Exfoliated transition metal dichalcogenide nanosheets for supercapacitor and sodium ion battery applications

Growing concerns regarding the safety, flammability and hazards posed by Li-ion systems have led to research on alternative rechargeable metal-ion electrochemical storage technologies. Among the most notable of these are Na-ion supercapacitors and batteries, motivated, in part, by the similar electrochemistry of Li and Na ions. However, sodium ion batteries (SIBs) come with their own set of issues, especially the large size of the Na⁺ ion, its relatively sluggish kinetics and low energy densities. This makes the development of novel materials and appropriate electrode architecture of absolute significance. Transition metal dichalcogenides (TMDs) have attracted a lot of attention in this regard due to their relative ease of exfoliation, diverse morphologies and architectures with superior electronic properties. Here, we study the electrochemical performance of Mo-based two-dimensional (2D) layered TMDs (e.g. MoS₂, MoSe₂ and MoTe₂), exfoliated in a superacid, for battery and supercapacitor applications. The exfoliated TMD flakes were interfaced with reduced graphene oxide (rGO) to be used as composite electrodes. Electron microscopy, elemental mapping and Raman spectra were used to analyse the exfoliated material and confirm the formation of 2D TMD/rGO layer morphology. For supercapacitor applications in aqueous electrolyte, the sulfide-based TMD (MoS₂) exhibited the best performance, providing an areal capacitance of 60.25 mF cm^-2. For SIB applications, TMD electrodes exhibited significantly higher charge capacities than the neat rGO electrode. The initial desodiation capacities for the composite electrodes are 468.84 mAh g^-1 (1687.82 C g^-1), 399.10 mAh g^-1 (1436.76 C g^-1) and 387.36 mAh g^-1 (1394.49 C g^-1) for MoS₂, MoSe₂ and MoTe₂, respectively. Also, the MoS₂ and MoSe₂ composite electrodes provided a coulombic efficiency of near 100 % after a few initial cycles.