Fiber‐Reinforced Ultrathin Solid Polymer Electrolyte for Solid‐State Lithium‐Metal Batteries

Reducing the thickness of solid polymer electrolytes can help to enhance the energy density for solid‐state batteries. However, ultrathin electrolytes still face difficulties in preparation methods, mechanical properties, and interface instability. Herein, a free‐standing, scalable, and ultrathin solid polymer electrolyte with a thickness of 10 µm is reported. It is achieved through in situ thermal curing after filling a porous electrospun polyacrylonitrile fiber membrane with poly(ethylene glycol) diacrylate‐based electrolyte. Impressively, it contributes to a high ionic conductivity of 8.8 × 10 −4 S cm −1 at room temperature. The membrane can not only provide good mechanical strength but also offer a Li 3 N‐enriched solid electrolyte interphase, thereby stabilizing the lithium metal anode. The pouch cell pairing the ultrathin electrolyte with Li foil and LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode of high mass loading can realize a gravimetric/volumetric energy density of 380 Wh kg −1 and 936 Wh L −1 . This investigation provides new insights into the potential of fiber‐reinforced membranes for high‐performance solid‐state batteries.