Conductive Holey MoO2-Mo3N2 Heterojunctions as Job-Synergistic Cathode Host with Low Surface Area for High-Loading Li-S Batteries.

Abstract

Li-S batteries have several advantages in terms of ultrahigh energy density and resource abundance. However, the insulating nature of S and Li2S, solubility and shuttle effects of lithium polysulfides (LiPSs), and slow interconversion between LiPSs and S/Li2S/Li2S2 are significant impediments to the commercialization of Li-S batteries. Exploration of the advanced S host skeleton simultaneously with high conductivity, adsorbability, and catalytic activity is highly desired. Herein, a heterojunction material with holey nanobelt morphology and low surface area (95 m2/g) is proposed as a compact cathode host to enable a conformal deposition of S/Li2S with homogeneous spatial distribution. The rich heterointerfaces between MoO2 and Mo3N2 nanodomains serve as job-synergistic trapping-conversion sites for polysulfides by combining the merits of conductive Mo3N2 and adsorptive MoO2. This non-carbon heterojunction substrate enables a high S loading of 75 wt % even under low surface area. The initial capacity of MoO2-Mo3N2@S reaches 1003 mAh/g with a small decay rate of 0.024% per cycle during 1000 cycles at 0.5 C. The long-term cyclability is preserved even under a high loading of 3.2 mg/cm2 with a reversible capacity of 451 mAh/g after 1000 cycles. The Li-ion diffusion coefficient for MoO2-Mo3N2@S is extremely high (up to 2.7 × 10-7 cm2/s) benefiting from LiPS conversion acceleration at heterojunctions. The affinity between LiPSs and heterojunction allows a dendrite-free Li plating at anode even after long-term cycling. Well-defined heterointerface design with job-sharing or job-synergic function appears to be a promising solution to high-performance Li-S batteries without the requirement of loose or high-surface-area carbon network structures.