2D Layered Transition-metal Dichalcogenides:Synthesis,Characterization And Their Application In Lithium/Sodium Ion Batteries

Lithium-ion batteries?LIBs?have been used as the main power source for portable electronics over the past two decades due to their high energy densities,eco-friendly as well as long lifespan,and are considered one of the most promising candidates for hybrid electric vehicles?HEVs?and electric vehicles?EVs?.Furthermore,sodium-ion batteries?SIBs?also have recently attracted tremendous interest owing to their great potential and similar insertion electrochemistry with their lithium-ion counterparts.Nevertheless,the high performance electrode material is one of the most key factors for both LIBs and SIBs.Recently,there is growing attention on 2D layered transition metal dichalcogenides?TMDs?for efficient energy storage and conversion.These materials have unique two-dimensional layered structure analogous to graphite and consisting of covalently bonded monolayers brought together via weak van der Waals interactions.The most well studied example among them is Mo S₂.In this paper,we mainly focus on the synthesis,characterization of TMDs,and have a deep insight into their electrochemical reaction mechanisms.The detailed works included the following parts:1?TiS₂ nanoplates were prepared with high yields using a high-temperature solution approach.Resulting products possess nanoscale dimensions,large surface areas and most importantly,open interlayer galleries at edges providing access to their internal space.From electrochemical studies as well as ex-situ XRD,XPS and STEM-EDS characterizations,we demonstrate that TiS₂ nanoplates can be reversibly sodiated and desodiated,reaching a large specific capacity close to full Na⁺intercalation.In addition,they also show high rate capability up to 10 C and satisfactory cycling stability at both low and high current rates.The favorable electrochemical performance is believed to be afforded by their advantageous nanostructures with large surface areas and reduced dimensions,which facilitate the diffusion of Na⁺ions and better accommodate the stress upon repeated charge and discharge cycles.2?We prepared high-quality Mo S₂ nanosheets in a large quantity using the facile liquid phase exfoliation?LPE?method.They were further reinforced with different ratio of SWNTs to construct of Mo S₂/SWNTs composites.We studied a wide range of electrode compositions observing large increases in electrical conductivity,mechanical toughness and lithium storage capacity.The 20%wt SWNTs composites show high porosity,excellent electrical conductivity and mechanical robustness.They demonstrate excellent specific capacity,long cycle life as well as rate capability.These results were achievable because of the presence of the highly conductive SWNT network,which percolates through the MoS₂ nanosheet network,rapidly shuttling electrons to/from all parts of the electrode.This allows the Mo S₂-based electrodes to operate effectively,even at very high rates.The nanotube network plays a dual role,also improving the mechanical properties of the electrodes.The dramatically increased toughness of the composites electrodes allows them to withstand the significant expansion/contraction cycles as so dramatically increase the electrode stability and so lifetime.3?We demonstrate that the Mo S₂/SWNTs dispersion composite material could be readily processed into flexible and free-standing membranes with tunable thickness via the vacuum filtration,and directly utilized as the battery electrode without any binder or current collector.And this membranes show high porosity,excellent electrical conductivity and mechanical robustness.When evaluated as the SIB anode material at the normal condition,they achieved striking specific capacity of>400 mAh/g,volumetric capacity of⁶50 mAh/cm³ as well as areal capacity of up to>6.0 mAh/cm².Our study here represents the first experimental report on the volumetric or areal capacities of SIB electrode materials as far as we are aware.These values are outstanding,and even superior to those of most LIB electrode materials,as enabled by the hierarchical architecture percolated with the SWNTs network.We believe our strategy of combined LPE and vacuum filtration could be extended to the preparation of other TMD composite membranes for high volumetric/areal capacity sodium-ion storage.4?We prepared five high-quality Mo-and W-based TMDs in a large quantity using the facile LPE method,then constructed their composites with SWNTs?wt 20%?.We have detailed characterizations for these five TMDs and their composite using SEM,TEM,and Raman.From these characterizations,we can find that they have the similar morphology,structure and component,which provide the possibility to compare their electrochemical performance under the same conditions.After concluding about the great electrochemical performances of TMDs nanosheet composites,we went on to investigate their abnormal capacities.Several plausible explanations were frequently attempted in literature to rationalize the abnormal capacities of Mo-based and W-based TMD materials.But unfortunately none of them appeared reasonable after careful scrutinization.Therefore,we come up with two new possibilities for the abnormal capacities.First,Li⁺ions could further react with converted metallic Mo/W nanoparticles via the alloying mechanism for the extra capacity.The second possible scenario was the induced underpotential deposition of Li metal similar to the mesoporous MoO₂.