Shutdown-Enabled Electrospun Polyimide Composite Film Prepared By Novel Spin-Coating Method with Polyethylene As Separator for Rechargeable Lithium-Ion Batteries

Abstract

Global demand for lithium-ion secondary batteries have been increasing due to new market growth from electric vehicles and power storage systems. Lithium-ion batteries possess high power density, energy density and long cycle life without memory effect and little self-discharge, and have been primary energy storage system for commercial electronics. However, recent accidents regarding battery safety, such as Samsung Note 7 issue, has raised doubt among public about lithium-ion batteries as option for energy storage. Separator is one of the primary component for lithium-ion battery safety. Commercial separators like PE or PP not only possess outstanding mechanical and electrochemical performance, but also have thermal shutdown function that provide safety measure by pore—blocking with melted polyolefin materials. Nevertheless, polyolefin materials suffered serious shrinkage at elevated temperature, which cause internal short-circuited of batteries ; also, polyolefin materials have relatively low electrolyte uptake, and could be major obstacle for future lithium-ion battery application. Hence it’s necessary to design new type of separator that guarantee battery safety and demonstrate better electrochemical performance In this study, we proposed a brand-new method to design composite separator with high thermal stability, electrolyte uptake ability and thermal shutdown function. Polyimide(PI) nanofiber film was used as substrate and produced by electrospinning technique, and ultra-thin Low-Density-Polyethylene(LDPE) layer was laid upon PI by simple spin-coating method. Spin-coating method provided unique advantages such as ultra thin coating layer (less than 1 micron), uniform coating structure and simple process, and was yet been applied for construction of composite film as lithium-ion battery separator. Coating parameter was tuned to obtain the optimized composite separator. Field-emission scanning electron microscope (FE-SEM) was used to investigate morphologies of separators ; Contact angle measurement was conducted to verify electrolyte affinity of different separators ; A.C. impedance method was used to measure conductivities and verify thermal shutdown of separators, and electrolyte uptake of different separators were measured by electrolyte soaking method in an argon-filled glovebox. The composite film had sub-micron average pore size, and possessed excellent physical properties, for instance, high porosity(73%), electrolyte uptake(1300%), ion conductivity(4.3*10-4 S/cm), air permeability(gurley value=0), and thermal stability(no shrink up to 150。C). Thermal shutdown function of composite film was also confirmed by impedance measurement at elevated temperature. It was noticed that physical properties between pristine PI and composite film were similar owning to ultra-thin coating layer, which mitigate performance loss from LDPE coating. Afterward, full cell battery tests were conducted with CR2032 type coin cell (Cathode : LFP, anode : MCMB, electrolyte : 1 M LiPF6 in EC/EMC/DMC=1:1:1+1%VC, 1C=2.4mAh/g). The first cycle charge-discharge curves, rate capability and cyclic performance were conducted. The composite film showed flatter first cycle charge-discharge plateau, better capacity retention at higher C-rate (Composite film :102 mAh/g at 1C, 79% retention, Celgard 2500 : 73 mAh/g, 51% retention , w.r.t. 0.1C), and higher discharge capacity at 0.5C 50th cyclic test. The higher porosity and electrolyte uptake of composite film facilitated ion conduction and were the main cause for enhanced battery performance. With enhanced thermal stability and electrochemical performance, we expected this type of composite separator to be promising for future lithium-ion battery application. Figure 1