Preparation Of Transition-metal Composites And Their Application In Lithium Sulfur Batteries

In order to meet the needs of the global electronics market,it is imperative to develop high-energy-density energy storage technologies.Lithium-sulfur batteries with a high theoretical specific capacity of 1675 m Ah g^-1 and an energy density of 2600 Wh kg^-1 are considered to be one of the most promising new energy storage devices and will replace lithium-ion batteries.However,sulfur cathodes still suffer from the poor electrical conductivity of S and Li₂S₂/Li₂S,the shuttle effect of polysulfides,and the slow conversion of lithium polysulfides.At the same time,the excessive change of the cathode volume during the process of redox reaction also hinders the improvement of battery performance.To solve the above problems and promote the practical application of lithium-sulfur batteries,this thesis carries out relevant experimental research on the design and preparation of transition-metal composite materials and their application in lithium-sulfur batteries.The main research contents of this paper are as follows:1.Mo₂C@CNF was prepared by electrospinning and carbothermal reduction.Mo₂C was located on the surface of carbon fiber with the good conductivity and catalytic activity.The composite material can promote the conversion of lithium polysulfide and adsorb lithium polysulfide.The structure of the 3D network also improves its performance.When it was used as the cathode host material for lithium-sulfur batteries,after 100 cycles at 0.1 C,the capacity was 706 m Ah g^-1,and the average decay rate was 0.37%.2.Co₃O₄@CNF and Mn O/Co-CNT@CNF composites were prepared by electrospinning and thermal treatment.Co₃O₄ has strong adsorption energy for lithium polysulfides.By loading it on carbon nano fibers,the advantages of both can be combined to achieve the synergy of physical confinement and chemical adsorption.The assembled lithium-sulfur battery exhibits an average decay rate of 0.61%per cycle at a current density of 0.1 C.Its poor cycling performance is due to the lack of the catalytic effect of Co₃O₄ for lithium polysulfides and the unsatisfactory electrical conductivity.To address this issue,a freestanding interlayer Mn O/Co-CNT@CNF was synthesized.Carbon nanofibers（CNF）and carbon nanotubes（CNT）form a three-dimensional network substrate that provides fast transport channels for electrons and ions.Mn O and Co can catalyze and adsorb lithium polysulfides.Benefiting from the coordination of each component,the capacity of the assembled lithium sulfur battery has a capacity of 1064 m Ah g^-1after 100 cycles at 0.2 C.It has a capacity of 713 m Ah g^-1 when the current density was up to 5 C.3.Fe₃O₄/Fe_xS_y composites were prepared by the combination of electrochemical deposition technology and vacuum heat treatment technology.This method has advantages of high controllability,simple manipulation and rapid synthesis.The synthesized composites can produce synergistic effects to promote the adsorption and transformation of lithium polysulfides.At the same time,good electrical conductivity can increase the utilization rate of lithium polysulfides.When it is used as a host material of lithium-sulfur batteries,the electrode maintains a capacity of 454 m Ah g^-1 after 500 cycles at a current density of 0.5 C,with a capacity decay rate of only 0.067%per cycle.