Janus Bilayer Separator With Opposite Charges for Lithium Metal Batteries

ABSTRACT Uncontrolled lithium ion transport and dendrite growth compromise the cycling stability and safety of lithium metal batteries (LMBs). Herein, a Janus bilayer separator with opposite charges is designed to enhance lithium ion transport and suppress dendrite growth. The sulfonated aramid nanofiber layer (S‐ANF) promotes selective lithium ion transport via enhanced desolvation kinetics and repulsion of anions, while the quaternary ammonium‐functionalized covalent organic framework layer (N‐COF) allows uniform lithium ion distribution and accelerates transport via cationic nanochannels. This S‐ANF/N‐COF separator demonstrates a high lithium ion transference number (0.67), enhanced ionic conductivity (1.39 mS cm −1 ) and exceptional thermal stability. Impressively, the assembled Li||LiFePO 4 cell achieves 72.6% capacity after 3000 cycles at 5C and 86.3% at 70°C after 100 cycles, outperforming conventional polyolefin separators. The assembled Li||Li symmetric cell shows stable lithium plating/stripping over 600 h. This work provides a novel ion‐regulated separator design strategy for high‐stability LMBs.