Design And Synthesis Of Dopamine-derived Carbon-based Nanocomposites For Cathodes Of Lithium-sulfur Batteries

Lithium-sulfur battery is considered to be one of the most promising energy storage devices due to its high theoretical energy density of 2600 W h kg-1,combined with the merits of the sulfur cathode including low cost,natural abundance,and nontoxicity.Despite these advantages,the sulfur cathode still suffer from some issues during the discharge and charge process.Firstly,the ionic and electrical insulation of sulfur and its solid discharge products(Li2S2/Li2S)results in the low utilization of active materials.Secondly,intermediary product polysulfides are soluble in the ether-based electrolyte and diffuse to anode,thus triggering"shuttle effect".Thirdly,owing to the different density of sulfur and LiS2/Li2S,the electrode endures large volumetric expansion and shrinkage,which adversely affects its integrity and stability.These issues finally lead to low capacity,poor rate capability and severe capacity decay,thus hindering the commercial application of lithium-sulfur battery.In this study,we focus on the structure design and surface modification of various dopamine-derived carbon-based nanocomposites as sulfur hosts to meet the requirement of the advanced cathodes for high-performance lithium-sulfur batteries.Nitrogen-doped tubular/porous carbon and graphene composites(NTPC-G)were synthesized via a polymerization/coating strategy with dopamine as nitrogen-doped carbon precursors,graphene sheets as the substrates and NiCo-carbonate hydroxide nanowires as templates and porogen.In the hierarchically sandwich-type architecture,graphene functioned as conductive networks and flexible frameworks,which are favorable for high electrical conductivity and structure stability of the cathode;Nitrogen-doped hollow porous carbon nanorods loading on the surface of graphene sheets and nitrogen-doped porous carbon coating layers served as polysulfide reservoirs,which can effectively suppress the diffusion of polysulfides via physical confinement and chemical adsorption.When employed as the cathode materials of lithium-sulfur battery,NTPC-G and sulfur composites(S@NTPC-G)delivered a high rate capability(563 mA h g-1@6 C)and a good cycle stability up to 600 cycles.Cobalt-embedded nitrogen-doped hollow carbon nanorods(Co@NHCRs)were synthesized by using cobalt carbonate hydroxide nanorods derived from a hydrothermal method as cobalt precursor and templates as well as dopamine as carbon and nitrogen source.Metal cobalt and highly graphitized carbon layer were favorable for high electrical conductivity of the cathode,which can facilitate the reaction kinetics.The hollow and porous structure of carbon nanorods provided enough space for confining sulfur and polysulfides.Theoretical calculation revealed that the introduction of nitrogen heteroatoms and cobalt nanoparticles can synergistically modulate the electron distribution of carbon surface and thus effectively strengthen the affinity for Li2S2/Li2S,which can ensure the good electrical contact between Li2S2/Li2S and conductive substrate.On integrating these fascinating benefits into one electrode,Co@NHCRs and sulfur composites(Co@NHCRs/S)achieved a high reversible capacity(971 mA h g-1@0.5 C),a good cycling stability(a high capacity retention rate of 73%after 100 cycles),and a good rate capability(747 mA h g-1@1 C).CoO/Co3O4 coupled with nitrogen-doped hollow carbon nanospheres(CoOx-NC)were synthesized via an in situ complexation and polymerization of Co ions and dopamine with polystyrene nanospheres as templates.The fabrication method that avoided additional steps to remove templates was facile and controllable.The strong coordination effect between metal ions and organic ligands as well as the nanoscale Kirkendall effect endowed CoO/Co3O4 with small particle size and hollow structure,which were beneficial to providing more polar sites for the adsorption of polysulfides and Li2S2/Li2S.Moreover,the intimate coupling between CoO/Co3O4 and the conductive carbon shells improved the electrical conductivity of metal oxides.When employed as the cathode materials of lithium-sulfur battery,CoOx-NC and sulfur composites(S@CoOx-NC)delivered a high capacity(655 mA h g-1)and a long cycling life up to 400 cycles.