Understanding formation of solid electrolyte interface film on LiMn2O4 electrode

Article abstract of DOI:10.1149/1.1516220

We studied formation of solid electrolyte interface (SEI) film on the surface of spinel LiMn₂O₄ electrodes by evaluating irreversible capacity and monitoring impedance change in the first charge and discharge cycle of a Li/LiMn₂O₄ cell. Results show that during the first cycle, LiMn₂O₄ produces 10-15% irreversible capacity in both 1 m LiPF₆ 3:7 ethylene carbonate/ethyl methyl carbonate (EC/EMC) and 1 m LiPF₆ 1:1:3 propylene carbonate (PC)/EC/EMC electrolytes. Formation of the irreversible capacity mainly takes place in two voltage regions of ～3.1 V, near open-circuit voltage of a fresh cell, and 3.7-4.2 V, in the voltage range of delithiation and lithiation of LiMn₂O₄. It is believed that the irreversible capacity is associated with the formation of SEI film on the surface of LiMn₂O₄ electrode. Impedance data indicate that electrolyte solvents greatly affect properties of the SEI film. The SEI film formed with PC/EC/EMC electrolyte is more resistive and more stable, while that formed with EC/EMC electrolyte is subjected to a reversible breakdown at voltages higher than 3.8 V. It is observed that after the cell is cycled, the SEI film becomes more conductive while the bulk resistance of electrolyte and electrode increases.

Full text of DOI:10.1149/1.1516220

Journal of The Electrochemical Society, 149 ͑12͒ A1521-A1526 ͑2002͒
A1521
0
013-4651/2002/149͑12͒/A1521/6/$7.00 © The Electrochemical Society, Inc.
Understanding Formation of Solid Electrolyte Interface Film
on LiMn O Electrode
2
4
,z
S. S. Zhang,* K. Xu,** and T. R. Jow*
United States Army Research Laboratory, Adelphi, Maryland 20783, USA
We studied formation of solid electrolyte interface ͑SEI͒ film on the surface of spinel LiMn O electrodes by evaluating irrevers-
2
4
ible capacity and monitoring impedance change in the first charge and discharge cycle of a Li/LiMn O4 cell. Results show that
2
during the first cycle, LiMn O produces 10-15% irreversible capacity in both 1 m LiPF6 3:7 ethylene carbonate/ethyl methyl
2
4
carbonate ͑EC/EMC͒ and 1 m LiPF 1:1:3 propylene carbonate ͑PC͒/EC/EMC electrolytes. Formation of the irreversible capacity
6
mainly takes place in two voltage regions of ϳ3.1 V, near open-circuit voltage of a fresh cell, and 3.7-4.2 V, in the voltage range
of delithiation and lithiation of LiMn O . It is believed that the irreversible capacity is associated with the formation of SEI film
2
4
on the surface of LiMn O electrode. Impedance data indicate that electrolyte solvents greatly affect properties of the SEI film.
2
4
The SEI film formed with PC/EC/EMC electrolyte is more resistive and more stable, while that formed with EC/EMC electrolyte
is subjected to a reversible breakdown at voltages higher than 3.8 V. It is observed that after the cell is cycled, the SEI film
becomes more conductive while the bulk resistance of electrolyte and electrode increases.
©
2002 The Electrochemical Society. ͓DOI: 10.1149/1.1516220͔ All rights reserved.
Manuscript submitted November 5, 2001; revised manuscript received May 24, 2002. Available electronically October 16, 2002.
Li-ion batteries, which are made of lithium transition metal ox-
ides as a cathode and carbonaceous materials as an anode, have been
widely used in portable electronics and are under intensive research
for application in hybrid electric vehicle ͑HEV͒ and electric vehicles
The formation process of the SEI film is studied by monitoring
impedance change of the Li/LiMn O cell during the first charge
and discharge cycle.
2
4
Experimental
Spinel Li Mn O (x ϭ 1.0, EM Industries, Inc.͒ was coated onto
͑
EV͒ because of their relatively high energy density and power
1-3
density. To maximize the energy density of Li-ion batteries, the
ratio of cathode and anode active materials must be optimized.
When determining this ratio, one should consider practical specific
capacity and irreversible capacity produced during the initial form-
ing cycles for both electrodes. Initial irreversible capacity of carbon-
aceous materials has been well understood and considered critical to
the normal operation of Li-ion batteries. This irreversible capacity is
known to result in the formation of a solid electrolyte interface ͑SEI͒
film on the surface of carbonaceous anodes which effectively pre-
x
2
4
an aluminum foil in a weight ratio of 82% LiMn O , 10% acetylene
2
4
black, and 8% poly͑vinylidene difluoride͒ ͑PVdF͒. The obtained
2
electrode film was punched into disks with an area of 1.27 cm and
dried at 120°C for 16 h under vacuum prior to use. LiPF ͑Ͼ99.9%,
6
Stella Chemifa Corp.͒, ethylene carbonate ͑EC, battery grade, Grant
Chemical͒, and propylene carbonate ͑PC, battery grade, Grant
Chemical͒ were used as received. Ethyl methyl carbonate ͑EMC,
water content Ͻ30 ppm, Mitsubishi Chemical Co.͒ was dried using
activated aluminum oxide before use. Electrolytes with a composi-
tion of either 1 m ͑mole solute per kilogram solvent͒ LiPF6 3:7
vents solvent decomposition and graphite exfoliation.4-7 Similar SEI
films, or surface layers as called by some authors, on lithium tran-
8,9
EC/EMC ͑wt %͒ or 1 m LiPF 1:1:3 PC/EC/EMC ͑wt %͒ were
sition metal oxide cathodes recently have been reported. Fourier
6
8
prepared in an argon-filled glove box. Karl-Fisher titration showed
that the electrolytes thus prepared had a water content of 10-15 ppm.
In the same glove box, a Li/LiMn O button cell was assembled and
transform infrared ͑FTIR͒, Raman, and surface-enhanced Raman
9
͑
SER͒ spectra show that the SEI film on the cathode is composed of
various lithium salts, including lithium carbonate (Li CO ), lithium
2
4
2
3
filled with 150 ␮L of electrolyte.
alkoxides ͑ROLi͒, and carboxylic lithium (RCO Li).
2
Galvanostatic cycling testing of the cell was carried out on a
Maccor Series 4000 tester at a constant current density of 0.5
In the last decade, LiCoO , LiNiO , LiMn O , and their doped
2
2
2
4
compounds have been most extensively investigated as candidates
2
mA/cm between 3.5 and 4.3 V. To describe experimental results, we
for cathode materials. Among these, spinel LiMn O4 is of great
2
took the irreversible capacity as the capacity difference between
charge and discharge, and coulomb efficiency ͑CE͒ as the capacity
percentage of discharge vs. charge. A Solartron SI 1287 electro-
chemical interface and SI 1260 impedance/gain-phase analyzer, con-
trolled by CorrWare and Zplot softwares ͑Scribner and Associates,
Inc.͒, were used to measure electrochemical impedance spectros-
copy ͑EIS͒ of the cells. The EIS was potentiostatically measured by
applying a dc bias, whose value equals the open-circuit voltage
importance due to its low cost, environmental harmlessness, high
natural abundance, and good safety in spite of its existing problems,
1
0-14
such as high capacity fading and self-discharge rate.
It has been
9
reported that formation of the SEI film on LiMn O electrodes
2
4
involves a series of spontaneous reactions between the cathode ac-
tive materials and the electrolyte solvents, and that the resulting film
greatly reduces electronic conductivity of the LiMn O particles and
2
4
͑OCV͒ of the cell, and an ac oscillation of 5 mV over the frequency
consequently causes capacity loss. However, little attention was pre-
viously paid to the SEI film of the cathodes since its presence is not
as essential as on the carbonaceous anode.
In order to improve the energy density of Li-ion batteries, it is
essential to understand and reduce the irreversible capacity of the
cathodes developed during the initial forming cycles. Therefore, in
this work we study the formation of SEI film on the surface of
range from 100 kHz to 0.01 Hz in every 50 mV of voltage intervals
between 3.5 V or the OCV of the newly assembled cell and 4.3 V.
The voltage at which EIS measurement was performed was
achieved by galvanostatically cycling the cell at a current density of
2
0
.1 mA/cm . The obtained EIS was analyzed using ZView
software.
LiMn O electrodes. For this purpose, galvanostatic cycling tests
are used to observe voltage dependence of the irreversible capacity.
2
4
Results and Discussion
Galvanostatic charge and discharge.—Li/LiMn O cells using 1
2
4
m LiPF 3:7 EC/EMC and 1 m LiPF 1:1:3 PC/EC/EMC electrolyte,
6
6
respectively, were assembled. We found that in most cases, the
OCVs of the cells were 2.95-2.96 V with negligible dependence on
the electrolyte solvents. Obviously, this value is rather lower as
*
Electrochemical Society Active Member.
*
* Electrochemical Society Student Member.
z
E-mail: szhang@arl.army.mil
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A1522
Journal of The Electrochemical Society, 149 ͑12͒ A1521-A1526 ͑2002͒
Figure 1. Voltage-capacity curve and
differential capacity-voltage plot of
Li/LiMn O4 cell using 1 m LiPF6 3:7
2
EC/EMC electrolyte, obtained from the
first cycle by cycling the cell at 0.1
mA/cm2 from OCV to 4.3 V, and then
back to 3.5 V.
compared to the voltage ͑i.e., ϳ3.5 V͒ of even a discharged
Li/LiMn O4 cell. We ascribe this phenomenon to a spontaneous
LiMn O and electrolyte solvents such as dimethyl carbonate
2
4
2
͑DMC͒. Also other insoluble compounds, such as CH OCH OLi,
3 2
lithiation of the LiMn O4 active materials upon contact with the
2
CH OCO Li, and Li CO , have been detected from the surface of
3
2
2
3
electrolytes, because the observed OCVs are very close to the pla-
8,9
LiMn O electrode.
Based on these facts, we speculate that
2
4
teau voltage ͑2.8-2.9 V͒ of the extended discharge of a Li/LiMn O
2
4
LiMn O and electrolyte solvents might undergo the following
2 4
1
5,16
cell.
In cycling testing, we attempted to minimize this spontane-
spontaneous reaction
ous lithiation of LiMn O by charging the cells as soon as they were
2
4
assembled. Figures 1 and 2, respectively, show voltage-capacity
2
LiMn O ϩ CH OCO CH → 2Mn O ϩ CH OCH OLi
2
4
3
2
3
2
3
3
2
curves of the first cycle of two Li/LiMn O4 cells using different
2
electrolytes. We see that these two cells still have a small amount of
irreversible capacity near their OCV. To better observe the presence
of this irreversible capacity, we also plotted the differential capaci-
ties of these two cells in Fig. 1 and 2, respectively, as a function of
the voltage. Initial irreversible capacities of the cells are clearly
indicated by a small peak of the differential capacities at ϳ3.1 V. It
is evaluated from Fig. 1 that with EC/EMC electrolyte, LiMn O
ϩ LiHCO3
͓1͔
Such a reaction leads to the formation of an electronically insulating
surface layer ͑or called SEI film͒ and its presence can deactivate
electrodes. Due to the presence of Mn O and CH OCO Li, we
2
3
3
2
consider that the irreversible capacity observed below 3.5 V likely
arises from an electrochemical oxidization of Mn O such as
2
4
2
3
produces an irreversible capacity of 8.3 mAh/g in the voltage range
below 3.5 V and a reversible capacity of 109 mAh/g between 3.5
and 4.3 V with an overall CE of 88%. For PC/EC/EMC electrolyte,
similar data are obtained from Fig. 2, i.e., the initial irreversible
capacity below 3.5 V is 6.1 mAh/g and the reversible capacity be-
tween 3.5 and 4.3 V is 115 mAh/g with an overall CE of 89%.
As discussed previously, the formation of irreversible capacity in
the first charge and discharge cycle of LiMn O electrode can be
Mn O ϩ 2CH OCO Li → LiMn O ϩ CH OCO CH ϩ CO
2
3
3
2
2
4
3
2
3
2
ϩ
ϩ Li ϩ e
͓2͔
While the irreversible capacity developed above 3.5 V probably
originates from the electrochemical oxidation of electrolyte solvents,
which is speculated to form an SEI film on LiMn O surface be-
2
4
divided into these two voltage regions of ϳ3.1 V, near OCV of the
2
4
fresh cell, and above 3.5 V, mainly between 3.8 and 4.3 V. It has
cause, a carbonates such as PC can be electrochemically oxidized at
9
ϩ
been reported that at elevated temperatures Mn O could be imme-
potentials as low as 2.1 V vs. Li /Li, and the rate of oxidation
2
3
1
7,18
diately formed on the surface of LiMn O electrode upon contact of
increases substantially above 3.5 V.
As a result of carbonate
2
4
Figure 2. Voltage-capacity curve and
differential capacity-voltage plot of
Li/LiMn O4 cell using 1 m LiPF6 1:1:3
2
PC/EC/EMC electrolyte, obtained from
the first cycle by cycling the cell at 0.1
mA/cm2 from OCV to 4.3 V, and then
back to 3.5 V.
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Journal of The Electrochemical Society, 149 ͑12͒ A1521-A1526 ͑2002͒
A1523
Figure 4. Equivalent circuit used to analyze the EIS of a Li/LiMn O cell.
2
4
4
͒, R is the bulk resistance of electrolyte, separator, and electrode;
e
R and C are the resistance and capacitance of the SEI film, respec-
f
f
tively; Rct and Cdl are the charge-transfer resistance and double-
layer capacitance, respectively; and W is the Warburg impedance
ϩ
arising from the semi-infinite diffusion of Li ions in LiMn O4
2
electrode.
Using the above-described model and Zview software, individual
values of R , R , and R of the Li/LiMn O cell were obtained
e
f
ct
2
4
through fitting. It is noticed that in both EC/EMC and PC/EC/EMC
electrolytes R constantly remained at 3.6-3.7 and 3.7-3.8 ⍀, respec-
e
tively, during the entire charge and discharge processes, being in a
negligible region of the deviation with the cycling status ͑voltage͒.
However, the R and R vary greatly with voltage. The R and R of
f
ct
f
ct
Figure 3. Selected Nyquist plots of Li/LiMn O4 cell using 1 m LiPF6 3:7
EC/EMC electrolyte at various charged states potentiostatically measured
during the first charge of a newly assembled cell.
2
the cell using EC/EMC electrolyte are plotted in Fig. 5 as a function
of voltage. The fresh cell has an initial R of 62 ⍀, which means that
f
an SEI film has been performed upon the contact of LiMn O elec-
2
4
9
trode and electrolyte. According to the early report, this film con-
sists of various products of the spontaneous reactions between
LiMn O and electrolyte solvents, such as Mn O , lithium alkox-
2
4
2
3
oxidation, both solid (Li CO ) and volatile gaseous (CO ) products
2
3
2
ide, and lithium carbonate, and its presence causes a noticeable de-
can be formed with solid products being the major component of
1
9
cline in the surface electronic conductivity of LiMn O4 particles.
SEI films.
2
With charging of the cell ͑i.e., increase of the voltage͒, R is in-
Contrary to what happens at the cathode, solvent reduction at the
graphite anode in a Li-ion battery simultaneously occurs, which is
responsible for the formation of critically important SEI films, also
with some gaseous products released. It is because of the formation
of gaseous products from both oxidization and reduction of carbon-
ates that a forming process needs to be completed before a freshly
assembled Li-ion battery can be sealed, especially for the large-size
battery. In the consideration of cell chemistry efficiency, the simul-
taneously irreversible oxidation and reduction at cathode and anode,
respectively, is complementary to each other. As Eq. 2 shows, the
f
creased very slowly in the voltage range below 3.7 V. It appears that
the initial irreversible capacity formed below 3.5 V hardly affects
R . This observation could be ascribed to these two reverse pro-
f
cesses during the initial charge: ͑i͒ oxidative dissolution of the pre-
formed SEI film, as described in Reaction 2, and ͑ii͒ increasing
growth of the SEI film. Interaction of these two reverse processes
results in a negligible voltage dependence of the Rf . A dramatic
increase in R occurs at 3.7 V, at which the reversible delithiation of
f
LiMn O is confirmed to have initialized. This observation reveals
2
4
ϩ
irreversible oxidization below 3.5 V on the cathode produces Li
that the electrochemical delithiation of LiMn O is accompanied by
2 4
ϩ
ions. The resulting Li ions can be used to offset the consumption of
the formation of a highly resistive SEI film. However, R is sharply
f
solvent reductive decomposition on the graphite anode, which oth-
fallen at 3.95 V, followed by a slow decrease above 4.15 V until the
ϩ
erwise has to consume the limited Li ion resource in the cathode.
end of charge process. It is seen that the dropped R may be revers-
f
Therefore, we believe that some irreversible capacities, which are
produced in the initial forming cycles of Li-ion batteries, indeed
result from the oxidization and reduction of the electrolyte solvents
on the cathode and graphite anode, respectively. This appears to
explain reasonably why the overall CE of a Li-ion battery is very
close to the lower one of either Li/graphite or Li/cathode cell.
ibly recovered when the cell is discharged. There are two possibili-
ties that may lead to the sharp drop of R at the voltage range
f
3.95-4.15 V: ͑i͒ resistive SEI film reversible breakdown ͑or dissolu-
tion͒ and ͑ii͒ increase in the surface electronic conductance of the
LiMn O electrode caused by the phase transitions of delithiation. If
2
4
the latter were predominant, the observed phenomenon should be
universal and independent of the electrolytes. Different voltage de-
EIS study on Li/LiMn O cell.—To understand the effect of irre-
2
4
pendence of the R , as shown in PC/EC/EMC electrolyte ͑to be
versible capacity on the formation of SEI film, we monitored the
impedance change of the cell during the first delithiation and lithia-
tion cycle. Nyquist plots of the cell are illustrated in Fig. 3, which
presents several selective plots, although the impedance was mea-
sured every 50 mV of voltage intervals for the entire cycle. It is seen
that these plots are composed of one or two overlapped semicircles
at high and medium frequencies and a straight sloping line at low
frequencies, whose profile greatly varies with the voltage. These
features are very similar to those observed from graphite anodes.
Figure 4 is an equivalent circuit, which has been successfully used
f
discussed latter͒, appears to rule out the second possibility. A slow
increase in R is still seen when the cell is discharged to 3.95 V and
f
lower. This means that the SEI film is continuously grown with
cycling. If comparing R at voltages below 3.8 V from the viewpoint
f
of time scale instead of voltage, one may find that the increase of Rf
with cycling time is continuous in the entire charge and discharge
processes.
Voltage dependence of Rct is interesting. We find that Rct change
with voltage is very similar to the plot of differential capacity vs.
voltage ͑see Fig. 1͒. Between 3.7 and 4.3 V, the Rct value for both
charge and discharge processes shows two minima at ϳ3.85 and
ϳ4.15 V, respectively, which corresponds to two differential capac-
ϩ
to analyze the electrochemical process of Li ions with graphite
7
,8,20,21
electrodes.
Therefore, we hereby adopt this equivalent circuit
to analyze the measured Nyquist plots. In the equivalent circuit ͑Fig.
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A1524
Journal of The Electrochemical Society, 149 ͑12͒ A1521-A1526 ͑2002͒
Figure 5. Voltage dependence of R and Rct , which were fitted from EIS of
f
a Li/LiMn O cell using 1 m LiPF 3:7 EC/EMC electrolyte during the first
Figure 6. Voltage dependence of R and R fitted from EIS of a Li/LiMn O
f ct 2 4
2
4
6
charge and discharge cycle: ͑a͒ Rf and ͑b͒ Rct , ͑᭺͒ charge and ͑᭹͒ dis-
charge.
cell using 1 m LiPF6 1:1:3 PC/EC/EMC electrolyte during the first charge
and discharge cycle: ͑a͒ R and ͑b͒ R , ͑᭺͒ charge and ͑᭹͒ discharge.
f
ct
ity peaks in Fig. 1. That is, higher differential capacity corresponds
Voltage dependence of the R and R for the cell using PC/EC/
f ct
to a lower R . In the voltage regions of the differential capacity
being vanished, such as between 3.1 and 3.5 V, the Rct becomes too
EMC electrolyte is plotted in Fig. 6, in which Rct behaves in a
similar manner as observed in Fig. 5. We notice that PC/EC/EMC
electrolyte results in a high and increasing Rf , especially in the
ct
large to be measured. As a result of the large R , two semicircles of
ct
the EIS are merged into a semicircle ͑Fig. 3͒. In addition, a rela-
tively low Rct of 612 ⍀ can be found at 3.0 V, which corresponds to
the small amount of initial irreversible capacity as shown in Fig. 1.
charge process. It is indicated that R starts with an initial value of
203 ⍀ and increases faster with voltage. Even between 3.9 and 4.3
f
V, R varies only in a small range, showing a better stability of the
f
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Journal of The Electrochemical Society, 149 ͑12͒ A1521-A1526 ͑2002͒
A1525
Figure 7. Charge and discharge capacities of the Li/LiMn O cell using 1 m
2
4
LiPF 3:7 EC/EMC electrolyte as a function of cycle number, measured by
6
2
cycling the cell at 0.5 mA/cm between 3.5 and 4.3 V. Inset: cyclic voltam-
mogram of the cell recorded at 0.01 mV/s ͑a͒ from second cycle and ͑b͒ after
2
00 galvanostatic cycles, respectively.
SEI film. Comparing Fig. 5 and 6, one may find that the SEI formed
with PC/EC/EMC solvent is more resistive and more stable. These
results have proven that the electrolyte solvents play a critical role in
affecting the properties of the SEI film on the surface of LiMn O
2
4
electrode.
Impedance change with cycling.—To observe the effect of
charge and discharge cycling on the stability of the SEI film, we
measured impedance of a cycled cell. Before impedance measure-
ment, the Li/LiMn O cell ͑same one as tested in Fig. 5͒ was cycled
2
4
2
200 times at 0.5 mA/cm between 3.5 and 4.3 V. Charge and dis-
charge capacities of the cell vs. cycle number are plotted in Fig. 7,
which indicates that the cell lost 9.7% of capacity after 200 cycles.
The capacity fading also is reflected by the cyclic voltammograms
͑
inset of Fig. 7͒, which were respectively recorded at the second
cycle and after 200 galvanostatic cycles. Figure 8 plots the voltage
dependence of R and R for the cell that had been cycled 200 times.
f
ct
Comparing Fig. 5a and 8a, we find that R has decreased to 40-60 ⍀,
f
while R increased to 5.7-5.8 from 3.6-3.7 ⍀ ͑not shown͒. The in-
e
creased R is likely related to the change of electrode conductance
e
and electrolyte composition such as cathode dissolution and solvent
decomposition. Decreased R suggests that the SEI film in the cycled
f
cell is more conductive. Large drop in R still exists in the voltage
f
range 3.9-4.2 V, but its changing range becomes much smaller. Un-
like the observation in the first cycle ͑see Fig. 5a͒, the R value of
f
the cycled cell decreases with the increase of voltage in the entire
voltage range for both charge and discharge processes. Compared to
R , R is less affected by cycling, however, its voltage dependence
f
ct
seems to be more complicated with the appearance of three Rct
minima ͑Fig. 8b͒.
Figure 8. Voltage dependence of the R and R after the Li/LiMn O cell
f
ct
2
4
Conclusions
using 1 m LiPF6 3:7 EC/EMC electrolyte was cycled 200 times at 0.5
mA/cm2 between 3.5 and 4.3 V: ͑a͒ R and ͑b͒ R , ͑᭺͒ charge and ͑᭹͒
In conclusion, the first cycle of spinel LiMn O generally pro-
duces 10-15% of irreversible capacities in both 1 m LiPF 3:7 EC/
f
ct
2
4
discharge.
6
EMC and 1 m LiPF 1:1:3 PC/EC/EMC electrolytes, which is asso-
6
ciated with the formation of a solid electrolyte interface ͑SEI͒ film
on the surface of LiMn O4 electrode. Irreversible capacity of
2
LiMn O is produced mainly in these two voltage ranges of near 3.1
and 3.8-4.3 V. EIS analysis shows that the SEI film can be sponta-
2
4
lyte solvents greatly affect conductance and stability of the SEI film.
It is found that the SEI film formed with PC/EC/EMC electrolyte is
more resistive and more stable. It is seen that the SEI film becomes
more conductive after the cell is cycled.
neously formed upon the contact of LiMn O and electrolyte and is
2
4
continuously grown during the charge and discharge cycle. In EC/
EMC electrolyte, the SEI film is mainly formed at 3.7-3.85 V during
delithiation ͑charge͒ of LiMn O , but the resultant film is subjected
2
4
The United States Army Research Laboratory assisted in meeting the
publication costs of this article.
to reversible breakdown ͑or dissolution͒ at higher voltages. Electro-
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A1526
Journal of The Electrochemical Society, 149 ͑12͒ A1521-A1526 ͑2002͒
1
1
2. D. H. Jang, Y. J. Shin, and S. M. Oh, J. Electrochem. Soc., 143, 2204 ͑1996͒.
References
3. M. M. Thackeray, Y. Shao-Horn, A. J. Kahaian, K. D. Kepler, E. Skinner, J. T.
Vaughey, and S. A. Hackney, Electrochem. Solid-State Lett., 1, 7 ͑1998͒.
4. H. Huang, C. A. Vincent, and P. G. Bruce, J. Electrochem. Soc., 146, 481 ͑1999͒.
5. S. H. Kang and J. B. Goodenough, J. Electrochem. Soc., 147, 3621 ͑2000͒.
6. J. Read, A. Driedger, D. Foster, J. Wolfenstine, and W. Behl, Electrochem. Solid-
State Lett., 4, A162 ͑2001͒.
1
2
3
. B. Scrosati, J. Electrochem. Soc., 139, 10 ͑1992͒.
. K. G. McColl, J. Power Sources, 48, 29 ͑1994͒.
1
1
1
. X. Zhang, P. N. Ross, Jr., R. Kostecki, F. Kong, S. Sloop, J. B. Kerr, K. Striebel, E.
J. Cairns, and F. McLarnon, J. Electrochem. Soc., 148, A463 ͑2001͒.
. E. Peled, D. Golodnitsky, and G. Ardel, J. Electrochem. Soc., 144, L208 ͑1997͒.
. D. Aurbach and A. Zaban, J. Electroanal. Chem., 348, 155 ͑1993͒.
. Y. Ein-Eli, Electrochem. Solid-State Lett., 2, 212 ͑1999͒.
. S. S. Zhang, M. S. Ding, K. Xu, J. Allen, and T. R. Jow, Electrochem. Solid-State
Lett., 4, A206 ͑2001͒.
. D. Aurbach, M. D. Levi, E. Levi, H. Teller, B. Markovsky, and G. Salitra, J.
Electrochem. Soc., 145, 3024 ͑1998͒.
4
5
6
7
1
7. P. Novak, P. A. Christensen, T. Iwasita, and W. Vielstich, J. Electroanal. Chem.,
2
63, 37 ͑1989͒.
1
1
2
8. M. Christie and C. A. Vincent, J. Appl. Electrochem., 26, 255 ͑1996͒.
9. E. Cattaneo and J. Rich, J. Power Sources, 43-44, 341 ͑1993͒.
0. N. Takami, A. Satoh, M. Hara, and T. Ohsaki, J. Electrochem. Soc., 142, 371
͑1995͒.
8
9
. Y. Matsuo, R. Kostecki, and F. McLarnon, J. Electrochem. Soc., 148, A687 ͑2001͒.
1
0. A. Yamada and M. Tanaka, Mater. Res. Bull., 30, 715 ͑1995͒.
21. T. Piao, S. M. Park, C. H. Doh, and S. I. Moon, J. Electrochem. Soc., 146, 2794
͑1999͒.
11. Y. Xia, Y. Zhou, and M. Yoshio, J. Electrochem. Soc., 144, 2593 ͑1997͒.
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