Research On Preparation And Properties Of Inorganic Composite Separator For Lithium-ion Batteries

Lithium-ion batteries are widely used as a power source for portable electronic devices and electric vehicles due to their excellent energy and power densities and long lifetime. The separator is placed between the positive and negative electrodes in order to prevent the short circuit and allowing the transportation of ions while wetted by electrolyte. Polyolefin microporous membranes are commonly used as separators in most commercial lithium-ion batteries due to their high mechanical strength and good chemical stability. Their poor thermal stability and electrolyte wettability lower the safety and electrochemical properties of the batteries. Inorganic separators have great high-temperature dimensional stability and electrolyte wettability, but the larger thickness increased impedance and lower the volume energy density. The inorganic composite separators combined advantages of these two separators mentioned before are supposed to have extensive application prospect in lithium-ion battery for their excellent insulating property, thermal stability, electrolyte wettability and flexibility.In order to improve the battery safety and electrochemical properties, porous Al₂O₃/PVDF, Al₂O₃/SBR, and paper-supported Al₂O₃/SBR separators were prepared in this dissertation. Field Emission Scanning Electron Microscopy （FESEM） was performed to analyze the surface topography of the separators. Use constant current charge and discharge test and AC impedance method to analyze the electrochemical properties of composite separators.Firstly, Al₂O₃ particles, binder PVDF and solvent NMP were mixed by ultrasonication and grinding. Al₂O₃/PVDF separators with the thickness 40μm and porosity 65% were prepared via a casting process. Improved separators were prepared by phase separation method, the wet membranes were dipped into deionized water, PVDF in the wet membrane separated out and formed as solidification state, the thickness of these separators was decreased to 20 μm and porosity was increased to 70% further. Both of these two separators have excellent thermal stability and electrolyte wettability. LiNi_1/3Co_1/3Mn_1/3O₂|graphite cells with the separators prepared by phase separation method showed excellent electrochemical properties.Secondly, Al₂O₃ particles, SBR, dispersant agent PEG and H2O were mixed by stirring. Al₂O₃/SBR separators with the thickness 37μm and porosity 68% were prepared via a casting process. Using aqueous adhesive SBR and H2O made the separator preparing process more environmental friendly. Introduction of PEG improved the stability of slurry. After removed by deionized water, PEG helped to form pores. Al₂O₃/SBR separators have good flexibility, electrolyte wettability and thermal stability. For LiNi_1/3Co_1/3Mn_1/3O₂|graphite cells with Al₂O₃/SBR separators, the first discharge capacity reached 156mAh g^-1, and the capacity still retained 90.3% after 100 cycles at 0.5C. The battery exhibited superior rate capability with 127.5 mAh g^-1 at 8 C.Lastly, composite separators were made by spraying an Al₂O₃/SBR slurry on both sides of rice paper. Compared with Al₂O₃/SBR separators, the paper-supported Al₂O₃/SBR separators kept excellent electrolyte wettability （electrolyte contact angle was 0°）, thermal stability （no thermal shrinkage under 130℃）, ion conductivity (0.398 mS cm^-1) and improved the mechanical strength with the resistance to tensile stress of 1.4MPa and 5.4% deformation quantity or even better. Assembled LiCoO2|graphite cell, the capacity retained 163.7 mAh g^-1 after 60 cycles at 0.5C. The battery exhibited good rate capability with 108 mAh g^-1 at 8 C, the discharge capacity returned to 162.6 mAh g^-1 when the C-rate went back to 0.1C.