Rational Synthesis Of Molybdenum Disulfide–based Composites And Their Electrochemical Properties

Molybdenum disulfide?MoS₂?,a typical layered transition metal dichalcogenide compound,has been exhibited preponderant in electrochemical fields.The layered structure endows it has a large specific surface area,high surface atomic ratio and multi-type electron structure,which are beneficial to synthesize ultra-thin structure and assemble hierarchical structure with high conductivity and electrochemical activity.Therefore,it is considered to be a very promising functional material using in electricity storage systems.The 2H phase MoS₂ with poor conductivity and the metastable metallic phase 1T MoS₂ usually lead to poor electrochemical reversibility during applications.In this thesis,we adopt the soft chemical synthetic technique to enhance the conductivity and stability of MoS₂ composites via optimize stuctures and the incorporation of transition metal with covalent bonds,which greatly improve the electrochemical performance in energy storage.Furthermore,the relationship among microstructure,electron structure and electrochemical properties of synthesized materials was also conducted an optimized study,which provide a theoretical basis and the expansion of new structural design methods for the development of novel and high-efficiency nanomaterials applied in energy storage and conversion.The details of this dissertation are summarized briefly as follow:?1?Peony-like MoS₂ hierarchical structure supported by garphene was synthesized by a simple solvothermal method,and the structure-function relationship between unique structure and electrochemical lithium storage was discussed thoroughly.The morphology characterizations show that MoS₂ with a peony-like hierarchical structure is assembled by ultrathin nanosheets and further composited with graphene in a new configuration of"face to face".This special structure with enhanced interactions between graphene and MoS₂ can improve the stability of whole materials,which promote the electron transfer between the graphene and the MoS₂ interface.At the same time,the poney-like structure prevents the graphene stack and ensures the electrolyte infiltration and Li⁺ion diffusion.The simultaneously optimized electron transfer and ion diffusion improve the conversion reaction reversibility of MoS₂ and inhibit the formation and dissolution of polysulfide ions.Thus,it shows an excellent electrochemical cyclic stability in lithium ions batteries?LIBs?,e.g.the capacity retention of MoS₂/graphene nanocomposites exhibits above 80%after 2000 cycles at the current density of 1 A g^-1.HAADF-STEM measurement shows that the nanocomposites not only exhibit the regeneration of MoS₂ nanoparticles after 2000^thh cycle,but also maintains a good dispersion in graphene substrate during the conversion reaction process.The result indicates that the simutaneously optimized electron/mass dynamic behaviors can efficently enhance the conversion reaction reversibility,and especially the regeneration of sulfides is the key factor in inhibiting the formation of polysulfides and improve the cyclic stability.?2?Co-Mo-S/graphene namocomposites have been designed and synthesized,which shows much more excellent rate capability and long-term cycling stability with cycles enhanced from 2000 to 3000.MoS₂/graphene nanocomposites as anode materials in LIBs due to the synergistic effect between Co and Mo.System research shows that the incorporation of cobalt?Co?can firstly narrow the energy gap from 1.24eV?2H MoS₂?to 0 eV?Co-Mo-S?via theoretical calculation,which leads to improving electrical conductivity of Co-Mo-S/graphene nanocomposites from 0.46 S m^-1?MoS₂/graphene?to 1.39 S m^-1.Then,the size of whole hierarchical structure and the thickness of nanosheets have been decreased,which is conducive to improve the utilization space for electrochemical reaction and ease its volume expansion.More importantly,the CoS_x and MoS₂ nanoparticles formed in-situ during the charge-discharge process inhibit themselves from stacking and agglomeration to each other and promote the surface lithium storage and ion diffusion channels in active materials.The conductivity,ions diffusion and electron transport kinetic of Co-Mo-S/graphene nanocomposites are optimized simultaneously,which can more effectively improve the reversibility of conversion reactions.Thereby,the materials show excellent electrochemical properties in LIBs.The reversible capability can be maintained¹200mAh g^-1(at 0.2 A g^-1)and⁹40 mAh g^-1(at 2 A g^-1)after 700^th and 3000^th cycle as anode in LIBs,respectively.The reversible capacity can be maintained about 400 mAh g^-1 at high current density of 50 A g^-1,which is still higher than commercialized graphite.In addition,the capacity and cycle performance of Co-Mo-S/graphene nanocomposites have also been greatly improved as anode in sodium ion batteries?SIBs?.?3?Metallic phase?1T?with poor intrinsic structural stability has inhibited the application potentials in enegy storage and conversion process such as supercapactors and electrocatalysis.We have developed a new bottom-up solvothermal synthesis strategy to synthesize the transition metal stabilized metallic 1T Co-MoS₂ hierarchical structure.The formed Co-MoS₂ by incorporation of the transition metal cobalt has a typical zigzag-chain superlattice of metallic 1T phase MoS₂ and a hierarchical structure assembled by ultrathin nanosheets characterized via HAADF-STEM,etc,which indicates the Co is doped as an electron donor.The results of differential scanning calorimetry?DSC?show that the incorporation of Co can inhibit the phase transformation from 1T MoS₂ to thermostable 2H MoS₂.The obtained Co-MoS₂nanomaterials incorporated by Co have unique physical and chemical properties such as lower crystallinity,higher-disorder structure,higher conductivity and larger surface area than pure 1T MoS₂ and CoS_x synthesized under similar system.Thereby,when the obtained applied to the supercapacitor electrode,the electrochemical reversibility and specific capacitance of metallic 1T Co-MoS₂ are greatly improved.What's more,it exhibits a long-term cyclic stability.The specific capacitance of obtained materials still has almost no decrease(431 F g^-1)at 10 A g^-1 even after 25000 cycles.The electrochemical lithium energy storage performance of Co-MoS₂ is also obviously better than the 1T MoS₂ and CoS_x nanocompounds due to its high conductivity and bimetal synergic effect,which finally exhibits reversibility capacity of⁸23 mAh g^-1after 700^th cycle at 0.5 A g^-1 in LIBs.The stabilized metallic 1T MoS₂ by incorporation of transition metal?Co?with excellent electrochemical performance under simply and low cost synthesis process provides much more choices to design high-efficient energy storage devices.?4?We have expanded and synthesized the transition metal iron?Fe?doped into MoS₂ with long-string hierarchial structures assembled by nanosheets,in which the metallic phase was stabilized with enhanced temperature of 450 ^oC and the controllable adjustment structure sizes is realized by the different dopants concentration of.The results of spectroscopic analysis,synchrotron radiation and DSC characterizations show that substitution of Fe for Mo can stabilize metallic phase.This special materials with high conductivity,hierarchical structure and bimetal synergistic effect lead to a good electrochemical performance in LIBs.Fe-doped MoS₂ with the optimal molar ratio of Fe/Mo?0.5:1?can have capacity of 1490 mAh g^-1 at 0.5 A g^-1 even after 700cycles.In addition,the long-string structure assembled by nanosheet along the Z axis direction can expose much more active sites,which improves the hydrogen envolution reaction property.The successful expanding synthesis of transition metal-doped stable metallic 1T MoS₂ stucture provides a theoretical reference and potential possibility for further development of new high-efficient function materials and other application areas.