## 2 Electrodes and Electrolytes for Lithium-Based Batteries
### Electrodes
#### 2.1.1 Anodes
[113] reported a low-temperature (190 \({}^{\circ}\)C) multi-phase synthesis of layered lithium titanate hydrate (Li\({}_{1.25}\)H\({}_{1.63}\)Ti\({}_{2}\)O\({}_{5.44-\sigma}\)) by removing loosely bound water (such as adsorbed and crystallographic) but keeping tightly bound pseudohydrates (hydroxides and hydronium ions or -OH, -H surface terminations). This method avoided detrimental structural coarsening. Xu et al. [114] reported a strategy to extend the ion-insertion channels within a titanate anode by doping Li\({}_{1.81}\)H\({}_{0.19}\)Ti\({}_{2}\)O\({}_{5}\)-nH\({}_{2}\)O with 3 nm MoS\({}_{2}\) quantum dots. A strong heterointerfacial effect between LTO and the coupled MoS\({}_{2}\) quantum dots created a space charge layer near the interface, reducing the diffusion distance due to the significantly altered redox potential distribution near the surface. Christensen et al. [115] reported that disordered nano-rutile transforms into a composite of \(\sim\)5 nm domains of a layered Li\({}_{x}\)TiO\({}_{2}\)\(\alpha\)-NaFeO\({}_{2}\)-type structure with \(\sim\)1 nm Li\({}_{x}\)TiO\({}_{2}\) grain boundaries with a columbite-like structural motif. Yang et al. [116] reported the synthesis of a TiO\({}_{2}\)@\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) core-shell nanostructure to deliver a high capacity of 820 mAh/g after 1000 cycles at 500 mA/g. Han et al. [117] reported a new insertion type anode material, TiNb\({}_{2}\)O\({}_{7}\) (TNO), in 2011. TNO has a monoclinic structure (C2/m) utilizing the Ti (IV)/Ti (III), Nb (V)/Nb (IV) and Nb (IV)/Nb (III) redox couples to insert 5 Li\({}^{+}\) per formula (theoretical capacity: 387.6 mAh/g). It is a high potential anode (1.3-1.5 V vs. Li/Li\({}^{+}\)); additionally, TNO has excellent cycling stability and its volumetric capacity is twice (1680 mAh/cm\({}^{3}\)) compared to that of graphite (837 mAh/cm\({}^{3}\)). Despite its high theoretical capacity, poor electronic and ionic conductivities [118] of TNO require additional materials engineering approaches such as doping [119], conductive layer coating [120] or nanomaterials formation [121] to improve the performance of batteries. Increasing the electrochemically active surface area by creating porous structures was effective in maintaining a long cycle life. Park et al. [122] synthesized porous TNO nanotubes by electrospinning. These nanotubes maintained a capacity of 210 mAh/g after 700 cycles at 1 C. Guo et al. [123] formed a porous TNO structure by using a sol-gel method and achieved a capacity of \(\sim\)200 mAh/g after 1000 cycles at 5 C. Jo et al. [124] synthesized mesoporous TNO crystals using poly(ethylene oxide)-b-poly(styrene) block copolymers reporting 190 mAh/g of capacity after 2000 cycles at 10 C. Summary of the discharge capacity among anode materials are provided in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**Discharge Rate (C)**} & \multicolumn{2}{c}{**Current Density**} & **Initial Discharge** \\  & & **(mA/cm\({}^{2}\))** & **Capacity (mAh/g)** & **Ref.** \\ \hline Si & 0.05 & & 2834 & [93] \\ Si & 0.2 & & 2433 & [95] \\ Si & & 0.2 & 3000 & [96] \\ Si & & 200 & 4122 & [98] \\ Si & 0.5 & & 4021 & [100] \\ Si & 0.05 & & 3400 & [101] \\ Li\({}_{1.25}\)H\({}_{1.63}\)Ti\({}_{2}\)O\({}_{5.44-\sigma}\) & 35 & & 130 & [113] \\ TiO\({}_{2}\) & 10 & & 131 & [109] \\ TiO\({}_{2}\) & 0.1 & & 343 & [112] \\ TiO\({}_{2}\) & 0.1 & & 320 & [115] \\ TiO\({}_{2}\)@\(\alpha\)–Fe\({}_{2}\)O\({}_{3}\) & & 100 & 800 & [116] \\ Cu\({}_{0.8}\)LTO & 0.1 & & 209 & [110] \\ TNO & 100 & & 184 & [119] \\ TNO & 1 & & 195 & [121] \\ TNO & 50 & & 230 & [122] \\ TNO & 0.1 & & 281 & [123] \\ TNO & 0.1 & & 289 & [124] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the discharge capacity among different anode materials.