CO 2 ‐Assisted Induced Self‐Assembled Aramid Nanofiber Aerogel Composite Solid Polymer Electrolyte for All‐Solid‐State Lithium‐Metal Batteries

All‐solid‐state lithium metal batteries (ASSLMBs) hold great promise for the development of next‐generation high‐safety, high‐energy‐density lithium batteries, but still face the challenges of lithium dendrite growth and thickness. Herein, the ultrathin PEO‐based composite solid polymer electrolyte (denoted as PAL) supported by a low‐density self‐supporting aramid nanofiber (ANF) aerogel framework is developed. The ANF aerogel obtained by a novel CO 2 ‐assisted induced self‐assembly method has a well‐designed bilayer structure with double cross‐linking degree. Benefiting from the intermolecular interaction between ANFs and PEO, the PAL achieves an ultrathin thickness (20 µm) with excellent thermal stability and mechanical strength. Meanwhile, due to the modulation of ionic pathways by the functionalized ANF, the PAL achieves uniform lithium deposition without dendrites, resulting in stable long cycling (1400 h) for symmetric cells. Consequently, the Li|PAL|LiFePO 4 (LFP) cell has excellent long‐term cycling stability (1 C, >700 cycles, Coulombic efficiency > 99.8%) and fast charge/discharge performance (rate, 10 C). More practically, the Li|PAL|LFP cell achieves an energy density of 180 Wh kg −1 due to the ability to match a high‐loading (8 mg cm −2 ) cathode. Furthermore, the double‐layer Li|PAL|LFP pouch cell demonstrates excellent flexibility and safety in cycling and abuse tests.