Study On Lithium Storage Properties And Mechanism Of Two-Dimensional Layered Nb₂CT_x And Its Composite

Lithium-ion storage devices have become an indispensable consumer product in our daily lives,and the vast majority of high-performance energy storage systems currently available for commercial production are assembled on the basis of lithium-ion storage devices.Of course,lithium-ion storage devices also inevitably face the problem of performance approaching the limit,especially with the commercial graphite anode,which is very close to its theoretical capacity.The development of new high-performance anode materials for lithium-ion storage is therefore essential.Transition metal carbon-nitrides have unique physical and chemical properties,including tunable surface terminal groups,abundant electrochemically active sites and good stability.In this thesis,the Nb₂CT_xwith high theoretical capacity is explored and developed,a series of Nb₂CT_xbased composites and their derived heterostructures are designed and constructed,its electrochemical properties and storage mechanism in lithium-ion storage are further studied.So far,Nb₂CT_xwas prepared by etching Nb₂AlC with hydrofluoric acid,resulting in Nb₂CT_xwith small interlayer spacing and low lithium-ion storage capacity.Therefore,in order to directly prepare Nb₂CT_xwith high lithium-ion storage capacity,a preparation method using a mixture of hydrochloric acid and lithium fluoride instead of hydrofluoric acid to etch Nb₂AlC was developed,and multilayer Nb₂CT_xwas successfully prepared.The gentle preparation process can reduce the damage to the crystal structure of Nb₂CT_x,and the terminal group-F content is greatly reduced.In addition,lithium-ion plays the role of pre-intercalation in the etching process,which leads to a large interlayer spacing.The results show that the Nb₂CT_xobtained by this preparation method has excellent lithium-ion storage properties.At 0.05 A g^-1,the lithium-ion storage capacity of the Nb₂CT_xelectrode was 425 m Ah g^-1,and the specific capacity remained basically unchanged during 1000 hours of continuous charging and discharging.The few-layer Nb₂CT_xnanosheets are promising materials for lithium-ion storage electrodes.However,due to the strong interaction between Nb-Albonds,the preparation of few-layer Nb₂CT_xnanosheets has been a challenge.In addition,the re-stacking and low conductivity of Nb₂CT_xnanosheets inhibited their electrochemical activity.In order to solve these problems,the preparation method was further optimized to obtain few-layer Nb₂CT_xnanosheets.The Ag-Nb₂CT_xnanosheets modified with Ag nanoparticles and C@Nb₂CT_xnanofibres coated with three-dimensional carbon were developed by a surface modification strategy.Ag nanoparticles can inhibit the re-stacking of Nb₂CT_xnanosheets,thus increasing their specific surface area.Moreover,the abundant free electrons of Ag nanoparticles improve the electron transfer ability of the electrode.Therefore,Ag-Nb₂CT_xnanocomposites exhibits excellent electrochemical properties,with specific capacities of 481 and 187 m Ah g^-1at 0.05 and 5.00 A g^-1,respectively,and exhibit excellent cyclic stability.C@Nb₂CT_xnanofibres are prepared by electrostatic spinning and annealing.They have a three-dimensional conductive network and self-supporting structure,which is advantageous for electrolyte penetration and diffusion,electron conduction,and can stimulate more electrochemical active sites in Nb₂CT_xnanosheets.In addition,C@Nb₂CT_xcan be used directly on electrodes without adding other additives（such as conductive adhesive,conductive agent,etc）.C@Nb₂CT_xexhibits superior electrochemical properties in lithium-ion storage,with a lithium-ion storage specific capacity of up to 500 m Ah g^-1.To enhance its electrochemical reaction kinetics,an interfacial modulation strategy was used to construct heterostructures based on Nb₂CT_xand its derived heterostructures.The built-in electric field is induced by modulating the interfacial electronic structure,which to drive its charge transfer kinetics during electrochemical reactions.Nb₂CT_x/Nb₂O₅Schottky heterostructures were constructed using in situ derivatization.The results show that the semiconductor Nb₂O₅has a low work function,which leads to the formation of an built-in electric field during the construction of the Nb₂CT_x/Nb₂O₅Schottky heterostructure with the transfer of electrons at the interface.Benefiting from the fast charge transport driven by the built-in electric field at the interface,the Nb₂CT_x/Nb₂O₅electrode has a superior lithium-ion storage capacity of 575 m Ah g^-1for200 cycles at 0.1 A g^-1and 290 m Ah g^-1for 1000 cycles at 2.00 A g^-1,both with no capacity degradation.Furthermore,in situ XRD and ex situ XPS analysis were used to reveal the structural derivatization of the Nb₂CT_x/Nb₂O₅Schottky heterostructure during lithium-ion storage,as well as the mechanism of its enhanced lithium-ion storage properties.In addition,two-dimensional Re S₂nanosheets were grown on Nb₂CT_xnanosheets using in situ growth and derivatization,while Nb₂CT_xwas derived to Nb₂O₅,constructing a new Re S₂/Nb₂O₅heterostructure,which coupled with Re S₂and Nb₂O₅with mutually compatible band structures and enriched oxygen vacancies.The unique heterostructure can facilitate the redistribution of charges to induce built-in electric fields and microlocalized electric fields.Benefiting from the double electric field effect,the Re S₂/Nb₂O₅heterostructure shows excellent reversible capacity for lithium-ion storage,maintaining a reversible capacity of 805 m Ah g^-1during more than 2400 h of cycling at a current density of 0.10 A g^-1.In addition,in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy analysis reveal the phase transition process and the mechanism to optimize lithium-ion storage reversibility of the Re S₂/Nb₂O₅heterostructure during the lithium-ion storage.This provides deeper insights into the construction of high-performance lithium-ion storage materials based on heterostructures with dual-electric field-driven charge transfer.In this thesis,the enhanced lithium-ion storage capacity of Nb₂CT_xand its composites is achieved through improved preparation methods,surface modification and interfacial modulation.Combining in situ and ex situ experimental means,the lithium-ion storage mechanism of the materials is analyzed and revealed,laying the foundation for the design and development of new high-performance electrode materials.