Construction Of Two-Dimensional Metal Sulfide Composites And Their Applications In Lithium/Sodium Ion Batteries

With the increasing demand for clean and economical electrical energy storage devices,developing rechargeable batteries such as lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs)has attracted significant scientific interest.Two-dimensional metal sulfide(2D MS₂)materials are considered as one kind of promising anode material for LIBs and SIBs due to their unique physical and chemical properties.However,the electrochemical performance of MS₂ materials is severely limited by their poor structural stability and electrical conductivity.In order to prepare promising 2D MS₂ based materials for LIBs and SIBs with a long-life span,high rate performance,high security,and low cost,the synthetic methodologies of hybrid nanostructures including one-dimensional(1D)TiO₂/MS₂ and carbon/MS₂have been widely developed.However,there are still some important issues that limit the practical application of MS₂ in LIBs and SIBs.For TiO₂/MS₂ composites,the poor electron/ion transport of TiO₂ with low aspect-ratio and the small interfacial contact area between 1D TiO₂ and 2D MS₂ need to be addressed.For carbon/MS₂ composites,the structural stability of MS₂ anchored on the surface of carbon(carbon@MS₂)and the electrochemical properties of electrodes are the main challenges for improvement.This dissertation provides fundamental information for the application of MS₂ in LIBs and SIBs through the design of surface,interface and electrode structure.In this dissertation,we design and develop the 1D TiO₂ sub-nanotubes with high aspect-ratio.The electron and ionic transfer are greatly improved.We also construct 2D TiO₂/MoS₂composites with“face-to-face”interface,which is beneficial to enhance the structural stability during discharge and charge.The sandwich-like C/MS₂/C(MS₂=MoS₂,ReS₂)composites are constructed to improve the conversion reversibility and electrochemical stability during lithium/sodium storage process.The 3D interconnected multi-walled carbon nanotubes(MWCNT)networks are introduced into porous MoS₂/C spheres electrode,leading to the enhanced electrical conductivity and Na⁺diffusion kinetics.The electrochemical properties,storage mechanisms,and electrochemical performance of these materials as anode materials of LIBs and SIBs are systematically investigated.The detailed information is listed below:(1)We synthesize the new 1D elongated TiO₂ sub-nanotubes by a convenient and scalable gel-derived method.During calcination,the elongated gel nanoribbons are rolled into a sub-nanotubes structure in the primary volatilization process and then the gel sub-nanotubes decompose and crystallize to form anatase/bronze TiO₂ at a temperature of 400?.The obtained 1D elongated TiO₂ sub-nanotubes exhibit,synchronously for the first time,a high aspect-ratio,open tubular interior,and anatase/bronze nanocrystal TiO₂ wall.This new 1D elongated TiO₂ sub-nanotubes show excellent properties of electron/ion transport and reaction kinetics.Therefore,this 1D elongated TiO₂ sub-nanotubes can benefit the application of MS₂ by enhancing its electrochemical performance.(2)We develop a new 2D micro-sized(1-4?m)ultrathin(?10 nm)TiO₂ nanosheet using a facile hydrothermal method,which employs graphene oxide(GO)as a support and dispersant,and HF as a morphology controlling agent.The GO/TiO₂ nanosheets can work as templates to grow 2D MoS₂ via hydrothermal method.The“face-to-face”interface between 2D TiO₂ and 2D MoS₂ is significantly beneficial for enhancing the structural stability of MoS₂.Moreover,the growth behavior and the surface defects of MoS₂ are optimized by controlling the amount of glucose and CH₄N₂S,respectively.The optimized 2D TiO₂/MS₂ composite successfully achieves the synergistic regulation of high structural stability,electrical conductivity,and Li⁺diffusion kinetics.As a result,the composite displays high-rate capability(582 mAh g^-1 at 2 Ag^-1)and cycling performance(818 mAh g^-1 at 0.1 Ag^-1 after 100 cycles)as the anode material of LIBs.Particularly,it achieves an outstanding long-life span cycling performance(648 mAh g^-1 at 1 Ag^-1 after 400 cycles).(3)We invent a scalable method to prepare layer-by-layer nitrogen-doped graphene/MoS₂/nitrogen-doped graphene(NDG/MoS₂/NDG)heterostructures by freeze-drying followed by thermal decomposition-reduction.This new NDG/MoS₂/NDG composite exhibits the high crystallinity of MoS₂ and dual NDG protecting layer,which can effectively host the electrochemical products of soluble lithium polysulfides and restrain the adverse reaction with electrolyte.As a result,it shows a high initial coulombic efficiency of 84.3%,a long-life span cycling performance(552 mAh g^-1 at 1 Ag^-1 after 600 cycles),and a high areal capacity(409mAh g^-1 at 8.73 mg cm^-2)when evaluated as the anode material of LIBs.(4)We obtain a porous MoS₂/carbon spheres anchored on the three-dimensional(3D)interconnected MWCNT network composite as the anode of SIBs by hydrothermal method using CS₂ as a S source and soft template.When compared with the MoS₂/C,the 3D MWCNT network in MoS₂/C-MWCNT shows a significant effect on the electrochemical properties.The optimized MoS₂/C-MWCNT composite features favorable structure of active materials,namely a few-layered MoS₂,abundant mesopores/macropores,and carbon incorporation,for enhancing Na⁺diffusion kinetics,electrical conductivity,and structural stability.In addition,the presence of 3D MWCNT network in the electrode structure further improves the interparticle and intraparticle conductivity,Na⁺diffusion kinetics,and structural stability.As a result,the optimized MoS₂/C-MWCNT anode shows a superior rate performance(324 mAh g^-1 at 20 Ag^-1)and a long-life span cycling performance of 416 mAh g^-1 at 2 Ag^-1after 1000 cycles.(5)We prepare a kind of new composite,in which ReS₂ nanosheets are confined in dual-carbon protective layers that comprise a reduced graphene oxide(r GO)inter-layer and a N-doped carbon coating-layer(r GO@ReS₂@N-C).When compared with the conventional r GO@ReS₂ nanostructures,the N-doped carbon coating-layer in r GO@ReS₂@N-C shows a significant effect on the structural stability.The strong interfacial interaction between carbon and ReS₂ increases overall conductivity and decreases Na⁺diffusion resistance,whilst the intended dual-carbon protective layers maintain the structural morphology and electrochemical activity during long-term cycling.As a result,when evaluating the new r GO@ReS₂@N-C as the anode of SIBs,it exhibits the highest rate performance reported so far for ReS₂ of 231 mAh g^-1 at 10Ag^-1.Significantly,this high performance occurs with a long-life span cycling of 192mAh g^-1 at 2 Ag^-1 after 4000 cycles.This dissertation contributes to the understanding of the design and fabrication of MS₂ based composites for applications in LIBs and SIBs.In addition,the results provide theoretical basis and technical approach for the practical application of MS₂ anode materials.