生物质基氮掺杂多级次孔碳材料制备及其锂硫电池性能

Biowaste-derived three-dimensional nitrogen-doped hierarchically porous carbon materials for lithium-sulfur batteries

锂硫电池因具有较高的理论比容量和能量密度、价格低廉等优势,在动力汽车和大规模储能系统中有广阔的应用前景.发展锂硫电池,设计制备低成本和高性能的电极材料是关键.本文利用廉价生物质废弃物(虾壳)作为原料,通过简单、绿色的水热碳化和二氧化碳碳化/活化过程制备了三维氮掺杂多级次孔碳材料,并将其作为载体负载硫制备锂硫电池正极材料.该制备过程避免了传统活化过程中危险化学品的使用及活化后材料清洗的过程.水热碳化和活化步骤均对材料孔结构的形成具有重要作用.通过氮气吸附、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线光电子能谱(XPS)、拉曼(Raman)等表征方法对材料形貌、孔结构、功能基团等进行了深入研究.研究结果表明,得到的材料是具有高孔隙率的微孔-介孔-大孔碳材料,比表面积高达1190m^2/g.由于材料具有多级次孔结构,有利于硫的有效负载.同时,材料中的石墨氮可以提高材料的导电率,吡咯氮和吡啶氮在限制多硫化锂(Li_2S_n,4≤n≤8)中发挥了重要作用,从而使得材料在锂硫电池中具有较好的倍率性能和循环性能.另外,本文为生物质或生物质废弃物转化为功能多孔碳材料提供了一种新的绿色途径.

Lithium-sulfur(Li-S) batteries have attracted particular interests owning to their high theoretical specific capacity and energy density.However,some key issues hinder the development of Li-S batteries.One is that sulfur is a low conductivity element,which leads to poor electrochemical contact and low utilization in Li-S batteries.Another one is "shuttle effect" brought by the dissolution of the polysulfides.Much effort have been paid to the development of low-cost and conductive material as matrix to disperse active sulfur for Li-S batteries.Hereby,we present a strategy to synthesize three-dimensional(3D) nitrogen-doped hierarchically porous carbon material(NHPC) from biowaste(e.g.,shrimp shell) via sustainable hydrothermal carbonization and CO2 activation process.The obtained material possesses 3 D micro-meso-macroporous structure with relatively large Brunauer-Emmet-Teller specific surface area(up to 1190 m2/g).The properties of the material,such as morphology,porous structure,elemental composition,degree of structural order,and functional group,were investigated.It is found that both the hydrothermal carbonization and CO2 activation procedures play virtual roles in the formation of hierarchically porous structure.During the HTC procedure,the chitin in shrimp shell was transformed into N-doped carbon.The carbon structure stabilized,which avoids the structure destroying during the acid washing and further activation procedures.The control material synthesized without hydrothermal carbonization is microporous material.The CO2 activation procedure introduce micromesopores into the carbon structure,the control material synthesized without CO2 activation(only hydrothermal carbonization and thermal treatment) possesses much lower specific surface area compared with NHPC.The obtained N-doped 3D hierarchically porous carbon was used as matrix to load sulfur(S@NHPC).The energy dispersive spectroscopy(EDS) mapping experiment was conducted to investigate the element distribution of the carbon,nitrogen,and sulfur.The results show that the carbon,nitrogen,and sulfur species are uniformly dispersed.This phenomenon indicates that the nitrogen and sulfur are well distributed in the carbon material,which results in good electrical contact of sulfur and the N-doped carbon matrix.The types of the carbon,nitrogen,and sulfur functionalities of the materials were further studied by X-ray photoelectron spectroscopy(XPS) technique.After the activation/carbonization process,the quaternary N was detected,which confirms that the structural order increases during the CO2 activation procedure.This phenomenon was further proven by X-ray diffraction(XRD) and Raman results.It was reported that the nitrogen functionalities not only enhance the electrical conductivity of the carbon materials,but also prevent the polysulfides shuttling in Li-S batteries.The electrochemical performance of the S@NHPC as cathode in Li-S batteries was studied.The micro-mesomacroporous structure is helpful for electrolyte immersion and diffusion.In addition,the nitrogen functionalities increase the conductivity of the carbon materials and confine the diffusion of the Li2Sn diffusion.The synergistic effect of the nitrogen,sulfur and 3D hierarchically porous structure makes the material possess good performance as cathode in Li-S batteries.Considering the material is biowaste-based,and the synthesis process is facile and sustainable,the 3D NHPC materials have the potential to be good electrode materials in electrochemistry.Furthermore,it is believed that the method used in this work supplies a sustainable and facile way for transforming polysaccharides and inorganic components containing natural biomass or biowaste into functionalized hierarchically porous carbon materials.