Preparation And Properties Of Modified PVDF/PMMA Electrospun Lithium Ion Battery Separators With Nano-SiO2 Particles

At present,the commercial lithium-ion battery?LIB s?separator with micro-porous structure still has its weakness,such as the poor wetting performance of liquid electrolyte,electrolyte uptake,and ionic conductivity.Because of its high porosity and excellent electrolyte-taken,electro-spinning fibrous membrane noxw is the most ideal material to prepare LIBs' separator.However,there is still much space to improve the mechanical and electrochemical properties of electrospun fibrous separators.Poly?methyl methacrylate??PMMA?is an amorphous polymer,known as the best electrophilic polymer so far as.Its interface impedance with lithium metal is low and its electrochemical performance is good.PMMA can absorb and retain a large amount of electrolyte,while its mechanical strength is weak.PVDF has good property of film-forming and good mechanical properties,but its structural regularity,high crystallinity,and electrochemical properties still needs to be strengthed;nano-SiO₂ is a typical inorganic ceramic particles,and it has advantages of electrochemical stability,insulation,and it can not only enhance the strength of electro-spun fiber,but also elevate the thermal stability of polymer separator.In this paper,the combination of PVDF,PMMA and SiO₂ was used to promote the strength and anti-heat performance of the separator,and the amorphous state of the component was used to enhance the Li+.And the multi-component interaction can enhance the performance of the electrospun membrane.Two different kinds of electrospun LIBs separators with multi-component were prepared by direct addition method and multi-layered composite method.One is to add nano-SiO₂ particles in the mixture of PVDF/PMMA,to prepare homogeneous single-layer fiber membrane,denoted as S-n membranes.The structure and properties of the membranes with different SiO₂ additions were investigated,including porosity,lyophilicity,mechanical properties and heat shrinkage.The surface morphology of the membrane was analyzed by SEM microscopy.The electrochemical impedance spectroscopy and the simple lithium cycle were used to analyze the electrochemical performance and the battery cycle performance assembled by the composite films.The other method is a multi-layer composite membrane.Two kinds of sandwiched structure membranes were obtained by sequential receiving:1)The outer layer is SiO₂/PVDF electrospun nanofibers,the middle is PMMA fiber layer,denoted as FS-n Membrane;2)The outer layer of pure PVDF fibers,the middle layer of SiO₂/PMMA nano-fiber,recorded as MS-n Membranes.The surface morphology of the membrane was analyzed by scanning electron microscopy?SEM?.The electrochemical performance and cycling performance of the two composite membranes were analyzed by AC impedance method and simple lithium cycle test.S-n Membranes have excellent physical properties,among which S-6 Membrane has the best overall performance.It has an electrolyte taken of 381.8%,and the heat shrinkage is only 2.16%.S-6 Membrane also has the highest tensile strength of 14.13 MPa.which is 112.80%higher than S-0 Membrane.The electrochemical performance of S-n Membrane was improved.The ionic conductivity of the S-6 Membrane was 2.88×10-3 S/cm.and the electrochemical window was 5.5 V,battery assembled by S-6 Membrane has a discharge capacity of 136.78 mAh/g in the first circle,with the battery cycle greatly improved.FS-n Membrane has good physical properties,among which FS-6 Membrane has the best comprehensive performance.Its tensile broken strength is 9.67 MPa,and the ionic conductivity of the related GPE is 3.68×10-3 S/cm.the electrochemical window is 5.4 V,and the retention rate of the assembled battery was 92.4%after 35 complete cycles.MS-n Membranes have excellent electrochemical properties,of which MS-4 Membrane is the best.The tensile strength of MS-4 Membrane is about 6.0 MPa.The discharge capacity of the first cycle reached 137.1 mAh/g,and after 50 times of discharge,the discharge capacity of the MS-4 Membrane is attenuated to 132.0 mAh/g.which was the first discharge 96.29%of the specific capacity.The properties of FS-n Membranes and MS-n Membranes are different,mainly due to the addition of inorganic nanoparticles.The addition of nanoparticles to the surface of the PMMA fibers promoted the strength of the PVDF fibers,while the addition of the nanoparticles to the PMMA fibers did not enhance the phsiscal performance significantly,but the electrochemical performance was improved significantly.LIBs prepared by MS-n Membranes have better cycle performance,while the FS-n Membranes have better physical properties.