In-situ Composite SiO₂、TiO₂/EVOH-SO₃Li Electro-spinning Lithium-ion Battery Separator

With the depletion of fossil fuel for human survived and increasing of environmental pollution,developing new clean pollution-free energy has been concerned widely.Lithium-ion battery as a clean reliable secondary renewable energy has become a research hotspot due to its high energy density,low selfdischarge,safe and reliable,etc.The diaphragm is an important component of the lithium-ion battery,its performance determines the battery interface structure,resistance,as well as the characteristics of battery capacity,etc.Therefore,the excellent performance diaphragm plays an important role in improving the comprehensive properties of the battery.The polymer electrolyte of poly（ethylene-co-vinyl alcohol）grafted lithium sulfonate（EVOH-SO₃Li）is a semicrystalline polymer with excellent barrier properties,and it has both good processing properties and the strong hydrophilicity.Furthermore,it has good barrier properties,and can be used as an excellent material for lithium-ion battery separator.Electro-spinning technology is an ideal method of preparation of ultrafine fiber separator,this separator has advantages of high porosity,strong hydrophilicity,can well make up for the disadvantages of poor liquid absorption rate and low ionic conductivity of commercial diaphragm.In-situ generation of inorganic nanoparticles to modify the electrospinning separator,can improve the mechanical properties,meanwhile,improve the thermal properties and electrochemical properties of the separator,etc.In this paper,under the action of Lithium Tert-butoxide,a graft polymer of poly（ethylene-co-vinyl alcohol）grafted lithium sulfonate（EVOH-SO₃Li）was synthesized from poly（ethylene-co-vinyl alcohol）and 1,3-propane sultone;Insitu generated inorganic nanoparticles（Si O₂,TiO₂）in EVOH-SO₃ Li solutionwere utilized to compound with EVOH-SO₃ Li,and the composite separators of EVOH-SO₃Li/Si O₂ and EVOH-SO₃Li/Ti O₂ were prepared by electro-spinning.Taking advantage of the interaction between the polar groups on the surface of nanoparticles and the polar groups of EVOH-SO₃ Li molecular chain to enhance the mechanical properties,meanwhile,improve the thermal properties and electrochemical properties of the separator.FT-IR was used to characterize the structure of separator;By consolidating the test results of micro-structure,porosity and electrolyte uptake,chemical stability and mechanical properties,analyzing the influence of in-situ Si O₂ and TiO₂ doping on each performance,and determining the optimum contents of Si O₂ and TiO₂ were 3 wt% and 2 wt%,respectively.Moreover,the separators based on them showed maximum tensile strengths of 2.36 MPa and 2.19 MPa,respectively.The thermal properties and electrochemical properties of the separators of EVOH-SO₃Li/Si O₂ with 3 wt% Si O₂,EVOH-SO₃Li/TiO₂ with 2 wt% TiO₂,EVOH-SO₃ Li and commercial diaphragm（pp diaphragm）were tested,analyzing the influence of in-situ SiO₂ and Ti O₂ doping on the thermal properties and electrochemical properties.The results showed that the thermal properties were improved,the melting point increased 15²0 ℃;Furthermore,the composite separators of EVOH-SO₃Li/Si O₂ and EVOH-SO₃Li/Ti O₂ exhibited a good electrochemical stability,and the electrochemical window were 4.7 V and 4.5 V,respectively,meeting the requirements of the commercial lithium-ion battery separator.The ionic conductivity of EVOH-SO₃Li/Si O₂ and EVOH-SO₃Li/Ti O₂ were 1.27×10-3 S/cm and 0.43×10-3 S/cm,that had different degrees of increase than EVOH-SO₃Li（0.16×10-3 S/cm）,better than that of commercial diaphragm（0.99×10-4 S/cm）;The lithium-ion transference number of the composite separators always stayed above 0.70,in-situ SiO₂ and Ti O₂ had no effect on it,and that was closer to the ideal state of lithium-ion transference number 1 than commercial diaphragm（0.50）.Experiments confirmed that the part of performances of the composite separators of EVOH-SO₃Li/Si O₂ and EVOH-SO₃Li/Ti O₂ had obvious advantages than commercial diaphragm,in-situ Si O₂ and Ti O₂ not only increased the basic performances,but also improved the electrochemical properties of the composite separators.