Salt-template-assisted Synthesis Of Transition Metal Dichalcogenides-based Nanostructures And Their Electrochemical Properties On Energy Storage

Two-dimensional nanomaterials have attracted enormous interests due to their unique physicochemical properties and promising applications in various fields.Molybdenum disulfide?MoS₂?,as a kind of typical layered transition metal dichalcogenides?TMDs?,has been widely studied and applied in electrochemical energy storage.MoS₂ possesses a high theoretical specific capacity of 669 mAh g^-1for lithium-ion battery anode.On hand,the layered structure of MoS₂ can shorten the transportation pathways of both metal ions and electrons as well as facilitate the fast insertion/extraction of lithium ions on the surface of active materials.On the other hand,its two-dimensional configuration is quite suitable for buffering the volume expansion caused by the lithium intercalation,thus maintaining the structural integrity and stability.However,owing to the low intrinsic electro-conductivity of MoS₂semiconductor coupled with the effect of strong van der Waals interaction and?-?conjugation,MoS₂ is easy to reaggregate and re-stack spontaneously during the charge/discharge cycles,which results in dramatic electrode pulverization and even loss of electrical contact from the current collector,thus leading to poor cyclability and rate performance of MoS₂.Construction of nanostructured MoS₂ and MoS₂-based composite are two effective ways to solve the above problems of MoS₂.Nanostructure engineering can promote the microstructure and accelerate the diffusion of metal ions and the transport of electrons,as well as improve the structural stability of materials.For composites,the advantages of every component can be reserved while the disadvantages can be overcome,in which combing with carbon nanomaterials is one of the most effective ways to promote the properties of active materials.Therefore,the practical performance of composites will be greatly improved by making full use of synergistic effect.In this work,to solve the low electronic conductivity and easy aggregation of MoS₂ and built the foundamental of high-performance TMDs-based anode materials,we proposed an in-situ chemical vapor deposition?in-situ CVD?strategy to prepare two-dimensional TMD nanosheets,honeycomb-like structured MoS₂ and three-dimensional porous graphene networks anchored with few-layer MoS₂nanosheets?3D FL-MoS₂@PCNNs?nanocomposite where the soluble Na Cl salt acted as the template to provide a 2D confined space between the surfaces of adjancet NaCl crystals.The effect of different raw materials,materials ratio and process parameters on the phase,microstructure and interfacial bonding of the products has been systematically investigated.It revealed the related synthesis factors of TMD nanosheets,honeycomb-like structured MoS₂ and 3D FL-MoS₂@PCNNs in this paper,followed by studing the growth mechanism of the different TMD-based nanocomposites.Moreover,their electrochemical properties together with the energy storage mechanism have also been explored.It was found that the precursor of TMD can be effectively dispersed in the confined space existed in 3D NaCl self-assembly.The amount of NaCl template had an obvious effect on the distribution state of TMD precursor,which directly determined the morphology and microstructure of products.The WS₂ nanosheets under the optimized preparation condition have achieved a high specific capacity of453 mAh g^-1 when evaluated as a sodium-ion battery anode.Meanwhile,the electrode of WS₂ nanosheets has also exhibited an excellent cycle stability and remarkably enhanced rate capability.As the current density increased from 0.1 to 0.2,0.5,0.8,1,2 and 5 A g^-1,the average reversible capacities were 447.1,429.2,401.6,390.3,388.5,371.3 and 350.8 mAh g^-1,respectively.The Na⁺reaction kinetics controlled by capacitance process enables the fast adsorption/desorption of Na⁺on the surface of nanorods derived from WS₂ nanosheets,and as a result,this WS₂-Na battery cell can demonstrate the excellent rate performance.The optimized condition for preparing 3D honeycomb-structured MoS₂ via the salt-template-assisted in-situ CVD strategy is that the molecular ratio between TMDs precursor and NaCl template is equal to 1:80.The robust honeycomb-structured MoS₂exhibited a large preliminary capacity of 938 mAh g^-1 and a quite high initial Columbic efficiency of 93.5%.Moreover,there wasn't obvious capacity loss appearing in the electrode after 50 discharge/charge cycles.The highly-crystalized incorporate network,intimate interfacial bonding between adjacent MoS₂ walls and unique hierarchical porous structure should be responsible for the excellent lithium storage of our honeycomb-like structured MoS₂.The optimized synthesis process of 3D FL-MoS₂@PCNNs is that C₆H₈O₇,?NH₄?₆Mo₇O₂₄,CH₄N₂S and NaCl were taken as the raw materials with a molecular ratio of 1:20:150 beween MoS₂,C and NaCl to prepare the freeze-dried precursor gel,followed by an in-situ CVD process at 750°C for 2 h under Ar.Surprisingly,3D FL-MoS₂@PCNNs manifested an outstanding long-life cycling capability at high rates,namely,a specific capacity as large as 709 mAh g^-1 was delivered at 2 A g^-1 and maintained⁹5.2%even after 520 deep charge/discharge cycles.The well-crystallized active materials,few-layer MoS₂ nanosheets,intimate MoS₂/C interfacial contact and highly conductive three-dimensional graphene matrix not only ensure the fast and stable transport of ions and electrons,but also effectively maintain the integrity and stability of 3D porous architecture,thus achieving the excellent electrochemical performance,especially the outstanding long-life cycling capability at high rates.It is also believed that this facile salt-template-assisted in-situ CVD synthesis strategy can be extended to construct other 2D/3D nanostructures or low/multi-dimensional nanocomposites which may exert their particular advantages in various applications.