Preparation And Characterization Of High-safety Inorganic Composite Separators For Lithium-ion Batteries

Lithium-ion batteries have been widely used in electronics,electric vehicles and energy storage systems due to their excellent comprehensive performance.Separator plays a significant role in lithium-ion batteries,which is to separate the positive and negative electrodes and provide channels for lithium ions transport.Although the separator itself does not take part in any cell reactions directly,its structure and properties can greatly affect the battery performance.The most widely used lithium-ion battery separator is polyolefin separator for its superior comprehensive properties.However,the thermal instability at high temperature and poor wettability toward electrolyte of the polyolefin separator hinder the electrochemical and safety performance of lithium-ion batteries.Thus,how to improve the performance of lithium-ion batteries from the perspective of the separator has become a hot topic in current research on lithium ion batteries.In this thesis,high-safety inorganic composite separators were prepared by introducing inorganic particles on the surface of commercial polyethylene?PE?separator and nonwoven to overcome the defects of the polyolefin separator and improve the performance of lithium-ion batteries.The details are as follows:1.Comparing with alumina?Al₂O₃?,boehmite?AlOOH?presents better hydrophobicity and lower density,which means AlOOH is more suitable as the separator coating material than Al₂O₃.Therefore,in the first part of this thesis,the AlOOH composited PE separator?APE separator?was prepared and the performance of the APE separator was investigated.The result shows that the introduction of AlOOH effectively improves the thermal stability and wettability of the APE separator.In addition,the APE separator shows better ionic conductivity and electrochemical stability than PE separator.Furthermore,the cell assembled with APE separator shows better cycling and rate performance than that with PE separator.2.In the second part of this thesis,polyimide?PI?nonwoven was chosen as the separator substrate instead of the PE separator,as the PI nonwoven has much higher porosity and better thermal stability than PE separator.The AlOOH composited PI nonwoven separator?API separator?was prepared by coating AlOOH on the surface of PI nonwoven and its performance was studied.The result presents that the AlOOH coating layer can efficiently block the macropores existing on the surface of PI nonwoven and optimize the pore size distribution of the PI nonwoven,further improve the wettability,ionic conductivity of separator,and the electrochemical stability as well.The cell assembled with API separator shows excellent battery performance with a capacity retention of 92.8%after 50 cycles at 1C,and a discharge capacity of 137 mAh/g at 8 C.Finally,if the battery system suffers thermal runaway,the nature of endothermic decomposition of AlOOH can quickly decrease the temperature of the battery system below the ignition point of electrolyte,which can significantly improve the safety of batteries.3.In order to decrease the costs and reduce the pollution of the environment,a cheap and environmentally friendly fiber paper nonwoven was applied instead of PI nonwoven as the substrate of the composite separator in the third part of this thesis.In this work,the paper-supported inorganic composite separator?PIC separator?was prepared by spraying Al₂O₃ onto the surface of the fiber paper nonwoven,and the performance of the PIC separator was studied.The result indicates that the Al₂O₃ inorganic particles effectively cover the oversized pores on the surface of the paper and the-OH functional groups in the paper fiber structure which can improve the work stability,electrochemical stability and initial coulombic efficiency of the cell.In addition,the PIC separator exhibits excellent wettability and high ionic conductivity because of the high porosity and liquid absorbency of the fiber paper.The cell assembled with PIC separator exhibits a higher discharge capacity at 0.5C and shows superior rate performance than that of PE separator.Impressively,the results of pouch cell penetration test show that no smoke or burning appeared and the voltage of pouch cell was also maintained at the normal level after nail penetrated,which exhibits excellent safety performance.