Janus Bilayer Separator With Opposite Charges for Lithium Metal Batteries

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||LiFePO₄ 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.