Study On Preparation And Properties Of High Temperature Resistant PAN Based Composite Separator For Lithium-ion Batteries

As the efficient renewable energy storages,lithium-ion batteries（LIBs）are prone to internal short circuits due to complex working conditions such as external forces,electrochemical reactions,and heat conduction,which can lead to safety problems such as thermal runaway or explosion.As a key component of LIBs,the separator plays a key role on battery safety by isolating the positive and negative electrodes to prevent short circuiting.Moreover,it has the ability to achieve the transfer of Li⁺between the electrodes.Polyolefin microporous materials such as PP and PE are typical candidates for separators,which are widely used in commercial LIBs because of their excellent chemical stability and mechanical properties.However,the application of the polyolefin separators in power batteries are severely limited due to their low ionic conductivity and weak thermal stability.This thesis focuses on the thermal stability and ionic conductivity of LIBs separator.The high temperature resistant PAN was used as a separator substrate,and the modification study of doping and coating for PAN by electrospinning and dip-coating process.The main research contents and results are as follows:（1）The PAN/cellulose composite separator was prepared by electrospining and mechanical rolling process.The thermal stability of the composite separator was increased by doping the high temperature resistant cellulose.Meanwhile,the addition of cellulose also weaken the interaction between the PAN polymer segments and improved the ionic conductivity of the separator.The research results showed that the PAN/cellulose composite separator doped with 10 wt%cellulose showed excellent performance.The heat-resistant temperature of the composite separator reached 210?C,but the PP separator had completely melted at this temperature.Compared with the PP separator,The liquid electrolyte-soaked PAN/cellulose（10%）composite separator had a higher electrochemical oxidation limit（5 V）and the ionic conductivity(1.99 m S cm^-1)at room temperature.The Li Fe PO₄ battery assembled with the PAN/cellulose（10%）composite separator had a specific discharge capacity of up to 160 m Ah g^-1 at 0.5 C,and the rate performance was excellent.（2）The PAN/cellulose/PEO composite separator was prepared by electrospining and dip-coating process.The PAN/cellulose composite fiber membrane was used as the high temperature resistant framework of separator,and then the fiber membrane was dip-coated with PEO.The highly flexible EO segment could effectively improve the transmission capacity of Li⁺,thereby improving the ionic conductivity of the composite separator.The composite separator still had high thermal stability after dip-coating PEO,and only 2.22%heat shrinkage after heat treatment at 210?C for 30 min.The ionic conductivity of the liquid electrolyte-soaked PAN/cellulose（10%）composite separator(2.21 m S cm^-1)was 15 times that of the liquid electrolyte-soaked PP separator(0.144m S cm^-1)at room temperature.The Li Fe PO₄ battery assembled with the PAN/cellulose（10%）composite separator had a specific discharge capacity of up to 164 m Ah g^-1 at0.5 C,which reached 96.5%of the theoretical specific capacity of Li Fe PO₄.In addition,the specific discharge capacity of the composite separator was 133 m Ah g^-1 at 3 C,which was higher than the commercial PP separator at 0.5 C.