Preparation Of Multi-doped Carbon Nanomaterials And Electrochemical Performance Of Lithium-sulfur Batteries

In recent years,with the continuous development of society,traditional lithium-ion batteries have become increasingly unable to meet the current demand for high energy density energy storage in terms of energy density.Due to the high theoretical specific capacity(1675 m Ah g^-1)and energy density(2600 Wh Kg^-1),lithium-sulfur(Li-S)batteries are used as a new generation of high performance energy storage systems.However,the development of lithium-sulfur batteries is mainly limited by several fatal shortcomings,such as the insulation of both elemental sulfur and Li₂S₂/Li₂S.lithium dendrite growth during electrode reaction,and the“shuttle effect”related to the dissolution of lithium polysulfide(Li PS)in the electrolyte.Therefore,the development of a new type of sulfur-based composite material that improves the conductivity of the sulfur cathode,restricts the growth of lithium dendrites,and effectively inhibits the dissolution of lithium polysulfide is the key to promote the performance of Li-S batteries.This article focuses on solving the problems of the insulation of sulfur and Li₂S₂/Li₂S,and the shuttle effect formed by the dissolution of lithium polysulfide,and mainly fabricates the multi-doped carbon nanocomposite materials to promote the conductivity,and the adsorption of Li PS through both physical restriction and chemical interaction by non-metals(B/N)heteroatoms and transition metal nanoparticles for high-performance Li-S batteries.Meanwhile,the functional mechanisms of these materials on enhancing the battery performance are revealed.Firstly,B/N-codoped hierarchically porous carbon nanosheets(B-N-CSs)was prepared via a direct pyrolysis of the mixture urea and boric acid as efficient sulfur host for lithium-sulfur battery.The graphene-like B-N-CSs provides high specific surface area and porous structure with abundant micropores(1.1 nm)and low-range mesopores(2.3 nm),thereby constraining the sulfur active materials within the pores.More importantly,the codoped B/N elements can further enhance the polysulfide confinement through strong Li-N and B-S interaction based on the Lewis acid-base theory.These structural superiorities significantly suppress the shuttle effect by both physical confinement and chemical interaction,and promote the redox kinetics of polysulfide conversion.When evaluated as the cathode host,the S/B-N-CSs composite displays the excellent performance with a high reversible capacity up to772 m Ah g^-1at 0.5 C and a low fading rate of?0.09%per cycle averaged upon 500cycles.In particular,remarkable stability with a high capacity retention of 87.1%can be realized when augmenting the sulfur loading in the cathode up to 4.6 mg cm^-2.Secondly,in order to further enhance the catalytic performance of host material for the electrochemical reaction of the sulfur cathode,porous carbon nanosheets modified with metallic cobalt and nonmetallic nitrogen/boron heteroatoms,denoted as Co-NBC,is reported as sulfur immobilizer.The presence of conductive metallic cobalt as well as the doped N/B heteroatoms can enhance the electronic conductivity of carbon matrix,and significantly improve the chemical entrapment of soluble polysulfides.Kinetics measurements show that metallic Co catalysts can greatly promote the polysulfide redox kinetics of both liquid-liquid and liquid-solid conversion.As a result,the S/Co-NBC composite realizes remarkable rate performance with high specific capacity of 509 m Ah g^-1at a high C-rate of 5 C,and excellent cycle stability with a low capacity fading rate of?0.09%per cycle within500 cycles.Moreover,the S/Co-NBC cathodes realize promising cycle performance even at harsh working condition of high sulfur loading(?4.6 mg cm^-2)or low electrolyte addition(?3?L mg^-1).This work proposes a meaningful strategy to obtain efficient carbon nanomaterials for high-performance Li-S batteries.Thirdly,we report an electrocatalyst based on nickel nanoparticles encapsulated in N-doped carbon nanotube(Ni@NCNT)as kinetics regulators for Li-S batteries.These metal Ni nanoparticles exhibit a durable electrocatalytic effect to propel the polysulfide-involving multiphase transformation's redox kinetics.When used as a modified layer on a commercial separator,the Ni@NCNT interlayer contributes to stabilizing both the S cathode and Li anode by retarding the dissolution and the diffusion of soluble polysulfides.Even with a 70 wt%sulfur loading at the cathode,the Li-S batteries based on Ni@NCNT interlayer achieve promising cycle stability at a low electrolyte/sulfur ratio of 3?L mg^-1.Moreover,a high areal capacity of 7.5m Ah cm^-2could also be achieved at a sulfur loading of 12.2 mg cm^-2.This work proposes a simple strategy to explore durable catalysts for high energy Li-S batteries under high sulfur loading and lean electrolyte conditions.Finally,A dual-regulation strategy by adopting B/N co-doped carbon nanotube-encapsulated nickel nanoparticles(Ni@BNCNT)as both sulfur host and separator coating is proposed for high-performance Li-S batteries.In the cathode side,the 3D conductive network structure of Ni@BNCNT is favorable for high sulfur utilization,and the synergistic effect between polar metal Ni nanoparticles(NPs)and B/N heteroatoms can enhance the chemical adsorption of polysulfide intermediates.In addition,these Ni NPs can serve as catalysts to promote the redox kinetics of polysulfide conversion.Moreover,the employment of the Ni@BNCNT interlayer can further capture the soluble polysulfides,make them convert quickly,and prevent them from diffusing toward Li anode.As a result,the Li-S batteries equipped with S/Ni@BNCNT cathode and Ni@BNCNT interlayer delivers high reversible capacity and good cycle stability.Even at high sulfur loading of 3.5 mg cm^-2and low electrolyte/sulfur ratio of 3?L mg^-1,the excellent performance can also be achivevd for the Li-S batteries.Therefore,based on the effective combination of catalytic host and separator coating,this work offers a new concept of constructing high-performance Li-S batteries.In summary,multi-doped carbon nanocomposites were designed and prepared for inhibiting the polysulfide dissolution and shuttling,and promoting the redox kinetics of lithium polysulfide conversion,These results might lay a theoretical foundation for the construction of high performance lithium sulfur battery energy storage system and its commercial application.