Single Step Synthesis of Carbon Quantum Dots/MnCo₂O₄ Heterostructures as a Multifunctional Janus‐Type Interlayer for Polysulfide Immobilization and Enhancing the Electrochemical Performance of Lithium‐Sulfur Batteries

Abstract Advanced separators are intensively researched to address sluggish reaction kinetics, polysulfide shuttling effect, and lithium‐dendrite growth in high‐capacity and long‐cycle life lithium‐sulfur batteries. Herein, a Janus‐type separator is fabricated on conventional polypropylene (PP) with a highly electronically conducting (≈7.44 × 10⁻⁴ s cm⁻¹) nanoflower‐structured carbon quantum dot anchored MnCo₂O₄ (CQDs/MCO@PP) interlayer facing the cathode and insulting PP facing the anode and observed beneficial charge storage behavior compared to bare or MCO@PP. The improved catalytic properties of the CQDs/MCO layer are shown to provide a strong Lewis acid‐base interaction that traps the lithium polysulfide, promotes higher Li⁺ ionic conductivity (≈1.34 × 10⁻³ s cm⁻¹), and helps uniform lithium deposition on the anode. Consequently, the lithium‐sulfur cells (cathode: composite of reduced graphene‐oxide nanoribbon, carbon nanotube, and lithium sulfide) containing the CQDs/MCO layer offered superior specific capacity and cycling stability (0.038% per cycle at 3C) than the control devices using bare and MCO@PP separators. A series of experiments is undertaken to validate the observed superior charge storage behavior, including polysulfide adsorption and diffusion tests, distribution of relaxation time, operando X‐ray diffraction (XRD), and time‐of‐flight secondary ion mass spectroscopy. A model is proposed for improved cycling behavior based on the above studies.