Construction Of Active Sites In Heterogeneous Framework Host And Kinetics Of Enhanced Conversion Reaction For Lithium-Sulfur Batteries

Due to the multi-electron conversion reaction of elemental sulfur from polysulfide to lithium sulfide(Li2S),lithium-sulfur batteries(LSBs)have a theoretical energy density of 2600 Wh kg-1,which is much higher than the current commercial lithium-ion batteries(LIBs),Moreover,the theoretical specific capacity of sulfur as active materials is as high as 1672 mAh g-1,which has the characteristics of non-toxic,cheap and easy to obtain,and rich natural reserves.These advantages make LSBs become the main research hotspot in the next generation of energy storage system.However,some shortcomings seriously hinder the practical application of LSBs,such as,the electronic insulation of sulfur and its discharge products(Li2S2/Li2S),volume change of cathode and anode during charging/discharging process,the dissolution of polysulfide and the Li dendrite growth on anode.In recent years,researcher have devoted to design polar porous materials or artificial SEI layer in order to improve the electrode conductivity,inhibit the dissolution of poly sulfides and alleviate the growth of lithium dendrite,but the sluggish polysulfide conversion reaction and Li-ions migration rate still hinder the improvement of the rate performance and actual energy density of LSBs.In order to solve these above problems,this thesis aims to design a series of heterogeneous framework host materials with affluent active sites(catalytic or lithiophilic sites),optimize the ability of sulfur cathode to adsorb polysulfides and improve the conversion reaction kinetics,reduce the migration barrier of Li-ions through SEI layer on Li anode,and regulate its plating/stripping process,so as to exert the synergistic effect of sulfur cathode and Li anode to enhance the multi-electron conversion reaction kinetics comprehensively thus realizing the commercialization of high-energy-density LSBs.This thesis has carried out the following research works:(1)Co/N co-doped corrugated 2D porous carbons(Co-CNCs)are synthesized by ionothermal method as sulfur cathode host materials,in which Co served as sulfiphilic catalytic sites and N as lithiophilic site.Its unique polar surface can realize the uniform loading and deposition of S/Li2S in the host framework provided by the corrugated 2D carbon layer,which has strong adsorption capacity for poly sulfides and can improve its conversion reaction kinetics.The composite sulfur cathode(S@Co-CNCs)applied to LSBs,which had a high initial specific capacity of 1290.4mAh g-1 at 0.2C,low capacity decay rate per cycle of 0.029%in 600 cycles at 2C,and excellent rate performance up to 20C.Even with sulfur content as high as 92 wt%and sulfur loading of 9.7 mg cm-2,the area specific capacity is still as high as 6.0 mAh cm-2 after 100 cycles.(2)Al-MOFs are used as precursors to prepare ordered ladder-like 3D carbon framework and then combined with loaded MoS2 nanosheets catalyst to construct a nanoreactor with built-in catalytic function(C@MoS2),which is served as sulfur host materials.Thanks to the excellent 2D interface compatibility between MoS2 nanosheets and the conductive carbon layer,C@MoS2 has good sulfur fixation ability and built-in catalytic effect on polysulfides conversion.The composite sulfur cathode S-C@MoS2 with the integration function of adsorption-catalysis-conversion is obtained,and its sulfur content was close to 80 wt%.The obtained LSBs has a high initial specific capacity of 1240.0 mAh g-1 at 0.2 C,long cycle stability of 1000 cycles at 2 C(capacity decay rate per cycle is 0.03%),and high-rate tolerance up to 20C.Even at high sulfur loading(6.0 mg cm-2)and low E/S ratio(5 μL mg-1),LSBs still has considerable capacity output and reversible cycling.(3)The open-architecture MOF film(OA-MOF)with stereoscopic lithiophilic sites is designed to enhance the SEI layer kinetics and optimize the Li-ions plating/stripping process of Li metal anode in LSBs.The appropriate thickness and vertical stacking morphology of MOF(Cu2(BDC)2)nanosheets are beneficial to the interface charge transfer and compact Li deposition morphology.DMF molecules trapped in the open-architecture structure can provide better wetting and dissolution effects,and promote the Li-ion to pass through Cu2(BDC)2.OA-MOF can regulate the Li-ions flux homogeneously,promoting more effective Li deposition.OA-MOF@Li anode shows high coulomb efficiency and low voltage hysteresis even at a high current density of 15 mA cm-2 in Li-Cu cells,and has a persistent small overpotential voltage distribution during the repeated plating/stripping process of Li-Li symmetric cells.The obtained LSBs still shows excellent stability and rate performance at a current density of 5C.(4)A 3D nanoporous coating(MgOx-C)crosslinked by oxygen defects-rich MgOx nanocrystals and carboxyl-rich amorphous carbon is designed by one-step thermal decomposition of Mg-MOF,serving as Li anode host and artificial SEI layer simultaneously,and realizing uniform Li deposition thus improving multi-electron conversion reaction kinetics in LSBs.The carboxyl groups serve as lithiophilic sites to guide the Li-ions flux uniformly,while the oxygen defects act as anionphilic sites to adsorb the electrolyte anion to reduce the depletion of space charge near the anode.The conversion and alloying reaction between Li and MgOx can form uniform Li nucleation center after lithium is removed,which is beneficial to improve the cycle stability of subsequent lithium anode.The results show that the Li@MgOx-C anode has stable Li plating/stripping process under capacity of 10mAh cm-2 and current density of 15mA cm-2,respectively,and has a low overpotential and a high coulomb efficiency.Even at a high rate of 5C,the two typical discharge plateau characteristics are well maintained in obtained LSBs.