Journal of Nanomaterials
Volume 2016 (2016), Article ID 8123652, 15 pages
http://dx.doi.org/10.1155/2016/8123652

Review Article

Recent Progress of TiO₂-Based Anodes for Li Ion Batteries

Yu Liu¹ and Yefeng Yang²

¹School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
²Department of Materials Engineering, College of Materials and Textile, Zhejiang Sci-Tech University, Hangzhou 310018, China

Received 9 November 2015; Accepted 3 January 2016

Academic Editor: Donglu  Shi

Copyright © 2016 Yu Liu and Yefeng Yang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

TiO₂-based materials have been widely studied in the field of photocatalysis, sensors, and solar cells. Besides that, TiO₂-based materials are of great interest for energy storage and conversion devices, in particular rechargeable lithium ion batteries (LIBs). TiO₂ has significant advantage due to its low volume change (<4%) during Li ion insertion/desertions process, short paths for fast lithium ion diffusion, and large exposed surface offering more lithium insertion channels. However, the relatively low theoretical capacity and electrical conductivity of TiO₂ greatly hampered its practical application. Various strategies have been developed to solve these problems, such as designing different nanostructured TiO₂ to improve electronic conductivity, coating or combining TiO₂ with carbonaceous materials, incorporating metal oxides to enhance its capacity, and doping with cationic or anionic dopants to form more open channels and active sites for Li ion transport. This review is devoted to the recent progress in enhancing the LIBs performance of TiO₂ with various synthetic strategies and architectures control. Based on the lithium storage mechanism, we will also bring forward the existing challenges for future exploitation and development of TiO₂-based anodes in energy storage, which would guide the development for rationally and efficiently designing more efficient TiO₂-based LIBs anodes.

1. Introduction

Lithium ion batteries (LIBs) are becoming the best choice in portable electronics, implantable devices, power tools, and hybrid/full electric vehicles (EVs) for their high working voltage, low self-discharge rate, long cycle life, high energy, and power density [1, 2]. Using electric vehicles instead of traditional gasoline powered transportation can significantly reduce pollution of combustion gas and increase energy security. More importantly, the high energy efficiency of LIBs also has potential application in various large electric grid applications, including improving the energy efficiency of wind, solar, tidal, and other clean energy; thus LIBs are expected to have a very favorable impact on building an energy-sustainable economy [3, 4]. Figure 1 shows the forecasted evolution of the LIBs demand in the future years [5]; we think we will see economical battery-driven electric vehicles sooner than most people expect.

Figure 1: Forecasted expansion in demand for lithium ion batteries. Reprinted from [5].

Up to now, the vast majority of commercial LIBs rely, at the cathode side, on transition metals oxides or phosphates active material (LiCoO₂ [6], LiNiO₂ [7], LiMnO₂ [8], LiFePO₄ [9], LiMnPO₄ [10], etc.), while graphite is commonly used as anode active material. Figure 2 is the principle of a typical lithium ion battery; both anodes and cathodes could shuttle lithium ion back and forth between them. The electrolyte is usually made of polypropylene/polyethylene which contains lithium salts (i.e., LiPF₆) in alkyl organic carbonates. The separator between anode and cathode can allow the diffusion of Li ions from cathode to anode during the charging and the reverse discharging process.

Figure 2: Schematic representation of lithium insertion/deinsertion mechanism for current rechargeable lithium battery.

The anode is a crucial part in LIBs; therefore, the research and the development on the current situation of anode materials are one of the most important factors to determine the performance of this device. An ideal anode material shall meet the following requirements [11, 12]: (1) high specific surface area and large exposed surface offering more lithium insertion channels, (2) low volume change during Li ion insertion/desertions process, which is important for good cycling stability, (3) large pore size and short paths for fast lithium ion diffusion with high speed, which is crucial for good rate capability, (4) low internal resistance which leads to fast charging and discharging, (5) low intercalation potential for Li, (6) low price, (7) environment friendly. Based on the Li ion storage mechanisms, anode materials can be classified into the following categories: carbon based materials, such as graphite, amorphous carbon, carbon nanotubes, and graphene; alloy/dealloy materials, such as Si, Sn, Ge, Al, and Bi; transition metal oxides (MxOy, M = Cu, Mn, Fe, Co, Ni, etc.); metal sulphides; metal phosphides and metal nitrides [13–15]. Figure 3 shows the potential versus Li/Li⁺ and the corresponding capacity density of some potential active anode materials. In the whole, transition metal oxides always have relatively higher potential and capacity.

Figure 3: Comparing of some potential anode materials for lithium ion batteries.

Among these transition metal oxides, TiO₂ is one of the most promising anode candidates for LIBs, which exhibits excellent structural stability, high discharge voltage plateau (more than 1.7 V versus Li⁺/Li), excellent cycling stability, environmentally friendly, high safety, and low cost [16, 17]. However, some limitations of TiO₂ exist as well, such as low capacity, low electrical conductivity, and poor rate capability. Table 1 compares advantages and limitations of TiO₂ and other anode materials. The reversible lithium ion insertion and extraction from TiO₂ occur according to the following reaction [18]:

where can range between 0 and 1, depending strongly on the TiO₂ polymorph, particle size, and morphology. Therefore, the electrochemical performance of TiO₂ highly depends on their structural parameters such as crystallinity, size, morphology, polymorphs, and specific surface area. Table 2 summarizes the structural and electrochemical profiles of various TiO₂ polymorphs [19]. Amongst these, the anatase, rutile, brookite, and bronze phases of TiO₂ have been reported for LIBs applications. However, there are some problems which exist in practical application, that is, low electrical conductivity (10⁻¹²–10⁻⁷ s cm⁻¹) and diffusion coefficient of lithium ions (10⁻¹⁵–10⁻⁹ cm² s⁻¹), always leading to the poor rate capability of TiO₂ anodes, which result from their low electric conductivity with the lack of open channels [20–22].

Table 1: Comparison of advantages and limitations of TiO₂ and other anode materials [11–15].

Materials
Theoretical capacity
(mA h g⁻¹)
Advantages
Common issues

TiO₂
330
Fast lithium ion diffusion; low cost; environmentally friendly; good safety
Low capacity; low electrical conductivity; poor rate capability

Metal oxides (CuO, NiO, Fe₃O₄, etc.)
500–1200
High capacity; high energy; low cost
Low coulombic efficiency; unstable SEI formation; low electrical conductivity; poor capacity retention

Carbon
372
Good working potential; low cost;
good safety
Low coulombic efficiency; high irreversible capacity

Si
4200
High specific capacities
Large irreversible capacity; poor cycling

Sn
990
Good safety; low cost; good electrical conductivity
Poor cycling

Table 2: Structural and electrochemical properties of various TiO₂ polymorphs [19].

Structure
Space group
Density (g cm⁻³)
Lattice parameter values
Lithiation quantity (mole)

Bulk
⁢Nano

Rutile
Tetragonal P4 ₂/mnm
4.13
,

0.1
⁢0.85

Anatase
Tetragonal 14 ₁/amd
3.79
,

0.5
⁢1.0

Brookite
Orthorhombic Pbcv
3.99
,
,

0.1
⁢1.0

TiO₂-B (bronze)
Monoclinic C2/m
3.64
,
,
,

0.71
⁢1.0

TiO₂-II (Columbite)
Orthorhombic Pbcn
4.33
,
,

TiO₂-H (hollandite)
Tetragonal 14/m
3.46
,

TiO₂-III (baddeleyite)
Monoclinic P2 ₁/c

,
,
,

TiO₂-R (ramsdellite)
Orthorhombic Pbmn
3.87
,
,

TiO₁-O I
Orthorhombic

TiO₂-O II
Orthorhombic

Based on the analysis of shortcomings of TiO₂ anodes, several different strategies have been developed to address these issues of TiO₂-based anodes and summarized in this review, such as designing different nanostructured TiO₂, coating or combining TiO₂ with carbonaceous materials and metal oxides to change the physical and chemical surface, and selective doping with heteroatoms to form more open channels and active sites for Li ion transport, as well as increasing the intrinsic conductivity. Indeed, these methods lead to many advantages in improving the capacity, cycling stability, and rate capability of TiO₂.

2. Research on the LIBs Property of TiO₂-Based Anodes

2.1. Different Structures

Different structures usually exhibit unique performance based on their surface and structural properties. Thus, various morphologies of TiO₂ have been synthesized to obtain superior electrochemical properties.

2.1.1. One-Dimensional Nanostructures

One-dimensional (1D) nanostructures including nanorods, nanoneedles, nanotubes, nanofibers, and nanowires could serve as an electron express way along the axial direction for electron collection due to a shorter collection time for the efficient electron transportation [31, 32]. For example, single-crystalline TiO₂ nanowires have an electron mobility of ~1 cm² V⁻¹ s⁻¹, nearly 1-2 orders higher than that of polycrystalline nanoparticles [33, 34]. Thus, 1D nanostructure is conductive to shorten the diffusion length for electrons and lithium, increase the electrode/electrolyte interfacial area, and accommodate volume changes arising from the lithium ion insertion/extraction process [35]. Moreover, due to the unique structural flexibility, 1D material with good mechanical properties has potential in various binder-free and flexible electronics and photonics [36–38].

1D TiO₂ with different nanostructure (Figure 4) including nanotubes, nanofibers, and nanorods has been designed for high performance anodes in LIBs. The significance of 1D TiO₂ on battery performance was demonstrated by several groups. Tammawat and Meethong reported that anatase TiO₂ nanofiber anodes were directly used as an anode active material in LIBs without an additive or a binder. The nanofibers exhibited a high lithium storage capacity, a stable cycle life, and good rate capability [39]. The enhanced reversible capacity and cycling performance of the anatase TiO₂ nanofibers are attributed to the large surface area of the nanofibers, small nanocrystalline size, large Li nonstoichiometric parameters, and the increased electronic conductivity. Armstrong et al. prepared TiO₂ nanowires; these unique structures gave a higher capacity of 305 mA h g⁻¹ compared to 240 mA h g⁻¹ of bulk TiO₂ [40–42]. The enhanced capacity closely related to the good electronic conductivity and large surface area. Wei et al. reported a highly ordered anodic TiO₂ nanotube arrays with a tube length of 9 mm. These nanofibers exhibited significantly better microbattery performance (i.e., areal capacities, rate capability, and cycling stability) than both previously TiO₂-based electrodes and other 3D microbattery electrodes. They suggested that the enhanced performance depends strongly on the long range ordering and crystallinity of the nanotube structures [18]. Wang et al. prepared a hybrid Li ion capacitor based on TiO₂ nanobelt array and graphene hydrogels cathode. It is found that the densities of the capacitor can reach an energy density of 21 W h kg⁻¹ and a high power density of 19 kW kg⁻¹ [43]. The above studies also show that self-ordered 1D nanoarchitectures grown directly on a current collector are helpful to have a regularly oriented property and good contact with the current collector, enhancing the lithium ionic and electrical conductivities. Designing 1D structure is an effective way to improve the Li storage properties of TiO₂.

(a)

(b)

(c)

(d)

Figure 4: SEM images of different kinds of 1D TiO₂ nanostructures. Thin wall TiO₂ nanotubes ((a) and (b)), nanofibers (c), and nanorods (d). Reprinted from [18, 38, 44].

2.1.2. Two-Dimensional Structure

Two-dimensional (2D) nanomaterials often have large exposed surfaces and specific facets, which is very effective in high energy storage applications such as LIBs and supercapacitors. More importantly, 2D nanostructures can offer short ion diffusion length and open charge transport channel for electrolyte penetration and buffer the volume variations during the Li ion intercalation/deintercalation process [45–48]. Lithium insertion in this kind of material is just like surface lithium storage; both sides of 2D structure can store lithium ion, which can meet the requirement of fast and more lithium storage. A large number of 2D nanomaterials have been explored as anodes for LIBs, including graphene [49, 50], transition metal dichalcogenides (MoS₂, WS₂) [51, 52], ternary transition metal carbides (Ti₃C₂, Ti₂C) [53–55], and metal oxides (V₂O₅, MoO₃) [56, 57].

For TiO₂, 2D structures could provide stable framework, effective grain boundaries, and short path for lithium ion diffusion and storage compared with 0D nanoparticles and 1D nanostructures. Significant efforts have been made on the fabrication of 2D TiO₂ materials. Li et al. synthesized mesoporous TiO₂ nanoflakes with size of 10–20 nm via hydrothermal methods using Ti(SO₄)₂ as titanium source and NaOH solution as alkaline medium. The result of electrochemical performance test shows that the prepared TiO₂ nanoflakes with shorter calcining time have high discharge specific capacity (261.5 mA h g⁻¹) and good cycling performance [25]. In the process of heat treatment, longer calcining time results in uneven nanometer size and obvious reunion phenomenon. Shorter calcining time usually leads to more stable structure and higher specific surface area. Thus, both the lithium storage specific capacity of TiO₂ and the cycling stability of the battery can be improved [25]. Zhu et al. first synthesized the mesoporous single-grain layer anatase TiO₂ nanosheets using a simple and easily reproducible method. The obtained TiO₂ nanosheets exhibited a discharge capacity of 73 mA h g⁻¹ with obvious voltage plateaus over 4000 cycles, highlighting them as promising anode material for long-term LIBs [58]. Wu et al. demonstrated a simple and green approach for the synthesis of anatase petal-like TiO₂ nanosheets; the unique structure showed high capacity and good cycling stability. This is because obtained petal-like TiO₂ nanosheets showed a comparative surface area of 28.4 m² g⁻¹, which should provide shorter diffusion distance for Li ions and should be beneficial for electrochemical performance of the electrode [29]. Some typical TiO₂ nanosheets used in the lithium storage were listed in Table 3. It can be seen that 2D TiO₂ nanosheets exhibit the superior capacities, improved cycling stability and rate capabilities, owing to unique exposed facets, shortened path, and reserved porous structures.

Table 3: The capacity of reported 2D TiO₂ materials for lithium storage.

Number
Structures
⁢Performance
Ref.

Reversible capacity
Charge/discharge rates

1
Carbon-supported ultrathin anatase TiO₂ nanosheets
∼150 mA h g⁻¹
850 mA g⁻¹
[23]

2
Anatase TiO₂ nanosheets
∼150 mA h g⁻¹
1675 mA g⁻¹
[24]

3
TiO₂ nanoflakes
∼261 mA h g⁻¹
33 mA g⁻¹
[25]

4
2D rutile TiO₂-MoO₃ hybrid structure
∼240 mA h g⁻¹
600 mA g⁻¹
[26]

5
Mesoporous TiO₂ nanobelts and graphene sheets
∼430 mA h g⁻¹
1500 mA g⁻¹
[27]

6
TiO₂ hollow spheres
∼148 mA h g⁻¹
850 mA g⁻¹
[28]

7
Mesoporous anatase TiO₂ sheets/rGO
∼161 mA h g⁻¹
335 mA g⁻¹
[12]

8
petal-like TiO₂ nanosheets
∼180 mA h g⁻¹
400 mA g⁻¹
[29]

∼170 mA h g⁻¹
850 mA g⁻¹

9
Sandwich-like, stacked ultrathin titanate nanosheets
∼155 mA h g⁻¹
1700 mA g⁻¹
[30]

∼135 mA h g⁻¹
3400 mA g⁻¹

2.1.3. Three-Dimensional Porous Structure

Recently, three-dimensional (3D) porous structure materials exhibiting interesting electrochemical performance in LIBs have attracted more attention, due to their special nature including highly exposed skeleton, tunable pore size, high porosity, high specific surface area, and low bulk density [59, 60]. As described, first, the unique structure is conductive to enhance the diffusion kinetics for its short diffusion paths for Li ions. Second, the pores are beneficial to enable easy infiltration of electrolyte and fast liquid-phase Li ion diffusion, reducing the concentration polarization and increasing rate performance and capacity of the cell. Third, the continuous network of 3D porous structure can provide better electrical conductivity compared to loosely connected particles. Forth, the porosity in 3D structure should help in accommodating volume change during charging/discharging process and maintaining the structural integrity of the electrode [61, 62]. Up to now, different hollow structures such as hollow spheres, nanoboxes, and nanotubes are explored to be used as high performance LIBs anodes [63–66]. And the same happens for TiO₂; the introduction of porosity into TiO₂ nanomaterials also can improve the cycling stability and increase the capacity at high charge-discharge rates due to the increased contact surface area and shortened path length for diffusion of Li ions [67–70]. Highly crystalline, nonordered mesoporous TiO₂ nanocrystalline with high specific surface area and having anatase as the dominant phase have been reported by Gerbaldi and other researchers, which showed very high rate capability and excellent stability upon very prolonged cycling [71, 72]. Besides, the storage characteristics of the mesoporous samples in lithium test cells were reported, and a close correspondence between the structural properties of materials and the electrochemical performance was studied. The presence of mesopores is thought to be important for high rate performances and favorable for electrolyte ions transport. Lou’s group recently reported the TiO₂ hollow spheres and submicroboxes, owing to the high surface area, porous shells, and small primary nanoparticles; these TiO₂ hollow structures possess significantly improved lithium storage properties with superior lithium storage properties in terms of high specific capacity, long-term cycling stability, and excellent rate capability [73, 74]. Figure 5 shows the typical morphology of different hollow TiO₂ structures, all of which exhibit outstanding electrochemical performance.

(a)

(b)

(c)

(d)

Figure 5: The morphology of different hollow TiO₂ structures. Reprinted from [67, 68, 73, 74].

2.2. Coating or Combining TiO₂ with Carbonaceous Materials

Carbon materials such as active carbon, carbon nanotubes, and graphene have been extensively used for sorption, sensing, photocatalyst, electrocatalyst, and energy storage applications, owing to their abundance, accessibility, low health risk, suitable surface areas, and extreme chemical and thermal stabilities [75–80]. Especially in LIBs and supercapacitors, carbon materials are very popular for their superior conductivity, good chemical stability, and mechanical property [81–84].

2.2.1. Carbon Coating

Carbon coating is an effective and common approach to improve the electrochemical performance of the anode materials. The role of carbon has also been studied, such as reducing the charge transfer resistance and improving the Li ions diffusion, enhancing electron transport, buffering the large volume changes during the charge/discharge process, and acting as a passivation layer to prevent the aggregation of active materials [66, 85, 86]. Some research has proved that the SEI (solid electrolyte interphase) film for carbon coated materials was found to be much thinner than the SEI film on uncoated active materials; thus, initial charge-discharge efficiency can be greatly enhanced [87, 88]. For example, Xia et al. investigated the effect of carbon coating on TiO₂; these TiO₂/carbon hybrids could enhance electronic conductivity and provide flexible space for suppressing the large volume expansion during cycling [89]. E. Portenkirchner reports that the anatase -C nanotubes demonstrate a superior Li storage capacity as high as 320 (±68) mA h g⁻¹, nearly twice as high as pure . Electrochemical impedance spectroscopy reveals smaller charge transfer resistances for -C nanotubes at the solid/liquid interface which improves the transfer of lithium ions from the electrolyte into the electrode [90]. Besides, the composites also showed higher initial charge-discharge efficiency compared to pure TiO₂; the reason can be ascribed to the formation of thinner SEI films. Zheng and coworkers prepared nitrogen-containing carbon modified porous TiO₂ composites. The as-prepared composites also exhibited enhanced rate performance and superior cyclability for LIBs compared to pure TiO₂ (Figure 6). The study indicates that N doping is favorable to improve the electronic conductivity and the composites possessed much lower charge transfer resistance than that of TiO₂ [91].

(a)

(b)

Figure 6: The morphology of carbon coated TiO₂ spheres and their cycling performance. Reprinted from [91].

2.2.2. Combining TiO₂ with Carbon Nanotubes (CNTs)

In recent years, CNTs have been approved to be a good anode material for lithium batteries due to their unique 1D structure, high conductivity (10⁶ S m⁻¹ for single-walled carbon nanotubes and >10⁵ S m⁻¹ for multiwalled carbon nanotubes), low gravity (0.8–1.8 g cm⁻³), high mechanical properties (Young’s modulus of the order of 1.2 TPa), and high surface area (>100 m² g⁻¹) [92–96]. Some studies showed that CNTs could exhibit reversible capacities anywhere from 300 to 1000 mA h g⁻¹ after chemical treatment; the value is significantly higher than the theoretical capacity of graphite (320 mA h g⁻¹) [97–100]. Numerous CNTs conjugated with a variety of nanostructured materials and metal oxides have been synthesized to obtain good electrochemical performance [101–103]. For example, CNTs@TiO₂ composites have been synthesized by controlled hydrolysis of titanium isopropoxide over CNTs (as shown in Figures 7(a) and 7(b)). When CNTs are used as lithium ion battery electrodes, their inclusion is beneficial for an extreme enhancement of the rate capability of lithium ion uptake and release in TiO₂; it also favors the interfacial lithium ion intake from the solution by reducing the inherent charge transfer resistance. CNTs efficiently provide electrons to the nanostructure through the formation of Ti-C bonds, then effectively assisting lithium ion incorporation [104]. Zhao’s group synthesized TiO₂/CNTs composite through chemical vapor deposition method. The in situ synthesized composite showed better electrochemical performance (high specific capacity and long-term cycling stability) than the pristine TiO₂. This is because CNTs in the composites not only supply an efficient conductive network but also keep the structural stability of the TiO₂ particles, ultimately resulting in the improved electrochemical performance [105].

(a)

(b)

Figure 7: (a) and (b) are the TEM images of CNTs@TiO₂ nanocomposite material. Reprinted from [105].

2.2.3. Combining TiO₂ with Graphene

Graphene is a single atomic plane of graphite and consists in a honey comb network of sp² carbons bonded into two-dimensional sheets with nanometer thickness, due to its unique properties, including high intrinsic carrier mobility (200 000 cm² V⁻¹ s⁻¹), relevant mechanical strength, excellent conductivity (5000 W m⁻¹ K⁻¹), high optical transmittance (~97.7%), large theoretical specific surface area (2630 m² g⁻¹), and superior mechanical strength which make graphene a suitable anode material for LIBs [105–111]. Besides, the rich functional groups on the surface of graphene make it an appealing 2D substrate for the anisotropic growth of different kinds of active materials [112, 113]. For example, Fang el al.’s group prepared novel mesoporous graphene nanosheets with an excellent reversible capacity of 833 mA h g⁻¹ after 60 cycles [114]; this capacity is much higher than the theoretical lithium storage of graphite. This can be ascribed to the high contact surface area for lithium ion adsorption and intercalation, as well as edges and other defects. Thus, many synthetic strategies have been reported for TiO₂/rGO hybrid nanostructures; Ti-C bond in the hybrids is crucial for rapid interfacial charge transferring. Etacheri et al. chemically bonded mesoporous TiO₂ nanosheets to rGO sheets through a photocatalytic reduction method, resulting in the formation of Ti³⁺-C bonds between TiO₂ and rGO. These TiO₂/rGO hybrid nanostructures demonstrate superior specific capacity, excellent rate capability, and capacity retention compared to a physical mixture of TiO₂ and rGO [115]. The reason can be attributed to the higher electrochemical performance of TiO₂/rGO hybrid nanostructures to efficient interfacial charge transfer between TiO₂ nanosheets and rGO, which is fostered by Ti³⁺-C bonds. Figure 8 shows the SEM and digital images of TiO₂/rGO hybrid films; insets in Figure 8(b) display the flexibility of the corresponding films upon bending. The high flexibility of graphene could be an excellent supporting matrix or coating layer to accommodate the volume change during the charge/discharge process. This is crucial for maintaining the good cycling stability [116].

(a)

(b)

Figure 8: (a) and (b) are the SEM and digital images of TiO₂/rGO hybrid films, respectively. Insets display the flexibility of the corresponding films upon bending. Reprinted from [116].

2.3. Combining Metal Oxides with TiO₂

Combining different physical and electrochemical properties of components with TiO₂ and utilizing the respective advantage to increase the capacitance are a feasible method, such as using high conductive materials (conducting polymers) [117, 118], increasing the surface area (carbon nanotubes) [104, 105], and using high performance redox-active transition metal oxides (MnO₂) [119]. Among the above materials, metal oxide coatings can efficiently improve the capacitive performance of the materials through intruding synergistic effects into an electrode system, such as in SnOx@TiO₂ core-shell composites, due to the nearly zero volume change of TiO₂ in insertion of Li⁺ ions process, making it suitable as a backbone or protective layer for SnOx to restrain the pulverization and achieve an excellent high rate cycling ability and good cycling stability [120–123]. Recently, synergistic TiO₂-MoO₃ core-shell nanowire arrays were prepared via a facile hydrothermal growth of ordered TiO₂ nanowires followed by a subsequent controllable electrodeposition of nano-MoO₃. The composites exhibited high gravimetric capacity, good rate performance, and cycling stability. Figure 9 is the SEM images of the pristine TiO₂ nanowire array and optimized TiO₂-MoO₃ hybrid array anode with different magnifications. The strong synergistic effect existing in this design can be summarized as follows: (1) nearly negligible lattice changes during Li ion insertion/extraction, which is crucial to maintain excellent cycling stability. (2) The electrodeposited MoO₃ shell provides both reversible large capacity and good electrical conductivity for its nanosize effect and intrinsic characteristics. (3) The TiO₂ nanowire array can provide direct electron transport pathway between active material and current collector; Li ions can easily intercalate into the composites, manifesting an excellent rate capability and a significantly improved cycling performance [124]. Other transition metal oxides coating TiO₂ composites were also deeply investigated, such as TiO₂-V₂O₅, TiO₂-CoO, and TiO₂-SnO₂ [125–128].

(a)

(b)

(c)

(d)

Figure 9: (a) SEM images of the pristine TiO₂ nanowire array. (b)–(d) SEM images of the optimized TiO₂-MoO₃ hybrid array anode with different magnification. Reprinted from [124].

2.4. Doping with Ion or Atom Dopants

For the low electrical conductivity and ion diffusivity of TiO₂, doping with appropriate ions or atoms is advantageous since this method can improve the intrinsic nature of TiO₂ by adjusting its electronic structure, increase the internal surface area and electrical conductivity, and form more open channels and active sites for Li ion transport via the expanding interplanar spacing of TiO₂ lattices [129, 130]. The reported dopants include Fe³⁺ [131], Ti³⁺ [132], Sn⁴⁺ [133], B [134, 135], and N [136], all of which show beneficial effect on increasing the electrical conductivity more or less.

2.4.1. Ion Dopants

Doping Ti³⁺ in the TiO₂ structure can provide conduction band electrons and undoubtedly improves its conductivity, which also helps to increase the reversible capacity. Ren et al. presented a simple and controllable method to prepare the Ti³⁺ doped TiO₂ by a solvothermal process at lower temperature. The doped TiO₂ nanoparticles showed much enhanced electrochemical performance in reversible capacity, rate performance, and stability comparing with the pure TiO₂ [132]. This is because Ti³⁺ doping can increase the electrical conductivity of TiO₂. Liu et al. synthesized Ti³⁺ doped TiO₂ nanotube arrays which also exhibited excellent lithium ion intercalation performance with an initial discharge capacity of 101 mA h g⁻¹ at a high current density of 10 A g⁻¹ [137]. The much improved lithium ion intercalation properties were attributed to the easy phase transition promoted by the surface defects, that is, Ti-C, Ti³⁺, and O²⁻ vacancies, which could serve as nucleation centers. In addition, the rate performance was also improved due to the enhanced electrical conductivity. Sn⁴⁺, Fe³⁺, and other metal ions were also investigated as dopants to improve the electrochemical performance [133, 138]. Kyeremateng and coworkers reported that the Sn doped TiO₂ nanotubes delivered much higher capacity values compared to simple TiO₂ nanotubes. The outstanding electrochemical behaviour is proposed to be related to the enhanced lithium diffusivity evidenced with Cottrell plots (Figure 10) and the rutile-type structure imparted with the Sn doping. The results showed that lithium ion insertion into Sn doped TiO₂ is about 40 times faster than into undoped TiO₂ [133].

Figure 10: Cottrell plots for the determination of Li⁺ diffusion coefficients in TiO₂ and Sn doped TiO₂. Reprinted from [133].

2.4.2. Atom Dopants

Atoms doping is also a useful technique to increase the internal surface area and electrical conductivity of anode materials. For example, boron (B) and nitrogen (N) doping had been proven to be an effective strategy for improving the electrochemical performance of carbon materials [139–143]. For example, B doped graphite has a larger lattice constant value, ₀, and a smaller ₀₀₂ distance than ideal graphite, due to replacement of the carbon atoms with boron [144], leading to increases in both the crystallinity and electronic property of carbon as a Li-host material. Jeong et al. synthesized B doped TiO₂ materials through a simple one-pot process. The doped sample containing a relatively large amount of B possesses cylindrical pores that are favorable for lithium ion transfer, leading to the highest diffusion coefficient. Consequently, the doped anodes exhibit significantly improved cyclic capacities compared to the nondoped TiO₂ sample [134]. Furthermore, nitrogen doping has been proven to be an effective strategy for improving the capacity of TiO₂. This is because nitrogen doping can improve the electric conductivity as well as the ionic conductivity; after introducing the N atoms, the distortion of Ti-O lattice can affect the electrochemical reactions on the interfaces between electrodes and the electrolyte, as well as lithium ion diffusion in the Ti-O lattice [68, 145–147].

3. Conclusions and Outlook

In summary, this review showed the amount of research efforts towards the development and improvement of TiO₂-based anode materials for LIBs. Several elegant strategies aiming to boost the electrochemical performance and promote the practical application of TiO₂ have offered, including fabrication of nanostructures with different morphologies and sizes, modification by various coating materials (carbon materials and metal oxides), elements doping. The unique design allows achieving high lithium storage and good cycling stability based on the high lithium ion flux at the electrode/electrolyte interface, low internal resistance, short paths for fast lithium ion diffusion, and low volume change during Li ion insertion/desertions process. When combining these exquisite features together, it is possible for maximizing their electrochemical advantages to meet the present energy demands.

Firstly, the performance of TiO₂ depends strongly on its particle size and morphology. Therefore, different structures of TiO₂ are explored to improve the electrochemical performance of TiO₂. In a second category, combining TiO₂ with carbonaceous materials such as active carbon, CNTs, and graphene, the composite anode materials can obtain moderate conductivity, large surface area, good chemical stability and mechanical property. In the third, metal oxides such as Fe₂O₃, SnO₂, and MnO₂ can provide larger capacities and high energy density compared to pure TiO₂, which had been combined to improve the overall anode performance. Fourthly, for the low electrical conductivity and ion diffusivity of TiO₂, doping with appropriate ions or atoms is advantageous since this method can improve the intrinsic nature of TiO₂ by adjusting its electronic structure and forming more open channels and active sites for Li ion transport via the expanding interplanar spacing of TiO₂ lattices.

Finally, from this short review, we can conclude that high energy density, high cycle life, and high efficiency battery will still be the mainstream in the future growth of lithium batteries. In order to utilize the TiO₂-based materials as effective anodes in commercial LIBs, interdisciplinary effort in this area is however required.

Although considerable advances have been achieved in improving the Li ion storage performance of TiO₂, several fundamental issues are still needed to be solved. For example, nanomaterials usually show large surface area, which leads to more significant side reactions and results in low coulombic efficiency. Besides, nanopowder has lower density compared to the block material, which would reduce the volumetric energy density of battery. The following two possible strategies may be helpful to solve the abovementioned problem: (1) adopting surface modification or coating to reduce unnecessary side reactions; (2) designing hierarchical structures to enhance the tap density of anode materials.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was financially supported by Start-Up Foundation of Jiangsu University (15JDG144), the National Natural Science Foundation of China (no. 51402262), the 521 Talent Project of Zhejiang Sci-Tech University, and Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant no. 13012143-Y.

References

Z. Yang, D. Choi, S. Kerisit et al., “Nanostructures and lithium electrochemical reactivity of lithium titanites and titanium oxides: a review,” Journal of Power Sources, vol. 192, no. 2, pp. 588–598, 2009.

L. Jabbour, R. Bongiovanni, D. Chaussy, C. Gerbaldi, and D. Beneventi, “Cellulose-based Li-ion batteries: a review,” Cellulose, vol. 20, no. 4, pp. 1523–1545, 2013.

P. Xiong, L. Peng, D. Chen, Y. Zhao, X. Wang, and G. Yu, “Two-dimensional nanosheets based Li-ion full batteries with high rate capability and flexibility,” Nano Energy, vol. 12, pp. 816–823, 2015.

D. Ma, Z. Cao, and A. Hu, “Si-based anode materials for Li-ion batteries: a mini review,” Nano-Micro Letters, vol. 6, no. 4, pp. 347–358, 2014.

A. Yoshino, “The birth of the lithium-ion battery,” Angewandte Chemie—International Edition, vol. 51, no. 24, pp. 5798–5800, 2012.

L. Xia, Y. Xia, and Z. Liu, “Thiophene derivatives as novel functional additives for high-voltage LiCoO₂ operations in lithium ion batteries,” Electrochimica Acta, vol. 151, pp. 429–436, 2015.

S. Muto, K. Tatsumi, Y. Kojima et al., “Effect of Mg-doping on the degradation of LiNiO₂-based cathode materials by combined spectroscopic methods,” Journal of Power Sources, vol. 205, pp. 449–455, 2012.

H. Ji, G. Yang, X. Miao, and A. Hong, “Efficient microwave hydrothermal synthesis of nanocrystalline orthorhombic LiMnO₂ cathodes for lithium batteries,” Electrochimica Acta, vol. 55, no. 9, pp. 3392–3397, 2010.

X. Yin, K. Huang, S. Liu, H. Wang, and H. Wang, “Preparation and characterization of Na-doped LiFePO₄/C composites as cathode materials for lithium-ion batteries,” Journal of Power Sources, vol. 195, no. 13, pp. 4308–4312, 2010.

T. Doi, S. Yatomi, T. Kida, S. Okada, and J.-I. Yamaki, “Liquid-phase synthesis of uniformly nanosized LiMnPO₄ particles and their electrochemical properties for lithium-ion batteries,” Crystal Growth & Design, vol. 9, no. 12, pp. 4990–4992, 2009.

S. Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. P. Zaccaria, and C. Capiglia, “Review on recent progress of nanostructured anode materials for Li-ion batteries,” Journal of Power Sources, vol. 257, pp. 421–443, 2014.

L. Bai, F. Fang, Y. Zhao et al., “A sandwich structure of mesoporous anatase TiO₂ sheets and reduced graphene oxide and its application as lithium-ion battery electrodes,” RSC Advances, vol. 4, no. 81, pp. 43039–43046, 2014.

N. Nitta, F. Wu, J. T. Lee, and G. Yushin, “Li-ion battery materials: present and future,” Materials Today, vol. 18, no. 5, pp. 252–264, 2015.

X. Zheng and J. Li, “A review of research on hematite as anode material for lithium-ion batteries,” Ionics, vol. 20, no. 12, pp. 1651–1663, 2014.

V. J. Babu, S. Vempati, T. Uyar, and S. Ramakrishna, “Review of one-dimensional and two-dimensional nanostructured materials for hydrogen generation,” Physical Chemistry Chemical Physics, vol. 17, no. 5, pp. 2960–2986, 2015.

X. Li, Y. Zhang, T. Li, Q. Zhong, H. Li, and J. Huang, “Graphene nanoscrolls encapsulated TiO₂ (B) nanowires for lithium storage,” Journal of Power Sources, vol. 268, pp. 372–378, 2014.

J. Jin, S.-Z. Huang, J. Liu et al., “Design of new anode materials based on hierarchical, three dimensional ordered macro-mesoporous TiO₂ for high performance lithium ion batteries,” Journal of Materials Chemistry A, vol. 2, no. 25, pp. 9699–9708, 2014.

W. Wei, G. Oltean, C.-W. Tai, K. Edström, F. Björefors, and L. Nyholm, “High energy and power density TiO₂ nanotube electrodes for 3D Li-ion microbatteries,” Journal of Materials Chemistry A, vol. 1, no. 28, pp. 8160–8169, 2013.

V. Aravindan, Y. Lee, R. Yazami, and S. Madhavi, “TiO₂ polymorphs in ’rocking-chair‘ Li-ion batteries,” Materials Today, vol. 18, no. 6, pp. 345–351, 2015.

P. Bottke, Y. Ren, I. Hanzu, P. G. Bruce, and M. Wilkening, “Li ion dynamics in TiO₂ anode materials with an ordered hierarchical pore structure—insights from ex situ NMR,” Physical Chemistry Chemical Physics, vol. 16, no. 5, pp. 1894–1901, 2014.

D. Yuan, W. Yang, J. Ni, and L. Gao, “Sandwich structured MoO₂@TiO₂@CNT nanocomposites with high-rate performance for lithium ion batteries,” Electrochimica Acta, vol. 163, pp. 57–63, 2015.

C. Yang, Z. Wang, T. Lin et al., “Core-shell nanostructured ‘black’ rutile titania as excellent catalyst for hydrogen production enhanced by sulfur doping,” Journal of the American Chemical Society, vol. 135, no. 47, pp. 17831–17838, 2013.

J. S. Chen, H. Liu, S. Z. Qiao, and X. W. Lou, “Carbon-supported ultra-thin anatase TiO₂ nanosheets for fast reversible lithium storage,” Journal of Materials Chemistry, vol. 21, no. 15, pp. 5687–5692, 2011.

J. S. Chen and X. W. Lou, “Anatase TiO₂ nanosheet: an ideal host structure for fast and efficient lithium insertion/extraction,” Electrochemistry Communications, vol. 11, no. 12, pp. 2332–2335, 2009.

T.-D. Nguyen-Phan, V. H. Pham, H. Kweon et al., “Uniform distribution of TiO₂ nanocrystals on reduced graphene oxide sheets by the chelating ligands,” Journal of Colloid and Interface Science, vol. 367, no. 1, pp. 139–147, 2012.

J. S. Chen and X. W. Lou, “Unusual rutile TiO₂ nanosheets with exposed (001) facets,” Chemical Science, vol. 2, no. 11, pp. 2219–2223, 2011.

H. Huang, J. Fang, Y. Xia et al., “Construction of sheet-belt hybrid nanostructures from one-dimensional mesoporous TiO₂(B) nanobelts and graphene sheets for advanced lithium-ion batteries,” Journal of Materials Chemistry A, vol. 1, no. 7, pp. 2495–2500, 2013.

S. Ding, J. S. Chen, Z. Wang et al., “TiO₂ hollow spheres with large amount of exposed (001) facets for fast reversible lithium storage,” Journal of Materials Chemistry, vol. 21, no. 6, pp. 1677–1680, 2011.

F. Wu, Z. Wang, X. Li, and H. Guo, “Simple preparation of petal-like TiO₂ nanosheets as anode materials for lithium-ion batteries,” Ceramics International, vol. 40, no. 10, pp. 16805–16810, 2014.

J. Liu, J. S. Chen, X. Wei, X. W. Lou, and X.-W. Liu, “Sandwich-like, stacked ultrathin titanate nanosheets for ultrafast lithium storage,” Advanced Materials, vol. 23, no. 8, pp. 998–1002, 2011.

J. Tian, Z. Zhao, A. Kumar, R. I. Boughton, and H. Liu, “Recent progress in design, synthesis, and applications of one-dimensional TiO₂ nanostructured surface heterostructures: a review,” Chemical Society Reviews, vol. 43, no. 20, pp. 6920–6937, 2014.

B. Yuan and L. Cademartiri, “Flexible one-dimensional nanostructures: a review,” Journal of Materials Science & Technology, vol. 31, no. 6, pp. 607–615, 2015.

X. Feng, K. Shankar, O. K. Varghese, M. Paulose, T. J. Latempa, and C. A. Grimes, “Vertically aligned single crystal TiO₂ nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications,” Nano Letters, vol. 8, no. 11, pp. 3781–3786, 2008.

Q. Tian, Z. Zhang, L. Yang, and S.-I. Hirano, “Facile fabrication of one-dimensional mesoporous titanium dioxide composed of nanocrystals for lithium storage,” Electrochimica Acta, vol. 138, pp. 155–162, 2014.

Y. Tang, Z. Liu, X. Lü, B. Wang, and F. Huang, “TiO₂ nanotubes grown on graphene sheets as advanced anode materials for high rate lithium ion batteries,” RSC Advances, vol. 4, no. 68, pp. 36372–36376, 2014.

S. Peng, L. Li, Y. Hu et al., “Fabrication of spinel one-dimensional architectures by single-spinneret electrospinning for energy storage applications,” ACS Nano, vol. 9, no. 2, pp. 1945–1954, 2015.

Y. Tan, Q. Gao, C. Yang, K. Yang, W. Tian, and L. Zhu, “One-dimensional porous nanofibers of Co₃O₄ on the carbon matrix from human hair with superior lithium ion storage performance,” Scientific Reports, vol. 5, Article ID 12382, 2015.

J. L. Xie, C. X. Guo, and C. M. Li, “Construction of one-dimensional nanostructures on graphene for efficient energy conversion and storage,” Energy & Environmental Science, vol. 7, no. 8, pp. 2559–2579, 2014.

P. Tammawat and N. Meethong, “Synthesis and characterization of stable and binder-free electrodes of TiO₂ nanofibers for Li-ion batteries,” Journal of Nanomaterials, vol. 2013, Article ID 413692, 8 pages, 2013.

A. R. Armstrong, G. Armstrong, J. Canales, and P. G. Bruce, “TiO₂-B nanowires,” Angewandte Chemie—International Edition, vol. 43, no. 17, pp. 2286–2288, 2004.

A. R. Armstrong, G. Armstrong, J. Canales, and P. G. Bruce, “TiO₂-B nanowires as negative electrodes for rechargeable lithium batteries,” Journal of Power Sources, vol. 146, no. 1-2, pp. 501–506, 2005.

A. R. Armstrong, G. Armstrong, J. Canales, R. García, and P. G. Bruce, “Lithium-ion intercalation into TiO₂-B nanowires,” Advanced Materials, vol. 17, no. 7, pp. 862–865, 2005.

H. Wang, C. Guan, X. Wang, and H. J. Fan, “A high energy and power Li-ion capacitor based on a TiO₂ nanobelt array anode and a graphene hydrogel cathode,” Small, vol. 11, no. 12, pp. 1470–1477, 2015.

H.-R. Xia, J. Li, C. Peng, W.-T. Sun, L.-W. Li, and L.-M. Peng, “Floating growth of large-scale freestanding TiO₂ nanorod films at the gas-liquid interface for additive-free li-ion battery applications,” ACS Applied Materials & Interfaces, vol. 6, no. 20, pp. 17376–17383, 2014.

T. Hu, X. Sun, H. Sun et al., “Flexible free-standing graphene-TiO₂ hybrid paper for use as lithium ion battery anode materials,” Carbon, vol. 51, no. 1, pp. 322–326, 2013.

C. Wang, D. Li, C. O. Too, and G. G. Wallace, “Electrochemical properties of graphene paper electrodes used in lithium batteries,” Chemistry of Materials, vol. 21, no. 13, pp. 2604–2606, 2009.

Y. Liu, W. Wang, L. Gu et al., “Flexible CuO nanosheets/reduced-graphene oxide composite paper: binder-free anode for high-performance lithium-ion batteries,” ACS Applied Materials & Interfaces, vol. 5, no. 19, pp. 9850–9855, 2013.

Y. Liu, W. Wang, H. Huang, L. Gu, Y. Wang, and X. Peng, “The highly enhanced performance of lamellar WS₂ nanosheet electrodes upon intercalation of single-walled carbon nanotubes for supercapacitors and lithium ions batteries,” Chemical Communications, vol. 50, no. 34, pp. 4485–4488, 2014.

L. Ma, X. Zhou, L. Xu, X. Xu, L. Zhang, and W. Chen, “Chitosan-assisted fabrication of ultrathin MoS₂/graphene heterostructures for Li-ion battery with excellent electrochemical performance,” Electrochimica Acta, vol. 167, pp. 39–47, 2015.

T. Zhang, F. Zhang, L. Zhang et al., “High energy density Li-ion capacitor assembled with all graphene-based electrodes,” Carbon, vol. 92, pp. 106–118, 2015.

H. Liu, D. Su, G. Wang, and S. Z. Qiao, “An ordered mesoporous WS₂ anode material with superior electrochemical performance for lithium ion batteries,” Journal of Materials Chemistry, vol. 22, no. 34, pp. 17437–17440, 2012.

H. Hwang, H. Kim, and J. Cho, “MoS₂ nanoplates consisting of disordered graphene-like layers for high rate lithium battery anode materials,” Nano Letters, vol. 11, no. 11, pp. 4826–4830, 2011.

A. N. Enyashin and A. L. Ivanovskii, “Structural and electronic properties and stability of MX enes Ti₂C and Ti₃C₂ functionalized by methoxy groups,” Journal of Physical Chemistry C, vol. 117, no. 26, pp. 13637–13643, 2013.

J. Come, M. Naguib, P. Rozier et al., “A non-aqueous asymmetric cell with a Ti₂C-based two-dimensional negative electrode,” Journal of the Electrochemical Society, vol. 159, no. 8, pp. A1368–A1373, 2012.

Y. Liu, W. Wang, Y. Ying, Y. Wang, and X. Peng, “Binder-free layered Ti₃C₂/CNTs nanocomposite anodes with enhanced capacity and long-cycle life for lithium-ion batteries,” Dalton Transactions, vol. 44, no. 16, pp. 7123–7126, 2015.

C.-L. Liu, Y. Wang, C. Zhang, X.-S. Li, and W.-S. Dong, “In situ synthesis of α-MoO₃/graphene composites as anode materials for lithium ion battery,” Materials Chemistry and Physics, vol. 143, no. 3, pp. 1111–1118, 2014.

Y. Qian, A. Vu, W. Smyrl, and A. Stein, “Facile preparation and electrochemical properties of V₂O₅-graphene composite films as free-standing cathodes for rechargeable lithium batteries,” Journal of the Electrochemical Society, vol. 159, no. 8, pp. A1135–A1140, 2012.

K. Zhu, X. Liu, J. Du et al., “In situ synthesis of mesoporous single-grain layer anatase TiO₂ nanosheets without additives via a mild and simple process for a long-term Li-ion battery,” Journal of Materials Chemistry A, vol. 3, no. 12, pp. 6455–6463, 2015.

X. Lü, F. Huang, J. Wu, S. Ding, and F. Xu, “Intelligent hydrated-sulfate template assisted preparation of nanoporous TiO₂ spheres and their visible-light application,” ACS Applied Materials & Interfaces, vol. 3, no. 2, pp. 566–572, 2011.

X. Lü, S. Ding, Y. Xie, and F. Huang, “Non-aqueous preparation of high-crystallinity hierarchical TiO₂ hollow spheres with excellent photocatalytic efficiency,” European Journal of Inorganic Chemistry, no. 18, pp. 2879–2883, 2011.

S. Ding, T. Lin, Y. Wang, X. Lü, and F. Huang, “New facile synthesis of TiO₂ hollow sphere with an opening hole and its enhanced rate performance in lithium-ion batteries,” New Journal of Chemistry, vol. 37, no. 3, pp. 784–789, 2013.

S. Ding, Y. Wang, Z. Hong, X. Lü, D. Wan, and F. Huang, “Biomolecule-assisted route to prepare titania mesoporous hollow structures,” Chemistry—A European Journal, vol. 17, no. 41, pp. 11535–11541, 2011.

G. Grinbom, D. Duveau, G. Gershinsky, L. Monconduit, and D. Zitoun, “Silicon/hollow gamma-Fe₂O₃ nanoparticles as efficient anodes for Li-ion batteries,” Chemistry of Materials, vol. 27, no. 7, pp. 2703–2710, 2015.

X. Wang, Y. Wang, L. Yang, K. Wang, X. Lou, and B. Cai, “Template-free synthesis of homogeneous yolk–shell TiO₂ hierarchical microspheres for high performance lithium ion batteries,” Journal of Power Sources, vol. 262, pp. 72–78, 2014.

W. Wei, L.-X. Song, and L. Guo, “SnO₂ hollow nanospheres assembled by single layer nanocrystals as anode material for high performance Li ion batteries,” Chinese Chemical Letters, vol. 26, no. 1, pp. 124–128, 2015.

S. Zhang, W. He, X. Zhang, and X. Yang, “Rational design of carbon-coated hollow MnO nanotubes for Li-ion batteries,” Journal of Materials Science: Materials in Electronics, vol. 26, no. 4, pp. 2189–2197, 2015.

Y.-C. Chang, C.-W. Peng, P.-C. Chen, C.-Y. Lee, and H.-T. Chiu, “Bio-ingredient assisted formation of porous TiO₂ for Li-ion battery electrodes,” RSC Advances, vol. 5, no. 44, pp. 34949–34955, 2015.

G. Hasegawa, T. Sato, K. Kanamori, K. Nakanishi, and T. Abe, “Synthesis and electrochemical performance of hierarchically porous N-doped TiO₂ for Li-ion batteries,” New Journal of Chemistry, vol. 38, no. 4, pp. 1380–1384, 2014.

H. Ming, Y. Yan, J. Ming et al., “Porous TiO₂ nanoribbons and TiO₂ nanoribbon/carbon dot composites for enhanced Li-ion storage,” RSC Advances, vol. 4, no. 25, pp. 12971–12976, 2014.

H.-W. Shim, D. K. Lee, I.-S. Cho, K. S. Hong, and D.-W. Kim, “Facile hydrothermal synthesis of porous TiO₂ nanowire electrodes with high-rate capability for Li ion batteries,” Nanotechnology, vol. 21, no. 25, Article ID 255706, 2010.

F. Di Lupo, A. Tuel, V. Mendez et al., “Mesoporous TiO₂ nanocrystals produced by a fast hydrolytic process as high-rate long-lasting Li-ion battery anodes,” Acta Materialia, vol. 69, pp. 60–67, 2014.

S. Casino, F. Di Lupo, C. Francia, A. Tuel, S. Bodoardo, and C. Gerbaldi, “Surfactant-assisted sol gel preparation of high-surface area mesoporous TiO₂ nanocrystalline Li-ion battery anodes,” Journal of Alloys and Compounds, vol. 594, pp. 114–121, 2014.

G. Zhang, H. B. Wu, T. Song, U. Paik, and X. W. Lou, “TiO₂ hollow spheres composed of highly crystalline nanocrystals exhibit superior lithium storage properties,” Angewandte Chemie—International Edition, vol. 53, no. 46, pp. 12590–12593, 2014.

X.-Y. Yu, H. B. Wu, L. Yu, F.-X. Ma, and X. W. D. Lou, “Rutile TiO₂ submicroboxes with superior lithium storage properties,” Angewandte Chemie—International Edition, vol. 54, no. 13, pp. 4001–4004, 2015.

S. Li and X. Wang, “Micro Li-ion capacitor with activated carbon/graphite configuration for energy storage,” Journal of Power Sources, vol. 282, pp. 394–400, 2015.

C. Yuan, L. Zhang, S. Zhu, H. Cao, J. Lin, and L. Hou, “Heterostructured core-shell ZnMn₂O₄ nanosheets@carbon nanotubes' coaxial nanocables: a competitive anode towards high-performance Li-ion batteries,” Nanotechnology, vol. 26, no. 14, Article ID 145401, 2015.

J.-M. Feng, L. Dong, Y. Han, X.-F. Li, and D.-J. Li, “Efficient exfoliation N-doped graphene from N-containing bamboo-like carbon nanotubes for anode materials of Li-ion battery and Na-ion battery,” Applied Physics A: Materials Science & Processing, vol. 120, no. 2, pp. 471–478, 2015.

R. Jin, G. Li, Z. Zhang, L. Yang, and G. Chen, “Carbon coated flower like Bi₂S₃ grown on nickel foam as binder-free electrodes for electrochemical hydrogen and Li-ion storage capacities,” Electrochimica Acta, vol. 173, pp. 458–464, 2015.

X. Lu, F. Yang, Y. Wang, X. Geng, and P. Xiao, “The influence of graphene/carbon mass ratio on microstructure and electrochemical behavior in the graphene–SnO₂–carbon composite as anodes for Li-ion batteries,” Journal of Alloys and Compounds, vol. 636, pp. 202–210, 2015.

C. Zhang, H. Song, C. Liu et al., “Fast and reversible Li ion insertion in carbon-encapsulated Li₃VO₄ as anode for lithium-ion battery,” Advanced Functional Materials, vol. 25, no. 23, pp. 3497–3504, 2015.

S. Fang, L. Shen, Z. Tong, H. Zheng, F. Zhang, and X. Zhang, “Si nanoparticles encapsulated in elastic hollow carbon fibres for Li-ion battery anodes with high structural stability,” Nanoscale, vol. 7, no. 16, pp. 7409–7414, 2015.

X. Lv, J. Deng, J. Wang, J. Zhong, and X. Sun, “Carbon-coated alpha-Fe₂O₃ nanostructures for efficient anode of Li-ion battery,” Journal of Materials Chemistry A, vol. 3, no. 9, pp. 5183–5188, 2015.

S. Wang, C. Xiao, Y. Xing, H. Xu, and S. Zhang, “Carbon nanofibers/nanosheets hybrid derived from cornstalks as a sustainable anode for Li-ion batteries,” Journal of Materials Chemistry A, vol. 3, no. 13, pp. 6742–6746, 2015.

S. Wang, Y. Xing, C. Xiao, X. Wei, H. Xu, and S. Zhang, “Hollow carbon-shell/carbon-nanorod arrays for high performance Li-ion batteries and supercapacitors,” RSC Advances, vol. 5, no. 11, pp. 7959–7963, 2015.

M. Ara, V. R. Chitturi, S. O. Salley, and K. Y. S. Ng, “Nitrogen-doped carbon-coated SnxOy (x=1 and y=0 and 2) nanoparticles for rechargeable Li-ion batteries,” Electrochimica Acta, vol. 161, pp. 269–278, 2015.

X. H. Wang, C. Guan, L. M. Sun, R. A. Susantyoko, H. J. Fan, and Q. Zhang, “Highly stable and flexible Li-ion battery anodes based on TiO2 coated 3D carbon nanostructures,” Journal of Materials Chemistry A, vol. 3, no. 30, pp. 15394–15398, 2015.

S. S. Tang, G. R. Li, B. H. Liu, and Z. P. Li, “Carbon-coated titanium dioxide micro-bowls as an anode material for Li-ion batteries,” Electrochimica Acta, vol. 125, pp. 199–205, 2014.

G. Lu, S. Qiu, H. Lv et al., “Li-ion storage performance of MnO nanoparticles coated with nitrogen-doped carbon derived from different carbon sources,” Electrochimica Acta, vol. 146, pp. 249–256, 2014.

T. Xia, W. Zhang, Z. Wang et al., “Amorphous carbon-coated TiO₂ nanocrystals for improved lithium-ion battery and photocatalytic performance,” Nano Energy, vol. 6, pp. 109–118, 2014.

J. Brumbarov, J. P. Vivek, S. Leonardi, C. Valero-Vidal, E. Portenkirchner, and J. Kunze-Liebhäuser, “Oxygen deficient, carbon coated self-organized TiO₂ nanotubes as anode material for Li-ion intercalation,” Journal of Materials Chemistry A, vol. 3, no. 32, pp. 16469–16477, 2015.

Y. Fu, H. Ming, Q. Zhou, L. Jin, X. Li, and J. Zheng, “Nitrogen-doped carbon coating inside porous TiO₂ using small nitrogen-containing molecules for improving performance of lithium-ion batteries,” Electrochimica Acta, vol. 134, pp. 478–485, 2014.

J. A. Puértolas and S. M. Kurtz, “Evaluation of carbon nanotubes and graphene as reinforcements for UHMWPE-based composites in arthroplastic applications: a review,” Journal of the Mechanical Behavior of Biomedical Materials, vol. 39, pp. 129–145, 2014.

Q. Wang and B. Arash, “A review on applications of carbon nanotubes and graphenes as nano-resonator sensors,” Computational Materials Science, vol. 82, pp. 350–360, 2014.

C. Kang, R. Baskaran, J. Hwang, B.-C. Ku, and W. Choi, “Large scale patternable 3-dimensional carbon nanotube-graphene structure for flexible Li-ion battery,” Carbon, vol. 68, pp. 493–500, 2014.

H. Liu, “Carbon nanotubes anchored with SnO₂ nanosheets as anode for enhanced Li-ion storage,” Journal of Materials Science: Materials in Electronics, vol. 25, no. 8, pp. 3353–3357, 2014.

H. Pan, “Graphitic carbon nitride nanotubes as Li-ion battery materials: a first-principles study,” The Journal of Physical Chemistry C, vol. 118, no. 18, pp. 9318–9323, 2014.

A. Varzi, C. Täubert, M. Wohlfahrt-Mehrens, M. Kreis, and W. Schütz, “Study of multi-walled carbon nanotubes for lithium-ion battery electrodes,” Journal of Power Sources, vol. 196, no. 6, pp. 3303–3309, 2011.

H. Zhang, G. Cao, Z. Wang, Y. Yang, Z. Shi, and Z. Gu, “Carbon nanotube array anodes for high-rate Li-ion batteries,” Electrochimica Acta, vol. 55, no. 8, pp. 2873–2877, 2010.

J. Chen, J. Z. Wang, A. I. Minett et al., “Carbon nanotube network modified carbon fibre paper for Li-ion batteries,” Energy & Environmental Science, vol. 2, no. 4, pp. 393–396, 2009.

A. Antic, V. Barone, and B. D. Fahlman, “Comparison of oxidized carbon nanotubes for Li-ion storage capacity,” Journal of Applied Electrochemistry, vol. 45, no. 2, pp. 161–167, 2015.

J.-C. Kim, I.-S. Hwang, S.-D. Seo, and D.-W. Kim, “Nanocomposite Li-ion battery anodes consisting of multiwalled carbon nanotubes that anchor CoO nanoparticles,” Materials Letters, vol. 104, pp. 13–16, 2013.

J.-C. Kim, I.-S. Hwang, S.-D. Seo et al., “Superior long-term cycling stability of SnO₂ nanoparticle/multiwalled carbon nanotube heterostructured electrodes for Li-ion rechargeable batteries,” Nanotechnology, vol. 23, no. 46, Article ID 465402, 2012.

L. Zhuo, Y. Wu, J. Ming et al., “Facile synthesis of a Co₃O₄-carbon nanotube composite and its superior performance as an anode material for Li-ion batteries,” Journal of Materials Chemistry A, vol. 1, no. 4, pp. 1141–1147, 2013.

P. Acevedo-Peña, M. Haro, M. E. Rincón, J. Bisquert, and G. Garcia-Belmonte, “Facile kinetics of Li-ion intake causes superior rate capability in multiwalled carbon nanotube@TiO₂ nanocomposite battery anodes,” Journal of Power Sources, vol. 268, pp. 397–403, 2014.

Y.-X. Wang, J. Xie, G.-S. Cao, T.-J. Zhu, and X.-B. Zhao, “Electrochemical performance of TiO₂/carbon nanotubes nanocomposite prepared by an in situ route for Li-ion batteries,” Journal of Materials Research, vol. 27, no. 2, pp. 417–423, 2012.

A. W. Anwar, A. Majeed, N. Iqbal et al., “Specific capacitance and cyclic stability of graphene based metal/metal oxide nanocomposites: a review,” Journal of Materials Science & Technology, vol. 31, no. 7, pp. 699–707, 2015.

S. M. Lee, J.-H. Kim, and J.-H. Ahn, “Graphene as a flexible electronic material: mechanical limitations by defect formation and efforts to overcome,” Materials Today, vol. 18, no. 6, pp. 336–344, 2015.

H. Miao, Y. Xue, X. Zhou, and Z. Liu, “Graphene-based oxygen reduction reaction catalysts for metal air batteries,” Progress in Chemistry, vol. 27, no. 7, pp. 935–944, 2015.

H. Wang, X. Yuan, G. Zeng et al., “Three dimensional graphene based materials: synthesis and applications from energy storage and conversion to electrochemical sensor and environmental remediation,” Advances in Colloid and Interface Science, vol. 221, pp. 41–59, 2015.

D. Li, Y. Liu, B. Lin, Y. Sun, H. Yang, and X. Zhang, “Graphene/metal oxide composites as electrode material for supercapacitors,” Progress in Chemistry, vol. 27, no. 4, pp. 404–415, 2015.

M. Mao, J. Hu, and H. Liu, “Graphene-based materials for flexible electrochemical energy storage,” International Journal of Energy Research, vol. 39, no. 6, pp. 727–740, 2015.

Y. Zhao, X. Li, B. Yan, D. Li, S. Lawes, and X. Sun, “Significant impact of 2D graphene nanosheets on large volume change tin-based anodes in lithium-ion batteries: a review,” Journal of Power Sources, vol. 274, pp. 869–884, 2015.

G. Fang, S. Kaneko, W. Liu et al., “Persistent cyclestability of carbon coated Zn-Sn metal oxide/carbon microspheres as highly reversible anode material for lithium-ion batteries,” Electrochimica Acta, vol. 111, pp. 627–634, 2013.

Y. Fang, Y. Lv, R. Che et al., “Two-dimensional mesoporous carbon nanosheets and their derived graphene nanosheets: synthesis and efficient lithium ion storage,” Journal of the American Chemical Society, vol. 135, no. 4, pp. 1524–1530, 2013.

V. Etacheri, J. E. Yourey, and B. M. Bartlett, “Chemically bonded TiO₂-Bronze nanosheet/reduced graphene oxide hybrid for high-power lithium ion batteries,” ACS Nano, vol. 8, no. 2, pp. 1491–1499, 2014.

J. Kim, W.-H. Khoh, B.-H. Wee, and J.-D. Hong, “Fabrication of flexible reduced graphene oxide-TiO₂ freestanding films for supercapacitor application,” RSC Advances, vol. 5, no. 13, pp. 9904–9911, 2015.

H. Aydın and A. Bozkurt, “Nanocomposite polymer electrolytes comprising PVA-graft-PEGME/TiO₂ for Li-ion batteries,” Journal of Materials Research, vol. 29, no. 5, pp. 625–632, 2014.

J. Cao, L. Wang, Y. Shang et al., “Dispersibility of nano-TiO₂ on performance of composite polymer electrolytes for Li-ion batteries,” Electrochimica Acta, vol. 111, pp. 674–679, 2013.

Q. Zhu, H. Hu, G. Li, C. Zhu, and Y. Yu, “TiO₂ nanotube arrays grafted with MnO₂ nanosheets as high-performance anode for lithium ion battery,” Electrochimica Acta, vol. 156, pp. 252–260, 2015.

S. Park, S.-D. Seo, S. Lee et al., “Sb:SnO₂@TiO₂ heteroepitaxial branched nanoarchitectures for Li ion battery electrodes,” The Journal of Physical Chemistry C, vol. 116, no. 41, pp. 21717–21726, 2012.

X. Wu, S. Zhang, L. Wang et al., “Coaxial SnO₂@TiO₂ nanotube hybrids: from robust assembly strategies to potential application in Li⁺ storage,” Journal of Materials Chemistry, vol. 22, no. 22, pp. 11151–11158, 2012.

C. Guan, X. Wang, Q. Zhang, Z. Fan, H. Zhang, and H. J. Fan, “Highly stable and reversible lithium storage in SnO₂ nanowires surface coated with a uniform hollow shell by atomic layer deposition,” Nano Letters, vol. 14, no. 8, pp. 4852–4858, 2014.

J.-H. Jeun, K.-Y. Park, D.-H. Kim et al., “SnO₂@TiO₂ double-shell nanotubes for a lithium ion battery anode with excellent high rate cyclability,” Nanoscale, vol. 5, no. 18, pp. 8480–8483, 2013.

C. Wang, L. Wu, H. Wang, W. Zuo, Y. Li, and J. Liu, “Fabrication and shell optimization of synergistic TiO₂-MoO₃ core-shell nanowire array anode for high energy and power density lithium-ion batteries,” Advanced Functional Materials, vol. 25, no. 23, pp. 3524–3533, 2015.

M. J. Armstrong, D. M. Burke, T. Gabriel et al., “Carbon nanocage supported synthesis of V₂O₅ nanorods and V₂O₅/TiO₂ nanocomposites for Li-ion batteries,” Journal of Materials Chemistry A, vol. 1, no. 40, pp. 12568–12578, 2013.

M. Madian, L. Giebeler, M. Klose et al., “Self-organized TiO₂/CoO nanotubes as potential anode materials for lithium ion batteries,” ACS Sustainable Chemistry & Engineering, vol. 3, no. 5, pp. 909–919, 2015.

D. Ma, P. Dou, X. Yu et al., “Novel hollow SnO₂ nanosphere@TiO₂ yolk-shell hierarchical nanospheres as anode material for high-performance lithium-ion batteries,” Materials Letters, vol. 157, pp. 228–230, 2015.

C. Chen, S. H. Lee, M. Cho, and Y. Lee, “Core-shell CuO@TiO₂ nanorods as a highly stable anode material for lithium-ion batteries,” Materials Letters, vol. 140, pp. 111–114, 2015.

X. Lü, W. Yang, Z. Quan et al., “Enhanced electron transport in Nb-doped TiO₂ nanoparticles via pressure-induced phase transitions,” Journal of the American Chemical Society, vol. 136, no. 1, pp. 419–426, 2014.

X. Lü, X. Mou, J. Wu et al., “Improved-performance dye-sensitized solar cells using Nb-doped TiO₂ electrodes: efficient electron injection and transfer,” Advanced Functional Materials, vol. 20, no. 3, pp. 509–515, 2010.

S. K. Das, M. Gnanavel, M. U. M. Patel, C. Shivakumara, and A. J. Bhattacharyya, “Anamolously high lithium storage in mesoporous nanoparticulate aggregation of Fe³⁺ doped anatase titania,” Journal of the Electrochemical Society, vol. 158, no. 12, pp. A1290–A1297, 2011.

Y. Ren, J. Li, and J. Yu, “Enhanced electrochemical performance of TiO₂ by Ti³⁺ doping using a facile solvothermal method as anode materials for lithium-ion batteries,” Electrochimica Acta, vol. 138, pp. 41–47, 2014.

N. A. Kyeremateng, F. Vacandio, M.-T. Sougrati et al., “Effect of Sn-doping on the electrochemical behaviour of TiO₂ nanotubes as potential negative electrode materials for 3D Li-ion micro batteries,” Journal of Power Sources, vol. 224, pp. 269–277, 2013.

J.-H. Jeong, D.-W. Jung, E. W. Shin, and E.-S. Oh, “Boron-doped TiO₂ anode materials for high-rate lithium ion batteries,” Journal of Alloys and Compounds, vol. 604, pp. 226–232, 2014.

H. Tian, F. Xin, X. Tan, and W. Han, “High lithium electroactivity of boron-doped hierarchical rutile submicrosphere TiO₂,” Journal of Materials Chemistry A, vol. 2, no. 27, pp. 10599–10606, 2014.

Y. Zhang, Q. Fu, Q. Xu et al., “Improved electrochemical performance of nitrogen doped TiO₂-B nanowires as anode materials for Li-ion batteries,” Nanoscale, vol. 7, no. 28, pp. 12215–12224, 2015.

D. Liu, Y. Zhang, P. Xiao et al., “TiO₂ nanotube arrays annealed in CO exhibiting high performance for lithium ion intercalation,” Electrochimica Acta, vol. 54, no. 27, pp. 6816–6820, 2009.

V. Gentili, S. Brutti, L. J. Hardwick, A. R. Armstrong, S. Panero, and P. G. Bruce, “Lithium insertion into anatase nanotubes,” Chemistry of Materials, vol. 24, no. 22, pp. 4468–4476, 2012.

H. Fujimoto, A. Mabuchi, C. Natarajan, and T. Kasuh, “Properties of graphite prepared from boron-doped pitch as an anode for a rechargeable Li ion battery,” Carbon, vol. 40, no. 4, pp. 567–574, 2002.

R. P. Hardikar, D. Das, S. S. Han, K.-R. Lee, and A. K. Singh, “Boron doped defective graphene as a potential anode material for Li-ion batteries,” Physical Chemistry Chemical Physics, vol. 16, no. 31, pp. 16502–16508, 2014.

M. Sahoo, K. P. Sreena, B. P. Vinayan, and S. Ramaprabhu, “Green synthesis of boron doped graphene and its application as high performance anode material in Li ion battery,” Materials Research Bulletin, vol. 61, pp. 383–390, 2015.

R. Gokhale, V. Aravindan, P. Yadav et al., “Oligomer-salt derived 3D, heavily nitrogen doped, porous carbon for Li-ion hybrid electrochemical capacitors application,” Carbon, vol. 80, no. 1, pp. 462–471, 2014.

T. Sharifi, M. Valvo, E. Gracia-Espino, R. Sandström, K. Edström, and T. Wågberg, “Hierarchical self-assembled structures based on nitrogen-doped carbon nanotubes as advanced negative electrodes for Li-ion batteries and 3D microbatteries,” Journal of Power Sources, vol. 279, pp. 581–592, 2015.

U. Tanaka, T. Sogabe, H. Sakagoshi, M. Ito, and T. Tojo, “Anode property of boron-doped graphite materials for rechargeable lithium-ion batteries,” Carbon, vol. 39, no. 6, pp. 931–936, 2001.

Y. Zhou, Q. Bao, L. A. L. Tang, Y. Zhong, and K. P. Loh, “Hydrothermal dehydration for the ‘green’ reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties,” Chemistry of Materials, vol. 21, no. 13, pp. 2950–2956, 2009.

C. D. Joyce, T. McIntyre, S. Simmons et al., “Synthesis and electrochemical evaluation of an amorphous titanium dioxide derived from a solid state precursor,” Journal of Power Sources, vol. 195, no. 7, pp. 2064–2068, 2010.

S. Chen, W. Chu, Y. Y. Huang, X. Liu, and D. G. Tong, “Preparation of porous nitrogen-doped titanium dioxide microspheres and a study of their photocatalytic, antibacterial and electrochemical activities,” Materials Research Bulletin, vol. 47, no. 12, pp. 4514–4521, 2012.