Synthesis And Electrochemical Performance Of The Cathode Materials In Lithium-sulfur Batteries

With the rapid development of the portable electronic devices,electric vehicles and large-scale smart power grids,the demands for high energy density storage devices are ever-increasing.Lithium-sulfur(Li-S)batteries are promising in the next-generation high-energy storage devices owing to the large theoretical energy density of 2600 W h kg-1 and the high theoretical specific capacity(1675 mA h g-1)of S.Additionally,S is abundant,low-cost,non-toxic and environmentally friendly.Despite of these perceived advantages,the commercialization of Li-S batteries is hindered by several intrinsic drawbacks,including the poor conductivity of sulfur and its discharge products,shuttle effect of polysulfide during the charge-discharge process as well as large volume expansion.In this dissertation,we design and synthesize high performance S-based cathode materials to improve cathodic properties of S-based electrodes in Li-S batteries and investigate their electrochemical properties.The main content and innovation points are concluded as following:(1)We have prepared biomass-based hierarchical porous carbon materials(HPCMs)via carbonization and subsequent removal of self-template mesoporous SiO2 nanoparticles in bamboo leaves by HF etching.The eliminated mesoporous SiO2 nanoparticles provide abundant 3D connected micro-nano pore in the HPCMs,which could load S with large mass loading and physically inhibit polysulfide for producing high performance S-based cathode materials.Moreover,the HPCMs have good conductivity rendering high S utilization and rate capability.The S loading in HPCMs is 70.26 wt%.The HPCMs/S composite exhibits a large initial discharge specific capacity of 1487 mAh g-1 at a rate of 0.05 C(1C = 1675 mA g-1)and a capacity of 707 mA h g-1 at 1 C for over 200 cycles with a capacity decay of only 0.014%per cycle.When the current density is increased 20 times from 0.2 C to 4 C,62.3%of the capacity is retained,revealing high rate capability of the HPCMs/S cathode.Our research provides a novel route for the low-cost and large-scale preparation of hierarchical porous carbon materials and promising applicaiton in Li-S batteries.(2)Owing to the weak absorption of nonpolar carbon materials towards polar poly sulfides,we propose to improve the electrochemical performance of the cathode materials of lithium-sulfur batteries by introducing TiO2 nanocrystals(NCs)on the surface of graphene carbon,in which TiO2 NCs could effectively inhibit the polysulfide shuttling effect via chemical absorption.Anatase mesoporous TiO2 NCs with dimensions of 10-20 nm are in situ prepared on the surface of the highly conductive reduced graphene oxide(rGO)to form two-dimensional(2D)TiO2@rGO composite by a one-pot hydrothermal technique using glucose as the dispersing agent and linker.The TiO2 NCs can effectively capture the polysulfide intermediates via strong chemical binding and the 2D rGO provides a fast channel for electron and ion transport.The 2D morphology of TiO2@rGO is still maintained after loading with S and the S mass loading is as high as 72 wt%.The S/TiO2@rGO cathode shows a high capacity of 1116 mA h g-1 at 0.2 C(1 C=1675 mA g-1)after 100 cycles,and the capacity of 831 mA h g-1 is maintained at 1 C after 200 cycles.When the current density is increased 20 times from 0.2 to 4 C,60%of the capacity is still preserved.Even the sulfur areal mass loading is increased to 3.9 mg cm-2,the S/TiO2@rGO cathode still maintains a high capacity of 550 mA h g-1 at a rate of 1 C after 200 cycles,indicating good cycling stability.The excellent electrochemical properties of the S/TiO2@rGO cathode are ascribed to the synergistic effects between the strong chemical absorption of the mesoporous TiO2 nanocrystals towards polysulfide and the high conductivity of the rGO nanosheets,which provide a good research foundation for designing high active materials of Li-S batteries.(3)S/C composite materials are conventionally prepared by S melt-diffusion method,by which S is generally loaded on the surface of carbon materials and is difficult to uniformly distribute into carbon matrix.We fabricate a novel S/C composite by a simple in situ oxidizing reaction using FeCl3 as oxidizing agent and organic-inorganic hybrid ZnS nanorod as precursor.The in situ produced S can adhere onto N-doped carbon(CNx)skeleton,forming 3D interconnected S/CNx network.The 3D conductive carbon network increases the electroactive interfaces to facilitate the transfer of the ions and electrons,thereby producing the high utilization ratio of the S active material,large specific capacity and high rate capability.In contrast to the conventional melt-diffusion method,the S in situ produced can uniformly distribute on the highly conductive CNx skeleton in our technique with an S content of 88 wt%.The as-prepared S/CNx composite delivers a specific capacity of 830 mA h g-1 at a rate of 1 C after 100 cycles with 98.8%of capacity retention.Even at a high current desity of 4 C,S/CNx composite cathode material exihibits a discharge capacity of 689 mA h g-1,indicating excellent rate capability.The novel synthetic route proposed in this work using metal sulfide/C precursor to prepare the S/C composite could provide a new thought for designing and developing high performance cathode material for Li-S batteries.