Essential Contribution of Li ⁺ Intercalation/Deintercalation Afforded by the First Layer of Coaxial Coated MoS ₂ Sheets on Carbon Nanotubes

Although the combined use of MoS2 and carbon nanotubes (CNTs) has been known to work as superior anodes for Li+ intercalation/deintercalation, the essential contribution was still blurred provided by the so-called synergistic effects. To make this question clearer, we fabricated coaxial coated CNTs by MoS2 of single- and multiple-layered sheets via a solution-based method implemented by ultrasonic agitation and co-use of the nonionic surfactant polyethylene glycol (PEG400). The synthetic conditions were found as concentrations >2.0 M of the precursor (NH4)2MoS4, a lower ultrasonic power (<500 W), and less ultrasonic treatment time (10–30 min), which yielded hybrids with multiple layers of MoS2-coated CNTs (multi MoS2@CNTs). The lower concentration of (NH4)2MoS4 (0.5–1.0 M) produced monolayer MoS2-coated NTs (mono MoS2@CNTs). The mono MoS2@CNTs were proved to have a reversible capacity >750 mA·h·g–1 for at least 100 cycles, and no decaying of Li+ intercalation/deintercalation capacity was observed. The multi MoS2@CNTs delivered a higher capacity (∼1050 mA·h·g–1) but suffered from a gradual fading in Li+ storage/release capacity. DFT calculations demonstrated that the typical Li+ diffusion energy barrier (0.5 eV) from Oh to Th sites between two sheets of MoS2 constructed in the multi MoS2@CNTs was greatly decreased to 0.20 eV for lithium transfer from H to T sites between the MoS2 sheet and the CNT surface which was constructed in mono MoS2@CNTs. The superior cyclability and high rate of lithiation for the hybrid of MoS2@CNTs were confirmed to stem mainly from the first layer of MoS2 that was conjugated with the sp2 surface of graphene.