Studies On The Preparation And Properties Of Wet-laid Nonwoven For Paper-based Lithium Ion Battery Separator

The market of lithium-ion batteries(LIBs),due to their high energy density,excellent cycle performance,and low selfdischarge rate,has grown tremendously over the past decade to keep pace with consumer electronics.High-power density lithium-ion batteries are finding their application in the power tool market and stationary and transportation applications.These new markets have aroused new demands and challenges on separators for lithium-ion batteries.In the construction of LIBs,a separator is placed between the cathode and anode electrodes,which prevents the physical contact of electrodes while allowing Li+ ion transport during the charge and discharge processes.The separator as a critical component of LIBs directly affects the battery performance,especially for cycling stability and power capability.Now,commercial separators used for LIBs are typically micorporous polyolefin polymer films(polyethylene(PE)and polypropylene(PP))due to their high mechanical strength,good electrochemical and chemical stability,low cost,and thermal shutdown property.However,the surface energy of the polyolefin is low,and the wetting property of the liquid electrolyte is poor,so that a high ionic resistance is formed between the separator and the electrode,which directly affects the cycle performance and power capacity of the battery:Therefore,it is necessary for the lithium-ion battery separator surface treatment to enhance the membrane on the electrolyte wetting performance and conductivity,reduce the impedance.The combination of chemical oxidation and physical coating method improved the fiber hydrophilic in this paper.This can oxidize polypropylene,implanting polar groups on its surface,increasing the interfacial bonding strength between the fibers and the coating in the physical coating.The modified polypropylene fiber and cotton fiber were mixed and wet-formed to form a paper-based lithium ion battery separator with high strength,good hydrophilicity and good thermal stability.The main research work in this thesis are as follows:1.The effects of oxidant dosage,reaction temperature and reaction time on the hydrophilic properties of polypropylene fibers were investigated by oxidation method.The results showed that polypropylene fiber could achieve the best hydrophilic performance under the condition as follows:temperature of 70°C,reaction time of 15min and with a 5%acid potassium permanganate solution.It was proved by FTIR that the polar group(-C=O)was formed on polypropylene fiber surface.Modified fiber wetting performance has improved,and the density slightly larger than the water.The floating phenomenon was significantly reduced.2.The above-mentioned polypropylene fibers were modified by a coating method.The results showed that polypropylene fiber could achieve the best coating effect and hydrophilic performance under the condition as follows:swelling time of 2h,swelling temperature of 60 ?,reaction time of 2h and with reaction temperature of 90?.The moisture absorption rate reached 4.08%,which indicated the obvious improvement of hydrophilicity.It was found by DSC that the basic thermal properties of polypropylene fiber were not changed.It was found by SEM that the surface of the modified polypropylene fiber would not only produce rough traces,but also the complex geometry of the polypropylene fiber and calcium carbonate greatly enhanced the adhesion of calcium carbonate package.Covering the ability to improve the moisture absorption performance of the fiber.3.Lithium ion battery separator was prepared from cotton fibre and polypropylene fiber which was modified with the method of oxidation and coating by wet non-woven papermaking.The influence of different ratio of fiber on tensile strength,porosity,electrolyte uptake,electrolyte retaining and thermal shrinkage of separator was studied,and the morphology and conductivity of the lithium ion battery eparator was analyzed.The results showed that,when the fiber ratio was 1:1,with 1.5%PEO,3%PVA(relative to the absolute of dry pulp),lithium ion battery separator with good performance was prepared including tensile strength(1.6471/KN.m-1),porosity(45.45%),electrolyte uptake(687.3%),capillary rise of paper(39.2mm),and electrolyte retaining(121.3%).The thermal stability of separator was improved by adding cotton fibre through performance test.It was found by SEM that the separator was interwoven with modified polypropylene fiber and cotton fiber,and the conductivity of eparator was 2.39x1 0-3S/cm.