Multifunctional Janus Separators for Safer and Dendrite‐Free Lithium‐Metal Batteries

Abstract The design of multifunctional separators can address crucial drawbacks of the lithium‐metal batteries. Here, a novel Janus separator (JS) is developed with tunable properties to prevent internal short circuiting and improve the cathode/electrolyte interface. JS is a P(VdF‐HFP) composite with LLZO facing lithium (Li) anode (Layer A) and N‐CQDs contacting the NMC cathode. Layer A acts as Li‐ion flux regulator to hinder dendrite propagation; Layer B plays multiple roles: i) promotion of better carrier migration, ii) protection from side reactions at the NMC cathode and iii) action as physical barrier against dendrite penetration. Galvanostatic cycling of Li/NMC cells with JS including 5 wt% CQDs demonstrated that JS was effective to avoid self‐discharge and internal short‐circuiting due to dendrite perforation, resulting in enhanced cell lifespan. Specific capacity >150 mAh g⁻¹ with capacity retention of 91 % was delivered over 130 cycles at 1C. This result contrasted with higher capacity loss (>25 %) and cell failure in case of P(VdF‐HFP) and Single Layer A separators, respectively. Post‐mortem SEM on JS confirmed that the introduced functionalities successfully intercept dendrites. ARC overheating tests on JS‐based cells demonstrated a remarkably higher thermal resistance and lower severity of the exothermic phenomena compared to Celgard separators.