Sulfur Atoms Bridging Few-Layered MoS2 with S-Doped Graphene Enables Highly Robust Anode for Lithium-Ion Batteries
Authors: Wang Xiaolei , Li Ge , Seo Min Ho , Chen Zhongwei 
Published: February 2020

Tremendous research interest from both academy and industry has been dedicated to the rechargeable lithium-ion batteries (LIBs) in the last decades for the upcoming era of portable electronics, electric vehicles (EVs) and hybrid electric vehicles (HEVs). As one of the favorite power sources, most commercial LIBs utilize natural or synthetic graphite as the anode material due to its low cost, high Coulombic efficiency, and flat and low average potential of 0.2 V (vs. Li/Li+), as well as long cycle life. However, its specific capacity of 372 mA h g-1 results in a device energy density of ~150 W h kg-1, which is much lower than that of internal-combustion engines and cannot meet the EVs requirements. Therefore, there is an urgent need to develop novel anode materials with high theoretical capacities to replace graphite in next generation high energy LIBs. So far, various materials have been extensively studies for LIBs anodes, including alloys (e.g. Si and Sn) and transition metal oxides (e.g. Li4Ti5O12 and SnO2). Although most of these materials possess a significant larger specific capacity, they suffer from either poor cycling life due to volume change associated with Li-ion insertion/extraction or sluggish electrode kinetics stemmed from slow ion diffusivity or intrinsic poor electron conductivity.
Compared to metal oxide materials, some transition metal sulfides possess high specific capacity and unique structures, and have been considered as promising candidates for high-performance anode materials. Among various candidates, a typical member of transition metal sulfide-molybdenum disulfide (MoS2) possesses a similar layered structure to graphite but a much larger interlayer spacing of 6.15 Å (vs. 3.35 Å of graphene) by stacking together through van der Waals interactions, which facilitates lithium-ion intercalation without a significant volume expansion. However, MoS2 still suffers from fast structural deterioration during lithiation/de-lithiation process and poor electrical/ionic conductivity, resulting in unsatisfactory cycling performance and rate capability in LIBs application. Therefore, the development of novel highly stable MoS2-based materials with fast kinetics remains challenging, owing to the lack of a ration design from molecular level. Moreover, it is also critical to correlate the performance with materials structure, and to understand the chemistry behind before its future practical applications.
Herein, we demonstrate a facile solvothermal synthesis of nanocomposites consisting few-layered MoS2 and covalently sulfur-doped graphene (MoS2/SG) with excellent electrochemical performance. We focus on not only the development of MoS2-based electrode materials but also the materials design based on both structure and chemistry considerations. The sulfur atoms covalently bonded to graphene sheets and effectively bridging two-dimensional (2D) few-layered MoS2 and graphene enable high robustness of the composite materials. Moreover, the intimate contact of MoS2 and highly conductive graphene provides efficient electron transfer pathways, while the high surface of assembled 2D structured materials allows fast access to active materials. Such a unique composite architecture derived from the “bridging effect” ensures the electrode with an exceptional cycling stability and superior rate capability, which is also interpreted by the density functional theory (DFT) calculations. A capacity retention of 92.3% can be achieved after 2000 cycles at a current density of 10 A g-1; even at a high current density of 20 A g-1, the electrode still possesses a specific capacity of 766 mA h g-1. This composite material with excellent electrochemical properties synthesized via a facile solvo-thermal approach holds great promise in the practical application of high-performance LIBs.

Figure 1