ARTICLE IN PRESS

Journal of Crystal Growth 267 (2004) 184–187

*Corresp

3601592.

E-mail

0022-0248/

doi:10.101

Fabrication and characterization of nanosizedsingle-crystalline LiNi0.5Mn0.5O2

Xiong Wanga, Fu Zhoua, Xuemei Zhaoa, Zude Zhanga,*, Mingrong Jia,Chenming Tangb, Tao Shenb, Huagui Zhenga,*

aDepartment of Chemistry, University of Science and Technology of China, Hefei 230026, ChinabHighstar Chemical Power Source Co., Ltd., Qidong 226200, China

Received 31 January 2004; accepted 23 March 2004

Communicated by M. Schieber

Abstract

Layered single-crystalline LiNi0.5Mn0.5O2 nanostructures were synthesized through a citric acid-assisted sol–gel

method. The results of XPS match well with the expected stoichiometric formula NiII0:5MnIV0:5: Electrochemical tests show

that it delivers a large discharge capacity of 160.7mAhg�1 vs. Li metal at 0.2mAcm�2 (voltage window 2.5–4.5V).

r 2004 Elsevier B.V. All rights reserved.

Keywords: A1. Nanostructures; B1. Lithium compounds; B1. Nanomaterials

1. Introduction

Since the commercialization of LiCoO2 by Sonyin 1990s, many efforts have been made to findother potential electrode materials to replace itbecause of the high cost and toxicity of LiCoO2.Recently, layered LiNi0.5Mn0.5O2 has been inten-sively investigated as an attractive cathode materi-al for rechargeable lithium batteries in severalaspects [1–5]. LiNi0.5Mn0.5O2 does not transformto spinel structure under electrochemical condi-tions and does not exhibit a distinct structuraldegradation even at higher voltages.

onding author. Tel.: +86-3607752; fax: +86-

address: libchem@mail.ustc.edu.cn (Z. Zhang).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

6/j.jcrysgro.2004.03.053

Due to the morphology of an electrode materialgreatly influencing the electrochemical perfor-mances and the diffusion mechanism of Li+ ions[6,7], nanosized electrode materials have beingattracting increasing interest among researchersduring the past few years [8–12].

Herein, in the paper we first report the synthesisof nanosized single-crystalline LiNi0.5Mn0.5O2 by asol–gel method. The valence states of Mn and Niin LiNi0.5Mn0.5O2 were clarified by X-ray photo-electron spectroscopy, and the electrochemicaltests were made.

2. Experimental procedure

All chemicals are analytical grade and usedwithout further purification. Lithium acetate

d.

ARTICLE IN PRESS

Fig. 1. XRD pattern of the as-prepared LiNi0.5Mn0.5O2.

X. Wang et al. / Journal of Crystal Growth 267 (2004) 184–187 185

(LiOAc �H2O), manganese acetate (Mn(CH3COO)2� 4H2O) and nickel acetate (NiCH3COO)2 � 4H2O)were employed as starting materials. Manganeseacetate (1mmol) and nickel acetate (1mmol), citricacid (8 g) and lithium acetate (2.14 g, 5% excess)were dissolved in distilled water, and the pH of thesolution was adjusted to be in the range 9–10 byadding ammonium hydroxide. Ethylene glycol wasadded dropwise into the continuously agitatingdissolved solution. The molar ratio of citric acid toethylene glycol was 1:4. A transparent solutionwas formed and then heated at 140�C for 5 h untila viscous gel was obtained. The resulting gelprecursor was heated to 450�C for 3 h in air with aramping rate of 2�C/min to decompose theorganic components. The product was finelyground in a mortar and calcined at 900�C for12 h, then quenched to room temperature.

The crystalline phase was identified by powderX-ray diffraction (XRD) using a Philips X’PertPro Super diffractometer with graphite mono-chromatized Cu Ka radiation (l ¼ 1:54178 (A).The morphologies of the samples were examinedby transmission electron microscopy (TEM, Hi-tachi, Model H-800) with an accelerating voltage200 kV. SEM image was taken on a Hitach (X-650) scanning electron microscope. X-ray photo-electron spectra (XPS) were collected on anESCALab MKII X-ray photoelectron spectro-meter, using nonmonochromatized Mg Ka X-rayas the excitation source and C 1 s (284.6 eV) as thereference line.

Electrochemical tests were conducted withTeflon cells. The positive electrodes were fabri-cated by pasting slurries of the as-preparedLiNi0.5Mn0.5O2 (85 wt%), carbon black (SuperP, 10wt%) and polyvinylidene (PVDF, 5wt%)dissolved in N-methyl-pyrrolidinone (NMP) onAl foil strips by doctor blade technique. Thenthe strips were dried at 100�C for 24 h in an airoven, pressed under 20MPa pressure and kept at100�C for 12 h in a vacuum. Metallic lithium wasused as anode. The electrolyte was 1M LiPF6 in a1:1 mixture of ethylene carbonate (EC)/diethylcarbonate (DEC); the separator was Celgard 2500.The cells were assembled in the glove box filledwith highly pure argon gas (O2 and H2O levelso5 ppm). The cells were galvanostatically cycled

in the 2.5–4.5V range at a current density of0.2mA cm�2.

3. Results and discussion

Fig. 1 shows the XRD pattern of the as-prepared LiNi0.5Mn0.5O2. All diffraction linescan be indexed by assuming a hexagonal lattice,indicating a single phase of LiNi0.5Mn0.5O2. Thelattice parameters were calculated to be a ¼0:2890; c ¼ 1:433 nm by a least squares method,which agree well with the reported values [1–3]. Noother peaks for impurities were detected. TheXRD pattern indicates that well-crystallized pureLiNi0.5Mn0.5O2 can be obtained through thepresent synthetic route.

The morphology of the particles was observedby TEM and SEM. The TEM and SEM imagesare shown in Fig. 2a and c, respectively, whichindicate that the particles are well dispersed withaverage diameter of 100 nm and the size distribu-tion is fairly narrow. The single-crystal and wellcrystallization of the as-synthesized LiNi0.5M-n0.5O2 is revealed by the electron diffraction(ED) pattern (Fig. 2b). All of the diffraction spotscan be indexed to a hexagonal phase, which areconsistent with the XRD results. Fig. 3 shows thehigh-resolution XPS spectra of Mn 2p and Ni 2pin LiNi0.5Mn0.5O2. As shown in Fig. 3a, the peakat 642.2 eV corresponds to Mn 2p3/2. The Ni 2p3/2

ARTICLE IN PRESS

Fig. 2. TEM image (a), electron diffraction (ED) pattern (b)

and SEM image (c) of the particles.

Fig. 3. High-resolution XPS core spectrum for Mn 2p (a) and

Ni 2p (b).

Fig. 4. Charge/discharge curves of (a) the as-prepared Li-

Ni0.5Mn0.5O2 and (b) LiNi0.5Mn0.5O2 powders prepared by

solid-state method.

X. Wang et al. / Journal of Crystal Growth 267 (2004) 184–187186

peak is at about 854.6 eV (Fig. 3b). These valuesmatch well with the reported data for Mn4+ andNi2+, respectively [2,13,14]. The relative amountsof Mn4+ and Ni2+ estimated from the area underthe peaks are equivalent, and compare well withthe expected stoichiometric formula.

The formation of nanosized LiNi0.5Mn0.5O2

particles may be ascribed to the sol–gel syntheticroute. The solution-based system can offer mole-cular level mixing of the starting materials, whichfacilitates the high degree of homogeneity withminimum particle size and high surface area.

Fig. 4 shows the voltage versus capacity for thecell between 2.5 and 4.5V at a current density of0.2mA cm�2. The reversible capacity of the as-prepared LiNi0.5Mn0.5O2 in the lithiation-delithia-tion cycles is around 160.7mAh g�1 (Fig. 4a),which is higher than that of LiNi0.5Mn0.5O2

prepared by solid-state method (139.5mAhg�1).The improved properties might result from the

ARTICLE IN PRESS

X. Wang et al. / Journal of Crystal Growth 267 (2004) 184–187 187

increased surface area of LiNi0.5Mn0.5O2, whichfacilitates the deintercalation and intercalation ofLi-ion.

4. Conclusions

In summary, single-crystalline LiNi0.5Mn0.5O2

nanoparticles were successfully synthesizedthrough a citric acid-assisted sol–gel method. Theobtained products were characterized by a varietyof techniques.

Acknowledgements

This work is supported in part by the Ministryof Science and Technology of China(2002BA322C).

References

[1] T. Ohzuku, Y. Makiura, Chem. Lett. (2001) 744.

[2] S.–H. Kang, J. Kim, M.E. Stoll, D. Abraham, Y.K. Sun,

K. Amine, J. Power Sources 112 (2002) 41.

[3] Y. Makimura, T. Ohzuke, J. Power Sources 119–121

(2003) 156.

[4] Y.–K. Sun, C.S. Yoon, Y.S. Lee, Electrochim. Acta 48

(2003) 2589.

[5] C.S. Johnson, J. Kim, A.J. Kropf, A.J. Kahaian, J.T.

Vaughey, L. Fransson, K. Edstr .om, M.M. Thackeray,

Chem. Mater. 15 (2003) 2313.

[6] B. Ammundsen, J. Paulsen, Adv. Mater. 13 (2001) 943.

[7] M. Yoshio, H. Nakamura, Y. Xia, Electrochim. Acta 45

(1999) 273.

[8] Y.S. Lee, Y.K. Sun, S. Ota, T. Miyashita, M. Yoshio,

Eelectrochem. Comm. 4 (2002) 989.

[9] L. Zhang, J. Yu, A. Xu, Q. Li, K. Kwong, L. Wu, Chem.

Commun. Vol. 2910, 2003.

[10] X. Wang, X. Chen, L. Gao, H. Zheng, M. Ji, C. Tang, T.

Shen, Z. Zhang, J. Mater. Chem. 14 (2004) 905. DOI:

10.1039/b310722a.

[11] S. Myung, S. Komaba, N. Kumagai, H. Yashiro, H.

Chung, T. Cho, Electrochim. Acta 47 (2002) 2543.

[12] C. Lu, H. Wang, J. Mater. Chem. 13 (2003) 428.

[13] K. Amine, H. Tukamoto, H. Yasuda, Y. Fujita, J. Power

Sources 68 (1997) 604.

[14] G.E. Muilenberg, C.D. Wager, W.MO. Riggs, L.E. Davis,

J.F. Moulder (Eds.), Handbook of X-ray Photo-

electron Spectroscopy, Perking-Elmer Corp., New

York, 1979.