Li-ion batteries have contributed to the commercial success of portable electronics, and are now in a position to influence higher-volume applications such as plug-in hybrid electric vehicles. Most commercial Li-ion batteries use positive electrodes based on lithium cobalt oxides. Despite showing a lower voltage than cobalt-based systems (3.45 V versus 4 V) and a lower energy density, LiFePO(4) has emerged as a promising contender owing to the cost sensitivity of higher-volume markets. LiFePO(4) also shows intrinsically low ionic and electronic transport, necessitating nanosizing and/or carbon coating. Clearly, there is a need for inexpensive materials with higher energy densities. Although this could in principle be achieved by introducing fluorine and by replacing phosphate groups with more electron-withdrawing sulphate groups, this avenue has remained unexplored. Herein, we synthesize and show promising electrode performance for LiFeSO(4)F. This material shows a slightly higher voltage (3.6 V versus Li) than LiFePO(4) and suppresses the need for nanosizing or carbon coating while sharing the same cost advantage. This work not only provides a positive-electrode contender to rival LiFePO(4), but also suggests that broad classes of fluoro-oxyanion materials could be discovered.

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.

Owing to cost and abundance considerations, Na-based electrode materials are regaining interest, especially those that can be prepared at low temperatures. Here, we report the low temperature synthesis of highly divided Na-based fluorophosphates ( normalNa2MPnormalO4F , M = Fe, Mn, or mixtures) in ionic liquid media. We show that this ionothermal approach enables the synthesis of these phases at temperatures as low as 270°C, while temperatures as high as 600°C are needed to obtain similar quality phases by solid-state reactions. Moreover, owing to their highly divided character, normalNa2FePnormalO4F powders made via such a process show better electrochemical performances vs either Li or Na than their ceramic counterparts. In contrast, regardless of how they were made, the normalNa2MnPnormalO4F powders, which crystallize in a three-dimensional (3D) tunnel structure rather than in the two-dimensional (2D)-layered structure of normalNa2FePnormalO4F , were poorly electroactive. Substituting 0.25 Fe for Mn in normalNa2normalFe1−xnormalMnxPnormalO4F is sufficient to trigger a 2D–3D structural transition and leads to a rapid decay of the materials electrochemical performances. A tentative explanation, based on structural considerations to account for such behavior, is given in this paper.

Ceramic processes are currently used to prepare most of today’s electrode materials. For energy-saving reasons, there is a growing interest in electrode materials prepared via eco-efficient processes, which has led to the resurgence of low temperature hydro- and solvothermal processes. This review will highlight how some of these processes have been successfully used to prepare today’s most praised electrode material: LiFePO4. Particular attention is paid to the recently developed ionothermal synthesis process. This will be done in order to stress the versatility and richness of ionothermal synthesis, its control over particle size and shape, and the ability of ionic liquids to provide stabilization to new metastable phases. We outline the pertinent questions that should be clarified for continued advancement of the ionothermal process which opens the door to innovative inorganic synthesis and to materials which have remained hidden for a long time.

Li-rich oxides continue to be of immense interest as potential next generation Li-ion battery positive electrodes, and yet the role of oxygen during cycling is still poorly understood. Here, the complex electrochemical behavior of Li4FeSbO6 materials is studied thoroughly with a variety of methods. Herein, we show that oxygen release occurs at a distinct voltage plateau from the peroxo/superoxo formation making this material ideal for revealing new aspects of oxygen redox processes in Li-rich oxides. Moreover, we directly demonstrate the limited reversibility of the oxygenated species (O2(n-); n = 1, 2, 3) for the first time. We also find that during charge to 4.2 V iron is oxidized from +3 to an unusual +4 state with the concomitant formation of oxygenated species. Upon further charge to 5.0 V, an oxygen release process associated with the reduction of iron +4 to +3 is present, indicative of the reductive coupling mechanism between oxygen and metals previously reported. Thus, in full state of charge, lithium removal is fully compensated by oxygen only, as the iron and antimony are both very close to their pristine states. Besides, this charging step results in complex phase transformations that are ultimately destructive to the crystallinity of the material. Such findings again demonstrate the vital importance of fully understanding the behavior of oxygen in such systems. The consequences of these new aspects of the electrochemical behavior of lithium-rich oxides are discussed in detail.

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.

We have recently reported a promising 3.6 V metal fluorosulphate (LiFeSO(4)F) electrode, capable of high capacity, rate capability, and cycling stability. In the current work, we extend the fluorosulphate chemistry from lithium to sodium-based systems. In this venture, we have reported the synthesis and crystal structure of NaMSO(4)F candidates for the first time. As opposed to the triclinic-based LiMSO(4)F phases, the NaMSO(4)F phases adopt a monoclinic structure. We further report the degree and possibility of forming Na(Fe(1-x)M(x))SO(4)F and (Na(1-x)Li(x))MSO(4)F (M = Fe, Co, Ni) solid-solution phases for the first time. Relying on the underlying topochemical reaction, we have successfully synthesized the NaMSO(4)F, Na(Fe(1-x)M(x))SO(4)F, and (Na(1-x)Li(x))MSO(4)F products at a low temperature of 300 degrees C using both ionothermal and solid-state syntheses. The crystal structure, thermal stability, ionic conductivity, and reactivity of these new phases toward Li and Na have been investigated. Among them, NaFeSO(4)F is the only one to present some redox activity (Fe(2+)/Fe(3+)) toward Li at 3.6 V. Additionally, this phase shows a pressed-pellet ionic conductivity of 10(-7) S x cm(-1). These findings further illustrate the richness of the fluorosulphate crystal chemistry, which has just been recently unveiled.

As opposed to ceramic methods, low-temperature solvothermal-hydrothermal methods using liquid media as reaction support are less energy demanding to design new electrode materials; therefore, they tend to replace ceramic routes. Here, we report the use of ionic liquids as both solvent and template to enable the growth of LiFePO 4 (LFP) powders with controlled size and morphology at temperatures at least 200 °C lower than those required for conventional ceramic methods, while showing excellent electrochemical performances versus lithium. An inherent advantage to the use of ionic liquids lies in the feasibility of carrying out the reaction at atmospheric pressure. Besides, the recovery of the powders from the reacting medium is particularly easy, as are the effluents and ionic liquid recycling. Additionally, it is shown that ionic liquids can be used as a structural directing agent to orient crystal growth and obtain powders adopting a single morphology. Needless to say, such a new approach, which is not specific to LiFePO 4 , offers great opportunities for the low-temperature synthesis of new electrode materials.