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

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.