**4. Titanium dioxide derivatives as effective electrode materials**

Recent years have seen intense development in research aimed at seeking new materials and design solutions to enable further progress in the technology of lithium-ion batteries, which are seen as one of the leading technologies for energy storage. Currently, the greatest challenge in the design of these batteries is to find an optimum combination of cathode and anode materials, as these largely determine the cell's parameters, including capacity, voltage, reversibility of the charge/discharge reaction, and chemical stability. The electrode materials must not only be compatible with each other but also should form a synergic system together with the electrolyte and separator [143–145].

Among a range of available materials, titanium dioxide and its derivatives have recently gained popularity as anodes for Li-ion batteries because they allow the design of operational devices with only minor safety concerns. This class of materials offers improved chemical and thermal stability, low cost, biocompatibility, relatively high surface area and porosity, a broad electrochemical window, rate capability and enhanced cyclic performance by virtue of their superior electrical conductivity. These features make titania-based derivatives a good candidate to replace the commonly used carbon (graphene) as an anode material in LIBs. However, limitations include the low capacity, low electrical conductivity, poor rate capability and poor cycling performance of titanium dioxide. Much research has been carried out to overcome the difficulties related to the use of TiO<sup>2</sup> as an electrode material. Numerous scientific centres worldwide are working on ways of improving the electrochemical behaviour of titania and its derivatives. The chief aim is to enhance the electronic conductivity by producing different titania nanostructures to increase its capacity through the incorporation of selected metal oxides into its structure. Another approach is to combine or coat TiO<sup>2</sup> with carbonaceous materials or to introduce anionic or cationic dopants to form more open channels and active sites for Li ion transport [143–149].

Moreover, the electrochemical performance and the lithium intercalation/de-intercalation processes of titania-based materials typically depend on their crystallinity, structure, morphology, particle size and surface area. In particular, it has been found that nanostructured titanium oxide leads to better capacity, longer cycling life and higher rate capability than the bulk materials. Titania has several allotropic forms, the best-known being tetragonal rutile and anatase, and orthorhombic brookite. Even though anatase has been considered the most electroactive form, other allotropes such as rutile and brookites are also widely studied for potential use as anodes. Moreover, synthesis of this type of system can effectively improve their capacitive performance by creating products with excellent high-rate cycling ability and stability. The application of such hybrid materials as anodes in lithium-ion batteries should lead to charge redistribution in the lattice, facilitate the diffusion of Li<sup>+</sup> , and finally increase lattice defects and conductivity [145–152].

the combination of TiO<sup>2</sup>

168 Titanium Dioxide

obtaining unique titania-based materials.

the electrolyte and separator [143–145].

the difficulties related to the use of TiO<sup>2</sup>

sites for Li ion transport [143–149].

with polymers or various forms of carbon nanotubes, fullerenes, gra-

as an electrode material. Numerous scientific centres

with carbonaceous

phene oxide (GO) or reduced GO (R-GO), with the aim of obtaining multifunctional materials with a wide range of applications. With this in mind, it should be emphasised how many opportunities and technological solutions are available for implementation with the goal of

Recent years have seen intense development in research aimed at seeking new materials and design solutions to enable further progress in the technology of lithium-ion batteries, which are seen as one of the leading technologies for energy storage. Currently, the greatest challenge in the design of these batteries is to find an optimum combination of cathode and anode materials, as these largely determine the cell's parameters, including capacity, voltage, reversibility of the charge/discharge reaction, and chemical stability. The electrode materials must not only be compatible with each other but also should form a synergic system together with

Among a range of available materials, titanium dioxide and its derivatives have recently gained popularity as anodes for Li-ion batteries because they allow the design of operational devices with only minor safety concerns. This class of materials offers improved chemical and thermal stability, low cost, biocompatibility, relatively high surface area and porosity, a broad electrochemical window, rate capability and enhanced cyclic performance by virtue of their superior electrical conductivity. These features make titania-based derivatives a good candidate to replace the commonly used carbon (graphene) as an anode material in LIBs. However, limitations include the low capacity, low electrical conductivity, poor rate capability and poor cycling performance of titanium dioxide. Much research has been carried out to overcome

worldwide are working on ways of improving the electrochemical behaviour of titania and its derivatives. The chief aim is to enhance the electronic conductivity by producing different titania nanostructures to increase its capacity through the incorporation of selected metal

materials or to introduce anionic or cationic dopants to form more open channels and active

Moreover, the electrochemical performance and the lithium intercalation/de-intercalation processes of titania-based materials typically depend on their crystallinity, structure, morphology, particle size and surface area. In particular, it has been found that nanostructured titanium oxide leads to better capacity, longer cycling life and higher rate capability than the bulk materials. Titania has several allotropic forms, the best-known being tetragonal rutile and anatase, and orthorhombic brookite. Even though anatase has been considered the most electroactive form, other allotropes such as rutile and brookites are also widely studied for potential use as anodes. Moreover, synthesis of this type of system can effectively improve their capacitive performance by creating products with excellent high-rate cycling ability and stability. The application of such hybrid materials as anodes in lithium-ion batteries should

oxides into its structure. Another approach is to combine or coat TiO<sup>2</sup>

**4. Titanium dioxide derivatives as effective electrode materials**

Kubiak et al. [153] investigated the electrochemical performance of a mesoporous TiO<sup>2</sup> synthesised via a sol-gel method utilising an ethylene glycol-based titanium precursor in the presence of an amphiphilic molecule as the templating agent. The obtained material presents pure anatase TiO<sup>2</sup> without the presence of other phases, with a monomodal pore diameter close to 5 nm and BET surface area of 92 m<sup>2</sup> /g. The mesoporous anatase titania shows excellent rate capability (184 mAh/g at C/5, 158 mAh/g at 2C, 127 mAh/g at 6C, and 95 mAh/g at 30C) and good cycling stability. The authors concluded that the electrochemical performance of anatase titania was determined not only by surface area and crystallite size but also by mesopore size. The presence of mesopores was important for high-rate performance and favourable to electrolyte ion transport.

Mancini et al. [154] found that new Cu or Sn/mesoporous anatase electrodes offer excellent electrochemical performance, especially in terms of fast insertion/extraction capacity. The capacity after 200 cycles is 123, 147 and 142 mAh/g for uncoated, Cu-coated and Sn-coated anatase electrodes, respectively, with capacity retention of about 80% for all electrodes. The good electrochemical behaviour of metal/mesoporous anatase TiO<sup>2</sup> is ascribed to the combined effects of mesopores and the electronically conductive metal layer. Moreover, the metal coating provides a lower polarisation of the electrodes, which indicates faster kinetics of the electrochemical processes. The researchers suggested that a thin metal coating may be a very promising method in the development of high-rate electrode materials for Li-ion batteries.

Kubiak et al. [155] produced nanosized rutile TiO<sup>2</sup> via a hydrolytic sol-gel route, applying a glycerol-modified precursor in the presence of an anionic surfactant. The proposed methodology led to rutile whiskers, which agglomerated to cauliflower-like aggregates of several micrometers, with a BET surface area of 181 m<sup>2</sup> /g. This interesting morphology of rutile titania favours contact between the active material and the electrolyte. The obtained material shows excellent electrochemical performance in terms of capacity, cyclability, stability and reversibility, especially at high charge/discharge rates. The authors demonstrated that this high rate capability can be ascribed to shorter transport lengths for both electronic and Li+ transport, as well as a larger electrode/electrolyte contact area due to the high surface area.

Mesoporous anatase TiO<sup>2</sup> was synthesised via a urea-assisted hydrothermal method by Jung et al. [156]. The authors investigated the influence of thermal treatment of mesoporous TiO<sup>2</sup> at 300, 400 and 500°C on its electrochemical performance. The prepared material was found to consist of monophasic TiO<sup>2</sup> sub-microspheres with uniform particle size (ca. 400 nm), a crystallite size of 14 nm and a BET surface area of 116 m<sup>2</sup> /g. The capacity for the mesoporous TiO<sup>2</sup> calcined at 400°C after 80 cycles is 154 mAh/g, with capacity retention of about 94.5%. It was concluded that the large surface area introduced by the highly porous nano-structured building blocks of each TiO<sup>2</sup> sub-microsphere assisted in creating an easy and shorter diffusion pathway for ionic and electronic diffusion. These results indicate the good power performance of the synthesised material.

Zhang et al. [157] showed that hierarchical nanostructures and composition play key roles in the electrochemical performance of TiO<sup>2</sup> hollow microspheres used as anode materials. Mesoporous hollow TiO<sup>2</sup> microspheres with controlled size and hierarchical nanostructures were synthesised by hydrothermal methods. The results show that the hollow microspheres composed of mesoporous nanospheres exhibit a very stable reversible capacity of 184 mAh/g at 0.25C and an extremely high power of 122 mAh/g at the high rate of 10C. It was also shown that the hollow structure and large mesoporous channels of the material facilitate electrolyte transportation and lithium ion diffusion, and the small mesopores and small-sized nanoparticles increase the lithium storage capacity.

Metal oxides are one of the promising classes of materials to replace graphite anodes for LIBs, because these materials have diverse chemical and physical properties and can deliver high reversible capacities between 500 and 1000 mAh/g. 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. To prevent these failures, TiO<sup>2</sup> is introduced into these electrodes to form TiO<sup>2</sup> /metal-oxide composites. The TiO<sup>2</sup> /metal-oxide composites for LIBs most commonly combine TiO<sup>2</sup> , which has good cycling performance and capacity for LIBs, with other metal oxides with high capacity for LIBs such as SiO<sup>2</sup> , ZnO, ZrO<sup>2</sup> , etc [154, 155, 158].

Opra et al. [159] obtained nanostructured Zr-doped (1 at.%) TiO<sup>2</sup> (anatase) via a template solgel method on carbon fibre. The obtained material consisted of microtubes (length 10–300 μm, outer diameter 3–5 μm) composed of nanoparticles with a size of 15–25 nm. Moreover, Zr-doped TiO<sup>2</sup> shows significantly higher reversible capacity (140 mAh/g) after 20-fold cycling at a rate of 0.1C in the range 3–1 V in comparison with undoped titania (65 mAh/g). It was reported that the transport of Li+ ions depends significantly on the structural characteristics of titania [17]. When Zr4+ ions are incorporated into the anatase structure, the difference in the ionic radii of the metal ions increases the lattice parameters and creates defects. The creation of defects leads to charge redistribution in the titania lattice and increases the conductivity (according to EIS results).

Gao et al. [160] reported the successful production of TiO<sup>2</sup> /ZnO nanocomposite arrays for lithium-ion battery application. The sandwich-like TiO<sup>2</sup> /ZnO framework with 3D interconnected construction shows stable cycling performance with a specific capacity of 340 mAh/g at a current density of 200 mA/g after 100 cycles. The authors noted that the uniform decoration of ZnO nanoparticles onto the TiO<sup>2</sup> nanosheet arrays plays a significant role in advancing their electrochemical performance.

Siwińska-Stefańska and Kurc [161] used a novel TiO<sup>2</sup> -SiO<sup>2</sup> -ZrO<sup>2</sup> (TSZ) ternary oxide system (with a TiO<sup>2</sup> :SiO<sup>2</sup> :ZrO<sup>2</sup> molar ratio of 8:1:1) synthesised via a sol-gel route as an anode material in a Li-ion battery. They combined titania with silica, which can react with a low discharge potential and can store a large quantity of lithium ions, as well as with zirconia, which is capable of suppressing SEI formation and enhancing electron transport to improve electrochemical performance. The specific discharge/charge capacity of the TSZ electrode is about 175 mAh/g.

Unique TiO<sup>2</sup> nanotube arrays (TNAs) grafted with MnO<sup>2</sup> nanosheets were synthesised as a Li-ion battery anode by Zhu et al. [162]. The obtained composite combines the advantages of both MnO<sup>2</sup> , with its high capacity (1230 mAh/g), and TNAs, with excellent cycle stability and superior electrical conductivity. Sample TM-10 demonstrated a capacity of 610 mAh/g at a current rate of 350 mA/g and a capacity of 385 mAh/g at a rate of 700 mA/g even after 700 cycles. It was proved that the layer thickness of MnO<sup>2</sup> has a major impact on electrochemical performance.

Zhang et al. [157] showed that hierarchical nanostructures and composition play key roles

were synthesised by hydrothermal methods. The results show that the hollow microspheres composed of mesoporous nanospheres exhibit a very stable reversible capacity of 184 mAh/g at 0.25C and an extremely high power of 122 mAh/g at the high rate of 10C. It was also shown that the hollow structure and large mesoporous channels of the material facilitate electrolyte transportation and lithium ion diffusion, and the small mesopores and small-sized nanopar-

Metal oxides are one of the promising classes of materials to replace graphite anodes for LIBs, because these materials have diverse chemical and physical properties and can deliver high reversible capacities between 500 and 1000 mAh/g. The electrodes of metal oxides, which have high specific capacity, are prone to fail during their reaction with Li ions during the

gel method on carbon fibre. The obtained material consisted of microtubes (length 10–300 μm, outer diameter 3–5 μm) composed of nanoparticles with a size of 15–25 nm. Moreover,

at a rate of 0.1C in the range 3–1 V in comparison with undoped titania (65 mAh/g). It was

of titania [17]. When Zr4+ ions are incorporated into the anatase structure, the difference in the ionic radii of the metal ions increases the lattice parameters and creates defects. The creation of defects leads to charge redistribution in the titania lattice and increases the conductivity

nected construction shows stable cycling performance with a specific capacity of 340 mAh/g at a current density of 200 mA/g after 100 cycles. The authors noted that the uniform decora-

in a Li-ion battery. They combined titania with silica, which can react with a low discharge potential and can store a large quantity of lithium ions, as well as with zirconia, which is capable of suppressing SEI formation and enhancing electron transport to improve electrochemical performance. The specific discharge/charge capacity of the TSZ electrode is about 175 mAh/g.

Li-ion battery anode by Zhu et al. [162]. The obtained composite combines the advantages

, with its high capacity (1230 mAh/g), and TNAs, with excellent cycle stability

shows significantly higher reversible capacity (140 mAh/g) after 20-fold cycling


molar ratio of 8:1:1) synthesised via a sol-gel route as an anode material


hollow microspheres used as anode materials.

is introduced into these elec-

, etc [154, 155, 158].

(anatase) via a template sol-

/ZnO nanocomposite arrays for

(TSZ) ternary oxide system

nanosheets were synthesised as a

/ZnO framework with 3D intercon-

/metal-oxide composites for LIBs most

microspheres with controlled size and hierarchical nanostructures

, which has good cycling performance and capacity for LIBs, with

, ZnO, ZrO<sup>2</sup>

ions depends significantly on the structural characteristics

nanosheet arrays plays a significant role in advancing

in the electrochemical performance of TiO<sup>2</sup>

ticles increase the lithium storage capacity.

charge and discharge processes. To prevent these failures, TiO<sup>2</sup>

Opra et al. [159] obtained nanostructured Zr-doped (1 at.%) TiO<sup>2</sup>

other metal oxides with high capacity for LIBs such as SiO<sup>2</sup>

Gao et al. [160] reported the successful production of TiO<sup>2</sup>

lithium-ion battery application. The sandwich-like TiO<sup>2</sup>

Siwińska-Stefańska and Kurc [161] used a novel TiO<sup>2</sup>

nanotube arrays (TNAs) grafted with MnO<sup>2</sup>

/metal-oxide composites. The TiO<sup>2</sup>

Mesoporous hollow TiO<sup>2</sup>

170 Titanium Dioxide

trodes to form TiO<sup>2</sup>

Zr-doped TiO<sup>2</sup>

(with a TiO<sup>2</sup>

Unique TiO<sup>2</sup>

of both MnO<sup>2</sup>

commonly combine TiO<sup>2</sup>

reported that the transport of Li+

tion of ZnO nanoparticles onto the TiO<sup>2</sup>

:ZrO<sup>2</sup>

their electrochemical performance.

:SiO<sup>2</sup>

(according to EIS results).

Research on anode materials for lithium-ion batteries has also been focused on carbonaceous materials and transmission semiconductors such as TiO<sup>2</sup> . Carbonaceous materials have high stability, but low volumetric capacity, mainly due to their large initial irreversible capacity. Metal oxide semiconductors have many advantages as electrode materials, including robustness, chemical and thermal stability, low cost, biocompatibility, and relatively high electronic conductivity. The synthesis of mesoporous oxide semiconductors like titania has become an important issue in the construction of smart nanosensors with electrodes decorated by metal oxides [163–166].

Qiu et al. [167] synthesised a mesoporous TiO<sup>2</sup> /graphene composite using graphene oxide (GO) and cheap TiOSO<sup>4</sup> as precursors, via a facile one-step hydrothermal route. The obtained material exhibited a high discharge capacity (141.7 mAh/g) at the current density 5000 mA/g, an impressive value which is among the highest measured for any TiO<sup>2</sup> /graphene composite. The authors suggested that the conductive graphene in the composite may facilitate electron transfer and contribute to the higher rate capability of the TiO<sup>2</sup> /graphene composite electrode compared with the blank TiO<sup>2</sup> electrode.

Carbon-coated TiO<sup>2</sup> /SiO<sup>2</sup> nanocomposites (CTSO) were produced using a simple hydrothermal approach by Zhang et al. [168] as anode materials for lithium-ion batteries. The CTSO anode exhibits superior high-rate capability and excellent cycling performance. The specific capacity of the obtained material is much higher than that of pure TiO<sup>2</sup> and silica-modified TiO<sup>2</sup> without carbon nanocoating (TSO), which indicates that the material and structural hybridisation has a positive synergistic effect on the electrochemical properties. CTSO (0.05) presented the best cyclability, with 264 mAh/g retained after 270 cycles at 30 mA/g, and superior high-rate performance (233 mAh/g at 150 mA/g after 600 cycles, and even 167 mAh/g at 300 mA/g after 1000 cycles).

Siwińska-Stefańska and Kurc [169] reported on the synthesis, electrochemical properties and performance of a new type of microsized titania/graphene oxide (TA/GO) composite applied as a new anode material in lithium-ion batteries. The material was characterised by the presence of microsized particles with anatase and rutile structure, and a BET surface area of 6.2 m<sup>2</sup> /g. The specific discharge and charge capacities of TA/GO electrodes are approximately 1850–2010 and 2050–2100 mAh/g, respectively. Strong Ti–O–C chemical bonds give the composite resilient strength to facilitate the ordered assembly of TiO<sup>2</sup> nanoparticles and formation of a mesoporous structure with a high tap density, enable the rapid transport of Li ions and electrons within the composite structure and maintain a stable mesoporous structure during the discharge/charge process of the resultant LIBs.

Mesoporous TiO<sup>2</sup> /CNTs 3D conductive network hybrid nanostructures were synthesised using a facile PEO-aided self-assembled process by Wang et al. [170]. The material demonstrated high Li storage capacity, superior rate performance and excellent long-term cycling stability. Mesoporous TiO<sup>2</sup> /CNTs exhibits a reversible specific capacity of 203 mAh/g at 100 mA/g and a stable capacity retention of 91 mAh/g at 8000 mA/g (47.6C) over 100 cycles. The obtained material also retained approximately 90% (71 mAh/g) of its initial discharge capacity after 900 cycles at an extremely high rate of 15,000 mA/g (89C).

As this review of the subject literature shows, the search for new electrode materials is the subject of ongoing worldwide research efforts. The work is largely oriented towards the development of new types of batteries with high energy density and cyclability, and with as fast a charging rate as possible. The search for new electrode materials, and modification of existing ones, to achieve increased electrical capacity and the possibility of operation over a wider range of potentials relative to the lithium electrode is part of a current trend in scientific research.

Based on the current state of knowledge and our own research, we foresee continued growth in research work oriented towards the synthesis of functional materials containing titanium dioxide. Studies to date show that the obtaining of such materials is particularly important from both a theoretical and a practical standpoint. The good availability of methods for obtaining titania-based systems, their interesting physicochemical properties and their broad range of possible applications mean that these materials are coming to be used more and more widely in various branches of industry. Also of key importance is the synthesis of hybrid materials aimed at improving the physicochemical properties of TiO<sup>2</sup> , including through the careful control of the quantities of particular components. This creates a wide area for potential research and represents an alternative to the popularly used methods of synthesis. Research in this area is directed towards obtaining functional materials with not only photocatalytic but also antibacterial properties, offering defined electrochemical or barrier effects. Also being intensively developed are combined methods, such as the microwaveassisted sol-gel process, which also introduce interesting theoretical considerations regarding the synthesis of titania-based materials. This creates better possibilities for control of the synthesis and of the physicochemical parameters of the products.

Analysis of the literature review presented confirms the justification for the continuation of research into the synthesis of functional materials based on TiO<sup>2</sup> .
