Synthesis Of N-doped Carbon Composites And Their Applications In Zinc-Iodine Batteries

Aqueous rechargeable zinc-iodine batteries(ZIBs)emerged as a promising energy storage devices has attracted considerable attention.However,ZIBs still suffer from the poor conductivity of iodine and severe shuttle effect of polyiodide,leading to the poor cycling lifetime and potential safety issues.To address these problems,facile synthesis methods were used to optimize the surface properties and chemical components of carbon-based materials,as well as porous strucutres.Through the dual protection of physical and chemical adsorption,the redox reactions of iodine would be enhanced in the cathode region,promoting the electrochemical performance of zinc-iodine battery.The main results are summiarized below:(1)The assembly of Al-based metal organic frameworks(Al-MOFs)in the presence of polyacrylonitrile(PAN)via electrospinning technique enables the formation of Al-MOF/PAN fibers.With the subsequent pyrolysis,the hierarchical porous carbon fibers with nitrogen doping(NPCNFs)were prepared for loading of iodine.Benefiting from the confinement effect of the highly porous carbon network and the nitrogen doping,the carbon nanofiber electrode is capable of inhibiting the shuttle effect of polyiodide species.Especially,the in-situ Raman spectroscopy reveals the reversible two-step conversion reaction between iodine and polyiodide,which enables the best cycling stability with negligible capacity attenuation.(2)The ultrafine MoN nanoparticles(NPs)were successfully embedded in N-doped porous carbon nanofibers to form a free-standing hybrid electrode(MoN/N-CNFs)by electrospinning process and subsequent pyrolysis,which was used for the immobilization of iodine species in ZIBs.During the pyrolysis process,the decomposition of PMMA and etching effect of MoN would contribute to the formation of porous structure,which enables a facile electrolyte infiltration and ion transportation.With physical confinement of hierarchical porous structures for iodine,the nitrogen doping and ultrafine MoN NPs in the carbon matrix would produce strong chemical interactions with iodine species,thus preventing the shuttle effect.When the flexible and free-standing I2@MoN/N-CNFs cathode was evaluated as cathode material for ZIBs,the batteries exhibited improved cycling performance and excellent rate performance.(3)Tungsten nitride-incorporated N-doped microporous carbon polyhedron(W2N/N-C)was prepared via the pyrolysis of phosphotungstic acid-loaded ZIF-8,PTA@ZIF-8.The obtained W2N/N-C materials were utilized as the iodine carrier for zinc-iodine batteries.Compared to N-doped carbon(N-C)derived from pure ZIF-8,the battery with I2@W2N/N-C exhibited tinier polarization,higher reversibility,and more stable cyclic performance.The initial discharge capacity at 5 C was up to 200.9 mA h g-1,and the reversible capacity was stable at 170.3 mA h g-1 after 2000 cycles,with a capacity retention rate of 85%.The improved electrochemical performance would be attributed to the confinement effect of W2N NPs and the highly porous carbon to entrap polyiodide,resulting in effective reversibility of active materials.