A nanocrystalline NiO thin-film electrode prepared by pulsed laser ablation for Li-ion batteries

Article abstract of DOI:10.1149/1.1481715

Nanocrystalline NiO thin-film electrodes were prepared by reactive pulsed laser ablation of a metallic Ni target in an oxygen ambient. X-ray diffraction (XRD) and scanning electron microscopy measurements demonstrated that the films deposited on stainless steel substrate exhibited nanocrystalline structure with average particle size of ～30 nm. Electrochemical properties of NiO films were examined by cyclic voltammetry and charge-discharge measurements. Excellent electrochemical performance, a reversible capacity as high as 700 mAh/g in the range of 0.01-3.0 V at high current density (80 μA/cm²) with a high capacity retention up to 100 cycles, could be achieved with optimized NiO films. The improved specific capacity, discharge rate, and cycling performance might be related to the nanosized character of the thin-film electrode of NiO. Combining the XRD and X-ray photoelectron spectroscopy results, we proposed an electrochemical replacement reaction mechanism for the nanocrystalline NiO film with lithium during the discharge/charge processes. This NiO thin film could be used as a promising anode material for all-solid-state thin film rechargeable Li-ion batteries.

Full text of DOI:10.1149/1.1481715

Journal of The Electrochemical Society, 149 ͑7͒ A873-A878 ͑2002͒
A873
0013-4651/2002/149͑7͒/A873/6/$7.00 © The Electrochemical Society, Inc.
A Nanocrystalline NiO Thin-Film Electrode Prepared by Pulsed
Laser Ablation for Li-Ion Batteries
,z
*
Ying Wang and Qi-Zong Qin
Department of Chemistry, Laser Chemistry Institute, Fudan University, Shanghai 200433, China
Nanocrystalline NiO thin-film electrodes were prepared by reactive pulsed laser ablation of a metallic Ni target in an oxygen
ambient. X-ray diffraction ͑XRD͒ and scanning electron microscopy measurements demonstrated that the films deposited on
stainless steel substrate exhibited nanocrystalline structure with average particle size of ϳ30 nm. Electrochemical properties of
NiO films were examined by cyclic voltammetry and charge-discharge measurements. Excellent electrochemical performance, a
reversible capacity as high as 700 mAh/g in the range of 0.01-3.0 V at high current density (80 ␮A/cm2) with a high capacity
retention up to 100 cycles, could be achieved with optimized NiO films. The improved specific capacity, discharge rate, and
cycling performance might be related to the nanosized character of the thin-film electrode of NiO. Combining the XRD and X-ray
photoelectron spectroscopy results, we proposed an electrochemical replacement reaction mechanism for the nanocrystalline NiO
film with lithium during the discharge/charge processes. This NiO thin film could be used as a promising anode material for
all-solid-state thin film rechargeable Li-ion batteries.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1481715͔ All rights reserved.
Manuscript submitted September 26, 2001; revised manuscript received January 16, 2002. Available electronically May 16, 2002.
Nanostructured materials often exhibit properties that are drasti-
cally different from those of conventional materials. In search of
novel electrode materials for Li-ion batteries, nanostructured anode
and cathode materials have been widely studied in recent years. It
has been demonstrated that the charge/discharge rate, specific capac-
ity, and cycling performance of nanostructured electrodes can be
obviously improved compared with conventional electrodes made of
the same materials.1,2 For example, the nanosized silicon composite
anode materials fabricated by high-energy mechanical milling ex-
hibited high electrochemical capacity and good lithium-ion transport
dynamics.3,4 In addition, nanostructured anode and cathode materi-
als were successfully fabricated using the template-synthesized
method, and they also exhibited good electrochemical
performance.5-7 These dramatically improved effects of nanostruc-
tured electrodes could be attributed to the high specific surface area
and the short diffusion distance for Li-ion transport in the solid state.
Recently, Tarascon’s group reported that nanosized transition
metal oxides ͑MO, where M is Co, Ni, Fe, and Cu͒ were found to be
promising anode materials with excellent electrochemical perfor-
mance for lithium ion batteries.8-10 Since the rock-salt structured
crystal of 3d transition metal oxides have no empty site for lithium
ions to intercalate into, nor can it form alloy with lithium, they
proposed a completely new mechanism different from the classical
ones. This mechanism involves the reversible formation and decom-
position of Li2O, accompanying the reduction and oxidation of
metal nanoparticles in the range 1-5 nm. Meanwhile Dahn’s group11
investigated the electrochemical displacement reaction of metal ox-
ides, including ␣-LiFeO2 , ␤-Li5FeO4 , and CoO with lithium using
͑PLA͒ with a nickel oxide target, thermal evaporation, metallorganic
chemical vapor deposition ͑MOCVD͒, and electrochemical
deposition.17-21 Of these, PLA has proven a powerful method to
prepare the thin films of pure and composite metal oxides. This
method has been successfully used for preparing thin-film electrodes
of LiCoO2 , LiMn2O4 , and V2O5 .22-24 We have also demonstrated
that the nanosized transition metal oxides such as Ta2O5 , Nb2O5 ,
TiO2 , and tin composite oxide ͑TCO͒ with good electrochemical
and electrochromic properties can be prepared by the PLA
method.25-28 Recent laser ablation experiments have shown that the
laser wavelength, intensity, ambient gas pressure, and target-
substrate distance are important experimental conditions required to
control the particle size distributions of the nanocrystalline semicon-
ductor films.29,30 Here we employed the reactive PLA method to
fabricate nanocrystalline NiO thin films with an average particle size
of about 30 nm. As we know, most previous works dealing with
nanosized electrode materials were investigated with nanosized
metal oxides of a few hundred nanometers, and only relatively few
studies focused on the novel electrochemical behavior of real nano-
structured oxides (Ͻ100 nm). The motivation of this paper is to
verify the excellent electrochemical performance of the thin-film
electrode of nanosized transition-metal oxides by examining the
nickel oxide ͑NiO͒ thin film composed of 30 nm nanoparticles. To
our knowledge, this is the first report using the PLA method to
prepare nanocrystalline NiO films, which can deliver a high specific
capacity of ϳ700 mAh/g with good capacity retention. A new elec-
trochemical mechanism, involving the electrochemical displacement
reaction of lithium with NiO and the reduction/oxidation of nickel
metal, is proposed to explain the electrochemical behavior of NiO
film electrode. Our results may be helpful to develop new electrode
materials for all-solid-state thin-film rechargeable Li-ion batteries.
¨
in situ X-ray diffraction ͑XRD͒ and in situ Mossbauer measure-
ments. From the data of in situ XRD measurements, they proposed
an attractive mechanism for the reaction between lithium and CoO,
in which cobalt oxide is immediately decomposed to form lithia and
the reduced Co metal during discharge, and cobalt metal displaces
lithium in lithium oxide to form CoO and metallic lithium during
charge. This process resembles an ion-exchange process, where the
Co2ϩ ions formed by the oxidation of the cobalt metal replace the
lithium ions in the Li2O lattice during cell charging.
Nickel oxide ͑NiO͒ with rock salt structure is an attractive ma-
terial for antiferromagnetic devices, electrochromic electrodes,
p-type transparent conducting films, chemical sensors, and active
electrode material of nickel battery systems.12-16 NiO thin films have
been prepared by many techniques, such as radio frequency ͑rf͒
sputtering with a nickel or nickel oxide target, pulsed laser ablation
Experimental
Nanocrystalline NiO films were prepared by reactive PLA in an
oxygen ambient using a pure nickel target. A detailed description of
PLA apparatus has been reported elsewhere.27 In our experiments, a
355 nm pulsed laser beam was provided by the third harmonic fre-
quency of a Q-switched Nd:YAG laser ͑yttrium aluminum garnet,
Spectra Physics GCR-150͒ with a laser fluence of ϳ2 J/cm2 and a
repetition rate of 10 Hz. The incident angle between the laser beam
and the target surface normal was 45°. The typical ambient O2 gas
pressure was controlled at 40 Pa by a needle valve. During deposi-
tion, the target was rotated and the substrate was mounted on a
holder. The distance between pure metallic nickel target and sub-
strate was 30 mm. The NiO film was deposited on a stainless steel
substrate heated to 600°C. After deposition NiO film was annealed
* Electrochemical Society Active Member.
z E-mail: qzqin@fudan.ac.cn
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Journal of The Electrochemical Society, 149 ͑7͒ A873-A878 ͑2002͒
fraction peaks corresponding to pure Ni and other Ni-containing
compounds. The average crystalline size of NiO particles on the film
surface was estimated by measuring the width of the XRD peaks,
which was used in the Scherrer equation. The average size of NiO
particles was found to be approximately 30 nm. To examine changes
of the nanocrystalline NiO during the electrochemical process, the
NiO film deposited on Au-coated Si͑100͒ substrate was discharged
in a NiO film/Li cell. Figure 1c presents a significant change in the
observed XRD pattern of the deposited NiO film after being dis-
charged to 0.01 V. It can be seen that characteristic diffraction peaks
of the NiO film vanished, only one peak appeared at about 52°
which can be assigned to ͑200͒ reflections of metallic Ni, and an-
other peak at 44.5° for Ni is overlapped with the broad diffraction
peak from the Au-coated Si͑100͒ substrate. The size of Ni particles
was found to be about 8-10 nm. This result implies that the nano-
crystalline NiO reacted with Li, leading to the formation of metallic
Ni after the first discharge.
The morphologies of the NiO films deposited on the Si͑100͒ and
stainless steel substrates as well as the film electrode after electro-
chemical cycling were examined by SEM. As shown in Fig. 2A, the
nanocrystalline NiO particles deposited on a Si͑100͒ substrate ap-
pear to be dense, fairly round, and uniformly distributed on the
surface. Their average size is found to be approximately 30 nm,
which is consistent with that estimated from the XRD data. A similar
result is shown in Fig. 2B, in which the NiO was deposited on a
stainless steel substrate. It implies that the NiO film with uniformly
distributed nanosized particles prepared by the PLA method is irrel-
evant to the substrate we used. In order to examine any changes in
the nanocrystalline NiO particles during electrochemical cycling, the
morphology of the film was also examined after cycling. As shown
in Fig. 2C, when the NiO film electrode was discharged/charged at
80 ␮A/cm2 after 60 cycles, an obvious agglomeration took place.
Compared to Fig. 2B, it is obvious that not only the larger-sized
particles appear to be in the range of 150-200 nm, but also the shape
of these particles changes obviously from round to needle-like
shape. It has been found that the agglomeration of the nanosized
particles caused by the electrochemical reactions with lithium is a
common phenomenon, but the nature of this process is unclear, and
we discuss it in the next section. The SEM image in Fig. 2D shows
a cross-sectional view of the NiO film deposited on a Si͑100͒ sub-
strate for 2 h. The thickness of the dense nanocrystalline NiO film
was found to be approximately 250 nm, which is close to the film
thickness measured by the profilometric method.
The voltage-specific capacity profiles of the NiO film/Li cells
were performed by constant current charge/discharge measurements.
Figure 3 shows the first three cycling curves of NiO film/Li cell
measured between 0.01 and 3.0 V vs. Li/Liϩ at 10 ␮A/cm2. In the
initial discharge, the potential drops rapidly to reach a well-
pronounced plateau at about 0.4 V, followed by a gradual decrease
to 0.01 V. The irreversible discharge capacity can be delivered to be
1800 mAh/g, which correspond to about 5 Li ions reacted with the
NiO film electrode. The first charge proceeds with a higher voltage
and less capacity. In the second discharge, the plateau appears at
about 0.9 V, and then the amplitude of the plateau reduces not ob-
viously in the subsequent discharge curves. The subsequent charged
plateau presents at about 2.0 V, which exhibits considerable polar-
ization compared to the discharge process. Our result revealed that
there was little change in the voltage-capacity profiles for the sub-
sequent cycles, and a high reversible capacity of 700 mAh/g was
achieved at a current density of 80 ␮A/cm2. These discharge and
charge curves for the NiO film/Li cell are similar to those for the 3d
transition oxides MO powder/Li cells reported previously.8-11 Nev-
ertheless, compared with other anode materials the voltage ranges of
the discharge and charge curves are wider for these metal oxide
electrodes. In addition, the irreversible capacity for the NiO film-
based lithium cell is much higher than that for the NiO powder
lithium cell upon the first discharge.
Figure 1. XRD patterns: ͑a͒ metallic Ni target, ͑b͒ NiO thin-film deposited
on a stainless steel substrate, and ͑c͒ NiO thin film used as an electrode in the
*
NiO film/Li cell discharged to 0.01 V. ͑ ͒ Au-coated Si͑100͒ substrate.
in an O2 ambient at 300°C for 2 h. In some cases, the film was
deposited on Si͑100͒ or Au-coated Si͑100͒ substrate for scanning
electron microscopy ͑SEM͒ and XRD measurements. The film depo-
sition rate estimated by the thickness of as-deposited film and abla-
tion duration was found to be about 2 nm/min.
XRD patterns of the NiO thin films were determined by using a
Rigata/max-C diffractometer with Cu K␣ radiation. The film mor-
phology was characterized by a Cambridge S-360 SEM. X-ray pho-
toelectron spectra ͑XPS͒ were obtained on a Perkin Elmer PHI
5000C ESCA system with monochromatic Al K␣ ͑1486.6 eV͒ irra-
diation. The XRD and XPS samples of the lithiated NiO film were
prepared by disassembling the cell, rinsing the film electrode with
dimethyl carbonate ͑DMC͒ to remove electrolyte, and drying the
film at room temperature in an Ar-filled glove box. Then the film
electrode was put in an argon-filled bag and opened right before
XRD and other measurements. The film thickness was measured by
a profilometer ͑Tencor Alpha-Step 200͒.
For the electrochemical measurements, two-electrode and three-
electrode cells were employed with the deposited NiO thin film as
working electrode, and two lithium sheets as both counter and ref-
erence electrodes. The electrolyte consisted of 1 M LiPF6 in a non-
aqueous solution of ethylene carbonate ͑EC͒ and DMC with a vol-
ume ratio of 1:1 ͑Merck͒. The cells were assembled in an Ar-filled
glove box. Charge-discharge measurements were performed at room
temperature with a Land BT 1-40 battery test system. The cells were
cycled between 0.01 and 3.0 V vs. Li/Liϩ at different current densi-
ties from 10 to 100 ␮A/cm2. Cyclic voltammetric measurements
were carried out with a CHI660a electrochemical work station ͑CHI
Instruments͒. The measurements were performed by a computer-
controlled potentiostat and galvanostat at room temperature.
Results
In order to examine the crystallinity of the NiO film prepared by
PLA before and after the first discharge, XRD measurements of the
metallic Ni target and NiO film were performed under different ex-
perimental conditions. As shown in Fig. 1a, the diffraction peaks of
Ni target appear at 2␪ of 44.48 and 51.87°, which are attributed to
͑111͒ and ͑200͒ reflections of metallic Ni, respectively. The diffrac-
tion pattern of NiO film deposited on the stainless steel substrate
shown in Fig. 1b can be characterized by a cubic structure of NiO
with two peaks at 37.41 and 43.47°, which corresponds to ͑111͒ and
͑200͒ reflections, respectively.17,31 Since the background peaks from
the stainless steel substrate were removed, there was no other dif-
To further study the electrochemical properties of the NiO film,
the film electrodes were discharged and charged at different current
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Journal of The Electrochemical Society, 149 ͑7͒ A873-A878 ͑2002͒
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Figure 3. Voltage-capacity profiles for the NiO film/Li cell cycled between
0.01 and 3.0 V at a current density of 10 ␮A/cm2.
densities in the range 10-100 ␮A/cm2. The dependence of revers-
ible capacity on the cycle number and the current density is pre-
sented in Fig. 4. It can be seen that the specific capacity decreases
rapidly in the first 20 cycles, then slowly decays, and finally remains
nearly a constant capacity after 100 cycles. At a current density of
20 ␮A/cm2, the NiO film electrode exhibits a large reversible ca-
pacity of 720 mAh/g, which corresponds to about 2 equiv mol of Li
ion per unit mole of NiO.
From the data in Fig. 4 the specific capacity of the NiO film
electrode as a function density after 100 cycles for NiO film/Li cell
is shown in Fig. 5. It can be seen that the reversible capacity de-
creases with increasing the current density. However, this film elec-
trode exhibits a capacity as high as about 600 mAh/g, even when
cycling at 100 ␮A/cm2. Apparently, the reversible capacity of the
NiO film electrode is quite high compared with the theoretical ca-
pacity of graphite ͑372 mAh/g͒.32 Because the density of nickel
oxide (6.67 g/cm3) is larger than that of graphite (2.26 g/cm3), the
Figure 2. SEM images for as-deposited NiO thin films: ͑A͒ deposited on a
Si͑100͒ substrate, ͑B͒ deposited on a stainless steel substrate, ͑C͒ NiO film
used as an electrode after 60 cycles at 80 ␮A/cm2, and ͑D͒ a cross-sectional
view of a NiO film deposited on a Si͑100͒ substrate for 2 h.
Figure 4. Discharge capacity of the NiO thin-film electrode as a function of
the cycle number and current density cycled between 0.01 and 3.0 V.
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Journal of The Electrochemical Society, 149 ͑7͒ A873-A878 ͑2002͒
Figure 7. XPS for Ni 2p and Li 1s: ͑a͒ as-deposited NiO film electrode, ͑b͒
lithiated NiO film electrode obtained from discharge to 0.01 V, and ͑c͒ Li 1s
of lithiated NiO film electrode.
Figure 5. Specific capacity of the NiO film electrode as a function of current
density after 100 cycles for NiO film/Li cell.
volumetric capacity of NiO is about six times that of graphite. The
improved electrochemical performance of the nanocrystalline NiO
film electrode could be attributed to the film consisting of particles
having sizes ranging about 30 nm and the highly ordered structure
of NiO in the film electrode.
In order to further understand the electrochemical mechanism of
the reaction of the nanostructured NiO film with lithium, a prelimi-
nary XPS measurement was performed to examine the valence
change of nickel for the NiO film before and after being lithiated. As
shown in Fig. 7a, the XPS spectrum of the as-deposited film exhibits
a peak at binding energy of 854.5 eV, corresponding to the Ni 2p
doublet, and this binding energy is essentially the same as that found
for the cubic NiO in the previous report ͑853.8-854.7 eV͒.33 Figure
7b shows the XPS spectrum of the lithiated NiO film, which was
assembled as a cell and removed from the cell after being discharged
to 0.01 V. After this film was lithiated, it was found that the binding
energy of Ni 2p3/2 shifts to about 852.4 eV, indicating a conversion
of NiO to metallic nickel. The Li 1s XPS spectrum for the lithiated
film is presented in the insert of Fig. 7. The binding energy peak of
55.8 eV can be assigned to Li 1s, which should be attributed to Li2O
͑55.6 eV͒, confirming the formation of Li2O. Our XPS result further
supported that the Ni metal and Li2O were formed in the initial
discharge process.
Cyclic voltammetric ͑CV͒ measurements were performed at a
sweep rate of 5 mV/s to examine the electrochemical properties of
the NiO film electrode. Figure 6 presents the CV curves cycled
between 0 and 3.5 V for the film electrode after different cycles. It
can be seen that in the second cycle a broad anodic peak is observed
at about 2.3 V on charge, and a fairly sharp cathodic peak is located
at 0.9 V vs. Li/Liϩ on discharge. These two peaks could be attrib-
uted to the one-step reversible electrochemical oxidation and reduc-
tion of NiO with lithium in the film electrode. Furthermore, some
modifications of the CVs appear in the cathodic peak upon the 5th
and 10th cycles, and the peak potential slightly decreases to 0.7 and
0.6 V, respectively. However, the NiO film electrode showed good
reversibility and repeatability after 20 cycles. The effect of the scan
rates on the CV profile is complicated and an investigation is in
progress.
Discussion
Nanostructured electrode materials exhibit more attractive prop-
erties compared with conventional electrode materials, such as very
small particle size, large exposed surface areas, and high surface
energy. These properties can reduce the diffusion distance of Li ions
in solid state, enlarge the contact area between the active particles,
enhance the electrochemical reaction rate, as well as results in the
agglomeration of nanosized particles. Our film characterization re-
sults indicate that the NiO thin film deposited by PLA is composed
of nanocrystalline NiO particles with average size of ϳ30 nm. The
high specific capacity of the NiO thin film electrode (ϳ700 mAh/g)
can be attributed to the high surface area of the nanocrystalline NiO
particles and the short diffusion distance for Li ions in the nanocrys-
talline NiO electrode. However, it should be noted that the morphol-
ogy of the NiO particles on the film surface changed obviously both
their shape and size when the NiO film/Li cell was cycled. It could
be caused by the fact that the agglomeration of nanoparticles takes
place on the surface of the film electrode and the larger aggregates
are formed by a solid electrolyte interface ͑SEI͒ surrounded.10
To get further insight into the electrochemical properties of the
NiO thin-film electrode, the reversible electrochemical mechanism
of nanocrystalline NiO film with lithium is discussed in detail. Re-
cently a new reaction mechanism for transition metal oxides with
lithium was proposed by Tarascon’s group8 and Dahn’s group.11
Figure 6. CVS of NiO film electrode cycled between 0 and 3.5 V at a scan
rate of 5 mV/min with 1 M LiPF6 in 1:1 EC:DMC electrolyte.
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Journal of The Electrochemical Society, 149 ͑7͒ A873-A878 ͑2002͒
A877
This mechanism totally differs from the classical reversible
insertion/deinsertion of lithium or lithium alloying processes. Both
of them have investigated CoO-based electrodes as a typical ex-
ample using effective physical and electrochemical techniques.
However, there are some different viewpoints between these two
groups on the ability to drive the electrochemical reaction of re-
duced metal with lithium oxide. Tarascon’s group suggested that the
reduction mechanism of the metal oxide ͑MO͒ with Li involved the
formation of 1-5 nm metal nanoparticles, which lead to the decom-
position of Li2O, and the metal converts back to metal oxide upon
the subsequent charge. Dahn’s group proposed a similar reaction
mechanism for the first discharge process, including the immediate
decomposition of metal oxide and the formation of nanosized metal
particles. But for the subsequent charge process, they proposed that
the reduced metal is oxidized to metal ions by a replacement reac-
tion where the metal ions displace the Li ion in Li2O via an ion-
exchange process. In this reaction, Li2O is not decomposed and the
original oxygen lattice of lithium oxide is preserved. According to
our experimental results and the mechanisms proposed by these two
groups, we suggest that the electrochemical reaction mechanism for
the nanocrystalline NiO film-based lithium cell involves the follow-
ing processes.
For the first discharge process, the reaction of lithium toward
NiO film takes place, in which the nanocrystalline NiO decomposes
to form Ni metal and Li2O. The resulting products were identified
by our XRD and XPS measurements. The particle size of reduced Ni
metal observed in our experiment could be larger than a few nano-
meters, which was suggested by Tarascon’s group for the CoO/Li
cell.8 They directly proved the formation of 1-5 nm metal particles
by transmission electron microscopy ͑TEM͒ and considered that
such small metal nanoparticles could not be observed by XRD mea-
surement. In contrast, the diffraction peak of Ni metal was observed
in our XRD pattern of the lithiated film as shown in Fig. 1c. It is
difficult to assume that only the reduced metal particles in a few
nanometers range can drive the electrochemical decomposition of
Li2O. In addition, there is no evidence of the diffraction peak of
Li2O in our XRD measurement, implying that the lithium oxide
might be amorphous. Overall, the electrochemical reaction during
the discharge process can be written as
ing, as we have discussed. For the nanocrystalline NiO film-based
lithium cell, we suggest that the metallic Ni produced from the
starting NiO nanoparticles on the film may still preserve the same
nanosized range or reduce to smaller nanoparticles. Then the highly
reactive nickel nanoparticles could convert back to NiO, accompa-
nying the decomposition of Li2O during the charging process.
Further cycling of the NiO film-based lithium cell proceeds via
the following reversible electrochemical replacement reaction
Discharge
NiO ϩ Li
ꢀ
Ni ϩ Li2O
Charge
This reaction mechanism shows that NiO could reversibly react
with 2 Li per formula unit, corresponding to a reversible specific
capacity of 714 mAh/g, which is very close to the observed capacity
of 700 mAh/g. Since the electrochemical mechanism of the NiO
film/Li cell is very complicated, more work needs to be done for the
further understanding of this reaction mechanism.
Conclusion
Thin films composed of cubic NiO nanocrystalline particles hav-
ing average size of about 30 nm were successfully prepared by
reactive PLA in an oxygen ambient using a metallic nickel target.
The NiO film electrode exhibited excellent electrochemical perfor-
mance with a reversible capacity as high as 700 mAh/g at high
current density and good reversibility upon cycling 100 cycles. Dur-
ing NiO film/Li cell discharge, the formation of the metallic nickel
and Li2O was identified by the XRD and XPS measurements. Based
on our experimental results, we extend the new electrochemical
mechanism proposed recently8,11 to the NiO film-based Li-cell. This
mechanism involves the electrochemical displacement reaction of
NiO with lithium, in which the nanocrystalline NiO is reduced to
form metallic nickel, accompanying the formation of Li2O. Our
results also showed that metal oxide nanocrystalline films not only
offer a significant advantage of improved performance of lithium-
ion batteries, but also provide an ideal geometry and ‘‘clean’’ elec-
trode material for fundamental research. Moreover, the successful
fabrication of nanocrystalline NiO film demonstrates that PLA is a
promising method for preparing thin-film electrodes of all-solid-
state rechargeable lithium-ion batteries.
NiO ϩ 2Liϩ ϩ 2eϪ → Ni ϩ Li2O
͓1͔
Acknowledgment
The authors thank Dr. W. L. Dai for his help with the XPS
measurements.
From this reaction, the theoretical predicted capacity corresponds
to 2 equiv mol of lithium ion per unit mole of NiO, but the irrevers-
ible capacity of NiO film electrode, as mentioned previously, is
much larger than the predicted value. This large excess capacity
could be caused by the decomposition reactions of electrolyte and
formation of the SEI. Recently, Tarascon’s group9,10 pointed out that
the growth of an organic layer resulted from the reaction of the
solvent electrolyte molecules and the surface of nanosized metal.
This phenomenon was supported by their in situ TEM and selected
area electron diffraction ͑SAED͒ results obtained at the end of the
first discharge plateau. Our SEM image of the lithiated NiO film
also revealed that the larger size and needle-like shape nanoparticles
uniformly distributed on the film surface. This result provided indi-
rect evidence for the nanoparticles agglomeration and the formation
of an SEI-like layer upon cycling, although these processes need to
be further clarified by TEM, infrared, and Raman spectroscopy stud-
ies being conducted in our laboratory.
References
1. J. Schoonman, Solid State Ionics, 135, 5 ͑2000͒.
2. R. Vacassy, H. Hofmann, N. Papageorgiou, and M. Gratzel, J. Power Sources,
¨
81-82, 621 ͑1999͒.
3. H. Li, X. J. Huang, L. Chen, G. G. Zhou, Z. Zhang, D. Yu, Y. J. Mo, and N. Pei,
Solid State Ionics, 135, 181 ͑2000͒.
4. I. Kim, P. N. Kumta, and G. E. Blomgren, Electrochem. Solid-State Lett., 3, 493
͑2000͒.
5. M. Nishizawa, K. Mukai, S. Kuwabata, C. R. Martin, and H. Yoneyama, J. Elec-
trochem. Soc., 144, 1923 ͑1997͒.
6. N. Li, C. J. Patrissi, G. Che, and C. R. Martin, J. Electrochem. Soc., 147, 2044
͑2000͒.
7. N. Li, C. R. Martin, and B. Scrosati, Electrochem. Solid-State Lett., 3, 316 ͑2000͒.
8. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, and J-M. Tarascon, Nature (London),
407, 496 ͑2000͒.
9. S. Grugeon, S. Laruelle, R. Herrera-Urbina, L. Dupont, P. Poizot, and J-M. Taras-
con, J. Electrochem. Soc., 148, A285 ͑2001͒.
10. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, and J-M. Tarascion, J. Power
Sources, 97-98, 235 ͑2001͒.
11. M. N. Obrovac, R. A. Dunlap, R. J. Sanderson, and J. R. Dahn, J. Electrochem.
Soc., 148, A576 ͑2001͒.
For the charging process, the XRD, XPS, and electrochemical
measurements of nanocrystalline NiO film electrodes demonstrated
that the reduced Ni metal displaces the lithium in Li2O to form NiO
and lithium. This electrochemical displacement reaction can be pre-
sented as follows
12. E. Fujii, A. Tomozawa, H. Torii, and R. Takayama, Jpn. J. Appl. Phys., Part 2, 35,
L328 ͑1996͒.
13. S. Passerini and B. Scrosati, J. Electrochem. Soc., 141, 889 ͑1994͒.
14. H. Sato, T. Minami, S. Takata, and T. Yamada, Thin Solid Films, 236, 27 ͑1993͒.
15. H. Kumagai, M. Matsumoto, K. Toyoda, and M. Obara, J. Mater. Sci. Lett., 15,
1081 ͑1996͒.
Ni ϩ Li2O → NiO ϩ 2Liϩ ϩ 2eϪ
͓2͔
16. P. Tomczyk, J. Wyrwa, and M. Mosialek, J. Electroanal. Chem., 463, 78 ͑1999͒.
17. M. Kitao, K. Izawa, K. Urabe, and T. Komatsu, Jpn. J. Appl. Phys., Part 1, 33,
6656 ͑1994͒.
However, it is quite difficult for this reaction to occur under normal
conditions, and the redox reaction mechanism still remains disturb-
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A878
Journal of The Electrochemical Society, 149 ͑7͒ A873-A878 ͑2002͒
18. M. Tachiki, T. Hosomi, and T. Kobayashi, Jpn. J. Appl. Phys., Part 1, 39, 1817
25. Z. W. Fu and Q. Z. Qin, J. Electrochem. Soc., 147, 4610 ͑2000͒.
26. Z. W. Fu, J. L. Kong, and Q. Z. Qin, J. Electrochem. Soc., 146, 3914 ͑1999͒.
27. Z. W. Fu and Q. Z. Qin, J. Phys. Chem. B, 104, 5505 ͑2000͒.
28. F. Ding, Z. W. Fu, M. F. Zhou, and Q. Z. Qin, J. Electrochem. Soc., 146, 3554
͑1999͒.
29. R. Serna, C. N. Afonso, J. M. Ballesteros, A. Naudon, D. Babonneau, and A. K.
Petdord-Long, Appl. Surf. Sci., 138-139, 1 ͑1999͒.
30. C. N. Afonso, R. Serna, J. M. Ballesteros, A. K. Petford-Long, and R. C. Doole,
Appl. Surf. Sci., 127-129, 339 ͑1998͒.
͑2000͒.
19. C. Morant, L. Soriano, J. F. Trigo, and J. M. Sanz, Thin Solid Films, 317, 59
͑1998͒.
20. E. Fujii, A. Tomozawa, S. Fujii, H. Torii, M. Hattori, and R. Takayama, Jpn. J.
Appl. Phys., Part 2, 32, L1448 ͑1993͒.
21. T. Nishina, K. Takizawa, and I. Uchida, J. Electroanal. Chem., 263, 87 ͑1989͒.
22. K. A. Striebel, C. Z. Deng, S. J. Wen, and E. J. Cairns, J. Electrochem. Soc., 143,
1821 ͑1996͒.
23. M. Antaya, J. R. Dahn, J. S. Prestan, E. Rossen, and J. N. Reimers, J. Electrochem.
Soc., 140, 575 ͑1993͒.
24. J. Zhang, J. M. McGraw, J. Tumer, and D. Ginley, J. Electrochem. Soc., 144, 1630
͑1997͒.
31. M. Yoshimura, K-S. Han, and S. Tsurimoto, Solid State Ionics, 106, 39 ͑1998͒.
32. J. R. Dahn, T. Zheng, Y. Liu, and J. S. Xue, Science, 270, 590 ͑1995͒.
33. M. Schulze, R. Reissner, M. Lorenz, M. Lorenz, U. Radke, and W. Schnurnberger,
Electrochim. Acta, 44, 3969 ͑1999͒.
Downloaded on 2014-11-24 to IP 155.33.16.124 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).