Transition-Metal-Based Nanocatalytic Materials As Advanced Cathodes For Constructing High-Performance Lithium-Sulfur Batteries

Lithium-sulfur battery?Li-S?is regard as a new-generation energy storage system with high theoretical energy density(2600 Wh kg^-1 or 2800 Wh L^-1)and fast-charging characteristics,which is composed of elemental sulfur cathode and metallic lithium anode.The elementary sulfur,as the main active material for cathodes,with abundant source,cost-effective,and environmental benignity shows broad application prospects.In particular,it shows competitive potential progress in the application fields of smart phones,flat panel displays,flexible wearable devices,electric vehicles,smart grids and aerospaces.At present,Li-S falls to large-scale commercial applications due to low actual cycle-life and low energy density output.In this dissertation,composite matrixes of catalytic transition-metal based nano-compounds and carbon materials were constructed for constructing advanced sulfur cathodes with the purpose of:1)improve the electronic conductivity and ion diffusion of sulfur and its discharge products;2)alleviate the huge volume expansion?⁸0%?during cycling which may result in the pulverization of the cathode;3)construct active polar sites for adsorbing soluble polysulfides through chemical/physical interactions to inhibit the polysulfide shuttle;4)improve the electrochemical reaction kinetic and promote the rapid conversion and transfer of intermediate products.For the structure design and regulation of sulfur cathodes,the electrochemical reaction interface with excellent conductivity,efficient polysulfide adsorption and catalysis,and controllability to improve the conversion efficiency and cycling stability of sulfur cathode were carried out.The main contents are as follows:1.A highly conductive and polar transition-metal-selenide of Co₃Se₄ was vapor-deposited on a nitrogen-doped three-dimensional interconnected carbon networks.It was used for the first time as a cathode material for Li-S batteries.The nitrogen-doped three-dimensional interconnected carbon network with a hierarchical porous structure can effectively reduce the interfacial resistance caused by particle agglomeration,increase the conductivity of sulfur cathode,and build a fast ion transfer channel.Ultrafine Co₃Se₄ nanoparticles grafted on the surface of nitrogen-doped three-dimensional interconnected carbon network as a sulfur framework can inhibit polysulfide shuttle reactions through physical barriers and chemical adsorption,alleviating the corrosion of lithium metal anodes,and improve the utilization of sulfur.The results demonstrated that the as-prepared N-CN-750@Co₃Se₄ shows good electrochemical performance in Li-S batteries.It cycled continuously for 5 months?800 cycles?at a current density of 0.2 C and retained a high specific capacity of above 500 mAh g^-1 with coulombic efficiency of exceeding 99.3%,showing good cycle stability.2.We synthesized Co₃Se₄ nanoparticles in the previous section,however,their particle size is still relatively large and their distribution is hard to manipulate.In this section,we have synthesized cadmium sulfide?CdS?quantum dots?QDs?with uniform particle size distribution by hydrothermal method.The CdS-QDs was modified by oleylamine ligands and then uniformly grafted onto surface of carbon nanotubes?CNTs?to construct a composite structure with high catalytic activity and high conductivity.By adjusting the ratios of CdS-QDs in CNTs,the sulfur matrix of CNT/CdS-QDs 30%with rich catalytic sites was obtained.In this devise,CNTs with high specific surface area are used as channels for electron/ion transfer and the main carrier of sulfur,which can alleviate the volume expansion in the reaction.Meanwhile,CdS quantum dot on the surface was used as the adsorption and catalytic sites for lithium polysulfides to retard their diffusion to electrolyte.The specific capacity of CNT/CdS-QDs/S 30%cathode decreases slowly from 1237.8 mAh g^-1 at 0.2 C to918.1 mAh g^-1 at 2.0 C,showing an excellent rate performance and a good capacity retention of 820.6 mAh g^-1 at 0.5 C for over 150 cycles with a coulombic efficiency of over 98.0%.The inclusion of the CdS quantum dots suppresses the shuttle effect and enhances the redox kinetics,thereby leading to the high utilization of sulfur,and thus providing new avenues for the design of advanced cathode materials for high-performance lithium-sulfur batteries.3.When the particle size is reduced to the nanoscale,they are prone to aggregate,resulting in the decrease of active sites and longer distance of electron and ion transfer that are not conducive to the contact with the electrolyte and polysulfides.To this end,we further explored transition-metal nanomaterials by adjusting coordinating ligands and introducing a metal-organic framework?MOF?of Ni₃?HITP?₂ with high conductivity and abundant active sites.Herein,the first attempt to utilize a highly conductive pristine MOF to trap and transform polysulfdes for high-performance Li-S batteries was made.The Ni₃?HITP?₂ was synthesized by a one-step hydrothermal method.By combining long-range conductive CNTs and short-range conductive MOFs,an electronic/ionic conduction network was constructed.Ni₃?HITP?₂ possesses a graphene-like structure and shows good adsorption properties for polysulfides.Besides,it can also accelerate the solid-liquid-solid conversion of sulfur in electrochemical reactions.The as-prepared S@Ni₃?HITP?₂ exhibits a high initial capacity of 1302.9 mAh g^-1 at a current density of 0.2 C.Highly reversible discharge capacities of 807.4 and 629.6 mAh g^-1 are also obtained at 0.5 and 1 C for 150 and300 cycles,respectively.