Y2O3包尖晶石

ORIGINAL PAPERExcellent cycling stability of spherical spinel LiMn2O4by Y2O3 coating for lithium-ion batteriesBowei Ju&Xianyou Wang&Chun Wu&Qiliang Wei&Xiukang Yang&Hongbo Shu&Yansong BaiReceived:28July2013/Revised:21August2013/Accepted:23August2013#Springer-Verlag Berlin Heidelberg2013Abstract The Y2O3nano-film is coated on the surface of the spherical spinel LiMn2O4by precipitation method and subse-quent heat treatment at550°C for5h in air.The structure and performance of the bare LiMn2O4and Y2O3-coated LiMn2O4 are characterized by powder X-ray diffraction,scanning elec-tron microscopy,transmission electron microscopy,energy dispersive analysis X-ray spectroscopy,galvanostatic charge–discharge,cyclic voltammetry,and impedance spec-troscopy.It has been found that the addition of Y2O3does not change the bulk structure of LiMn2O4,and the thickness of the Y2O3coating layer is approximate to3.0nm.The1wt% Y2O3-coated LiMn2O4electrode reveals excellent cycling performance with80.3%capacity retention after500cycles at1C at25°C.When cycling at elevated temperature55°C, the as-prepared sample still shows76.7%capacity retention after500cycles.These remarkable improvements indicate that thin Y2O3coating on the surface of LiMn2O4is an effective way to improve the electrochemistry performance. Besides,the suppression of Mn dissolution into the electrolyte via the Y2O3coating layer can be accounted for the improved performances.Keywords Lithium ion batteries.Spherical spinel lithium manganese oxide.Surface coating.Yttrium oxide.Cycling stabilityIntroductionLithium-ion batteries,which use the lithium transition metal oxides as the positive electrode,have become attractive energy storage systems for portable electronic devices and the powering of emission-free vehicles[1–3].Many studies of lithium rechargeable batteries have been performed in attempts to develop a nontoxic cathode material with a high power density and excellent thermal stability.In this regard, LiMn2O4with a spinel structure is one of the most promising cathode materials due to its good intrinsic characteristics,such as high energy density,low cost,environmental friendliness, and high thermal stability under severe environmental condi-tions[4,5].However,LiMn2O4suffers fast capacity fading during cycling,especially at elevated temperatures,which is the major obstacle to its further commercialization.The rea-sons for capacity fading are ascribed to several factors:Jahn–Teller distortion[6–8],Mn3+dissolution[9],and electrolyte oxidation[10].To overcome the aforementioned problems,two types of methods can be employed.One is the elemental substitution in crystal lattice to stabilize the crystal structure[11–13], although the elemental substitution cathode materials still suffer from significant capacity decline at elevated tempera-ture(50–60°C).The other method is to coat LiMn2O4with various protective layers.Many researchers have attempted to improve the cyclic performance by means of metallic oxide coating layers,which inhibit the Mn3+dissolution of LiMn2O4,such as Al2O3[14–16],SiO2[17],La2O3[18], TiO2[19,20],ZrO2[21,22],CeO2[23–25],MgO[26],and LiAlO2[27].Among these,Arumugam et al.[18]have re-ported that the2.0wt%SiO2-coated LiMn2O4exhibits a good cyclic performance.The capacity retention is95.2%after 100cycles at1at30°C.Another report by Ha et al.[25] describes the improvement of the cycle behavior of2wt% CeO2-coated LiMn2O4,and the capacity retention of the coat-ed LiMn2O4is more than82%after150cycles between3.0 and4.4V at1C at room temperature.Over the past several years,yttria oxide has been used as various ceramics and luminescent materials because of its high corrosionresistance, B.Ju:X.Wang(*):C.Wu:Q.Wei:X.Yang:H.Shu:Y.BaiKey Laboratory of Environmentally Friendly Chemistry andApplications of Ministry of Education,School of Chemistry,Xiangtan University,Xiangtan,Hunan411105,Chinae-mail:wxianyou@J Solid State ElectrochemDOI10.1007/s10008-013-2241-xoutstanding thermostability,and excellent chemical inertness.If these merits could be applied in the regards of lithium ion batteries,it will be a significant work.It will significantly reduce the direct contact area of LiMn 2O 4/electrolyte interface and decrease the dissolution of manganese effectively.Bai et al.[28]coated Y 2O 3on the LiMn 2O 4by mixing Y(NO 3)3with commercial LiMn 2O 4,and the electrochemical perfor-mance of the formless Y 2O 3-coated LiMn 2O 4synthesized via this simple method is relatively limited.Moreover,the analy-sis of the electrochemical properties is also insufficient.In this paper,Y 2O 3have been successfully coated on the surface of spherical LiMn 2O 4sample by precipitation method.The ef-fects of coating on the structural and electrochemical proper-ties are investigated in detail.ExperimentalPreparation of the spherical spinel LiMn 2O 4The precursor MnCO 3is synthesized by the coprecipitation method.An aqueous solution of MnSO 4with a concentration of 1.6mol L −1is pumped into a continuously stirred tank reactor.At the same time,a 1.6mol L −1Na 2CO 3solution (aq)and the desired amount of NH 4OH solution (aq)as the chelating agent are also separately fed into the reactor.The obtained spherical MnCO 3precursor is then filtered,washed,and dried at 80°C.Then,the as-prepared powder and an appropriate amount of Li 2CO 3are mixed homogeneously and calcined at 750°C for 20h to obtain the spherical spinel LiMn 2O 4.Synthesis of the Y 2O 3-coated LiMn 2O 4The Y 2O 3-coated spherical LiMn 2O 4are prepared via precip-itation method.Figure 1shows the schematic diagram ofthe formation process of the Y 2O 3-coated LiMn 2O 4.Y(NO 3)3·6H 2O are dissolved in distilled water,and the pH value is adjusted to about 9.0by aqueous buffer solution of NH 4Cl/NH 3·H 2O with stirring.Subsequently,the as-prepared LiMn 2O 4powders is added to the solution with vigorous stirring for 2h,sonicated for 2h to obtain a suspension,then filtered and washed to obtain a black deposit,and finally evaporated at 80°C.The obtained coated LiMn 2O 4powder is heated at 550°C for 5h.The expected amount of the Y 2O 3coating is 1and 2wt%of the total LiMn 2O 4powder weight,respectively.Physical characterizationsThe phase identification of the samples is performed with a diffractometer (D/Max-3C,Rigaku,Japan)using Cu K αra-diation (λ=1.54178Å)and a graphite monochromatoratFig.1Schematic diagram of the formation process of the Y 2O 3-coated LiMn 2O4Fig.2X-ray diffraction patterns of the bare LiMn 2O 4and Y 2O 3-coated LiMn 2O 4samplesFig.3TG and TGA curves for the Y(OH)3-covered LiMn 2O 4materialsJ Solid State Electrochem36kV ,20mA.The scanning rate is 8°min −1,and the scanning range of diffraction angle (2θ)is 10°≤2θ≤80°.Thermal gravimetric analysis (TG/TGA)of Y(OH)3covered sample is made on a TG-DTA analyzer (Model TAS-200,Rigaku,Tokyo,Japan)in flowing air atmosphere (200ml min −1)with a heating rate of 10°C min −1.The morphology of the sample is observed using scanning electron microscopy (SEM,JEOL JSM-5600LV)and high-resolution transmission electron mi-croscopy (HRTEM,JEOL JEM 2010).Energy dispersive analysis spectroscopy is obtained in conjunction with TEM to roughly determine the element content of powders.Electrochemical characterizationsThe electrochemical tests are examined using CR2025coin-type cells.In all cells,the cathode consisted of a mixture of active material (80wt%),acetylene black (10wt%),graphite (5wt%),and polyvinylidene fluoride (5wt%)as the binderagent;lithium metal is used as the counter and reference electrodes;a Celgard 2400is used as the separator;and the electrolyte is a 1mol L −1LiPF 6solution in ethylene carbon-ate –dimethyl carbonate (1:1,v /v ).The charge –discharge mea-surements are carried out on a Neware battery test system (BTS-XWJ-6.44S-00052;Newell,Shenzhen,China).The electrochemical impedance spectroscopy (EIS)of the cells is measured on a VersaSTAT3electrochemical workstation (Princeton,America)in the frequency range of 10kHz to 10mHz with an AC voltage of 5mV .Results and discussionStructure and morphology analysisFigure 2shows the X-ray diffraction (XRD)patterns of the bare LiMn 2O 4and Y 2O 3-coated LiMn 2O 4samples.All of thea bdc e fFig.4SEM images of a the bare LiMn 2O 4,b 0.5wt%,c 1.0wt%,d 2.0wt%Y 2O 3coated LiMn 2O 4samples at high magnification and e the bare LiMn 2O 4,f 1.0wt%Y 2O 3-coated LiMn 2O 4samples at low magnificationJ Solid State Electrochemdiffraction peaks correspond to a well-defined cubic spinel structure with space group F d 3m except for the low intensity peaks at 2θ=21–24°that assigned to Y 2O 3in 2.0wt%Y 2O 3-coated LiMn 2O 4.The impurity diffraction peaks cannot be observed in 0.5and 1.0wt%Y 2O 3-coated LiMn 2O 4samples.It is possible because the content of Y 2O 3is low and the nanothin film of Y 2O 3on the surface of LiMn 2O 4may be too thin to be detected.Furthermore,the lattice constant of uncoated LiMn 2O 4and 0.5,1.0,and 2.0wt%Y 2O 3-coated LiMn 2O 4samples are0.825,0.824,0.824,and 0.824nm,respectively.Small changes in the lattice parameter for all samples show that Y 2O 3coating does not enter the spinel structure but is just deposited on the surface of LiMn 2O 4.TG/TGA is used to determine the decomposition tempera-ture of the Y(OH)3covered sample.In order to observe clearly weight loss of TG curve and estimate the decomposition temperature,the 50wt%Y(OH)3covered sample are purpose-fully chosen for the measurement.As seen in Fig.3,the first weight loss is seen between 150and 210°C due to thec dba eFig.5TEM images of a the pristine LiMn 2O 4and b 1wt%Y 2O 3-coated LiMn 2O 4,HRTEM images of c the pristine LiMn 2O 4and d 1wt%Y 2O 3-coated LiMn 2O 4,EDAX results of e 1wt%Y 2O 3-coated LiMn 2O 4J Solid State Electrochemevaporation of hydrate from the powder[29].Then,a distinct decrease is seen at500°C,which may be attributed to the decomposition and combustion of yttrium hydroxide and slight yttrium nitrate.The third weight loss in the range of 780–840°C should be related to a partial decomposition of the LiMn2O4to Li–Mn–O compounds containing a higher con-centration of Mn3+[7].However,no obvious change is ob-served at above900°C.Therefore,the heat treatment temper-ature of550°C is chosen for after coating procedure.The surface morphology of the bare LiMn2O4and Y2O3-coated LiMn2O4particles are presented in Fig.4.As shown in Fig.4a,the sample is made up of secondary particles formed by the primary particles(<1μm)aggregating together. Moreover,in Fig.4b–d,the edges and corners of the coated primary particles become blurred and the surfaces of the coated samples become more and more coarse.The reason is due to presence of the Y2O3coating layers,LiMn2O4 particles can prevent from contacting with the electrolyte,thusinhibiting Mn3+dissolution.Furthermore,it can be observed in Fig.4e and f that the particle sizes of bare spinel are uniform,and there are not clear size differences between the bare and the1.0wt%Y2O3coated LiMn2O4particles.To further identify the surface morphology of the bare LiMn2O4and Y2O3-coated LiMn2O4,TEM(HRTEM)and energy dispersive analysis X-ray spectroscopy(EDAX)are car-ried out.The spherical particles are ground by agate mortar before placed on a carbon film-coated TEM grid.Figure5a and b presents the TEM images of the bare LiMn2O4and 1.0wt%Y2O3-coated LiMn2O4cathode materials.The pristine LiMn2O4exhibits smooth surface,while the surface-treated LiMn2O4shows coarse surface and is covered by a uniform nanofilm.It may be caused by the electrochemically inactive Y2O3layer.Examination at higher resolution(Fig.5d)further reveals that the coating layers are uniformly distributed on the particle surfaces and the thickness approximate to2.9nm.It manifests that Y2O3coating has been successfully deposited on the surface of the spherical LiMn2O4.Figure5e shows the EDAX results of1.0wt%Y2O3-coated LiMn2O4.The existence of Y suggests that Y2O3may exist in the surface of the treated LiMn2O4.Electrochemical performanceThe initial charge–discharge curves of the bare LiMn2O4and 0.5,1,and2wt%Y2O3-coated LiMn2O4samples at a rate of 0.5C between3.0and4.4Vat room temperature are shown in Fig.6.All the charge–discharge curves exhibit two obvious discharge plateaus associated with the two-stage mechanism of the electrochemical lithium intercalation and extraction at 3.9–4.0V and4.0–4.1V[30].This indicates that the Y2O3 coating does not change the intrinsic structure of LiMn2O4 during the insertion and extraction of lithium ions.Instead,the surface coating can help to stabilize the structure of LiMn2O4and prevent fast capacity decay,which will be further con-firmed and supported by cyclic voltammetry(CV)curves. Compared with the bare one,the Y2O3-coated LiMn2O4sam-ples show slightly low capacity.It is probably because Y2O3 coating is electrochemically inactive.Moreover,the Y2O3 layer may both increase the contact resistance between parti-cles and the charge transfer resistance between the cathode and the electrolyte.The electrochemical cycling performances of the bare LiMn2O4and the0.5,1,and2wt%Y2O3-coated LiMn2O4at 1C between3.0and4.4V at room temperature are shown in Fig.7.The Y2O3-coated sample shows a much better cycling capability than the uncoated sample.The discharge capacity of the bare LiMn2O4fades from127.4to74.0mA h g−1after 500cycles,58.2%of the original capacity is only reserved. However,the capacity retentions are70.0%for0.5wt%, 80.3%for1.0wt%,and70.2%for2.0wt%Y2O3coating Fig.6Initial charge–discharge curves of the bare and Y2O3-coated LiMn2O4at0.5C in the voltage range of3.0–4.4VFig.7Cycling performances of the bare LiMn2O4and Y2O3-coated LiMn2O4cycle at1C between3.0and4.4V at room temperatureJ Solid State Electrochemafter 500cycles,respectively.These results apparently manifests that the Y 2O 3coating can facilitate the diffusion of lithium ions with the appropriate content.It also can be clearly observed that the cycle performance of the Y 2O 3-coated LiMn 2O 4samples increases with the content of coating increasing from 0.5to 1.0wt%,while it decreases with the content of Y 2O 3coating increasing from 1.0to 2.0wt%.With a too low Y 2O 3coating amount,the LiMn 2O 4surface may not be fully protected,and results in inferior cycling performance.Besides,as Y 2O 3increases beyond the optimal coating con-tent,the excess Y 2O 3will hinder the transportation of lithium ions,leading to the evident decay of the capacity.However,there is not notable decrease in the initial discharge capacity between the bare LiMn 2O 4(127.4mA h g −1)and 1.0wt%Y 2O 3-coated sample (123.9mA h g −1).Based on the above analysis,1.0wt%Y 2O 3-coated LiMn 2O 4is the optimum composition to enhance the stability and cycling performances of LiMn 2O 4.The elevated temperature (55°C)performance is usually an important evaluation criterion for lithium ion battery.At an elevated temperature of 55°C,1.0wt%Y 2O 3-coated mate-rials exhibit distinctly improved capacity retention compared with the pristine LiMn 2O 4.The cycling performances of the pristine LiMn 2O 4and 1.0wt%Y 2O 3-coated LiMn 2O 4sample at 55°C are shown in Fig.8.The discharge capacity of the bare LiMn 2O 4fades from 127.2to 43.0mA h g −1with a retention of 34.1%after 500cycles between 3.0and 4.4V at 1C.However,the discharge capacity of the 1.0wt%Y 2O 3-coated LiMn 2O 4clearly shows an improved cycling be-havior compared with the bare one in the same conditions.The discharge capacity of the 1.0wt%Y 2O 3-coated LiMn 2O 4fades from 117.5to 90.2mA h g −1with a retention of 76.7%of its initial capacity.In addition,this is markedly superior to the results modified by Al 2O 3[14–16],SiO 2[17],La 2O 3[18],TiO 2[19,20],ZrO 2[21,22],CeO 2[23,25],MgO [26],and LiAlO 2[27].Therefore,it is proposed that 1.0wt%Y 2O 3coating is an outstandingly effective way for improving the elevated temperature capacity retention of LiMn 2O 4.In order to investigate the effect of Y 2O 3-coating on the mechanism of spinel LiMn 2O 4,cyclic voltammetry tests are conducted on the pristine LiMn 2O 4and 1.0wt%Y 2O 3-coated sample at a scan rate of 0.1mV s −1in the voltage range of 3.0–4.4Vat 25°C.As seen from Fig.9,two cyclic voltammograms with two couples of well-defined redox peaks and similar shape further reveal that the Y 2O 3coating does not alter the electro-chemical mechanism of LiMn 2O 4during cycling.These two couples of redox peaks reflect the typical redox processes of these electrodes in the potential region from 4.0to 4.2V ,which can be ascribed to electrochemical insertion and extraction reactions of Li +proceed in two steps [31–33].In addition,the CV peaks related to the 1.0wt%Y 2O 3-coated samples are much sharper and appear with much smaller hysteresis,compared to the peaks of the uncoated LiMn 2O 4.These differences mean that the kinetic properties of the Y 2O 3-coat-ed LiMn 2O 4are better than those of uncoated material [19].Fig.9Cyclic voltammetric curves of initial and 500th cycle for a the bare LiMn 2O 4and b 1.0wt%Y 2O 3-coated LiMn 2O 4cycled between 3.0and 4.4V at a scan rate of 0.1mV s −1at roomtemperatureFig.8Cycling performances of the bare LiMn 2O 4and 1.0wt%Y 2O 3-coated LiMn 2O 4cycled between 3.0and 4.4V at elevated temperature (55°C)J Solid State ElectrochemTo further discuss the above results,the CVs of the coated and uncoated samples with the galvanostatic charge –discharge measurements after 500cycles at room temperature are shown in Fig.9.As being noted,the oxidation and reduction peaks related to 1.0wt%Y 2O 3-coated LiMn 2O 4electrode are much steadier compared to the peaks of the bare electrode after 500cycles.Consequently,1.0wt%Y 2O 3-coated LiMn 2O 4has better reversibility and stability than the pristine LiMn 2O 4.In order to further analyze the electrochemical properties about the electrode process and the conductivity of the elec-trode materials,EIS measurements are employed.Figure 10shows the Nyquist plots of the pristine LiMn 2O 4and 1.0wt%Y 2O 3-coated LiMn 2O 4after first,100th cycle and 500th cycle with fully charged state (4.4V)at 1C.The two samples exhibit typical Nyquist characteristics.An intercept in the high frequency region of the Z real axis corresponds to the ohmic resistance (R s ),the combined resistance of the electrolyte and the contacts of the cell [27].The semicircle in high-middle frequency region indicates the charge transfer process on the electrode interface,revealing the lithium transfer rate param-eters as well as the capacitance of the SEI (solid electrolyte interface)/electrolyte double layer [34].The inclined line in the lower frequency region represents the Warburg impedance (Z w )[35],which is associated with the diffusion of Li +in LiMn 2O 4particles.The equivalent circuit used for fitting Nyquist plots is shown in the Fig.10c in which constant phase element stands for the double-layer capacitance.The plots are fitted,and the results are listed in Table 1.The R s for 1.0wt%Y 2O 3-coated LiMn 2O 4exhibits slightly larger than the bare LiMn 2O 4at the first fully charged state.The reason is because the inactive Y 2O 3coating on the sample surface will slightly increase the resistance of the electrolyte and the contacts of thecell before cycling.In the cycling process,the Mn 3+dissolu-tion occurred by the corrosive reaction,which causes the partial electrolyte decomposition and then makes the value of the resistance increased.However,the Y 2O 3coating can inhibit the Mn 3+dissolution and then decrease the tendency of the R s increase.Thus,the R s values of pristine sample became larger than coated sample after cycling.Moreover,the charge transfer resistance (R ct )of the pristine LiMn 2O 4is greatly increased during the cycles.On the contrary,the R ct value of the 1.0wt%Y 2O 3-coated LiMn 2O 4is relatively reduced.The R ct value of the pristine LiMn 2O 4electrode at the first cycle is 25.51and significantly increased to 51.21after 100cycles and 99.37after 500cycles.The coated electrode varied slightly from 26.16at the first cycle to 33.68after 100cycles and 47.64after 500cycles.It means less deposition of Mn and LiF on the electrode surface.Therefore,the enhanced cycling stability of Y 2O 3-coated material should be linked to the improved interfacestability.Fig.10Nyquist plots of a the bare LiMn 2O 4and b 1.0wt%Y 2O 3-coated LiMn 2O 4at 1st,100th,and 500th fully charged state at 1C.c The equivalent circuit modelTable 1The AC impedance fitting results for pristine LiMn 2O 4and 1.0wt%Y 2O 3-coated LiMn 2O 4R s /ΩR ct /ΩPristine LiMn 2O 4at 1st fully charged state2.0325.511.0wt%Y 2O 3-coated LiMn 2O 4at 1st fully charged state 2.1126.16Pristine LiMn 2O 4at 100th fully charged state3.6551.211.0wt%Y 2O 3-coated LiMn 2O 4at 100th fully charged state2.6533.68Pristine LiMn 2O 4at 500th fully charged state5.0199.371.0wt%Y 2O 3-coated LiMn 2O 4at 500th fully charged state3.5247.64J Solid State ElectrochemConclusionsA uniform Y2O3nanofilm has expectedly been coated on the spinel LiMn2O4,and the effects on electrochemical perfor-mance are pared with the pristine LiMn2O4, 1.0wt%Y2O3-coated LiMn2O4exhibits much better cycle performance especially at an elevated temperature(55°C). The capacity retentions of1.0wt%Y2O3-coated LiMn2O4at 1C after500cycles are80.3and76.7%at25and55°C, respectively,while those of pristine LiMn2O4are58.0and 34.1%,respectively.The improved performance of the surface-coated sample is because the Y2O3coating on the surface of LiMn2O4can prevent direct contact between the LiMn2O4particles and electrolyte,thus reducing the dissolu-tion of Mn3+and the oxidation of electrolyte.EIS also shows that the coated sample possesses a lower charge transfer resistance(R ct)than the uncoated sample during cycling. 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