Study On Silica Enhanced Thermal Stability And Electrochemical Properties Of Pe Separator For Lithium Battery

Lithium-ion batteries?LIBs?have the advantages of high working voltage,high energy density,and long service life.They are promising power sources for mobile devices and large-scale power storage applications such as electric vehicles.The separator is the core component.It not only absorbs the electrolyte and provides micropore channels for lithium ion migration,but also plays a key role in preventing short circuits by cutting off the positive and negative electrodes.Currently commercially available LIBs separators are made of polyolefins such as polypropylene?PP?and polyethylene?PE?.However,the surface-energy limitation of the wettability of traditional polyolefin separators may result in poor wettability and retention of liquid electrolytes.In addition,when the operating temperature is too high,the separator may be damaged and cause LIBs to explore when short-circuited.Insufficient thermal stability and electrochemical performance of the separator seriously affect the safety and cycling performance of battery.Among the various methods to solve these shortcomings,the introduction of ceramic composite layers due to its advantages of preventing thermal shrinkage of the separator and improving the wettability of the electrolyte have been extensively studied.In this paper,a coating solution consisting of nano-silica?SiO₂?inorganic particles with high surface energy and polyvinyl alcohol?PVA?as a binder was used to modify the commercial PE separator.The high thermal stability of SiO₂ and good affinity with the electrolyte enhanced the safety performance of the PE separator and improve the electrochemical performance of the separator.First,a 4%PVA aqueous solution was prepared as a binder,and a PE/SiO₂ composite separator was prepared by coating nano-SiO₂ on the surface of a PE separator to improve the thermal stability and electrolyte affinity of the separator.Inspired by the"sandstone"structure of concrete,PE/MSiO₂ composite membranes were prepared by blending different proportions of micro-silica powder and nano-SiO₂ with PVA aqueous solution and coating them on PE membranes.Because the micro-silica powder is irregular particle,its addition increased the porosity of the PE composite film.When the solution of PVA,nano-SiO₂ and micro-silica?15:1:1:1?were applied on the surface of the PE separator,PE/MSiO₂0.5,PE/MSiO₂1.0,and PE/MSiO₂1.5 composite separators were prepared.The thermal shrinkage of PE/MSiO₂1.0 at 150 ^oC was 21%,which was a significant improvement over the PE separator?74%?,and the electrolyte absorption rate was 109%,which was much better than that of the PE separator?70%?.The electrolyte wettability and thermal stability of the separator can improve the electrochemical properties of the PE separator.In order to further improve the thermal stability and ionic conductivity of PE/SiO₂composite separators,bistrifluoromethanesulfonimide lithium salt?LiTFSI?was used to modify the PE separator.The effects of LiTFSI content on the electrolyte wettability,dimensional stability,and ionic conductivity of the composite separator were investigated.The PE separator was immersed in LiTFSI/ethanol solution?1%,2%,4%?,and then coated with nano-SiO₂/PVA?1:15?solution after drying.PE/L1SiO₂,PE/L2SiO₂,and PE/Li4SiO₂composite were thus prepared.The thermal shrinkage of PE/L2SiO₂ composite separator at150 ^oC was 20%,which was a significant improvement over PE separator?74%?,and the electrolyte absorption rate was 108%,which was also greatly improved compared to PE separator?70%?.The electrolyte wettability and thermal stability of the separator can improve the electrical properties of the separator.The ion conductivity and discharge capacity of PE/L2SiO₂ were 15.6×10^-44 S·cm^-1 and 159 mAh/g,respectively,which were higher than those of PE separator(10.1×10^-44 S·cm^-1 and 145 mAh/g).The electrical performance was significantly improved.Furthermore,PE/SiO₂CNT and PE/LSiO₂CNT composite separatorswere obtained through the coating of nano-SiO₂/CNT-OH solution to further improve the electrolyte wettability,thermal stability,and electrical properties of the composite separators.Hydroxylated carbon nanotubes?CNT-OH??5%?were heated and blended with PVA to prepare PVA?4%?aqueous solutions with the CNT-OH content of 0.1%and 0.3%.PE membrane was immersed in LiTFSI/ethanol solution?2%?.After drying,nano-SiO₂/?CNT-OH/PVA??1:15?was applied to the unmodified PE membrane.PE/SiO₂CNT0.1,PE/SiO₂CNT0.3 and PE/L2SiO₂CNT0.1 and PE/L2SiO₂CNT0.3 composite separators were prepared.The thermal shrinkage of PE/SiO₂CNT0.3 and PE/L2SiO₂CNT0.3 composite separators at 150 ^oC was 21%and 14%,respectively,which was a significant improvement over PE separators?74%?.The electrolyte absorption rates were 108%and 110%,respectively.The conductivity and discharge capacity of PE/SiO₂CNT0.3 and PE/L2SiO₂CNT0.3 composite separators were 15.5×10^-44 S·cm^-1,16.9×10^-44 S·cm^-1,and 162mAh/g,164 mAh/g,The electrical performance is significantly improved.