Novel Separator Prepared By Electrospinning for Lithium Metal Battery

Abstract

With the ever-growing demand for high energy density, anode materials with even higher reactivities, such as Li metal, are being researched and drawn extensive attention owing to its high theoretic capacity (3,860 mAh∙g-1) and low electrode potential (−3.04 V versus standard hydrogen electrode). During the charging process of lithium metal batteries (LMBs), lithium will be electrodeposited on the anode current collector as metallic lithium, and be stripped back to the electrolyte in the discharge process. The reaction is purely lithium deposit/stripping, but several side reactions will deteriorate the reversibility, and the major issues include the incomplete dissolution of metallic lithium and formation of solid electrolyte interphase (SEI). After the discharge process, some dead lithium residue will remain on the anode current collector surface. These inactive substances will gradually accumulate after cycling, which will increase the internal resistance and block ions transportation. The dead lithium was composed of metallic Li within the inner layer, and wrapped by insulating SEI on the outer layer. The formation of SEI will consume lithium, and moreover, the metallic lithium embedded by non-conductive SEI can no longer be stripped back to the electrolyte, and both phenomenon will reduce the Coulombic efficiency. An important factor influencing the reversibility of Li is its electrodeposition morphology on the current collector. If the deposited Li patterns are needle-like and the porosity between each grain is large, the amount of metallic lithium embedded in insulating SEI after discharging will be enormous, and therefore, the consumption rate of usable lithium will be very fast. On the other hand, if lithium can be electrodeposited with large and granular grains in a densely-packed formation, the efficiency of lithium dissolution will be enhanced. The overpotential required for nucleation and growth of lithium during the electrodeposition step will affect the deposition pattern, and evidences from theoretical calculation and experiments have confirmed that lower overpotential can deliver better Li deposition morphology. An important component influencing the internal resistance is the separator. Normally, separators soaked with electrolyte will be placed in the middle of cathode and anode, and act as the agent to transport ions during the charge and discharge process. Most microporous membrane separators are made of polyethylene (PE), polypropylene (PP), or their combinations such as PE/PP and PP/PE/PP. For LMBs, if the electrolyte was conventional LiPF6 salt dissolved in the carbonate-based solvent, the Li deposition pattern with commercially available separator was mostly dendritic. To solve this issue, we proposed a novel type of separator synthesized by electrospinning strategy. The polyamic acid (PAA) precursor solution was used to fabricate polyimide (PI) separator. We compared the in-house synthesized separator with the commercial PP/PE/PP separator, and the results showed that both the volume of electrolyte uptake and ionic conductivity were improved. Also, when using electrospun PI as the separator, the overpotential required for Li nucleation in LMB was clearly lower than commercial one, and PI could ameliorate the lithium deposition morphology, leading to better Li reversibility. In this research, scanning electron microscopy (SEM) was employed to analyze the Li deposition morphology and dead Li after cycling. The battery performance with both types of the separator was evaluated by coin cell configuration, and the cycling ability of LMB was successfully promoted with the PI separator.