Layered Li–Mn oxides with the O2 structure: preparation of Li2/3[Mn1−xMx]O2 (M=Li, Cr, Mg, Al)...

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Solid State Ionics 161 (2003) 133–144

Layered Li–Mn oxides with the O2 structure: preparation of

Li2/3[Mn1�xMx]O2 (M=Li, Cr, Mg, Al) by ion exchange

Min Wei, Yanluo Lu, D.G. Evans, Xue Duan*

Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education,

Beijing University of Chemical Technology, Beijing 100029, China

Received 7 October 2002; received in revised form 23 June 2003; accepted 4 July 2003

Abstract

Layered sodium manganese bronzes, Na2/3[Mn1� xMx]O2 (M=Li, Cr, Mg, Al) with the P2 structure have been investigated.

In the case of M=Li, the location of the dopant was identified on the basis of ion-exchange properties and chemical analysis.

With the increase doped Cr content, the P2-structure Na2/3[Mn1� xCrx]O2 gradually underwent a transformation to

orthorhombic Na4[Mn9� yCry]O18. X-ray photoelectron spectroscopy (XPS) was used to measure the oxidation state and the

composition near the surface. The compositions near the surface and in the bulk were found to vary with the identity of the

doped element. The O2-structure lithium manganese oxides were prepared from Na-bronzes by ion exchange, but the

crystallinity decreased after the ion exchange as a result of stacking faults.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Layered Li–Mn oxides; O2 structure; Preparation; Ion exchange

1. Introduction rials which are cheaper, safer, more abundant and

The investigation of materials for rechargeable

lithium batteries has become a major topic of materi-

als research worldwide, in part due to the rapid growth

of lithium battery technology. Since 1990, much

attention has been paid to the layered cathode materi-

als, because a layered structure allows ready interca-

lation and decalation of lithium ions. Sony success-

fully launched the commercial lithium-ion cell in

1991, using layered LiCoO2 as the positive material

[1]. However, identifying other layered cathode mate-

0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0167-2738(03)00276-5

* Corresponding author. Tel.: +86-10-64435271; fax: +86-10-

64425385.

E-mail address: [email protected] (X. Duan).

nontoxic represents a key challenge in this area.

In recent years, interest in the layered Li–Mn

oxides as intercalation electrodes has grown rapidly

[2–5]. Doping LiMnO2 by replacing part of the Mn

with transition metal elements such as Co, Ni and Cr

has been carried out [6–9], and it has been reported

that doping layered LiMnO2 improves its perfor-

mance, including capacity and retention. However,

these compounds were found to convert to a spinel

phase during cycling. The reason is that all such

compounds are structurally analogous to the O3

structure and consist of cubic close-packed oxide ions

which are identical to the arrangement in spinel.

Consequently, only a minor cation rearrangement

occurs during conversion to the more stable spinel

structure [10].

M. Wei et al. / Solid State Ionics 161 (2003) 133–144134

According to Delmas et al. [11], the bronzes

AxMnO2 can exist in another two layered polymorphs,

with P2 and O2 structures. Generally, lithium man-

ganes oxides having the O2 structure are obtained by

ion exchange of P2 type sodium manganese bronzes.

The O2 structure has an oxygen stacking sequence

ACAB which differs fundamentally from the ABC

stacking of the O3 form. Thus, the transformation of

the O2 form to a spinel structure is rather difficult and

unlikely to happen, since it requires the breaking of all

the Mn–O bonds. Thus, it can be expected that the a

lithium manganese oxide with the O2 structure will

retain higher capacity and cycle better than that of with

the O3 structure. This has been confirmed by Dahn

[12–15] who studied a variety of layered Li2/3[Mn1� x

Mx]O2 species in which Mn was substituted by Co, Ni,

Li, and found no conversion to spinel during cycling.

In this work, doped sodium manganese bronzes

based on Na2/3MnO2 with the P2 structure were

prepared. Li, Cr, Mg and Al were chosen as dopants,

since they all can replace manganese on octahedral

sites leading to equilibrium phases Na2/3[Mn1 � x

Mx]O2 (M=Li, Cr, Mg, Al). Li2/3[Mn1� xMx]O2 was

subsequently obtained by ion exchange. Moreover, the

effect of varying the content of the dopants was also

investigated. The lithium-doped Li2/3[Mn1� xLix]O2

material has previously been reported by Paulsen et

al. [10], but it was not clear whether lithium substitutes

for manganese on octahedral sites for two reasons: first,

lithium is a very light element, so it cannot readily be

determined by X-ray diffraction; second, the structure

of the compound Li2/3[Mn1� xLix]O2 is rather com-

plex. There are two possible sites for lithium: one is the

octahedral site occupied by manganese in the parent

structure; the other is the octahedral site between the

layers. Consequently, confirmation of the location of

the lithium ions is desirable for this compound. In this

work, different experimental methods were used to

determine the location of the lithium ions.

Table 1

Preparation of sodium bronze precursors for the ion exchange

reaction

Precursor Reaction 1 Reaction 2

Na2/3[Mn1� xLix]O2 24 h, 800 jC, ground 24 h, 800 jCNa2/3[Mn1� xCrx]O2 24 h, 1000 jC, ground 24 h, 1000 jCNa2/3[Mn1� xMgx]O2 24 h, 850 jC, ground 24 h, 850 jCNa2/3[Mn1� xAlx]O2 24 h, 1000 jC, ground 24 h, 1000 jC

2. Experimental

2.1. Preparation of Na2/3[Mn1�xMx]O2 (M=Li, Cr,

Mg, Al)

Samples of Na2/3[Mn1� xMx]O2 were prepared by

solid-state reaction of stoichiometric mixtures of

Mn2O3,Li2CO3(orCr2O3, (MgCO3)4�Mg(OH)2�6H2O,

Al2O3), and Na2CO3. The details are given in Table 1.

Thepowdersweremixedbygrinding,andwereheated in

abox furnace for 24h in air.Thepowerswere cooled and

re-milled, and the solid-state reaction was repeated.

After the second heating and cooling steps, small par-

ticles were obtained by grinding.

2.2. Preparation of Liy[Mn1�xMx]O2 (M=Li, Cr,

Mg, Al)

Samples of Liy[Mn1� xMx]O2 were prepared by

ion exchange of sodium by lithium. The ion exchange

was performed by one of two routes. In the molten salt

method, the ion exchange was made in a molten salt

((LiNO3)0.88(LiCl)0.12). Three grams of Na2/3[Mn1� xMx]O2 was added to 14 g of the molten salt

at 280 jC. After 3 h, the melt was poured into distilled

water, and the solution was filtered. The powder was

dried subsequently at 100 jC for 8 h in air. In the n-

hexanol method, the Liy[Mn1� xMx]O2 was obtained

by refluxing Na2/3[Mn1� xMx]O2 with an excess of

LiBr in n-hexanol at 145–150 jC for 8 h. After

cooling to room temperature, the product was filtered

under suction and washed, first with n-hexanol and

then with ethanol, and dried.

2.3. Characterization

The prepared samples were investigated by XRD

with a Siemens D5005 diffractometer using Cu Ka

radiation (40 kV, 40 mA). SEM studies of the samples

were carried out using a Cambridge S-250 scanning

microscope. TEM micrographs were recorded on a

HITACHI H-800 transmission microscope. Elemental

analysis was performed with a Shimadzu ICPS-7500.

Average oxidation state of Mn was determined by

redox titration using ferrous ammonium sulfate/

KMnO4. The X-ray photoelectron spectroscopy (XPS)

M. Wei et al. / Solid State Ionics 161 (2003) 133–144 135

spectra were collected on a PHI 5300 ESCA spec-

trometer using Mg Ka (hm = 1253.6 eV) and Al Ka

(hm = 1486.6 eV) as X-ray sources.

3. Results and discussion

3.1. Determination of the location of the lithium ions

in Na2/3[Mn1�xLix]O2

Paulsen et al. [10] have reported the preparation of

layered Li2/3[Mn5/6Li1/6]O2 with the O2 structure.

This rather complex compound has two possible

lithium sites. Because lithium is such a light element,

it was not clear whether lithium could occupy the

octahedral sites, as manganese and other transition

elements do. In this work, the location of the lithium

ions was determined on the basis of ion-exchange

properties and elemental analysis. The precursor, Na2/3[Mn5/6Li1/6]O2 was prepared by a solid-state reaction,

and ion exchange was subsequently carried out in

molten Mg(NO3)2�6H2O at 90 jC (or Mg(NO3)2might be dissolved in the water of crystallization).

XRD and elemental analysis were used to characterize

Fig. 1. XRD patterns of (A) Na2/3[Mn5/6Li1/6]O2. (B) Mg-phase obtained

exchange of Na+ by Li+.

the product. There are two possibilities: (1) If Li+ was

situated between the layers, it could be exchanged by

Mg2 +, since it is smaller than Na+. As a result, the

lithium content in the product would change greatly

compared with the precursor. (2) If Li+ was situated in

the Na–Mn oxide bronze layers, it would not partic-

ipate in an ion exchange with Mg2 +, since a high

energy is needed to break the Li–O bond which is

unlikely to happen at low temperatures.

Fig. 1A and B shows the XRD patterns of the

precursor Na2/3Mn5/6Li1/6O2 and the Mg-phase after

ion exchange with Mg2 +, respectively. The XRD

pattern of Na2/3Mn5/6Li1/6O2 (Fig. 1A) can be indexed

as a structure with the most symmetric hexagonal

space group P63/mmc. It can be seen from Fig. 1 that

the 002 peak of Na2/3Mn5/6Li1/6O2 was at 16.0j,while it moved to 18.7j for MgyMn5/6Li1/6O2 after

the ion exchange. Because the diameter of Na+ is

larger than that of Mg2 +, the d002 spacing of the

precursor is larger than that of the product. XRD thus

suggests that most of the Na+ between the layers was

exchanged by Mg2 +.

The composition of the precursor and the product

were found to be Na0.69Li0.11Mn0.88O2 and Mg0.32

by ion exchange of Na+ by Mg2 +. (C) Li-phase obtained by ion

M. Wei et al. / Solid State Ion136

Na0.02Li0.10Mn0.87O2, respectively. The content of

lithium did not change significantly after the ion-

exchange reaction, also implying that most of the

lithium occupies octahedral manganese sites and

thus the structure is best described as Na0.69[Li0.11Mn0.88]O2.

Fig. 1C shows the XRD pattern of Liy[Mn5/6Li1/

6]O2 obtained from Na2/3[Mn5/6Li1/6]O2 by ion ex-

change of lithium by sodium. The intensity of the

002 peak (at 16j) of the Na phase is very low,

indicating that the ion-exchange reaction was almost

complete. If the pattern is indexed in the P3ml

space group, the calculated Bragg peak positions

are in good agreement with the observed pattern.

Chemical analysis showed its composition was

Li0.64Na0.02[Li0.11Mn0.90]O2.

Fig. 2 shows the TEM micrographs of the crys-

talline O2-Li0.64Na0.02[Li0.11Mn0.90]O2. It can be seen

from Fig. 2A that the particles have a hexagonal

shape and range from 1 to 4 Am in diameter.

However, the edges of the particles appear rather

irregular, which might be due to stacking faults in the

crystals. As shown in Fig. 2B, a layered structure

could be observed with some disfigurement at larger

magnification, indicating the existence of the sacking

faults.

Fig. 2. TEM micrographs of Li0.64Na0.02[Li0.11Mn0.90]O2. (

3.2. Lithium manganese bronzes doped with Cr

3.2.1. The structure of Liy[Mn1�xCrx]O2

To investigate the effect of the content of chromi-

um dopant on the crystal structure, a series of Na2/3[Mn1� xCrx]O2 samples with different dopant levels

(x = 0,0.03, 0.05, 0.07 and 0.10) were prepared at

1000 jC, and their XRD patterns are shown in Fig. 3.

At low Cr concentrations (xV 0.05), the resulting

products have a layered P2 structure, just as for a-

Na0.70MnO2.05 with space group P63/mmc. When

x = 0 and 0.03, the products have the pure hexagonal

P2 structure. When x = 0.05, there are weak diffrac-

tion peaks at 2h= 6.8j, 14.0j, 19.5j and 61.2j(marked by arrows in Fig. 3), which can be attributed

to orthorhombic Na4[Mn9 � yCry]O18. With the in-

crease of the dopant content, the diffraction peaks of

Na4[Mn9� yCry]O18 became stronger while those of

the layered P2 structure became weaker. For x = 0.10,

the product was almost pure orthorhombic Na4[Mn9� yCry]O18, and no P2 phase was observed.

A refinement of cell parameters of orthorhombic

Na4[Mn9� yCry]O18 was made, and the results were

displayed in Table 2. Some distinct rules can be found

from Table 2. Cell parameters a and b decrease while

c increases with the increase value of y. Orthorhombic

ics 161 (2003) 133–144

A) 5� 103 magnification; (B) 2� 105 magnification.

Fig. 3. XRD patterns of Na2/3Mn1� xCrxO2 obtained with different stoichiometry x. (A) x= 0; (B) x= 0.03; (C) x= 0.05; (D) x = 0.07; (E)

x= 0.10.


Na4Mn9O18 has a double-tunnel structure which con-

tains both MnO6 octahedra and MnO5 square pyra-

mids. Na+ locates in the tunnel [16]. With the increase

of doped Cr content, the average oxidation state of Mn

increases, which means the increase of Mn4 +. Com-

pared with the ion radius of Mn3 + (0.62 A), Mn4 + has

a smaller ion radius (0.52 A) while Cr3 + possesses a

larger one (0.64 A). The comprehensive effect of

doped Cr3 + leads to the decrease of a and b and the

increase of c. Because of the different change of a, b

and c with the increase value of y, the change of cell

volume has no obvious rules.

The results obtained above indicate that the content

of chromium dopant has a profound effect on the

crystal structure. A layered P2 structure was produced

Table 2

Cell parameters of orthorhombic Na4[Mn9� yCry]O18

Nominal Cr Cell parameters

content ( y)a/A b/A c/A V/A3

y= 0 [16] 9.100 26.340 2.821 676.180

y= 0.05 9.088 26.208 2.842 676.807

y= 0.07 9.083 26.034 2.870 678.753

y= 0.10 9.082 25.750 2.902 678.556

for 0 < xV 0.05, while orthorhombic Na4[Mn9� yCry]

O18 was obtained for 0.5 < xV 0.10. This might be due

to the stabilization of the orthorhombic type structure

by Cr.

Fig. 4 displays the diffraction patterns of Liy[Mn1� x

Crx]O2 obtained by ion exchange of Na2/3[Mn1� xCrx]

O2 (x = 0, 0.03 and 0.05). The ion-exchange reaction

was successful, and it was estimated to be 90–95%

complete, based on the small residual 002 peak from

Na2/3[Mn1� xCrx]O2 in the XRD pattern. The diffrac-

tion peaks of both compounds can be indexed on the

basis of the O2 structure. When x = 0.05, the product

contains a small quantity of impurities.

The chemical composition of the precursors and

ion-exchange products are shown in Table 3. It can be

seen that Cr/Mn ratios of the precursors and ion-

exchange products are the same as those in the reagent

mixture, while the alkali metal content decreased to a

small degree and average oxidation state of Mn

increased after ion exchange.

3.2.2. SEM micrographs of Liy[Mn1�xCrx]O2

Fig. 5 shows the SEM micrographs of Liy[Mn1� xCrx]O2 (x= 0, 0.03 and 0.05). It can be seen

Fig. 4. XRD patterns of Liy[Mn1� xCrx]O2 obtained by ion exchang


that the particles have a flake-like morphology with a

particle size of 2–10 Am in diameter in the absence of

chromium. When x = 0.03, the particle size was

smaller and a hexagonal morphology was observed.

When x = 0.05, the particle size became rather large,

more than 10 Am in diameter, and the particles possess

very regular layered hexagonal morphology. Some

needle-like crystals could also be seen in SEM mi-

crograph when x = 0.05. These might be related to the

small quantity of the impurity, orthorhombic Na4[Mn9� yCry]O18, suggested by the XRD pattern (Fig.

3, curve C).

It can be concluded that the content of chromium

dopant has a very significant influence on the crystal

structure, morphology and properties of lithium

bronzes. Further work on the comparison of the

electrochemical performance is under way in our

laboratory.

Table 3

Chemical composition and average oxidation state of Mn of the precurso

Nominal Cr Precursors

content (x)Chemical composition Average oxidation

state of Mn

x= 0 Na0.69MnO2 3.31

x= 0.03 Na0.72[Mn0.97Cr0.03]O2 3.32

x= 0.05 Na0.72[Mn0.95Cr0.05]O2 3.35

3.2.3. Surface properties of Liy[Mn1�xCrx]O2

Active cathode materials should allow reversible

lithium intercalation/decalation and should be chem-

ically stable when in contact with the organic electro-

lyte. In principle, the transfer of lithium ions across

the interface between the cathode material and the

electrolyte is believed to be the rate-determining step.

Consequently, the surface properties and state have an

important influence on the optimization of battery

performance.

X-ray photoelectron spectroscopy can provide

chemical information such as the oxidation state, as

well as the semi-quantitative composition of the

surface, and is thus a very useful method of studying

the surface conditions.

Fig. 6 shows the XPS results for Li0.57Na0.02[Mn0.95Cr0.05]O2. The binding energy of Mn 2p3/2 is

at 642.2 eV, which agrees well with that of MnO2 in

e of Na2/3[Mn1� xCrx]O2. (A) x= 0; (B) x= 0.03; (C) x= 0.05.

rs and ion-exchange products with doped Cr

Products

Chemical composition Average oxidation

state of Mn

Li0.59Na0.02Mn0.98O2 3.46

Li0.57Na0.05[Mn0.97Cr0.03]O2 3.42

Li0.57Na0.02[Mn0.95Cr0.05]O2 3.48

Fig. 5. SEM micrographs of Liy[Mn1� xCrx]O2. (A) x= 0; (B)

x= 0.03; (C) x= 0.05.


the literature [17]. Furthermore, the Mn 2p3/2 peak is

broad and highly asymmetric towards higher energy

due to multiplet splitting effects, as expected for

transition metal 2p peaks [18]. Because the binding

energy of Mn3 + is about 0.7–0.9 eV less than that of

Mn4 +, there might be an Mn3 + signal overlapping the

tail of the major Mn4 + peak. It is almost impossible to

separate Mn3 + from Mn4 + by curve fitting however,

so this is somewhat uncertain. It can be concluded (at

least) that the predominant Mn species present near

the surface is Mn4 +. It has been suggested [19] that

the rapid capacity fade of the spinel-structure lithium

manganese oxide cathode material LiMn2O4 is due to

surface dissolution of Mn3 + in the organic battery

electrolyte. The predominance of Mn4 + on the surface

of Cr-doped lithium manganese oxide would prevent

manganese from dissolving into the organic electro-

lyte, which could make the cathode material more

stable.

The binding energy of Cr 2p3/2 is at 576.2 eV (Fig.

6), attributed to CrO2. Although the raw material used

to prepare Na bronze is Cr2O3, the predominant Cr

species present near the surface is Cr4 +. This indicates

that the metals existing near the surface are always in

higher oxidation state.

The XPS measurements using a semi-quantitative

method show that the molar ratio of Mn/Cr near

surface was 88:12, much less than that of the stoichi-

ometry in the bulk materials (Mn/Cr = 95:5). This

result implies that the compound involves a phase

separation into a Cr-rich oxide phase near the surface

and an Mn-rich oxide phase in the bulk. Regan et al.

[20] have studied the difference between surface and

bulk composition of Al-doped lithium manganese

oxides, and concluded that any component of a

multicomponent oxide intercalation compound that

tends to separate will lead to the formation of surface

layers. In this work, the evolution of a Cr-rich surface

layer could protect the manganese oxide phase from

contact with the electrolyte and consequently prevent

surface dissolution. As a result, it can be suggested

that the predominance Mn4 + of Cr-rich nature of

surface layer could improve the stability of the Cr-

doped lithium manganese oxide when used as a

cathode material. Further studies are being carried

out on the electrochemistry of these materials.

3.3. Lithium manganese bronzes doped with Mg

Fig. 7 shows the XRD patterns of Mg-doped

precursors Na2/3[Mn1� xMgx]O2 (x= 0.05, 0.10 and

0.15). All of them can be indexed in the space group

P63/mmc, and the calculated Bragg peak positions are

in agreement with the observed values. The diffraction

Fig. 6. X-ray photoelectron spectra of Mn 2p3/2, Mn 2p1/2, Cr 2p3/2 and Cr 2p1/2 in the Cr-doped Li0.57Na0.02[Mn0.95Cr0.05]O2.


peaks became stronger with increasing Mg content,

indicating that the crystal structure becomes more

complete and the stacking faults in the layered com-

pound are reduced.

Fig. 7. XRD patterns of Na2/3[Mn1� xMgx]O2

The XRD patterns of lithium manganese oxides

obtained by ion exchange of Na bronzes, displayed in

Fig. 8, can all be indexed as the layered O2 structure.

The diffraction peaks are broader compared with the

. (A) x = 0.05; (B) x= 0.10; (C) x= 0.15.

Fig. 8. XRD patterns of Liy[Mn1� xMgx]O2 obtained by ion exchange of Na2/3[Mn1� xMgx]O2. (A) x= 0.05; (B) x= 0.10; (C) x= 0.15.


precursors, indicating that the particle size decreases

after ion exchange. The ion exchange of precursors

with x = 0.10 and 0.15 was not as complete as the

precursor with x = 0.05, indicated by the preserve of

strong 002 peaks from the Na-phase at 2h = 16j,which might be due to the different particle size

between them. Generally, ion diffusion is more rapid

in small particles than in large ones.

Fig. 9. X-ray photoelectron spectrum of Mn 2p3/2, Mn 2p

X-ray spectra of Mn 2p and Mg 1s in the compound

with stoichiometry Li0.56Na0.04[Mn0.91Mg0.09]O2 (on

the basis of chemical analysis) are shown in Figs. 9

and 10, respectively. The binding energy of Mn 2p3/2is at 642.2 eV, consistent with the presence of Mn4 +.

The binding energy of Mg 1s is at 1302.7 eV, as

expected for Mg2 +. The molar ratio of Mn/Mg near

the surface was 90:10 by the semi-quantitative XPS

1/2 in the Mg-doped Li0.56Na0.04[Mn0.91Mg0.09]O2.

Fig. 10. X-ray photoelectron spectrum of Mg 1s in


measurement, which is consistent with the results

of the chemical analysis of the bulk (91:9). This

indicates that the composition near the surface and

in the bulk were almost the same; in other words,

there is no multicomponent oxide mixture in this

Mg-doped lithium manganese oxide, which is very

different from the Cr-doped lithium manganese

oxide.

Fig. 11. XRD patterns of Na2/3[Mn1� xAlx]O2

3.4. Lithium manganese bronzes doped with Al

Figs. 11 and 12 display the XRD patterns of Al-

doped precursors Na2/3[Mn1� xAlx]O2 and the ion-

exchange products, Liy[Mn1� xAlx]O2 (x = 0.05, 0.10,

0.15), respectively. The precursors and products can

be indexed in the hexagonal space groups P63/mmc

and P3ml as having P2 and O2 layered structures,

the Mg-doped Li0.56Na0.04[Mn0.91Mg0.09]O2.

. (A) x = 0.05; (B) x = 0.10; (C) x = 0.15.

Fig. 12. XRD patterns of Liy[Mn1� xAlx]O2 obtained by ion exchange of Na2/3[Mn1� xAlx]O2. (A) x= 0.05; (B) x= 0.10; (C) x= 0.15.


respectively. After ion exchange, the diffraction peaks

of lithium manganese oxides became weaker and

broader, indicating an increase in stacking faults and

the decrease of the particle size. The ion exchange

was estimated to be more than 90% complete for the

three layered compounds, on the basis of the small

residual 002 peak from the Na-phase.

4. Summary

P2-sodium manganese bronzes doped with Li, Cr,

Mg and Al have been prepared. O2-lithium manga-

nese oxides were obtained by ion exchange of sodium

by lithium. For the Li-doped compound, the doped Li

was shown to occupy the octahedral manganese site

on the basis of the ion-exchange properties and

chemical analysis. XPS measurements show that

Mn4 + and Cr4 + predominate near the surface of the

Cr-doped lithium manganese oxide, and the com-

pound involves a phase separation into a Cr-rich oxide

phase near-surface region and an Mn-rich oxide in the

bulk, while there is no such a phase separation in the

Mg-doped lithium manganese oxide compound. The

different element distribution in the two kinds of

compounds might be a consequence of the different

electronegativity and diameter of the dopant ions.

Questions remain concerning the effect of the doped

element on the structure and properties of the materi-

als. We are addressing these issues at present and will

return to them in a later paper.

Acknowledgements

This project was supported by the 863 Foundation

of China (2001AA323020).

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Layered Li–Mn oxides with the O2 structure: preparation of Li2/3[Mn1−xMx]O2 (M=Li, Cr, Mg, Al)...

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