Hybrid MoS_2+x Nanosheet/Nanocarbon Heterostructures for Lithium-Ion Batteries

Modulation of electron/ion transport in electrodes through the appropriate mesoscale electrode structural design is essential to achieving effective utilization of nanoscale electroactive materials. Herein, nanosheet MoS2+x/carbon [one-dimensional (1D) carbon nanotube (CNT) or two-dimensional (2D) graphene nanoplatelet (GNP)] heterostructures are prepared via a simple, one-step hydrothermal method, resulting in high-loading (16.2–21.0 mg/cm2) binder-free three-dimensional (3D) porous electrodes. In lithium-based batteries, an anionic S22––S2– redox system is demonstrated based on combined structural characterization using X-ray photoelectron spectroscopy, Raman spectroscopy, and in situ synchrotron-based X-ray absorption spectroscopy to elucidate the electrochemical behavior of the Mo and S centers. MoS2+x-GNP electrodes delivered 177 mAh/g (2.9 mAh/cm2) in the first cycle and 78 mAh/g (1.3 mAh/cm2) after 100 cycles at a current of 3.2 mA/cm2, representing high capacities despite such a high material loading for a sulfur-equivalent system, with 44% capacity retention and good rate capability. Conversely, the MoS2+x-CNT heterostructure displayed lower capacity and more capacity fade at all rates, attributed to aggregation of the active and carbonaceous materials in these electrodes and poor access to MoS2+x edge sites, as visualized via 3D Raman mapping and electron microscopy. The significantly improved capacity retention of the MoS2+x-GNP system is attributed to the (i) morphology because the arrangement of the 2D MoS2+x nanosheets on the GNP substrate allows for edge sites with excess sulfur to be exposed, (ii) increased stability of the structure during cycling, and (iii) homogeneous dispersion of the active and carbonaceous materials, resulting in good electrical contact.