**5.2. Lithium-ion batteries (LIBs)**

Lithium-ion batteries (LIBs) are rechargeable batteries widely used in laptop, mobile phones, and electric vehicles. These batteries are characterized by high-energy density, low maintenance, little self-discharge, and no memory effect, which means that it is not necessary to completely discharge them before charging. In a conventional LIB cell, lithium metal oxide (e.g., LiCoO<sup>2</sup> ) is used as cathode, while graphite is used as preferred anode. The two electrodes are separated by a porous membrane and soaked in a nonaqueous liquid electrolyte. During insertion (or intercalation), ions move into the electrode. During the reverse process, extraction (or de-intercalation), ions move back out. Upon charging, the lithium ions move from the cathode to enter the anode, while in the discharging phase, the reverse phenomenon takes place. A variety of metal oxides, in particular TiO<sup>2</sup> , have been investigated as potential electrode materials for LIBs. Compared with the currently commercialized graphite anode, these metal oxide materials have demonstrated various advantages, such as very high capacity, widespread availability, good stability, and environmental benignity. Generally, the reversible reaction between Li and TiO<sup>2</sup> can be expressed by the following equation:

$$\text{xLi}^+ + \text{TiO}\_2 + \text{xe}^- \rightarrow \quad \text{Li}\_{\text{x}} \text{TiO}\_2 \text{(} 0 \le \text{x} < 1\text{)}$$

where x is the mole fraction of Li in TiO<sup>2</sup> . This redox reaction typically takes place at around 1.7 V vs. Li<sup>+</sup> /Li. Lithium ions reversibly insert/extract into/from the interstitial vacancies of the TiO<sup>2</sup> framework, with a specific percentage depending on the TiO<sup>2</sup> crystalline form and morphology. Specifically, anatase is probably the most electrochemically active form of TiO<sup>2</sup> for this purpose [8]. Moreover, it has been demonstrated that anatase exposing (001) facets exhibits efficient Li<sup>+</sup> ion diffusion along this direction (c-axis) facilitating a fast lithium insertion/extraction [70].

TiO2 presents many advantages in its usage as anode material for LIBS, such as the low-volume expansion upon lithiation (<4%), good stability, and lack of lithium plating. However, TiO<sup>2</sup> is also characterized by some limitations, including a limited Li<sup>+</sup> ion diffusion, low capacity, and low electrical conductivity [71]. A possibility to overcome these drawbacks is represented by the nanostructuration of TiO<sup>2</sup> , which provides a higher specific surface area and shorter diffusion pathways for electrons and Li<sup>+</sup> ions, compared to the corresponding bulk materials [72]. In this context, vertically oriented TiO<sup>2</sup> nanotubes have been exploited as anode materials for LIBs by many groups [73–75]. However, the preparation of nanostructured TiO<sup>2</sup> in the form of mesoporous thin films could offer some advantages, such as higher specific surface area and thinner walls than a TiO<sup>2</sup> nanotube array [76]. For example, Ortiz et al. prepared mesoporous titania thin films on titanium substrates, with a hexagonally ordered porous structure, and they tested these samples as anode materials for LIBs, reporting an improved electrochemical performance, without the necessity of additives to enhance the transport properties of the electrode [77]. The enhanced electrochemical activity was ascribed to the higher area and volumetric capacity of these films due to the presence of the 3D-ordered mesostructure.
