Precise Tuning Of MOF-derived Carbon Materials And Their Application In Lithium-sulfur Batteries

With the rapid development of the global economy,lots of energy and environmental issues are emerging,and the search for new energy storage systems has become a general trend.Recyclable lithium-ion batteries have been widely used because of their long battery life,rechargeable capacity,and high safety performance.However,limited by its theoretical capacity and high cost,lithium-ion batteries can no longer meet the increasing energy demand.Therefore,lithium-sulfur(Li-S)batteries have attracted widespread attention due to the high theoretical specific capacity of 1675mAh g^-1and a specific energy density of 2600 Wh kg^-1.However,there is a core problem in Li-S batteries:the“shuttle effect”.That is,during the charging and discharging process,part of the intermediate products(polysulfides)are subjected to the combined effect of concentration polarization and electric field force,and shuttle between the positive and negative electrodes,thereby resulting in a“shuttle effect”.Shuttled polysulfide may deposit on the surface of the negative electrode and generate non-conductive lithium sulfide,resulting in short cycle life and low efficiency.In addition,the poor conductivity of sulfur,the growth of lithium dendrites and the volume change of sulfur during the electrochemical process also affect the practical application of Li-S batteries.In the past few decades,some strategies have been proposed to alleviate the above problems,among which positive electrode modification and separator modification are the most common and effective methods.Cathode modification refers to the combination of sulfur and conductive materials.Conductive materials are generally used as the main host of sulfur to relieve the volume expansion of sulfur during charging/discharging and improve the conductivity,thereby achieving the purpose of improving battery performance;Separator modification generally refers to the design of a suitable material to be coated on the separator,with the containment of the modified material and the chemical bonding principle of non-polar functional groups to stop and reuse the polysulfides that are to be shuttled.Recently,carbon materials derived from metal-organic frameworks(MOFs)are very advantageous to be host or separator modifier in Li-S batteries due to the high BET surface areas,good conductivity,tailorable pore structure,and easy functionalization(e.g.heteroatoms doping and active species loading).The research is carried out from two aspects of cathode and separator modification for the“shuttle effect”of Li-S batteries.Firstly,a new MOF-derived nitrogen-doped carbon nanotube/nanoparticle network structure containing active Fe₃C nanoparticles(referred to as Fe CN)is designed and synthesized.The Fe CN are used as sulfur host to be applied to positive electrode modification,and the influence of componential and structural properties of the carbon as the main sulfur host of Li-S batteries on the“shuttle effect”is studied.Secondly,a ZIF-derived fluorine and nitrogen co-doped porous carbon material(referred to as F-N-C)is designed and prepared.Then,F-N-C was coated on the surface of the separator,and the heteroatom doping effect of the F-N-C coating separator on the“shuttle effect”is explored.The specific research contents are as follows:(1)Iron graphene quantum dots(FeGQDs)are prepared by a hydrothermal method,and then FeGQDs are introduced into the crystal structure of ZIF-8 to obtain Fe GQD@ZIF-8 composite precursors.After carbonization in a nitrogen atmosphere,carbon nanotubes are in-situ formed by catalytic growth by FeGQDs,thus a nitrogen-doped carbon nanotube/nanoparticle network is constructed.Next,the temperature-dependent pyrolysis of the precursor was carried out to investigate the structure evolution of the Fe CN.Finally,the influence of components and structure of Fe CN on the“shuttle effect”as the host in Li-S battery is discussed.The electrochemical results show that when the carbonization temperature is 900?,the prepared sample S/Fe CN-900 gets the best electrochemical performance due to its optimized active components and most developed network structure.At a current density of 0.5 C,the initial discharge specific capacity reached 1252 mAh g^-1,and the specific capacity remained at 533 mAh g^-1 after 500 cycles,and the decay rate per cycle was 0.11%.(2)We successfully prepared a fluorine and nitrogen co-doped porous carbon material via a fluorinating process of ZIF-8 derived nitrogen doped carbon nanoparticles(N-C-1000)by a hydrothermal method.Then,it was coated on one side of the separator as a membrane modification material.The introduction of F brings F-N-C-1000 more electronegativity and polarization,so that polysulfides can be effectively adsorbed and transformed before passing through the separator,with enhanced Li-S batteries performance.Compared with batteries using N-C coated separators,batteries using F-N-C coated separators have greatly improved cycle and rate performance.After 500 charge-discharge cycles at 1 C,a reversible specific capacity of 640 mAh g^-1 is retained;at a high rate of 5 C,the specific discharge capacity reaches 500 mAh g^-1.