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

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.