TiO2 Microboxes with Controlled Internal Porosity for High-Performance Lithium Storage

Article abstract of DOI:10.1002/anie.201506357

Titanium dioxide (TiO₂) is considered a promising anode material for high-power lithium ion batteries (LIBs) because of its low cost, high thermal/chemical stability, and good safety performance without solid electrolyte interface formation. However, the poor electronic conductivity and low lithium ion diffusivity of TiO₂ result in poor cyclability and lithium ion depletion at high current rates, which hinder them from practical applications. Herein we demonstrate that hierarchically structured TiO₂ microboxes with controlled internal porosity can address the aforementioned problems for high-power, long-life LIB anodes. A self-templating method for the synthesis of mesoporous microboxes was developed through Na₂EDTA-assisted ion exchange of CaTiO₃ microcubes. The resulting TiO₂ nanorods were organized into microboxes that resemble the microcube precursors. This nanostructured TiO₂ material has superior lithium storage properties with a capacity of 187 mAh g^-1 after 300 cycles at 1 C and good rate capabilities up to 20 C.

Full text of DOI:10.1002/anie.201506357

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506357
German Edition: DOI: 10.1002/ange.201506357
TiO Electrodes
2
TiO Microboxes with Controlled Internal Porosity for High-Perfor-
2
mance Lithium Storage
Xuehui Gao, Gaoran Li, Yangyang Xu, Zhanglian Hong, Chengdu Liang,* and Zhan Lin*
Abstract: Titanium dioxide (TiO ) is considered a promising
use of thick electrodes. Rapid mass transport is crucial to high
power batteries, particularly for using thick electrodes. To
2
[
3]
anode material for high-power lithium ion batteries (LIBs)
because of its low cost, high thermal/chemical stability, and
good safety performance without solid electrolyte interface
formation. However, the poor electronic conductivity and low
achieve the rapid mass transport of lithium ions, nano-
+
structuring of TiO is widely sought to improve Li diffusion
2
at the active material level. Nevertheless, the close packing of
nanomaterials leaves very limited porosity at the electrode
level to allow rapid mass transport through the interior of the
lithium ion diffusivity of TiO result in poor cyclability and
2
lithium ion depletion at high current rates, which hinder them
from practical applications. Herein we demonstrate that
hierarchically structured TiO2 microboxes with controlled
internal porosity can address the aforementioned problems
for high-power, long-life LIB anodes. A self-templating
method for the synthesis of mesoporous microboxes was
[
4]
electrode to the electrolyte. Therefore, high rate cycling of
high energy batteries will always struggle with a limited mass
transport rate between the electrode and electrolyte. A
phenomenon of lithium ion depletion occurs at the internal
[5]
porosity of practical electrodes.
developed through Na EDTA-assisted ion exchange of CaTiO3
To alleviate mass transport at the electrode level, a few
approaches have been reported to control the porosity and
tortuosity of the electrode. For example, Chiang and co-
workers used a co-extrusion method to produce porous
2
microcubes. The resulting TiO nanorods were organized into
2
microboxes that resemble the microcube precursors. This
nanostructured TiO2 material has superior lithium storage
À1
[6]
properties with a capacity of 187 mAhg after 300 cycles at 1C
electrodes by the sacrificial use of organic templates.
and good rate capabilities up to 20C.
However, only limited success has been achieved on the
control of electrode porosity because of the nature of forming
dense structures of random particles. Herein, we manipulate
the formation of electrode porosity through the morphologic
engineering of active materials. This bottom-up approach
allows the control of porosity of individual particles of active
materials. The electrode porosity is thus predefined by the
porosity and structure of the active material, instead of pore
formation through conventional methods which are defined
by the slurry coating process of the electrode after prepara-
tion treatments. Hollow microboxes of TiO2 have been
designed and synthesized to demonstrate the concept of
preformed internal porosity of active materials.
O
wing to its unique advantages of low cost, nontoxicity,
small volume expansion (< 4%), and high thermal and
chemical stability, titanium dioxide (TiO ) has attracted
great interest as an attractive substitute of the conventional
graphite anode for high-power lithium ion batteries (LIBs).
Furthermore, a stable solid electrolyte interface layer is not
2
[1]
required at the relatively high TiO working voltage (about
2
+
1
.5 V vs. Li/Li ), enabling TiO to meet safety requirements of
2
[
2]
LIB anodes for electric vehicles. However, such a high
working voltage of anode reduces the working voltage of full
cells, thus compromising the energy density of practical
batteries. One of the solutions of high energy density is the
Hollow micro/nano-structures with high surface area,
large nanoscale interiors, and good mass permeability exactly
meet the need of controlling the interior porosity of high-
[
*] X. H. Gao, G. R. Li, Y. Y. Xu, Prof. Z. Lin
Key Laboratory of Biomass Chemical Engineering of
Ministry of Education
[7]
energy electrodes. Various synthesis methodologies are well
established for mesoporous hollow structures, including the
[
8]
[9]
Ostwald ripening method, galvanic replacement, and
College of Chemical and Biological Engineering
Zhejiang University
Hangzhou 310027 (P.R. China)
[10]
templating. Generally, these methods require high temper-
atures, special conditions, and/or template removal.
[11]
All
E-mail: zhanlin@zju.edu.cn
these synthesis and processing conditions increase costs and
have limited capability for mass production. A template-free
synthesis approach of nanostructured active material is highly
desirable for practical applications. Herein, we first report
a straightforward, effective route for the synthesis of hollow
TiO microboxes through a Na EDTA-assisted ion exchange
X. H. Gao, Prof. Z. L. Hong
State Key Laboratory of Silicon Materials
School of Materials Science and Engineering
Zhejiang University
Hangzhou 310027 (P.R. China)
2
2
Dr. C. D. Liang
(
Figure 1). CaTiO was first grown into microcubes with good
3
Center for Nanophase Materials Sciences
Oak Ridge National Laboratory
Oak Ridge, TN 37831 (USA)
E-mail: liangcn@ornl.gov
uniformity using a solvothermal method, and they served as
[12]
self-sacrificial templates for TiO microboxes. Then an ion
2
exchange reaction occurred with the assistance of Na EDTA.
2
The initial ion exchange generated a thin H TiO layer on the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506357.
2
3
surface of the CaTiO , which quickly transformed into TiO -
3
2
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14331

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Communications
Figure 1. Schematic of the formation of hollow TiO microboxes.
2
coated CaTiO intermediates. A subsequent ion exchange
3
+
between CaTiO and H ions led to the formation of hollow
3
structures with mesoporous shells. The outward diffusion rate
2
+
of Ca ions is much faster than the inward diffusion rate of
+
2+
H
ions, which leads to the evacuation of Ca in the inner
[13]
region, generating a hollow interior. The whole reaction is
described in Equation (1):
Figure 2. a) XRD pattern, (b, c) FESEM, and d) TEM images of CaTiO
3
microcubes.
CaTiO
3
þ H
2
O þ Na
2
EDTA ! TiO
2
þ CaEDTA þ 2 NaOH
ð1Þ
hollow microboxes. Note that the use of a mixed solvent of
ethylene glycol (EG) and deionized (DI) water is critical for
As a result, mesoporous TiO microboxes were success-
2
fully obtained, although the conversion of complex metal
the formation of uniform TiO hollow microboxes. When the
2
[14]
oxides into simple metal oxides was reported before. The
detailed procedure is provided in the Supporting Information.
Benefiting from unique structural features, the mesoporous
synthesis was carried out in an aqueous solution, a mixture of
TiO2 and CaTiO3 was obtained (Supporting Information,
Figure S2). When the synthesis was carried out in an EG
TiO microboxes exhibit superior lithium storage properties
with a stable cycling capacity at 187 mAhg over 300 cycles
at 1C and enhanced rate capabilities up to 20C, which is
solution, the only product was CaTiO (Supporting Informa-
2
3
À1
tion, Figure S3). The formation of the mesoporous TiO2
hollow microboxes was observed only in mixed solvents of
À
among the best cycling performances of TiO anodes for LIBs
EG and DI water. EG combines with more OH and, as
2
+
2+
to date.
a result, abundant H participates in ion exchange with Ca
in CaTiO and facilitates the formation of TiO . The relatively
Uniform CaTiO microcubes were synthesized as self-
3
3
2
sacrificial templates using a facile solvothermal method
high viscosity of EG effectively slows down the reactivity and
diffusion rate of reagents and favors structural integrity
during the reaction.
(
Supporting Information). The morphological and crystallo-
[15]
graphic structures of the CaTiO microcubes are shown in
3
Figure 2. The X-ray diffraction (XRD) of CaTiO microcubes
The high crystallinity and phase purity of the resulting
material were confirmed by XRD (Supporting Information,
3
(
Figure 2a) was indexed to its orthorhombic pattern (JCPDS
card No. 22-0153) with no impurity peaks detected. The
morphology of the phase-pure CaTiO3 microcubes was
characterized by field-emission scanning electron microscopy
Figure S4), which was indexed to anatase TiO (JCPDS card
2
No. 21-1272). TiO microboxes, uniformly about 0.9 mm in
2
size, were constructed by the roughly 0.1 mm shells of self-
assembled nanorods (Figure 3a,b). Their hollow interiors and
detailed geometrical structure were further elucidated by
TEM (Figure 3c,d), which shows that they duplicated well the
(
FESEM) and transmission electron microscopy (TEM;
Figure 2b–d). A panoramic FESEM image shows the high
uniformity of these CaTiO3 microcubes with an average
length of 0.9 mm (Figure 2b). With high uniformity and
size and shape of CaTiO precursor microcubes. The solid
3
relatively large size, the CaTiO microcubes serve as tem-
CaTiO microcubes were fully converted into porous TiO
2
3
3
plates for the formation of hollow TiO microboxes through
hollow structures, in good agreement with the SEM observa-
tion. Figure 3e shows a high-resolution TEM image of the
microboxes, in which an interplanar spacing of 0.35 nm is
2
the Na EDTA-assisted ion-exchange route.The FESEM
2
image at higher magnification further reveals the microcubic
structure of CaTiO with a smooth surface (Figure 2c), and
assigned to the (101) lattice plane of anatase TiO . The
3
2
the TEM image confirms its solid texture (Figure 2d).
selected area diffraction pattern for TiO hollow microboxes
2
Using the CaTiO microcubes as self-sacrificial templates,
(Figure 3 f) indicates their high level of crystallinity, and each
3
we synthesized hollow TiO2 microboxes with mesoporous
diffraction ring is also well indexed to anatase TiO . The
2
shells through a Na EDTA-assisted ion exchange procedure
under hydrothermal conditions (Figure 1). In the absence of
chemical compositions were confirmed by energy-dispersive
x-ray spectroscopy (EDX) analysis (Figure 3g), which shows
2
Na EDTA during the reaction, the resulting product was only
only pure Ti and O signals. The self-assembled TiO nanorods
2
2
CaTiO (Supporting Information, Figure S1), which indicates
formed mesoporous structures, as confirmed by N adsorp-
3
2
the important role of Na EDTA in the formation of TiO2
tion/desorption isotherm analysis (Supporting Information,
2
1
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optimum porosity of high power and high energy LIBs.
Moreover, the structures of TiO microboxes can be designed
2
for different sizes and porosities to meet the requirements of
[12]
real LIB anodes for practical applications. It is anticipated
that the mesoporous channels on the wall of the microboxes
will improve Li-ion diffusion and ensure intimate contact
between the active material and the electrolyte for high-rate
of mass transport at the TiO anodes in LIBs.
2
TiO hollow microboxes were evaluated as anode materi-
2
als for lithium storage properties in LIBs. The electrochemical
lithium insertion properties of the microboxes were inves-
tigated in a Li/TiO half-cell configuration by cyclic voltam-
2
metry (CV) between 1 and 3 V at a slow scan rate of
À1
0
.1 mVs (Figure 4a). The cathodic/anodic peaks located at
Figure 3. (a, b) FESEM and (c–e) TEM images of hollow TiO micro-
2
boxes (ca. 0.1 mm shell) with f) corresponding SAED pattern, and
g) EDX analysis.
Figure S5). A Brunauer–Emmett–Teller (BET)-specific sur-
2
À1
face area of 30.3 m g was obtained and the sizes of the
mesopores were centered around 4 nm (Supporting Informa-
tion, Figure S5, inset). Such a value of surface area is high for
À3
TiO considering its relatively high bulk density of 3.78 gcm .
2
The microboxes contain two types of porosities: the meso-
pores that can be measured by the BET theory, and the
macropores as the hollow interior, which can be estimated
through the wall thickness and the size of the microboxes.
The importance of the internal porosity for high rate LIBs
with practical loading is illustrated as follows by comparing
the porosities of spheres, hollow spheres, cubes, and hollow
microboxes, respectively. In Figure 3a and 3b, the size of the
Figure 4. a) CV curves of hollow TiO microboxes between 3 and 1 V
2
À1
with a scan rate of 0.1 mVs . b) Cycling performance of hollow TiO
microboxes at a current density of 1C with corresponding coulombic
2
efficiency. c) Rate performance of anatase TiO microboxes at various
2
current rates from 1 to 20C.
+
1.68 and 2.06 V (vs. Li/Li ) are associated with lithium
TiO microboxes is uniformly about 0.9 mm with the 0.1 mm
insertion and extraction, respectively, in the anatase lattice,
which is consistent with previously reported data. No obvious
change in cathodic peak potential was observed after the first
cycle, demonstrating the good reversibility of the structures in
electrochemical reactions. The galvanostatic charge-discharge
2
shells of self-assembled nanorods. The porosity of the micro-
boxes is calculated as roughly 47.1%. As a comparison, the
closest theoretical packing of the cubes is zero in the porosity.
For spherical particles, the porosity of the cubic closest
[16]
packing (CCP) is approximately 25.9%. We assume that
profiles of the TiO microbox electrode with different cycles
2
the hollow sphere is also 0.9 mm in the diameter with the
(for example, cycle number 1, 2, 10, 100, and 300) were
demonstrated (Supporting Information, Figure S6). These
profiles exhibit two voltage plateaus at 1.75 V for lithium
insertion and 2.0 V for lithium extraction, respectively, which
are consistent with the CV curves. Figure 4b shows the cycling
0
.1 mm shell for comparison. The calculated porosity of the
hollow spheres is 47.1%. When taking the theoretical
porosity of the whole electrode into consideration, the
porosities for the spheres, hollow spheres, cubes, and hollow
microboxes are 25.9%, 73.0%, 0%, and 47.1%, respectively.
When taking both the energy density and power density into
consideration, a porosity of 50% of cell volume is required for
performance and the coulombic efficiency of TiO hollow
2
À1
microboxes at a current density of 1C (1C = 170 mAg ). The
À1
initial discharge capacity was 205 mAhg , indicating a cou-
[6]
high energy density of LIBs. Given the 47.1% porosity from
the hollow core and 1% additional porosity from the
mesoporous wall, the total porosity is approximately 48.1%
lombic efficiency of 89.1%. The discharge capacity then
À1
decayed to 193 mAhg in the first 10 cycles and remained
À1
very stable up to 300 cycles at 187 mAhg . The coulombic
for hollow TiO microboxes. This means that the design of the
hollow microboxes matches well with the requirements for
efficiency remained nearly 100% after the first few cycles,
which demonstrate the high reversibility of TiO2 hollow
2
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microboxes for lithium storage during cycling. The FESEM
microboxes (0.15 mm shell) and TiO microboxes (crashed),
2
images of the TiO microbox electrode after 300 cycles show
respectively, which were utilized for comparisons in cycling
performance (Supporting Information, Figure S10). TiO2
2
there are no obvious morphological changes in the electrode,
although there are some inevitable minor structural defects in
microboxes during cycling (Supporting Information, Fig-
ure S7). As a result, the stability in the microbox structure
and the integrity in the electrode are responsible for the stable
performance for lithium storage in long-term cycling. TiO2
microbox electrodes were also investigated for rate capability
microboxes (0.15 mm shell) and TiO microboxes (crashed)
2
À1
delivered discharge capacity of 166 and 116 mAhg after 100
cycles, respectively, which is much lower than that of TiO
microboxes with 0.1 mm shell (197 mAhg ). Their rate
2
À1
capabilities were also much poorer than that of TiO micro-
2
boxes with 0.1 mm shell (Supporting Information, Figure S11).
It is worth noting that not only the lower internal porosity but
also structural instability are responsible for the worse cycling
(
Figure 4c). They showed high capacities at different current
À1
rates; even at a very high current rate of 20C (3400 mAg ),
a reversible capacity of 63 mAhg was still delivered. When
the current was cycled back to 1C, a capacity of 190 mAhg
À1
performance of TiO microboxes (crashed).
2
À1
In summary, we demonstrated the concept of predefined
internal porosity through morphological engineering of each
individual particle of TiO2 as high performance anode
materials for LIBs. Comparing with previous micro/nano-
structured metal oxides (Supporting Information, Table S2),
the synthesis of hollow TiO2 microbox materials by self-
sacrificial templates offers practical value for low-cost mass
production of electrode materials. A hierarchically porous
was resumed, indicating outstanding high-rate cycling perfor-
mance resulting from their good structural stability. The
cycling stability of TiO microbox electrodes at high current
2
rates of 10 and 20C was also shown (Supporting Information,
Figure S8). There were almost no obvious discharge capacity
decays up to 1000 cycles, which further confirm excellent
structural stability of TiO microboxes for long-term cycling
2
even at high rate capabilities. The high cycling stability of
structure of TiO microboxes with controlled internal porosity
2
TiO hollow microboxes is superior to that of other anatase
meets the optimum porosity of LIB electrodes with high
2
TiO micro/nanostructures, and the high rate performance is
power and high energy. The hollow TiO microbox anodes
2
2
among the best demonstrated (Supporting Information,
showed significant improvement of lithium storage, compared
with other TiO2 meso/microporous structures, with stable
cycling capacity at 187 mAhg over 300 cycles at 1C and
Table S1). These findings indicate TiO hollow microboxes
2
À1
are capable of stable electrochemical cycling with high power
density.
enhanced rate capability up to 20C. This concept presents the
battery field a new approach for the fine tuning of electrode
internal porosity. The uniform hollow structures might be
model systems for basic research of the mass transport
phenomenon in porous electrodes and a broad scope of
applications in which mass transport is a key parameter, for
Clearly, the outstanding electrochemical performance of
TiO electrodes originates from the porosity control through
2
hollow structure and the advantages of microbox structures.
The hollow microboxes are composed of small nanorods,
between which finite mesoporous shells enable easy diffusion
+
[17]
of Li ions by shortening transport length. The high surface
examples, catalysis, sensor, and separation.
area provides more active sites and a large electrolyte-
+
electrode contact area for Li insertion and surface Li storage.
The robust, micro-size shell structure effectively prevents the Acknowledgements
undesirable aggregation of conventional nanoparticles, which
ensures the integrity of the electrode and improves capacity
retention upon prolonged cycling. Finally, the large porosity
in the hollow structure provides enough space to accommo-
Z. Lin thanks the funding support from Chinese government
under the “Thousand Youth Talents Program”. C. Liang’s
effort was sponsored by the Division of Materials sciences and
engineering, Office of Basic Energy Sciences U.S. Depart-
ment of Energy (DOE). A portion of the electrochemical
analysis was conducted at the Center for Nanophase Materi-
als Sciences, which is a DOE Office of Science User Facility.
+
date a liquid electrolyte, which supplies the necessary Li ions
+
during cycling at high current rates to avoid Li depletion.
These features ensure TiO anodes with high specific capacity,
2
a long cycle life, and promote fast, reversible Li insertion and
extraction at high current rates.
To confirm the importance of controlled internal porosity
for high-performance lithium storage, we intentionally
Keywords: battery · hydrothermal · lithium ion · microbox · TiO2
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14331–14335
Angew. Chem. 2015, 127, 14539–14543
designed TiO microboxes with different internal porosities.
2
As discussed above, we synthesized TiO microboxes with
2
0
4
.1 mm shells and the internal porosity was calculated as
7.1%, when volume ratio of EG to DI water is 1:3. The
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