Preparation Of Highly Safe Separators And Their Foundational Application In Lithium Ion Batteries

Separator plays the vital role of safety and electrochemical performance for lithium-ion batteries,although they do not participate in the electrochemical reaction directly.The commercial polyolefin separators still possess two well-known drawbacks:poor wettability and thermal stability.When the temperature overheated,the poor thermal stability will induce the shrinkage of the separators,making the batteries shorted and even cause fire.Meanwhile,the low affinity between electrolyte and polyolefin separators will influence the uptake and retention of the electrolyte,and finally affect the rate capability and cycling performance.Moreover,the poor electrolyte wettability limits the application of batteries in electric vehicles and Energy Storage Systems.Furthermore,polyolefin separators must be matched with flammable liquid electrolyte,which will lead to a security risk.In order to solve these problems,we chose two methods to improve the safety and electrochemical performance of the lithium-ion batteries.The specific studying process and results as follows:First,improve the thermal stability of the separator.A thin and lightweight silica filled in polyimide?PI?nanofibers membranes are prepared by electrospinning.The microstructure,porosity,electrolyte wettability,mechanical flexibility,thermal stability and electrochemical performance have been studied.The results show that:compared with the commercial polypropylene separator,PI-SiO₂ membrane with high porosity?90%?presents excellent electrolyte wettability and large electrolyte uptake?up to 2400%?.In addition,PI-SiO₂membrane displays good mechanical flexibility and enhanced thermal stability up to 250°C,which significantly improve the safety of lithium-ion batteries when used as a separator.The LiMn₂O₄/Li cell with the PI-SiO₂ separator exhibits highly improved rate capability and cycling stability at different temperatures?25°C and 55°C?,which make PI-SiO₂ membrane as a promising secure separator candidate for high-performance and safety lithium-ion batteries.Second,using the solid polymer electrolyte replace the liquid electrolyte.Li₇La₃Zr_1.75Ta_0.25O₁₂?LLZTO?nanoparticles doped PEO?LiClO₄?-x LLZTO?x=10 wt%,15wt%,20 wt%?solid polymer electrolytes are prepared using solution casting method.The influence of LLZTO nanoparticles to the crystallinity and ionic conductivity is investigated.The results show that:the adding of LLZTO decreased the crystallinity of PEO?LiClO₄?matrix and improved the ion conductivity.Through the Linear Sweep Voltammetry and Electrochemical Impedance Spectroscopy results,adding 15 wt%LLZTO obtained the hightest electrochemical stability and ion conductivity.According to the above analysis,the LiFePO₄/Li cell using the PEO?LiClO₄?-15%LLZTO show good electrochemical performance.Meanwhile,using the solid polymer electrolytes will solve the leakage and flammable of the liquid electrolyte and ensure safety of the lithium-ion batteries.