## 2 Electrodes and Electrolytes for Lithium-Based Batteries
### Electrodes
#### 2.1.1 Anodes
Using in situ TEM imaging, they revealed an in-plane lattice constant of 3.1 A, which is an unexpected result, compared to theordinarily required very low temperatures and high pressures for such dense packing. Although it may be thermodynamically unfavorable for ions with larger radii than lithium to form intercalation compounds in bulk graphite, Nandi and Das reported the use of exfoliated graphite (foam) used with Cu and Al cathodes in an aqueous electrolyte [104]. Yang et al. [105] reported the use of a graphite anode for use with a water-in-salt electrolyte. A specific capacity of 243 mAh/g at 4.2 V vs. Li/Li\({}^{+}\) was reported for mixed lithium halide (Br, Cl). Graphdiyne with natural pores could be a high capacity carbon-based alternative to graphene-based anodes. Mortazavi et al. [106] predicted that P- and N-doped triphenylene-graphdiyne nanosheet monolayers could yield capacities well above graphite. From first principle calculations, they suggested capacities of 1979 mAh/g and 2664 mAh/g for P-TpG and N-TpG, respectively. Ti-based compounds are promising anode materials known for their outstandingly high-rate capacity, negligible volume change and cycling stability, as well as improved safety over graphite [107], Ti-based materials, however, suffer from low specific capacity, low electronic/ionic conductivity and relatively low gravimetric/volumetric energy density. To compete with commercial anode materials, several approaches have been studied including morphologic control, doping, surface modification, increasing the working voltage by using high-voltage cathodes and maximization of electroactive materials [108]. Among TiO\({}_{2}\)-based anodes that exist as eight different phases (Figure 2), the predominant three phases of interest for lithium insertion anodes are rutile, anatase and bronze phase (TiO\({}_{2}\)(B)) with typical operating potentials \(\sim\)1.3-1.8 V vs. Li/Li\({}^{+}\). Li et al. [109] reported the synthesis of porous nanospheres of TiO\({}_{2}\) (anatase and B phase) coated with graphene nanoribbons to have a high initial charge capacity of 390 mAh/g and 79.5% coulombic efficiency. Spinel Li\({}_{4}\)Ti\({}_{5}\)O\({}_{12}\) (LTO) has also received considerable attention because of its excellent cycling stability originating from small volume changes during Li insertion/extraction reactions. However, its low electronic and ionic conductivities required further engineering to enhance the power performance of batteries. For example, Bai et al. [110] reported the enhanced performance of batteries by substituting Cu for Li in LTO in which Cu\({}_{0.8}\)LTO showed a capacity of 138.5 mAh/g at 40 C rate. Furthermore, capacity retention of 90.4% (132.3 mAh/g) was obtained after 2500 cycles at 10 C with Cu\({}_{0.8}\)LTO. Recent progress on Ti-based anode was achieved by porosity control, structural water removal or composite formation. Liu et al. [112] studied the interface between a dual anatase/(B) phase nanofiber heterostructure (pore size \(<\) 10 nm) formed by a one-step hydrothermal processing method. Figure 2: TiO\({}_{2}\) crystal structures of (**a**) rutile, (**b**) anatase, (**c**) bronze, (**d**) brookite, (**e**) columbite, (**f**) hollandite, (**g**) baddeleyite and (**h**) ramsdellite phases. Reprinted from ref. [111] with permission of Elsevier. According to this study, TiO\({}_{2}\)(B) doping of anatase can effectively improve the pseudocapacitive behavior lowering polarization. Using density functional theory calculations, the authors also suggested that the interface can act as a cationic site reservoir. The highest reported discharge capacity was 343.9 mAh/g attributed to vacancies in the Ti\({}^{3+}\)-O bonds, Ti-C bonds and high porosity. Wang et al.