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Particuology 22 (2015) 89–94

Contents lists available at ScienceDirect

Particuology

jo ur nal home page: www.elsev ier .com/ locate /par t ic

ighly dispersed Mn2O3 microspheres: Facile solvothermal synthesisnd their application as Li-ion battery anodes

ianfei Yua, Lin Zhua,c, Cheng Fana, Cheng Zanb, Ling Hua, Shuhui Yanga, Qiang Zhanga,∗,ancheng Zhuc, Lin Shid, Fei Weia

Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University,eijing 100084, ChinaState Key Laboratory of Enhanced Oil Recovery, Research Institute of Petroleum Exploration & Development, China National Petroleum Corporation,eijing 100007, ChinaDepartment of Chemical Engineering, Qufu Normal University, Shandong 273165, ChinaKey Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University,eijing 100084, China

r t i c l e i n f o

rticle history:eceived 21 August 2014eceived in revised form 5 October 2014ccepted 11 October 2014

eywords:n2O3

a b s t r a c t

Nanostructured transition metal oxides are promising alternative anodes for lithium ion batteries. Li-ionstorage performance is expected to improve if high packing density energy particles are available. Herein,Mn2O3 microspheres with a ca. 18 �m diameter and a tapped density of 1.33 g/cm3 were synthesizedby a facile solvothermal–thermal coversion route. Spherical MnCO3 precursors were obtained throughsolvothermal treatment and they decomposed and converted into Mn2O3 microspheres at an annealingtemperature of 700 ◦C. The Mn2O3 microspheres consisted of Mn2O3 nanoparticles with an average 40 nm

icrospheresi-ion batteriesolvothermal synthesisanostructures

diameter. These porous Mn2O3 microspheres allow good electrolyte penetration and provide an ion bufferreservoir to ensure a constant electrolyte supply. The Mn2O3 microspheres have reversible capacities of590 and 320 mAh/g at 50 and 400 mA/g, respectively. We thus report an efficient route for the fabricationof energy particles for advanced energy storage.

© 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy ofSciences. Published by Elsevier B.V. All rights reserved.

Alo(hcelfltes

. Introduction

Rechargeable lithium ion batteries (LIBs) have become the dom-nant power source used in portable electronic devices because ofheir excellent low self-discharge rate, high coulombic energy effi-iency, and the absence of a memory effect (Deng, Wan, Xie, Qin, &hen, 2014; Su & Schlogl, 2010;). However, commercial LIB anodeaterials such as graphite have a relatively low Li storage capacity

f only 372 mAh/g. Much effort has been put into the developmentf better electrode materials for LIBs to meet ever-growing perfor-ance demands (Cheng et al., 2013b; Gao et al., 2013; Jiang, Tang,u, Lin, & Qu, 2013; Zhang et al., 2014c).

Nanostructured transition metal oxides are the most widely

nvestigated alternative anode materials for LIBs because of theirarge theoretical specific capacities (Yuan, Wu, Xie, & Lou, 2014).

∗ Corresponding author. Tel.: +86-10-62789041; fax: +86-10-62772051.E-mail address: zhang-qiang@mails.tsinghua.edu.cn (Q. Zhang).

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ttp://dx.doi.org/10.1016/j.partic.2014.10.007674-2001/© 2015 Chinese Society of Particuology and Institute of Process Engineering, C

mong all transition metal oxides, manganese-based oxides haveower operating voltages (average discharge and charge voltagesf 0.5 and 1.2 V, respectively). Anode materials with lower chargebattery discharge process) voltages versus Li/Li+ can deliver aigher energy density. Manganese oxides (MnOx) are an importantlass of the metal oxide family and are believed to be promisinglectrode materials because of their abundance, nontoxicity, andow cost. For instance, manganese oxides with excellent structuralexibility combined with unique chemical and physical proper-ies are widely used in fields like heterogeneous catalysis (Hant al., 2008; Xu et al., 2012), electrocatalysis (Cheng et al., 2013a),upercapacitors (Xu, Kang, Li, & Du, 2010; Yan et al., 2010), andechargeable batteries (Dai, Jiang, Hu, & Li, 2013; Wang et al.,014; Zhang et al., 2014b). Among these, Mn2O3 possesses a highheoretical specific capacity (∼1018 mAh/g) as an anode mate-

ial for LIBs. Recently, Mn2O3 nanorods (Shen, Ji, Miao, Yang, &hen, 2011), nanocubes (Wang et al., 2012), nanoplates (Zhangt al., 2014b), stacking nanoflowers (Zhang, Qian, Zhu, & Tang,014a), porous spheres (Chang et al., 2013; Deng et al., 2012;

hinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

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iao, Yu, Jin, Guan, & Chen, 2014), nanochains (Salavati-Niasari,ohandes, Davar, & Saberyan, 2009), nano-ovals (Qiu et al., 2011),

nd nanocones (Dai et al., 2013) have been reported. Both thetructure and morphologies of Mn2O3 nanostructures can be eas-ly tuned. However, their packing density is usually not very highnd the volumetric density of the as-obtained electrodes shoulde further improved. An efficient method to fabricate high pack-

ng density Mn2O3 nanostructures with excellent Li-ion storageerformance is thus required.

In this contribution, Mn2O3 microspheres were fabricatedhrough the facile solvothermal synthesis from MnCO3 precursorsnd subsequent calcination. Microspheres have a high tapped den-ity and, therefore, are widely employed in the battery industry.he reason we selected solvothermal synthesis is because of advan-ages such as high efficiency, low cost, extraordinary homogeneity,bility to mix starting compounds on the molecular level, and flex-bility to effectively tailor products by tuning synthetic parametersLiu et al., 2012). As-obtained Mn2O3 microspheres exhibit goodlectrochemical performance as anode materials for LIBs.

. Experimental

.1. Synthesis of Mn2O3 microspheres

Mn(NO3)2, ethylene glycol (C2H6O2, 99.9%), and HNO3 (99.9%)ere purchased from Beijing Chemical Company (China) and used

s received. In the preparation of Mn2O3 microspheres, 2.0 mL.0 M Mn(NO3)2 and 2.0 mL 5.0 M HNO3 solution were added tothylene glycol. After stirring for 10 min, the mixtures were trans-erred to a 100 mL Teflon-lined vessel, which was sealed in anutoclave and then treated at 180 ◦C for 10.0 h. As the autoclaveas cooled to room temperature naturally, the Mn2O3 precur-

ors were collected from the reaction solution by centrifugation at0,000 rpm for 10 min. The precursors were washed several timesith ethanol and dried in air at 100 ◦C. The final Mn2O3 micro-

pheres were obtained upon heat treatment of the precursors at00 ◦C for 2.0 h in air.

.2. Characterization

The as-prepared Mn2O3 microspheres were examined by X-rayiffraction (XRD, D8 Advance, Bruker, Germany; CuK�), scan-ing electron microscopy (SEM, JSF7401, JEOL, Japan; 3–20 kV),ransmission electronic microscopy (TEM, JEOL2010, JEOL, Japan;00 kV), Raman spectroscopy (Horiba JY, France; 633 nm laser), X-ay photoelectron spectroscopy (XPS, 250Xi, ESCALAB, USA; MgK�),nd thermogravimetric analysis (TGA, TGA/DSC1, Mettler Toledo,witzerland; elevating heat rate: 10 ◦C/min, O2 atmosphere).

.3. Electrochemical measurements

The composite electrodes consisted of 80% Mn2O3, 10% car-on nanotubes (high purity, made by fluidized bed technology ofsinghua University), and 10% polyvinylidene fluoride (PVDF, 761A,rkema, France) by weight and were prepared by coating the mix-

ure on copper foil. The 2025 coin cells use lithium foil as a counterlectrode, a polypropylene microporous membrane as a separa-or and 1.0 M LiPF6 dissolved in ethylene carbonate (EC), dimethylarbonate (DMC), and ethylene methyl carbonate (EMC) (1:1:1,

/v/v) as the electrolyte. The cells were assembled in an argon-lled glove box. Charge–discharge measurements were carried outsing a Neware battery testing system (CT3008 W) at 100 mA/g inhe range of 0.01–3.0 V versus Li/Li+.

c66m

22 (2015) 89–94

. Results and discussion

.1. Structure of the Mn2O3 microspheres

MnCO3 microspheres with a diameter of ca. 20 �m (Fig. 1(a))ere formed as precursors in the solvothermal process withn(NO3)2 and HNO3 as oxidant, as well as ethylene glycol as theedium. Ethylene glycol provides abundant hydroxy ligands to

oordinate with manganese ions, which reduces the hydrolysisate of the manganese ions and induces the formation of sphericalnCO3 precursor (Liang, Xu, Kuang, & Wang, 2008). The carboxyl

roups produced by HNO3 oxidation inhibit precursor hydrolysisnd favor the formation of microspheres. The rough surfaces of thes-prepared MnCO3 microspheres are composed of tiny grains. Thepherical morphology of the MnCO3 microspheres is attributed tonergy minimization principle in these systems. After heat treat-ent under an air atmosphere at 700 ◦C for 2.0 h, the MnCO3icrospheres decomposed into Mn2O3 microspheres. The Mn2O3icrospheres had an average diameter of about 18 �m. A tapped

ensity of 1.33 g/cm3 was obtained.The crystal structures of the as-synthesized samples were

etermined by XRD (Fig. 1(c)). All the diffraction peaks of therecursor were indexed to rhombohedral MnCO3 (JCPDS Cardo. 85-1109, space group: R-3c, a = b = 4.72926 A, c = 15.48718 A,

= = 90◦, � = 120◦) without other impurities. All the diffractioneaks of the precursors that were annealed at 700 ◦C were perfectly

ndexed to the cubic phase of Mn2O3 (JCPDS Card No. 24-0508,pace group: Ia3, a = b = c = 9.4091 A, = = � = 90◦). According tohe Scherrer formula, the as-obtained Mn2O3 had an average sizef 40.6 nm. A high-resolution TEM image of Mn2O3 is shown inig. 1(d). The lattice fringe with an interplanar distance of 0.473 nmorresponds to the (200) plane of the cubic �-Mn2O3 structure.

To investigate the growth mechanism of the Mn2O3 micro-pheres, concentration-dependent shape evolution studies werelso carried out. When using a 1.0 mL Mn(NO3)2 solution thenCO3 (Fig. S1) were found to be irregular grains because ofstwald ripening. The thermal decomposition behavior of thenCO3 precursors was investigated by TGA and differential scan-

ing calorimetric (DSC) methods (Fig. 2(a)). Three weight loss stepsre present in the TGA profile. The first weight loss is between0 and 250 ◦C in the TGA curve and comes from the desorptionf physically absorbed water, glycol molecules, glycol oligomers,nd other organic molecules (Chen, Yu, & Liang, 2011). The sec-nd sharp weight loss step between 250 and 400 ◦C correspondso the decomposition and oxidation of chemically bonded organicpecies and the partial thermal decomposition of MnCO3 to MnOn the precursors. The third weight loss step between 400 and50 ◦C comes from the transformation of MnCO3 and MnO ton2O3. In particular, 500 ◦C was found to be a critical tempera-

ure for the simultaneous decomposition of MnCO3 and oxidationf MnO (Chang et al., 2013). The DSC curve exhibited an exother-ic peak centered at about 260 ◦C and this corresponds well to

he rapid weight loss in the TGA curve. Consequently, annealinghe Mn2O3 precursors at 700 ◦C for 2.0 h induced the formationf pure �-Mn2O3. The specified surface area of the as-obtained-Mn2O3 was 12.6 m2/g and the pore volume was 0.018 cm3/g

Fig. S2).The Raman spectra of the Mn2O3 precursors and the samples

nnealed at 700 ◦C for 2.0 h are shown in Fig. 2(b). The Raman bandsf around 200–1000 cm−1 come from the Mn O vibration modes ofanganese oxides. The Raman spectrum of the Mn2O3 precursors

ontains Mn O vibrations of MnCO3 and these are located at 953.1,43.2, and 347.8 cm−1. Three Raman bands are centered at 948.8,28.6, and 335.5 cm−1 respectively, and these fit the Mn(III) Oodes observed in the reference Mn2O3 well (Han et al., 2008).

J. Yu et al. / Particuology 22 (2015) 89–94 91

Fig. 1. Structure and morphology of the MnCO3 precursor and the Mn2O3 microspheres: (a) SEM image of the MnCO3 microsphere precursor produced by the solvothermalprocess; (b) SEM image of the Mn O microspheres obtained by thermal decomposition of the MnCO precursor annealed at 700 ◦C for 2.0 h; (c) XRD patterns; (d) HRTEMi

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An XPS measurement of the as-prepared Mn2O3 microspheresas carried out to identify the chemical state of Mn as well as

he composition and purity of the samples (Fig. 2(c) and (d)). Theinding energy (BE) values of Mn 2p1/2 and Mn 2p3/2 in the sam-le were 653.2 and 641.3 eV, respectively (Fig. 2(c)). Spin-orbitplitting was responsible for the difference between the BE val-es of the Mn 2p3/2 and Mn 2p1/2 levels. The observed spin-orbitplitting value was about 11.9 eV, which was nearly the same ashat in manganese oxides. The BE of Mn 2p3/2 (641.3 eV) andpin-orbit splitting (11.9 eV) were nearly the same as the valueseported for Mn2O3 (Salavati-Niasari et al., 2009). Oxygen spectrare also shown in Fig. 2(d). The O 1s peaks at 529.7 and 531.1 eVre attributed to oxygen O2− in the lattice of Mn O Mn.

.2. Mn2O3 microsphere formation mechanism

The formation of uniform Mn2O3 microspheres was investi-ated as follows. First, spherical MnCO3 precursors were obtainedhrough a soft-chemical process. Glycol acted as both a solvent and

ligand. Glycol was oxidized by HNO3 and a series of organic lig-

nds such as ethane diacid, ethanoic acid, and poly(ethylene glycol)ormed. Manganese is oxyphilic and, therefore, it coordinates withhese organic ligands to form hybrid precursors in the initial period.ecause of the minimum surface energy principle, the precursors

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abricated by the soft-chemical process have spherical morphol-gy. During the annealing process, the noncrystalline precursorsonvert into the crystalline cubic phase of Mn2O3. The organic lig-nds decompose and combust leading to the maintenance of theicrosphere’s morphology (see Fig. 3).

.3. Li-ion storage performance of Mn2O3 microspheres

The electrochemical performance of an as-prepared Mn2O3icrosphere electrode as an anode material for LIBs was investi-

ated. Cyclic voltammetry (CV) profiles of the Mn2O3 microspheresre shown in Fig. 4(a). The voltammograms were measured at aweeping rate of 0.1 mV/s in the potential range from 3.0 to 0.02 Vs. Li+/Li at room temperature. The CV curve for the first cycle wasifferent from that of subsequent cycles. The strong peak centeredt 0.36 V in the first cathodic process was associated with the reduc-ion of Mn2+ to Mn0. In subsequent cycles the main cathodic peakhifted to about 0.30 V. For the anodic process, two peaks wereresent at 1.29 and 2.35 V, and these come from the oxidation ofn0 to Mn2+ and Mn2+ to Mn3+, respectively. The first anodic peak

hifted to 1.40 V because of the polarization induced by solid elec-rolyte formation during the first cycle. Compared with the firstischarge process, the peak current decreased and this indicatesapacity loss during the charging process. The cathodic peak for

92 J. Yu et al. / Particuology 22 (2015) 89–94

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ig. 2. Decomposition of the MnCO3 precursors to Mn2O3 microspheres: (a) TGA air; (b) Raman spectra; (c) Mn 2p and, (d) O 1s XPS spectra of the as-prepared Mn2O

n3+ to Mn2+ was faint. However, as shown in the discharge curvesn Fig. 4(b), the slope at 1.5–0.4 V and the voltage plateau at 0.4 Vome from Li-ion insertion for Mn3+ to Mn2+ and Mn2+ to Mn0,espectively. The lithium storage mechanism of the Mn2O3 micro-pheres is as follows (Chang et al., 2013; Deng et al., 2012):

Li+ + 3Mn2O3 + 2e → 2Mn3O4 + Li2O (1)

Li+ + Mn3O4 + 2e → 3MnO + Li2O (2)

nO + 2Li+ + 2e → 4Li2O + Mn (3)

With an increase in current density the discharge capacity of then2O3 anode decreases gradually. Specific discharge capacities of

90, 435, 377, and 320 mAh/g at 50, 100, 200, and 400 mA/g, respec-ively, were obtained (Fig. 4(c)). This is similar to the Li-ion storageerformance reported for porous Mn2O3 spheres (∼300 mAh/g at00 mA/g for Mn2O3 calcined at 700 ◦C) (Chang et al., 2013), straw-

heaf-shaped Mn2O3 (∼370 mAh/g at 400 mA/g) (Qiu et al., 2011),nd hollow Mn2O3 nanocones (380 mAh/g at 400 mA/g) (Dai et al.,013). This good rate performance is attributed to rapid ion diffu-ion through the mesopores of the Mn2O3 microspheres.

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Fig. 3. Proposed formation mechanism

C profiles of the Mn2O3 precursors recorded from room temperature to 900 ◦C inrospheres.

The cycling stability of the Mn2O3 microspheres was eval-ated at a current density of 100 mAh/g. Fig. 4(d) showslots of the discharge capacity and coulombic efficiency versusycle number. A rapid capacity fading was detected overhe initial five cycles, and this stabilized over the follow-ng 95 cycles. The coulombic efficiency was nearly 100% forhe whole test. These results indicate that the Mn2O3 micro-pheres possess impressive Li-ion storage performance, whichs better than that of reported pure porous Mn2O3 spheresChang et al., 2013) and also superior to nanocarbon anodesCheng et al., 2013b).

The good Li-ion storage performance of Mn2O3 microspheresan be attributed to the following aspects. First, the porousn2O3 microspheres allow good electrolyte penetration and an

on buffer reservoir to ensure a constant supply of electrolyte,ven at high rates. Second, microspheres are ideal structureso accommodate the stress induced by volume changes duringycling, and the close packing of microspheres results in an anodeith a high volumetric density. Li-ion storage performance can

e further improved using a combination of nanocarbon struc-

ures as short-range current collectors from nanosized Mn2O3anoparticles.

for the Mn2O3 microspheres.

J. Yu et al. / Particuology 22 (2015) 89–94 93

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ig. 4. (a) The CVs of the Mn2O3 electrode from 0.02–3.0 V versus Li+/Li at a scan raterom 50–400 mA/g; (d) long-life cycling performance at 0.10 C (1.0 C = 1018 mAh/g)

. Conclusions

Mn2O3 microspheres of ca. 18 �m in size were synthesized by aeneral, facile solvothermal–thermal coversion route. The porousn2O3 microspheres allow good electrolyte penetration and an

on buffer reservoir to ensure a constant supply of electrolyte.hey exhibit a reversible capacity of 590 and 320 mAh/g at 50 and00 mA/g, respectively. Microspheres with a packing density of.33 g/cm3 are ideal structures to accommodate the stress. Mn2O3icrospheres have potential application as anodes in lithium ion

atteries with high energy density. This work offers a facile methodo fabricate advanced functional particles for energy storage.

cknowledgments

This work was supported by the National Natural Scienceoundation of China (21422604), the PetroChina Technology R&Droject on New Technology and the Method for Oil & Gasevelopment (2011A-1006), and CNPC Innovation Foundation

2014D-5006-0207).

ppendix A. Supplementary data

Supplementary material related to this article can beound, in the online version, at http://dx.doi.org/10.1016/.partic.2014.10.007.

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