Performances of lithium manganese oxide prepared by hydrothermal process

J. Cent. South Univ. (2014)21: 1279 - 1284包SpringerDOI: 10. 1007/511771-014-2063-8.Performances of lithium manganese oxide prepared by hydrothermal processKONG Long(孔龙)"2, LI Yunjao(李运姣) ?, ZHANG Peng(张鹏)，HUANG Hai-hua(黄海花)’, YE Wang-qi(叶万奇)', LI Chun-xia(李春霞)，21. School of Metallurgy and Environment, Central South University, Changsha 410083, China;2. Citic Dameng Mining Industries Limited, Nanning 530028, China;3. Changsha Research Institute of Mining and Metallurgy Corporation Limited, Changsha 410083, ChinaC Central South University Press and Springer-' Verlag Berlin Heidelberg 2014Abstract: A simple hydrothermal process followed by heat treatment was applied to the preparation of spinel Li1.osMn1.9sO4 In thisprocess, electrolytic manganese dioxide (EMD) and LiOHH2O were used as starting materials. The physiochemical properties of thesynthesized samples were investigated by thermograv imetry-diffrential scanning calorimetry (TG-DSC), X-ray diffractometry(XRD), and scanning electronic microscopy (SEM). The results show that the hydrothermally synthesized precursor is an essentialamorphous. The precursor can be easily transferred to spinel powders with a homogeneous structure and a regularly-shapedmorphology by heat treatment. Li1.osMn1.9sO4 powder obtained by heat treating the precursor at 430 °C for 12 h and then calcining at800 °C for 12 h shows an excellent cycling performance with an initial charge capacity of 118.2 mA:hrg' obtained at 0.5C rate and93.8% of its original value retained after 100 cycles.Key words: lithium ion batteries; LiMn2O4; hydrothermal method; heat treatmentsuch advantages as high potential, low cost, good1 Introductionthermal stability and low toxicity, over compounds basedon cobalt or nickel [10- 12]. It is known that the qualityLithium-ion batteries are efficient, light-weight, andof the LiMn2O4 powders is greatly related to the startingrechargeable power sources for consumer electronics,materials and the preparation route， including thesuch as digital cameras, laptop computers, and cellularprecursor synthetic method and the following heatphones. Lithium cobalt oxide, LiCoO2, is currently thetreatment procedure. The physical and chemicalpopularly used positive electrode material in commercialproperties of the LiMn2O4 materials, such as latticeLi-ion batteries, due to its high reversible capacityparameters, particle size, stoichiometry and average Mn(130- 150 mAh:g ), high working voltage (3.6 V), longvalence as well as electrochemical performance, arecycle-life (300- -500 cycles) and easy preparation [1highly associated with the preparation conditions. So far,Unfortunately, the safety and high cost issues, the poorLiMn2O4 is only commercially obtained by solid staterate performance and the toxicity of cobalt associatedreactions. However, the final products contain largewith LiCoO2 limit its use in electrical hybrid vehicles .irregular particles as well as impurity phases, which have(HEVs) and plug-in hybrid electric vehicles (PHEVs).negative effects on electrochemical performance. InThis limitation has motivated the study of otheraddition, it is difficult to control the morphology,compounds that contain less or no cobalt at all, such ashomogeneity, and microstructure of particles. Recently,Li-Ni-O [2], and Li-Mn-O systems [3-4]. LiNiO2, as asoft methods such as microemulsion routes [15]positive electrode material, possesses low cost and hightopochemical methods [16]， and rheological-phase-rechargeable capacity [5]. However, its drawbacks, suchassisted microwave methods [17] could address thoseas the difficulty to prepare a pure material [6], the lowproblems to some extent, but usually lead to complexthermal stability in the charged state [7- -8], and the poorprocesses and high cost of reagents. Hydrothermalcapacity retention upon long range cycling [9], have toprocess, as a useful method for the preparation 0be overcome before commercialization.advanced materials, has lots of advantages [18]. It hasLiMn2O4 has been atracted as an important cathodebeen used for the preparation of the precursors with highmaterial for rechargeable Li-ion batteries since it haspurity, great hor中国煤化工which mayFoundation item: Project(50174058) supported by the National Natural Science Foundation of ChineMHC N M H Ge Glorious LarelScholar Program of Guangxi Zhuang Autonomous Region, ChinaReceived date: 2012-11-09; Accepted date: 2013-11-11Corresponding author: LI Yun-jiao, Professor; Tel: +86-73 1-88830476; Fax: +86-73 1-88710171; E-mail: yujaoli6601@hotmail.com1280J. Cent. South Univ. (2014) 21: 1279- 1284affect the phase transformation with different routes in85% (mass fraction) active material, 10% acetylenethe subsequent heat treatment.black and 5% polyvinylidene fluoride (PVDF) in N-In this work, a hydrothermal process was used formethyl-2-pyrolidinone (NMP) was coated onto anthe synthesis of the lithium manganese oxide precursoraluminum foil and dried at 120 °C for 12 h underusing EMD and LiOHH2O as raw materials. Thevacuum. The cathode was punched as a circular diskprecursor was subsequently subjected to heat treatmentfrom the foil, and Li metal disk was used as the anode.to obtain spinel lithium manganese oxides as positiveThe electrolyte was based on 1 mol/L LiPF6 in a 1:1materials for Li-ion batteries. The experimental results(volume ratio) mixture of ethylene carbonate (EC) and)n the physiochemical properties and the cycledimethyl carbonate (DMC). The thin polypropylene filmperformances of the materials obtained by different heatwas used as the separator. The cell assembly set-up wastreatment procedures were mainly reported in detail.performed in an argon-filled glove box. Galvanstaticcharge/discharge studies were carried out at 0.5C2 Experimental(1 C=148 mA:hg ") rate between 3.0 and 4.3 V at roomtemperature with Land (CT2001A) cell systems.2.1 Synthesis and characterizationThe lithium manganese oxide precursor was3 Results and discussionprepared by a simple hydrothermal process using EMD(w(Mn)259.36%，Xiangtan Chemical Industry, China)3.1 Morphology and structure of materialsand LiOHH2O (purity>98.9%, Sichuan Tianqi LithiumFigure 1 shows the TG-DSC curves for the thermalIndustries, Inc, China) as starting materials. A certaindecomposition of the complex precursor prepared by theamount of EMD, typically 200 g, was pre-treated andhydrothermal method. These results display four massthen added to 1.0 L stainless steel autoclave (GCF-1 Lloss temperature regions: 30- 130.5, 130.5- -431.7, 431.7-Weihai Jingda Chemical Machinery Co, Ltd.) with786.4 and 786.4- -900 °C. The lttle (about 1.02%) massrequired amounts of LiOH:H2O and deionized water. Theloss of the first region may be attributed to the superficialautoclave was then sealed and heated up to the designedwater loss due to the hygroscopic nature of the precursortemperature to fulfill the hydrothermal process. At thecomplex [20]. The mass loss (about 3.64%) of the secondend of the test, the slurry was discharged and subjectedregion may be attributed to the loss of chemically bondedto solid- -liquid separation to obtain the precursor. Thwater which locates between the sheets of [MnO;]detailed procedure was described in our previous studyoctahedra in the samples coupled with release of oxygen[19].gas. The mass loss of the second (about 3.64%) and theThe obtained lithium manganese oxide precursorsthird (about 1.86%) regions between 130.5 and 786.4。Cwere heat treated in a muffle furnace at a temperatureis a complex thermal process that includes the crucialbetween 400 and 900 °C for 5 h to get the information onformation step for spinel-structured Li .osMn| 9sO4.he phase transformation. In order to obtain theTypical DSC data show an exothermic peak aroundspinel-structured Li.osMn1.9sO4 powders with mor426.1 °C, which corresponds to the transformation 0uniform morphology and better electrochemical cycliMn(+4) to Mn(+3) with the release of oxygen gas. Theperformance，the precursor was pre-heat treated atmass loss of the last region may due to the formation of430 °C for 12 h and then calcined at 700, 750, 800 andoxygen-lack Li.osMn2O4s [21].850 °。C for 12 h, respectively.The XRD patterns of the precursor and the samplesThe structure of the as-prepared powders was obtained at different heat temperatures are shown incharacterized by X-ray diffraction (XRD， RigakuD/max-2500) with Cu Ka radiation. The scanning angle106426.1。Cwas varied from 10° to 80° and the scanning velocitywas 1 (°)/min. Furthermore, the particle morphology ofthe products was examined by means of scanning会心Mass change: -1.02%electron microscopy (SEM， JEOL JSM-6360LV)昌98-130.5°Mass change: -3.64%1 3 ;operated at 20 kV. The thermogravimetric analysis (TG)Mass change:94一431.7°C 1 .86%and differential scanning calorimetry (DSC) were786.4°C> aperformed using a NETZSCH STA 449 C instrument in复90Mass change: -1.02%7王the temperature range of 30- 900 °C at a scan rate of10 °C/min.86_中国煤化工--CNMHG9002.2 Electrochemical measurementMYHFor electrochemical testing, a slurry mixed withFig.1 TG-DSC data of as-synthesized Li-Mn-O materialJ. Cent. South Univ. (2014)21: 1279- 12841281Fig.2. The powder XRD pattern data reveal that theprecursor is an amorphous material with low crysallinity.Besides, some weak unidentified peaks appear at about20=42° and 53°, which are consistent with Ref. [22]. A .further increase in the reaction temperature to 500 °C-。拿，induces a progressive reductin of Mn*+ to Mn2*. This850°results in the pure cubic spinel Li.osMn.9gsO4 formedwithout any other impurity peaks detected. Notably,when the precursor synthesized by hydrothermal methodis heat treated between 500 and 900 °C for 5 h, no800 Lmnimpurities, such as Mn2O3 or Mn;O4, are detected. Thisresult strongly suggests that the present hydrothermalmethod is much superior to conventional solid-statereaction for obtaining spinel lithium manganese oxide750°clJL_ mnpowders. The fact may be attributed to the followingfactors. First, the Lit ion diffusion rate in such aliquid-solid reaction system is much higher than that in a700°Chk_msolid state system, which results in the easy formation of102030405(50 70 80the chemically combined lithiated manganese dioxidewith uniform and atomic-scale distribution of metals,Fig. 3 XRD patterns of precursor pre-heated at 430。C for 12 h .namely，Li-Mn-O precursor. Second, the as-preparedand then calcined at different temperatures for 12 hprecursor has an amorphous structure with high activity,which makes it easily transform to designated spinel8.244LiMn2O4 structure at relatively low temperature.8.242◆Unknown(g多8.240(1)e); 8.238 t8.236 t8.23470075800850、(c)200)2 56Temperature/PCFig.4 Lattice parameters of precursor pre-heated at 430 °C for(a)2(30 40 5(500812 h and then calcined at different relatively higher20/(9)temperatures for 12 hvarious temperatures for 5 h at rising rate of 3 °C/min:obtained from the Rietvelt refinement on the XRD data.(a) Precursor; (b) 400。C; (c) 500。C; (d) 600 °C; (e) 700 °C;It can be seen that the values of the lattice parameters of(f) 800 °C; (g) 900 °C (Insert: Expanded view of samplesthe samples increase with increasing the temperature.heat- treated at 400。C and 500。C detected at 20-40°-55)The phenomena might be owing to the increase in Mn3+concentration in the materials, which has a larger ionicThe representative X-ray diffraction patterns of eachradius (r= =0.645 A) than Mn4+ (1- =0.530 A). It has beensample preheated at 430 °C for 12 h and then calcined atreported that Mn2+ is more stable than Mn+ at higherdifferent temperatures for 12 h are shown in Fig. 3. Alltemperatures [22].diffraction peaks can be indexed by the cubic spinel. InFigure 5 shows the surface morphology and theaddition, the XRD patterns for the sample heatedgrain size of Li1.osMn.9sO4 investigated by SEM. It isbetween 700 and 850 。C show high-intensity peaksfound that the spinel crystals can be observed from thecorresponding to planes (111), (311) and (400). ThisSEM images of the powders obtained at 700 °C. Withconfirms the occupancy of lithium ions in tetrahedral 8aincreasing the t中国煤化工stals becomesites, and the manganese ions in 16d sites as well assmooth and vCN M H Gsie becomesoxygen ions in 32e sites [23]. Figure 4 shows the effectlarger. The spine, cryouunsil a uivii, nearly cubicof the heat treating temperature on the lattice constant,structure morphology and a narrow size distribution1282J. Cent. South Univ. (2014) 21: 1279- 1284(bd0.5 umFig. 5 Typical SEM photographs of Li osMn.9sO4 powders preheated at 430。C for 12 h and then calcined at different relativelyhigher temperatures for 12 h: (a) 700 °C; (b) 750 °C; (c) 800 °C; (d) 850 °C4.4perfectly form in the relatively higher temperature. Thismeans that the morphology and the particle size of theLi.osMn2O4 spinel powder can be controlled by altering≥4.0the calcination temperature. The results indicate that theformation temperature of spinel phase is rather low, butthe growth of the crystalline and the well-shaped兰3.6-1 700 oCparticles needs rather a high temperature [24]. The4- 850°C .characteristics of the materials， such as a highS 3.2-homogeneity and a granular shape, can be atributed tothe homogeneous distribution of the metal ions at an12 43atomic scale in the precursor.2.8δ 20 4060801001203.2. Elctrochemical performanceFigure 6 shows the initial discharge curves at 0.5CFig.6 Initial discharge curves of Li osMn1.9sO4 powderspre-heated at 430 °C for 12 h and then calcined at differentof the samples obtained at different calcinedtemperatures for 12 htemperatures. The discharge curves display two plateaus,which is typical electrochemical characteristic o1LiMn2O4 [25- 26]. It can also be seen that the dischargefrom the loss of oxygen may play an important role andthus lead to the microstructure defects. In fact, wecapacity of all samples increases with temperature up todefinitely detect a mass loss from the TG curve above800 °C and slightly decreases at 850 °C. The increase in800 °C, as shown in Fig. 1. It is suggested that twoinitial capacity is attributed to the improvement of thehomogeneity and the crystallinity of the materials.influence the electrochemical performances. TheHowever, it should be observed that the capacityimprovement of the homogeneity and the crystallinity ofdecreases from 118.2 mA:hrg' to 113.4 mA:h:g 1 as thethe materials taking place between 700 and 800 °C maytemperature increases from 800 to 850 °C, indicating thatgive the first中国煤化工formation ofthere are other factors in the thermal treatment process .oxygen-lack Lidefects thatthat affect the electrochemical behavior in a negative way.result in the eledMYHCNM HG,:terioration atAmong them, altered structure of the powders that results850 °C may occupy the vital factor.J. Cent. South Univ. (2014)21: 1279- 12841283The discharge characteristics, as a function of cyclenumber at a rate of 0.5C between 3.0 and 4.3 V, of theReferencessamples obtained under different heat treatmentconditions are shown in Fig. 7. In general, all theWINTER M, BESENHARD J O, SPAHR M E, NOV K P. 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