REVIEW

Advanced titania nanostructures and compositesfor lithium ion battery

Xin Su • QingLiu Wu • Xin Zhan • Ji Wu •

Suying Wei • Zhanhu Guo

Received: 3 August 2011 / Accepted: 17 September 2011 / Published online: 29 September 2011

� Springer Science+Business Media, LLC 2011

Abstract Owing to the increasing demand of energy and

shifting to the renewable energy resources, lithium ion

batteries (LIBs) have been considered as the most prom-

ising alternative and green technology for energy storage

applied in hybrid electric vehicles (HEVs), plug-in hybrid

electric vehicles (PHEVs), and other electric utilities.

Owing to its environmental benignity, availability, and

stable structure, titanium dioxide (TiO2) is one of the most

attractive anode materials of LIBs with high capability,

long cycling life, high safety, and low cost. However, the

poor electrical conductivity and low diffusion coefficient of

Li-ions in TiO2 hamper the advancement of TiO2 as anode

materials of LIBs. Therefore, intensive research study has

been focused on designing the nanostructures of TiO2 and

its composites to reduce the diffusion length of Li-ion

insertion/extraction and improve the electrical conductivity

of the electrode materials. In this article, the development

of TiO2 and its composites in nano-scales including

fabrication, characterization of TiO2 nanomaterials, TiO2/

carbon composite, and TiO2/metal oxide composites to

improve their properties (capacity, cycling performance,

and energy density) for LIBs are reviewed. Meanwhile, the

mechanisms for influences of the structure, surface mor-

phology, and additives to TiO2 composites on the related

properties of TiO2 and TiO2 composites to LIBs are dis-

cussed. The new directions of research on this field are

proposed.

Introduction

Owing to the worldwide concerns on the environmental

pollutions, global warming, and increasing price of the

traditional energy sources [1], lithium ion batteries (LIBs)

are being considered as the most promising green gen-

eration of energy storage technologies applied in hybrid

electric vehicles (HEVs) [2], plug-in hybrid electric

vehicles (PHEVs) [3, 4], and storage systems for renew-

able and intermittent energy sources including solar,

wind, and nucleation power [5, 6]. The great success of

LIBs in portable electronic divisions have been achieved

since its first commercialization by Sony at early 1990s

[7]. However, several aspects including relatively high

cost and poor performance limit their further wider

applications [8]. For example, the current cost of 18,650

cells is more than twice of the target price for HEV and

PHEVs [5], more than 80% of which is attributed to the

cost of materials [8]. From this view, materials process is

crucial to reduce the cost of LIBs [8] and the employment

of titanium dioxide (TiO2) as anode materials of LIBs can

effectively benefit further development of LIBs because

of its low cost [9, 10], environmental benignity and easy

availability [11, 12].

Xin Su and QingLiu Wu contributed equally to this study.

X. Su � Q. Wu � J. Wu

Department of Chemical and Materials Engineering, University

of Kentucky, Lexington, KY 40506, USA

X. Zhan

Department of Chemistry, University of Kentucky,

Lexington, KY 40506, USA

S. Wei

Department of Chemistry and Biochemistry,

Lamar University, Beaumont, TX 77710, USA

Z. Guo (&)

Integrated Composites Laboratory (ICL), Dan F. Smith

Department of Chemical Engineering, Lamar University,

Beaumont, TX 77710, USA

e-mail: zhanhu.guo@lamar.edu

123

J Mater Sci (2012) 47:2519–2534

DOI 10.1007/s10853-011-5974-x

For the aspect of power/energy density, graphite has

been the sole anode material, which has been widely

commercialized for LIBs’ electrodes. However, the theo-

retical capacity of graphite is as low as 372 mAh g-1 (the

practical value is lower), which is far lower than the

requirements for HEVs, PHEVs, etc. To solve this prob-

lem, other materials including high capacity Si (4,200

mAh g-1), Sn (980 mAh g-1), and their oxides, have been

investigated aiming to meet the requirements for LIBs [1].

However, these high-capacity materials and graphite suffer

from the capacity degradation over a long time cycling,

which limits their applications as LIBs’ electrodes in HEVs

[1]. The theoretically calculated capacity of TiO2 is 330

mAh g-1, which is a little lower than that of graphite [13].

However, the volume change of TiO2 is less than 4% as Li-

ion inserted into the TiO2 electrodes [14], which affords

TiO2 outstanding structure stability after Li-ion insertion,

and thus, extremely long cycling life as electrodes in LIBs.

Safety is another crucial concern for LIBs utilized in

HEVs, PHEVs, and especially for stationary energy storage

applications, in which the safety and stability are consid-

ered as priority over the energy/power density [15]. How-

ever, the working potential for most high-capacity

materials including graphite, Si, and Sn is in the region of

0–0.5 V (versus Li/Li?) as they are employed as anodes in

LIBs [1]. In such a low operating voltage region, the

electrolyte is prone to decompose and form an unstable

solid–electrolyte interface (SEI) layer on the anodes’ sur-

face, which in turn will promote the decomposition of

electrolyte. Concurrently occurring with the decomposition

of electrolyte, gases are released and build up pressure in

the cell. This situation will exacerbate and endanger the

safety of the battery system as gases accumulate with

increasing cycling time [15]. On the contrary, TiO2 oper-

ates at a relatively higher voltage (1.5–1.8 V versus Li/

Li?), and generates less energy than that from the previ-

ously discussed high-capacity materials of LIBs. However,

the formation of SEI layer on the surface of anode can be

effectively avoided at such a high working potential, and

which thus greatly improves the overall safety of the bat-

tery. In this regard, the utilization of TiO2 as electrode

materials of LIBs in HEVs, PHEVs, and stationary energy

systems has more benefits over the conventional anode

materials to achieve higher safety and stability.

Compared with conventional anode materials, TiO2 has

advantages including lower cost, relatively higher safety,

and longer cycle life; however, its applications as anode

materials in LIBs have been historically hampered by its

low ionic and electrical conductivity (*1 9 10-12 S m-1)

[16, 17], resulting in low energy/power densities [13].

Therefore, in recent decades, approaches to create TiO2

with various structures have been explored for serving as

anode materials of LIBs by improving the Li-ion diffusion

and electrical conductivity [18]. The nanostructured TiO2,

which is superior to conventional bulk materials, including

large specific surface area, short diffusion length, and fast

kinetics, paves the way for its wide utilization as electrode

material of LIBs with high power/energy density. Owing to

the intensive research interest and results, we found a

former review in 2009 of Yang et al., which [13] focused

on the electrochemical reactivity of lithium titanites and

different TiO2 polymorphs for LIB. Presented here, it is the

first review that categorized the TiO2 nanostructures

(nanoparticles, nanowires, nanoporous) and their compos-

ite with carbon, metal oxides, and silicon for LIB.

Mechanism and advantages of TiO2 for LIB

The eight polymorphs of TiO2 are well known as rutile,

anatase, brookite, TiO2-B, TiO2-R, TiO2-H, TiO2-II, and

TiO2-III [19]. Among these, rutile is generally reported as

available with the most stable structure and in common

natural form at low pressure [20, 21]. However, anatase is

generally considered to be the most suitable candidate for

Li-insertion host, while the Li insertion into bulk rutile is

usually negligible at room temperature [19, 22–24]. For

instance, while the particle sizes are micrometers, only

0.1–0.25 mol Li per mol TiO2 can be inserted into bulk

rutile at room temperature [25]. As the size of TiO2 falls in

the nanometers, anatase is probably more stable than rutile,

owing to the differences in particle surface tension, size,

and shape [26, 27]. Furthermore, nanostructures bring a lot

of benefits for TiO2 as Li-insertion hosts. There are

increasing reports on rutile, brookite, and TiO2-B with

nanofeatures as anode materials of LIBs.

Regardless of various polymorphs of TiO2, the insertion

reaction of Li-ion into TiO2 can be expressed as [28]

TiO2 þ xLiþ þ xe� ! LixTiO2 ð1Þ

In this redox reaction, the insertion of positively charged

Li? is balanced with an uptake of electrons to compensate

TiIII cations in the TiIV sublattice, which usually results in a

sequential phase transformation occurring in original TiO2

as a function of Li? content. This is predicted by theoret-

ical calculations [29–35] and has been observed in X-ray

photoelectron spectroscopy (XPS) experiments [36, 37].

Since almost all reports about the TiO2 polymorphs as

anode materials of LIBs mainly involve anatase, rutile, and

TiO2-B, the following discussion will focus on these three

polymorphs.

The details of TiO2 polymorphs structure were discussed

by other review literatures [13, 38, 39]. The structure of

anatase TiO2 can be considered as a stacking of one-

dimensional zigzag chains consisting of distorted

edge-sharing TiO6 octahedrals. Along [100] and [010]

2520 J Mater Sci (2012) 47:2519–2534

123

directions, this stacking is equivalent and composed of

empty zigzag channels in the whole framework, which

provides paths for Li-ion insertion occupying the interstices

of TiO6 octahedrals to form LixTiO2 [19, 24, 40]. Since the

Li–Li are interacting strongly [24], the originally tetragonal

anatase phase (space group I41/amd) undergoes a phase

transition with an orthorhombic distortion [40] as the ratio

of Li-ion insertion is larger than 0.05 [24]. The change of

symmetry involves a decrease of the unit cell along the

c axis and an increase along the b axis [23]. The overall

distortion of the atom positions accompanying the phase

transition is relatively small, which leads to the volume

change of unit cell less than 4% [41].

In the lattice framework of an ideal rutile crystal, TiO6

octahedral shares edges in the c-direction, while corners in

the ab-plane [38]. It is more difficult for Li ions to reach

the TiO6 octahedral interstitial sites in this special lattice

framework [39], which explains the diffusion of Li-ion

along other than c-direction to octahedral interstitial sites.

Li? has to migrate through the tetrahedral site, which is

neighbor to the octahedral site in the ab-plane. Therefore,

Li? diffusion in rutile is highly anisotropic that the diffu-

sion along the c-direction is fast (diffusion coefficient is

about 10-6 cm2 s-1), while Li diffusion in the ab planes is

very slow (diffusion coefficient is about 10-14 cm2 s-1)

[32, 42, 43]. The anisotropic diffusion of Li ions in rutile

limits the amount of Li ions distributed along ab-planes

and separates Li? in the c channels [30, 32, 38]. Even along

c-direction, further Li-ion insertion would be blocked

because of the repulsive forces between Li ions in

c-direction and trapped Li-ion pairs in the ab-planes [44].

Therefore, the Li-ion insertion into bulk rutile TiO2 is very

difficult and only a negligible number of Li ions have been

reported to be accommodated in the bulk rutile except

those operated at high temperature up to 120 �C [45].

The idealized TiO2-B has the same structure as a shear

derivative of the ReO3-type structure, which is composed

of corrugated sheets of edge- and corner-sharing TiO6

octahedrals [46–48]. In this special framework, TiO2-B

possesses a one-dimensional infinite channel, which indi-

cates that the structure of TiO2-B is more open than other

polymorphs [13]. Furthermore, the existence of parallel

infinite channels in TiO2-B lattice can also accommodate

volume change without any significant distortion of the

structure during Li? insertion [49]. In addition, Zukalova

et al. proposed that, the diffusion of Li? in TiO2-B is a

pseudocapacitive faradic process, which is a faster process

compared with the solid-state diffusion process, which

controls the Li? diffusion in anatase [48]. All these prop-

erties suggest that TiO2-B is an outstanding candidate as

anode materials of LIBs.

In addition to anatase, rutile, and TiO2-B, brookite have

also been investigated as electrode materials in LIBs

[50–55]. However, the low-delivered reversible capacity

(\170 mAh g-1 even for particles with size *10 nm) [52]

limits its application and development as anode materials

in LIBs. Therefore, the further discussion will be mainly

focused on anatase, rutile, and TiO2-B.

TiO2 nanomaterials for LIBs

Regardless of various polymorphs, the practical attainable

capacities of bulk TiO2 are reported to be only half of the

theoretical value (330 mAh g-1). The main reason is that

the further Li-ion insertion in TiO2 is blocked because of

the strong repulsive force between Li ions as the insertion

ratio is greater than 0.5 (as x [ 0.5 in LixTiO2) [49, 56,

57], which greatly limits the applications and develop-

ment of TiO2 as electrode materials in LIBs. Fortunately,

it has been predicted by theoretical simulations and fur-

ther proved by experiments that the lithium-intercalation

activity and cycling stability of titania-based electrode can

be improved dramatically as the scale of employed

materials moves into the region of nanometers [13, 56,

58–62]. Therefore, numerous approaches and strategies

have been investigated to achieve the objective of

reducing lithium diffusion length by fabrication of nano-

structured TiO2 materials, such as nanosized anatase

titania [63], TiO2-B nanowires [64], and mesoporous

rutile [65].

TiO2 nanoparticles

It is obvious that as the size of particles moves into the

nanometer ranges, the diffusion length of Li-ion will

effectively reduce in addition to improving surface areas

and finally leading to the improved capacity [66]. Fur-

thermore, simulation results from multi-scale theoretical

modeling reveal a larger Li-ion conductivity in TiO2 dra-

matically for particle size smaller than 20 nm, and lower

conductivity for larger nanoparticles [67]. This has been

corroborated by various experimental results. For instance,

the study done by Wang et al. [68] indicates that the total

energy storage, which comes from double layer effects and

intercalation process, increased with decreasing the size of

nanocrystalline anatase from 5 to 10 nm. The amount of

stored charges derived from Li-ion intercalation processes

was reduced because of the decrease of TiO2 particle size;

however, this can be more compensated by increased

capacities from both double layer and pseduocapacitive

effects derived from smaller particles. Moreover, reducing

particle size to the nanoscale regime led to faster charge/

discharge rates because the diffusion-controlled Li-ion-

intercalation process was replaced by faradic reactions

J Mater Sci (2012) 47:2519–2534 2521

123

which occur at the surface of the material TiO2 [68]. This

phenomenon is also observed on the anatase TiO2 with

smallest primary particle size of *8 nm obtained from a

hydrothermal treatment on sol–gel precipitates of TiO2

[69]. These particles with smallest primary size deliver the

highest charge capacity as 140 mAh g-1, and the capacity

decreased with increasing the primary particle size

obtained through higher temperature calcination [69]. A

higher initially discharge capacity of 203 mAh g-1 is

reported for anatase TiO2 NPs with a particle size of about

20 nm, which were prepared by hydrolysis of titanium

tetraisopropoxide in pure water, and followed by calcined

at high temperature above 600 �C [63]. However, the ini-

tial loss of capacity is as high as 14% between the insertion

and the extraction of Li ions, and the reversible capacity

after 40 cycles decreases to 148 mAh g-1 [63]. Higher

reversible capacity is reported on the anatase TiO2 pow-

ders with same size *20 nm, which were prepared by

hydrothermally treating titanium butoxide at 120 �C, and

followed by thermal treatment between 200 and 600 �C

[66]. The sample with particle size of *20 nm yields a

specific capacity on the first discharge of 180 mAh g-1,

and only 5% loss was observed after the second cycle, and

there is no appreciable capacity fading even after 100

cycles [66].

Electrochemical performances do not always benefit

from decreasing particle sizes. Poizot et al. studied metal-

oxide systems and found that there exists an optimal size

range for the metal-oxide particles to obtain the best

electrochemical properties [70]. Exnar et al. also found

that the optimal anatase particles size in practical TiO2/

LiCo0.5Ni0.5O2 button cells is about 10–15 nm [71]. This

has been corroborated by more evidences provided by

other researchers. Most recently, Kang et al. presented the

so-called polyol-based method to fabricate anatase TiO2,

in which the triethylene glycol solution of titanium iso-

propoxide was refluxed to fabricate well-dispersed anatase

TiO2 NPs with fairly homogeneous particle size distri-

butions [72]. The effect of particle size on the electro-

chemical performances were elucidated, and they

proposed that, under investigated experimental conditions,

8–25 nm appeared to be the critical particle size for

anatase TiO2 with sufficient crystallinity and considerable

electrochemical performances [72]. Besides approaches

pursuing higher capacity and stability, the laser-ablation

method, presented by Tsuji et al., can produce anatase

TiO2 NPs for fabricating electrodes directly from this

product [73]. The NPs with an average size of 6 nm

were derived from laser irradiation on the suspensions

with micropowders anatase TiO2 in acetone, and the

final products can be easily deposited on a substrate

using an electrophoresis technique for electrochemical

measurements.

TiO2 nanowires, nanorods, and nanowhiskers

Since the diffusion coefficient of Li ions along c-direction

is almost eight orders higher than that along ab-plane [32,

42, 43], the Li-ion insertion into rutile TiO2 can be con-

sidered as a nearly one-dimensional diffusion. Conse-

quently, the restricted diffusion into three-dimensional

volume makes poor electrochemical performance of the

bulk rutile TiO2 as anodes of LIBS. However, the Li-ion-

intercalation capacity can be significantly improved as

rutile TiO2 possesses morphologies as nanorods, nano-

wires, or nanowhiskers, especially as particles grow along

the c-direction [25, 74–76]. For instance, Hu et al. reported

that up to 0.8 mol of Li can be inserted into 1 mol needle-

shaped rutile TiO2 with *10 nm in diameter and

30–40 nm in length, in addition to excellent retention of

capacity on cycling and a high rate performance. The

c-direction was found to lie along the needle length of these

rutile TiO2 [25]. Similar shapes including nanoneedles,

nanowhiskers, and nanorods are also reported with excel-

lent electrochemical performances by other researchers.

For instance, rutile TiO2 nanowhiskers with 4–5-nm

diameter and 40–50-nm length synthesized via a hydrolytic

sol–gel route are reported to have excellent specific

capacities of above 100 mAh g-1 even at a constant charge

rate of 30 C [77–79]. This high rate capability is the result

of this nanowhisker morphology, which provides shorter

transport lengths for both electronic and Li? transport as

well as higher specific surface areas for more electrode–

electrolyte contact area [77–79]. Ultra-fine rutile nano-

whiskers with similar size were also fabricated by Chen

et al. through a modified wet-chemical method, and dem-

onstrated similarly outstanding electrochemical perfor-

mances [80]. The fabrication of rutile TiO2 nanoneedles,

with a width of 20–25 nm and length of [100 nm, was

reported by Khomane through a reverse microemulsion-

mediated sol–gel method at room temperature. The elec-

trochemical measurements show that the nanoneedles

deliver an initial capacity of 305 mAh g-1, and the

capacity value retains as high as 128 mAh g-1 after 15

cycles. The highlights of this method is that it provides a

simple energy-saving method to fabricate rutile NPs [81].

Qiao et al. developed a method to prepare flower-like rutile

TiO2 nanorods through mixing TiCl4 precursor with etha-

nol and distilled water in sequence. These flower-like

structures were composed of many nanorods with

10–15 nm in diameter and 50–70 nm in length, and the

growth direction of nanorods was found to be parallel to

(110) crystal planes. These flower-like rutile TiO2 nano-

rods demonstrated a stabilized charge capacity of 183

mA g-1 after 30 cycles [82]. Most recently, Dong et al.

reported a method to fabricate rutile TiO2 nanorod arrays

(grew along c-axis) on Ti foil substrates using a template-

2522 J Mater Sci (2012) 47:2519–2534

123

free method. These nanorods exhibited significantly elec-

trochemical performance with capacity of about ten times

higher than that of rutile TiO2 compact layer [83]. The

higher capacity was ascribed to the greatly improved

contacts between electrode materials and electron collec-

tors leading to a significant resistance reduction. Even at

25 �C, the maximum insertion amount of lithium ions is

about 0.85 Li per Ti. This makes TiO2-B a superior

intercalation host for lithium ions compared with rutile and

anatase [84], and several studies have reported the fabri-

cation of TiO2-B with excellent electrochemical perfor-

mances. TiO2-B was first synthesized by Marchand et al. in

1980 through a three-step procedure [47]: at first, K2Ti4O9

was prepared through a solid-state reaction of KNO3 and

anatase TiO2 at a high temperature; then ion exchanging

K? with H? in K2Ti4O9 in acidic solutions was used to

form a hydrate hydrogen titanate; and finally, TiO2-B was

generated after thermal treatment at 500 �C. This proce-

dure is adopted and well developed by other researchers to

prepare TiO2-B nanoparticles, nanowires, and nanorods

aiming at high capacities in LIBs [85–87]. For instance, as

the heating temperature increased from 170 to 450 �C, the

initially needle-like structure with 400 nm in diameter and

2 lm in length were transformed to large particles with

average size 1 lm. The electrochemical test demonstrated

that the initial discharge capacity can be dramatically

improved from 193 to 291 mAh g-1 as the final resulting

hydrate hydrogen titanate with less water content was

obtained under higher heating temperature [86].

Armstrong et al. creatively proposed a facile way to

synthesize TiO2-B through replacing the solid reaction by

a hydrothermal treatment of anatase TiO2 in concentrated

NaOH solutions [64, 88–90]. Using this hydrothermal

strategy, TiO2-B nanowires with 20–40 nm in diameter

and several micrometers in length can be fabricated

through this initially hydrothermal process followed by

ion-exchange and heat treatment on the as-synthesized

products. The finally calcined products exhibited a high

charge capacity of 305 mAh g-1, corresponding to a

composition Li0.91TiO2, and excellent cyclability. This

approach is widely adopted by most researchers to prepare

TiO2-B with different synthesizing parameters including

titanium precursors [91], thermally treating temperature

[92], and hydrothermal conditions including temperature,

time, and the composition of solution [93, 94]. For

instance, in the presence of ethanol in hydrothermal solu-

tion, Wang et al. produced TiO2-B nanowires with

40–80 nm in diameter and 1.5 lm in length. However,

these nanowires exhibited a little lower rate capacity of

280 mAh g-1 after 40 cycles with the columbic efficiency

approximately being 98% [94]. TiO2-B nanowires with

larger diameters (300–1,000 nm) and longer length

(2.5–10 lm) can be achieved at higher temperatures during

calcinations process. However, these nanowires exhibited

further lower first discharge capacity of 232 mAh g-1.

More recently, Li et al. proposed that replacing NaOH

solution by KOH solution in hydrothermal process for

preparation of TiO2-B nanowires with ultrahigh surfaces

area up to 210 m2 g-1. They found that these nanowires

grow along \110[ direction with shorter b- and c-axis

channels (Fig. 1). The electrochemical measurement

demonstrated that these nanowires deliver the highest ini-

tial discharge capacity (388 mAh g-1 at constant current

density 10 mA g-1) that have been reported for TiO2

nanowires until now (Fig. 2) [95]. More interestingly, Liu

et al. grew vertically oriented single-crystalline TiO2-B

nanowire arrays on titanium foil over large areas by

placing titanium substrates in hydrothermal solution during

hydrothermal process. These arrays exhibited a high

capacity of 120 mAh g-1 even as the charge/discharge rate

is up to 1.8 C and excellent cycling stability beyond 200

cycles [93].

Besides autoclave hydrolysis strategies, other approa-

ches have also been reported to synthesize TiO2-B under

mild conditions. For instance, Xiang et al. produced

TiO2-B with ultra-thin nanosheet form from mixing TiCl3with ethylene glycol followed by heating mixture at

150 �C [96]. Estruga et al. produced TiO2-B nanoparti-

cles through the hydrolysis of an ionic liquid with titania

precursor at ambient pressure and low temperature [97].

However, no electrochemical characterization was repor-

ted. Wessel et al. proposed that pure TiO2-B nanoparti-

cles with 20–25 nm in diameter can be fabricated by

mixing 1-hexadecyl-3-methylimidazolium chloride

(C16mimC1)/1-butyl-3-methylimidazolium tetrafluorobo-

rate (C4mimBF4) with TiCl4. It exhibited an outstanding

high capacity of 100 mAh g-1 even at a high current rate

of 10 C [98].

In addition, the preparation of anatase TiO2 nanorods

with improved capacities has been reported as well. For

instance, Lan et al. reported a facile way to prepare anatase

TiO2 nanorods from rutile powders directly, which exhibit

a discharge of 198 mAh g-1 [99]. In this strategy, hydro-

thermal treatment was used for treating rutile powders

dispersed in 10 M NaOH, and nanotubes/nanorods were

obtained at 150/180 �C. After calcinations at 500 �C,

anatase TiO2 is obtained without morphology changes

[99]. Using similar procedures, Kim et al. found that the

nanotubes transformed to nanorods as the temperature of

thermal treatment on the as-prepared nanotubes (hydro-

thermally treated at 150 �C) increased from 300 to 400 �C

[100]. However, nanorods exhibited lower initial discharge

capacity (178 mAh g-1 at the 0.5 C rate) and faster

capacity decay (110 mAh g-1 at 10 C rate) compared to

nanotubes (205 and 180 mAh g-1 at 5 and 10 C, respec-

tively) [100]. The significance of this approach is that it

J Mater Sci (2012) 47:2519–2534 2523

123

provides a facile route to fabricate anatase TiO2 nanorods

directly from industrial raw materials, which always exists

in the rutile phase.

It is well known that nanoparticles possess high surface

energy compared with the bulk counterpart and incline to

aggregate either during synthesis or galvanostatic cycling.

The aggregation of nanoparticles decreases the surface area

and increases the difficulty for electrolyte solution diffu-

sion within aggregates reaching the surface of particles. It

will result in the reduction of the total storage energy.

However, it poses a dilemma as scientists attempt to pro-

duce more loosely packed nanoparticles aiming at exposing

more surface to the electrolyte solution and improving the

diffusion of electrolyte solution within aggregates of TiO2

nanoparticles. Therefore, a better way to avoid this situa-

tion is to prepare nanoporous TiO2, especially with verti-

cally aligned nanotubes arrays of thin films. This alignment

will favor the reduction of electrical resistance derived

from very good connections of nanoparticles as well as

higher surface area and the short diffusion length from

porous structures.

Nanoporous TiO2

There is a limit to improve electrochemical performance,

which is limited by reducing the size of particles in that

the intrinsic propensity of nanoparticles, nanorods, nano-

wires, nanowhiskers, etc. The aggregated particles will

make diffusion of electrolyte difficult within aggregates.

The emergence of porous structures would shorten the

diffusion length of Li ions further and thus lead to higher

capacity. For instance, the capacity of anatase nanotubes

has been observed to be higher than that of anatase

nanorods [99, 100]. Therefore, numerous studies have

been accomplished to produce nanoporous TiO2 including

nanotubes and mesoporous structures, aiming at higher

capacities in LIBs.

Similar to the conditions as for the nanowires, most of

TiO2 nanotubes are synthesized by the so-called three-step

hydrothermal treatment on anatase of rutile powders. The

main difference is the temperature and volume of solution,

which are lower and less for preparing nanotubes [84, 99–

106]. Among all these TiO2 nanotubes, the best perfor-

mance of TiO2 nanotubes as anode materials in LIBs was

reported by Armstrong et al. In this case, TiO2-B nanotubes

with 100–200 nm in diameter and 1–2 lm in length were

fabricated through hydrothermally treating anatase pow-

ders in 15 M NaOH, and they demonstrated near theoretical

capacities (338 mAh g-1 at the rate of 10 mA g-1), and

excellent rate capability [84].

Similar to nanotubes, mesoporous structure can also

effectively reduce the diffusion length of Li ions and

increase the specific surface areas. Methods of fabrication

of mesoporous TiO2 can be roughly categorized to tem-

plate-assistant [65, 107–109] and template-free [110–114].

In template-free routines, hydrothermal treatment was

Fig. 1 Low- (a) and high-

resolution (b, inset is the

corresponding fast Fourier

transfer (FFT) patterns) TEM

images of TiO2-B nanowires.

Below is the schematic model of

side (c) or cross section (d) view

of a single TiO2-B nanowire.

The white octahedral represents

one TiO6 unit. The examined

sample was obtained from

hydrothermally treating anatase

TiO2 powder in 10 M solution

of KOH at 200 �C for 24 h

followed by ion-exchanging

process in 0.1 M nitric acid at

above 100 �C for 24 h

(permission from reference of

[95])

2524 J Mater Sci (2012) 47:2519–2534

123

always utilized to create a core–shell mesoporous TiO2

spheres with micrometers in diameter [110, 111, 113].

Among all products fabricated from template-free

approach, microspheres produced by Wang [112] demon-

strated the highest initial discharge capacity of 265 and 151

mAh g-1 at 0.06 and 1.2 C, respectively, even after 50

cycles. Compared to the template-free approach, the

employment of templates can create well-ordered and

uniform mesopores which can more effectively accom-

modate the volume change during Li-ion insertion/de-

insertion process. In general, surfactants including tri-block

polymers [115, 116], cationic surfactant molecules [107],

anionic surfactant molecules [109], and polystyrene (PS)

colloids [108], etc. are widely used as templates for gen-

erating mesopores. The relationship between mesoporous

properties and electrochemical performances has been

elaborated by Saravanan et al. [107]. Through varying the

chain length of surfactants, the average pore size of mes-

oporous TiO2 increases from 5.7 to 7.0 nm with increasing

the specific surface area from 90 to 135 m2 g-1. It is

believed that a high specific surface area benefits electro-

chemical reactions because of the enhanced active sites for

electrolytes, and it is found that the reversible capacity

increases with increasing pore size and specific surface

area reaching the maximum capacity of 268 mAh g-1 for

the samples with a specific surface area of 135 m2 g-1 and

pore size of 7 nm. A remarkable high rate performance of

107 mAh g-1 up to 30 C is also exhibited by this sample

[107].

All the aforementioned discussions focused on reducing

the diffusion length of Li ions through decreasing the

particles size or generating porous structure, aiming to

overcome the shortcoming of low diffusion of Li ions in

TiO2. However, the poor electrical conductivity limits the

development of TiO2 as electrode materials in LIBs.

Reducing particles size or creating porous structure in the

individual particles has limited improvement on the elec-

trical conductivity because of the high resistance existing

between particles derived from interstices among particles.

There are two strategies to resolve this problem. One is to

dope TiO2 with elements possessing high electrical con-

ductivity to form composites, which will be discussed later.

The other one is to grow vertically aligned nanotubes

arrays on metal substrates or films with a 2D porous

structure and channels vertically to the substrates. The

arrangement of arrays of films will effectively avoid the

generation of interstices between particles leading to an

improved electrical conductivity, without affecting the

shortened diffusion length of Li ions benefited from the

porous structures.

Of all the available nanostructured TiO2 materials, well-

oriented nanotube arrays are perhaps one of these promising

forms for LIB anodes because of its several advantages. The

first is that the porous structure offers a short Li? diffusion

length in the solid phase and high accessibility for the

transport of electrolyte ions into the material framework,

thus potentially lowering the polarization and enhancing the

rate of charging and discharging. The second advantage is

that the large specific surface area of the porous structure

increases the electrode/electrolyte contact area and decrea-

ses the current density. The third potential advantage is that

the tubular structure can effectively accommodate the

expansion/contraction occurring during the lithium ion

insertion/removal process. In fact, the last mentioned

advantage of TiO2 nanotube arrays as anodes for LIBs has

been already utilized by several researchers. For instance,

Fang et al. reported that amorphous TiO2 nanotube arrays

with an average pores size of 54 nm and wall thickness of

10 nm (Fig. 3) exhibited a high capacity and excellent

Fig. 2 The initial charge/discharge voltage profiles (a, current

density if 10 mA g-1) and discharge capacities at various rates

(b) exhibited from TiO2-B nanowires. The examined sample was

obtained from hydrothermally treating anatase TiO2 powder in 10 M

solution of KOH at 200 �C for 24 h. TiO2(B)-t represented the

as-synthesized samples were treated by ion-exchanging process in

0.1 M nitric acid at t �C for 24 h. TiO2(B)-c indicated that the sample

was obtained by replacing KOH by NaOH and as-synthesized sample

was calcined at 400 �C (permission from reference of [95])

J Mater Sci (2012) 47:2519–2534 2525

123

capacity retention, with less than a 5% loss of capacity after

100 cycles [117]. Ortiz et al. demonstrated an improved

capacity retention of up to 90% over 50 cycles by utilizing

amorphous and anatase TiO2 nanotube arrays as LIB elec-

trodes, which had an average diameter of 80 nm and wall

thickness of 20 nm [118]. More recently, Wei et al. dem-

onstrated that excellent durability of 96.4% capacity reten-

tion over 140 cycles could be achieved in anatase TiO2

nanotubes arrays with an average pore diameter of 50 nm

and wall thickness of 25 nm [119]. However, the capacity

retention dropped quickly to 53% as the pore diameter and

wall thickness increased to 100 and 40 nm, respectively;

further decay of the capacity was observed as the pore

diameter and wall thickness continued to increase [119]. A

combined feature of these reports is that excellent capacity

retention (over 90%) can be observed over many cycles

when the wall thickness of TiO2 nanotubes is below 40 nm;

above this thickness, drastic capacity fading can be

observed, irrespective to the pore diameter. However, the

effects of wall thickness or the ratio of inner to the outer

diameter of the tubes, on the durability of titania nanotube

arrays electrodes has not been directly elucidated in the

above reports. Furthermore, Fang et al. [117] and Furukawa

et al. [120] separately reported that, compared with anatase

TiO2, amorphous TiO2 has higher capacity for lithium ions

because of the large density of defects in the disordered

structure.

Among various approaches explored to fabricate TiO2

nanotubes [121–124], anodic oxidation is the most widely

used since it was first applied to fabricate self-ordered TiO2

nanotube arrays by Grimes et al. in 2001 [125]. This

approach provides the possibility of controlling pore size,

length of nanotubes, and uniformity over large areas at low

cost. However, the electrolyte used for anodization was

aqueous hydrofluoric acid in the initial studies; therefore, it

was difficult or impossible to achieve nanotube arrays with

length greater than 500 nm in this electrolyte solution

[126]. Later, utilizing aqueous fluoride-containing solutions

with high pH values or nonaqueous solution consisting of

organic solvents allowed the fabrication of TiO2 nanotube

arrays with lengths up to hundreds of micrometers [127–

131]. However, the average pore diameters of nanotubes

derived from these electrolyte solutions are almost all above

50 nm and the wall thicknesses are above 15 nm. Schmuki

et al. reported that using neutral fluoride solutions com-

prised of (NH4)2SO4(1 M) and NH4F (0.5 wt%) as the

electrolyte solution can lead to TiO2 nanotube arrays with

lengths up to micrometers [132]. This strategy successfully

takes advantage of a relatively mild environment to reduce

the dissolution rate of TiO2 in the electrolyte solution

containing fluoride ions during anodization [133].

Besides anodizing technique, vertically aligned meso-

porous TiO2 nanotubes arrays with 100–200 nm in diam-

eter are also reported through surfactants-templated sol–gel

approach with the assistance of anodic aluminum oxide

(AAO) membranes. For these mesoporous nanotubes, a

remarkable specific capacity of 150 mAh g-1 was

obtained even at a current of 40 A g-1 (about 240 C), and

excellent cycling stability can be observed up to 100 cycles

[103].

The well-ordered mesoporous structure of TiO2 (Fig. 4)

makes it as another promising candidate for using as an

anode material in LIBs. Compared to nanotube arrays,

mesoporous structures afford TiO2 a thin films with higher

Fig. 3 SEM images of

anodized titania nanotubes

arrays before (a, top view; b,

cross section) and after

annealing at 480 �C for 3 h.

Samples were obtained through

anodizing Ti foil in the

electrolyte containing NH4F at

the constant voltage of 15 V for

17 h (permission from reference

of [117])

2526 J Mater Sci (2012) 47:2519–2534

123

specific surface area, thinner walls and a highly ordered

interconnected network of solid phase. Higher capacity

retention would be expected for electrodes fabricated from

these mesoporous TiO2 films. Several groups have reported

the advantages of using mesoporous TiO2 as electrode

materials in LIBs. For instance, Jiao et al. studied the

electrochemical performance of low temperature LiCoO2

as a cathode. The mesostructured LiCoO2 has higher initial

discharge capacity than that of nanowires, although both

forms exhibited superior capacity retention on cycling

compared with normal low temperature LiCoO2 [134]. Guo

et al. found that mesoporous TiO2 spheres have the capa-

bility to reversibly accommodate Li up to Li0.63TiO2 (210

mAh g-1) at 1–3 V versus Li?/Li, thus leading to a higher

capacity and better cycling performance compared with

commercial TiO2 [114]. Kim et al. demonstrated that the

capacity retention of SnO2 nanowires was 31% at a 10 C

rate (which is equal to 4,000 mA g-1), while that of

mesoporous SnO2 was improved up to 98% [135]. Porous

structures in these electrode materials not only provide

channels for the transport of electrolytes, but also act as a

buffer zone during the volume contraction and expansion

of Sn leading to a better retention of the capacity [135].

Wang et al. recently reported that mesoporous crystalline

rutile, obtained via a low temperature solution growth route

on TiO2 nanoparticles within an anionic surfactant matrix,

shows excellent capacity retention with less than 10%

capacity loss after more than 100 cycles [65]. The high

durability of this material is considered to be the result of

maintaining a stable mesostructure within the electrode

material over Li? ion insertion cycles [65]. Similar effect

of increased capacity retention could be expected while

using mesoporous TiO2 thin films as electrode materials in

LIBs. Therefore, it can be imagined that the utilization of

thin films with well ordered porous structure in a large area

as an electrode should have significant effects on the

reduction of strain energy and will open new opportunities

for the development of electrode materials with outstand-

ing electrochemical performance.

TiO2/carbon composite for LIB

It is well known that there is a conflict between the dis-

advantage of TiO2 and requirement of LIBs for electrode

material to store high quantity of Li ions with very fast

charge and discharge rates. All related reports mentioned

above demonstrated that it is challenging to compromise

the conflict just by tuning the structure and surface mor-

phology of TiO2. Thus, TiO2/carbon composite has been

developed to assist the storage rate of TiO2 by offering the

electrons to TiO2 [107, 136–138].

Ti2/CNTs composites

The carbon nanotubes (CNTs) with high conductivity and

stability can be directly used as electrodes materials for

LIBs after surface modification and are the ideal candi-

dates for the TiO2/carbon composites for LIBs [138–140].

Cao et al. have developed an TiO2/CNTs shell–core

nanostructure by a controlled hydrolysis of tetrabutylti-

tanate. The TiO2/CNTs composites demonstrated three

times of specific capacity (238 mAh g-1 at current density

of 5,000 mA g-1) for LIBs than that of the TiO2 without

CNTs (62 mAh g-1 at the current density of 5,000

mA g-1). The shell–core nanostructure of the TiO2/CNT

in the fully lithiated and delithiated states were shown in

TEM images, Fig. 5. The reason for improving the specific

capacity of this TiO2 composite is that the CNTs offer

sufficient electrons for TiO2 nanoshell to store the Li, and

TiO2 nanoshell provides Li ions to CNT for the storage

[136]. Meanwhile, Wang et al. reported that CNT–TiO2

nanowire composites can offer the double energy density

than that of CNT for super capacitor and LIB [141]. It is

also reported that MWCNTs and TiO2 nanoparticles

composites have been used for the electrode for LIBs.

According to the report by Reedy et al., these composites

have more than two times specific capacity of MWCNTs

[142].

TiO2/graphene composites

Graphene owns high conductivity on heat and electron

transport. Thus, the TiO2/graphene composites have been

investigated for the LIBs to reduce internal resistance and

irreversible heat during the charge and discharge process of

LIBs. Choi et al. have reported that the TiO2/graphene

Fig. 4 TEM image of mesoporous TiO2 thin film obtained from

P123-templated sol–gel process through dip-coating technique

J Mater Sci (2012) 47:2519–2534 2527

123

Fig. 5 TEM images of a fully

lithiated (a) and a fully

delithiated (b) bare CNTs; a

fully lithiated (c), and a fully

delithiated (d) core–shell

structure of CNT/TiO2 after ten

discharge/charge cycles under a

current density of

3,000 mA g-1 (permission from

[136])

2528 J Mater Sci (2012) 47:2519–2534

123

composites, which were synthesized by the approach of self-

assembly, show very ideal stability on specific capacity at

low energy density (125 mAh g-1 for 700 cycles) for LIBs

[143]. Furthermore, it was found that the structure and sur-

face morphology of TiO2 in TiO2/graphene composites for

LIBs could be tuned by the temperature during the atomic

layer deposition [144]. According to the references [25, 145],

reducing the particles for TiO2 will be helpful in increasing

the capacity, electroactivity, and cycling performance of the

electrodes. For the TiO2/graphene composites, graphene not

only supports good conductivity to the composites but also

prevents the aggregation of the TiO2 nanoparticles. For

example, Qiu et al. have carried out tuning the particles size

of anatase TiO2 nanospindles from 15 to 300 nm in the

composites of anatase@oxynitride/titanium nitride–graph-

ene for LIBs. It was achieved by the hydrothermal dissolu-

tion/recrystallization process. The composites performed the

better discharge capacity (314 mAh g-1 of composites at

initial, 287 mAh g-1 TiO2 nanospindles at initial) and

cycling performance of capacity, especially at high charge/

discharge rate (130 mAh g-1 at 12 C of composite, 75

mAh g-1 at 12 C of TiO2 nanospindle). This is mainly

because that the conductive matrix improves the electron

transport and prevents the aggregation of TiO2 nanoparticles

[146]. As shown in Fig. 6, no matter whether it is anatase or

rutile TiO2, the self-assembled composites of TiO2/func-

tionalized graphene was also proved to improve the specific

capacity two times more than that of pure TiO2 at high charge

rate of 30 C by Wang et al. [147]. Meanwhile, from the

cycling performance of the composite, Fig. 6c, f, the specific

capacity of the composite can be maintained at initial value

up to 100 cycles.

Other TiO2/carbon composites

Besides CNTs and graphene, other forms of carbon can

also be used for TiO2/carbon composites to improve the

properties (capacity, cycling performance, energy density,

etc.) for LIBs [148]. Carbon doped TiO2 nanotubes have

been reported by Xu et al., which were fabricated by a two-

step approach. It was done by the sol–gel method by fol-

lowing hydrothermal process. The TEM image, Fig. 7,

shows surface morphology and structure of TiO2 nanotubes

doped with carbon. From the initial sixth charge and dis-

charge curve in Fig. 8, it is obvious that the specific

capacity of carbon-doped TiO2 nanotubes was 30% higher

than that of the pure TiO2 nanotubes. They found that the

Li ions’ intercalation specific capacity can reach 291.7

Fig. 6 a Charge and discharge curve of pure rutile TiO2 and rutile

TiO2/graphene hybrid nanostructures at C/5 charge_discharge rates;

b specific capacity of pure TiO2 and the TiO2/graphene hybrids at

different charge/discharge rates; c cycling performance of the rutile

TiO2/grapheme composites up to 100 cycles at 1 C charge/discharge

rates after testing at various rates; d charge and discharge curve of

pure anatase TiO2 and anatase TiO2/grapheme hybrid nanostructures

at 5 C charge/discharge rates. e Specific capacity of pure anatase TiO2

and the anatase TiO2/grapheme composites at different charge/

discharge rates. f Cycling performance of the anatase TiO2/grapheme

composites up to 100 cycles at 1 C charge/discharge rates after testing

at various rates (permission from reference of [147])

J Mater Sci (2012) 47:2519–2534 2529

123

mAh g-1 at current density of 70 mA g-1 with columbic

efficiency of 91.7%. Furthermore, cycling performance of

the composites indicated that the specific capacity of

composites could stay at 211 mAh g-1 after 30 cycles. The

related CV and impendence measurement demonstrated

that the conductivity and charge-transfer rate of carbon-

doped TiO2 were greatly improved [149]. It seems that the

opposite structure of the carbon-doped TiO2 composites is

another more commonly used TiO2/carbon composites, in

which TiO2 is dispersed in the carbon matrix. The advan-

tages of this structure are the improvement of the electron

transport by a conductive matrix and prevention of the

aggregation of TiO2 nanoparticles, which will improve the

specific capacity and cycling performance of the compos-

ites as the electrode [148]. For example, the TiO2 nano-

particles were encapsulated in the porous carbon to form

TiO2/carbon nanowires. The composites were synthesized

by two-step hydrothermal method. The reversible capacity

of the composites as the electrode for LIBs could maintain

560 mAh g-1 at a current density of 30 mA g-1 and 200

mAh g-1 at a current density of 750 mA g-1 after 100

cycles [150]. The composites, in which TiO2 nanoparticles

were dispersed in carbon by the sol–gel method, have also

reported to possess specific capacity of 125 mAh g-1 at a

current density of 10,000 mA g-1 [120]. The TiO2/carbon

core–shell nanocomposites synthesized by polymerization

and carbonization process also have been proved to

improve diffusion coefficient of Li ions during the charge/

discharge process [151]. In addition to the chemical

methods for loading TiO2 on carbon matrix mentioned

above, the TiO2 nanoparticles have also been loaded by

simple mechanical method. The specific capacity reached

300 mAh g-1 at the charge rate of 0.25 C [148]. Not only

nanoparticles but also TiO2 nanotubes were used to dis-

perse in carbon to from TiO2/carbon composites, which

demonstrated a higher specific capacity at different

charging rate and better cycling performance than those of

pure TiO2 nanotubes [17].

Meanwhile, the mesoporous was a kind of the fantastic

structure of TiO2/carbon composites for the LIBs. The

mesoporous TiO2/carbon nanocomposites with several

nanometer pore sizes have been fabricated by Ishii et al.

with an approach of tri-constituent co-assembly and show a

maximum reversible capacity for the LIBs of 179 mAh g-1

at a current density of 50 mA g-1 and 109 mAh g-1 at a

current density of 1,000 mAh g-1 [137]. The similar

mesoporous microspheres of TiO2/carbon composites also

were obtained by self-assembled solvothermal synthesis.

The composites demonstrate a high specific capacity o 334

mAh g-1 at a current density of 66 mA g-1 and stable

cycling performance. The relative better properties of the

composites for LIBs is due to the specific nanostructure of

the composites [152].

TiO2/metal oxide and silicon composites for LIB

Metal oxides have also been considered as the anode

materials for the LIBs, which have respective advantages to

be used as the electrodes for the LIBs [70, 153, 154]. To

use the synthetic advantages of different metal oxides for

LIBs, the metal-oxide composites have been developed for

LIBs [155, 156]. The electrodes of metal oxides, which

have high specific capacity, are prone to fail during their

reaction with Li ions during the charge and discharge

processes. The TiO2 is introduced to these electrodes to

form TiO2/metal-oxide composites to prevent these fail-

ures. Thus TiO2/metal-oxide composites for LIBs are the

most common to combine the TiO2, which has good

cycling performance on capacity for LIBs, and other metal

Fig. 7 TEM image of the TiO2 nanotubes doped with carbon

(permission from [149])

Fig. 8 The initial discharge/charge curves of the carbon-doped TiO2

nanotube electrode and the undoped TiO2 nanotube electrode

(permission from [149])

2530 J Mater Sci (2012) 47:2519–2534

123

oxides with high capacity for LIBs are ZnO, SnO2, RuO2,

etc. [155–160]. The objective of these studies is to obtain

electrodes with high capacity and good stability on the

cycling performance to meet the application requirements

of LIBs for industry.

TiO2/ZnO composites

Many TiO2/ZnO composites have been fabricated for the

solar cell because of their advantages, such as flexibility of

ZnO on morphology, chemical stability of TiO2, and their

suitable band gap [161–163]. For the LIBs, ZnO also offers

very high capacity (978 mAh g-1) in theory. However, the

stability of cycling performance of ZnO for LIBs is very

poor. TiO2 has been well known for its good cycling per-

formance. Thus, TiO2/ZnO composites have been developed

to obtain the combination of advantages of TiO2 and ZnO

[156]. Recently, Lee et al. have reported that the composites

of ZnO nanorods coated with TiO2 via atomic layer depo-

sition demonstrated more stable cycling performance than

that of ZnO nanorods for LIBs. The structure and surface

morphology of the ZnO nanorod arrays uniformly coated

with TiO2 were observed, Fig. 9. The specific capacity of the

ZnO/TiO2 composite can maintain at about 450 mAh g-1 at

a current density of 50 mA g-1 after 30 cycles [155].

TiO2/SnO2 composites

Owing to the high capacity of 781 mAh g-1 of SnO2 for

LIBs, TiO2/SnO2 composites have been developed for the

LIBs [157–159]. Du et al. have succeeded on depositing

5-nm SnO2 nanocrystals into TiO2 nanotubes by solvo-

thermal approach to obtain the TiO2/SnO2 composites as

electrode for LIBs. The TiO2/SnO2 composites demon-

strated two times higher capacity than that of pure TiO2

nanotubes. Furthermore, the capacity of the composites can

be maintained at 70.8% of the initial value after 100 cycles.

The capacity of the composites can be tuned by the length

of TiO2 and loading amount of SnO2 [158]. Thin film of

TiO2/SnO2 was fabricated by Roginskaya et al. for LIBs by

the thermohydrolytic decomposition of mixed hydrochloric

acid solutions of chlorides of tin and titanium. The ratio of

TiO2 in composites was varied from 0 to 20%. The higher

the content of TiO2 in the composites, the more the stable

capacity the composites demonstrated for LIBs. The

mechanism is that the interface of TiO2/SnO2 hampers the

growth of b Sn to prevent the degradation of SnO2 during

the charging/discharging process of LIBs [159].

TiO2/RuO2 composites

RuO2 has very high specific capacity for LIBs, which is

about 1,130 mAh g-1 [164]. However, RuO2 is expensive.

Thus, RuO2 has been used as additive to TiO2/RuO2

composites for the supercapacitors and LIBs [160, 165].

The mesoporous TiO2/RuO2 composites have been syn-

thesized by loading the RuO2 nanoparticles to mesoporous

TiO2 spheres (300 nm in diameter) with hydrolysis of

RuCl3 and flowing by oxidation under O2. The specific

Fig. 9 SEM image of ZnO

nanorod array (a, b) and TiO2-

coated them (c, d) (permission

from [155])

J Mater Sci (2012) 47:2519–2534 2531

123

capacity of the mesoporous TiO2/RuO2 composites was

nine times higher than that of pure mesoporous TiO2 [160].

TiO2/silicon composites

The objective of TiO2/other-oxide composites for LIBs is to

combine the advantages of cycling performance stability

and low cost of TiO2 and high specific capacity of other

oxides. Thus, besides oxides of ZnO, SnO2, and RuO2,

silicon, with the highest capacity (4,200 mAh g-1) in the-

ory, has been used in TiO2 composites for LIBs [166, 167].

However, silica has poor cycling performance due to its

large volume expansion during the intercalation of lithium

ions. Park et al. have loaded the silica nanoparticles on

mesoporous TiO2 by sol–gel method. The well-organized

mesoporous TiO2 can prevent the failure of Si during the

charging/discharging processes. The specific capacity of Si/

TiO2 composites can be maintained at four times higher

than that of the pure Si nanoparticles after five cycles [168].

Conclusion

The LIB has already been commercialized for the past two

decades. To explore the application of the LIB in industry

like vehicle, numerous research studies have been focused

on the electrodes for the battery to improve its reliability,

energy density, and cost reduction, etc. TiO2 has great

advantages for LIB like low cost, and long and safe-cycling

life. However, the practical specific capacity of TiO2 is

relatively low. To improve the practical specific capacity of

TiO2, a series of the experiments about TiO2 nanomaterials

and TiO2 composites for LIB have been performed. The

related research study, mechanisms, and progresses have

been summarized in review. In conclusion, the low practical

specific capacity and energy density are due to the poor

electron and ion-transfer rate in TiO2, which prevent the

application of LIB with TiO2 as electrode. There are two

strategies for improving the specific capacity: (1) improving

the nanostructure of TiO2 like nanoparticles, nanorods, and

nanopores; (2) loading carbon, metal oxide, and silicon on

TiO2 to form the TiO2 composite. For improving the

nanostructure of TiO2 and loading carbon on TiO2 to form

TiO2 composites, both these strategies try to enhance the

specific capacity of the TiO2/TiO2 composite electrodes by

improving the ion and electrode conductivity. For loading

the high capacity materials like ZnO, SnO2, RuO2 and sil-

icon on the TiO2 matrix to form the composites, we tried to

obtain the synthetic advantage of high capacity and stable

cycling performance. To meet the requirement for broad-

ening the applications of LIB in industry like hybrid elec-

trical vehicle (HEV), the directions of research of TiO2 and

TiO2 composites for LIB in future could form in two

strategies. First, it is to develop more facile methods to

improve the nanostructures of TiO2 to get the high specific

capacity. Second, the more promising strategy is to combine

the materials of high specific capacity with TiO2 possessing

a stable cycling performance.

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