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Energy Storage
Ternary Sn–Ti–O Based Nanostructures as Anodes for
Lithium Ion Batteries
Hongkang Wang,* He Huang, Chunming Niu, and Andrey L. Rogach
From the Contents
1. Introduction ........................................1365
2. TiO2–SnOx (x = 0, 1, 2) Nanocomposites as
Anode Materials ..................................1366
3. Nanoscale SnxTi1-xO2 (0 < x < 1) Solid
Solutions .............................................1378
4. Conclusions and Outlook .....................1380
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S
nOx (x = 0, 1, 2) and TiO2 are widely considered to be potential
anode candidates for next generation lithium ion batteries. In terms
of the lithium storage mechanisms, TiO2 anodes operate on the
base of the Li ion intercalation–deintercalation, and they typically
display long cycling life and high rate capability, arising from the
negligible cell volume change during the discharge–charge process,
while their performance is limited by low specific capacity and low
electronic conductivity. SnOx anodes rely on the alloying–dealloying
reaction with Li ions, and typically exhibit large specific capacity
but poor cycling performance, originating from the extremely large
volume change and thus the resultant pulverization problems.
Making use of their advantages and minimizing the disadvantages,
numerous strategies have been developed in the recent years to design
composite nanostructured Sn–Ti–O ternary systems. This Review
aims to provide rational understanding on their design and the
improvement of electrochemical properties of such systems, including
SnOx–TiO2 nanocomposites mixing at nanoscale and nanostructured
SnxTi1-xO2 solid solutions doped at the atomic level, as well as their
combinations with carbon-based nanomaterials.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1. Introduction
Lithium ion batteries (LIBs) have received considerable
attention in the last decades, owing to their attractive features such as high energy density and low self-discharge,
and have become the dominant power source for portable
electronics and recently for electric vehicles.[1,2] In order
to satisfy the requirements of high-energy and high-power
applications of LIBs, fundamental improvements on the
electrochemically active electrode materials are needed
in order to achieve higher specific capacity, energy and
power density, longer cycle life, lower cost, and improved
safety.[3] In commercial LIBs, the cathode materials are
usually mixed metal oxides or phosphates such as LiCoO2,
LiMnO2, or LiFePO4, while the anode is graphite. These
cathode and anode materials can react with Li ions via an
intercalation–deintercalation mechanism: Li ions or atoms
reversibly insert into and extract from the interstitial sites
within the host lattices, which result in small strain and
minimal structural changes and thus ensure long LIB cycle
life. However, the theoretical specific capacity of graphite is
relatively low (≈370 mA h g−1) due to the limited interstitial sites, which constrains its further applications in electric
vehicles.[4] Based on the alloying–dealloying mechanism,
tin-based nanomaterials such as SnO2, SnO, and metallic
Sn have been widely considered as potential anode materials for LIBs, due to their high theoretical specific storage
capacity (SnO2: ≈790 mA h g−1;[5] SnO: ≈875 mA h g−1;[6]
Sn: ≈990 mA h g−1[7] as compared with that of commercial
graphite.[1,5–9]
SnO 2 + 4Li + +4e − → Sn + 2Li 2 O
(1)
SnO + 2Li + +2e − → Sn + Li 2 O
(2)
Sn+xLi + + xe − → Li x Sn(0 ≤ x ≤ 4.4)
(3)
Equations 1, represent the irreversible reduction of
SnO2 and SnO into metallic Sn in the first discharge process, during which SnO2 and SnO undergo crystal structure
destruction (amorphization) and formation of metallic Sn
dispersed in the amorphous Li2O matrix. Such irreversible
reactions often result in the capacity loss in the first discharge–charge cycle for Sn-based electrodes. Equation illustrates the reversible alloying–dealloying process between
Sn and Li which happens at lower voltages (V ≤ 1 V vs Li+/
Li), showing that one unit of Sn can store upto 4.4 units of
Li, which determines the high Li storage capacities of Snbased materials. However, the reversible Li-Sn alloying–
dealloying process also involves large volume changes (as
high as 300%), which eventually leads to the electrode disintegration (namely, electrochemical pulverization) and
fast capacity fading upon long-term cycling.[8,10,11] In order
to overcome this drawback, strategies have been developed
towards the size, morphology and composition control of
the tin-based materials[12–17] or their nanoscale mixing with
carbon-based materials[18–24] and structurally stable materials such as TiO2.[4,25–28]
small 2015, 11, No. 12, 1364–1383
Other potential anode materials for LIBs, TiO2 with
anatase, rutile or TiO2(B) polymorphs have received considerable attention because of its long cycle life, low cost and
eco-friendliness. The lithium storage and cycling performance
of TiO2 anodes can be described by the reaction on the basis
of an intercalation–deintercalation mechanism. The lithium
insertion/extraction happens at higher voltages (>1.5 V vs
Li+/Li):[1]
TiO 2 + xLi + + xe − → Li x TiO 2 ( x ≤ 1)
(4)
Generally, TiO2 with small particle size, high surface area
and porous structures can deliver stable and near theoretical
capacity of 335 mA h g−1, according to Equation 4.[1] However, several studies have confirmed that ≈0.5 Li is found to
be cyclable without significant capacity fading, corresponding
to a theoretical specific capacity of ≈170 mA h g−1.[1,29] Moreover, the Li-rich Li0.55(TiO2) phase with an orthorhombic
structure only exhibits a small net increase in the unit cell
volume by ≈4% as compared to the pristine anatase TiO2,
which contributes to the superior rate capability and long
cycle life of TiO2 anodes but results in the lower theoretical
capacity.
Considering that SnOx-based and TiO2 based materials
possess complementary characteristics as anode materials
in LIBs, development of hybrid Sn–Ti–O systems through
a variety of approaches is important in order to improve
their electrochemical performance by combining their
merits,[11,30,31] so that Sn-based materials contribute to the
high capacity, while TiO2 accommodates the volume expansion and maintains the structural integrity of the electrode.
In this respect, development of the proper synthetic strategies is crucial to realize the desired property control. For the
hybrid Sn–Ti–O-based structures, different approaches have
been adopted, including wet chemical synthesis such as solvo/
hydrothermal, sol–gel, co-precipitation and electrospinning
methods,[4,32,33] and chemical vapor deposition such as atomic
layer deposition.[34] Solvo/hydrothermal synthesis under
high temperature and pressure has been the most widely
used so far, owing to the exceptional versatility of the morphologies of the resulting nano/microstructures, which can
be conveniently achieved by introducing growth-controlling
Dr. H. Wang, Prof. C. M. Niu
Center of Nanomaterials for Renewable Energy (CNRE)
State Key Lab of Electrical Insulation
and Power Equipment
School of Electrical Engineering
Xi’an Jiaotong University
Xi’an, China
E-mail: hongkang.wang@mail.xjtu.edu.cn
H. Huang, Prof. A. L. Rogach
Department of Physics and Materials Science & Centre
for Functional Photonics (CFP)
City University of Hong Kong
Hong Kong S.A.R
DOI: 10.1002/smll.201402682
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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systems.[35]
agents into the reaction
Sol–gel method has been
a traditional approach in fabrication of metal oxide nanoparticles; combined with the electrospinning this technique
particular facilitates the fabrication of 1D nanofibers with
readily tunable compositions.[32]
This Review aims to provide a comprehensive summary
on the ternary Sn–Ti–O systems mixed or doped at the
nanoscale or atomic levels, which makes it different from
previous review articles on pure phase SnO2- or TiO2-based
anodes.[1,2,36,37] In Section 2, fabrication and lithium storage
properties of TiO2–SnOx (x = 0, 1, 2) nanocomposites will be
surveyed, where TiO2 and Sn-based components including
SnO2, SnO and metallic Sn are combined at the nanoscale
while retaining their separately independent phases. In Section 3, nanoscale solid solutions of the general composition
TixSn1-xO2 (0 < x < 1), including Sn-doped TiO2 and Ti-doped
SnO2 nanostructures will be discussed. In the both sections,
Sn–Ti–O systems further modified with different carbonaceous nanomaterials including amorphous carbon, carbon
nanotubes and graphene will also be surveyed. This Review
intends to provide rational understanding on the design principles of these nanomaterials and their electrochemical properties associated with their different architectures which are
important for their use as anodes in LIBs. Table 1 summarizes the representative Sn–Ti–O based anode nanomaterials
which can be found in literature nowadays from the point of
view of their structural characteristics and electrochemical
properties.
2. TiO2–SnOx (x = 0, 1, 2) Nanocomposites as
Anode Materials
The pulverization problem of the tin-based anode materials during cycling is a major concern, which deteriorates
the electrical conductivity of the electrode and causes the
rapid capacity fading.[11] One approach to avoid these problems is to incorporate tin compounds into structurally stable
matrixes, especially electrochemically active anode materials
such as TiO2, which can not only contribute to the lithium
storage capacity, but also improve the structural stability of
the composite electrodes. For this purpose, Sn-based compounds such as SnO2, SnO, and metallic Sn have been combined with TiO2, in order to develop high performance anode
materials for lithium ion batteries, such as summarized in this
section. TiO2 anodes with high crystallinity are most preferable for Li intercalation–deintercalation reaction; among the
different possible polymorphs of TiO2, the crystalline TiO2
with anatase phase is usually referred to in this section if not
otherwise specified.
2.1. TiO2–SnO2 Based Anodes
Among the three common Sn-based anode materials, SnO2
has received the most considerable investigation, due to its
easier fabrication as compared to metallic Sn. In the following, we introduce three kinds of SnO2 based anodes:
i) SnO2–TiO2 core–shell nanostructures, ii) TiO2–SnO2
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Hongkang Wang is an Assistant Professor at
the Center of Nanomaterials for Renewable
Energy in the School of Electrical
Engineering at Xi'An Jiaotong University,
China. He received his B.S. degree in
Materials Science from Henan University
of Science and Technology in 2007, M.S.
degree in Materials Science from the East
China University of Science and Technology
in 2010, and Ph.D. degree in Materials
Science from City University of Hong
Kong in 2013. His research topics are the
design and characterization of metal oxide
nanostructures and their energy conversion
and storage applications (ex. lithium ion batteries, solar cells and so on).
core–shell nanostructures, and iii) combinations of SnO2TiO2 with carbon based nanomaterials.
2.1.1. SnO2–TiO2 Core–Shell Nanostructures
Due to the negligible volumetric change of TiO2 in the
course of Li-intercalation (≈4%), rigid TiO2 shells coated
on SnO2 cores can maintain the mechanical stability of the
composite electrodes during cycling. Further rational design
of such core–shell structures is vital to achieve their good
electrochemical performance. In particular the combination of TiO2 shells and hollow inner space helps to confine
the volume expansion of encapsulated SnO2 nanoparticles
during cycling. Therefore, both morphological and compositional engineering of active materials enable us to improve
their structural stability and electrochemical characteristics. 1D and 3D SnO2–TiO2 core–shell micro/nanostructures
with either hollow or solid interiors have been reported
recently,[26,34,38–41] as introduced in details below.
Tian et al. developed a structure of SnO2 nanoparticles
encapsulated into hollow TiO2 nanowires (denoted as
SnO2@TiO2).[40] As schematically illustrated in Figure 1A,
SnO2 nanoparticles were firstly prepared by hydrothermally
treating tin dichloride dehydrate in the mixed solvent of distilled water and ethanol (volume ratio = 1:1) with the presence of ammonium hydroxide, followed by centrifugation,
washing, drying and calcination in air. Then carbon nanowires
with embedded SnO2 nanoparticles were prepared via in
situ polymerization and subsequent carbonization of pyrrole
by calcination under argon. Finally, hollow TiO2 nanowires
were prepared by hydrolysis and condensation of tetrabutyl
titanate on SnO2/C nanowire templates, followed by calcination in air. Such hollow TiO2 nanowires encapsulating SnO2
nanoparticles exhibited good lithium storage performance
and excellent cyclability, delivering a high reversible capacity
of 445 mA h g −1 at 800 mA g −1 after 500 cycles in the
voltage range between 0.01 and 3 V (vs Li/Li+) (Figure 1B).
TEM images showed that the curved wires-like 1D structure
of the SnO2@TiO2 composite was still preserved after 200
cycles at 800 mA/g (Figure 1C vs D), but the primary distinct
void space between the shell and cores shrank due to the
volume expansion of SnO2 during cycling. Encapsulation of
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1. Structural characteristics and electrochemical properties of Sn–Ti–O based anode nanomaterials.
Sn–Ti–O based systems
Elemental
ratio
Particle size/Surface
area
Current rate
Voltage
range
Initial
Coulombic
efficiency
Reversible capacity
after n cycles
Year
encapsulation of SnO2 nanoparticles Sn/Ti ≈ 86
at%
into hollow TiO2 nanowires
SnO2: ≈15 nm, TiO2:
≈200 nm
800 mA g−1
0.01–3 V
≈46.8%
445 mA h g−1 (n = 500)
2014[88]
SnO2 encapsulated TiO2 hollow
nanofibers
Sn/Ti ≈ 33
at%
diameter of ≈200 nm,
shell thickness of
≈30 nm
0.2 C
(not stated)
0.01–3 V
≈64.5%
517 mA h g−1 (n = 100)
2012[89]
SnO2@TiO2 double-shell nanotubes
Sn/Ti ≈ 48
at%
thicknesses of 12 nn
(SnO2) and 22 nm
(TiO2)
800 (1500)
mA g−1
0.01–3 V
≈40% (/)
300 (200) mA h g−1
(n = 50)
2013[34]
TiO2-coated SnO2 hollow spheres
Sn/Ti ≈ 32
at%
hollow sphere diameter of 320 nm,
30.4 m2 g−1
0.04 mA
cm−2
0.01–2.5 V
≈44%
220 mA h g−1 (n = 23)
2011[41]
SnO2@TiO2 hollow spheres
Sn/Ti ≈ 3
at%
diameter of 1000 nm
10 C (not
stated)
1–3 V
/
125 mA h g−1 (n = 100)
2010[38]
SnO2@TiO2 core–shell composites
Sn/Ti ≈ 57
at%
60–80 nm
3000 mA g−1
0.005–2 V
≈45%
380 mA h g−1 (n = 30)
2013[26]
≈30 wt%
SnO2
wire diameter of
30−120 nm
30 mA g−1
0.01–3 V
∼58.5%
463 mA h g−1 (n = 50)
2011[47]
TiO2-supported-SnO2
Sn/Ti = 99
at%
≈10 nm, 100 m2 g−1
0.2 C (not
stated)
0.05−1V
<30%
320 mA h g−1 (n = 100)
2012[28]
TiO2 nanocones@SnO2
nanoparticles
Sn/Ti =
14.5 at%
91.6 m2 g−1
0.5 C (1C =
167 mA g−1)
0.05–2.5 V
Nearly
100%
294 mA h g−1
(n = 100)
2012[45]
SnO2 nanoflakes on TiO2 nanotubes
(array)
≈20 wt%
TiO2
tube diameter of
90 nm, thickness of
20 nm
1600 mA g−1
1 mV-2.5 V
≈70%
580 mA h g−1
(n = 50)
2014[46]
mesoporous C–TiO2–SnO2
nanocomposites
19.9 wt%
TiO2
281 m2 g−1
0.1 C
(not stated)
0–2.5 V
≈43%
275 mA h g−1 (n = 40)
2012[59]
graphene–TiO2–SnO2
nanocomposites
TiO2:SnO2
= 9:81 wt%
particle size of ≈5 nm
50 mA g−1
0.01–3 V
≈49%
537 mA h g−1 (n = 50)
2013[57]
mesoporous graphene-based TiO2/
SnO2 hybrid nanosheets
TiO2:SnO2:
graphene
= 1:3:0.16
wt%
size of ≈3 nm,
138 m2 g−1
160 mA g−1
0.02–3 V
≈49%
600 mA h g−1 (n = 300)
2013[56]
diameters of 80−
150 nm and length of
a few µm to mm
30 mA g−1
0.01–3 V
≈70%
442.8 mA h g−1 (n = 100)
2013[58]
5 nm size Sn and 3 nm
size rutile TiO2
100 mA g−1
0–2.5 V
≈77%
610 mA h g−1 (n = 100)
2009[90]
mesoporous TiO2-Sn@C core–shell
microspheres
166.6 m2 g−1
500 mA g−1
0.01–2.5 V
≈48%
206.2 mA h g−1 (n = 2000)
2013[4]
mesoporous TiO2–Sn/C core–shell
nanowire arrays
coated carbon shell
thickness of 4 nm,
45.2 m2 g−1
335 mA g−1
3350 mA g−1
0.01–3 V
≈65%/
459 mA h g−1 (n = 160)
160 mA h g−1 (n = 100)
2014[25]
Diameters of 12 nm,
210 m2 g−1
4000 mA g−1
1–2.5 V
/
176 mA h g−1 (n = 60)
2008[68]
SnO2 nanocrystals on self-organized
TiO2 nanotube arrays
SnO2 less than 5 nm
20 µA cm−2
0.05–2.5 V
30 µA h cm−2 (n = 100)
2010[27]
SnO2@TiO2 nanotube arrays
thickness of ≈1.8 µm
100 µA cm−2
0–2.5 V
≈42%
113 µA h cm−2 (n = 50)
2012[43]
pore size of 40–70 nm
200 µA cm−2
0.005–2.5 V
∼64%
142 µA h cm−2 (n = 50)
2013[44]
thickness of 1.6 µm
100 µA cm−2
0.01−1.2 V
≈40%
162 µA h cm−2 (n = 50)
2009[64]
0.01−1.2 V
≈40%
cm−2
2010[61]
0–2.7 V
/
SnO2 nanocrystals on TiO2(B)
nanowires
TiO2/SnO2/carbon hybrid nanofibers
nanostructured Sn/TiO2/C
composites
TiO2@Sn core–shell nanotubes
Sn:TiO2:C
= 45:15:40
wt%
20 wt% Sn
SnO2@C-doped TiO2 nanotube
arrays
SnO on nanotubular titania matrix
SnO nanorod on TiO2 nanotubes
Sn/TiO2 nanowire array composites
(annealed)
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Sn/TiO2 ≈
22 wt%
cm−2
thickness of 2 µm
100 µA
Sn particle size of
≈8 nm
150 mA cm−3
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
140 µA h
(n = 50)
1006 mA h cm−3
(n = 300)
2013[67]
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Table 1. Continued
Sn–Ti–O based systems
Elemental
ratio
Particle size/Surface
area
Current rate
Voltage
range
Initial
Coulombic
efficiency
Reversible capacity
after n cycles
Year
lengths of ≈1.2 µm
and diameters of
≈90 nm
70 µA cm−2
0.05–0.6 V
≈45%
43 µA h cm−2
(n = 50)
2012[91]
Sn/Ti ≈ 100
at%
thickness of 1.2 µm
70 µA cm−2
1–2.6 V
≈50%
51.6 mA h cm−2 µm−1
(n = 50)
2014[29]
2013[85]
mesoporous Sn-doped TiO2 thin
films
Sn/Ti ≈ 6
at%
diameter of ≈10
nm/68.2 m2 g−1
84 mA g−1
0.01–3 V
≈45%
252.5 mA h g−1
(n = 80)
2013[92]
(Sn-Ti)O2 solid solution
nanoparticles
Sn/Ti ≈ 10
at%
width of 10 nm and
length of 80 nm
30 mA g−1
0.1–3 V
≈47%
217 mA h g−1
(n = 50)
2013[76,77]
Ti2/3Sn1/3O2 nanocrystallites
Sn/Ti ≈ 50
at%
≈5 nm
C/20 (not
stated)
0.02–3 V
≈55%
300 mA h g−1
(n = 100)
2011[33]
nanoparticulate Sn-doped TiO2
Sn/Ti ≈ 27
at%
10–30 nm
250 mA g−1
0.01–2.5 V
≈43%
300 mA h g−1
(n = 300)
2014[79]
Ti(IV)/Sn(II) co-doped SnO2
Nanosheets
Ti/Sn ≈ 17
at%
thickness of ≈20 nm
250 mA g−1
0.01–2 V
≈45%
319 mA h g−1
(n = 35)
2013[17]
Ti-doped SnOx encapsulated in
carbon nanofibers (CNFs)
Ti/Sn ≈ 10
at%
2–4 nm Ti-doped
SnOx, ≈300 nm in
diameter for CNFs
200 mA g−1
0.005–3 V
≈77%
670.7 mA h g−1
(n = 60)
2014[87]
Sn-doped TiO2 nanotubes
Ti1/2Sn1/2O2 nanotube arrays
SnO2 nanoparticles into highly crystalline hollow TiO2 nanostructures was useful for accommodating the large volume
expansion of SnO2 and maintaining the structural integrity,
resulting in a stable cycling and improved Li storage capacity.
Electrospinning is a simple and versatile method for generating nanofibers with tunable composition and diameters
(10–1000 nm) under the influence of an electrostatic field.
The composition can be readily controlled by adjusting the
precursor solutions and design of the spinneret (such as a
dual syringe system in a side-by-side or coaxial fashion).[42]
Tran et al. prepared three types of SnO2/TiO2 composite
nanofibers (homogeneous SnO2/TiO2, heterogeneous SnO2/
TiO2 and SnO2 nanoparticles/TiO2) via the electrospinning
method combined with a sol–gel chemistry.[32] Park et al.
made TiO2 hollow nanofibers with encapsulated SnO2 nanoparticles by coaxial electrospinning method,[39] with the Sn-
Figure 1. A) Schematic illustration of the fabrication process of SnO2@TiO2 composite. B) Comparative cycling performance of SnO2@TiO2
composite and SnO2 nanoparticles (NPs) at 800 mA g−1 between 0.01 V and 3.0 V. C,D) Transmission electron microscopy (TEM) images of SnO2@
TiO2 composite: as-prepared (C) and after 200 cycles at 800 mA g−1 (D). Reproduced with permission.[40] Copyright 2014, Elsevier.
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Figure 2. A) Schematic cartoons illustrating morphological changes during the lithiation/delithiation and B–D) volumetric change of SnO2/TiO2
hollow nanofibers with different molar ratios of Sn to Ti during the lithiation process. Reproduced with permission.[39] Copyright 2012, Elsevier.
and the Ti-precursor solutions separately prepared in advance
and then loaded into syringes connected to a metallic needle
with dual nozzle. The molar ratio of Sn to Ti could be controlled by adjusting the feeding rates of the inner and outer
solutions. As shown in Figure 2, low Sn content was not
enough to reach the maximum capacity, while high Sn content resulted in a significant capacity increase but large
amounts of SnO2 in the hollow space led to fast capacity
fading since the hollow inner space could not accommodate
the volume expansion of SnO2. The composite with SnO2
encapsulated into TiO2 hollow nanofibers demonstrated a
high discharge capacity of ∼517 mA h g−1 after 100 cycles at
0.2 C in the voltage range of 0.01–3 V as compared with the
TiO2 hollow nanofibers. Jeun et al. reported fabrication of
SnO2@TiO2 double-shell nanotubes with tunable shell thickness by applying atomic layer deposition, using electrospun
polyacrylonitrile nanofibers as templates.[34] Figure 3A shows
the schematic fabrication process of the SnO2@TiO2 doubleshell nanotubes, where TiO2 outer shell encapsulated the SnO2
inner nanotube (Figure 3B). The SnO2@TiO2 double-shell
nanotubes exhibited reversible capacities of 300 and
200 mA h g−1 after 50 cycles at the current densities of
800 and 1500 mA g−1 in the voltage range of 0.01–3.0 V
(Figure 3C).
3D spherical SnO2–TiO2 core–shell hollow structures
were prepared as well via template-based approaches. Yi et
al. reported the synthesis of TiO2-coated SnO2 hollow spheres
using SiO2 spheres as templates (Figure 3D).[41] Uniform
SnO2 shells were firstly deposited on the SiO2 shells through
the hydrolysis and polycrystallization of Na2SnO3, followed
by removal of SiO2 by NaOH etching. TiO2 outer layers were
then deposited via hydrolysis of titanium (IV) butoxide with
small 2015, 11, No. 12, 1364–1383
the assistance of sodium dodecyl sulfate, followed by calcination. The TiO2-coated SnO2 hollow spheres (Figure 3E) demonstrated a discharge capacity of 220 mA h g−1, about two
times as much as that of SnO2 hollow spheres (113 mA h g−1)
after 23 cycles, showing improved electrochemical performance after applying TiO2 coating. Chen et al. reported a
template synthesis of SnO2@TiO2 hollow spheres assembled from ultrathin anatase TiO2 nanosheets with exposed
high-energy (001) facets, using 500 nm SiO2@SnO2 core–
shell spheres as templates, where SiO2 was removed by HF
etching.[38] It was suggested that SnO2 shells can better sustain the template-removal process, giving additional mechanical support to the SnO2@TiO2 hollow architecture, which
resulted in good rate performance with a reversible capacity
of 125 mA h g −1 after 100 cycles at the current rate of 10 C in
the voltage range of 1–3 V.
Ji et al. designed nanostructured SnO2@TiO2 core–shell
composites, where the thin crystalline TiO2 has been converted into conductive LiyTi1-yO2 shell on a core aggregated from SnO2 nanoparticles, producing reversible Li-ion
storage hosts furnished with their own current collectors
(Figure 4A,B).[26] Electrodes based on these SnO2@TiO2
core–shell composites exhibited high electrical conductivity
even after expansion and contraction of the active material
during cycling. Figure 4C-D demonstrates a uniform TiO2
coating layer on the core SnO2 aggregated nanoparticles.
Electrochemical impendence spectroscopy revealed that
the SnO2@TiO2 core–shell composite without any conductive additive showed smaller charge transfer resistance
than that of the pristine SnO2 mixed with 10 wt% carbon
conductive additive (Figure 4E,F). It delivered a charge (delithiation) capacity of 735 mA h g −1 at 1 A g −1 in the first
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Figure 3. A) Schematic diagram of the fabrication procedures and Li-ion insertion/extraction in SnO2@TiO2 double-shell nanotubes. B) High
and low (inset) magnification TEM images of SnO2@TiO2 double-shell nanotubes. C) Cyclability of SnO2 and SnO2@TiO2 nanotube electrodes
at 800 and 1500 mA g−1, respectively. D) Preparation process of the TiO2-coated SnO2 hollow spheres and () their TEM image. Reproduced with
permission.[34,41] Copyright 2013, Royal Society of Chemistry; Copyright 2011, Springer.
cycle with 69% capacity retention after 30 cycles without any
conductive additives in the voltage range of 0.005–2.0 V. By
increasing the current density to 3 A g−1, the charge capacity
was 649 mA h g−1 in the first cycle with 58% capacity retention after 30 cycles, indicating the efficacy in minimizing the
capacity loss in cycling at high current densities.
Well-aligned TiO2 nanotube arrays (TNTs) grown on
Ti foils have attracted considerable attention in the energy
conversion and storage application areas, owing to their high
surface-to-volume ratio, excellent electrochemical activity,
and good contact with the current collector. However, further
improvements of both electronic conductivity and specific
capacity of TNTs are still desirable, which can be achieved by
filling their interior region with high capacity materials (such
as SnO2), as schematically illustrated in Figure 5A. Such a
configuration with SnO2 embedded in the space-confined
tubular structures of TNTs can enhance the capacity, while
the tubular TiO2 shell can accommodate the volume expansion and maintain the structural integrity of the electrode.
Wu et al. developed two strategies for the synthesis of
coaxial SnO2@TiO2 nanotubes, by utilizing either electrochemical or solvothermal methodology.[43] TNTs were prepared by anodization of Ti foils in the electrolyte of ethylene
glycol/NH4F/H2O and crystalized by annealing under air at
450°C. Electrochemical fabrication of SnO2-core layers were
performed in the two-electrode system (the annealed TNTs
as working electrode and Pt as counter electrode) in the electrolyte consisting of SnCl2·2H2O and Na3C6H5O7·2H2O, so
that metallic Sn layers were formed first and then converted
into SnO2 by annealing (Figure 5B,C). The second strategy is
the solvothermal treatment of TNTs in SnCl2·2H2O-alcohol
solution (Figure 5D,E). Du et al. also reported the deposition
of SnO2 nanocrystals (less than 5 nm in size) within TNTs,
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which has been realized by solvothermally treating TNTs
in SnCl2·2H2O-ethanol solution, with the loading amount
of SnO2 controlled by varying the reaction time.[27] Meng
et al. prepared TNTs with improved electrical conductivity by
annealing the amorphous TiO2 nanotubes in CO atmosphere
instead of air, which resulted in C-doping.[44] SnO2 on C-TiO2
NTs was grown by the hydrothermal method, performed in
SnCl4·5H2O-NaOH aqueous solution. These SnO2-in-TNTs
nanostructures demonstrated remarkably enhanced Li+ ion
storage performance, originating from the synergistic effects
of improved conductivity and lithium storage by both components, highlighting their potential for LIBs.
2.1.2. TiO2–SnO2 Core–Shell Nanostructures
Instead of using TiO2 shells as protective layers to confine
the volume expansion of SnO2 during the Li-Sn alloying process and maintain the structural integrity of the electrodes,
TiO2 was also used as a supporting matrix for growth and
deposition of SnO2 nanostructures.[28,45–47] The TiO2 (core)–
SnO2 (shell) composite architectures have been thus developed to improve the Li+ storage capacity and cyclability of
SnO2-based nanomaterials making use of both buffering and
mechanical support of TiO2 cores.
Zhu et al. reported the deposition of SnO2 nanoflake
branches onto robust TiO2 nanotube stems (Figure 6A).[46]
Self-supported Co2(OH)2CO3 nanorod arrays on Ni foam
were firstly prepared by hydrothermal method, serving as
sacrificial templates for the atomic layer deposition of TiO2
coaxial layers. They provided a scaffold for the hydrothermal
growth of SnO2 nanoflakes, with the resulting core-branch
nanostructured electrodes demonstrating a high capacity of
580 mA h g−1 at a current density of 1.6 A g−1 in the voltage
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Figure 4. A,B) Schematic diagram providing an explanation for the good rate performance of the SnO2@TiO2 core–shell composite. C,D) TEM images
of the pristine SnO2 (C) and SnO2@amorphous TiO2 composite (D). E,F) Nyquist plots of the SnO2+C (E) and SnO2@TiO2 composite electrodes (F) for
different stages of discharging and charging at 0.5 A g−1 (Test conditions: voltage range of 0.005–2 V). Reproduced with permission.[26] Copyright
2013, Wiley-VCH.
range of 0.01-2.5 V, which favorably compared to that of
the commercial SnO2 powder electrode (Figure 6B). The
rate capability was much improved with a specific capacity
of 498 mA h g−1 at a high discharge current density of
3.2 A g−1, and the discharge capacity was as high as
1600 mA h g−1 at 0.2 A g−1 after 50 cycles. Lei et al. reported
a synthesis of TiO2 hollow spheres consisting of anatase TiO2
nanocones with exposed high-energy {001} facets through a
liquid-phase deposition reaction, using polystyrene spheres
as templates.[45] The nanocone TiO2 hollow spheres served
as buffers architectures for solvothermal deposition of SnO2,
which has been grown in the cavities surrounding the TiO2
nanocones as shown in Figure 6C. These mesoporous hollow
structures (Figure 6D–F), not only provided numerous cavities allowing for the deposition of SnO2 nanoparticles, but
also helped to accommodate their volume changes during the
Li ion charge–discharge cycling, which contributed to their
improved capacity after SnO2 loading (Figure 6G).
Amorphous TiO2-derived LixTiO2 has been reported
to show higher rate capacity than anatase TiO2 even with
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similar morphology, owing to the faster Li+ diffusion in this
material.[26,28] Lin et al. reported amorphous TiO2-supported
SnO2 nanocomposite (TiO2–SnO2) (Figure 7A,B) prepared
by a cetyltrimethylammonium bromide assisted co-precipitation of equimolar stannic chloride and titanium(IV)
butoxide.[28] The powder X-ray diffraction (XRD) pattern
revealed no peak shift or splitting, pointing on the formation
of the two-phase composite consisting of crystalline SnO2
and amorphous TiO2, but not of a SnxTi1-xO2 solid solution
(Figure 7C). They found that the cycling stability can be
improved by limiting the voltage window of the charge–discharge cycles to the range of 5 0mV−1.0 V rather the 1.5 V.
It has been suggested that LixTiO2 becomes damaged by the
excessive lithiation/delithiation during charging to up to 1.5V.
Moreover, the Li2O matrix which acts as the buffer to retard
the aggregation of tin may undergo decomposition, resulting
in the capacity fading. The Li+ conducting LixTiO2 layer
formed in the TiO2–SnO2 composite helps to mechanically
support the composite anode, which exhibited a reversible
capacity of ≈500 mA h g−1 (based on the weight of SnO2) or
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Figure 5. A) Schematic representation of the SnO2@TiO2 coaxial nanotubes (SnO2@TNTs) and the advantages of using such nanoarchitectured
electrodes for Li+ storage. B) Scanning electronic microscopy (SEM) image of the electrochemically synthesized coaxial SnO2@TNTs and C) their
TEM image with inset showing a SAED pattern. D) Top-view SEM of the solvothermally synthesized coaxial SnO2@TNTs and E) their TEM image.
Reproduced with permission.[43] Copyright 2012, Royal Society of Chemistry.
≈320 mA h g−1 (based on the weight of TiO2–SnO2) at 0.2 C
after 100 cycles (Figure 7D).
Another polymorph of TiO2 is TiO2(B), which adopts a
monoclinic structure with the space group C2/m and has a
lower density (3.7 g cm−3) in comparison with the anatase
(3.89 g cm−3) and rutile (4.25 g cm−3) polymorphs.[1] All these
polymorphs have a TiO6 octahedron as a building block, but
with a different arrangement mode. TiO2(B) has a more open
framework structure composed of corrugated sheets of edgeand corner-shared TiO6 octahedra, which enables easier Li+
transport and results in a lower intercalation/deintercalation voltage and larger reversible capacity. TiO2(B) can be
obtained by heat treatment of H2Ti3O7, which is made by
the proton exchange of sodium titanate (Na2Ti3O7). Various
1D sodium titanate nanostructures have been prepared by
treating TiO2 and NaOH under hydrothermal conditions.[47–50]
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Yang et al. reported the fabrication of SnO2 nanocrystal
shells on TiO2(B) nanowire cores, which were prepared by
hydrothermal reaction between NaOH and TiO2 powder, followed by a liquid phase reaction process in SnCl2·2H2O and
urea solution, and finally by dehydrating and drying.[47] The
TiO2(B)@SnO2 hybrid nanowires demonstrated excellent Li
storage capacity and good cyclability, delivering a reversible
capacity of 463 mA h g−1 with a high Coulombic efficiency of
nearly 100% after 50 cycles at a current density of 30 mA g−1.
A reversible discharge capacity as high as 477 mA h g−1 was
obtained when cycled at a current density of 1000 mA g−1.
Such particular architecture of TiO2(B)@SnO2 hybrid nanowires with open continuous channel along the nanowire axis
facilitated lithium ion diffusion, and provided mechanical
support for the TiO2(B) core, alleviating the stress caused by
the Li–Sn alloying–dealloying.
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Figure 6. A) Schematics of the fabrication process of TiO2 nanotube@SnO2 nanoflake core-branch nanostructures, and B) their rate performance
against the cycle number at various current densities (unit: mA g−1) compared to the commercial SnO2 powder electrode. C) Schematic illustration
of the preparation of TiO2-SnO2 nanocone hollow spheres, with their corresponding D) FE-SEM image, E) TEM image, and F) EDS spectrum.
G) Cycling performance of TiO2–SnO2 and TiO2 nanocone hollow spheres, and SnO2 nanoparticles conducted with a voltage window of 0.05–2.5 V.
Reproduced with permission.[45,46] Copyright 2014, Elsevier; Copyright 2012, Royal Society of Chemistry.
2.1.3. SnO2–TiO2 Combined with Carbon Nanomaterials
Carbon-based nanomaterials, such as graphene and carbon
nanotubes (CNTs) have been widely investigated as anode
materials in LIBs, in particular as buffering support for metal
oxides.[51,52] 2D graphene nanosheets possess large specific
surface area, excellent charge carrier mobility and advantageous mechanical properties. Both SnO2 or TiO2 grown
on graphene has been investigated as anode materials for
LIBs,[8,53–55] and demonstrated improved electrochemical
properties. However, the breakdown and aggregation of
SnO2 on the graphene surface during lithiation/delithiation
is still hard to avoid, especially for the high density loading
of SnO2. Due to its low volume variation (<4%) during the
Li intercalation–deintercalation process, combination of TiO2
with carbon nanomaterials as mechanical support can help to
suppress the volume expansion and agglomeration of SnO2
during the Li-alloying/dealloying process.
Tang et al. reported the fabrication of graphene-based
TiO2/SnO2 hybrid nanosheets (denoted TiO2@SnO2@GN),
with both SnO2 and TiO2 nanoparticles loaded on a conductive
graphene substrate.[56] As schematically shown in Figure 8A,
SnO2 nanoparticles on graphene nanosheets (SnO2@GN)
were firstly synthesized via in situ hydrolysis of SnCl2·2H2O
in the presence of graphene oxide (GO) and then the controlled nucleation-crystallization of TiO2 was performed
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at low temperatures. Jiang et al. prepared graphene–TiO2–
SnO2 ternary nanocomposites via different strategy, illustrated by scheme in Figure 8E.[57] Firstly, graphene-TiO2
nanocomposites were prepared by solvothermal treatment
of graphene oxide in tetrabutyl titanate/isopropanol/H2O
solution. Then graphene–TiO2–SnO2 nanocomposites were
prepared by hydrothermal reaction between graphene-TiO2
and SnCl4·5H2O. By comparing the TEM images (Figure
8B,C vs F,G), the deposition of SnO2 and TiO2 nanoparticles resulted in different morphologies. With monodisperse nanosheets, low graphene content (≈5 wt%), defined
mesopores, high specific surface area (138 m2 g−1) and large
pore volume (0.133 cm3 g−1), the TiO2@SnO2@GN manifested excellent cycling performance with discharge capacity
stabilized at 600 mA h g−1 after 300 cycles at a current density of 160 mA g−1 and excellent rate performance of 260
mA h g−1 at an higher current density of 4000 mA g−1 in
the voltage range 0.02–3.0 V (Figure 8D). Graphene–TiO2–
SnO2 ternary nanocomposites delivered a capacity of 537
mA h g−1 at 50 mA g−1 and good reversible capacity of 250
mA h g−1 at the current density 1000 mA g−1 in the voltage
range 0.01–3.00 V (Figure 8H). Furthermore, their cycling
performance could be improved by limiting the voltage
window to the range of 0.01−1.0 V, which is the optimum
voltage range for the alloying–dealloying reaction between
Li and Sn.
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Figure 7. A,B) TEM images of the TiO2-SnO2 nanocomposite at low magnification (A) and high magnification (B). C) X-ray powder diffraction patterns
of the SnO2 nanoparticles and SnO2/TiO2 nanocomposite. D) Reversible capacities of the TiO2–SnO2 electrodes and SnO2 nanoparticle electrodes
cycled at 0.2 C between 50 mV−1.5 V and 50 mV−1.0 V. Scales on the left y-axis apply to both the SnO2 electrodes and the TiO2–SnO2 electrodes,
while those on the right y-axis apply to the TiO2–SnO2 electrodes only. Reproduced with permission.[28] Copyright 2012, Royal Society of Chemistry.
Combining CNTs with SnO2 or TiO2 is another useful
strategy towards active materials for lithium storage.[51,52]
The interconnected CNTs provide a mechanically robust
3D matrix with a superior electronic conductivity, which can
effectively buffer the large volume change of SnO2 during
the charge–discharge cycling. Ding et al. were first to report
the formation of TiO2 nanosheets on SnO2@CNT coaxial
nanocables, even though their lithium storage properties
were not investigated.[19] In their synthesis, acid-treated
multiwalled CNTs were mixed with tin dichloride dihydrate,
urea, and HCl solution, followed by the hydrothermal treatment to obtain SnO2@CNT. The subsequent coating of TiO2
nanosheets on SnO2@CNT was performed by their solvothermal treatment in a mixture of isopropyl alcohol, diethylenetriamine and titanium (IV) isopropoxide.
Yang et al. prepared TiO2/SnO2/carbon nanofibers by a
combination of electrospinning and the subsequent thermal
treatment.[58] The electrospinning solution of a polymer was
prepared by dissolving polyacrylonitrile in N,N-dimethylformamide, followed by addition of a mixture containing
titanium(IV) isopropoxide with anhydrous ethanol and acetic
acid, and then tin(II) 2-ethylhexanoate. Both white TiO2/
SnO2 nanofibers and black TiO2/SnO2/carbon nanofibers can
be prepared by annealing the electrospun samples in an air
and argon atmospheres, respectively. The as-prepared TiO2/
SnO2/carbon nanofibers exhibited high porosity and a large
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surface-to-volume ratio, and delivered reversible capacity of
442.8 mA h g−1 for up to 100 cycles at a current density of
30 mA g−1 in the voltage range of 0.01–3.0 V. Both TiO2 and
carbon matrix in the TiO2/SnO2/carbon nanofibers not only
provided mechanical support to effectively buffer the volume
changes of Sn during Li+ insertion-extraction, but also protected the tin crystals from agglomeration and contributes
to the capacity of the composite material within a certain
voltage range. In addition, the good conductivity of carbon
in the composite helped to improve its electrical conductivity
and to further enhance the electrochemical performance.
Ordered mesoporous carbon also attracted attention as
a component for electrodes in LIBs, usually combined with
electrochemically active metal oxides. The fast diffusion
of Li+ and the easy transport of the electrolyte through its
ordered mesopores are both promising for improving the
electrochemical properties of LIBs. Zhou et al. reported a
mesoporous C–TiO2–SnO2 nanocomposite, prepared via
a tetra-constituent co-assembly method on the basis of
the acid-base interaction between Ti and Sn precursors, in
which P123 (EO20-PO70EO20) was employed as the growthdirecting agent, and SnCl4 and Ti(OEt)4 as Sn and Ti sources,
respectively.[59] Structural changes of the composite upon
increasing SnO2 content were studied, with a finding that
an ordered mesoporous structure can be maintained for low
amount of SnO2, but it is collapsed at 64 wt% SnO2. The
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Figure 8. A) Schematic illustration of the stepwise synthesis of TiO2@SnO2@GN: a) adsorption of Sn2+ ions on the GO surface; b) oxidization
of Sn2+ ions by GO and crystallization of SnO2 nanoparticles on a reduced GO surface; c) hydrolysis of Ti(OBu)4 to form TiO2@SnO2@GN.
B) TEM and C) high resolution TEM (HRTEM) images of TiO2@SnO2@GN. The inset in (C) is the corresponding SAED pattern. D) Variation of charge–
discharge specific capacity versus cycle number for SnO2@GN, TiO2@SnO2, and TiO2@SnO2@GN at 160 mA g−1. E) Synthesis steps and structure
of graphene-TiO2-SnO2 ternary nanocomposites. F) TEM and G) high-magnification TEM images of graphene–TiO2–SnO2. The inset in G) shows the
associated SAED pattern, where T corresponds to TiO2 and S corresponds to SnO2. H) Comparison of charge–discharge cycling performance of
graphene-SnO2 and graphene–TiO2–SnO2 electrodes at a current density of 50 mA g−1. Reproduced with permission.[56,57] Copyright 2013, Royal
Society of Chemistry; Copyright 2013, Royal Society of Chemistry.
nanocomposite electrode with 28.8% TiO2 demonstrated a
high rate performance and a very stable cycle performance
(95.3% retention at 40th cycles).
2.2. TiO2–SnO Based Anodes
SnO is another promising anode material for LIBs because
of its high theoretical capacity of 875 mA h g−1.[6,60–62]
However, due to its easy oxidization into SnO2, stable SnO
nanostructures are difficult to obtain, so that only a few
approaches have been reported towards SnO nanostructures for LIBs. Ning et al. synthesized nanoparticle-attached
SnO nanoflowers via decomposition of an intermediate
product Sn6O4(OH)4 with assistance of free metal cations,
such as Sn2+, Fe2+, and Mn2+.[6,62] Aurbach et al used a sonochemical method to prepare amorphous SnO nanoparticles,
which were then annealed in N2 flow.[60] We obtained singlecrystalline SnO nanosheets by sonochemical method in the
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aqueous SnCl2-NaOH solution, where polyvinylpyrrolidone
was introduced as an additive to hinder the spontaneous formation of the truncated bipyramidal SnO microcrystals and
exfoliate them into layer-by-layer hierarchical structures and
further into separate SnO nanosheets.[63]
Due to the relatively large size of the reported SnO structures, their electrochemical properties are far away from being
satisfied, due to the severe volume changes and the related
pulverization problems. Ortiz et al. used TNT arrays as a
matrix to accommodate the large volume change of SnO.[64]
Composite materials consisting of either metallic Sn, or SnO
nanowires have been grown on TiO2 nanotubes by electrodeposition.[61] They highlighted that Sn and the related SnO
nanowires can only be obtained when using a TiO2 nanotubes
matrix as a seed layer. The electrochemical measurements
implied that the SnO/TiO2 electrode was capable of providing
75 µW h cm-2 of energy density and 85 µW cm-2 peak power.
Also here the TiO2 nanotube matrix helped to accommodate
the volume expansion of Li-Sn alloys and thus enhance their
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electrochemical performance as compared with tin-only based
electrodes.
2.3. TiO2-Sn Based Anodes
Metallic tin, with its theoretical capacity of 991 mA h g−1
or 7313 mA h cm-3 has been regarded as one of the most
promising anode materials for the next generation LIBs, as
it is much higher than that of commercial graphite anode
materials (372 mA h g−1 or 833 mA h cm−3).[4] However, the
volume expansion of Sn is huge (up to 360%), so that dramatic mechanical stress easily causes cracking and pulverization of the active material and the loss of conductivity. To
overcome this problem, the use of Sn nanoparticles has been
suggested[65] but this still resulted in poor cycling stability, as
such small particles are prone to agglomeration into larger
structures during both fabrication and cycling processes.
Composites of Sn nanoparticles and mesoporous TiO2 can
be helpful to both prevent the aggregation of Sn into large
grains and to buffer volume change and structural stress
during their discharge–charge process.
Peng et al. prepared a mesoporous TiO2 with high structural stability (stable up to 500 °C), large BET surface area
of 603 m2 g−1 and pore size of app. 7 nm, via a surfactanttemplate synthesis using tri-block copolymers as growthdirecting agents and titanium butoxide as Ti precursor.[66]
These mesoporous nanostructures with large surface area
and pore volume could accommodate large quantity of electrochemically active materials such as SnO2 and Sn. In their
work, SnO2 was introduced into the mesoporous TiO2 using
a sol–gel process and then reduced into metallic Sn by calcination under H2 atmosphere, resulting in the formation of
Sn–TiO2 composite.
As discussed above, embedding Sn-based anode materials
into a carbon matrix is a good strategy to improve the cycling
stability.[52] Chen et al. reported a carbon coated mesoporous
TiO2–Sn (TiO2–Sn@C) core–shell microspheres for LIBs,[4]
illustrated in Figure 9A. Amorphous TiO2 microspheres were
used as precursors, which were transformed into mesoporous
TiO2 under hydrothermal treatment. Using K2SnO3 as a tin
precursor, SnO2 was in situ generated within the mesoporous
TiO2 microspheres, forming TiO2–SnO2 composite structures
(step I), which were then coated with carbon layer through
the hydrothermal carbonization of glucose (step II). Finally,
TiO2–Sn@C structures were obtained by heat treatment of
TiO2–SnO2@C under Ar atmosphere (step III), with both
TiO2 matrix and carbon shell preventing the aggregation of
metallic Sn particles. In Figure 9B, cyclic voltammograms of
TiO2–Sn@C show a pair of cathodic-anodic peaks located
at 1.73 and 2.15 V, which correspond to Li+ insertion into
and extraction from the interstitial octahedral sites of TiO2,
Another pair of cathodic–anodic peaks located at 0.6 V and
0.12/0.54 V correspond to the Li-Sn alloying and de-alloying
processes, showing that both TiO2 and Sn were electrochemically active in the composite structure and they both contributed to lithium storage. However, owing to an irreversible
lithium intercalation into TiO2 and amorphous carbon below
1 V, degradation of the electrolyte, and the capacity loss of
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Sn, a large irreversible capacity loss was observed for the
composite in the initial cycles (Figure 9C,D). The cycling
performance showed that TiO2–Sn@C exhibited the largest
reversible capacity of 246.5 mA h g−1 in the voltage range
of 0.01–2.5 V at the 200th cycle, compared with TiO2, Sn@C,
and TiO2–SnO2 structures. Furthermore, a reversible capacity
of 206.2 mA h g−1 could be retained after 2000 cycles at
500 mA g−1, and the capacity loss in each cycle from 200 to
2000 cycles was merely 0.022 mA h g−1. Figure 9E revealed
the much better high-rate cyclability of TiO2–Sn@C
(131.8 mA h g−1 at 5000 mA g−1) than that of the artificial
graphite (MAG) and carbon microspheres (CMS). These
properties were attributed to the mesoporous structure of
TiO2 matrix with simultaneous carbon coating, which provided enough void space to accommodate volume expansion
of Sn and buffer the resultant stress. Park et al. reported a
nanostructured Sn/TiO2/C composite made by mechanochemical reduction using SnO, Ti and carbon via the reaction 2SnO + Ti → 2Sn + TiO2. Nanocrystalline Sn particles
and rutile TiO2 were uniformly dispersed in the amorphous
carbon buffer matrix; the Sn/TiO2/C electrode showed a
stable capacity of 610 mA h g−1 in the voltage range of 0–2.5 V
over 100 cycles with the capacity retention of approximately
82.4%.
Li-ion microbatteries based on self-supported TiO2
nanoarray thin films as an anode material can be integrated
into microelectronic circuit boards to meet special energy
requirement of devices such as medical implants and remote
sensors.[25,29,67] Self-supported nanoarrays are favorable for
both ion- and electron transport, while their direct growth
on the current collecting substrates such as Ti foil helps to
simplify the battery assembly procedure and to avoid the
use of the conductive carbon black and the binder. Liao
et al. reported 3D mesoporous TiO2–Sn/C core–shell
nanowire arrays on Ti foil and their use as anodes for
LIBs.[25] Firstly, mesoporous TiO2-based nanowire arrays
were prepared by hydrolysis of H2Ti2O2·H2O nanowire
arrays on Ti foil (30 µm thick) in water at 100 °C, which were
then hydrothermally treated in K2SnO3·3H2O-urea ethanolwater (1:1) solution to obtain TiO2-SnO2 composite nanowire
arrays. Carbon coating via hydrothermal carbonization of
glucose and reduction of SnO2 into Sn followed up to prepare TiO2–Sn/C composites, where Sn was encapsulated
into TiO2 nanowires with the carbon layer coating outside.
These additive-free, self-supported anodes exhibited a discharge capacity of over 160 mA h g−1 (capacity retention rate
of 84.8%) at 10 C rate after 100 cycles and 459 mA h g−1 at
1 C rate after 160 cycles in the voltage range of 0.01–3.0 V.
Electrochemical impedance spectroscopy revealed that these
TiO2–Sn/C nanowire arrays had a low ohmic resistance and a
lower activation energy for Li+ diffusion, which contributing
to their enhanced high-rate performance over the TiO2–SnO2
composites.
Wei et al. reported on another kind of Sn/TiO2 nanowire
arrays as anode material for LIBs.[67] Oriented rutile TiO2
nanowire array was grown on Ti substrate by solvothermal
method without use of any soft or hard templates, and Sn
metal was chemically deposited into the interspace of the
arrays, forming a Sn/TiO2 composite. Due to the difficulty of
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 9. A) Schematic illustration of the fabrication process, and corresponding TEM images of TiO2 microspheres and the resulting TiO2–SnO2,
TiO2–SnO2@C, and TiO2–Sn@C composites. B) Cyclic voltammogram of TiO2–Sn@C at a scan rate of 0.5 mV s−1 between 2.5 and 0 V. C) Cycling
performances of TiO2, Sn@C, TiO2–SnO2, and TiO2–Sn@C microspheres at 500 mA g−1. D) Long-term cycling performance of TiO2–Sn@C at
500 mA g−1. E) Rate performance of TiO2–Sn@C. Reproduced with permission.[4] Copyright 2014, Royal Society of Chemistry.
the determination of the accurate weight, volumetric capacity
was adopted. The Sn/TiO2 electrode exhibited a reversible
volumetric capacity of 1610 mA h cm-3 for the initial cycle
and 1006 mA h cm-3 after 300 cycles in the voltage range of
0–2.75 V, which was 4 times of that of the bare TiO2 nanowire
arrays. Such enhancement was in part attributed to a partial oxidization of Sn into SnO2. In the initial discharge processes, a TiO2–Li2O framework was formed which could both
accommodate Sn and lessen the mechanical strain during
cycling. Besides, the 1D orientation of TiO2 nanowires and
the high electrical conductivity of the tin component both
facilitated the rapid discharge/charge process.
A challenge for the practical applications of TiO2 anodes
is their low intrinsic electrical conductivity (10−12 S cm−1),
which results in the limited rate capability.[55,68] In order to
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improve the conductivity of TiO2 and thus its power density,
metallic Sn coated TiO2 nanotubes have been made.[68,69]
Instead of its function as an electrochemically active material,
metallic Sn coating in this case merely worked as a conductor
by applying a lower cutoff voltage of 1.0 V in the charge–discharge process, as lithium storage of Sn via alloying–dealloying takes place at potentials V ≤ 1.0 V versus Li/Li+. TiO2
nanotubes were prepared in a powder form by an alkaline
hydrothermal method (Figure 10A, B),[70,71] followed by
the deposition of a thin Sn coating layer (Figure 10C,D)
via a wet chemical route using SnCl4 as a tin source and
NaBH4 as a reducing agent. The resulting TiO2@Sn core–
shell nanotubes demonstrated the Li-ion storage capability of
176 mA h g−1 in the voltage range of 1.0–2.5 V even at high
current rate of 4000 mA g−1 (charge and discharge within
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Figure 10. A) TEM and B) HRTEM images of TiO2 nanotubes; C) TEM and D) HRTEM images of TiO2@Sn core–shell nanotubes. E) Voltage profiles
and F) rate capabilities of the TiO2 nanotubes and TiO2@Sn core–shell composites between 2.5−1 V at different C rates from 0.1 C to 20 C
(1 C = 200 mA/g). Reproduced with permission.[68] Copyright 2008, Elsevier.
5 min) (Figure 10E,F), and the corresponding volumetric
energy densities of 317 mA h cm-3, which was 3.5 times larger
than that of the bare TiO2 nanotubes. Furthermore, the Sn
coating allowed to reduce the amount of the conducting
additive carbon, while showing a significant improvement of
the capacity retention at high C rates. Kim et al. reported the
fabrication of Sn/TiO2 nanotube arrays via electrochemical
anodization followed by sputtering and calcination,[69] which
demonstrated high reversible capacity and Coulombic efficiency in the first cycle.
3. Nanoscale SnxTi1-xO2 (0 < x < 1) Solid
Solutions
In the previous sections, we considered composite materials
made out of SnOx (x = 0, 1, 2) and TiO2 where the two components were phase separated. Caused be this phase separation, the disadvantages of each constituent still show up in
the electrochemical performance of the resulting composite,
while the atomic level mixing of Sn and Ti in their oxide
state(s) may be a useful pathway to overcome this. In Section
3, we thus review the fabrication strategies and the lithium
storage properties of nanoscale SnxTi1-xO2 (0 < x < 1) solid
solutions, which can be classified into Sn-doped TiO2 and Tidoped SnO2 materials according to the relative concentration
of these two chemical elements.
3.1. Sn-doped TiO2 Nanostructures
Among the various polymorphs of TiO2, the anatase TiO2 phase
has been highlighted as the most electroactive host for lithium
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intercalation–deintercalation.[1,72] However, Sn-doped TiO2 of
anatase phase was hard to prepare, and it was suggested that
Sn4+ substitution for Ti4+ in SnxTi1-xO2 may facilitate the phase
transition from anatase to rutile at a very low Sn contents
already.[73,74] Aldon et al. made 5% tin-doped TiO2 anatase by
simultaneous precipitation of TiCl4 and SnCl4·5H2O in ethanol,
and studied particle size effects on the lithium insertion into Sndoped TiO2.[75,119] Sn Mössbauer spectroscopy revealed that Sn
atoms were substituted into Ti network in the doped system,
which was inactive upon cycling for the studied potential
window of 1.2–2.2 V, serving as a local probe for investigation of
Li insertion into TiO2. The Li insertion into Sn-doped TiO2 was
suggested to follow a three-step mechanism: i) for x < 0.07, topotactic insertion forming LixTiO2 with a small distortion of the
anatase structure; ii) for 0.07 < x ≤ 0.5, a two-phase mechanism
with final formation of orthorhombic Li0.5TiO2 phase; and iii) for
x > 0.5, formation of a diffusion layer in Li0.5+xTiO2 via the
solid-solution mechanism.
Rutile-phase Sn-doped TiO2 nanostructures undergo
lithium insertion similar to that of the bare rutile TiO2. Sndopant concentrations as well as their morphological and
structural characteristics are all important for their electrochemical properties. Chang’s group reported the synthesis
of rutile-type (Ti, Sn)O2 nanorods by varying the calcination temperatures of tin-modified titanium dioxide (Sn/TiO2)
nanocomposites under a nitrogen atmosphere.[76,77] To make
Sn/TiO2, commercial needle-like rutile TiO2 powder was
dispersed in an aqueous precursor solution of Sn(BF4)2,
followed by addition of Na2S2O3·5H2O and HBF4 as the
reducing agents. The atomic ratio of Sn/Ti in these structures
was estimated to be 1/9. After sintering at 500 °C, the solid
solution of (Ti, Sn)O2 exhibited a reduction of the a- and
b-axes but an increase of the c-axis as well as the overall cell
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 11. TEM images of A) H-titanate and B) Sn-titanate nanotubes. Insets show photographs of the powdered products, as well as their
corresponding EDS spectra. C) TEM bright field image of Sn-doped TiO2 nanostructures. D) Cycle performance of TiO2 and Sn-doped TiO2 electrodes
at 250 mA g−1 in 0.01–2.5 V range. Reproduced with permission.[79] Copyright 2014, Wiley VCH.
volume expansion. Enlargement of the c-axis may facilitate
the diffusion of Li+ ions within the (Ti, Sn)O2 solid solution.[78] As a consequence, the (Ti, Sn)O2 nanorods delivered
a specific capacity of about 300 mA h g−1 and showed minimal capacity fading even at a high current density of 3 A g−1.
Issac et al. prepared Ti2/3Sn1/3O2 nanocrystallites with size
of about 5nm by co-precipitation of Ti(isopropoxide)4 and
SnCl4·5H2O followed by calcination at 600 °C.[33] Increasing
Sn/Ti atomic ratio to 1/2, specific capacities of 300 mA h g−1
were observed after 100 cycles at the current density of
C/20 in the voltage range of 0.02–3.0 V, but at the same time
a faster capacity fading happened upon increasing the current density, indicating the poor rate capability. Our group
recently reported nanoparticulate Sn-doped TiO2 anodes
with synergistically improved capacity and rate performance,
using layered titanate nanostructures as precursors.[79] Such
layered titanate nanostructures, including 1D wire- and
tubular-like as well as 2D sheet-like morphologies, can be
prepared by hydrothermal treatment of TiO2 with highly
concentrated NaOH, and have been widely used as precursors for the fabrication of TiO2 with different polymorphs
by thermal treatment of proton-exchanged titanates.[49,80]
Titanates with exchangeable cations (such as K+ and Na+)
located in the interlayers have been extensively studied as
ion-exchangeable materials for the collection of valuable
trace elements and the removal of radioactive ions from
waste water.[81–84] By utilizing this ion-exchangeable properties of titanates, we demonstrated the synthesis of Sn-titanate
nanotubes by reacting hydrogen titanate with SnCl2 via the
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ion-adsorption-incorporation steps in the aqueous solution at
room temperature (Figure 11A,B).[79] H-titanate nanotubes
with large surface area of 300 m2 g−1 and negatively charged
layered structures are prone to adsorb Sn2+ ions, both at the
surface and within their interlayers, allowing for the uniform
distribution of Sn element with a Sn/Ti atomic ratio of up
to 27%. Annealing of these Sn-titanate nanotubes at 500 or
600 °C resulted in the formation of Sn-doped TiO2 nanostructures (Figure 11C) of either antase-rutile biphase or
highly crystalline rutile phase, respectively. The latter offered
an enhanced rate capability and improved capacity of over
300 mA h g−1 after 300 cycles in the voltage range 0.01–2.5 V
at current density of 250 mA g−1 (Figure 11D), attributed to
the small size of constituting nanoparticles, their high crystallinity, and uniform Sn-doping.[79]
Kyeremateng et al. prepared Ti1/2Sn1/2O2 nanotube arrays
by the electrochemical anodization of co-sputtered Ti-Sn
thin films on Si substrates (Figure 12A,B).[29,85] The Sn/Ti
atomic ratio for these samples was high, reaching 1/1. Upon
annealing at 450 °C, Ti1/2Sn1/2O2 nanotubes transformed into a
rutile phase, while the undoped TiO2 nanotubes transformed
to anatase as a result of the same treatment. No Li-Sn phase
was formed during the discharge and no decomposition of
the Ti1/2Sn1/2O2 structure occurred during their cycling, corroborating that the electrochemical reaction was exclusively
due to the Li+ insertion in the voltage range 1–2.6 V, which
was revealed by 119Sn Mössbauer spectroscopy. Figure 12C
shows that Sn-doped TiO2 delivered much higher capacity
values than undoped TiO2 at a current of 70 mA cm-2 (1 C)
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Figure 12. A) Targeted 3D microbattery design based on TiO2 nanotubes (top view and cross section). B) Top-view SEM image of Ti1/2Sn1/2O2
nanotubes (inset: cross section). C) Galvanostatic cycle life performance at 70 mA cm-2 for the undoped and the Sn-doped TiO2 in the 1.0 <
U/V < 2.6 voltage range. The capacity values are given in mAh cm-2 µm−1 (closed symbols) and in mA h g−1 (open symbols). Reproduced with
permission.[29,85] Copyright 2014, Royal Society of Chemistry; Copyright 2013, Elsevier.
and cut-off potential of 1 V. These improved electrochemical properties were attributed to the enhanced lithium diffusivity achieved through the Sn doping, as revealed by the
Cottrell plots. Li+ insertion into Sn-doped TiO2 was proposed
to be about 40 times faster than into undoped TiO2. This is
consistent with the assertion that the increased bond lengths
lead to more easy lithium ion diffusion in the structure.
3.2. Ti-doped SnO2 Nanostructures
Usually, the incorporation of electrochemically inactive
oxides into the SnO2 helps to buffer the unit cell volume
changes during the Li–Sn alloying–dealloying, thus enabling
better cyclability. However, a disadvantage of the mixed
oxides is that the theoretical reversible capacity will be lowered. As demonstrated in our recent work,[17] spherical hierarchical SnO2 nanostructures composed of Sn(II)-doped
SnO2 nanosheets were prepared by hydrothermal method,
using SnCl2 as a tin precursor and Sn(II) dopants and NaF
as the morphology controlling agent (Figure 13A), and were
used as reference samples.[86] The wire-like hierarchical
nanostructures composed of Ti(IV)/Sn(II)-codoped SnO2
nanosheets were prepared by introducing hydrogen titanate
(H-titanate) nanowires as both the directing agent and Tidopant source in the above reaction system (Figure 13B).[17]
XRD, HRTEM, and elemental mapping analysis data evidenced the uniform Ti-doping within the SnO2 nanosheets,
with the atomic concentration of 15–20%. Electrochemical
measurements revealed that the substitution of Sn atoms
by Ti decreased the lithium storage capacity but showed
improved capacity retention, owing to the alleviation of the
volume expansion of SnO2-based anode materials by substituting some Sn sites with Ti dopants. However, the overall
lithium storage properties of these materials were still far
from being satisfied.
Liu et al. reported Ti-doped SnOx nanoparticles with
varied molar ratios of Ti/Sn (0.05, 0.1, and 0.2), which were
embedded in the carbon nanofibers through electrospinning
technique and the subsequent thermal treatments.[87] The
precursor solution for the electrospinning was prepared by
mixing polyacrylonitrile dissolved in N,N-dimethylformamide
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with another solution containing anhydrous TiCl4 and
SnCl4 in ethylene glycol, where Ti/Sn ratio could readily
be adjusted. The resulting Ti-doped SnOx nanoparticles
with a very small particle size of 2–4 nm were uniformly
encapsulated into the carbon nanofibers (Figure 14A, B).
Elemental mapping revealed the uniform distribution of
Sn and Ti within the fibers (Figure 14B). Among the samples studied, the electrodes with the Ti/Sn molar ratio of 0.1
delivered the best reversible capacity of 670.7 mA h g−1 at
the 60th cycle and they also displayed good rate performance
(302.1 mA h g−1 at 2 A g−1). The enhanced lithium storage
properties of Ti-doped SnOx/carbon nanofibers were attributed to the uniform encapsulation of ultrafine SnOx nanoparticles in the conductive carbon matrix, as well as the
doping with Ti4+.
4. Conclusions and Outlook
In this Review, we have considered Sn–Ti–O ternary based
nanomaterials as anodes for high performance lithium ion
batteries, which should ideally combine the high capacity
of Sn-based anode material such as metallic Sn, SnO, and
SnO2, with the long cycle life and high rate capability of
TiO2. On their drawback side, SnOx (x = 0, 1, 2) suffer from
severe pulverization problems arising from the large volume
change (upto 300%) during the Li-Sn alloying–dealloying
in the discharge–charge process; while TiO2 have their limitations in low theoretical capacity and low conductivity.
Started from SnOx/TiO2, we have discussed the fabrication
methods and the lithium storage properties of a wide range
of nanostructured composites of SnO2/TiO2, SnO/TiO2, and
Sn/TiO2. A number of specific architectures have been considered, such as TiO2-encapsulated SnOx and TiO2-supported
SnOx. Nanocomposites with structurally unstable SnOx (x
= 0, 1, 2) incorporated into a stable TiO2 hollow matrix can
significantly improve both the cycling performance and the
specific capacity of anodes. Furthermore, SnOx/TiO2 nanocomposites combined with different carbonaceous supports
such as amorphous carbon, graphene and carbon nanotubes
have also been surveyed. In those systems, SnOx/TiO2 nanocomposite is usually successively grown on the graphene
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 13. Field emission SEM images of A) 3D hierarchical Sn(II)-doped SnO2 nanoflowers and B) quasi−1D hierarchical Ti(IV)/Sn(II) co-doped
SnO2 nanostructures. C) XRD patterns of the Sn(II)-doped (blue) and Ti(IV)/Sn(II) co-doped (black) hierarchical SnO2 nanostructures. Tetragonal
structure of SnO2 (cassiterite, JCPDS#41−1445) is shown as a line spectrum in red. D) Cycling performance for the hierarchical SnO2 nanoflowers
and Ti-doped SnO2 nanowires. Reproduced with permission.[17] Copyright 2013, Royal Society of Chemistry.
or carbon nanotube sustrates, while the amorphous carbon
coating is commonly performed by the hydrothermal carbonization of carbonaceous organics, such as glucose. Successive
annealing in the inert atmospheres (N2 or Ar) can further
result in the reduction of oxidized Sn into metallic Sn. Due to
the lower electric conductivity of TiO2, deposition of metallic
Sn onto TiO2 can greatly enhanced the rate capability, and
the metallic Sn produced by the reduction of tin oxides in the
first discharge process also contributes to this improvement.
Instead of the nanoscale mixing of SnOx and TiO2 separate phases, the fabrication of nanostructured SnxTi1-xO2 solid
solutions mixed at the atomic level is yet another strategy we
have surveyed. According to their relative concentration in
the SnxTi1-xO2 solid solutions, Ti-doped SnO2 and Sn-doped
TiO2 have been discussed. Ti-doping in SnO2 results in the
decrease of the specific capacity but improves the cycling
performance of the resulting anodes to some extent. However, their overall electrochemical performance is still far
from satisfactory. On one hand, the alleviation of the severe
volume changes of Sn-based anodes by small amount of Tidoping is limited. On the other hand, the often large building
block sizes of the nanostructured anode materials often deteriorate their long-term cycling performance. Size-minimizing
and hybridizing with carbon-based matrixes appear to be a
direction towards high performance Sn-based anode materials. In contrast, Sn-doping of TiO2 can greatly improve the
small 2015, 11, No. 12, 1364–1383
specific capacity while keeping satisfied rate capability and
long cycling performance, especially when embedded into
carbon matrix, which not only buffers the mechanical stress
induced by the volume change in the charge–discharge process but also improves the electrical conductivity of the Sn–
Ti–O ternary systems.
Besides the architecture design of Sn–Ti–O ternary
systems, the fabrication methods also have a significant
influence on their electrochemical properties. Especially,
among a number of approaches such as solvo/hydrothermal,
sol–gel, atomic layer deposition, and co-precipitation, electrospinning method has shown a great potential in fabrication of high performance anode materials, where both SnOx/
TiO2 nanocomposites and SnxTi1-xO2 doped systems with or
without carbon coating can be readily prepared by adjusting
the Sn- and Ti-precursor concentrations and controlling the
post thermal treatment in different atmospheres.
Despite the encouraging reports on Sn–Ti–O ternary
systems with an improved specific capacity, long cycle life,
and high rate capability by proper engineering of their morphology, composition and their further combination with
carbon-based nanomaterials, they still suffer from the large
irreversible capacity loss in the initial cycles with a low initial Coulombic efficiency. On one hand, formation of solid
electrolyte interphase (SEI) contributes to the irreversible
capacity loss due to the chemical reaction between Li and
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Figure 14. Characterization of Ti-doped SnOx/carbon nanofibers (Ti/Sn = 0.1) sample: A) SEM image; B) Ultrathin cross section for the TEM
mapping and the corresponding elemental mapping of carbon, oxygen, tin, and titanium. The inset on the left shows the HRTEM image; C) Cyclic
performances of all samples at 200 mA g−1; D) rate performances of Ti-doped SnOx/carbon nanofibers (Ti/Sn = 0.1) and SnOx/carbon nanofibers.
Reproduced with permission.[87] Copyright 2014, Elsevier.
electrolyte at the electrode surfaces. On other hand, oxidized
Sn also consumes Li so as to be electrochemically reduced
into metallic Sn, which irreversibly produces Li2O. Therefore,
combination of metallic Sn with TiO2 in a composite structure which avoids the irreversible formation of Li2O should
be able to improve the initial Coulombic efficiency. Furthermore, carbon coating already demonstrated great potential
on improving the initial Coulombic efficiency up to >60%, as
illustrated by data presented in Table 1. However, in order to
achieve their practical applications, further improvements of
such characteristics as capacity, cycle life, rate and safety are
important to fully realize the potential of Sn–Ti–O ternary
systems, which can be achieved by an advanced nanostructure design, composition engineering, and the development
of green synthetic fabrication methods for their large-scale
production.
Acknowledgments
The authors acknowledge the financial support of the City University of Hong Kong through Strategic Research Grant 7004010,
National Science Foundation of China (Grant No. 51402232) and
Fundamental Research Funds for the Central Universities in China
(Grant No. 08143072).
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: September 5, 2014
Revised: October 9, 2014
Published online: December 12, 2014
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