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Solid State Ionics 176

Layered O2-Li2/3(Ni1/3�xMn2/3�xM2x)O2 (M=Cr, Co, x=0.05) cathode

materials for Li-ion rechargeable batteries

Xiong Wanga, Xiaohong Yangb, Huagui Zhenga, Tao Shenc, Zude Zhanga,TaDepartment of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China

bDepartment of Chemistry, Chizhou Normal Teaching College, Chizhou 247000, ChinacHighstar Chemical Power Source Co., Ltd., Qidong 226200, People’s Republic of China

Received 27 February 2004; received in revised form 20 December 2004; accepted 16 January 2005

Abstract

Layered O2-phase lithium manganese oxides, Li2/3(Ni1/3�xMn2/3�xM2x)O2 (M=Cr, Co, x=0.05) were prepared by ion exchange of Li for

Na in P2-Na2/3(Ni1/3�xMn2/3�xM2x)O2 precursors, which were synthesized by a citric acid-assisted sol–gel method. The as-synthesized

compounds were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS),

and galvanostatic tests. The discharge capacity of O2-Li2/3(Ni1/3�xMn2/3�xCo2x)O2 (x=0.05) is around 188 mA h g�1 (vs. Li metal; voltage

window 2.5–4.6 V) at a current density of 0.2 mA cm�2.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Layered structure; O2-phase; Electrode materials; Lithium ion batteries

1. Introduction

In recent years, layered O2-type lithium manganese

oxides, Li2/3(Mn1�xMx)O2 (M=Li, Ni, Co) have attracted

increasing attention [1–5]. In the case of Li2/3(Mn1�xMx)O2

with O2 structure (octahedral Li+ ion coordination and two

MO2 sheets per unit cell), every O-M-O layer is mirrored

and hence the O2-phases are stable against transformation to

spinel structure under electrochemical cycling as compared

to the O3-phases.

Dahn’s group first synthesized layered O2-Li2/3(Ni1/3Mn2/3)O2 by a high temperature solid-state reaction [4–7]

P2-Na2/3(Ni1/3Mn2/3)O2 precursor (trigonal prismatic Na+

ion coordination and two MO2 sheets per unit cell) was

prepared by calcining stoichiometric amounts of Na2CO3,

EMD (electrolytic manganese dioxide) and Ni(OH)2 at 900

8C for 32 h and then refluxed in a solution of LiBr in

hexanol. O2-Li2/3(Ni1/3Mn2/3)O2 was found to give a re-

0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2005.01.003

* Corresponding author. Tel.: +86 551 3607752.

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

versible capacity of 180 mA h g�1 between 2.5–4.6 V vs. Li

at C/40 rate [5,8]. However, the first charge capacity was

only about 100 mA h g�1 corresponding to ~1/3 mol of

Li+ ion extraction. Higher extraction capacity (190 mA h

g�1) was obtained via incorporating extra Lithium (~1/3

mol) into the compound Li2/3(Ni1/3Mn2/3)O2 using LiI as the

reducing agent [8].

Here, we have successfully synthesized the O2-Li2/3(Ni1/3�xMn2/3�xM2x)O2 (M=Cr, Co, x=0.05) compounds

by a citric acid-assisted sol–gel method to improve the

electrochemical performances. The structural and electro-

chemical properties of the as-prepared compounds were

investigated.

2. Experimental

The P2-Na2/3(Ni1/3�xMn2/3�xM2x)O2 (M=Cr, Co, x=

0.05) precursors were prepared via a sol–gel method using

citric acid as a chelating agent. Appropriate amounts of

Na2CO3, Mn(CH3COO)2d 4H2O, Ni(CH3COO)2d 4H2O,

Co(CH3COO)2d 4H2O (or Cr(NO3)3d 9H2O) and citric acid

(2005) 1043–1049

Fig. 1. TGA curve of P2-Na2/3(Ni1/3Mn2/3)O2 dried-gel precursor.

X. Wang et al. / Solid State Ionics 176 (2005) 1043–10491044

were dissolved in distilled water. The PH of the solution

was adjusted to be in the range of 9–10 by dropping

ammonia solution. Ethylene glycol was added dropwise

into the continuously agitating dissolved solution. The

molar ratio of citric acid to ethylene glycol was 1:4. A

transparent solution was formed and then heated at 140 8Cfor 3 h until a viscous gel was obtained. The resulting gel

was calcined at 450 8C for 5 h to decompose the organic

components. Then the obtained powders were finely

ground in a mortar, calcined at 850 8C for 10 h and then

quenched.

Fig. 2. XRD patterns of P2-Na2/3(Ni1/3�xMn2/3�xM2x)O2 synthesized by the

The prepared powders were added into hexanol with a

10-fold excess of lithium bromide and refluxed at 180 8C for

8 to 18 h. After the ion exchange, the slurry was filtered

under suction and washed repeatedly with methanol. Then

the resulting product was dried at 100 8C for an hour in a

vacuum oven.

The powders were identified by X-ray diffraction (XRD)

using a Philips X’Pert Pro Super diffractometer with Cu Ka

radiation (k=1.541784 2). The morphology of the samples

was examined by scanning electron microscopy (SEM,

Hitachi, X-650). Thermal gravimetric analysis (TGA) of the

sol–gel method; (a) x=0, (b) M=Co, x=0.05, and (c) M=Cr, x=0.05.

Fig. 3. XRD patterns of the as-prepared O2-Li2/3(Ni1/3�xMn2/3�xM2x)O2 (x=0.05) obtained by ion exchange from the Na-phases; (a) M=Cr, (b) M=Co.

X. Wang et al. / Solid State Ionics 176 (2005) 1043–1049 1045

dried gel was carried out on a Shimadzu TA-50 thermal

analyzer at a heating rate of 10 K min-1 from room

temperature to 1000 8C in air. X-ray photoelectron spectra

(XPS) were collected on an ESCALab MKII X-ray photo-

electron spectrometer, using nonmonochromatized Mg Ka

X-ray as the excitation source.

Electrochemical tests were conducted with Teflon cells.

The positive electrodes were fabricated by pasting slurries

of the as-prepared active material (85 wt.%), carbon black

(Super P, 10 wt.%) and poly(vinylidene fluoride) (PVDF, 5

wt.%) dissolved in N-methyl-pyrrolidinone (NMP) on Al

foil strips by doctor blade technique. Then the strips were

dried at 140 8C for 24 h in an air oven, pressed under 20

MPa pressure and kept at 100 8C for 12 h in a vacuum.

Metallic lithium was used as anode. The electrolyte was 1 M

LiPF6 in a 1:1 mixture of ethylene carbonate (EC)/diethyl

carbonate (DEC); the separator was Celgard 2500. The cells

Fig. 4. SEM images of the as-prepared O2-Li2/3(Ni1/3�

were assembled in the glove box filled with highly pure

argon gas (O2 and H2O levels b5 ppm). The cells were

galvanostatically cycled in the 2.5–4.6 V range at a current

density of 0.2 mAd cm�2 at room temperature.

3. Results and discussion

Fig. 1 shows the TGA curve of the P2-Na2/3(Ni1/3Mn2/3)

O2 dried gel. There are two distinct weight loss steps in the

temperature range of 100–250 and 250–450 8C, respec-

tively. The first weight loss may be attributed to the

emission of the residual H2O in the gel, whereas that at

250–450 8C may be ascribed to the decomposition of the

organic components.

The XRD patterns of P2-Na2/3(Ni1/3�xMn2/3�xM2x)O2

are shown in Fig. 2. The patterns can be readily indexed to

xMn2/3�xM2x)O2 (x=0.05); (a) M=Cr, (b) M=Co.

X. Wang et al. / Solid State Ionics 176 (2005) 1043–10491046

hexagonal phase with lattice parameters a=2.87, c=11.20

2 (Space Group: P63/mmc) without impurities, which

match well with those reported by Dahn [4]. Effects of the

doping on the lattice parameters are very small and

negligible. Fig. 3 shows the XRD patterns of O2-Li2/3(Ni1/3�xMn2/3�xM2x)O2 (M=Cr, Co, x=0.05) synthesized

by ion exchange of P2 (Na) phases. From the patterns, a

small fraction (~1%) of Na- and O3-phase can be noted

even increasing the exchange time, similar phenomenon

also observed by Dahn [1] and Doeff [9].

Fig. 4 shows the SEM images of O2-Li2/3(Ni1/3�xMn2/3�x

M2x)O2 (M=Cr, Co, x=0.05). It was found that the particles

Fig. 5. XPS spectra of (a) Mn 2p3/2 and (b) Ni 2p3/2 in the a

have a flake-like morphology with a particle size of 2–5 Am in

diameter. For the Cr-doped compound (Fig. 4a), there are

some small crystallites on the surface. The Co-doped phase

has a regular layered hexagonal morphology and a smaller

particle size (Fig. 4b).

XPS has been extensively used to characterize the cathode

materials [10–12]. Binding energies (BE, w.r.t. C1s as the

internal standard) and relative area under the peaks provide

relevant information. Fig. 5 shows the Mn and Ni XPS core

spectra for O2-Li2/3(Ni1/3�xMn2/3�xCo2x)O2 (x=0.05). The

Mn 2p3/2 spectrum fits to a single peak with a BE of 642.1 eV,

which is close to the value for Mn4+ in MnO2 and LiMn2O4

s-prepared O2-Li2/3(Ni1/3�xMn2/3�xCo2x)O2 (x=0.05).

Fig. 6. Charge–discharge curves of the as-synthesized O2-Li2/3(Ni1/3�xMn2/3�xM2x)O2 (vs. Li metal); (a) x=0, (b) M=Cr, x=0.05, and (c) M=Co, x=0.05.

X. Wang et al. / Solid State Ionics 176 (2005) 1043–1049 1047

Fig. 7. XRD patterns of O2-Li2/3(Ni1/3�xMn2/3�xCo2x)O2 (x=0.05) before charge (a) and after 30 cycles (b) and spinel LiMn2O4 (c).

X. Wang et al. / Solid State Ionics 176 (2005) 1043–10491048

[13,14]. The Ni 2p3/2 peak with a BE of 854.9 eV matches

well with those reported for Ni2+ in NiO and LiMn1.5Ni0.5O4

[15]. The relative amounts of Ni2+and Mn4+ estimated from

the area under the peaks are 31% and 69% (0.28 Ni2+and 0.62

Mn4+), respectively, and compare well with the expected

stoichiometric formula Li2/3I (Ni0.28

II Mn0.62IV Co0.1

II )O2.

Electrochemical performances of the compounds in the

cell configuration O2-Li2/3(Ni1/3�xMn2/3�xM2x)O2 (M=Cr,

Co, x=0.05)/Li have been evaluated. Fig. 6 shows the

voltage versus capacity for the cell between 2.5 and 4.6 V

at a current density of 0.2 mAd cm-2. The first charge

capacity of Li2/3(Ni1/3Mn2/3)O2 is only 105 mA h/g,

corresponding to the extraction of 1/3 mol Li+ ion from

the compound. While the discharge curves have two

plateaus centered at 4.0 and 2.75 V with a first discharge

capacity of 185 mA h/g. Since the intercalation of 1/3

mol of extra Li+ ion into the lattice, the first discharge

capacity is higher than the first charge capacity. The

charge and discharge processes can be expressed as

follows [5,7,8]:

Charge process

LiI2=3ðNiII1=3MnIV2=3ÞO2YLiI1=3ðNiIII1=3MnIV2=3ÞO2 þ ð1=3ÞLiþ

þ ð1=3Þe�

or

LiI2=3ðNiII1=3MnIV2=3ÞO2YLiI1=3ðNiII1=6NiIV1=6MnIV2=3ÞO2

þ ð1=3ÞLiþ þ ð1=3Þe�

Discharge process

LiI1=3ðNiIII1=3MnIV2=3ÞO2 þ ð1=3ÞLiþ þ e�

Yð1=3ÞLiI2=3ðNiII1=3MnIV2=3ÞO2

or

LiI1=3ðNiII1=6NiIV1=6MnIV2=3ÞO2 þ ð1=3ÞLiþ þ e�

Yð1=3ÞLiI2=3ðNiII1=3MnIV2=3ÞO2

LiI2=3ðNiII1=3MnIV2=3ÞO2 þ ð1=3ÞLiþ þ ð1=3Þe�

YLiI2=3ðNiII1=3MnIII1=3MnIV1=3ÞO2

The charge–discharge curves of Li2/3(Ni1/3�xMn2/3�x

M2x)O2 (M=Cr, x=0.05) shows the first charge capacity of

119 mA h/g and the first dischage capacity of 162 mA h/g

(Fig. 6b). While the first charge capacity of Li2/3(Ni1/3�x

Mn2/3�xCo2x)O2 (x=0.05) was found to be 167 mA h/g and

the first discharge capacity 188 mA h/g. The discharge

curves are almost smooth up to 2.5 V. The XRD pattern of

the Co-doped compound after 30 cycles (Fig. 7) shows the

as-synthesized compound is stable and does not convert to

spinel structure under electrochemical conditions. During

the 30 charge–discharge cycles, the average capacity

retention of the Co-doped sample is about 97.5% per cycle

due to the metal lithium as anode.

The transition-metal ion doping can remarkably influ-

ence the electrochemical behaviors of the layered O2

compound. A small amount Co substitution would

facilitate the removal of more lithium ions from the O2

lattice in the charge process and improve the electro-

chemical performances.

4. Conclusions

Layered O2-phase lithium manganese oxides, Li2/3(Ni1/3�xMn2/3-xM2x)O2 (M=Cr, Co, x=0.05) were pre-

X. Wang et al. / Solid State Ionics 176 (2005) 1043–1049 1049

pared by ion exchange of Li for Na in P2-Na2/3(Ni1/3�x

Mn2/3�xM2x)O2 precursors, which were synthesized by a

citric acid-assisted sol–gel method. The Co-doping can

remarkably improve the electrochemical performances.

The first charge capacity of Li2/3(Ni1/3�xMn2/3�xCo2x)O2

(x=0.05) can reach 167 mA h/g. The discharge curves

are almost smooth with reversible discharge capacity of

188 mA h/g.

Acknowledgements

This work is supported in part by the Ministry of Science

and Technology of China. We acknowledge the assistance

from the Highstar Chemical Power Source Company

Limited.

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