Preparation Of Composite Separator Of Polyimide Lithium Battery

As one of the four major components of lithium-ion batteries(LIB),the separator plays a vital role in the performance of the battery,especially safety and power density.Polyolefin separators have a dominant position in the battery separator market due to their good mechanical properties and stable electrochemical properties.However,the conventional polyolefin separator has problems such as poor thermal stability and poor wettability of the electrolyte due to the properties of the material itself,which greatly limits the application of the lithium ion battery.In this paper,soluble polyimide was synthesized to prepare polyimide separator and compounded with traditional polyolefin separator,so as to improve the thermal stability of traditional polyolefin separator,the wettability of electrolyte and other performance,and increase the safety of lithium ion power battery,charging and discharging efficiency.The main research contents are as follows.(1)Preparation of soluble polyimideIn this paper,two kinds of soluble polyimides were prepared.One was synthesized from diamine 2,2-bis[4-(4-aminophenoxy)phenyl]propane(BAPP),diaminodiphenyl ether(ODA)and maleic 4,4'-diphenyl ether(ODPA)at a ratio of moles to 1:1:2.The other is the PTEI with better bonding property,and was prepared by using 3,3'-dimethyl-4,4'-diaminodiphenylmethane(MDT)and three kinds of isomeric diphenyl sulfide(m-tdpa)as raw materials through two-step method.The structure and properties of polyimides were characterized by Fourier-transform infrared spectroscopy(FT-IR)and thermal gravimetric analysis(TGA).The results show that the two synthesized polyimides are completely imidization and have good heat resistance and high glass transition temperature.(2)Preparation and characterization of polyimide battery separatorThe ratio of moles to the diamine 2,2-bis[4-(4-aminophenoxy)phenyl]propane,diaminodiphenyl ether,dianhydride 4,4'-diphenyl ether dianhydride is 1:1:2.The soluble polyimide was synthesized,and the polyimide battery separators with different porosity were prepared by phase transfer method using ethanol and dimethylacetamide(DMAC)as coagulation baths.The properties of the separator were characterized by TGA,FT-IR,scanning electron microscopy(SEM)and electrochemical methods.The results show that the porosity of the prepared polyimide battery separator decreases as the polyimide solution concentration increases.Compared with Celgard2500,the polyimide separator is heat treated at 180°C for 0.5 h,and its heat shrinkage rate is less than 1%,while Celgard2500 was completely melted.At the same time,Celgard2500 is of a contact angle of 50.1°,while the contact angle of polyimide separator was less than 10°,showing a better wettability for the electrolyte.In terms of electrochemical performance,the interfacial impedance and bulk resistance of polyimide separator is lower than that of Celgard2500,showing better ionic conductivity.The battery rate performance and battery charge and discharge cycle performance of polyimide separator are superior to that of Celgard2500,and the separator prepared with 10 wt%soluble polyimide solution has the best performance.(3)Preparation and characterization of polythioetherimide composite separatorWe describe a sandwich-like composite separator with two sheath layers of PTEI and a core layer of Celgard2400 to improve the heat resistance and wettability of polypropylene(Celgard2400)separators for LIBs.The porous PTEI membranes were prepared by a wet phase inversion process and then coated with Celgard2400 via engaging a simple process without adhesive.The separators were characterized by TGA,SEM,contact angle measurements,and the electrochemical performance was evaluated.The results show that the PTEI/Celgard2400/PTEI composite separators exhibited excellent thermal stability(-37%at 180°C),better wettability,liquid electrolyte uptake(298%)and ionic conductivity(0.59m Scm^-1).Versus the Celgard2400 separator cell,the PTEI/Celgard2400/PTEI composite separators exhibit almost resemble electrochemical properties.Such composite separators with high heat resistance and wettability properties have an applications in high safety LIBs.