Construction Of Porous Carbon-based Materials And Their Application In High-performance Room Temperature Sodium-sulfur Batteries

Room temperature sodium-sulfur（RT Na-S）batteries have been considered as one of the most promising candidates for next generation energy storage technology owing to their high theoretical energy density up to～1274 Wh kg^-1 and low cost of the ecofriendly sulfur.However,the commercialization of RT Na-S batteries is still severely impeded by some technical obstacles,such as low utilization of sulfur,poor cycling performance and rate performance.These issues are mainly caused by the nature of insulated sulfur and its nonconductive discharge products（Na2S2 and Na2S）,volume expansion of active materials（170%）,as well as the dissolution of of sodium polysulfide intermediates and the resulting“shuttling effect”during the charge/discharge processes.Therefore,the rational design and and preparation of multifunctional sulfur host materials with high conductivity,strong polysulfide-trapping capability and high catalytic activity for polysulfides conversion are the key factor to improve the electrochemical performance of RT Na-S batteries.In this paper,in order to improve the conductivity of sulfur cathode,inhibit the shuttle effect and catalyze the conversion of sodium polysulfides,three kinds of multifunctional carbon materials based on the structure tailoring and components modification are developed as sulfur host materials,thus achieving improvement in rate performance and cycling performance of RT Na-S batteries.The main research contents and results are displayed as follow:（1）A nitrogen-doped porous carbon（NPC-T）sheet was prepared by adjusting the activation temperature,in which polyvinyl pyrrolidone（PVP）simultaneously acted as a carbon and nitrogen source,and Zn Cl2 acted as an activator.It was found that the specific surface area and nitrogen doping amount of carbon nanosheets increased firstly and then decreased with the increase of temperature.When the temperature was controlled to be 700 ^oC,the NPC-700material had the highest specific surface area(918.3 m² g^-1)and doping nitrogen content（5.5at%）.Benefiting from the large specific surface area,abundant microporous structure,and nitrogen-doping,the obtained NPC-T sheets can significantly boost space confinement for sulfur molecules and effectively alleviate the dissolution of sodium polysulfides during the charge-discharge processes in RT Na-S batteries.As a result,the optimum sample（S@NPC-700）was used as the cathode of a RT Na-S battery,which delivered a high initial specific capacity of 1213.9 m Ah g^-1 at 0.1 C with a high capacity retention of 74.22%（based on 2 nd cycle）after 200 cycles,as well as a high rate performance with a specific capacity of 280.9m Ah g^-1 at 2.0 C and a reversible capacity of 418.9 m Ah g^-1 at 0.5 C after 400 cycles.（2）A new type of three-dimensional（3D）hierarchical porous carbonaceous nanocubes（CNTs/Co@NC）derived from bimetallic Zn Co-ZIFs were prepared as efficient sulfur hosts,composed of carbon nanotubes（CNTs）and Co nanoparticles（NPs）uniformly embedded into a nitrogen-doped carbon matrix（NC）.Such 3D hierarchical porous carbon nanocubes inherited the multiple advantages,including high specific surface area(637.2 m² g^-1),large degree of graphitization,and uniformly distributed Co-N active sites.Firstly,the 3D hierarchical porous structure provided abundant space to accommodate sulfur loading and volume change.Secondly,the growth of CNTs can efficiently enhance the conductivity and stability of the carbon skeletons.Thirdly,Co NPs-embedded and N-doped carbon with strong polarity not only significantly increased polysulfides immobilization,but also efficiently catalyzed sulfur redox reactions,as confirmed by experimental results and DFT calculations.When tested in a RT Na-S battery,the S@CNTs/Co@NC-0.25 cathode demonstrated outstanding electrochemical performance,achieving high initial specific capacity of 1200.3 m Ah g^-1 at 0.1 C,remarkable rate capability up to 5.0 C(474.2 m Ah g^-1),and superior cyclic performance of 292 m Ah g^-1after 400 cycles at 5.0 C.Even at a high areal sulfur loading of 3.0 mg cm^-2,the S@CNTs/Co@NC-0.25 cathode still showed an outstanding cyclability with a reversible capacity of 427.6 m Ah g^-1 after 200 cycles at 0.5 C.（3）A template-assisted pyrolysis strategy was designed to synthesizeα-Mo C1-x decoratedhoneycomb nitrogen-doped carbon hollow spheres（Mo C@HNC）.Novel core-shell architectures（PS/PMA@ZIF-8）consisting of polystyrene（PS）cores and ZIF-8 composite shells encapsulated with phosphomolybdic acid hydrate（PMA）by self-assembling were synthesized,followed by a thermal treatment to fabricate hollow materials composed ofα-Mo C1-x nanoparticles homogeneously distributed in honeycomb nitrogen-doped carbon shells.The as-prepared Mo C@HNC-15 possessed high specific surface area(680.9 m² g^-1),large degree of graphitization,and uniformly distributedα-Mo C1-x nanoparticles.Benefiting from the structural and compositional advantages,the designed hollow architecture of Mo C@HNC-15can encapsulate more sulfur and relieve volume expansion of sulfur during cycling.Secondly,the nitrogen-doped carbon shell derived from the stacking of PMA@ZIF-8 can effectively improve the electrical conductivity and provide more active sites for polysulfides adsorption.Finally,the modified polarα-Mo C1-x nanoparticles not only had a strong chemical anchoring for polysulfides,but also manifested exceptional electrocatalysis for polysulfides,realizing the synergistic effect of structure-adsorption-catalytic regulation.Therefore,the structural and compositional advantages rendered S/Mo C@HNC-15 with a high rate capability of 351.8 m Ah g^-1 at 10.0 C,and long cycling stability（a low capacity decay of 0.028%per cycle at 1.0 C after1000 cycles）.Even at a high areal sulfur loading of 3.0 mg cm^-2,the S/Mo C@HNC electrode also showed a good rate performance.