Surface Modification,Doping And Electrochemical Performance Of MXene

MXene,an emerging family of conductive two-dimensional transition metal carbon/nitride,is an important candidate electrode material for energy storage devices,and has shown an attractive prospect in the field of lithium-ion batteries.It has been demonstrated that the intercalated Li ions are supposed to stay in the interlayers of MXene during the charge and discharge.However,a complete understanding of the intrinsic storage mechanism underlying the charge/discharge behavior remains elusive.What is the specific Li-ion(de)intercalation process?What are factors that affect lithium storage performance.Moreover,the current research work is mainly focused on the lithium storage capability of MXene,leaving extremely limited in-depth comprehending of the lithium storage mechanism for MXene,which limits not only the practical application of MXene but also the property-oriented design of late-model electrode materials with superior performance.This thesis takes MXene as the research object,proceeds from the understanding of lithium storage mechanism of MXenes,and aims to shed light on the key factors that affect lithium storage performance.Thus,it points out the direction for the preparation of high-performance MXene.The main contents and conclusions are as follows:(1)The surface chemistry and structure of MXene was modified by controlled atmosphere via detailed electrochemical,structural,kinetic,and spectroscopic studies,in tandem with DFT calculations,we provide in-depth insights into Ti3C2Tx as an intercalation anode material.The characteristics of the Ti3C2Tx structure can be modified systematically by calcination in various atmospheres,and second,these structural changes greatly affect Li-ion storage behavior.Ti3C2Tx as an intercalation anode material stores charge highly depending on the terminal Ti atom,which is definitely evidenced by the fact that the transition metal bonded with various surface groups undergoes specific redox reactions at well-recognized voltages.Specifically,via ammonization,the interlayer spacing gets dilated and uniform,giving rise to only one redox couple.In stark contrast,there are two well-recognized redox couples corresponding to two interlayer spacings in pristine Ti3C2Tx MXene,in which Li-ion(de)intercalation occurs between interlayers in a sequential manner as evidenced by ex situ X-ray diffraction(XRD).Notably,the XRD diffraction peaks shift hardly in the whole range of charge/discharge voltage,indicating a zero-strain feature upon Li-ion(de)intercalation.(2)Based on the understanding of the lithium storage mechanism,we show that microwave irradiation of MXene particulates in NH4VO3-containing ethylene glycol is efficient for nitrogen and vanadium incorporation into the Ti-deficient Ti3C2Tx MXene.The introduction of N and V decrease the content of-F groups and increase the concentration of-O groups,which is benefited to the electrochemical performance.N-incorporation Ti3C2Tx causes uniformly increased interlayer spacing due to the fact that N-containing species act as pillars between the interlayers as well as increases the ability of change in the oxidation state of Ti.V can be doped or substituted not only in surface functional groups but also in Ti vacancies.V shows a higher ability of valence change than Ti,and V patch up the Ti vacancies,which can enhance charge storage capability.Thus,the integrated effect of N and V coincorporation enables the rate capability and exceptional cycling performance.(3)Based on the understanding of the lithium storage mechanism,the lithium storage capability of MXene was improved by two-step N,S-doping.Firstly,MXene causes uniformly increased interlayer spacing due to the fact that N-containing species act as pillars between the interlayers.Moreover,the uniformly increased interlayer spacing can provide larger space for Li-ion transport.S element are introduced into the Ti3C2Tx MXene interlayer in the form of C-Ti-S,which is favourable for lithium ion transport.The synergistical effect of N and S dual-doping contribute to rate capability and cycling performance.For example,the NS0.3-Ti3C2Tx anode in lithium ion batteries(LIBs)exhibited excellent comprehensive performance with?89%capacity retention after 1000 charge/discharge cycles at a rate of 3C.When paired up with a LiFe0.5Mn0.5PO4/C cathode,the full cell delivers reversible capacity of 199.3 mAh g-1 after 200 cycles at a rate of 1C.(4)Based on the understanding of the lithium storage mechanism of Ti3C2Tx MXene,via detailed electrochemical,structural,and spectroscopic studies,we provide in-depth insights into Nb2CTx as an intercalation anode material that possess the best-performing MXene anodes in LIBs.Specifically,the root of Nb2CTx excellent electrochemical performance mainly relate to the special architecture of-O dominant terminal group.More importantly,the electrochemical reaction mechanism is elucidated thoroughly through investigation of ex-situ XRD,which demonstrate only a scarce interlayer spacing change occurs during lithium ions intercalation/deintercalation into Nb2CTx,implying zero-strain feature in Nb2CTx upon lithium ions intercalation/deintercalation,which may be due to the stronger interlayer coupling of MXene.By coupling with a LiFePO4/C cathode,as-assembled full Li-ion cell presented superior rate capability and cycling stability as well,the full cell delivers reversible capacity of 120.2 mAh g-1 after 200 cycles at a rate of 1C.