The Application Research On Nano-magnesia-Modified Crab Shell-based Hierarchical Porous Carbon Materials In Lithium-sulfur Battery

The rapid development of electronic products and electric vehicles has aroused the demands for secondary batteries with high specific energy and high cycle stability.Lithium-sulfur batteries are widely used because of their high specific energy(2800Wh L-1),which is as five times as that of lithium-ion batteries.However,due to the insulation of the positive electrode sulfur and the dissolution-deposition reaction mechanism of the lithium-sulfur battery,there is a problem that the utilization rate of the active material is low and the cycle is unstable,which restricts its commercial development.Therefore,this work intends to design and prepare in-situ synthesized nano-magnesia-modified crab shell-based hierarchical porous carbon(n-MgO?CSHPC)to ensure the good contact between polysulfide and conductive structure while efficiently fixing polysulfide.This material can achieve the balance of polysulfide adsorption and reuse,promote the utilization of active materials,and improve the electrochemical performance of lithium-sulfur batteries.Firstly,based on the composition and structure of the crab shell,a hierarchical pore carbon material with good pore structure,microporous pore volume,high specific surface area and hetero atom doping was designed.The magnesium source was fixed with doped heteroatoms,and the nanometer magnesium oxide was successfully loaded in situ in the carbon pores.The structural characterization proves that the magnesium oxide has a uniform particle size distribution and good dispersibility in the carbon material.A controllable preparation method for preparing magnesium oxide modified carbon materials has been developed.Secondly,the nano-magnesia-doped crab shell-based hierarchical porous carbon material was used to prepare carbon-sulfur composite materials,which was used in the cathode of lithium-sulfur battery.Through electrochemical performance tests,the battery with modified cathode was proved that the effective use and fixation of the active material in cathode ensures long-cycle stability.5%nano-magnesia-doped crab shell-based graded porous carbon/sulfur composite cathode has a initial capacity up to 1468.3mAh g-1 at 0.1C,and a initial capacity of 1053.9mAh g-1 at 0.5C and the remaining capacity after 400 cycles is 575.5mAh g-1.Thirdly,the material is used as a functional coating of separator assisting in blocking the shuttling effect of polysulfide and the reconstruction of the positive side structure.During this research,it had been found the modified separator can significantly reduce polarization,promote transportation of lithium ion and improve long cycle performance.The battery with separator modified by 5%nano-magnesia-doped crab shell-based hierarchical porous carbon performs best in fully utilizing the active material and forming the structure of positive electrode side.This battery with the initial discharge capacity up to 949.0 mAh g-1 at 1C can ensure 74.71%capacity retention rate after 300 cycles,even charge and discharge normally after 1000 cycles.