Controllable Preparation Of Heteroatom-doped Porous Carbon Materials And Their Lithium/sodium Storage Propertie

Energy-storage technology is the key to the energy transition away from fossil fuels to renewable energy and the wide application of electric hybrid vehicles,which in turn will affect the global carbon neutrality and net zero targets.Secondary batteries represented by lithium-ion batteries（LIBs）have been widely used in portable electronic devices and electric vehicles,while sodium-ion batteries（SIBs）are more suitable for grid-level renewable energy storage due to their lower cost and security of raw material supply,among other advantages.However,the commercial graphite anode has been reaching to its theoretical specific capacity in LIBs and shows the limited specific capacity in SIBs.Therefore,the development of high-performance,low-cost LIBs/SIBs anode materials has drawn intensive attentions in the field of energy industy.Carbon-based materials are the primary choice for anode materials in LIBs/SIBs because of low cost,high safety,non-toxicity,as well as their low charge/discharge voltage.However,the limited reversible capacity,poor rate performance,and low initial Coulombic efficiency of carbon-based anodes need to be further optimized.In this work,a hollow porous amorphous carbon material was prepared by the template method.It shows flower-like morphology assembled by two-dimensional hollow porous carbon microcapsules.Engineering the pore structure,binary-heteroatom doping,and conductive polymer coating were employed to explore the effective way to promote the electrochemical Li/Na-storage performance of this carbon material.（1）The hollow N-doped carbon-flower（HNC）was synthesized with Ni_0.5Co_0.5（OH）（OCH₃）as the template,and polydopamine as the nitrogen and carbon sources.The material retains the flower-like morphology of the template and the"petals"are two-dimensional hollow microcapsule-like structure.Moreover,the pore structure and defect concentration of HNC can be controlled by the pyrolysis temperature.As a result,the optimized product,HNC-600,exhibits excellent specific capacity,rate performance and cycling stability.The excellent electrochemical performance of this porous carbon material can be ascribed to its unique hierachical structure:（i）the two-dimensional structure increases the contact area between carbon and carbon and between carbon and electrolyte,which is conducive to electron and ion transfer;（ii）the hollow structure provides large electrode/electrolyte contact area and abundant active sites for charge storage,as well as buffers volume change;（iii）mesopores on the surface facilitates electrolyte penetration and ion diffusion.（2）To enhance the reversible capacity of HNC,we introduced sulfur atom into its framework to obtain S and N co-doped porous carbon S-HNC.Sulfur doping can enlarge the layer spacing of the material to enhance the ability of lithium/sodium intercalation,thereby increasing reversible capacity.Moreover,the covalent sulfur atoms in carbon skeleton can be involved as active materials in electrochemical reaction,providing additional reversible capacity.As a result,the reversible capacity of S-HNC in LIBs/SIBs is significantly improved compared to HNC.In addition,the phenomenon of"voltage failure"of S-HNC in ether electrolyte was investigated.It is proposed that the cell failure may be due to the escape of polysulfides from the electrode.These escaped polysulfides will migrate to the seperator during cycling,which in turn impedes the transport of Na⁺between the two electrodes,and finally cause cell failure.（3）Although the introduction of the pore structure facilitates the improvement of the specific capacity and rate performance,it also intensifies the decomposition of the electrolyte on the electrode surface,resulting in the reduction of the initial Coulomb efficiency.Therefore,we explored the feasibility of enhancing the comprehensive performance of porous carbon materials by conducting polymer coating.By coating polypyrrole（PPy）on HNC surface,the electrode/electrolyte contact area is reduced while PPy itself has little steric hindrance to ion transfer without degrading the rate performance due to its flexible structure.Moreover,PPy’s high conductivity also facilitates the promotion of specific capacity and rate performance.Therefore,compared to HNC in SIBs,the polypyrrole-coated HNC@PPy exhibits improved initial Coulombic efficiency,specific capacity,and rate performance.The kinetic analysis results demonstrate that the PPy coating can enhance charge storage transfer without sacrificing on the diffusion coefficient of Na⁺.