A pair of researchers from Zhejiang University (China) have synthesized layered MoS2/graphene (MoS2/G) composites using a facile biomolecular-assisted process for use as high-performance anode materials in Li-ion batteries. The MoS2/G composite with a Mo:C molar ratio of 1:2 exhibited the highest specific capacity of ~1100 mAh/g at a current of 100 mA/g, as well as excellent cycling stability and high-rate capability.
In a paper published in the journal ACS Nano, researchers Kun Chang and Weixiang Chen attributed the electrochemical performances of the MoS2/G composites to their robust composite structure and the synergistic effects between layered MoS2 and graphene.
Graphite, widely used in current commercial Li-ion batteries (LIB), is limited by a small theoretical specific capacity (372 mAh/g). Graphene, a flat, one-atom-thick monolayer exfoliated from graphite, shows excellent electronic behavior and mechanical properties, as well as a large specific surface area, and has attracted considerable research interest for many applications, the authors note.
Graphene nanosheets and their composites thus have been intensively investigated for their electrochemical properties to determine their suitability as anode materials for LIBs, with high capacities from ~600 to 1000 mAh/g having been observed.
To date, numerous studies on metal and metal oxides supported on graphene have been conducted, in which their electrochemical performance as anode materials for LIBs was considerably enhanced. However, research on layered metal sulfides supported on graphene as LIB anode materials has hardly been reported thus far.
As a typical layered transition metal sulfide, MoS2 has the analogous structure of graphene; this structure is composed of three stacked atom layers (S–Mo–S) held together by van der Waals forces. This layered structure enables the convenient intercalation and exfoliation of Li+ ions.—Chang and Chen
Chang and Chen synthesized their layered MoS2/G composites by an L-cys-assisted solution phase method and subsequent annealing in a H2/N2 atmosphere at 800 °C for 2 h. The layered MoS2 are supported on the graphene surface, which then form the MoS2/G composites.
The MoS2/G composites exhibit a 3D architecture morphology consisting of curved nanosheets, attributed to the self-assembling of graphene hydrogel during the hydrothermal process. In particular, the MoS2/G (1:2) composite delivers a 3D sphere-like architecture.
In addition to the high specific capacity, the composites showed excellent cyclic stabilities. After 100 cycles, the reversible capacities of the MoS2/G (1:1), MoS2/G (1:2), and MoS2/G (1:4) electrodes remained at 734, 1187, and 978 mAh/g, respectively.
Among the different composite materials fabricated, MoS2/G (1:2) also demonstrates better rate performance. Even at a high current density of 1000 mA/g, the specific capacity remains at ~900 mAh/g, which is still higher than that of MoS2 at a low current density of 100 mA/g. Additionally, the extraordinary cycling stabilities of the three electrodes are exhibited at various current densities, the authors found.
The electrochemical evaluations reveal that all the MoS2/G composite electrodes exhibit much higher specific capacities and more cyclic stability than bare MoS2 electrodes…the present results suggest that this novel kind of MoS2/ G composite holds great potential as an anode material for LIBs.—Chang and Chen
- Kun Chang and Weixiang Chen (2011) L-Cysteine-Assisted Synthesis of Layered MoS2/Graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano doi: 10.1021/nn200659w