| The continuously growing demand of electric vehicles and renewable energy storage has driven the development of energy-storage technologies to go beyond lithium-ion batteries and attain a higher energy density and lower cost with more eco-friendly chemical composition.Lithium–sulfur?Li–S?batteries,using metallic lithium and elemental sulfur as the anode and cathode materials respectively,have drawn extensive attentions in the pursuing of the next generation of rechargeable batteries due to the high theoretical energy density(2600 kW h kg-1)as well as the low costs and environmental friendless of Li-S system.However,there are still several tough obstacles standing between the appealing theoretical benefits and practical application,including the low conductivity of elemental sulfur and lithium sulfide,diffusion of soluble intermediate lithium polysulfides?LiPS?and drastic volume variation during cycling,which inevitably result in low utilization of active material and fast capacity attenuation.To solve these issues,a series of multifunctional graphene-based hierarchical porous carbon material with novel micro-nano structure and modified surface chemistry were developed as sulfur host materials for the comprehensive improvement the capabilities of electron/ion transfer,LiPS immobilization and volume variation accommodation of the sulfur cathode.Due to the rational multiscale structure design of the hosts,the obtained carbon/sulfur composites exhibit remarkable comprehensive electrochemical performance.The main research contents and results are concluded as follow:?1?A simple synthetic strategy was proposed for the controllable synthesis of 3D hierarchical porous N-doped graphene?3D-PNG?with optimized hierarchical porous structures and high N doping content.In this strategy,urea was employed as both self-removed template and nitrogen source to realize synergetic pore structure optimization and nitrogen doping of 3D porous graphene derived from reduction-induced self-assembly of GO.Compared with conventional 3D rGO areogel,the as-synthesized 3D-PNG possesses enhanced mesoporosities and surface polarity as well as mechanical structure stability while maintaining the 3D interconnected macroporous channel,which makes it a preferable host material for Li-S battery.Owing to the highly efficient multiscale electron delivering and ion transport network as well as improved adsorption ability for polysulfide,the 3D-PNG/S based electrode exhibits high sulfur utilization(1311 mA h g-1 at 0.2 C),high rate capability(950,762 and 580 mA h g-1 at 1,2 and 3C respectively)and excellent cycling stability with high sulfur mass loading(714 mA h g-1 at 1.5mA cm-2 after 400 cycles for a sulfur loading of 4 mg cm-2).?2?A 3D graphene-based N-doped porous carbon?3DG@NPC?with interconnected hierarchical porosities and large accessible surface modified with high-level N doping was prepared as sulfur host material.In this structure,macro-/meso-/micropores are orderly integrated into a 3D conductive network by in-situ growth of hydrochar on self-assembled rGO skeleton and subsequent carbonization and activation,meanwhile the KOH activation process,coupling with urea pyrolysis,creates massive active sites to stimulate high-level N doping?18at%?.The 3D interconnected conductive network,abundant hierarchical porosities and highly polarized surface endow 3DG@NPC comprehensive structural advantage for electrons/ions transport and sulfur accommodation as well as LiPS immobilization.The obtained 3DG@NPC/S composite with 70%sulfur exhibits a high reversible capacity of 1280mA h g-1 at 0.2 C,outstanding rate capability up to 3C and a prolonged cycle life over 500cycles,demonstrating much enhanced electrochemical activity and cycling stability comparing with the counterparts with less pore hierarchy and surface polarity?3DG/S and 3DG@NC/S?.What's more,an impressive specific capacity of 859 mA h g-1 along with remarkable cycling stability can still be maintained with a high sulfur content of 90wt%and a high areal loading of3.6 mg cm-2,revealing a promising potential of 3DG@NPC for practical application.?3?Starting with the uniform deposition of nanosized bimetallic CoZn-ZIFs on GO,3D hierarchical architected Co,N-codoped graphene@CNTs hybrids?Co,N-G@CNT?were synthesized as multifunctional sulfur hosts for Li-S batteries with CoZn-ZIFs serving as the spacers for anti-stacking of graphene and the precursors for self-catalyzed CNTs growth as well as Co and N doping source.The novel hybrid structure constructed by separated graphene sheets and densely distributed small-diameter CNTs with covalent conjunctions possesses high accessible surface(636 m2 g-1)and large nanopore volume(3.42 cm3 g-1)for sulfur accommodation,highly efficient 3D hierarchical network for electron/ion transfer,and robust structure integrity over volume variation as well as enriched doping sites for enhanced chemical interaction with LiPS.These synergetic structure merits endow Co,N-G@CNT/S electrode with high initial discharge capacity(1398 mA h g-1 at 0.2 C),high rate performance(1124,953,803,and 611 mA h g-1 when cycled at,1,2,4 and 6 C respectively)and excellent cycling stability(retained 659 mA h g-1 at 1C even after 1500 cycles with a ultralow capacity degradation rate of 0.035%).Furthermore,a high and stable reversible capacity of 816 mA h g-1(3.42 mA h cm-2)can be achieved after 200 cycles with a practical acceptable sulfur loading of 4.2 mg cm-2.?4?A facile strategy was proposed for the synthesis of nitrogen-doped graphene-based porous carbon embeded with highly exposed CoO nanoparticles as a multifunctional sulfur host material for Li-S batteries,in which multiscale nanostructure construction and surface chemistry modification of carbon matrix were simultaneously achieved by simple pyrolysis process of GO-supported ZnCo-ZIFs.The obtained NGPC@CoO possess microscale interconnected conductive skeleton for highly efficient electron transfer,hierarchical porosities for effective sulfur accommodation and fast mass transportation and a large specific surface with massive active sites for LiPS adsorption and conversion catalysis.The NGPC@CoO/S composites with a high sulfur content of 79wt%exhibits a high reversible capacity of 1303 mA h g-1 at 0.1 C,outstanding rate capability up to 2C and a prolonged cycle life over 600 cycles,demonstrating much enhanced electrochemical activity and cycling stability comparing with the counterparts without CoO hybridization or pore architecture optimization. |
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