Metal-hexamine Coordination Frameworks Derived Nitrogen-rich Carbon Materials For Sodium-ion Storage

As one of the most promising candidates for large-scale energy storage devices,sodium-ion batteries(SIBs)have received great interest due to the abundance and low cost of sodium resources.The overall electrochemical performance of SIBs strongly depends on the electrode materials.Among various anode materials,carbon materials are the most widely used and likely closer to commercialization.Nevertheless,the lack of intrinsic chemical activity and weak polarity greatly limit their practical applications.To this end,numerous strategies have been developed to tailor the properties of the pristine carbons.In particular,N-doping can efficiently modify the electronic structure of carbon materials to elevate their charge carrier density,surface energy,and chemical affinity,thereby enhancing the electron transfer ability and ion diffusion kinetics.However,the structural design of high-level N-doped carbon nanomaterials remains a significant challenge.Recently,metal-organic coordination frameworks(MOFs)are emerging as promising precursors to prepare functional carbon-based nanomaterials.Due to the highly selectable elemental species,various heteroatoms can be easily introduced into the carbons.For N-doped carbons,the common used MOF precursors are mostly focused on the zeolitic imidazolate frameworks(ZIFs).However,the inherent coordination mode greatly limits the designability of ZIFs,which always exhibit a monotonous polyhedral morphology.Based on these points,in this work,we developed a new and efficient ligand with a high N content,hexamine(HMT),that we used to construct metal-HMT coordination frameworks(MHFs).HMT,in which four N atoms are situated at the corners of a tetrahedron,is capable of adopting different coordination modes that span from the terminal monodentate to the bridging bi-,tri-,and tetradentate modes,which provides a high diversity of topologies of MHFs including 1D chains,2D layers as well as 3D nets.Furthermore,HMT,as a low-cost industrial raw material,is favorable for large-scale production.Thus,by selecting the proper metal clusters and controlling the coordination modes,we developed a series of MHFs with controllable morphologies and structures.Through a subsequent pyrolysis,the MHFs can be converted to highly N-doped carbon-based materials.The electrochemical energy storage performance of MHFs-derived N-rich carbon-based materials is further investigated,and the relationships between the structural composition and electrochemical performance are also revealed.These results shed some light on the further design of N-rich carbon materials for excellent sodium storage.(1)Highly N-doped carbon materials(HNCs)were successfully prepared by using Cu-HMT coordination frameworks as precursors.The highest N-doping level is up to 29.42 at%.Furthermore,the N content and configuration can be effectively adjusted by controlling the carbonization temperature and holding time.When used as anode materials for SIBs,the HNCs exhibit a high sodium storage capacity of 142 mAh g'1 at 5 A g'1.Based on systematic analysis,new insight into the roles of the different N configurations in Na-ion storage is proposed.The adsorption of Na ions on pyridinic-N(N-6)and pyrrolic-N(N-5)is fully irreversible,whereas the adsorption on graphitic-N(N-Q)is partially reversible and the adsorption on N-oxide(N-O)is fully reversible.More importantly,the N-6/N-Q ratio is an intrinsic parameter that reflects the relationship between the N configurations and carbon textures for N-doped carbons prepared from in situ pyrolysis of organic precursors.Both the cyclic stability and rate-performance improve with decreasing N-6/N-Q ratio.(2)2D Zn-HMT coordination frameworks were designed for the first time.Through a simple pyrolysis,the 2D Zn-HMT coordination frameworks can be converted to N-rich porous carbon nanosheets(NPCNs).Remarkably,the N-doping level of NPCNs can attain 16.54 at%.In addition,the thickness of the carbon nanosheets can effectively be tuned by simply adjusting the molar ratio of the Zn(NO3)2/HMT molecules,and the Zn species can serve as a self-removable template to further promote a porous structure.As a proof-of-concept application,the NPCNs as an anode material for SIBs exhibit an ultra-fast sodium storage capability of 194 mAh g-1 even at 10 A g-1.To the best of our knowledge,such a high-rate capability has been rarely achieved in previous studies on carbonaceous anode materials for Na-ion storage.(3)A thermal exfoliation strategy was developed for the design of a series of 2D N-rich carbon-based nanomaterials by employing layered metal-hexamine framework microcrystals as precursors.Through a facile pyrolysis,the sharply generated gases escape from the layered precursors,leading to the exfoliation of the layers and successful preparation of 2D carbon-based nanomaterials regardless of the morphologies of precursors.In contrast to conventional route(first preparation of 2D MOFs and subsequent pyrolysis to obtain 2D carbon-based derivatives),this strategy can readily skip the complicated morphology engineering process of 2D MOFs to produce 2D N-rich carbon-based nanomaterials on a large scale.(4)N-rich hollow carbon-onion-constructed nanosheets(HCONs)were developed from Co-hexamine coordination frameworks through simple pyrolysis and subsequent acidification process.The HCONs have the size and the thickness of ca.3 ?m and 5 nm,respectively,and consist of interconnected hollow carbon onions with a high N content of 16.54 at%.This kind of 2D nanosheets significantly enhance the diffusion kinetics of Na ions and facilitate the electron transport through the conductive carbon onion networks.In particular,the hollow onion-like carbon textures possess a better structural stability for the long-term cycling and can also mitigate the volume change during the sodiation/desodiation process.Remarkably,when used as an anode for SIBs,the HCONs exhibit a superior high-rate capacity of 131 mAh g-1 at 10 A g-1.Even after 10000 cycles at 5 A g-1,a high capacity of 151 mAh g-1 can still be retained,with a capacity retention of 96%.(5)2D NiSe2/N-rich carbon nanocomposite(NiSe2/NC)was successfully prepared by simple pyrolysis and subsequent selenization process.In the 2D NiSe2/NC nanocomposite,the NiSe2 nanoparticles are homogeneously decorated on the N-rich carbon matrix,and the N content is up to 12.82 at%.The 2D nanocomposites exhibit a high reversible capacity of 410 mAh g-1 at 1 A g-1 and maintains a value of 255 mAh g-1 even at 10 A g-1.The outstanding electrochemical performance should be ascribed to the synergistic effects between the N-rich carbon nanosheets and NiSe2 nanoparticles:the NiSe2 possesses a high reversible capacity and outstanding high-rate performance,while the N-rich carbon matrix can efficiently enhance the contacts between the electrode and electrolyte as well as mitigate the volume expansion during the electrochemical process.