Defect Engineering And Heterostructure Desigin Of Transition Metal Compounds And Their Applications In Lithium-sulfur Batteries

With the development of economy and advancement of lifestyles,global energy consumption has increased dramatically.Lithium-ion batteries are the most widely used energy storage device,however their energy density have reached the theoretical limit value(?400 Wh kg^-1)due to the limitation of lithium insertion and removal mechanism.Therefore,it's urgent to find alternative battery technologies to achieve higher energy density and specific capacity.Lithium sulfur batteries have been universally acknowledged as the most promising high-energy secondary batteries due to their unparalleled energy density(?2500Wh kg^-1)and theoretical specific capacity(?1675 m Ah g^-1).However,some serious problems in Li-S batteries have hindered their development,such as low conductivity of sulfur and lithium sulfide,volume change of the cathode material during the charge and discharge process,and the shuttle effect of lithium polysulfide intermediates.In this thesis,we take advantage of the chemical adsorption of transition metal materials to lithium polysulfide,regulate its surface structure,and rational design cathode materials with defects and heterostructure for high-performance lithium-sulfur battery.Combined with phase characterization,electrochemical testing,and theoretical calculations,we have comprehensively explained the reaction mechanism.Main conclusions are summarized as follows:?1?we present a facile method to prepare the defect-rich honeycomb-like Co₃S₄hexagonal nanosheets?Co₃S₄-DHS?as sulfur hosts through anion exchange.The abundant honeycomb-like porous structures of the resultant Co₃S₄-DHS not only provide a high specific surface area to relieve the negative effects of large volume fluctuation of sulfur during operation,but also maximize the exposure of catalytically active sites toward Li PSs conversion.Density functional theory?DFT?calculations are conducted to get deeper insight into the effect of sulfur-atom vacancy defects in CO₃S₄-DHS on the electronic structure and adsorption capability of lithium polysulfides,which verifies that CO₃S₄-DHS is more effective in anchoring Li PSs intermediates.The Co₃S₄-DHS/S based cathode materials in Li-S batteries exhibit a reversible specific capacity of 1090 m Ah g^-1at 0.1 C and 750 m Ah g^-1at 1C,together with an ultra-low capacity decay rate of 0.17%per cycle within 400 cycles at 1 C.This work enrichs the way to fabricate defect-rich transition metal,and provides new guidance for the application of defect engineering in efficient sulfur host matrix for Li-S batteries.?2?we report the design and fabrication of ultrathin MoO₂nanosheets wrapped Fe₄O₃-Fe₃C nanotube heterostructures?MoO₂/Fe₄O₃-Fe₃C@NC?via the thermal treatment of Prussian blue analogue Fe₄[Fe?CN?₆]₃,which has been coated with Mo-polydopamine?Mo-PDA?.The ultrathin MoO₂render considerable electroactive surface area and sufficient active sites,which significantly accelerate the ion mobility or mass transport and promote the catalytic reactions.Meanwhile,the excellent conductivity framework of polar Fe₄O₃,Fe₃C and N-doped carbon can also propel redox reaction kinetics through catalyzing Li PSs conversion.Moreover,the heterojunction can enhance surface polarization and lower the reaction energy barriers of surface catalysis,eventually leading to strong chemical trapping and realized smooth trapping-diffusion-conversion of Li PSs.Benefitting from these favorable advantages,the MoO₂/Fe₄O₃-Fe₃C@NC as a sulfur host delivers superior electrochemical performances,i.e.,high initial specific capacity(1169 m Ah g^-1at 0.1 C)and superior cycling stability(0.055%capacity decay cycle^-1up to 900 cycles at 1 C),holding a great promise in promoting the practical application of Li-S batteries.This idea of synergistic catalysis through a multi-component heterostructure catalyst provides a new direction for the design of high-performance lithium-sulfur battery cathode materials.