Preparation Of Aramid Nanofiber Composite Membranes And Their Dendrite Inhibition Mechanism

The use of metal lithium as the negative electrode can significantly increase the energy density of lithium batteries,but there is a serious problem of lithium dendrites,and the growth of lithium dendrites will penetrate the separator,resulting in short circuit of the battery,which seriously affects the safety and cycle life of lithium metal batteries.Aramid nanofiber(ANF)has the advantages of stable chemical properties,high specific strength,rich amide groups,and good thermal stability.It can use its rich functional groups to regulate the distribution of anions and cations,making the deposition of lithium ions more uniform,and ANF Higher mechanical strength can also have an inhibitory effect on lithium dendrites.In view of this,this paper intends to use ANF as the substrate,and add lithium aluminum titanium phosphate(LATP)and boron nitride(BN)nanoparticles to prepare ANF composite separators to further improve the electrical and mechanical properties of the separator and inhibit dendrite growth.ability to improve the safety and cycle performance of lithium metal batteries.Firstly,an ANF/LATP composite separator with controllable thickness was prepared by vacuum filtration method,and the effect of LATP content on the microstructure,morphology and performance of the separator was explored.The composite separator has abundant pores and a layered structure in cross section with a thickness of 7-35 μm.When the content of LATP was 40 wt%,the tensile strength and Young’s modulus of the composite separator reached 33.30 MPa and 2.63 GPa,respectively,which were significantly higher than the commercial Celgard separator(8.84 MPa and 0.09 GPa).The thermal stability of the composite separator is also excellent,and the thermal decomposition temperature is as high as 550℃.Moreover,the ionic conductivity and lithium ion transfer number of the composite separator reached 0.18 mS/cm and 0.64,respectively.The addition of LATP can also significantly broaden the electrochemical window of the separator(4.5 V),which is much higher than that of ANF(4.2 V)and commercial Celgard separator(4.1 V).Due to its high lithium ion migration number and mechanical strength,the composite separator has a strong ability to inhibit the growth of lithium dendrites,and the lithium/lithium pair battery based on the composite separator has a smaller overpotential(0.009 V)at 1 mA/cm2(1 mAh/cm2),and can cycle stably for 3000 h,which is much higher than that of cells assembled based on Celgard separator(450 h).The ANF/BN composite membrane with a thickness of 10-30 μm was further prepared by vacuum filtration,and the influence of BN content on the properties of the composite membrane was explored.Among them,the composite separator containing 25 wt%BN has the best mechanical properties,with tensile strength and Young’s modulus reaching 37.50 MPa and 2.73 GPa,respectively,which are better than ANF/LATP composite separator(33.30 MPa and 2.63 GPa).The thermal decomposition temperature of the ANF/BN composite separator is also as high as 550℃.In terms of electrical properties,the ionic conductivity of the composite separator can reach 0.28 mS/cm,which is better than that of the ANF/LATP composite separator(0.18 mS/cm)and Celgard separator(0.14 mS/cm).The electrochemical window of the ANF/BN composite separator is 4.2 V,which is significantly improved compared with the Celgard separator(4.1 V).Moreover,the lithium ion migration number of the ANF/BN composite separator is as high as 0.8,so the ANF/BN composite separator can effectively inhibit the growth of lithium dendrites.The lithium/lithium pair battery based on the ANF/BN composite separator can cycle stably for 2000 h at 1 mA/cm2(1 mAh/cm2),and the overpotentials are all lower than 0.025 V.After the charge-discharge cycle,the surface morphology of the metal lithium electrode was characterized by an electron microscope.It was found that the surface of the metal lithium electrode corresponding to the ANF/BN composite separator was flat and without obvious dendrites,while the surface of the metal lithium electrode corresponding to the Celgard separator was vertical and horizontal.The dendrites are densely covered,which further confirms the strong ability of the ANF/BN composite separator to inhibit the growth of lithium dendrites.