduced by CA reduced the crystallinity of the separators,resulting in the ionic conductivity of 3.3 mS/cm for PVDF/CA separator,which of PE separator is only 0.5 mS/cm.At the same time,the electrochemical stability window of PVDF/CA separator is 4.7V,and the interface impedance is 210Ω.Assembled into Li/LiFePO4 batteries,it has higher specific capacity and better cycle performance than PE separator.Secondly,the addition of TPP is used to make the membranes flame retardant.Since the benzene ring of TPP forms hydrogen bond with polymer macromolecule,PVDF/TPP/CA composite separator has the tensile strength of 8.5 MPa and elongation at break of 155%.Because it has good compatibility with the electrolyte,the ionic conductivity of PVDF/TPP/CA composite separator is 4.4 mS/cm,and the electrochemical stability window is 4.9V.Assembled into batteries,the initial discharge capacity is 141.4mAhg-1,while that of PE separator is 139.7mAhg-1.After 100 cycles,the capacity retention rate of PVDF/TPP/CA composite separator(86.9%)is higher than that of PE(72%).Moreover,the battery with PVDF/TPP/CA composite separators has better rate performance than that of PE separators.Finally,CA/TPP@PVDF core-shell nanofiber separator was prepared by coaxial electrospinning.Because the main material of the shell is CA,which has better lyophilic,the liquid absorption rate of CA/TPP@PVDF nanofer bseparator can reach 410%,and the liquid cotact,angle of it is only 10.9°.Because of TPP in the shell of fiber,the membrane also has flame retardant.And the ionic conductivity can reach 4.5mS/cm.In the cycle test,the Coulomb efficiency of CA/TPP@PVDF separator is stable between 98-99%,while that of PE separator slightly floated.After 100 cycles,the discharge capacity of PE separator was attenuated to 72%(100.7 mAhg-1),while the discharge capacity of CA/TPP@PVDF was 123.7 mAhg-1,which was still 87.6%of the first cycle.In the rate performance test,the capacity of the coaxial electrospinning separator was always higher than that of PE,so CA/TPP@PVDF separator has better cycle performance and rate performance than PE. |
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