Oxide Nanostructures for Energy Storage
Yuan Yang, Jang Wook Choi, Yi Cui
Department of Materials Science and Engineering, Stanford University
12.1 Introduction
Energy is becoming a critical societal issue nowadays, which
greatly impacts world economy, environment, and human life. The
global energy consumption will keep growing in the following decades. Although traditional combustion-based energy technologies,
including coal, oil and natural gas, dominates in energy needs, alternative energy sources and technologies are imperative due to the
disadvantages of traditional counterparts, such as emission of greenhouse gases and long-term environmental consequences. Moreover,
the rapid depletion of fossil fuels could result in severe economic
and political conflictions between countries. As a result, solving energy issues becomes one of the greatest challenges in the 21st century. To conquer this challenge, alternative sustainable and clean energy sources and technology should be explored and utilized. In
general, energy technology can be divided into two categories, energy conversion and energy storage. The former one deals with how to
efficiently convert energy from one form to another, such as solar
2
cells, wind power plants and nuclear power plants, which convert
various kinds of energy to electricity. The latter one stores energy in
a steady state. The most widely used energy storage devices include
batteries and electrochemical capacitors. Energy can be stored chemically in these devices for long time and convert directly into electrical energy with little or no impact on the environment. Furthermore,
the efficiency of such devices could go beyond the Carnot limit.
Batteries and electrochemical capacitors have wide applications,
such as portable devices, power tools and electric vehicles. However, devices with better performance, such as higher energy density,
longer cycle life, and faster charge/discharge rate, are still desired or
necessary for certain applications. Design of new materials and
techniques are crucial to achieving these goals. In view of this,
nanostructured materials and nanotechnology offer great promise
because of the unusual properties resulting from confining their dimensions and the combination of bulk and surface properties to the
overall behavior. Dramatic improvement in power rate, cycling life,
energy density and other aspects has been observed [1, 2]. In the following part, progress in nanosized materials, especially nanostructured oxides, for lithium-ion batteries and electrochemical capacitors
will be presented.
12. 2 Nano Oxides for Li-Ion Batteries
3
Rechargeable batteries play an important role in energy storage
and have found applications at different scales [3], including on-chip
power supplies (10-6-10-3 Wh), portable devices (100-102 Wh), vehicle electrification (104-105 Wh) and storage for electric grid and
buildings (106-108 Wh). Among different types of rechargeable batteries, lithium-ion batteries can offer the highest energy density and
power density and show potential for further improvement [4]. As a
result, much attention has been given to lithium ion batteries. In
commercialized Li-ion batteries, LiCoO2 and graphite are the most
common cathode and anode, respectively. As shown in Fig. 1,
LiCoO2 and graphite both have a layered structure and lithium ions
can move readily in the two-dimensional interlayer spacing. During
charge, lithium is removed from LiCoO2 and intercalates into graphite. The reverse process occurs during discharge. The practical capacity of LiCoO2 is 140 mAh g-1, as only half the lithium can be
used to avoid cracking of the layer structure. Graphite has a practical
capacity of 320-360 mAh g-1, which is close to the theoretical capacity of 372 mAh g-1, corresponding to the conversion of graphite to
the LiC6 phase [5]. The voltage of LiCoO2 and graphite are ~3.9 V
and ~0.1 V vs Li/Li+, respectively. More details on the operation
principles of Li-ion batteries can be found in other references [3, 5,
6].
Fig. 1 The illustration of the structure of a lithium ion battery with graphite and
LiCoO2 as anode and cathode, respectively [1]
4
The LiCoO2/graphite system has been successfully utilized in the
field of portable electronics, such as cell phones and laptop computers. However, lithium-ion batteries are still far from optimization,
and significant improvement is necessary for use in applications
such as vehicle electrification. Necessary improvements include
higher energy density, faster discharge/charge rates, longer lifetimes,
better environmental benignity and safety. Much research has been
dedicated to accomplishing these goals by either synthesizing new
materials or designing new material structures and morphologies [7].
Scaling the size of existing materials down to the nanoscale offers
a variety of advantages. The relatively small size of nanomaterials
leads to shorter diffusion length and higher surface area for reaction,
which can enhance the kinetic behavior and thus the power density
of batteries. Generally, nanomaterials can also accommodate larger
stress without fracturing, which enables the use of new materials that
undergo large volume changes during reaction with lithium, such as
silicon and tin. Moreover, many exotic nanostructures have been
created, including nanowires [8-10], nanorings [11, 12], and mesoporous materials [13, 14], providing various choices for the rational
design of electrodes. For example, mesoporous LiMn2O4 as a cathode material could trap dissolved Mn ions inside the porous network
to suppress capacity fading, which is the bottleneck for this material
[15, 16]. As another example, silicon nanowires used as an anode
material provide one-dimensional electron transport pathways and
facile relaxation of stress along the radial direction [17, 18]. These
5
examples illustrate the promise of nanomaterials used in Li-ion batteries.
It is worth pointing out that nanomaterials also have certain disadvantages in battery applications. For instance, the high surface area of nanomaterials also promotes side reactions, which have a parasitic effect on a battery’s performance. Also, the density of
nanopowder film is typically less than that for micrometer-sized particles. As a result, the volumetric energy density is lower in nanomaterials-based electrodes compared to conventional battery electrodes, which usually contain micron-sized powder. However, these
problems may be minimized by utilizing specific techniques, such as
surface coatings to reduce the side reactions [19]. In the following
sections, we will review recent progress in the development of
nanostructured oxides for lithium batteries.
12.2.1 Spinel LiMn2O4
Spinel LiMn2O4 is a promising cathode material due to its low
cost, low toxicity and high stability with respect to thermal runaway.
The typical capacity of this material is 110-120 mAh g-1 [6].
LiMn2O4-based battery products have been fabricated [20, 21] and
several companies are exploring its use as the cathode material for
batteries designed for electric vehicles. However, its power density
is not high since the diffusivity of lithium ions in LiMn2O4 (10-9~1011
cm2/s) [22, 23] is much lower than of the diffusivity in LiCoO2
6
(10-7~10-9 cm2/s) [24, 25]. As a result, shrinking the size of LiMn2O4
down to the nanoscale can shorten the diffusion distance of lithium
and result in enhanced power density. Recent work has shown that
the power performance of LiMn2O4 can be improved by utilizing
nanostructures, such as nanowires, nanorods and nanoparticles [2630].
Eiji Hosono, et al have synthesized LiMn2O4 nanowires by a
three-step method that consists of hydrothermal synthesis of
Na0.44MnO2 nanowires, ion exchange with lithium ions in molten
salt and high temperature annealing [30]. The as-synthesized nanowires have a diameter of 50-100 nm and are hundreds of micrometers in length (Fig. 2(a)). These nanowires exhibit excellent highpower performance: the discharge capacity is 118 mAh g-1 at 0.1 A
g-1 (~0.7 C), and when the current rate increases to 20 A g-1
(charge/discharge in ~16 seconds), 75% of the initial capacity (88
mAh g-1) still remains (Fig. 3b). In comparison, commercial micronsized LiMn2O4 particles show capacity less than 40 mAh g-1 at 20 A
g-1. Besides nanowires, LiMn2O4 nanorods also show much better
power performance than micron-sized LiMn2O4 particles [27].
Fig. 2 (a) A SEM image of as-synthesized single crystalline LiMn2O4 nanowires
(b) The discharge curve of LiMn2O4 nanowire-based cathodes and commercial
samples at different current rates. (c) The rate-dependent 2nd discharge capacity
of LiMn2O4: single crystalline nanowires (red circle), Honjyo Chemical (green
box), Mitsui Metal (blue triangle), and Aldrich (black box) [30]
7
Another issue related to the use of LiMn2O4 as a cathode material
is its fast capacity fading, especially at high temperature (e.g. 50-60
ºC). There are three primary reasons for this fade in capacity [3133]. The first is the dissolution of Mn2+ ions into the electrolyte after
the disproportionation of LiMn2O4 [34, 35]. The trace amount of water inside the electrolyte can react with LiPF6 to form HF. The acidic
environment promotes the disproportionation of Mn3+ ions in
LiMn2O4 (Mn3+ -> Mn2+ + Mn4+). The second reason is the decomposition of the electrolyte, which also generates H2O and leads to
disproportionation of LiMn2O4 [32, 36]. The final reason is JahnTeller distortion. It should be noted that the high surface area inherent in nanomaterials can enhance the dissolution of Mn ions and the
side reactions with the electrolyte, which lead to capacity fading.
These facts raise concerns regarding the use of LiMn2O4 nanostructures in cathodes even though better power performance can be realized. However, recent research has shown that nanostructured
LiMn2O4 can also exhibit good cycling performance [15, 16, 28, 29].
Though the reasons are not yet clear, some have reported that the
good cycling data might result from the high crystallinity of the incorporated nanostructures; the degree of crystallinity plays an important role in the stability of LiMn2O4 [37, 38]. For example, stoichiometric LiMn2O4 nanoparticles show 83% capacity retention
after 200 cycles at 55 ºC [29], which is much better than some reports on micron-sized particles [39]. Moreover, excellent cycling retention has been demonstrated in mesoporous LiMn2O4 as dissolved
Mn ions can be trapped inside the porous materials. This study
8
shows that at room temperature, 94% of the initial capacity is retained after 500 cycles and the fading per cycle is less than 0.005%
after the first 100 cycles [15]. At 50 ºC, the capacity retention is over
95% after 100 cycles [16].
In addition to providing better performance, nanostructures also
facilitate the use of new methods to study battery materials. Common techniques used to study battery materials are based on the
electrochemical testing of ensemble electrodes. However, the heterogeneous nature of ensemble electrodes averages all information and
can not provide a direct correlation of electrochemical properties
with the local morphology, structure and chemical composition. Investigations at the single particle level can significantly expand the
scope of understanding for specific battery materials. The difficulties
of studying a single particle include distinguishing a particle in the
mixed film-like electrode and contacting probes, such as metallic
electrodes, onto such small particles (typically less than 20 m for
battery materials). Nanostructures, especially one-dimensional nanorods and nanowires, can solve these problems due to the following
reasons: 1) In contrast to micrometer-sized particles, it is feasible to
make contact with metallic probes to study their transport properties
[40]. 2) The single-nanostructure devices can be characterized by a
variety of electron microscopy and in-situ techniques [40, 41]. 3)
Nanomaterials can be highly crystalline[42, 43], which provides
well-defined nanoscale domains for testing intrinsic properties. Recently, direct observation of the dissolution of LiMn2O4 nanostruc-
9
tures in an electrolyte has been observed [44]. Electron beam lithography was used to deposit metallic electrode contacts to fix and identify single LiMn2O4 nanorods. SEM images show that pure LiMn2O4
is etched in organic electrolyte (1M LiPF6 in EC/DEC) after a few
hours at 60 ºC while Al-doped LiMn2O4 is much more stable, as
shown in Fig. 3(a). I-V measurements also record that the resistivity
of pure LiMn2O4 nanorods increase significantly while that of Aldoped sample remains basically the same (Fig. 3(b)). These results
correlate well with their electrochemical performance: pure
LiMn2O4 nanorods show only 69 % capacity after 100 cycles at 55
ºC while LiAl0.1Mn1.9O4 nanorods exhibit a capacity retention of 80
% after 100 cycles [44].
Fig. 3 (a) SEM images of nanorod devices in the organic electrolyte (1 M LiPF6
in EC/DEC). (b) The evolution of the normalized conductance of nanorods in the
electrolyte [44]
12.2.2 Manganese Dioxide
Spinel LiMn2O4 is favored as a cathode material because of its
low cost, environmental compatibility and superior safety characteristics. However, the capacity of this material is limited to ~120 mAh
g-1. Meanwhile, manganese dioxide (MnO2), which is widely used in
primary lithium batteries [45], could accommodate up to ~0.7 lithium [46], corresponding to a capacity of ~210 mAh g-1. However,
due to structural changes and volume expansion, the reaction between lithium and MnO2 is not fully reversible, which makes this
10
material unsuitable for rechargeable batteries. Recently, research has
shown that the reversibility and capacity of MnO2 can be improved
by utilizing nanostructured MnO2, which is likely due to better accommodation of strain during structural changes.
Manganese dioxide has many different polymorphs, including the
and phases. The structures of MnO2 polymorphs can
be described as various stacking arrangements of linked Mn-O octahedra, as shown in Fig. 4 [47]. One dimensional tunnels exist in the
and phases . The tunnel sizes are different for different phases of MnO2; these tunnel sizes largely determine the electrochemical
properties of the material. For example, -MnO2 contains a so-called
1×1 tunnel, which is too small for the transport of lithium ions. As a
result, this phase is commonly considered inactive towards lithium
[48]. The and phases have 2×2 and 1×2 channels, respectively
[47]. These channels are of sizes that are suitable for lithium
transport, and these phases are electrochemically active in lithiumbased batteries [49]. Besides pure MnO2, there is another family of
layered MnO2-related materials, AxMnO2 (A = H+ or cations). These
materials have a layered structure similar to  - MnO2 (birnessite)
with a basal spacing of ~ 0.7 nm, allowing high mobility of the interlayer cations with fast kinetics and slight structural changes [50].
Fig. 4 Schematics of and-MnO2. and -MnO2 present channels
with different sizes. -MnO2 has a layered structure. -MnO2 has a cubic structure
11
[47]
MnO2 nanostructures have been synthesized by various methods,
especially hydrothermal reactions, and they exhibit good electrochemical performance [49-55]. For example, and-MnO2 nanorods have been synthesized and show reversible capacity [49]. Ma et
al. synthesized nanorods of layered MnO2 (birnessite) nanobelts
[50]. The nanobelts are 5-15 nm in width and several to tens of micrometers in length. The initial discharge capacity reaches 375 mAh
g-1 in the voltage range of 1.0-4.8 V vs Li/Li+, corresponding to 1.3
lithium per unit of MnO2. A considerable capacity loss (85 mAh g-1)
was observed in the second cycle, which is similar to bulk birnessite.
However, the capacity loss of the following cycles is only ~ 0.7%
per cycle, much less than bulk birnessite [50]. Moreover, the layered
MnO2 nanobelts do not undergo the phase transformation to spinel
structure ( phase) during cycling, which occurs in bulk birnessite
MnO2 [56] and should be avoided in practical applications.
-MnO2 is the most stable phase at room temperature [57, 58], but
it can only accommodate a small amount of lithium due to the narrow 1×1 channels. However, Feng Jiao and Peter G. Bruce reported
that a considerable amount of lithium can be reversibly inserted into
-MnO2 by utilizing mesoporous structures [48]. In their study,
mesoporous MnO2 is formed by using a KIT-6 mesoporous silica
template (space group Ia3d). The as-synthesized mesoporous MnO2
has highly ordered 3D pore structure (Fig. 5) with a lattice parameter
12
of 24.9 nm. The reported pore size was 3.65 nm and the BET surface
area was 127 m2 g-1. The mesoporous MnO2 exhibits an initial discharge capacity of 284 mAh g-1 (0.92 lithium per MnO2) at 15 mA g1
. After some initial decay, the capacity stabilizes around 200 mAh
g-1. Furthermore, the capacity only decreases by 19% when the rate
is increased from 15 to 300 mA g-1. In contrast, bulk MnO2 shows a
capacity less than 10 mAh g-1, indicating very poor electrochemical
activity.
Fig. 5 Characterization of Mesoporous -MnO2. Left: TEM images of (a) and (b)
as-prepared mesoporous -MnO2; (c) and (d) after first discharge. Right: capacity retention for mesoporous -MnO2 cycled at (a) 15 mA g-1, (b) 30 mA g-1, (c) 300
mA g-1, and (d) bulk -MnO2 cycled at 16 mA g-1 [48]
12.2.3 Vanadium Pentoxide (V2O5)
V2O5 was one of the first oxides investigated for use in rechargeable lithium-ion batteries [6]. Its orthorhombic crystal structure can
be visualized as layers (ab-plane) of VO5 square pyramids that share
edges and corners. The sixth V-O bond in the c-direction consists of
weak electrostatic interactions, which facilitates the insertion of lithium ions between layers [59]. In theory, this material can intercalate
up to three lithium ions per unit of V2O5, corresponding to a capacity
of about 440 mAh g-1. However, the drastic structural transformation
during intercalation is complicated and not fully reversible [60].
Several phases of LixV2O5 that contain different amounts of lithium
have been observed. The - and -phases, with x < 0.1 and 0.35 < x
13
< 0.7, respectively, maintain the orthorhombic phase while the interlayer distance increases with increasing x. At x = 1, the -phase exists, resulting from one layer gliding out of two. At x > 1, the irreversible -phase is formed. When x is further increased to 3, the
structure of LixV2O5 transforms to the rock salt -phase. The delithiation of Li3V2O5 is limited to the phase and is not fully reversible in bulk materials, where x cannot reach lower than 0.4.
While fully reversible insertion of lithium in the range of 0<x<3
cannot be accomplished in bulk V2O5, studies show that it is likely
to be realized in nanostructured V2O5. For example, Chan et al. synthesized V2O5 nanoribbons by chemical vapor deposition and observed that the chemical lithiation process is fully reversible in the
nanoribbons [59]. These nanowires grew along the <020> direction
and the c-axis is parallel to the height. In their study, n-butyllithium
is used to chemically lithiate the nanoribbons while Br2 is employed
as the reagent for delithiation. This chemical process is considered to
give nearly the same results as electrochemical lithiation and is used
to determine the full lithiation capacity of the nanoribbons [61]. Exsitu TEM techniques were used to track the phase transformation.
During lithiation, the width of nanoribbons affects the phase transformation. When the width is on the order of hundreds of nanometers, -Li3V2O5 dominates the diffraction pattern, but another intermediate phase, -Li2V2O5, is also identified. Meanwhile, when the
width increases to several micrometers, the pristine V2O5 phase and
14
-Li3V2O5 show similar intensity in the diffraction pattern even after lithiation times of several days. These results indicate a diffusion
barrier which was more easily overcome in the narrower regions
than the wider region. In the process of delithiation, a complete removal of lithium by Br2 was observed and the structure transforms
back to the pristine orthorhombic V2O5 phase (Fig. 6). In comparison, using Br2 can only remove the lithium to x = 0.1 in bulk V2O5,
and the resulting material maintained a disordered cubic structure
[60]. In addition, the effect of the thickness of nanoribbons (the caxis dimension) on the Li insertion was studied, which was overlooked before as lithium is considered to diffuse along the a-b plane.
When 100 nm thick and 400 nm wide nanoribbons were lithiated for
only 10 s, only the pristine V2O5 phase was observed, suggesting no
significant amount of Li in the NR layers. In contrast, in a nanoribbon with a width of 740 nm but smaller height (~20 nm), -Li3V2O5
was clearly observed as the dominant phase. These observations indicate that the thickness along the c-direction has a significant impact on the phase transformation. The authors suggest that the small
thickness helps overcome the activation barrier of distortion during
the simultaneous structural transformation from the orthorhombic
phase to the rock salt cubic structure [59].
Fig. 6 Chemical lithiation in V2O5. a) Schematic of the insertion/deinsertion process in nanoribbon and bulk materials. b) TEM image of an as-synthesized nanoribbons. C) TEM image of a LixV2O5 nanoribbons treated with Br2. Inset: electron
15
diffraction shows the orthorhombic structure for pristine V2O5, indicating a full
delithiation [59]
In another study, Patrissi et al. synthesized V2O5 nanowire films
by a template-assisted method [62]. The nanowire film exhibits a
higher capacity and better power performance than a V2O5 thin film
made by a sol-gel method. The nanowire film can accommodate
about 0.05 more lithium (0.95 vs 0.90) than the thin film. Also, the
capacity retention at a rate of 1021 C is 40 % compared to C/20 for
the nanowire film. In comparison, the capacity retention is less than
20 % at 793 C in a V2O5 thin film. A V2O5/Carbon nanotube nanocomposite was synthesized by Sakamoto et al. and also showed improved rate performance [63].
12.2.4 Titanium Oxide
Titanium oxide has advantages for use in batteries such as ample
availability, low toxicity, and safety. Titanium oxide also has a theoretical capacity of 335 mAh g-1 or 1.0 lithium per TiO2 unit [64].
However, bulk TiO2 materials always exhibit limited capacity and
poor cycling performance, which is partially attributed to the low
diffusivity of lithium in TiO2 [64]. Recently, it has been found that
nanostructured TiO2 can enhance the capacity and reversibility of
this material significantly [64].
Table 12.1 The structure of TiO2 polymorphs
Structure
Space
Density (g
Unit cell (A)
Reference
16
group
cm-3)
Rutile
P42/mnm
4.13
a = 4.59, c = 2.96
[64-66]
Anatase
I41/amd
3.79
a = 3.79, c = 9.51
[64-66]
Brookite
Pbca
3.99
a = 9.17, b = 5.46,
c = 5.14
[64, 66, 67]
a = 12.17, b =
TiO2 (B)
C2/m
3.64
3.74, c = 6.51,  =
107.29
TiO2 (R)
Pbnm
3.87
[64, 66, 67]
o
a = 4.9, b = 9.46, c
= 2.96
[64, 66, 67]
There are many polymorphs of TiO2, such as rutile, anatase,
brookite and TiO2-B (bronze), as summarized in Table 12.1. The
typical charge/discharge voltage of these phases is 1.5-2.0 V vs
Li/Li+. Rutile is believed to be the most stable phase, but can only
accommodate limited amounts of lithium (< 0.1 Li per TiO2) [20,
68]. The diffusivity of lithium is anisotropic inside rutile TiO2; the
diffusivity is 10-6 cm2 s-1 along the c-axis but only 10-15 cm2 s-1 along
the ab-plane [69-71]. Therefore, the low diffusivity along the abplane restricts the reaction process and limits the intercalation of
lithium. In contrast, nanostructured rutile shows significantly improved capacity. Hu et al. reported more than 0.8 lithium (about 270
mAh g-1) insertion per TiO2 in rutile nanorods (10 nm × 40 nm)
while only around 0.1 lithium (~30mAh g-1) can be inserted into particles with diameter of 20 m at a current rate of C/20, as exhibited
in Fig. 7 [72]. Moreover, the capacity remains around 220 mAh g-1
at a 1 C rate and over 100 mAh g-1 at a 10 C rate. The success is
mainly due to the very short diffusion length along the ab-plane in
17
the synthesized nanorods. The ab-plane is parallel to the cross section of the nanorods and thus the diffusion length is only ~5 nm.
Furthermore, the authors found that the surface storage of lithium is
also favored in nanoparticles.
Fig. 7 TEM and electrochemical characterization of rutile TiO2 nanorods. (a)
HRTEM and electron diffraction pattern of TiO2 nanorods. (b) Voltage profiles of
rutile TiO2 with different sizes: 20 m (--), 0.5 × 5 m (┅), 500 nm (–•–) and 10
× 40 nm(—) [72]
The Bruce group has also investigated TiO2-B nanostructures [7376]. TiO2-B has a layer structure that is more open than the structures of rutile, anatase or brookite, making it a better host for Li+ intercalation [76]. TiO2-B nanowires and nanotubes can be synthesized by mixing TiO2 powder and concentrated NaOH aqueous
solution (e.g. 15 M) together and heating to 150-170 oC [75, 76].
TiO2-B nanowires exhibit an initial capacity around 200 mAh g-1
with stable capacity upon cycling. A capacity loss of no more than
0.1% per cycle occurred during the first 100 cycles, and only 0.06%
from cycle 20 to 100 [76]. The TiO2-B nanotubes even reach a capacity as high as 328 mAh g-1 in the first discharge at a rate of 10
mA g-1, corresponding to Li0.98TiO2 [75].
12.2.5 Metal Oxides with Displacement Mechanism
18
In the metal oxides already discussed, the reaction mechanism
with lithium is mainly based on intercalation. Meanwhile, many
metal oxides can react with lithium by a displacement mechanism
involving the formation of lithium oxide (Li2O). This mechanism
can be summarized by the half reaction x/2 Li+ + MOx + x/2 e- →
x/2 LiO2 + M0, and the composition can be written as MxOy/M0/Li2O
[77]. Though some kinds of displaced metals can further react with
lithium to form alloy compounds LixMy, such as SnO2 [78] and ZnO
[79, 80], the displacement reaction is universal in these metal oxides.
These metal oxides include Fe3O4, CoO, SnO2, ZnO, CuO, Cu2O
and MoO2. Some oxides, such as MnOx, can also react with lithium
by this mechanism [81, 82], but with less interesting characteristics.
Table12.2 The equilibrium potential of some metal oxides by displacement mechanism [82]
Material
Voltage vs Li/Li+
(V)
Capacity (mAh/g)
Material
EMF (V)
Capacity (mAh/g)
Fe2O3
1.631
1007
CoO
1.802
715
SnO2
1.582
711
CuO
2.248
674
Ga2O3
1.188
858
ZnO
1.252
659
Displacement reactions of metal oxides with lithium have a typical voltage of 1-2 V vs Li/Li+, as summarized in Table 12.2 [82]. As
a result, these oxides can act as the anode in lithium ion batteries.
These oxides have theoretical capacities between 600 – 1200 mAh
g-1, which is twice to three times that of graphite, the most common
anode material [4, 82]. Moreover, many metal oxides, such as iron
19
oxide and copper oxide, are very cheap. These advantages make
them attractive for use in lithium ion batteries, and much research
has been dedicated to this end. However, the insulating nature of
Li2O and the strong Li-O bonds result in marked hysteresis in voltage and significant capacity fading during cycles [77, 83]. By reducing the size of materials down to nanoscale, the spatial dimensions
of insulating Li2O can be significantly reduced. In addition, the
pathways for electrons and ions becomes much shorter. As a result,
by designing rational and efficient nanoarchitectures, such as metal/metal oxides hybrids and porous structures, the electrochemical
performance can be improved dramatically [77, 84-86].
Taberna et al. developed a strategy to form metal/metal oxide
core/shell nanowire arrays. In their design, the metal core acts as the
efficient electron collector, and the thin oxide shell facilitates the
transport of ions. A Cu/Fe3O4 core/shell nanowire array was demonstrated as the example. The nanowire arrays were fabricated by a
two-step method. First, Cu nanowires were electrodeposited on Cu
foil with the confinement of an anodic aluminum oxide (AAO) template. Then Fe3O4 was uniformly electrodeposited onto the Cu nanowires. The dimension of the copper nanowires and the thickness of
magnetite coating were tuned by the size of the AAO template and
the deposition conditions. Typically, the copper nanowires had a diameter of 200 nm and uniform length of several micrometers. These
hybrid core/shell nanowire arrays exhibit significantly improved
power performance (Fig. 8). For instance, the sample with Fe3O4 de-
20
posited for 150 s results in much higher capacity than Fe3O4 powders at high rate. At a 1C rate (8 Li+/h), more than 85% capacity is
retained compared to C/32 (0.25 Li+/h). Meanwhile, the capacity retention at a 1 C rate is only ~45% for Fe3O4 powders. Even at a
higher rate, such as 8 C, the capacity retention for the Cu/Fe3O4
core/shell nanowires is still 75%, while that of Fe3O4 powder is only
about 25%. These nanowire arrays also show good capacity retention. No decay in capacity was observed after 50 cycles. Another
advantage of such core/shell nanowires is the higher mass loading
due to the nanowire geometry. The Fe3O4/Cu nanowires exhibit similar rate performance as the planar Fe3O4 film [87]. However, the
mass loading of the planar film was only 0.136 mg, which corresponds to a capacity of ~0.05 mAh cm-2. In contrast, the nanowire
structure showed a mass of 0.820 mg and a capacity of about 0.3
mAh cm-2.
Fig. 8 Fe3O4/Cu core/shell nanostructure for lithium-ion batteries. (a) Cross section of Cu nanostructured current collector before (left) and after (right) Fe3O4
coating. (b) The cycling performance of a Fe3O4 film electrodeposited onto nanoarchitectured copper for 150 s. The electrode was cycled first at C/32 for 15 cycles followed by a higher rate of C/16. (c) The rate-dependent capacity of
Fe3O4/Cu core/shell nanostructure with different time of electrodeposition: 120 s
(t1), 150 s (t2), 180 s (t3), 230 s (t4), 300 s (t5). Fe3O4-Cu indicates a Fe3O4 film on
planar copper substrate. Fe3O4 powder is made by 60 wt% Fe3O4, 22 wt% carbon black, 18 wt% PVDF. (d) The rate-dependent capacity (mAh cm-2) of a Fe3O4based Cu-nanostructured electrode (deposition time: 150 s) and a Fe3O4-based
Cu planar electrode [77]
21
Among oxides that react according to the displacement mechanism, tin oxide (SnO2) is an especially attractive material. SnO2 has
a high theoretical capacity of 1494 mAh g-1 due to the two-step reaction with lithium. First, lithium replaces tin to form lithium oxides (4
Li + SnO2 -> 2 Li2O + Sn, ~1.6 V vs Li/Li+). Second, lithium alloys
with tin to form Li4.4Sn (4.4 Li + Sn -> Li4.4Sn, < 1.0 V vs Li/Li+).
As a result, one unit of SnO2 could accommodate 8.4 lithium. However, the first step is highly polarized and not reversible due to the
formation of lithium oxide. Moreover, the second step involves large
volume expansion. To overcome these issues, various nanostructures
have been designed and significant improvement has been observed.
Kim et al. studied the size effect of SnO2 nanoparticles for lithium
ion batteries [85]. They synthesized SnO2 nanoparticles of various
sizes (3, 4 and 8 nm) by hydrothermal methods. 3 nm nanoparticles
show a discharge capacity of 740 mAh g-1 in the voltage window of
0-1.2 V with capacity loss less than 10 mAh g-1 over 60 cycles. In
comparison, 4 nm nanoparticles show a capacity fade over 100 mAh
g-1 under the same conditions. When the particle size further increases to 8 nm, the corresponding discharge capacity decreases to
less than 100 mAh g-1 after 30 cycles. The authors attribute the good
cycling performance of 3 nm particles the fact that these smaller nanoparticles can undergo reversible volume changes without aggregation into larger Sn clusters during cycling.
22
To minimize the influence of pulverization of SnO2 during
charge/discharge, structures which can undergo large volume changes have been fabricated, such as macroporous/mesoporous and hollow SnO2. Lou et al. synthesized hollow SnO2 nanospheres with diameter of 50-200 nm and wall thicknesses of ~ 10 nm [88]. The
hollow nanospheres exhibit an initial discharge capacity of nearly
1200 mAh g-1, which is much higher than SnO2 nanoparticles. Lee et
al. fabricated macroporous SnO2/carbon materials from a close
packed PMMA nanosphere template; the porous composite exhibit
moderate capacity retention with cycling [89]. Besides porous
nanostructures, one dimensional structures of SnO2, such as nanowires and nanorods, have also been utilized and improved electrochemical performance has been observed [78, 84, 90-93].
12.2.6 Nano-Oxide Coatings
Besides acting as the active material in electrodes of lithium ion
batteries, nanostructured oxides can also be employed as a surface
modification agent for battery materials, especially for cathodes.
Typically, these oxide coatings have a thickness of several to tens of
nanometers and are distributed uniformly on the surface of active
material particles. This inert oxide layer can reduce the contact between active electrode and the electrolyte. There are two common
types of coatings, and they are distinguished by their functions and
targeting materials: 1) Coatings to enhance the stability of layered
oxides (e.g. LiCoO2, Li1+xNiyMnzCowO2) at high potential, and 2)
23
coatings to suppress the dissolution of active materials, such as
LiMn2O4 and LiFePO4.
As mentioned at the beginning of this chapter, only half the lithium atoms in LiCoO2 are utilized in commercial lithium ion batteries
due to the chemical instability between LixCoO2 and the organic
electrolyte when x < 0.5. One way to increase the stability is to coat
or modify the surface of LiCoO2 with an inert material, which could
reduce or prevent the direct contact between LiCoO2 and the electrolyte. Nano-oxide coatings on the surface of LiCoO2, including Al2O3
[94-96], TiO2 [97, 98], MgO [96, 99], ZnO [100], SnO2 [101] and
ZrO2 [97, 102], have been realized, and better stability of LixCoO2
with the electrolyte at high voltage has been observed. For example,
after coating LiCoO2 with a ~5 nm thick amorphous Al2O3 film, the
reversible capacity reaches 190 mAh g-1 over 20 cycles in the voltage range of 3.0 – 4.5 V vs Li/Li+, corresponding to ~0.7 Li atoms
utilized per formula unit instead of 0.5 Li atoms per LiCoO2 formula
unit [94]. In comparison, bare LiCoO2 shows a capacity of only 118
mAh g-1 after 20 cycles, which is just 60% of the initial capacity.
Mixed layer oxide cathodes, such as Li1+xNiyCozMnwO2, have recently become a much-studied system as they exhibit capacity higher than 180 mAh g-1 [103]. For instance, the solid solution between
Li2MnO3 (Li[Li1/3Mn2/3]O2) and LiNixCoyMnzO2 can reach a capacity of about 250 mAh g-1, corresponding to 0.8-0.9 lithium utilized
per unit formula of LiNixCoyMnzO2 [104, 105]. However, such high
24
capacity requires charging up to about 4.8 V, which corresponds to
the oxidation of O2- to neutral oxygen. This process is irreversible
during the following discharge/charge. Consequently, an irreversible
capacity loss related to the release of oxygen occurs. This capacity
loss can be as large as 50 – 80 mAh g-1, resulting in a coulomb efficiency less than 80% [104, 105]. Moreover, the release of oxygen
can also lead to safety concerns [1].
Diminution of this capacity loss has been achieved by nano-oxide
coatings. For example, Wu et al. coated the solid solution of (1-x)
Li[Li1/3Mn2/3]O2 – x Li[Ni1/3Mn1/3Co1/3]O2 (0.3 ≤ x ≤ 0.7) with a
layer of amorphous alumina less than 5 nm thick [106]. After coating, the irreversible capacity loss in the first cycle decreased markedly. In the material with x = 0.4 (Li[Li0.2Mn0.54Ni0.13Co0.13]O2), the
capacity loss was 41 mAh g-1 after coating, while a 75 mAh g-1 capacity loss was observed for the bare samples (Fig. 9). The initial
discharge capacity after coating was as high as 280 mAh g-1, which
is twice that of LiCoO2. Moreover, the capacity loss during the following cycles was reduced from 0.53 mAh g-1 to 0.34 mAh g-1 per
cycle after coating.
Fig. 9 (a) TEM image of Al2O3-coated Li(Li0.2Mn0.54Ni0.13Co0.13)O2. The light color
at the interface indicates an Al2O3 coating. (b) First charge-discharge profiles of
the layered (1-x) Li[Li1/3Mn2/3]O2 – x Li[Ni1/3Mn1/3Co1/3]O2 solid solutions before
and after surface modification with 3 wt.% nanostructured alumina [106]
25
In addition to stabilizing layered oxides, oxide coatings can also
suppress the dissolution of LiMn2O4 and LiFePO4. The main bottleneck to the commercial use of LiMn2O4 is its dissolution in organic
electrolytes, which leads to the dissolved Mn3+ ions chemically attacking the graphite anode, especially at high temperature (discussed
in the LiMn2O4 section). The trace amount of water in the electrolyte
can react with the commonly used solute LiPF6 to generate HF,
which results in an acidic environment to disproportionate Mn3+ ions
in LiMn2O4 and thus cause the dissolution of this material [107,
108]. Previous studies has shown that when temperature increases
from 25 ºC to 55 ºC, the concentration of Mn ions in the electrolyte
increases dramatically from 7.6 ppm to ~400 ppm [109]. Moreover,
many reports have shown that the capacity of LiMn2O4 fades much
faster at high temperature [29, 107]. Accordingly, surface coating
can reduce the contact between LiMn2O4 and the electrolyte and
thus the reduce dissolution of Mn. Various oxide coatings, including
Al2O3 [110, 111], TiO2 [112], ZrO2 [113], SiO2 [114], MgO [111],
ZnO [19, 115, 116], VOx [26], CeO2 [117] and LiCo1-xNixO2 [111,
118, 119], have been realized and better cycling performance has
been observed. The battery material LiFePO4 undergoes similar dissolution mechanisms; Fe ions can also be dissolved in the electrolyte
and commonly deposit on the carbon anode. Recent reports have
shown that oxide coatings, such as ZnO and TiO2, can improve the
cycling behavior of LiFePO4 at high temperature [120, 121].
26
For
example,
Han
et
al.
coated
doped
LiMn2O4
(Li1.05Al0.1Mn1.85O3.95F0.05) with ZnO using aqueous zinc acetate solution as a precursor [19]. By introducing a 2 wt % ZnO coating, the
capacity retention of LiMn2O4/graphite full cells improves significantly at high temperature (55 ºC). In bare LiMn2O4/graphite cells,
the retained capacity after 300 cycles was only about 40% of its initial capacity. Bare Li1.05Al0.1Mn1.85O3.95F0.05/graphite full cell
showed a retention of ~ 50% after 300 cycles, although bare
Li1.05Al0.1Mn1.85O3.95F0.05/lithium half cell exhibited 97 % capacity
retention after 50 cycles at 55 ºC. In comparison, In the case of 2%
ZnO-coated Li1.05Al0.1Mn1.85O3.95F0.05/graphite cells, the capacity retention increased to about 80% after 500 cycles. Furthermore, there
is little specific capacity loss resulting from the dead weight of the
added
ZnO
coating
layer.
ZnO-coated
Li1.05Al0.1Mn1.85O3.95F0.05/graphite cells also exhibited a smaller IR
drop and better performance at high power rate. For instance, in half
cell tests with lithium metal as the anode, the capacity of ZnOcoated Li1.05Al0.1Mn1.85O3.95F0.05 half cells at a 5 C rate is over 75
mAh g-1, while the capacity of bare LiMn2O4 and bare
Li1.05Al0.1Mn1.85O3.95F0.05 is less than 60 mAh g-1 for both.
Though oxide coatings can significantly enhance the stability of
cathode materials, it is worth pointing out that the coating layer can
have long-term durability issues. As a result, developing stable and
robust coatings remaining intact even under aggressive charge and
discharge conditions is the key for the success of this technique [1].
27
12.3 Nano Oxides for Electrochemical Capacitors
Various metal oxide nanostructures have also been used as another type of energy storage device, electrochemical capacitors
(ECs). Applications of ECs have expanded in recent years, most of
which utilize them as complementary energy storage devices to batteries, as they can operate at fast charge-discharge rates. Specific examples that have either been demonstrated already or which progressively will rely more on ECs in the future include hybrid electrical
vehicles (HEV), heavy duty cranes, and uninterruptible power supply (UPS) [122].
In general, ECs are divided into two classes, double layer ECs
and pseudocapacitors, depending on their charge storage mechanism[123]. Double layer ECs store charges in the electrical double
layer on the electrode surface via electrostatic interactions. Thus,
only the surfaces of electrodes contribute to capacitance, and materials with large surface areas, such as activated carbons[122, 124129], have attracted the most attention. On the other hand, the primary charge storage mechanism in pseudocapacitors involves faradaic redox reactions that control the oxidation states of host materials. Because pseudocapacitors can store charge into a certain depth
within the active materials, rather than merely on their surfaces,
pseudocapacitor specific capacitances are typically much larger than
those of double layer ECs. These higher specific capacitances have
attracted increasingly more attention to pseudocapacitors, as both
28
scientific and non-scientific communities are constantly exhibiting
higher demand for environmentally friendly energy storage devices
with large energy densities.
Pseudocapacitance has been observed mainly with conductive
polymers and metal oxides. In the pseudocapacitance process, ions
in the electrolyte intercalate into the lattices of host materials to a
certain extent and facilitate redox reactions. Pseudocapacitors are
similar to batteries in that charge storage is based on redox reactions
in the electrode materials. However, different aspects also exist: (1)
Charging and discharging in batteries take place mostly at relatively
constant voltages that are determined by the redox potentials of the
materials; in the case of pseudocapacitance, the potential between
both electrodes changes continuously during charging and discharging. (2) Most pseudocapacitors operate only in aqueous electrolytes,
whereas batteries can operate in both aqueous and organic electrolytes. This is mainly because redox reactions in pseudocapacitors
are associated with protons and other ions soluble only in the aqueous phase.
In general, metal oxide pseudocapacitors exhibit the following
characteristics: (1) Most metal oxides have reasonably good electrical conductivities due to oxygen vacancies. Nevertheless, the conductivities of some metal oxides such as manganese oxide are not
sufficiently high, so composite electrodes with conductive nanomaterials are often used. (2) Transition metals have multiple oxida-
29
tion states between which electron hopping can take place. (3) Protons can freely intercalate into oxide lattices to allow reduction of
O2- ↔ OH-. (4) The redox reaction can reversibly take place over
many cycles.
Similarly to Li ion batteries, nano-dimensional electrodes provide several important advantages in pseudocapacitor operations. In
most cases, the large surface-to-volume ratio inherent in nanostructures allows for easier ionic access into the active materials, and
thus, nanostructured pseudocapacitors show higher capacities and
better rate capabilities compared equivalent bulk-scale materials[130]. The benefits of nanostructures are more prominent for materials whose electrical or ionic conductivities are poor. The decreased dimensions provide shorter pathways for electron and ion
diffusion and therefore lead to significantly improved specific capacitances and power densities. In the case that metal oxides are integrated with conductive nanomaterials in the form of composites[122, 131-134], nanostructures allow for more uniform mixing,
and therefore, the effect of the conductive nanomaterials becomes
more remarkable. Although pseudocapacitor performance can be
improved monumentally by using nanostructured materials, some
disadvantages cannot be ignored. One of the most critical problems
is that electrode materials can be dissolved in the electrolyte due to
the large surface areas of the nanostructures which are exposed to
the electrolyte. This dissolution problem results in a limited cycle
life, which is one of the most important parameters in EC operations.
30
12.3.1 Ruthenium Oxide (RuO2)
Among a number of pseudocapacitor materials, ruthenium oxide
(RuO2) has shown the highest specific capacitances up to 800 F/g
[129]. Both hydrated and crystalline forms of RuO2 have been tested, but the hydrated form has shown the highest specific capacitances, which are about twice as high as those of the crystalline form.
Typically, hydrous RuO2 is a mixed conductor that conducts both
protons and electrons in acidic solution, while crystalline RuO2 conducts only electrons. During the charging–discharging process, the
protons and electrons are transferred between RuO2 and the electrolyte solution. Thus, it is desirable to have high conductivities for
both electrical and protonic transports. In addition, the stable potential range of RuO2, ~ 1.4 V, is relatively wider than other materials[135-138]. However, the excellent performance of the hydrated
form has been demonstrated only in highly acidic electrolytes such
as sulfuric acid. The superior specific capacitance is attributed to the
high availability of protons in strong acids because protons have better access to both the surface and interior of the electrode than other
larger alkali ions such as K+ and Na+. In fact, compared to these
larger alkali ions, proton diffusion in hydrated RuO2 is exceptionally
fast, leading to higher power densities[139]. The chemical diffusion
coefficient of hydrogen in bulk crystalline RuO2 is about 5 x 10-14
cm2/s, and thus the penetration of protons during a typical charging
process must still be shallow[140, 141]. Therefore, nanostruc-
31
tures[142] in the hydrous form are expected to improve the gravimetric capacitance because the surface area is significantly larger.
Another critical drawback of RuO2 is the scarcity of the underlying
metal, which makes it too expensive for commercialization. Due to
the cost burden, current research is focused rather on other abundant
metal oxides.
A variety of RuO2 structures have been prepared using different
methods. As mentioned above, RuO2 is prepared in either its hydrous or crystalline form. These structures are typically determined
by the oxidation condition. Preparation of the hydrous RuO2 is usually initiated from a hydrous ruthenium precursor such as hydrous
ruthenium chloride (RuCl3·xH2O). First, precursors are crushed into
fine particles and then precipitated in the form of RuOxHy. Next, the
precipitants are transformed into a hydrous RuO2 solid through annealing steps at 150 ~ 400 ˚C under oxygen flow. The oxidation processes for hydrous RuO2 can be also accomplished by electrochemical oxidation steps such as cyclovoltammetry sweeps in a sulfuric
acid solution[143]. On the other hand, crystalline RuO2 is initiated
from anhydrous precursors and transformed into the crystalline
structure by longer annealing steps[144].
As in most EC characterization processes, RuO2 EC performance is typically tested by galvanostatic, cyclovoltammetric, and
impedance measurements. Representative data from a galvanostatic
measurement is presented in Figure 10a. The areal capacitance was
32
obtained by measuring the BET (Brunauer-Emmett-Teller) surface
area. Hydrous RuO2 has shown areal capacitances as high as 260 μC
/cm2, which is about ten times larger than those of activated carbons.
This significantly increased specific capacitance verifies that charges
are stored in the bulk of electrodes through insertion reactions. Although RuO2 nanostructures have been developed using various
methods, EC performance is more directly related to whether the
structure is hydrous or crystalline. Indeed, the highest specific capacitance values (700 ~ 900 F/g) are achieved from hydrous RuO2 in
both micrometer[129] and nanometer[145] dimensions. Figure 10b
from reference 129 shows that a certain range of surface area increases does not improve the gravimetric capacitance. In addition,
the insertion depth of guest ions into host structures is often directly
correlated to the cycle life. The deep insertion of guest ions during
cycling can lead to volume changes of electrodes and thus impair the
cycle life.
Fig. 10 (a) Galvanostatic (5 mA constant current) charging-discharging curves
for RuO2· xH2O pseudocapacitors. Each electrode contains ~ 0.145 g of RuO2·
xH2O powder that was annealed at 150 ˚C. (b) Specific capacitances for RuO2·
xH2O with different BET surface areas. The specific capacitance is constant in
these BET surface area ranges. Both graphs are from reference 129
12.3.2 Manganese Oxide (MnO2)
Despite their excellent capacitances, the high material cost of
RuO2 has been a primary limitation in its development toward commercial products. To overcome the cost issue, research has been
33
driven to investigate relatively low cost materials such as manganese
oxide (MnO2) and nickel oxide (NiO). Manganese oxides especially
have been low cost useful materials in various applications including
catalysis[144, 146] and energy storage[55, 142, 147]. However, their
typically poor electrical conductivities need to be enhanced[148,
149] for EC operations.
As covered in the battery sections, manganese oxides have been
synthesized as various nanostructures, including dendritic clusters[150], nanocrystals[151-154] with different shapes, nanowires[26, 30, 155], nanotubes[156], nanobelts[157, 158], and
nanoflowers[132]. Also, various electrochemical and chemical
methods have been developed for thin film MnO2 electrodes[159,
160]. Among numerous chemical methods, the chemical reduction
of KMnO4 is one of the most well known processes to produce
MnO2 nanostructures. Kim et al. chemically reduced KMnO4 with
Mn/Ni/Pb acetate solutions[161] to generate both pure MnO2 and
Pb,Ni-mixed MnO2 nanostructures. Similarly, KMnO4 was also reduced using potassium borohydride (KBH4), sodium dithionate
(Na2S2O6), and sodium hypophosphite (NaPO2H2)[162]. MnO2
nanostructures were also prepared by simple precipitation methods.
The precipitation is enabled by mixing aqueous solutions, such as
KMnO4-MnSO4[163] and Mn(CH3COO)2-KMnO4[164]. Lee et al.
also developed a simple thermal decomposition of finely ground
KMnO4 powders at different temperatures ranging 300-1000
°C[165]. Among various MnO2 pseudocapacitors, one of the most
34
representative structures and operation data are presented in Fig.
11[157]. Similarly to RuO2 cases, as hydrothermal reaction times are
increased, the sizes of structures become smaller and thus BET surface areas increase. From a crystal structure perspective, longer hydrothermal reaction times lead to a crystal structure transition from
amorphous to crystalline and due to this transition, specific capacitances decrease from 150 to 70 F/g. SEM images in Fig. 11a show
the nanostructure evolution at different hydrothermal reaction
points. The nanostructure generated after a 6 hour hydrothermal reaction was electrochemically characterized, and a galvanostatic
curve is shown in Fig. 11b. Contrary to RuO2 pseudocapacitors,
MnO2 pseudocapacitors can also be operated in various aqueous
electrolytes beyond acid, such as sodium sulfate (Na2SO4).
Fig. 11 (a) SEM images of MnO2 nanostructures after different hydrothermal reaction times. An aqueous solution of MnSO4·H2O and KMnO4 underwent a hydrothermal reaction at 140 ˚C. (b) Galvanostatic charging-discharging data for the
sample after a 6 hour hydrothermal reaction. The current rate was 200 mA/g. (a)
and (b) are from reference [157]
12.3.3 Other Metal Oxides
Beyond RuO2 and MnO2, other metal oxides such as iridium oxide (IrO2), nickel oxide (NiO), and cobalt oxide (CoO) have also
been tested as pseudocapacitor electrode materials.
NiO has been prepared not only as crystalline nanoparicles[166]
and electrochemically deposited films[167], but also as mesoporous
35
particles[168]. In most cases, nickel hydroxide (Ni(OH)2) is made by
heating initial nickel precursors or by electrochemically inducing
precipitation. Then, Ni(OH)2 is transformed into NiO by calcination
processes typically around 300˚C. The mesoporous particles are prepared via similar procedures, but templates such as sodium dodecyl
sulfate (C12H25SO4Na) are included in initial mixtures for uniform
pore size. The template-based methods yield uniform pore sizes usually around several nanometers. The highest specific capacitance of
around 300 F/g was reported[166] from crystalline nanoparticle
electrodes. Other mesoporous particles or film electrodes have exhibited relatively lower specific capacitances of 50 ~ 100 F/g[17,
167-169].
CoO has also shown capacitive behavior when tested as pseudocapacitor electrodes. However, CoO has not been studied as much
as the aforementioned metal oxide materials. Still, CoO was prepared in the form of xerogel[170], or film[171, 172] using sol-gel
processes or electrochemical deposition. The preparation steps are
similar to those of other metal oxide nanostructures. In the sol-gel
reactions, precursors such as cobalt chloride (CoCl2) undergo a couple of reactions at increased temperatures to form cobalt hydroxide
(Co(OH)2) precipitants. The precipitants are calcinated at ~150 ˚C to
form the final CoO nanostructures. As in other metal oxide cases,
longer or higher temperature calcinations can lead to further oxidation toward crystalline Co3O4 and thus reduced specific capacitance.
Lin et al.[170] prepared xerogel electrodes that showed excellent
36
specific capacitances of ~300 F/g. In addition, cobalt has been used
to form binary metal oxides with other metal elements such as Mn
and Ni. These binary metal oxides have also been prepared through
similar processes and tested as pseudocapacitor electrodes[102, 173176]. With these binary metal oxides, specific capacitances of 50 ~
300 F/g have been demonstrated.
Iridium oxide (IrO2) has been also studied as a material for
pseudocapacitor electrodes since the 1970’s[177]. However, material scarcity and its relatively small potential range have prevented its
research from being active. Currently, the research focus is rather on
other metal oxides.
12.3.4 Hierarchical Metal Oxide-Carbon Composites
In some pseudocapacitor metal oxides, poor electrical conductivity is one of most limiting factors in the EC performance. In fact,
due to this limited conductivity, the gravimetric capacitance strongly
depends on the film thickness. In the case of MnO2 films, while thin
(10 ~ 100 nm) films exhibit very high specific capacitances of 700 ~
1300 F/g[159, 163, 178], thicker films or larger structure electrodes
show relatively lower specific capacitances of 150 ~ 250 F/g. In order to overcome the poor conductivity, various carbon nanostructures have been integrated with metal oxide nanostructures in the
form of nanocomposites. Thus, carbon nanostructures such as nanoparticles, nanotubes, nanofibers, and mesoporous materials not only
contribute to increasing the conductivity, but also function as dou-
37
ble-layer EC electrodes utilizing inherently large surface areas.
Fischer et al.[142] integrated MnO2 with porous carbon nanofoams
using electroless self-limiting deposition, but specific capacitance
(60 ~ 120 F/g) was not improved significantly. Jang et al.[143] impregnated a ruthenium precursor into mesoporous carbon and then
electrochemically oxidized the precursor to RuO2. Their specific capacitance was as high as 250 F/g even with a large mass loading. 1D
nanostructures such as carbon nanotubes[132, 179-181] and carbon
nanofibers[182] are also incorporated with metal oxide nanostructures to utilize their unique advantages of large surface areas and 1D
nanostructures to improve the film conductivities. Zhang et al. [132]
developed more organized carbon-MnO2 composites by electrochemically depositing MnO2 nanoflower structures onto aligned carbon nanotube arrays. As shown in Fig. 12a, MnO2 nanoflowers were
attached to the carbon nanotube surfaces. Their specific capacitances reached 200 F/g. Moreover, the composite structures exhibited an
excellent cycle life such that the capacitance loss after 20000 cycles
was only 3 % relative to the original (Fig. 12b). Our group also developed carbon-MnO2 composite cells[162]. But, we used cotton
textiles, instead of flat metal substrates, to reduce the overall cell
weight and also to improve the electrolyte accessibility to the electrode surface, which was driven by capillary forces within the 3D
porous structures of textile fibers. We first prepared a carbon nanotube ink and then dipped the textile into the ink. After several cycles
of a dipping and drying process, a sheet resistance as low as 1 Ω/sq
was reached. Then, MnO2 nanoflowers were electrochemically de-
38
posited onto the carbon nanotube networks. Not only was a high
specific capacitance of 230 F/g achieved, but compatibility of energy storage devices with stretchable platforms was also demonstrated.
An excellent capacity retention was demonstrated up to 40000 cycles (Fig. 12d). Lee et al.[182] first transformed RuCl3 to ruthenium
ethoxide (Ru(OC2H5))3, and then added carbon fibers to the
(Ru(OC2H5))3 solution. As this solution was annealed at 180 ˚C,
RuO2 films were formed conformally on the carbon nanofibers (Fig.
12e). They also compared their composite cells with control samples
without carbon nanofibers. The improved specific capacitances (>
1000 F/g vs. ~ 400 F/g) verified the importance of carbon nanostructures as electrically conductive pathways.
Fig. 12 (a) An SEM image of a MnO2-carbon nanotube composite electrode.
MnO2 nanoflowers are electrochemically attached onto aligned carbon nanotube
arrays. (b) Capacity retention data for the cell shown in (a). Only 3 % of the initial capacitance was lost after 20000 cycles. (c) An SEM image of MnO 2-carbon
nanotube energy textiles. MnO2 nanostructures were electrochemically attached
onto carbon nanotube networks that were integrated into cotton textiles. (d) Capacity retention data for the cell shown in (c). No visible capacitance decay was
observed after 40000 cycles. (e) An SEM image of a RuO2-carbon nanofiber
composite electrode. RuO2·xH2O is conformally coated onto carbon nanofibers.
(a), (b), and (c) are from reference 132, 162, and 182, respectively
12.4 Summary
39
This chapter reviews how nanostructured oxide materials can impact the performance of energy storage devices, such as lithium ion
batteries and electrochemical capacitors. Recent developments in
designing new materials and structures, and improvement on known
materials are discussed. Significant effects have been observed in
utilizing nanostructured materials. Higher power rate, better cycling
performance and larger capacity have been observed in many studies
for both lithium ion batteries and electrochemical capacitors.
In lithium ion batteries, since the transport lengths of both ions
and electrons are reduced significantly in nanostructured materials,
much higher power rate is achieved in nanostructured materials,
such as LiMn2O4, TiO2 and V2O5. Some inactive materials even turn
to active toward lithium by reducing size down to nanoscale, such as
-MnO2. Besides improving electrochemical properties of materials,
nanosized oxides also act as a protective layer to enhance the stability of active materials in the electrolyte.
Nanostructured metal oxides also result in improved performance
in electrochemical capacitor operations. Similarly to lithium ion batteries, nanostructures decrease electrical and ionic diffusion lengths
significantly so that power and energy densities can be improved
compared to bulk-scale electrodes. Also, nanostructures allow uniform mixing with conductive carbon nanomaterials in the metal oxide-carbon nanocomposites. These composite structures exhibit fur-
40
ther improved specific capacitances compared to the bare metal oxide cases.
In summary, nanostructured materials offer great opportunities for
the next generation energy storage devices. Novel design of composition, structures and synthetic conditions are crucial for the success
of nanostructured materials.
41
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