Since the commercialization of the Li-ion battery by SONY Corporation in 1991, research has focused on identifying better electrode and electrolyte materials. SONY combined LiCoO2 as a positive electrode material with a carbonaceous material as a negative electrode, and LiPF6 in a carbonate solution as the electrolyte. Initially, research focused on replacing the relatively expensive LiCoO2 with other transition-metal oxides where the most important structural groups include spinel and layered transition-metal oxides. Layered Li transition-metal oxides, LiMO2 with M = Mn, Co, and Ni, represent one of the most successful classes of positive — electrode materials. The layered topology offers easily-accessible two-dimensional ion diffusion pathways. In particular, the LiCo1/3Ni1/3Mn1/3O2 composition [18] results in high capacity, safety, and lower material costs than LiCoO2.

An important alternative to LiCoO2 is the spinel LiMn2O4 with the inexpensive and environmentally-benign Mn, which functions with the Mn3+/4+ redox couple. LiMn2O4 operates at 4.1 V versus Li/Li+ offering excellent safety and high power due to its three-dimensional lattice, in principle allowing three-dimensional Li-ion diffusion. Substitution of Mn by M = Co, Cr, Cu, Fe, and Ni, has led to the discovery of the high-voltage spinels LiM0 5Mn15O4 and LiMMnO4 with potentials between 4.5 and 5 V versus Li/Li+ [19], exemplified by spinel LiNi0 5Mn15O4 operating at 4.7 V versus Li/Li+ [20, 21]. However, Mn-based spinels have been plagued by capacity fade, generally considered to be the result of Mn dissolution into the electrolyte and Jahn-Teller distortion of Mn3+. Mn dissolution has been largely inhibited by substitution of dopants in the spinel structure [22].

The introduction of LiFePO4 [14] has initiated research on polyanion-based positive electrodes with the structural formula LiM(XO)4 (M = Fe, Mn, Co, and X = S, P, Si). The strong covalent X-O bonds result in the large polarization of oxygen ions towards the X cation, leading to larger potentials compared to oxides. Phosphates, in particular LiFePO4, have been extensively explored because of their favourable electrode properties, reasonably high potential and capacity, stability against overcharge or discharge, and their composition of abundant, cheap, and non-toxic elements. To date, almost a thousand papers have been devoted to the understanding and improvement of conductivity in the semiconductor LiFePO4. These have encompassed studies of surface-conductive phases [23, 24], modifica­tion of crystallite size [25], and elegant fundamental mechanistic and modelling studies [26-28]. Compared to the phosphates, the silicates (LiMSiO4) exhibit lower electrode-potentials and electronic conductivity [29]. Other promising groups of positive-electrode materials include fluorophosphates with the A2MPO4F (A = Alkali metal) stoichiometry [29].

For many years graphite has been the main negative-electrode material allowing Li-ion intercalation between graphene sheets at *0.3 V versus Li/Li+. The low potential results in a high battery-voltage, partly responsible for the high energy and power density of Li-ion batteries. The main disadvantage of the low voltage of a negative electrode is the restricted stability of the commercial carbonate-based electrolyte solutions at low potentials versus Li/Li+. Depending on the electrolyte — electrode combination a kinetically stable SEI layer can be formed. For graphite this was achieved by the addition of ethylene carbonate [30] which lead to the success of the LiCoO2-C battery. Efforts to develop low-voltage negative electrodes with capacities that exceed that of C have concentrated on reaction types other than insertion reactions. Metals such as Si and Sn form alloys with Li in so-called conversion (or alloying) reactions [31-34]. Although the (gravimetric) capacities of these reactions can reach almost 10 times that of graphite, the volumetric energy — density is not significantly improved. Moreover, the large volume changes inher­ently related to this type of reaction make it a challenge to reach good cycleability. The oxide insertion hosts offer much better stability, highlighted by the Li4Ti5O12 spinel [35] that has almost no volume change upon Li insertion. Other titanium oxides that attract considerable attention as negative electrodes are anatase, brookite, and rutile TiO2, particularly in the nanostructured form [36, 37] and in combination with carbonaceous materials providing good with electronic-conduc­tivity [16, 36]. The disadvantage of titanium oxides is their relatively high negative- electrode potentials, reducing the overall working voltage of the battery and thereby reducing both energy and power density. For example, titanium oxides operate at *1.6 V versus Li/Li+, over four times higher in voltage than graphite. In this context, an interesting oxide exceeding the graphite capacity at similarly low potentials is the layered transition-metal oxide Li1+xV1-xO2 [38].

One of the key strategies that improved electrode performance in general is the nano-sizing of the electrode crystal particles. The nano-sized electrode particles reduce both solid state (Li-ion) diffusion and electron conduction. The latter is achieved by mixing and coating with conducting phases (i. e. with carbonaceous materials). However, numerous recent observations indicate that nano-sizing elec­trode particles also has a large impact on electrode materials properties [39, 40] creating both opportunities and challenges for enhanced Li-ion storage. Changes in properties that are observed upon nano-sizing include smearing of the voltage profile [41—43], changing solubility limits and phase behaviour [41, 44—46], unexpected kinetics [47], and larger capacities [45, 48—51]. The downside of the large surface-area of nanostructured materials is the relative instability of nano­materials, which can promote electrode dissolution and the increased reactivity towards electrolytes at commonly used voltages, e. g. below 1 V versus Li/Li+, which may adversely affect performance. Another potential disadvantage is the lower packing density of electrodes, leading to lower volumetric energy-densities.

Among the materials that benefit from the possibilities of nano-sizing are the relatively stable transition-metal oxides and phosphates operating well within the stability window of the electrolyte.