Positioning and deciphering "goldilocks zone" of molybdenum carbide surface oxidation: optimizing molybdenum trioxide...

Although polar molybdenum carbide (MoC) possesses promising electrocatalytic properties for lithium‑sulfur (Li-S) batteries' S redox reactions, their surface oxidation is often considered detrimental. Contrary to this conventional wisdom, we demonstrate that controlled surface oxidation of MoC to molybdenum trioxide (MoO₃) can be leveraged to construct high-performance MoC/MoO₃-X (X represents oxidation hours) core-shell heterostructures. Theoretical and experimental analyses reveal that the optimally oxidized sample (MoC/MoO₃-1) exhibits superior interfacial synergy: the MoO₃ shell strongly chemisorbs polysulfides (with binding energies 40-65% higher than MoC), while the metallic MoC core facilitates efficient electron transfer, collectively lowering the energy barrier for Li₂S decomposition. The engineered MoC/MoO₃-1 for commercial polypropylene separator modification (MoC/MoO₃-1/PP) enables remarkable battery performance, including high-rate capability (645.0 mAh g^-1 at 3C), exceptional long-term cycling stability (419.0 mAh g^-1 after 700 cycles at 3C), and outstanding performance under high S loading and limited electrolyte dosage (a low capacity decay rate of 0.3% per cycle at 3.87 mg cm^-2 and 10.3 μL mg^-1). This work interprets surface oxidation from a limitation into an asset, proposing controlled oxidation as a general and efficient strategy for designing advanced electrocatalytic interfaces in metal-S batteries.