二氧化锰催化剂

CHEM．RES．CmNESE UNIVERSITIES 20 10，26(2)，283—286 Comparative Studies on Structure and Electronic Properties Between Thermal Lithiated Lio ． 5M nO2 and LiM n204 wANG Deng．pan ，cHEN Hong ’-，Du Fei ，BIE Xiao·fe LIU Li—na ，wEI Ying-jin ， CHEN Gang an dⅥrANG Chun—zhong J．Department ofMaterials Science，College ofMaterials Science and Engineering, dilin University,Changchun 130012 P R China； 2一College ofPhysics,Beihua University,Jtlin 132013,P．R China； 3．College ofPhysics,4．State Key Laboratory ofSuperhard Materials， Jilin University ，Changchun 130012，P R．China Abstract M onoclinic Lio 5MnO2 was synthesized by solid state reaction and the spectral and magnetic properties were studied in comparison with those of spinel LiMn2O4．The XI paUem an d Raman spectrum of Lio ． 5MnO2 are different from those of LiMn2O4，which indicate the different long-range and short-ran ge crystal structure．XPS result shows the binding energies of 2p3／2 an d 2pl／2 in Li0 ， 5MnO2 are located at 642．3 and 653．6 eV,respectively．Through fitting the XPS spectra,the valence state of Mn ion in Li0 ． 5MnO2 coincides with that in LiMn204．The high-temperature susceptibil 哆of Li0 5MnO2 can be fitted by Curie-Weiss law whose Curie and Weiss constants are 33 A·m2．K／(mol·T)and-277(6)K，respectively．Although Li0 ． 5MnO2 shows spin glass ground state，the transition temperature ofLin Mn02 is about 9 K lower than that ofLiMn204． Keywords Li0 ． 5MnO2；Raman ；XPS；Spin glass Artiele ID 1005．9040(20101-O2-283-O4 1 lntroduction Lithium ion batteries have been extensively used in portable electronics．Man ganese-based oxides are considered to be one of the most promising alternative cathode materials due to their economical and envi- ronmental advan tages[¨ ． M oreoveL the basic under- standing of the physical properties of man gan ese— based oxides is also of significan t interest because many of electrochemical properties such as charge capacity an d capacity retention upon cycling are closely related to cation ordering and phase stabi- lity【2’3t ． Spinel LIMn204 is a typical cathode material ex- hibiting not only good electrochemical performan ce but also interesting magnetic groun d state 】 ． For example LiMn204 has space group Fd3m and the average valence ofMn ions is 3．5 based on the X-ray photoemission spectrum results．Spin glass state is suggestedat alow temperature of25K duetothefru- stration competition among ferromagn etic M n4 一 02_一Mn4 ．antiferromagnetic 1~]113 一O2_一Mn and direct interaction Mn 忡 ．Mn [4】． Recently,a new mangan ese-based compoun d， Li0 ． 33Mn02，has been synthesized as 3 V cathode ma- teria1 by 1ow．temperature solid state syn thesis．which delivers a reversible discharge capacity of 1 40— 1 9O mA·h·g- 【oJ ． The arrangement of【MnO6]octahedron within the hexagonal close--packed oxygen lattice pro-- videdtwodifferent sitesforMnions，nam edMno)an d Mrs2)．respectively．The formal valency of Mn ions in Li0 ． 33M NO2 is suggested to be +3．67 based on the study of Mn K—edge XANES，which is higher than tllat of LiMn204(+3．51．In this work,we synthesized Lio 5MnO2 material by low-temperature solid state method brought forward in Ref．『61 and studied the structural，spectral an d magnetic properties by XRD， Ram an scattering，XPS and SQUID in contrast with those of spinel LiMn204． 2 Experimental Li0 ． sMnO2 was prepared by the conventional so- lid state reaction．Details of the sample preparation Corresponding author．E-mail：wcz@jlu．edu．en Received August 24，2009；accepted October 1 6，2009． Supported by the National Natural Science Foundation of China(No．5067203 1)，the Special Funds for Major State Basic Research Project of China(No．2OO9CB220 lO4)，Program for Changjiang Scholar and Innovative Research Team in Universities of China(No．IRT0625)and Jilin Province Project of Research and Development,China(Nos．200605 1 1 and 20075007)． 284 CHEM．RES．CHINESE UNⅣ ERSITlES V_o1 ．26 were reported in Ref f61．Powder X-ray difiraction (XRD)was employed to analyze the produced phase an d the crystal structure on a Bruker AXS difhacto． meter with Cu Ka l radiation =0．1 5406 nm)．The X-ray photoemission spectrum(XPS)was performed on an ESCALAB spectrometer(VG Scientific1 equipped with XR5 Monochromated X—ray Gun(15 kV, 1 50 W、．Magnetic characterization was perform ed by a superconducting quantum interference device mag— netometer(Quantum DesignⅣ MS．XL1． 3 Results and Discussion 3．1 XRD and SEM Fig．1(A)illustrates the powder XRD pattern of Lin 5M nO2 material Obtained by low．temperature solid state method．which reveals that the sample is fl single phase with monoclinic space group C／2m ’“ ． As can be seen in Fig．1(A)，the XRD peak is broad，indicating the bad crystallization of the material due to the low sintered temperature．In order to study the process of Li0 ． 5M nO2 phase form ation，the sam ples sintered at different temperatures were obtained with the XRD patterns listed in Fig．1(B1．It has been found that the higher the sintered temperature，the'sharper the peak intensity and the narrower the half-peak breadth． en the sintered temperature increased to 660 。C． the sample changed into the spinel structure LiMn204 with space group翮 ．After indexing．the lattice param eters ofLiM n204 are a= b： c= 0．82409 inn， similar to those in the Refs．【1】and【21． 2 (。) 20／(。) Fig．1 XRD patterns of Lio ． sMnO2(A)and samples sintered at different temperatures(B) a．460。C；b．560。C；c．660。C． Furthermore，ICP was employed to ascertain the ion ratio of the low—temperature synthesized sam ple． The mass percentages of Li and Mn ions were calcu— lated to be 3．27％ and 54．86％．respectively,with an error of 1％ from ICP measurement．The compositio- nal form ula of our prepared material is suggested to be Li0 5MnO2，where the oxygen content is set to be 2．0． In sunimary,although t ． he low·-temperature synthe-· sized sam ple has the similar molecular form ula to that of LiMn204，the long—ran ge crystal structure is diffe- rent． Fig．2 shows the SEM photos of two samples sin— tered at different temperatures．The SEM photo of Li0 5Mn02 form s large particle cluster which is in cot- respondence with the broad peak in the XRD pa~ern and bad crystallization．W hile the sam ple of LiMn204 shows good crystallization and particle size at about 】O0 nn1． Fig．2 SEM images of Lio ．sMnO2(A，360。C)and LiMn2O4(B，660。C) 3．2 Raman Scattering Fig．3 shows the Ram an pattems oftw o sam ples sintered at different temperatures．It can be seen that Raman pattem of Li0 ． 5MnO2 displays one broad peak at 634 cm- ，belonging to the A mode，which is in cOrrespOndence with the stretch vibration Of Mn—O—Mn in the[MnO6】octahedra．In comparison with the Raman patterns of Li0 33MNO2 “，the peak position of Li0 5Mn02 shows a blue—shift．Such a phe- nomenon indicates the stronger interaction between M n and O ions．which may induce be~er electro- chemical performance than that of Lio ． 33MNO2．Fur— thermore，the Ram an pattcm of LiMn204 is also pre· sented in Fig．3．It shows a similar pattem to the pre- vious reported one【 in the line shape and peak posi- tion．In comparison wim that Of the Raman spectrum Ramltn shift／cm一 Fig．3 Raman spectra of Lio ．sMnO2(a，360。C) and LiMn2O4(6，660。C) No．2 WANG Deng-pan etal 285 of LiMn204，peak position of Lio 5Mn02 displays a blue shift，which may imply【MnO6】octahedra in the monoclinic phase is more stable than that in the or。 thorhombic system． 3．3 XPS Study X-ray photoelectron spectroscopy has been widely used to characterize the valence state of mate- rials．Binding energies are used to identify different elements and their valence states．M oreoveL using the relative area under the deconvoluted XPS bands．we can obtain a semiquan titative estimation of the va- lence states of the elements in the mixed-valent com— pounds． Fig．4 presents the high-resolution XPS spectra of Mnap for Lio ． 5Mn02．The Mn2p core-level spectra show a typical two-peak structure(2p3／2 and 2p1／2)due to the spin—orbit splitting．The XPS spectrum is referenced to the C1 line，which is located at 285 eV．After standar- dizing with C1 peak position，the peak s at 642．3 and 653．5 eV are attributed to Mn2p3／2 an d Mn2pl／2，respec- tively．Chowdari et a1．【6J has repoged that the M n2p3／2 XPS binding energy of Mn” an d Mn4 ions are 641 ．9 an d 643．2 eV,respectively．In our experiment，the Mn2p3／2 binding energy of Li0 5MnO2 is in this scope， which indicates the mixed valence state of Mn ions． The relative amount of Mn” an d Mn” ions in Li0 5MnO2 can be estimated via deconvoluting the asymmetric Mn2p3／2 XPS spectra,using the dominant Mn” an d Mn4+binding energy values of 641．9 an d 643．2 eV,respectively,as shown in Fig．4．The percen— tages of Mn an d Mn4 ions of the Li0 ． 5Mn02 are 43％ an d 57％，respectively,with the M n average valence state of 3．57，giving a satisfied semiquan tita- tive estimation of stoichiometric LiMn2O4 with an error ofwithin 5％ EdeV Fig．4 M n2p XPS spectra of Lio ． sM n02 3．4 M agnetic Properties The temperature dependences of the Zero-field cooled(ZFC)magnetization and field cooled(FC) magnetization in an applied field of 0．05 T for Li0 ． sMnO2 are shown in Fig．5，together with the reci- procal ZFC／FC curve．The high-temperature region of reciprocal ZFC curve can be well fitted by Cu- rie．W leiss law =(1／( 1 (1) where is the susceptibility,C is Curie constan t an d 0 is Weiss constant ．The Weiss constant was calcu- lated to be-277(6)K which indicates strong antifer- romagnetic interaction．W hile the Curic constant was fitted to be 33 A·m2．K／(mol·T、with which the effec- tive moment eit=5．1(4)／~B／f．u．is obtained by means of Eq．(2) C=NpZeff／3 (2) where N is the number density of magnetic ions per unit gram， is the Boltzmann’s constant【 。， ¨ ． The ZFC／FC curves increase with the decrease of temper- ature．an d show a cusp at about 9 K in the ZFC bran ch iust below the bifurcation temperature of 1 5 K．The presence of bifurcation between MZFC and MFC indi- cates history dependence of the magnetization processes．These features suggest spin glass state in Li0 ． 5MR02 at low temperatures． 0 50 100 150 200 250 300 Fig．5 ZFC and FC susceptibility as a function of temperature between 5 K and 300 K in an ap- plied magnetic field of 0．05 T Inset shows the reciprocal susceptibility,including a Curie·Weiss law fittingtothedataabove200K Furthermore． it is important to compare the magnetic behavior of Lio 5MnO2 with tllat of spinel LiMn204．It is well known that spinel LiMn204 shows spin glass ground state due to the frustrated exchange interaction among Mn ions[12—14】 ． The spin glass tran． sition temperature is about 25 K higher than that of Li0 ． 5MnO2，which indicates the different magnetic be- havior between the two compoun ds． Although Li0．sMn02 has similar valence state of magnetic Mn ions as LiMn204，the difference in long—ran ge an d short—range structures would induce the different spin—glass behavior．In addition，a simple method to 9 8 7 6 5 4 3 2 l O 【【_ ．【_ H m．《一 286 CHEM ．RES．CHINESE UNIVERSITIES V_o1 ．26 estimate the effect of geometrical frustration iS to cal— culate lOl／Tf,where 0 iS Weiss constant and Tf iS the peak temperature in the ZFC curve．It has been pro． posed that the somewhat arbitrary condition，101／Tf>10， be taken as a criterion for the presence of frustra- tion[ ’16】|In Li0 ．5M1102，the frustration parameter f was calculated to be about 30，indicating that the magnetic state of Li0 ． sMn02 is dominated by geome— trical frustration effect．In contrast．although geome． trical frustration has been suggested in cubic LiMn204， the frustration parameter is comparatively lower than that of Li0 5MnO2【13,14】 ． References [1】Gong J．，Wang C．Z．，Liu W．，et a1．，Chem Chinese Universities， 2002，23(8)，1462 [2】WeiY J．，Kyingw N．，KwangB．K．，eta1．，SolidStatelonics，2006， J77-29 【3】3 [4】 Chen B．，Fu Q．，Huang X． ，et a1．，Chem． Chinese Universities， 2003，24(12)，2260 Wei Y J．，Yan L．Y，Wang C．z．，et a1．， 咖 Chem，2004， [12】 [13】 [14] [15] 【16】 108(48)，18547 Hon Y M．，Chung H．Y，Fung K．Z．，et a1．，J=． ffdState 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