Promotion of reversible Li+ storage in transition metal dichalcogenides by Ag nanoclusters

Abstract

After years of development, improvement in the electrochemical properties of MoS2 through structural modifications has reached its limit. Further improvements in the Li+ storage properties of MoS2 must be based on an understanding of the Li+ storage mechanism in MoS2. On this basis, we have developed a novel ternary composite of graphene, MoS2 nanosheets and a small amount (1 wt%) of silver nanoclusters (NCs; MoS2/G/Ag). The presence of the Ag NCs in the composite is, however, instrumental and serves several purposes: immobilization of sulfur, increased association with Li+ and increased spacing between graphene sheets. The presence of this small amount of Ag NCs was able to increase the Li+ storage capacity of MoS2 by 60% (with a discharge capacity of ~1300 mAh g−1 at 0.5 A g−1 compared with 800–850 mAh g−1 for a MoS2/graphene composite without the Ag NCs (MoS2/G)). The MoS2/G/Ag composite also exhibited a very impressive rate performance: discharge capacities of 1040 and 850 mAh g−1 at the very high current densities of 1 and 5 A g−1, respectively (the corresponding values from MoS2/G without the Ag NCs are 790 and 580 mAh g−1). A high-capacity alternative to graphite electrodes commonly used in lithium-ion batteries has been made by scientists in Singapore and China. Molybdenum disulfide (MoS2) has a two-dimensional layered structure resembling graphite, but with a significantly higher affinity for lithium ions. When combined with graphene, a conductive sheet-on-sheet material is formed which maintains a high capacity over many charge–discharge cycles. Jim Yang Lee and colleagues now report a way that uses silver nanoparticles to raise the lithium-ion storage of MoS2/graphene oxide electrodes even higher, by over 60%. The team employed a simple and scalable carbon monoxide reduction reaction to insert uniform layers of sub-2-nanometre silver spheres between the MoS2 and graphene sheets. The expansion in interlayer distance and the improved sulphur immobilization and lithium-ion association are likely causes for the jump in storage capacity. A minute presence of silver nanoclusters in a MoS2 nanosheet–graphene composite was found to significantly increase the reversibility of Li+ storage in the composite through Li+ association, pillaring of graphene and the immobilization of S species.