Porous Structure Regulation Of The Heteroatoms Dopped Carbon Drevied From Biomass For Lithium-sulfur Batteries

Due to the high theoretical capacity(1675 mA h g^-1)and energy density(²600 W h kg^-1)of sulfur as well as its abundant resource,low-cost,environmental benign and low toxicity,lithium–sulfur batteries?LSBs?have been recognized as promising next-generation batteries and attracted substantial research interests in recent years.Nevertheless,there are still several hindrances should be further addressed for the commercialization of LSBs.Firstly,the inferior electrical conductivity of sulfur/lithium sulfides restricts the efficientive electron transfer,which hampers the active materials utilization,leading to the sluggish kinetics as well as low capacity and rate performance.During the charge–discharge process,soluble intermediates lithium polysulfides?Li₂S_x?x?4?,LiPS?is likely to dissolve in electrolyte and migrate toward anode driven by the concentration gradient,which could not only lead to the severe active materials loss,but the reduced insoluble Li₂S depositing on the lithium surface,giving raise to serious lithium anode erosion.Intuitively,the notorious shuttle effect will substantially cause the low utilization of the active materials,as well as the poor columbic efficiency and rapidly capacity decay.Moreover,the severe volume expansion of sulfur during the charge/discharge inevitably leads to the electrode pulverization and poor contacts between the active materials and conductive framework,which further results in the rapid capacity fading and poor rate performance.This paper devoted to address the shuttle effect from physical and chemical aspects by synthesizing biomass-derived carbon materials with tailored porous structure and heteroatoms doping amount,which could achieve superior cycling stability,high sulfur utilization and low capacity degeneration.Our main works including the following fractions:?1?Dual-funcational SiO₂ layer was adopted as strcutural regent to regulate the pore distribution and geometrical architecture of the hierarchical porous carbon sheets?HPC?using bagasse bio-waste as carbon source.The SiO₂ layer were homogeneously coated on the surface of bagasse via the classical st?ber method taking advatge of the rich surface bonding groups,and then removed by HF after pre-carbonization.Optimiazed HPC with expanded mesopore distribution and thin sheet-like patterns were generated after the KOH activation,which could provide shorter mass transfer paths for accelerated ion transfer,and alleviate the volume expansion of sulfur during lithiation.In addition,the nitrogen/oxygen co-doping sites with intrinsic affinity towards LiPS could well prohibit the shuttle effect by forming Li-N/Li-O bonds,and serves as catalytic nucleation sites for the uniform deposition and propagation of the insolvable Li₂S.With respect to the synergistic effects,the porous electrode delivers an initial capacity of 1123 mAh g^-1 and maintains reversible capacity of 794 mAh g^-1 at 0.2 C after 200 cycles.At high current density of 1 C,an initial capacity of 863 mAh g^-1 and reversible capacity of 467 after 800 cycles with only 0.057%average capacity decay per cycle can further be achieved,demonstrating the great potential of this unique structural sulfur host for high-steady and prolonged-life LSBs.?2?Two-step pre-carbonization was adopted to prepare porous carbon sheet well preserving the initial porosity from the puffed rice biomass.Benefiting from the two-step pre-carbonization,the honeycomb-like structure and the heteroatoms inherited from the structure and composition of the amylum particles in the rice seeds could be preserved.After activation,the as-obtained N,O co-doped carbon?MPNC?material possesses ultrahigh surface area of 3593.1 m² g^-1 and large pore volume of 1.91 cm³ g^-1,which could be used as porous conductive matrix for lithium-sulfur battery physically cathodes to host the high sulfur loading(1.5-3.0 mg/cm^-2).Moreover,the N,O co-doping sites could synergistically enhance chemical adsorption of the polysulfide and increasing the electronic conductivity of the carbon framework,so as to reduce the shuttle effect by chemically bonding the polysulfide on the surface.Therefore,the as designed C/S hybrid cathode shows a reversible capacity of 920.8 mAh g^-1 after 100 cycles at a current density of 0.2C with high coulombic efficiency?100%?.In particular,a prolonged cycle life with excellent cycling stability is delivered at high current density of 1 C,presenting an initial capacity of 877.4 mAh g^-1 and maintaining a reversible capacity of 702.1 mAh g^-1 after 300 cycles with low capacity decay of 0.066%per cycle.This two-step pre-carbonization demonstrates a rational and general method to yield high quality carbon materials from biomass for high-performance LSBs cathodes.?3?Bagasse as bio-waste generated in sugar industry was used as precursor to prepare nitrogen and oxygen dual-doped mesoporosity dominated carbon materials using a fermentation mediated process.The metabolism of the yeast could selectively digest the irregularly distrubted sucrose on the surface of the biomass,and continuous proliferation also leads to the crosslink among the precousr particles,which leads to the high specific surface area(²940 m² g^-1),enhanced mesoporous distribution(².10 cm³ g^-1),increased nitrogen content?3.92 at%?,and the colvently bonding amonge the carbon motifes after the pre-carbonization and activation processes.The introduced heteroatoms with high electron donating behavior noticeably improve the conductivity of carbonaceous materials,and provide affluent polysulfide chemisorption sites through Li-N and Li-O bonds.The significantly improved mesoporous pore volume provides effective mass transfer channels for electrolyte transport,enabling rapid ions migration,and enhances the space for the adsorption,storage and reactivation of the dead sulfur spices by trapping in the modified separators.Taking account of the multiple effect,the battery adoping modified glass fiber films as separator demonstrates excellent sulfur utilization,robust architecture and strong physical and chemical immobilization and reactivation ability to inhibite the shuttle effect,which finally deliviers superior initial capacity of 1157 mAh g^-1 at 0.3 C and high capacity maintainment of 87%(1007 mAh g^-1)after 100 cycles.Furthermore,battery with modified separator presents initial capacity of 1011 mAh g^-1 at 1 C and retains reverse capacity of 567.1 mAh g^-1 after prolonged cycles of 500 with slight capacity loss of 0.087%per cycle.