Study On Sodium/potassium Storage Properties Of Carbon Composites With In-situ Grown Transition Metal Carbides/nitrides

Environmental pollution and energy crisis are important factors that hinder the sustainable development of social economy.Finding good energy storage systems for efficient storage of renewable energy is an important way to solve the above obstacles.Lithium-ion batteries are considered a driving force in the age of electrification because of their high energy density and wide range of applications.However,the gradual depletion of lithium resources and rising prices have severely hindered the development of large-scale energy storage applications.The resource rich sodium/potassium ion batteries,because of its working principle similar to lithium-ion batteries,is expected to be directly applied to the current mature industrial system.If sodium/potassium ion batteries can be successfully applied in the field of large-scale energy storage,which will basically meet human’s demand for electric energy storage facilities in the era of electrification.However,the larger radius of sodium/potassium ions in the deintercalation process can cause the electrodes to produce large internal stress,resulting in the actual capacity of conventional anodes is low,easy to occur a large structural damage in the long cycles.Therefore,the exploration and research of anode materials for sodium/potassium ion batteries is imminent.In this paper,according to the unique structural characteristics of transition metal compounds,such as oxides,nitrides and carbides,micro/nanostructure design,heterostructure construction and carbon protection are carried out.The influence of these strategies on the storage properties of composites is systematically studied,and some anode materials for sodium/potassium ion batteries with significantly improved properties are obtained,which provides a good reference for subsequent researches.Specific research includes the following two aspects:（1）Through the self-polymerization and two-step calcination process,Mo₂C nanocrystals are grown in situ in nitrogen-doped carbon nanobundles and constructed into hierarchical composites（Mo O₂@Mo₂C/C）with surface-modified Mo O₂nanoparticles.In this architecture,the flexible protection of nitrogen-doped carbon nanobundles（N-CNBs）significantly improves the conductivity and structural stability of the composites,and effectively alleviates the volume expansion and structural collapse of transition metal compounds.Mo₂C has good metal properties,and its in-situ growth further improves the charge transfer ability and structural stability of Mo O₂/C.Importantly,the active sites of Mo₂C nanocrystals are gradually activated during the sodium-ion storage process,significantly increasing the effective capacity during long cycles.After 8000 cycles at 5 A g^-1,the reversible capacity of Mo O₂@Mo₂C/C gradually increases from 126.2 to 419.1 m Ah g^-1,with a capacity retention of up to332.4%.These results suggest that the transition metal carbides with metal properties have great potential advantages in metal ion batteries,and can provide a good reference for the development of energy storage.（2）By using the self-polymerization and heat-treatment process,the defective heterostructures(WO_3-x@W₂N)with bimetallic characteristics are constructed on mono-faceted nanorods and in situ cross-linked in nitrogen-doped carbon nanosheets(WO_3-x@W₂N/CNSs).It is found that the WO_3-x nanorods have many unsaturated vacancy defects and dangling bonds in the facets,which effectively promote the directional charge transfer.The heterostructure with rich self-built electric fields can adjust the vacancy defects of WO_3-x and the electronic structures of W₂N,and significantly improve the efficiency of charge transfer and storage under the protection of CNSs.Utilizing defect and interface engineering,WO_3-x@W₂N/CNSs obtain a large reversible potassium storage capacity(366.6 m Ah g^-1 at 0.1 A g^-1),with high-rate capacity and ultralong lifespan(113.2 m Ah g^-1 after 12000 cycles at 10 A g^-1).Through many characterizations and first-principles calculations,the storage mechanism and kinetics behaviors of the composites are elucidated in detail.It is found that the defective heterostructures with bimetallic properties have promising potentials in high-performance metal ion batteries.