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1、Synthesis and electrochemical properties of high performance yolk-structured LiMn2O4microspheres for lithium ion batteries Yu Qiao, Si-Rong Li, Yan Yu and Chun-Hua Chen* Yolk-structured microspheres of spinel LiMn2O4are successfully prepared by a specially designed multi-step synthesis procedure inv
2、olving precipitation, controlled oxidation, selective etching and chemical lithiation. Solid-structured and hollow-structured LiMn2O4are also synthesized by a similar method for comparison. X-ray diff raction, scanningelectron microscopy, transmission electron microscopy, BrunauerEmmettTeller method
3、 and IR spectroscopy are employed to study their structures and compositions. The electrochemical properties of the LiMn2O4/Li cells are also tested. The results indicate that LiMn2O4powder composed of yolk-structured microspheres possesses remarkable high rate capability and outstanding high capaci
4、ty retention not only at room temperature but also at elevated temperatures. This study may provide signifi cant new insight into restraining the capacity fading of LiMn2O4electrodes and the yolk- structured LiMn2O4may be used for the next generation of lithium ion batteries. 1Introduction Inthepast
5、fewdecadeswehavewitnessedtherapiddevelopment of lithium ion batteries (LIBs) in response to the increasing needs of energy storage and conversion.13For example, they have made it possible for traditional gasoline-powered automo- biles to evolve into a generation of electric vehicles (EV). As the mos
6、t suitable substitute to replace the conventional LiCoO2 cathodematerial,spinel-structuredLiMn2O4hasbeen spotlighted due to its high power density, natural abundance, low material cost and environmental benignity.47However, it exhibits a serious capacity fading during charge and discharge, especiall
7、yatelevatedtemperatures.Thefadingcanbeattributed toseveralfactorssuchasJahnTellerdistortionduetoMn3+ions, spinel dissolution into the electrolyte and electrolyte decompo- sitioninthehighpotentialregions.812Thus,theimprovementin theelectrochemicalreversibilityandstabilityofLiMn2O4athigh rates and ele
8、vated temperatures becomes more and more important for scientists.1315 Recently, LiMn2O4nano-materials with various morphol- ogies have been prepared to improve the cycle life and rate capability,1620because the poor performance of LiMn2O4is also attributed to the long distance for lithium ions to d
9、iff use in the case of large particle sizes. Although the nano-structured LiMn2O4 is very eff ective in improving the rate capability, the low tap density of nano-sized powders directly leads to the low energy density of a cell. To achieve a high tap density, electrode materials are preferred to hav
10、e micron-sized particles, espe- cially with spherical shapes which can packmore densely.2125In our study, we want to nd a balance between high tap density and short diff usion distance. Inspired by the work of Qian et al. who have synthesized hollow structures of Mn2O3, MnO2and Mn2O3microspheres,262
11、8we design and synthesize a special yolk-structured LiMn2O4microsphere (Scheme 1b). In this structure model, the outer shell is porous and composed of many nanoparticles, which can enlarge the specic surface area of the electrode and provide more reaction sites for lithium insertion and extraction.
12、The core is relatively dense to increase the volumetric energy density. The spacing between the shell and the core can buff er any volume change of the core during heating/cooling or charge/discharge. In the present work, we rst synthesize a yolk-structured Mn2O3as a precursor and then follow a simp
13、le solid-state reaction or chemical lithiation to produce the yolk-structured LiMn2O4microsphere (LMO-Y) (Scheme 1a). Moreover, we compare the electrochemical performance of LMO-Y with those of hollow LiMn2O4microspheres (LMO-H) (Scheme 1c) and solid LiMn2O4microspheres (LMO-S) (Scheme 1d). 2Experim
14、ental 2.1Synthesis of MnCO3microspheres Manganese sulphate (MnSO4$H2O, 0.507 g) and sodium bicar- bonate (NaHCO3, 2.32 g) were separately dissolved in 210 ml of CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technolog
15、y of China, Anhui Hefei 230026, China. E-mail: ; Fax: + 86 551 3601592; Tel: + 86 551 3606971 Electronicsupplementaryinformation(ESI)available:Samplesurface morphology,electrodethicknessmeasurement,andthree diff erent yolk-structured LiMn2O4samples (conditions, structures and cycling properties) are
16、 given. See DOI: 10.1039/c2ta00204c Cite this: J. Mater. Chem. A, 2013, 1, 860 Received 6th September 2012 Accepted 25th October 2012 DOI: 10.1039/c2ta00204c www.rsc.org/MaterialsA 860 | J. Mater. Chem. A, 2013, 1, 860867This journal is The Royal Society of Chemistry 2013 Journal of Materials Chemis
17、try A PAPER Published on 25 October 2012. Downloaded by Jilin University on 11/04/2014 13:27:09. View Article Online View Journal | View Issue distilled water. About 21 ml of ethanol was then added to the MnSO4solution under vigorous magnetic stirring and the NaHCO3solution was then directly added t
18、o cause precipita- tion. The mixture solution was maintained at room temperature (nearly 25 ?C) for 3 h. A precipitate of MnCO3 was obtained by centrifuging and washed with distilled water and absolute alcohol for several times, and then dried at 80 ?C for 1 h in a vacuum. 2.2Synthesis of yolk-struc
19、tured MnCO3(core)MnO2(shell) microspheres The as-prepared MnCO3(0.2 g) was dispersed in 40 ml of distilled water. Then 20 ml of 0.032 mol L?1KMnO4solution was added under vigorous stirring for 40 min to form a homo- geneous solution. Then 20 ml of 0.6 mol L?1HCl was added to the above suspension and
20、 the mixture was maintained with stirring for 2 min followed by a rapid centrifuging step. The as- obtained powder was washed with distilled water and absolute alcohol for several times. It was nally dried at 80 ?C for 1 h in a vacuum. 2.3Synthesis of hollow MnO2microspheres The synthetic process wa
21、s similar to the steps above to prepare the yolk-structured MnCO3(core)MnO2(shell) microspheres. The diff erence was that the concentration of the added HCl solution was 4.2 mol L?1instead of 0.6 mol L?1to dissolve completely the MnCO3core and leave only a hollow MnO2shell remained aer this step. 2.
22、4Synthesis of LMO-S, LMO-Y and LMO-H The above as-prepared MnCO3microspheres, yolk-structured MnCO3(core)MnO2(shell) microspheres and hollow MnO2 microspheres were separately calcined at 530 ?C for 15 h. The obtained Mn2O3 powders with diff erent structures were sepa- rately mixed and ground with li
23、thium acetate (CH3COOLi$H2O) in the stoichiometric molar ratio (Li : Mn 1 : 2). Then the mixtures were calcined at 285 ?C for 3 h and subsequently cal- cind at 650 ?C for 6 h. All the heat treatment processes were carried out in air atmosphere. Finally, three spinel LiMn2O4 powders with diff erent m
24、icrostructures, LMO-S, LMO-Y and LMO-H, were prepared. It should be mentioned that the synthesis methods here for LMO-S and LMO-H are similar to previous reports.29,30 2.5Morphology and structure characterization Powder X-ray diff raction (XRD) was performed on a diff rac- tometer (Philips Xpert Pro
25、 Super with Cu Ka radiation) at room temperature. The XRD patterns were recorded in the 2q range from 10?or 20?to 80?at a scan rate of 2?min?1. A scanning electron microscopy (SEM, JSM-6390 LA, JEOL) study of the Scheme 1Schematic illustration of the synthetic process of a yolk-structured LiMn2O4mic
26、rosphere (a) and the sphere structures of yolk-structured (b), hollow (c), and solid LiMn2O4(d). This journal is The Royal Society of Chemistry 2013J. Mater. Chem. A, 2013, 1, 860867 | 861 PaperJournal of Materials Chemistry A Published on 25 October 2012. Downloaded by Jilin University on 11/04/201
27、4 13:27:09. View Article Online powders was performed to analyze their morphologies. The samples were analyzed under a Hitachi H-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. Fourier transformation infrared (FTIR) spectroscopy was also conducted with a Magna-IR 750
28、 spectrometer in the range of 4004000 cm?1with a resolution of 4 cm?1. The specic surface area of the powders was determined by the BrunauerEmmett Teller (BET) method using a Beckmancoulter SA3100 specic surface area and aperture tester. 2.6Electrochemical performance tests The LiMn2O4active materia
29、ls, acetylene black and poly- (vinylidene uoride) (80 : 10 : 10, w/w/w) were mixed into slur- ries in N-methyl-2-pyrrolidone in an agate mortar. They were then cast on aluminum foils with a doctor blade and dried at 70 ?C for 10 h to obtain several electrode laminates. Discs (diameter 14 mm) of the
30、laminates were punched, dried at 70 ?C for 2 h in a vacuum oven and then were transferred to an argon-lled glove box (MBraun Labmaster 130). A typical loading of the active material was 6.06.5 mg. Aerwards, CR2032 type coin-cells with Li as the counter electrode were assembled in the glove box with
31、the electrolyte of 1 M LiPF6 solution in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1, w/w, Zhuhai Smoothway Electronics Materials Co. Ltd). The cyclic voltammograms (CV) of the cells were measured on a CHI 604A electrochemical workstation from 3.2 to 4.35 V at a scan rate of 0.1 mV s
32、?1. The cells were also cycled on a multi- channel battery test system (Neware BTS-2300, Shenzhen) in the voltage window from 3.2 to 4.35 V. The room temperature was controlled at 25 ?C (?3?C) by an air-conditioner. The high temperature performances of the cells were also tested by laying them in an
33、 oven at 55 ?C (?1?C). 3Results and discussion 3.1Design of the synthesis procedure for yolk-structured LiMn2O4 As shown in Scheme 1a, a simple precipitation step is employed to prepare dense MnCO3spheres. Then, a controlled oxidation by KMnO4of these MnCO3spheres is followed to form a MnO2 (shell)M
34、nCO3(core) structure. The oxidation reaction can be written as 2KMnO4+ 3MnCO3 5MnO2+ 3CO2+ K2O. In the solution environment, K2O can react with water to form KOH which is soluble. Therefore, the products CO2and K2O can cause the micro-porosity of the MnO2shell, as evidenced by SEM analysis (Fig. S1a
35、). In the following step, dilute hydro- chloric acid does not react with MnO2but can penetrate through the micropores of the MnO2shell and react with the MnCO3cores. With the control of the HCl amount and the reaction time, the MnCO3core is partially etched and the yolk- structuredMnCO3(core)MnO2(sh
36、ell)microspheresare generated (Fig. S1b). Then, these spheres are heat-treated in air at a high enough temperature (530 ?C) to convert both the MnCO3and MnO2into Mn2O3. Thus, yolk-structured Mn2O3 spheres can be produced. Note that the Mn2O3shells should be with micropores (Fig. S1c). In the last st
37、ep, yolk-structured LiMn2O4spheres are prepared by a simple reaction between Mn2O3and lithium acetate at a high temperature (650 ?C). Again, the LiMn2O4shell is expected to be porous, which is conrmed by our experiments (Fig. S1d). 3.2Structures of MnCO3, MnCO3(core)MnO2(shell) and LiMn2O4 The cryst
38、al structure of the as-synthesized product was exam- ined by powder X-ray diff raction (XRD) and the result is shown in Fig. 1. The pattern of the synthesized MnCO3precursor (Fig. 1a) can be indexed as a pure phase (JCPDS 44-1472). The intermediate products MnCO3MnO2(Fig. 1b) and MnO2 (Fig. 1c) show
39、 only two broad peaks of a poorly crystallized MnO2, in agreement with the literature.26,28In Fig. 1df, the XRD patterns of the powders show that all LiMn2O4samples are well crystallized and can be identied as a pure spinel LiMn2O4 structure (JCPDS 35-0782). Fig. 2 shows the particle sizes and morph
40、ologies of the LiMn2O4samples and their precursors. As shown in these SEM images, the three LiMn2O4samples and their corresponding precursors are all rather homogeneous spherical spheres with a diameter of about 1.5 mm, which are very useful for further processing. The TEM images of the LiMn2O4powde
41、rs (Fig. 2df) conrm the successful syntheses of their designed microstruc- tures LMO-S, LMO-Y and LMO-H. In fact, the SEM observations of some broken spheres in LMO-Y and LMO-H also conrm the presence of hollow structures. It can also be seen that aer the calcinations and reaction with lithium aceta
42、te, the LiMn2O4 samples (Fig. 2df) are found to be smallerthan theirprecursors (Fig. 2ac) and the surface becomes rougher, which is probably caused by the emission of CO2gas. Particularly, the LiMn2O4 shells in LMO-Y and LMO-H are even rougher than LMO-S, suggesting that the LiMn2O4shells are compos
43、ed of many nanocrystalline grains. The TEM images also reveal that the shell thickness of LMO-Y and LMO-H is all nearly 100 nm and the inner diameter of LMO-Y is about 700 nm. Fig. 1 X-Ray diff raction patterns of (a) MnCO3microspheres, (b) MnCO3MnO2 coreshell structured precursor for LMO-Y, (c) hol
44、low MnO2precursor for LMO-H, (d) LMO-S, (e) LMO-Y, and (f) LMO-H. 862 | J. Mater. Chem. A, 2013, 1, 860867This journal is The Royal Society of Chemistry 2013 Journal of Materials Chemistry APaper Published on 25 October 2012. Downloaded by Jilin University on 11/04/2014 13:27:09. View Article Online
45、 3.3Electrochemical properties of solid, hollow and yolk- structured LiMn2O4powders The cycling stabilities of the samples at room temperature are tested by galvanostatically charging/discharging the cells under diff erent rates. Fig. 3ac reveal the cycle performance of the three LiMn2O4samples at 2
46、5 ?C at 0.2 C, 2 C and 10 C between 3.2 and 4.35 V. The rst discharge capacities of LMO-S, LMO-Y and LMO-H are 132.4, 128.9 and 130.2 mA h g?1at 0.2 C rate, with the capacity retention of 92.9%, 98.2% and 96.2% aer 100 cycles at 25 ?C. The discharge capacity of LMO-S decreases quickly with increasin
47、g the discharge rate from 0.2 C to 10 C. Moreover, the capacity retention of LMO-S falls to 88% while those of LMO-Y and LMO-H are both above 96% aer 100 cycles at 25 ?C at 2 C rate (Fig. 3b). When the rate increases to 10 C, the rst discharge capacities of LMO-S, LMO-Y and LMO-H are 73.4, 89.7 and
48、94.7 mA h g?1, with the capacity retention of 76.8%, 95.3% and 87.9%, respectively (Fig. 3c). In Fig. 3d, the discharge capacities as a function of discharge rates of 0.2 C, 1 C, 2 C, 5 C and 10 C are compared. It obviously reveals that the discharge capacities and cycle stability of Fig. 2SEM image
49、s of microspheres of (a) MnCO3, (b) MnCO3(core)MnO2(shell), (c) yolk-structured MnCO3(core)MnO2(shell), (d) LMO-S, (e) LMO-Y, and (f) LMO-H. The TEM images of (d) LMO-S, (e) LMO-Y and (f) LMO-H are also given as insets. Fig. 3Cycling performance of the LiMn2O4samples at the rate of (a) 0.2 C, (b) 2 C, and (c) 10 C at 25 ?C and (d) plots of discharge capacity values of the LiMn2O4 samples at diff erent rates from 0.2 C to 10 C. This journal is The Royal Society of Chemistry 2013J. Mater. Chem. A, 2013, 1, 860867 | 863 PaperJournal of Materials Chemistry A Published on 25 October 20
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