Design And Construction Of Metal-Nitrogen-Carbon Based Composite Electrodes For The Sodium-Ion Storage

Under the goals of "carbon peaking and carbon neutralization",renewable energy has gradually changed into the leading energy.Among the key technologies to promote the utilization of renewable energy,the development of energy storage technology has become one of the important supporting technologies to achieve this goal.Compared with lithium-ion batteries with relatively mature technology,sodium-ion batteries（SIBs）show broad application prospects in the field of large-scale energy storage due to their abundant resources,low cost,and high theoretical capacity.However,due to the large radius of sodium ions（Na⁺）,the storage capacity in traditional anode materials is low,which seriously restricts the industrialization of SIBs.Therefore,the design and development of low-cost,high-capacity,and high-rate anode materials has become an urgent problem to be solved.At present,hard carbon as an anode electrode material has a series of advantages compared with commercial graphite materials,and it is the most promising anode material for SIBs.However,the rate and capacity of hard carbon materials are not satisfactory,which limits their practical application and commercial promotion.In addition,the storage mechanism and the structure-activity relationship between structure and properties of hard carbon materials are still controversial.In order to enhance the diffusion and adsorption of Na⁺by hard carbon materials,it is one of the most effective methods to increase the layer spacing and construct Na⁺storage sites with the help of a structural engineering strategy.On the other hand,evolution of the solid electrolyte（SEI）interface plays an important role in the charge/discharge process,but the relationship between electrode material enhanced sodium storage and SEI has not been clarified.To solve these problems,this paper takes metal-nitrogen-carbon（M-N-C）based materials as the research object and prepares a series of M-N-C-based composites to deeply explore the structure-activity relationship and energy storage mechanism of M-N-C-based composites in Na⁺storage.The specific works are outlined as follows:（1）Fe-N-C graphitic layer-encapsulating Fe₃C species within hard carbon nanosheets（Fe-N-C/Fe₃C@HCNs）are rationally engineered by pyrolysis of self-assembled polymer.Impressively,the Fe-N-C/Fe₃C@HCNs exhibit outstanding rate capacity and prolonged cycling stability.Theoretical calculations unveil that the Fe₃C species enhance the electronic transfer from Na to Fe-N-C,resulting in the charge redistribution between the interfaces of Fe₃C and Fe-N-C.Thus,the optimized adsorption behavior towards Na⁺reduces the thermodynamic energy barriers.The synergistic effect of Fe₃C and Fe-N-C species maintains the structural integrity of electrode materials during the sodiation/desodiation process.The in-depth insight into the advanced Na⁺storage mechanisms of Fe₃C@Fe-N-C offers precise guidance for the rational establishment of confinement heterostructures in SIBs.（2）The capacity enhancement of FeN_X/C anodes was first clarified by employing in-situ temperature-dependent Nyquist plots and ex-situ X-ray photoelectron spectroscopy.It was evidenced that physiochemical evolution of the SEI and surface carbonaceous materials made a significant contribution to the improved sodium storage performance through the following three mechanisms:（a）FeN_X catalyzed the reversible conversion of SEIs,beneficial to the storage and release of extra Na ions,（b）a large number of spin-polarized charges were stored on the surface of the reduced Fe species,and（c）the carbon delivered additional capacity through the surface-capacitive effects.As a result,the FeN_X/C anode provided a high capacity of 217 m A h g^-1 after1000 cycles at 2000 m A g^-1.Therefore,the FeN_X species catalyzed the reversible conversion reaction of SEIs,which contributed novel avenues to the design of conversion-type electrode materials.（3）Ti₃C₂ MXene nanodots with uniform dispersion were obtained on FeNC nanosheets by high-temperature pyrolysis method,and MXene/FeNC anode materials with heterostructure were constructed.Mossbauer spectroscopy and XPS studies show that rich Ti-O-Fe bonds help Ti in Ti₃C₂ MXene to electronically regulate Fe in FeNC,change the spin state of FeNC,so as to adsorb more metal ions,and reduce the activation energy of metal ions passing through SEI interface.In addition,due to the fragmentation of Ti₃C₂ MXene during the in-situ synthesis of FeNC,the MXene/FeNC heterostructure exposes more active sites.Therefore,MXene/FeNC heterostructure can provide a convenient ion diffusion path,rich ions can be embedded into active sites and higher conductivity,and finally accelerate ion diffusion and promote electron transport process,so it shows excellent energy storage performance.（4）A MoS₂/Fe-N-C anode material was designed by using the metal carrier electron effect.Fe-N-C was used as the carrier to inject electrons into MoS₂ through the Fe-S bond,which enhanced the adsorption capacity of MoS₂ to Na⁺,and laid a foundation for the subsequent sodiation/desodiation process.The results show that a uniform small-size 1T/2H phase MoS₂ will be formed on the surface of the carrier Fe-N-C during charging,which indicates that it significantly enhances the reaction kinetics of MoS₂ sodiation/desodiation.In addition,Fe-N-C with ferromagnetic spin polarization can significantly promote the decomposition of Na₂S and Na MoS₂ during charging,so as to realize the efficient and reversible conversion of 1T/2H phase MoS₂.Therefore,MoS₂/Fe-N-C with a unique electronic structure and the promotion of Fe-N-C on the reversible evolution of MoS₂ show excellent magnification performance and cycle stability.Summary,the research in this paper not only broadens the application of carbon materials in the field of energy storage,but also has important guiding significance for the construction of efficient electrode materials,lays a solid theoretical and experimental foundation for the development of negative electrode materials for a new generation of SIBs,and will help the rapid development of SIBs.