Design Of Key Materials And Electrochemical Performance Study For Sulfur Cathode/Lithium Metal Anode Of Lithium-Sulfur Battery

In recent years,with the rise and vigorous development of various kinds of portable energy storage equipment,developing novel energy storage devices with high safety,high specific energy and long-cycling life has become a current research focus.Among many new energy storage systems,lithium-sulfur battery is widely considered to be one of the most promising lithium secondary batteries due to its ultra-high theoretical capacity,low-cost of raw materials and environmental friendliness.However,during the charging and discharging process of lithium-sulfur battery,the poor electronic conductivity of sulfur and lithium sulfides,the“shuttle effect”of lithium polysulfide,the uncontrollable growth of lithium dendrites,and the huge volume changes existing in both positive and negative electrodes would lead to low utilization of the active materials,poor cycling stability of the batteries,as well as the possible safety hazards like fire,these unsatisfied factors severely restrict its large-scale commercial application.To address these above-mentioned issues,this paper mainly focuses on two aspects:designing innovative cathode materials to improve the utilization of sulfur and inhibit the“shuttle effect”of polysulfides intermediates,thus improving the cycling stability of sulfur cathode.And constructing composite lithium metal anodes or adopting interface protection to guide the uniform lithium deposition and alleviate the volume expansion of lithium metal anode,thereby significantly improving the Coulombic efficiency and cycling life.Afterwards,the improved sulfur cathode is paired with the selected composite lithium metal anodes to build high-stability lithium-sulfur battery.The details are summarized as following:（1）To effectively restrain high solubility and shuttle effect of polysulfides during cycling,Co S_x in a composite with nitrogen doped carbon and sulfur（Co S_x-NC-S）has been successfully synthesized from zeolitic imidazolate framework-67（ZIF-67）,applied in the lithium-sulfur battery as cathode.The porous N-doped carbon polyhedra can not only improve the electronic conductivity,but also relieve the volume expansion of sulfur during cycling.Moreover,the physical restriction of polysulfides by N-doped carbon polyhedra could cooperate with the powerful attraction of polysulfides by Co S_x nanoparticles embedded in the polyhedra to significantly inhibit the dissolution and diffusion of polysulfides,which is beneficial to the stable cycling behavior of electrodes.Benefiting from these advantages,the Co S_x-NC-S electrode delivers enhanced electrochemical performance.Even at the high rate of 2 C,the capacity of CoS_x-NC-S electrode can still remain at 570 mAh g^-1 after 600 cycles.（2）To prevent the formation of Li dendrites,a flexible three-dimensional（3D）conductive scaffolds（Zn@NC@CC）composed of Zn nanoparticles uniformly embedded in N-doped carbon polyhedra homogeneously built on carbon cloth has been successfully proposed,applied to loading Li metal.Based on theoretical calculation and experimental observation,lithiophilic Zn nanoparticles and N-doping inside of the as-synthesized Zn@NC play a synergistic role in enhancing the adsorption capacity with Li,thus could guide metallic Li uniformly deposite on Zn@NC@CC from top to bottom layers along the carbon fibers and completely suppressed the growing of Li dendrites.Moreover,the porous N-doped carbon polyhedras uniformly distributed on carbon cloth effectively relieves the volume change of Li upon repeated Li stripping/plating process,which contributes to preserving the structural integrity of the whole electrode and hence enhancing its long-term cycling stability.Benefiting from these synergistic effects,after loading the Li capacity of 12 m Ah cm^-2,the Li-Zn@NC@CC electrode delivers a prolonged lifespan of over 1200 h at 1 m A cm^-2 with an areal capacity of 1 m Ah cm^-2 in symmetric cells.（3）A versatile interlayer composed of LiN₃ doped PAN nanofibers（Li N₃@PAN）has been prepared by electrospinning method to further protect the lithium metal anode.During the charging and discharging process,Li₃N with ultra-high ionic conductivity(6×10^-3 S cm^-1)could be obtained from the decomposition of Li N₃,which can greatly improve the ionic conductivity of the PAN nanofibers,thus accelerating the transfer kinetics of Li ions.Meanwhile,the formation of N₂ can react with Li⁰ resulting in additional Li₃N film in-situ covered on the surface of lithium matel to provide protection.In addition,the electrically insulating PAN matrix can not only adapt to the volume change of Li during cycling,but also help to attain uniform deposition of metallic Li and growth from bottom to top on the substrate under the guidance by the polar surface functional groups,thus eliminating the growth of lithium dendrites.Therefore,under the protection of LiN₃@PAN interlayer,the Li metal anode shows an ultra-long stable cycling life of over 4000 h at 1 m A cm^-2 in symmetric cells.（4）A novel“House Strategy”has been proposed to design a stable Li metal anode.That is utilizing the 3D Ni O nanosheets decorated nickel foam as the frame to absorb molten lithium and the organic[Li NBH]_n chains with high ionic conductivity and mechanical strength as the artificial protective proof to establish a stable dendrite-free Li NBH-Li@Ni anode.Benefiting from the synergistic effects of 3D Ni skeleton and the[Li NBH]_n protective layer,the Li NBH-Li@Ni electrode could not only accommodate the volume change and maintain the structural integrity,but also avoid the severe corrosion of the electrolyte or polysulfides from the positive side.Therefore,the Li NBH-Li@Ni electrode presents a long-term cycling lifespan of over 800h at both 1 m A cm^-2 and 3 m A cm^-2 with an high areal capacity of 5 m A h cm^-2 in symmetric cells.Impressively,upon coupling LiNBH-Li@Ni anode with a CoS_x-NC-S cathode,this Li NBH-Li@Ni//Co S_x-NC-S full cells also display an excellent cycling stability.