In the race for better Li-ion batteries, research on anode materials is very intensive as there is a strong desire to find alternatives to carbonaceous negative electrodes. A large part of these studies is devoted to alloying reactions, which have been known for more than thirty years but that have regained great interest by downsizing particle sizes, moving to nano-textured/ nanostructured composites, or designing new electrode concepts. It is not the scope of this review to retrace twenty-five years of research, but rather to highlight recent advances that have been made in the use of Sn or Si-based electrodes together with the remaining challenges to be addressed and issues to be solved prior to such electrodes being commercially implemented in Li-ion cells.

Li-ion batteries have empowered consumer electronics and are now seen as the best choice to propel forward the development of eco-friendly (hybrid) electric vehicles. To enhance the energy density, an intensive search has been made for new polyanionic compounds that have a higher potential for the Fe²⁺/Fe³⁺ redox couple. Herein we push this potential to 3.90 V in a new polyanionic material that crystallizes in the triplite structure by substituting as little as 5 atomic per cent of Mn for Fe in Li(Fe(1-δ)Mn(δ))SO₄F. Not only is this the highest voltage reported so far for the Fe²⁺/Fe³⁺ redox couple, exceeding that of LiFePO₄ by 450 mV, but this new triplite phase is capable of reversibly releasing and reinserting 0.7-0.8 Li ions with a volume change of 0.6% (compared with 7 and 10% for LiFePO₄ and LiFeSO₄F respectively), to give a capacity of ~125 mA h g⁻¹.

Alpha-Fe(2)O(3) has been synthesized with an ordered mesoporous structure and crystalline walls that exhibit a near-single crystal-like order. The unique magnetic behavior of the material, distinct from bulk nanoparticles of alpha-Fe(2)O(3) or mesoporous Fe(2)O(3) with disordered walls, has been established. Magnetic susceptibility, Mössbauer, and neutron diffraction data show that the material possesses the same long-range magnetic order as bulk alpha-Fe(2)O(3), despite the wall thickness being less than the 8 nm limit below which magnetic ordering breaks down in nanoparticulate alpha-Fe(2)O(3), yet the Morin transition of bulk alpha-Fe(2)O(3) is absent. It is also shown by TEM, PXRD, and EXAFS that alpha-Fe(2)O(3) with the same ordered mesoporous structure but disordered walls contains small crystalline domains. Mössbauer and magnetic susceptibility data demonstrate that this material exhibits no long-range magnetic order but superparamagnetic behavior.

Ordered mesoporous Fe(3)O(4) with crystalline walls (inverse spinel structure) has been synthesized for the first time, representing to the best of our knowledge, the first synthesis of a reduced mesoporous iron oxide. Synthesis was achieved by reducing ordered mesoporous alpha-Fe(2)O(3) (corundum structure) to Fe(3)O(4) spinel then to gamma-Fe(2)O(3) by oxidation, while preserving the ordered mesostructure and crystalline walls throughout. Such solid/solid transformations demonstrate the stability of the mesostructure to structural phase transitions from the hexagonal close packed oxide subarray of alpha-Fe(2)O(3) (corundum structure) to the cubic close packed subarray of Fe(3)O(4) spinel and gamma-Fe(2)O(3). Preliminary magnetic measurements reveal that the spins in both Fe(3)O(4) and gamma-Fe(2)O(3) are frozen at 295 K, despite the wall thickness (7 nm) being less than the lower limit for such freezing in corresponding nanoparticles (>8 nm).

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Polyanionic electrode materials offer a compelling combination of safety benefits and tunable redox potentials. Thus far, phosphate-based phases have drawn the most interest with a subsequent surge of activity focused on the newly discovered family of fluorosulfate phases.Here, we report the preparation of a new potassium-based fluorosulfate, KFeSO 4 F, which, with removal of K, leads to a new polymorph of FeSO 4 F crystallizing in the high-temperature structure of KTiOPO 4 . This new phase which contains large, empty channels, is capable of reversibly inserting 0.9 Li + per unit formula, and can accommodate a wide variety of alkali ions including Li + , Na + , or K + . This finding not only expands the rich crystal chemistry of the fluorosulfate family, but further suggests that a similar strategy can apply to other K-based polyanionic compounds in view of stabilizing new attractive hosts structures for insertion reactions.

Fluoride-based materials have regained interest within the field of Li-ion batteries largely due to the advent of nanosizing which has transformed the insulating insertion compounds into attractive electrode materials. Herein we demonstrate the effectiveness of ionothermal synthesis in the preparation of nanometric LiFePO4F and LiTiPO4F phases with structures isotopic to tavorite LiFePO4(OH) at temperatures of only 260 °C, while temperatures of 600−700 °C are required to obtain coarse powders via the ceramic method. However, the redox-active phases, which are obtained in a high state of division, have lower redox voltages than LiFePO4 despite the presence of fluorine. Additionally, LiTiPO4F shows staircase charge/discharge profiles with Ti2/3+ and Ti3/4+ couples. Though quite unusual in lithium intercalation oxides, a hint of a Li-driven Fe3/4+ transition has been detected in LiFePO4F.

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