Configurational Design And Electrochemical Performance Of Cathode Materials For Novel Lithium-sulfur Batteries

Lithium-sulfur(Li-S)batteries are a novel energy storage technology with the merits of high theoretical specific capacity,high energy density,low cost and environmental benign.In recent years,Li-S batteries have developed rapidly,and how to further enhance the electrochemical performance of Li-S batteries becomes the research topic in current energy storage fields.Based on the perspective and advancement of Li-S batteries and the research progress of our group,this dissertation will concentrate on the design of sulfur host of cathode and protection of lithium anode during charge/discharge processes,and propose several strategies for enhancing the electrochemical performance of Li-S batteries.The main contents of this dissertation are summarized as follows:1.Cerium oxides(Ce02)embedded micro-mesoporous carbon nanospheres as the sulfur host material for Li-S batteries.An effective sulfur host was prepared by implanting CeO2 nanocrystals homogeneously into well-designed bimodal micro-mesoporous nitrogen-rich carbon(MMNC)nanospheres.With the high conductivity and abundant hierarchical pore structures,MMNC nanospheres can effectively storage and entrap sulfur species.Moreover,the inserted polar CeO2 nanocrystals and high nitrogen content of MMNC can synergistically solve the hurdle of the polysulfide dissolution and furthermore significantly promoting stable redox activity.By combining these advantages,CeO2/MMNC cathode(1.4 mg cm-2 sulfur)based Li-S batteries exhibit high reversible capacities,good rate capability(737 mAh g-1 at 2.0 C),and high cycle stability.Increasing the areal sulfur loading to 3.4 mg cm-2,a high and stable discharge Lapacity of 611 mAh g-1 is achieved after cycling for 200 cycles.2.Flexible tubular sulfur host material with strong capillary and chemical adsorption ability for Li-S batteries.A novel highly-flexible sulfur host that can simultaneously meet the flexibility,capacity and stability requirements of flexible Li-S batteries was prepared.The host is consisted of crisscrossed network of carbon nanotubes(CNTs)reinforced CoS nanostraws(CNTs/CoS-NSs).The CNTs/CoS-NSs with large inner space and high conductivity enables a large loading content and efficient utilization of sulfur.Moreover,the strong capillarity effect and chemisorption of CNTs/CoS-NSs with sulfur species can efficiently suppress the shuttle effect and promote the redox kinetics of polysulfides.It was revealed that the S@CNTs/CoS-NSs cathode based Li-S batteries exhibit excellent electrochemical performance including high capacity,remarkable rate capability and stable cycling performance.Intriguingly,the soft-packaged Li-S batteries based on S@CNTs/CoS-NSs cathode with high areal sulfur loading show excellent flexibility and stability upon bending(5.05 mAh Cm-2 at 0.1 C).This study opens up the possibility to integrate flexible Li-S batteries with smart wearable devices.3.Porous-shelled VN nanobubbles as the host materials for high-capacity and long-cycling Li-S batteries.Porous-shell vanadium nitride nanobubbles(VN-NBs)were taken as the efficient sulfur host for Li-S batteries.The large inner space of VN-NBs can afford a high sulfur content and accommodate the volume expansion,and the high electrical conductivity of VN-NBs ensure the effective utilization and fast redox kinetics of polysulfides.Moreover,VN-NBs present strong chemical affinity/adsorption with polysulfides,thus can efficiently suppress the shuttle effect via both capillary confinement and chemical binding,and promote the fast conversion of polysulfides.Benefiting from the above merits,the Li-S batteries based on sulfur-filled VN-NBs cathodes exhibit impressively high areal/specific capacity(5.81 mAh cm-2),superior rate capability(632 mAh g-1 at 5.0 C)and long cycling stability,even when the areal sulfur loading is very high(5.4 mg cm-2).4.Three-dimensional spongy frameworks as polysulfide adsorption and reservoir layers for Li-S batteries.An advanced strategy by incorporating a three-dimensional spongy framework as the polysulfide reservoir layer was proposed to enhance the polysulfide retention in Li-S batteries.The spongy framework is a hierarchical architecture composed of highly-conductive Ni foam/graphene/carbon nanotubes/MnO2 nanosheets(NGCM).The strongly-interconnected Ni foam,graphene and carbon nanotubes of NGCM sponge ensure the fast electron transfer during charge/discharge processes,and the porous Mn02 nanosheets provide strong chemisorptive and electrocatalytic effects on polysulfides(as confirmed theoretically and experimentally).The NGCM as a polysulfide reservoir layer attached on conventional sulfur-mixed carbon nanotubes(S/CNTs)cathode can bring significantly-improved reversible capacity,rate capability(593 mAh g-1 at 3.0 C)and cycling stability.5.Nanoporous regenerated eggshell membrane(RESM)film for suppressing the formation and growth of dendritic lithium.A novel battery separator(RESM)was fabricated from eggshell membrane(ESM)to alleviate the influence of dendritic lithium on the electrochemical performance of Li-S batteries.The separator possesses several apparent advantages including porous structure,high lyophilic,high thermal stability and good mechanical properties,which can meet all the requirements of an ideal battery separator.It was revealed that the separator can efficiently suppress the growth of dendritic lithium,and protect the lithium metal anode from been attacked by dissolved polysulfides.Therefore,compared with commercial separator,the Li-S batteries with RESM separator exhibited greatly enhanced electrochemical performance in terms of discharge capacity,rate capability and cycling performance.