Freestanding polyimide fiber network as thermally safer separator for high-performance Li metal batteries

A stable and high-performance polyimide (PI) fiber mat was prepared via a facile two-step procedure, electrospinning deposition followed by an imidization process. The PI fiber separator presents higher thermal stability (>300 °C) than the commercial polypropylene (PP) separator (<160 °C). In Li/Cu half-cell configuration, PI fiber separator shows long cycle life performance with a coulombic efficiency of 82% at 0.5 mA cm^–2 after 100 cycles and an impedance of 29 Ω. It also showed suppression of sharp Li dendrite growth. While the commercial PP separator delivers a poor coulombic efficiency of 24% after 100 cycles and an impedance of 99 Ω because of the formation of dendritic Li. Moreover, the Li/LiFePO₄ full cell with a PI fiber separator exhibits a lower polarization and charge transfer impedance than the commercial PP separator. The PI fibers enhanced the electrochemical cycle performance and stability of the full cells that deliver a charge capacity of 118 mAh g⁻¹ at a C-rate of 0.5 C with a capacity retention of 86% and a coulombic efficiency of > 99% after 200 cycles. Additionally, the multimode calorimetry (MMC) studies under in-situ mode revealed that the freestanding PI fiber separator presents high thermal stability in Li/LiFePO₄ full cells.