Zinc-Doping-Induced Electronic States Modulation of Molybdenum Carbide: Expediting Rate-Determining Steps of Sulfur Conversion in Lithium-Sulfur Batteries.

Abstract

Enhancing Li₂S deposition and oxidation kinetics in lithium-sulfur batteries, especially the potential-limiting step under lean electrolyte, can be effectively achieved by developing conductive catalysts. In this study, by using ZnMoO₄ as precursors, Zn-doped molybdenum carbide microflowers (Zn-Mo₂C) composed of speared porous sheets are fabricated with a hierarchically ordered structure. Density functional theory calculations indicate that Zn doping shifts the d-band center on Mo atoms in Mo₂C upward, promotes the elevation of certain antibonding orbitals in Mo─S bonds above the Fermi level, enhances d-p interaction between lithium polysulfides (LiPSs) and catalysts, weakens both S─S and Li─S bonds of LiPSs. Incorporating Zn significantly reduces the Gibbs free energy barrier for the rate-limiting step of the Li₂S₂ → Li₂S conversion, from 0.52 eV for Mo₂C to just 0.05 eV for Zn-doped Mo₂C. Thus, the synthesized Zn-Mo₂C demonstrates impressive bifunctional electrocatalytic performance, significantly advancing sulfur reduction and Li₂S decomposition. Moreover, this modification enhances charge transfer within the Zn-Mo₂C/LiPSs system, synergistically accelerating the kinetics of Li₂S₄ to Li₂S reduction and Li₂S oxidation. The Zn-Mo₂C/S cathode demonstrates impressive electrochemical performance, achieves remarkable cycling stability with a minimal capacity decay of 0.021% per cycle over 1000 cycles at 5 C, underscoring its potential for high-energy applications.