Preparation And Performance Of PEI/PVDF Coaxial Fiber-based Lithium Ion Battery Separator

Polyolefin separators have poor thermal stability,poor electrolyte affinity,and low porosity,which seriously affect the battery’s safety performance and rate and cycle performance.Due to their large specific surface area,high porosity,and high composition controllability,electrospun fiber membranes have been used as new lithium ion battery separators.However,electrospun fiber membranes are randomly stacked with fibers,and there is almost no binding force between the fibers,resulting in poor mechanical properties.In addition,safety issues such as fire or explosion due to thermal runaway of the battery are key factors limiting the development of power lithium-ion batteries.Solving the problem of thermal runaway of lithium-ion batteries is the focus of current researchers.Aiming at the above two problems,this paper uses electrospinning technology to prepare coaxial lithium ion battery separators and modify them to improve mechanical properties,and tries to apply functional microspheres on the surface of the coaxial fiber membrane to give it an autonomous thermal shutdown function in order to effectively prevent thermal runaway.In this paper,a coaxial fiber membrane with a core-shell structure is prepared by electrostatic spinning technology,in which polyvinylidene fluoride（PVDF）is used as the shell layer and polyetherimide（PEI）is used as the core layer.The hot-pressing technology was applied to the caxial fiber membrane,the appropriate hot-pressing proameters were discussed,and its mechanical properties were tested.It was found that the mechanical properties of the coaxial fiber membrane were greatly improved after hot pressing,and the breaking strength was increased by nearly 4-6 times.At 0.08MPa and 170°C,the coaxial fiber membrane was hot-pressed.The breaking strength is 11.49 MPa,which is 4times that of the non-hot-pressed treatment.Based on the results of liquid absorption and porosity measurement and electrochemical performance test,the most suitable hot-pressed parameters are finally selected as 0.08MPa and 170°C.The separator assembled battery treated by hot-pressing at 0.08MPa、170°C showed that its rate performance and cycle performance are better than commercial separators.At 0.5C,the specific discharge capacity after 100 cycles is 138.2m Ah·g^-1,and the capacity retention rate is 90%,indicating that the coaxial diaphragm after hot pressing has greatly improved its mechanical properties without damaging its battery performance.In this paper,PE microspheres were prepared by solvent evaporation,and their structures were characterized by infrared,XRD,SEM,and DSC.The results showed that the PE microspheres exhibited a regular spherical shape with a narrow size distribution and an average diameter of about 7μm.They crystallized during the preparation process.The degree of PE microspheres was increased from 16.3%to 29.9%.PE microspheres have a melting point of about 115°C.This result provides a basis for thermal shutdown processing temperature.The PE microspheres were coated on one side with PVDF/PEI coaxial diaphragms.The effect of performance and its performance test showed that after coating PE microspheres,the liquid absorption rate and liquid retention rate of the coaxial diaphragm did not have a significant negative effect,the coating rate increased,and the PE-coaxial The liquid absorption rate of the diaphragm increased,but its liquid retention rate decreased accordingly.Through linear scanning voltammetry test and AC impedance test,it was found that PE coaxial diaphragms with different coating rates have almost no difference between the electrochemical window and 5V before coating.At room temperature,its bulk resistance remains below 4Ω,indicating that the coated microspheres have no adverse effect on its electrochemical performance.At high temperatures,when the coating rate is greater than 6mg/cm²,the PE-coaxial diaphragm bulk resistance increases by three orders of magnitude.Decidable PE-Coaxial diaphragms can achieve autonomous thermal shutdown.When the coating rate is 11.4mg/cm²,the discharge voltage will quickly become the set value and the discharge current will be 0 within 3 minutes,and the thermal shutdown function is successfully achieved.