Investigation On The Effects Of Transition Metal Nitrides In High-rate Lithium Storage

With the massive consumption of traditional fossil energy,social activities and production have an increasing demand for new energy.Therefore,it is imminent to develope the new energy storage technologies.Among many electrochemical energy storage devices,lithium-ion batteries(LIBs)are widely used in various fields due to their higher energy density and longer cycle life.However,their lower power density cannot meet the needs of the society for rapid energy use in the future.Therefore,the development of the next generation LIBs with high capacity/rate performance and long-cycle stability has become a major strategy.However,the development of LIBs anode materials with rapid lithiation and delithiation dynamics and excellent structural stability is the focus and difficulty in the current research and development.Among many negative electrode materials,Transition Metal Nitrides(TMNs)with high theoretical specific capacity and high electron/ion transfer rate have become a research hotspot.Moreover,TMNs are mainly used as active materials and conductive materials in LIBs.Therefore,this thesis focuses on TMNs as the main body of research.Firstly,TMNs,which are mainly used as active materials,are hindered by the fewer reactive sites and volume expansion during lithium storage.Therefore,a strategy of composite modification with conductive carbon materials is proposed to improve the electrochemical performance.Secondly,when the TMNs are as conductive materials,TMOs/TMNs nano-heterostructures are accurately constructed by the interface engineering strategy.Then,it is discussed the effect of TMNs in depth.This thesis comprehensively studies the electrochemical performance of TMNs as active materials and conductive materials,and deeply explores the structure-activity relationship between the structure and electrochemical properties of the materials.The main research contents are as follows:(1)A simple hydrothermal method was used to prepare TMOs(M=Fe,Nb)nanomaterials with uniform and controllable morphology,which were composited with graphene oxide(GO)by freeze-drying method.And then the TMNs/r GO nanocomposite with controllable mass ratio and uniform morphology was successfully prepared by ammoniating treatment.The research results showed that there was a tight interface between TMNs and r GO and M(TMNs)-O(r GO)-C(r GO)bond was formed,which effectively promotes electron transfer and enhances the electrochemical reaction kinetics of the electrode material.Therefore,the TMNs/r GO nanocomposite shows a high reversible specific capacity.Specifically,the Nb₄N₅/r GO and Fe₂N/r GO electrode were 434.8 m Ah g^-1 and 463.0 m Ah g^-1 at 0.1 A g^-1,respectively.More impressively,the capacity retention of Nb₄N₅/r GO electrode was 96.3%after 1000 cycles,and the Fe₂N/r GO electrode showed the capacity retention of 97%over 2000 cycles at 1.0 A g^-1.Moreover,the effect of r GO in electrochemical energy storage was futher investigate through quantitative kinetic analysis.It not only facilitates the transmission kinetics of electrons and ions,but also provides more active sites and buffers the strain during the process of lithiation/delithiation.Importantly,the above experimental results also showed that the method of synthesizing TMNs/r GO nanocomposites was universal and expandable.However,the Initial Coulombic Efficiency(ICE)of TMNs/r GO nanocomposite was low resulting from the introduction of r GO,which severely limits practical applications.Thus,the improvement of ICE has a great significance for improving the performance of electrode materials and solving practical applications.Therefore,transition metal oxide/nitride TMOs/TMNs(M=Fe,Nb,W)nano-heterojunctions were designed and synthesized to improve the ICE.(2)The precise structure of TMOs/TMNs nano-heterojunctions was successfully built through effective nitridation strategies.The research results revealed that the tight interfaces between TMNs and TMOs with different band gap were formed.More impressively,the formation of M-O-N bonds was induced by the heterostructure through the hybrid of TMOs/TMNs with the corresponding TMOs by the certain electron interaction,which can promote electron transfer and enhance electrochemical kinetics.Therefore,the TMOs/TMNs nanocomposites have excellent electrochemical performance.Specifically,Fe₃O₄/Fe₂N-60 electrode materials exhibit a specific capacity of 712.3 m Ah g^-1 at 0.1 A g^-1,and a specific capacity retention of 75%at 2.5A g^-1.More importantly,The Nb₁₂O₂₉/Nb₄N₅ and WO_2.92/WN electrodes are also evaluated duo to the fabrication of Fe₃O₄/Fe₂N electrode with superior electrodechemical performance.Specifically,the Nb₁₂O₂₉/Nb₄N₅-15 electrode presents the excellent capacity retention of 50%at high current density of 2.5 A g^-1 and superior cycling performance for nearly 100%capacity retention after 1300 cycles.Similarly,the WO_2.92/WN-30 electrode also possesses impressive electrochemical performance which is 51%capacity retention at 2.5 A g^-1.According to the quantitative kinetic analysis,TMOs/TMNs(M=Fe,Nb,W)nano-heterojunction composites have higher lithium ion diffusion coefficient(D⁺_Li),excellent rate capability and better cycle stability than their corresponding oxides.The main reason is that conductive heterogeneous layers(TMNs)can promote rapid electron and ion transfer to enhance surface reaction kinetics.Moreover,the rigid secondary structure can reduce internal stress changes and maintain the integrity of the structure during cycling.Moreover,the results of density function theory(DFT)displays the formation of the built-in electric field(between(200)facet of Fe₃O₄ and(002)facet of Fe₂N)in the p–n junction significantly decrease diffusion barrier to facilitate the charge transfer kinetics.Compared with TMNs/r GO nanocomposites,TMOs/TMNs show better ICE and rate performance.This work not only provides an effective strategy for precisely controlling the preparation of metal oxide/nitride nanoheterojunctions,but also proposes a new insight to understand the mechanism of interface engineering for enhancing electron and charge transfer kinetics to achieve high-rate lithium storage.