Local crystal structure around manganese in new potassium-based nanocrystalline manganese oxyiodide

Article abstract of DOI:10.1021/jp012704g

A new nanocrystalline potassium-based lithium manganese oxyiodide has been prepared by using Chimie Douce route at room temperature. According to the electrochemical measurements, this nanocrystalline sample shows a large initial capacity up to a??340 mAh/g at a constant current density of 0.2 mA/cm², which is much larger than that of sodium-based homologue. The X-ray diffraction analysis demonstrates that the amorphous character of the nanocrystalline compounds is maintained before and after chemical lithiation reaction. The local crystal structure around manganese in these materials has been determined by performing the combinative micro-Raman and X-ray absorption spectroscopy. From the Mn K-edge X-ray absorption near-edge structure and micro-Raman results, it becomes certain that manganese ions are stabilized in the rhombohedral layered lattice consisting of edge-shared MnO₆ octahedra, and the crystal symmetry is changed into a monoclinic symmetry upon reaction with n-BuLi. The Mn K-edge extended X-ray fine structure analysis reveals that the structural distortion caused by lithiation process is less significant for these nanocrystalline compounds than for the spinel lithium manganate. In this context, the great discharge capacity of the nanocrystalline materials is attributable for the pillaring effect of larger alkali metal ion than lithium ion, providing an expanded interlayer space available for Li insertion. In addition, the I L_I-edge X-ray absorption near-edge structure results presented here make it clear that iodine is stabilized as iodate species on the grain boundary or the surface of the nanocrystalline manganese oxyiodide, which helps to maintain the nanocrystalline nature of the present materials before and after Li insertion.

Full text of DOI:10.1021/jp012704g

J. Phys. Chem. B 2002, 106, 4053-4060
4053
ARTICLES
Local Crystal Structure around Manganese in New Potassium-Based Nanocrystalline
Manganese Oxyiodide
Seong-Ju Hwang,† Chai-Won Kwon,† Josik Portier,† Guy Campet,*,† Hyo-Suk Park,‡
Jin-Ho Choy,*,‡ Pham V. Huong,§ Masahiro Yoshimura,| and Masato Kakihana|
Institut de Chimie de la Matie`re Condense´e de Bordeaux (ICMCB) du CNRS, 87 AV. du Dr. A. Schweitzer,
33608 Pessac, France, National Nanohybrid Materials Laboratory, School of Chemistry, Seoul National
UniVersity, Seoul 151-747, Korea, Laboratoire de Physico-Chimie Mole´culaire, UniVersite´ Bordeaux I,
351 Cours de la Libe´ration, 33405 Talence, France, and Department of Materials Science and Engineering,
Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
ReceiVed: July 13, 2001; In Final Form: January 17, 2002
A new nanocrystalline potassium-based lithium manganese oxyiodide has been prepared by using Chimie
Douce route at room temperature. According to the electrochemical measurements, this nanocrystalline sample
shows a large initial capacity up to ∼340 mAh/g at a constant current density of 0.2 mA/cm2, which is much
larger than that of sodium-based homologue. The X-ray diffraction analysis demonstrates that the amorphous
character of the nanocrystalline compounds is maintained before and after chemical lithiation reaction. The
local crystal structure around manganese in these materials has been determined by performing the combinative
micro-Raman and X-ray absorption spectroscopy. From the Mn K-edge X-ray absorption near-edge structure
and micro-Raman results, it becomes certain that manganese ions are stabilized in the rhombohedral layered
lattice consisting of edge-shared MnO6 octahedra, and the crystal symmetry is changed into a monoclinic
symmetry upon reaction with n-BuLi. The Mn K-edge extended X-ray fine structure analysis reveals that the
structural distortion caused by lithiation process is less significant for these nanocrystalline compounds than
for the spinel lithium manganate. In this context, the great discharge capacity of the nanocrystalline materials
is attributable for the pillaring effect of larger alkali metal ion than lithium ion, providing an expanded interlayer
space available for Li insertion. In addition, the I LI-edge X-ray absorption near-edge structure results presented
here make it clear that iodine is stabilized as iodate species on the grain boundary or the surface of the
nanocrystalline manganese oxyiodide, which helps to maintain the nanocrystalline nature of the present materials
before and after Li insertion.
Introduction
related to the structural transition from cubic spinel to tetragonal
one, leading to a structural breakdown and to a formation of
Recently, special attention has been paid for lithium man-
ganese oxides as promising cathode materials for lithium
rechargeable batteries, which is due to the low toxicity, rich
abundance, and low price of manganese.1 In this context, various
types of lithium manganates have been explored for the purpose
of developing new cathode materials with better electrochemical
performance.2-4 However, most of lithium manganates experi-
ence a common structural modification to a spinel-type cation
ordering in the course of electrochemical cycling.5,6 This
thermodynamically stable spinel phase has been well-known
to suffer from capacity fading during repeated charge-discharge
cycling.1 The capacity loss of this cathode material is closely
electronically isolated zones.7 In fact, this sort of problem
originating from the phase transition is considered to be more
or less inevitable for well-crystallized lithium metal oxides. Such
a speculation gives an impetus to develop amorphous phases
as alternative cathode materials, since this type of compounds
would be much more tolerable for the repeated Li insertion-
deinsertion process. For example, very recently we have found
that nanocrystalline spinel compound exhibits an enhanced
electrochemical performance for 3 V region corresponding to
Li insertion into 16c site in the spinel phase, which would be
understood by the grafting mechanism of lithium on the surface
of nanocrystalline material.8 In addition, there have been some
reports on nanocrystalline lithium manganese oxides showing
superior electrochemical properties over well-crystalline homo-
logues.9-12 Among these nanocrystalline materials, sodium-
based manganese oxyiodides possess a quite large discharge
capacity and the best cyclability, and moreover their electro-
chemical performances can be improved by ball-milling pro-
cess.9 On the other hand, Whittingham, M. S. et al. reported
* To whom correspondence should be addressed.Guy Campet: Phone:
+33-5-56-84-62-97. Fax: +33-5-56-84-27-61. E-mail: campet@
icmcb.u-bordeaux.fr. Jin-Ho Choy: Phone: +82-2-880-6658. Fax:
+82-2-872-9864. E-mail: jhchoy@plaza.snu.ac.kr.
† Institut de Chimie de la Matie`re Condense´e de Bordeaux (ICMCB) du
CNRS.
‡ Seoul National University.
§ Universite´ Bordeaux I.
| Tokyo Institute of Technology.
10.1021/jp012704g CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/30/2002

4054 J. Phys. Chem. B, Vol. 106, No. 16, 2002
Hwang et al.
that a potassium-based manganate with rhombohedral layered
structure is more stable for electrochemical lithiation-delithia-
tion reaction compared to sodium- or lithium-based ones.13
Nonetheless, to our knowledge, there is no systematic study on
potassium-based nanocrystalline manganese oxyiodide although
it is worthwhile to probe the effect of alkali metal on the
electrochemical properties of these nanocrystalline manganese
oxyiodide. From the viewpoint of solid-state chemistry, it is
also important to determine the local structure around manganese
and the role of iodine species in understanding the mechanism
of Li insertion in this nanocrystalline material. However, it is
quite difficult to probe the local cation ordering of nanocrys-
talline compounds because of their X-ray amorphous character.
In our recent papers, it was demonstrated that a combination of
micro-Raman and X-ray absorption spectroscopy (XAS) is very
effective in probing the local atomic structure of electrode
materials with poor crystallinity as well as in examining the
effect of Li insertion-deinsertion process on the chemical
bonding character of these compounds.5,14,15
Figure 1. X-ray diffraction patterns of (a) nanocrystalline K-Li-
Mn-O-I compound and its derivatives (b) after the heat treatment at
600 °C and (c) after lithiation, together with (d-f) the corresponding
data of Na-homologues. In (b) and (e), the indexed reflections are
assigned on the basis of cubic spinel structure, and the circle and square
symbols represent the reflections from KxMnO2‚yH2O and Na0.7MnO2
phases, respectively. The asterisk symbols in (c) and (f) denote the
peaks originating from LiOH‚H2O.
In this work, we have prepared a new potassium-based
nanocrystalline manganese oxyiodide and characterized its
physicochemical properties in comparison with those of sodium-
based homologue. In addition, an attempt has been made to
determine the local geometric and electronic structures around
manganese before and after chemical lithiation by using micro-
Raman and Mn K-edge XAS analyses.16 The chemical bonding
characters of iodine species in these nanocrystalline materials
have also been studied by performing the X-ray absorption near-
edge structure (XANES) analysis at I LI-edge.
range of 1.5-4.3 V at the constant current densities of 0.2 and
0.5 mA/cm2 (10 and 25 mA/g).
Micro-Raman Spectroscopy. The micro-Raman spectra
presented here were recorded on a Dilor-Omars microspec-
trometer coupled with an optical microscope (spatial resolution
of 1 µm2) and an intensified 1024-channel photodiode array
detector. The 514.5-nm line from an argon ion laser Spectra
Physics model 2016 was used as an excitation source. All the
present spectra were measured by backscattering from the
freshly fractured surfaces of the pellet or from the powdered
sample. To prevent possible thermal damage of the samples,
the power of incident laser light was maintained at less than 1
mW. After each measurement, the sample surface was thor-
oughly checked to remove the possibility of spectral modifica-
tion caused by surface degradation.
X-ray Absorption Measurement. The XAS experiments
were performed for the nanocrystalline A-Li-Mn-O-I (A
) K and Na) compounds and their lithiated derivatives by using
the extended X-ray absorption fine structure (EXAFS) facility
installed at the beam line 7C at the Photon Factory in Tsukuba.17
The XAS data were collected at room temperature in a
transmission mode using gas-ionization detectors. All the present
spectra were calibrated by measuring the spectra of Mn metal
foil and KI. The data analysis for the experimental spectra was
carried out by the standard procedure as reported previously.5
Experimental Section
Sample Preparation. The nanocrystalline A-Li-Mn-O-I
(A ) K and Na) samples were prepared by reacting the aqueous
solution of KMnO4 or NaMnO4•H2O with 1.5 equiv LiI at room
temperature, similar to the previously reported Chimie Douce
method.9,10 The mixed solution was maintained for 1 day under
constant stirring. The resulting precipitate was washed with
water and was dried at 130 °C in a vacuum. The chemical
lithiation reaction for these amorphous samples was carried out
by reacting them with 1.6 M n-BuLi in hexane for 48 h. Prior
to the physicochemical characterization, the lithiated samples
were washed thoroughly with hexane and ethanol and were dried
in a vacuum.
Sample Characterization. The crystal structures of A-Li-
Mn-O-I (A ) K and Na) and their chemically lithiated
products were studied by X-ray diffraction (XRD) measurement
using Ni-filtered Cu KR radiation with a graphite diffracted
beam monochromator. The chemical compositions of these
samples were determined by using atomic absorption (AA)
spectrometry, inductive coupled plasma (ICP) spectrometry, and
electron probe for microanalysis (EPMA). The water contents
were estimated by performing thermogravimetric analysis (TGA)
under O2 flow. The weight losses of ∼3% and ∼6% up to ∼150
°C are observed for the Na- and K-based nanocrystals,
respectively, indicating that these materials possess about 0.2-
0.4 mole water per unit formula.13 The electrochemical mea-
surements were performed with the cell configuration of Li/
1M LiPF6 in EC:DEC (50:50 v/v)/composite cathode, which
was assembled in a drybox. The composite cathode was prepared
by mixing thoroughly the active A-Li-Mn-O-I (A ) K and
Na) material (20 mg, 70 wt %) with 25 wt % of acetylene black
and 5 wt % of PTFE (poly(tetrafluoroethylene)). All the
experiments were carried out in a galvanostatic mode with Arbin
BT 2043 multichannel galvanostat/potentiostat in the voltage
Results and Discussion
Powder XRD and Chemical Analyses. The powder XRD
patterns of as-prepared A-Li-Mn-O-I (A ) K and Na)
compounds are shown in Figure 1, together with those of their
heated and lithiated products. No distinct reflection can be
observed for the nanocrystalline samples heated at 130 °C in
vacuum. After the heat-treatment at 600 °C, an amorphous phase
is changed into a mixed crystalline phase, that is, a mixture of
rhombohedral layered KxMnO218 and cubic-spinel Li1+yMn2-yO4
for the potassium-based nanocrystal and a mixture of Na0.7
MnO2 and cubic-spinel Li1+yMn2-yO4 for the sodium-based
-
19

Local Crystal Structure around Manganese
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4055
Figure 2. Discharge potential profiles of the nanocrystalline A-Li-
Mn-O-I compounds with A ) (a) K and (b) Na. The electrochemical
measurements were carried out in the potential range of 1.5-4.3 V
with the applied current density of 0.2 mA/cm2.
Figure 3. Differential capacity profiles of the nanocrystalline A-Li-
Mn-O-I compounds with A ) (a) K and (b) Na. The electrochemical
measurements were carried out in the potential range of 1.5-4.3 V
with the applied current density of 0.2 mA/cm2.
TABLE 1: Chemical Formulas of Nanocrystalline
discharge potential profiles of the nanocrystalline A-Li-Mn-
O-I (A ) K and Na) compounds measured at a constant current
density of 0.2 mA/cm2 are plotted in Figure 2. For both
compounds, a small capacity of ∼110 mAh/g corresponding to
the oxidation of trivalent manganese ions is commonly observed
(not shown in the figure), which is much smaller than the
capacity of the following discharge process. This underlines that
the electrochemical activity of these manganese oxyiodides is
attributed mainly to the insertion of lithium ion from Li anode
rather than to the reinsertion of lithium ions which already exist
in the as-prepared materials. Both nanocrystals show nearly
identical curve shape, which is very close to that of the
previously reported amorphous manganese dioxide,9-11 confirm-
ing the amorphous character of the present manganese oxy-
iodides. The observed smooth decrease of potential during
discharge process is a characteristic of nanocrystalline electrode
materials such as SnO2, Fe2O3, and so forth.20,21 This can be
explained by the fact that the existence of surface defects or
surface dangling bonds in nanocrystalline materials gives rise
to the formation of many sub-bands between valence and
conduction bands, leading to a smooth change of Fermi energy
and hence a slow potential change.8 In comparison with the
sodium-based manganese oxyiodide, the potassium-based one
shows a greater discharge capacity for each cycle. For a more
accurate inspection of discharge behaviors, the differential
capacity profiles of the nanocrystalline A-Li-Mn-O-I
(A ) K and Na) compounds are presented in Figure 3. As the
cycle proceeds, no significant change in curve shape except a
slight depression of the peak at ∼2.7 V can be observed for
both compounds, which suggests that the electrochemical
discharge-charge process does not induce any marked structural
transition in these nanocrystalline materials. However, the
present nanocrystals commonly show a significant capacity
fading, which is fairly contrasted with the previous report on
Manganese Oxyiodides and Their Lithiated Derivatives
sample
as-prepared
lithiated
K-Li-Mn-O-I Li0.54K0.31MnO2.5-δI0.10 Li2.45K0.34MnO2.5-δI0.13
Na-Li-Mn-O-I Li0.52Na0.62MnO2.5-δI0.07 Li2.48Na0.79MnO2.5-δI0.08
nanocrystal. In the lithiated derivatives, there is no XRD peak
related to the crystalline lithium manganate, except several weak
reflections corresponding to a small amount of lithium hydroxide
hydrate formed by a reaction with n-BuLi and water of the
samples. This finding indicates that the amorphous structure of
the present nanocrystalline manganese oxyiodides is highly
tolerable for Li insertion reaction.
The chemical formula of the nanocrystalline A-Li-Mn-
O-I (A ) K and Na) compounds and their lithiated derivatives
estimated from AA, ICP, and EPMA analyses is summarized
in Table 1. The concentration of alkali metal ion is determined
to be smaller for the potassium-based nanocrystal than for the
sodium-based one, whereas the iodine content is larger for the
former compared to the latter. As listed in Table 1, the lithiation
process results in the incorporation of about two moles of lithium
ions per unit formula commonly for both nanocrystalline
materials. This reflects their greater ability to accommodate
lithium ions compared to well-crystallized electrode materials
such as LiMn2O4 spinel, even though a fraction of lithium ions
are consumed for the reaction with the sample water, leading
to the formation of lithium hydroxide.
Electrochemical Performance. The cycling characteristics
of the nanocrystalline A-Li-Mn-O-I (A ) K and Na)
compounds have been examined to probe the effect of alkali
metal on the electrochemical performance of the lithium
manganese oxyiodides. Since both pristine materials possess an
open circuit potential of ∼3.4 V (vs Li/Li+), the electrochemical
cycling starts with the first charge process up to 4.3 V. The

4056 J. Phys. Chem. B, Vol. 106, No. 16, 2002
Hwang et al.
Figure 4. Discharge capacities of the nanocrystalline A-Li-Mn-
O-I compounds with A ) K (squares) and Na (circles). The
electrochemical measurements were carried out in the potential range
of 1.5-4.3 V with the applied current density of 0.5 mA/cm2.
Figure 5. (a) Mn K-edge XANES spectra for the nanocrystalline
A-Li-Mn-O-I compounds with A ) (i) K and (ii) Na and (iv-v)
their lithiated derivatives, in comparison with the references (iii) cubic
spinel LiMn2O4, (vi) tetragonal spinel Li2Mn2O4, (vii) λ-MnO2, (viii)
nanocrystalline manganese oxide, and (ix) Mn2O3. The magnified
spectra for preedge region are presented in (b).
the sodium-based materials prepared in acetonitrile solvent.9 This
may be attributed not to the structural transition but to the
presence of water leading to the formation of LiOH and HF
via the reaction with Li anode and electrolyte, respectively. In
this context, now we are trying to prepare potassium-based
nanocrystal in nonaqueous solvent and to examine the effect of
ball milling on its electrochemical property, with a view to
improving the cyclability and discharge capacity of this promis-
ing nanocrystalline material. On the other hand, we have also
measured the electrochemical performances of the present
nanocrystalline samples up to 20 cycles with a higher current
density of 0.5 mA/cm2, to examine the long-term cycling
characteristics as well as to probe the effect of current density.
As presented in Figure 4, the potassium-based phase shows the
high initial discharge capacity of 185 mAh/g with a notable
capacity decrease to 120 mAh/g after the 20th cycle, whereas
the sodium-based one exhibits a better capacity retention from
178 mAh/g at the first cycle to 155 mAh/g at the 20th cycle.
This is surely contrasted with the results obtained with the lower
current density of 0.2 mA/cm2. Such different long-term cycle
characteristics of these nanocrystals are suggested to originate
from their dissimilar water contents or from the contribution of
other chemical species such as Na/K or iodine to discharge
capacity. However, it is quite difficult to conclude here the origin
of the observed electrochemical behavior of the present nano-
crystals, since there are so many factors to be considered
influencing the long-term cycling performance. To answer this
question, now we are performing the detailed electrochemical
measurements with various experimental conditions. In this
context, here we limit our further discussion to why the
potassium-based phase shows a greater “initial” discharge
capacity than the sodium-based one. In fact, prior to the
electrochemical measurements, we expected that the potassium-
based compound possesses inferior initial discharge capacity
to the sodium-based one, judging from the heavier atomic weight
of potassium compared to sodium. But this is not true for the
nanocrystalline materials presented here. To understand such
an unexpected finding, we have examined the local structure
around manganese in both nanocrystalline compounds by
performing micro-Raman and XANES/EXAFS tools comple-
mentarily.
illustrated in Figure 5a, in comparison with the reference spectra
of cubic spinel LiMn2O4, tetragonal spinel Li2Mn2O4, λ-MnO2,
nanocrystalline manganese oxide,22,23 and Mn2O3. The position
of the edge jump for the pristine manganese oxyiodides is a
little bit higher than that for LiMn+IVMn+IIIO4 but slightly lower
than that for λ-Mn+IVO2, which indicates that manganese ions
in these materials have an average valence state of >+3.5.
Actually, the sodium-based manganese oxyiodide has been
reported to possess the average Mn oxidation state of about
+3.8.9 In the lithiated manganese oxyiodides, the edge position
is rather in accordance with that of tetragonal spinel
Li2Mn+III2O4, confirming the reduction of manganese ions after
the n-BuLi-treatment. In the preedge region, all the spectra
presented here show small preedge peaks (denoted P and P′),
which are assigned as the transitions from the core 1s level to
unoccupied 3d states. Even though they are not allowed by the
electronic dipolar selection rule, ∆l ) (1, the preedge peaks
could be discerned either because of quadrupole-allowed
transitions or because of the mixing of 4p and 3d states.5 In
this context, the weak intensity of these features suggests that
all the manganese ions in the samples under investigation are
stabilized in octahedral site with an inversion center. The
position and shape of these preedge peaks are well-known to
be closely related to the oxidation state of the absorbing ion
and the local arrangement of backscattering ions, respectively.5
As can be seen clearly from Figure 5b, the relative energies of
the preedge peaks are wholly consistent with the order of the
main-edge positions. It is also evident that the spectral shape
of preedge features in all the samples presented here can be
classified into two groups; two preedge peaks P and P′ are
discernible for some materials containing considerable amount
of tetravalent manganese ions, whereas only one feature P at
lower energy can be observed for the others with trivalent
manganese ions. In fact, the latter characteristic preedge feature
for trivalent manganese compounds can be interpreted as a result
of the splitting of t2g and eg energy levels modified by Jahn-
Teller deformation.24 For the nanocrystalline manganese oxy-
iodides under investigation, both the pristine materials exhibit
Mn K-Edge XANES Analysis. The Mn K-edge XANES
spectra of the pristine nanocrystalline A-Li-Mn-O-I (A )
K and Na) compounds and their lithiated derivatives are

Local Crystal Structure around Manganese
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4057
nearly the same spectral features in preedge region, whereas
there are marked spectral differences between the corresponding
lithiated derivatives. That is, the lithiation reaction gives rise
to a smaller change in the preedge features of the potassium-
based nanocrystal compared to the sodium-based one, which
suggests that the Jahn-Teller deformation of the MnO6
octahedra is smaller in the former. In the main-edge region,
there are some peaks (denoted A, B, and C), which are assigned
as the dipole-allowed transitions from the core 1s level to
unoccupied 4p states. In contrast to the well-crystallized
LiMn2O4 and λ-MnO2,25 both the pristine nanocrystalline
compounds do not show the fine features A and B in this region,
which is attributed to the absence of long-range crystal order.
This finding is well consistent with the recent XANES study
for Na-based nanocrystal.26 On the contrary, a distinct peak A
is clearly observed for both the lithiated derivatives as well as
for tetragonal spinel Li2Mn2O4, although its spectral weight is
weaker for the former than for the latter. Since the strong peak
A is an indicator of trivalent manganese ion stabilized in the
Jahn-Teller distorted octahedra,5 the observed weak intensity
of peak A indicates that the Jahn-Teller deformation around
manganese is less prominent for the nanocrystalline manganese
oxyiodide than for the well-crystallized tetragonal spinel, as
reported recently.26 Moreover, a closer inspection on the present
XANES spectra reveals that K-based nanocrystal exhibits
smaller intensity of peak A than Na-based one, which is well
correlated with less prominent change of preedge features after
lithiation in the former. On the other hand, an intense and sharp
peak C appears commonly for all the nanocrystalline manganese
oxyiodides including the lithiated derivatives as well as for the
spinel-structured compounds. Considering the fact that the
intensity of this peak is proportional to the relative concentration
of edge-shared MnO6 octahedra with respect to corner-shared
one,27 it can be supposed that the crystal structure of nano-
crystalline manganese oxyiodides consists of edge-shared MnO6
octahedra like spinel-structured compounds. In contrast, only a
broader and less intense peak C can be discerned for another
nanocrystalline manganate without iodine and potassium species
(sample viii in Figure 5), indicating this compound has the
intergrowth structure of edge- and corner-shared MnO6 octa-
hedra. On the basis of this finding, it can be supposed that iodine
and potassium ions may act as stabilizers for edge-shared MnO6
network. It has been known that there are some kinds of lithium
manganate phases such as spinel LiMn2O4, layered LiMnO2,
and so forth, whose crystal structures constitute with edge-shared
MnO6 octahedra. However, on the basis of the present Mn
K-edge XANES results, it is very difficult to determine
conclusively the local structure around manganese in the
nanocrystalline manganese oxyiodides. To solve this problem,
we have applied micro-Raman spectroscopy.
Figure 6. Micro-Raman spectra for the nanocrystalline A-Li-Mn-
O-I compounds with A ) (a) K and (b) Na and (e-f) their lithiated
derivatives, in comparison with the references (c) rhombohedral layered
LiCoO2, (d) cubic spinel LiMn2O4, (g) monoclinic layered LiMnO2,
and (h) tetragonal spinel Li2Mn2O4.
Eg, and A1g mode, respectively, whereas four characteristic
phonon lines can be detected for the tetragonal spinel phase in
the frequency range 200-650 cm-1. The Raman spectrum of
tetragonal spinel Li2Mn2O4 can be clearly distinguished from
that of monoclinic layered LiMnO2, even though both the
lithium manganates show almost the same XRD patterns.29 This
is because the Raman spectrum is strongly dependent on the
crystal symmetry rather than on the structure factor. In this
regard, the Raman spectroscopy enables us to easily identify
various lithium manganate phases with closely related crystal
structures such as layered LiMnO2 (C2/m), tetragonal spinel Li2-
Mn2O4 (I41/amd), orthorhombic LiMnO2 (Pmnm), and so forth.15
In the nanocrystalline manganese oxyiodides, two very broad
features are discernible around 490 and 630 cm-1. Considering
that the nanocrystalline nature and the mixed Mn oxidation state
of the present manganese oxyiodides are surely responsible for
the very broad Raman feature, the Raman spectra of these
nanocrystals are somewhat similar to that of rhombohedral
LiCoO2 rather than that of spinel LiMn2O4. However, the
considerable broadness of features makes it very difficult to
definitively determine the local structure of nanocrystalline
materials on the basis of the present Raman spectra. In this
regard, we have also examined the Raman spectra of lithiated
manganese oxyiodides, since the lithiation usually increases the
signal-to-noise (S/N) ratio of Raman data by depressing the
electrical conductivity. As can be seen clearly from Figure 6,
there is remarkable spectral consistency between the nano-
crystalline materials and the monoclinic layered LiMnO2,
highlighting that the lithiated derivatives possess the layered
structure with monoclinic distortion. Generally the reduction
of Mn oxidation state induces a Jahn-Teller distortion around
manganese, giving rise to a structural transition from rhombo-
hedral layered to monoclinic layered, or from cubic spinel to
tetragonal spinel. Moreover, it is impossible to change from a
spinel structure to a layered one during lithiation process, since
the spinel structure is thermodynamically more stable than the
layered one. In this regard, we can conclude that the manganese
ion in nanocrystalline manganese oxyiodide crystallizes locally
in rhombohedral layered cation ordering not in the spinel-type
one, and the lithiation reaction results in a local structural change
into monoclinic layered structure.
Micro-Raman Spectroscopy. The Raman spectra of the
nanocrystalline A-Li-Mn-O-I (A ) K and Na) compounds
are represented in Figure 6, together with the reference spectra
of rhombohedral layered LiCoO2, cubic spinel LiMn2O4, mono-
clinic layered LiMnO2, and tetragonal spinel Li2Mn2O4. As
predicted by the factor group analysis, the rhombohedral LiCoO2
compound exhibits two A1g and Eg phonon lines at 486 and
596 cm-1, whereas three Raman peaks corresponding to Raman-
active 2Ag + Bg modes are observed for the layered LiMnO2
oxide at 422, 481, and 603 cm-1 15,28
In the spinel-structured
.
compounds, a larger number of Raman peaks appear because
of their complex crystal structure. As shown in Figure 6, the
present cubic spinel LiMn2O4 sample shows four Raman peaks
at 270, 340, 454, and 631 cm-1, which are assigned as 2T2g,

4058 J. Phys. Chem. B, Vol. 106, No. 16, 2002
Hwang et al.
Figure 8. Fourier filtered Mn K-edge EXAFS spectra for the
nanocrystalline A-Li-Mn-O-I compounds with A ) (a) K and (b)
Na and (c-d) their lithiated derivatives. The solid lines and empty
circles represent the fitted and experimental data, respectively.
TABLE 2: Results of Nonlinear Least-Squares
Curve-Fitting Analysis for the Mn K-edge EXAFS Spectra
of Nanocrystalline Manganese Oxyiodides and Their
Lithiated Derivatives
sample
bond
CNa R (Å) σ2 (10-3 × Å2)
K-Li-Mn-O-I
(Mn-O)
(Mn-Mn)
6.0
4.2
6.0
4.3
4
1.90
2.86
1.90
2.85
1.90
2.33
2.87
3.02
1.90
2.27
2.83
3.02
4.89
6.22
4.95
6.91
3.21
10.60
2.82
9.99
4.08
15.31
5.73
15.11
Figure 7. (a) k3-weighted Mn K-edge EXAFS spectra and (b) their
Fourier transforms for the nanocrystalline A-Li-Mn-O-I compounds
with A ) (i) K and (ii) Na and (iii-iv) their lithiated derivatives. The
range over which the Fourier filtering has been made is shown by the
arrows.
Na-Li-Mn-O-I (Mn-O)
(Mn-Mn)
(Mn-Oeq)
Lithiated
K-Li-Mn-O-I
(Mn-Oax)
(Mn-Mn)
(Mn-Mn)
(Mn-Oeq)
(Mn-Oax)
2
1.4
2.8
4
Mn K-Edge EXAFS Analysis. The local structural variation
of the nanocrystalline A-Li-Mn-O-I (A ) K and Na)
compounds upon chemical lithiation process has been examined
quantitatively using Mn K-edge EXAFS spectroscopy. The k3-
weighted Mn K-edge EXAFS spectra for the nanocrystalline
A-Li-Mn-O-I (A ) K and Na) compounds and their
lithiated derivatives are represented in Figure 7a and the
corresponding Fourier transforms (FTs) in Figure 7b. As can
be seen from Figure 7a, the overall spectral features of the
EXAFS oscillation of the pristine nanocrystalline compounds
are quite similar to each other, indicating the same local structure
around manganese in both compounds. On the other hand, the
lithiation process gives rise to a spectral change in EXAFS
oscillation which is less prominent for K-based material than
for Na-based one, as observed in XANES region. In the FT
diagrams (Figure 7b), the nanocrystalline manganese oxyiodides
exhibit two intense FT peaks at ∼1.5 and ∼2.5 Å, which are
attributed to the (Mn-O) and (Mn-Mn) shells, respectively.
Upon lithiation reaction, the FT peaks become depressed
especially for (Mn-Mn) shell, which can be understood from
the fact that the Jahn-Teller distortion around Mn causes a
damping of EXAFS signal. However, in contrast to the
previously reported results for the spinel phase,25,30 the splitting
of FT peaks corresponding to (Mn-Mn) shells induced by
lithiation is not obvious for the present nanocrystalline com-
pounds, which is well correlated with the XANES results.26
Between both lithiated nanocrystals, the splitting of (Mn-Mn)
shells is more well-resolved for Na-based material than for
K-based one. These findings can be regarded as further evidence
for the excellent endurance of the amorphous character of these
nanocrystalline materials during Li insertion reaction, especially
for the K-based nanocrystal. The peaks at 4-6 Å originated
Lithiated
Na-Li-Mn-O-I (Mn-Mn)
(Mn-Mn)
2
1.4
2.8
a While the coordination numbers for (Mn-O) shells are fixed to
the crystallographic values, those for (Mn-Mn) shells are set as
variables in the curve-fitting analyses.
from the multiple scattering paths with focusing effect are nearly
suppressed in the present compounds, in contrast to the well-
crystalline LiMnO2 and LiMn2O4 ones.5,25 This confirms the
nanocrystalline nature of the present manganese oxyiodides. To
determine the structural parameters such as coordination number
(CN), bond distance (R), and Debye-Waller factor (σ2), these
FT peaks were isolated by inverse Fourier transform to a k space
and then were curve-fitted. In light of micro-Raman results, we
have tried to reproduce the spectra of nanocrystalline manganese
oxyiodides and their lithiated derivatives on the basis of the
crystal structures of rhombohedral layered and monoclinic
layered phases, respectively, in such a way that we were able
to get reasonable fitting results.31 The best fitting results are
compared to the experimental k3ø(k) Fourier filtered data in
Figure 8, and the obtained structural parameters are listed in
Table 2. While the manganese ion in the nanocrystalline A-Li-
Mn-O-I (A ) K and Na) compounds is stabilized in a regular
MnO6 octahedron with six neighboring oxygen ions at the same
distance, the manganese ion in the lithiated derivatives exists
in a Jahn-Teller distorted MnO6 octahedra, as supposed from
micro-Raman study. Moreover, the average (Mn-O) and
(Mn-Mn) bond distances of the nanocrystalline materials are
found to be elongated by lithiation reaction, confirming the
decrease of Mn oxidation state after reaction with n-BuLi. In
addition, the coordination numbers are determined to be smaller

Local Crystal Structure around Manganese
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4059
boundary prevents an effective crystal growth of lithium
manganese oxide grains, it would help to maintain the nano-
crystalline nature of this material before and after Li insertion.
Summarizing the present experimental findings, it is evident
that manganese ion in the present nanocrystalline compounds
is stabilized in the rhombohedral layered-type local structure,
which is maintained by the presence of alkali metal pillar rather
than by the existence of iodate cluster in interlayer space. In
this context, a greater initial discharge capacity of K-based
nanocrystal compared to Na-based one is interpreted as a result
of the expanded interlayer spacing sustained by larger potassium
ion, giving a better ability to accommodate the lithium ion in
the interlayer space.
Conclusion
In this study, an attempt has been made not only to determine
the local structure of manganese in the nanocrystalline electrode
materials but also to probe the effects of Li insertion on the
atomic and electronic structures of these compounds by
performing systematic micro-Raman and XAS analyses. The
electrochemical measurements demonstrate that a newly pre-
pared potassium-based lithium manganese oxyiodide shows a
great initial discharge capacity of ∼340 mAh/g at 0.2 mA/cm2,
which is larger than that of sodium-based homologue. According
to the XRD analysis, the amorphous nature of the present
samples is maintained before and after lithiation reaction. The
Mn K-edge XANES and micro-Raman results reveal that the
manganese ion is stabilized in the rhombohedral layered lattice,
and the crystal symmetry is changed into a monoclinic symmetry
upon lithiation reaction. Despite layered local crystal structure,
the present nanocrystalline materials exhibit no significant
change in discharge potential profiles during lithium insertion-
extraction process, which is surely contrasted with the layered
LiMnO2 phase with well-crystalline structure showing a struc-
tural transition to spinel-type cation ordering. This can be
regarded as evidence of the better structural stability of
nanocrystalline materials with respect to well-crystalline ones.
The Mn K-edge EXAFS analysis confirms that the structural
distortion is less prominent for the nanocrystalline compounds
than for the spinel lithium manganate, which gives an explana-
tion for the greater initial discharge capacities of these nano-
crystalline materials. On the other hand, I LI-edge XAS spectra
show that the iodine is stabilized as iodate species in the grain
boundary or on the surface of the nanocrystalline manganese
oxyiodide. Such an iodate species in the grain boundary is
presumed to contribute to the maintenance of the nanocrystal-
linity of the present materials before and after Li insertion.
Figure 9. I LI-edge XANES spectra for the nanocrystalline A-Li-
Mn-O-I compounds with A ) (a) K and (b) Na and (g-h) their
lithiated derivatives, in comparison with the references (c) KIO3, (d)
KIO4, (e) I2, and (f) KI.
for (Mn-Mn) shells than for (Mn-O) shells, which reflects
the nanocrystalline nature of the present samples leading to an
incomplete coordination of manganese by distant shells.
I LI-Edge XANES Analysis. As described in the Mn K-edge
XANES section, another type of nanocrystalline manganate
without iodine and potassium species crystallizes with edge-
and corner-shared network of MnO6 octahedra, in contrast to
the present manganese oxyiodides. In this regard, it is a very
interesting question what the role of iodine is. To give an answer
to this question, we have studied the chemical bonding character
of iodine species by performing I LI-edge XANES analysis. The
I LI-edge XANES spectra of the nanocrystalline manganese
oxyiodides and their lithiated derivatives are presented in Figure
9, in comparison with the reference spectra of KI, I2, KIO3,
and KIO4. All the reference spectra except for KI exhibit a
characteristic white line (WL) peak at around 5185-5195 eV,
which is ascribed to the transition from the 2s core level to the
unoccupied 5p state.32 Since the intensity of this WL feature is
dependent on the density of the unoccupied 5p final state, the
most intense peak is discernible for both KIO3 and KIO4
references having completely empty I 5p orbitals. On the
contrary, this 2s f 5p transition is not possible for KI because
of the full occupancy of I 5p orbital. In the pristine nanocrys-
talline materials, this WL peak shows almost the same position
and intensity as the reference KIO3, which indicates that the
iodate species is stabilized in nanocrystalline manganate oxy-
iodides, as reported very recently.26 There are two possibilities
Acknowledgment. This work was supported in part by the
Korean Ministry of Education (BSRI-99-3413) with the Brain
Korea 21 fellowship for H. S. Park and by the Korean Ministry
of Science and Technology through the National Research
Laboratory Project (1999). C. W. Kwon thanks CROUS of
Bordeaux in France for the financial support. Authors are also
grateful to Prof. M. Nomura for helping us to get the XAS data
in the Photon Factory.
-
about the site of IO3 cluster in these nanocrystalline com-
pounds; one is the interlayer site and the other is the grain
boundary or the sample surface. As shown in Figure 9, a notable
depression of peak A upon lithiation can be observed for both
nanocrystalline materials, suggesting that most of iodate ion is
reduced to a negatively charged iodide ion. Considering the
electrostatic repulsion between iodide and oxide anions, it is
impossible for iodide ion to exist in the interlayer octahedral
site of rhombohedral layered lattice. However, the elemental
analyses reveal that there still remain lots of iodine species after
lithiation, which implies that most of iodine species in nano-
crystalline manganese oxyiodide is located in the grain boundary
or on the surface. Since the iodate ion stabilized in the grain
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