2Dtitaniumcarbide(MXene)electrodeswithlowerFsurfac

Journal of Energy Chemistry 31 (2019) 148-153ELSEVIERContents lists available at ScienceDirectJourndl of Energy Chemistryjournal homepage: /locate/jechemJGC■IOURNAI OF GNCRGY C HCMISTRYhttp://www.journals.elsevier.8m/journal-of-energy-chemistry/2D titanium carbide (MXene) electrodes with lower-F surface for high performanee lithium-ion batteriesMing Lu&b, Haojie Li bc , Wenjuan Han bd , Junnan Chen b , Wen Shi b , Jiaheng Wang c , Xiang-Min Meng e , Jingang Qi f , Haibo Li d , Bingsen Zhang b *, Wei Zhang 3-*, Weitao Zheng 3*a Key Laboratory of Automobile Materials MOE, School of Materials Science & Engineering. Electron Microscopy Center, International Center of Future Science. Jilin University, Changchun 130012, Jilin, Chinab Shenyang National Laboratory for Materials Science. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Liaoning, Chinac Department of Mechanical Engineering, Hefei University of Technology, Xuancheng 242000, Anhui, Chinad Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, Jilin, Chinae Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, Chinaf School of Material Science & Engineering, Liaoning University of Technology, Jinzhou 121001, Liaoning, ChinaARTICLE INFOABSTRACTArticle history:Received 20 April 2018Revised 30 May 2018Accepted 30 May 2018Available online 14 June 2018Keywords:Ti 3C 2MXene Li-ion Hydrogen -F contentMXene has shown distinctive advantages as anode materials of lithium-ion batteries. However, local sur ­face chemistry, which was confirmed that can block ion transfer and limit redox reaction, has a significant effect on electrochemical performance. Herein, annealing MXene under hydrogen was employed for re ­moving -F and turning -OH to -0 terminations. We dem o n s trate that 让 improves the kinetics of Li-ion transport between the electrolyte and electrode. As a result, a lower interfacial charge transfer impedance was obtained. The electrochemical measurement exhibited that a nearly 2-fold increase of specific capac ­ity was achieved for the annealed MXene.© 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Publishedby Elsevier B.V. and Science Press. All rights reserved.1. IntroductionCommercial rechargeable lithium-ion batteries (LIBs) widely adopt graphite as anodes. However, graphite anodes cannot meet the booming demand of energy storage due to the poor Li ion storage capacity. Therefore, searching new anode materials for LIBs becomes urgent [1-3]. In 2011, Naguib et al. obtained the earliest 2D transition metal carbides, known as MXene (T13C2), by selective etching Al layer from Ti 3AlC 2, and predicted that it can be a host for Li storage [4].Ti 3AlC 2 + 3HF = A1F 3 + 3/2H 2 + Ti 3C 2 (1)Ti 3C 2 + 2H 2O = Ti 3C 2(OH)2 + H 2(2)Ti 3C 2-F2HF = Ti 3C2F2 + H 2 (3)' Corresponding authors.E-mail addresses: bszhang@ (B. Zhang), weizhang@ (W. Zhang), wtzheng@ (W. Zheng).Then, the multilayer T13C2, the Ti 3C 2 paper, and the functional ­ized derivatives were experimentally explored as anode materials for LIBs. Inspired by the investigation of other 2D materials, the 3D porous structure [5], "pillar effect" [6], and hybrid materials were employed for enhancing the electrochemical performance of Ti 3C 2 MXene [7], In addition, a series of computational investiga ­tions indicated that surface functionalization has significant impact on both structural and electronic properties of MXenes. For pin- poiting the surface chemistry of Ti 3C 2, X-ray photoelectron spec ­troscopy (XPS) analysis was employed to unravel the presence of -0, -OH and -F terminations. Besides, energy-dispersive X-ray spectroscopy (EDS) and nuclear magnetic resonance (NMR) charac ­terizations were used to quantify the ratios of Ti 3C 2, (OH)0.i2« F0.8 and O0.54 [8]. Meanwhile, extensive works were conducted to un- derstand the role of surface functional groups on Li ion storage. By using density functional theory (DFT) calculation, the surface func- tionalization of -F and -OH blocks Li transport and decreases Li storage capacity from 320 of bare Ti 3C 2 to 130 mAh/g for Ti3C 2F 2, and 67 mA h/g for Ti 3C 2 (OH)2 [9], and a lower Li diffusion barrier was confirmed for bare MXene [10]. However, the -0 terminations were found to enable significantly improving the electrochemical performanee due to a redox storage mechanism [11 ]. To alleviatehttps:///10.l0l6/j.jechem.20l8.05.0172095-4956/© 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rightsreserved.M.Lu et al./Journal of Energy Chemistry31(2019)148-153149this deficiency,removing-F and converting-OH to-0were con­ducted for achieving a high electrochemical performance.In this work,to understand the mechanism between MXene surface chemical groups and electrochemical performences,a hy-drogen annealing approach was used to remove the-F termination on the surface of Ti3C2.The results are benefitial.Thus,a universal strategy is proposed to interfacially enhance the ion transport for 2D and other layered materials.2.Experimental2.1.Synthesis of low-F Ti3C2MXeneMultilayer Ti3C2was synthesized by etching MAX parent phases of Ti3AlC2(400mesh,purchased from11Technology Co.,Ltd.)in HF,as described elsewhere[12].1g of Ti3AIC2powder was im­mersed slowly in a high density polyethylene beaker containing 20mL of an aqueous concentrated HF(40%)at room temperature (RT).Then,the suspension was stirred at a speed of700rpm for 24h at RT.Afterward the concentrated HF solution was decanted and the solid was filtered.Fresh deionized(DI)water was then added to wash the obtained powder several times until the pH value of liquid was larger than 5.The washed powder was vac­uum filtered to facilitate drying at RT.The as-prepared Ti3C2pow­der was then armealed at500°C under hydrogen atmosphere for 30min.To delaminate the MXenes,100mg of the annealed Ti3C2was added into100mL of deoxidated DI water.The mixture solution was further sonicated(the power of the sonifier is above1200W) in a cold bath for5h under Ar,and then the centrifugation was re­peated at3000rpm for5min.The supernatant was filtered under vacuum in order to fabricate the flexbile,free-standing Ti3C2films (a Ti3C2film of12mg was obtained by using〜24mL supernatant). The prefabricated films were stored in an argon-filled glove box.2.2.Structural characterizationThe morphologies and structures of MXene sheets were exam­ined by using scanning electron microscope(SEM,Hitachi SU8010) and transmission electron microscope(TEM)(FEl Tecnai G2Spirit). Powder X-ray diffraction(XRD)data were recorded using a Rigaku D/Max-2500diffractometer w让h graphite monochromatized Cu Ka radiation(入=0.15406nm)at40kV and200mA.Chemical compo­sition and oxidation state of the samples were further analyzed using high-resolution XPS with monochromated Al Ka radiation (hv=1486.6eV).Bin d ing energies were referenced to the C Is peak of(C-C,C-H)bond,which was set at284.8eV.X-ray absorp­tion spectra(XAS)of Ti K edge(4.966keV)were collected in the range of4.766-5.766keV at Shanghai Synchrotron Facility(SSRF) on beamline BL14W[13].The SSRF storage ring was operated at an electron energy of3.5GeV w让h beam current of250MA.A Si (111)double-crystal monochromator was applied.All the data were collected with transmission mode at ambient temperature.X-ray absorption near-edge structure(XANES)and extended X-ray ab­sorption fine structure(EXAFS)were analyzed by ATHENA software package|14].2.3.Electrochemical characterizationThe working electrodes were the prefabricated Ti3C2film,and cells were assembled into coin cells(CR2032)in which Li sheets were used as the counter and referenee electrodes.The elec­trolyte was1M LiPFg solution in ethylene carbonate(EC)/dimethy catbonate(DEC)/ethyl methy carbonate(EMC)(1:1:1,v:v:v).In addition,conducting resin was employed for enhancing con t act between Ti3C2flexible film and electrode shell.The cells were assembled in an argon-filled glove box with the concentrations of moisture and oxygen below0.5ppm.Cyclic voltammetry(CV) was performed on CHI660E Electrochemical Workstation(Shang-hai Chenhua,China)from0.05to3V versus Li+/Li at0.5mV/s. Galvanosratic charge/discharge(GCD)measurements were carried out using a LAND battery tester(Wuhan LAN HE,China).Electro­chemical impedance spectroscopy(EIS)was performed at open cir­cuit potential with a10mV amplitude(10mHz-200kHz)via VMP3 Electrochemical Workstation(bio-logic,France).3.Results and discussionAs shown in Scheme1,Ti3C2was prepared by etching MAX parent phases of Ti3AlC2(400mesh)in HF.The multilayer Ti3C2 powder was annealed under hydrogen to generate the low—F ter-mination Ti3C2.Ti3C2MXene sheet aqueous suspension can be ob­tained by sonication-assisted exfoliation.After a simple vacuum fil­tration process,the low-F terminated Ti3C2,flexible free-standing film electrode can be obtained.Fig.1shows the XRD patterns of Ti3AlC2,Ti3C2and the low-F termination pared with the patterns of MAX which are consistent with the JCPDS52-0875,the disappearance of the most intense diffraction peak at〜39。