The Study Of Organic-inorganic Composite Nanofiber Separator For Lithium-ion Battery By Electrospinning

The properties of the separator with high porosity, fine wettability, high strength,good thermal stability, suitable shut down temperature and high melt temperaturewere required to high power output performance and safety for high powerlithium-ion battery. The research topic of lithium-ion battery was to balance therelation of high power output performance and safety.The polyamic acid (PAA), the precursor of polyimide (PI), was synthesized bypyromellitic dianhydride (PMDA) and4,4′-oxidianiline (ODA). Because of the PIinsolubilization in organic solvent, the PAA nanofiber membrane was prepared byelectrospinning of the solution of PAA. In order to improve the strength of membrane,the PET membrane was used to receptor. The final PI/PET composite membrane wasobtained after a thermal imidization process. The results showed that the porosity,liquid electrolyte uptake and room temperature ionic conductivity of PI/PETcomposite membrane decreased by10%~30%comparing with that of PI membrane,but its tensile strength improved by400%. The PI/PET composite membrane showedonly about2%of thermal shrinkage at180℃under air atmosphere. It showed goodthermal stability. The low thermal shrinkage could reduce the risk of shorting at a hightemperature. PI/PET composite membrane showed wide electrochemical stabilitywindow, which voltage was stable in0~5.5V, can be applied to high voltage forlithium-ion battery. The cyclic voltammetric curve repeatability of PI/PET compositemembrane was very well. It showed that the membrane had better reversibility oflithium deposit and strip and interfacial stability. The performance of the cell withPI/PET composite membrane was superior to one of the Celgard2400membrane. Thefirst coulomb efficiency of the cell with PI/PET composite membrane was83.8%, the discharged capacity retention rate was99.4%after50cycles and discharged capacitywas119mAh·g-1at20C. The instability of the PAA solution, the PAA solutionwould be degradation in the process of placed, can result to viscosity loss andstructure change, effect the quality and the morphology of electrospinning nanofiber.In order to overcome the instability of the PAA solution, PI nanofiber can beprepared by electrospinning the soluble PI solution directly. The influences of PImolecular weight, applied voltage, receiving distance and environment temperature onnanofiber membrane structure and surface morphology were investigated. It wasfound that the nanofiber diameter become uniform and distribution get narrow withincreasing PI molecular weight. The PI nanofiber diameter was decreased withincreasing the applied voltage. When the applied voltage was fixed, the receivingdistance increased was means to decreasing the applied voltage. According to thebasic theory of electrospinning, the nanofiber diameter could be decreased. Theexperimental result was just opposite to the theory. The PI nanofiber diameter wasdecreased with increasing the environment temperature. There was certainrelationship between nanofiber diameter and separator performance. The membranebody space was decreased with increasing the average nanofiber diameter. It canresult to the porosity decreased, so the permeability, weight uptake and ionicconductivity were also decreased.The studies had shown that PI material can be a good candidate as separator forlithium-ion battery, but the mechanical strength of prepared nanofiber membrane waspoor and was needed to improve. Add the inorganic materials can improve themechanical strength. On the other hand, it can increase ionic conductivity, so havesuperior electrochemical performance. By the way of adding the TiO2sol, it was welldispersed in PI matrix, and there was no aggregation of TiO2particles in the nanofiber.The average fiber diameter of the PI/TiO2composite membrane was decreased a half.The structure of throughout network was formed by PI and TiO2composite. Itimproved the mechanical strength of membrane. It has good affinity for liquidelectrolyte and increased the weight uptake. So PI/TiO2composite membrane has ahigher the ionic conductivity than PI nanofiber membrane. When the TiO2content was20wt.%, the tensile strength of PI/TiO2composite membrane was18.65MPa,and the ionic conductivity was2.146mS·cm-1. High ionic conductivity showedsuperior cells performance. The first coulomb efficiency of cells with PI/TiO2composite membrane was85.3%, the discharged capacity retention rate was98.6%after50cycles and discharged capacity was138mAh·g-1at20C.The polymer electrolyte was obtained by activated the nanofiber membrane inthe liquid electrolyte. The swell behavior of polymer electrolyte was studied. Aftersoaking in liquid electrolyte and electrolyte solvent EC, PI/TiO2nanofiber diametergot larger than before. The average fiber diameter was changed from85nm to913nm.The structure model of polymer electrolyte was provided. The microporous polymerelectrolyte of electrospinning was a concomitant system with a liquid phase, gel phaseand solid phase. It showed that the conductive behavior of the PI/TiO2compositepolymer electrolyte was obeyed to Arrehenius equations. The PI/TiO2compositepolymer electrolyte showed the higher fitting apparent activation energy than theliquid electrolyte and Celgard2400membrane systems, which mainly attributed tocharge interaction between Li ion and O atom besides of resistance of microporousmatrix. Moreover, the effect of the polymer electrolyte’s structure parameters on theionic conductivity of polymer electrolyte was also investigated. It was found that theionic conductivity increase with the increasing of porosity. The PI/TiO2and PIpolymer electrolyte all had low tortuosity less than2.0, in which the PI/TiO2polymerelectrolyte showed lower tortuosity between1.4and1.8.