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To meet the growing global demand for energy while preserving the environment, it is necessary to drastically reduce the world's dependence on non-renewable energy sources. At the core of this effort will be the ability to efficiently convert, store, transport and access energy in a variety of ways. Batteries for use in small consumer devices have saturated society; however, if they are ever to be useful in large-scale applications such as automotive transportation or grid-storage, they will require new materials with dramatically improved performance. Efforts must also focus on using Earth-abundant and nontoxic compounds so that whatever developments are made will not create new environmental problems.In this Account, we describe a general strategy for the design and development of new insertion electrode materials for Li(Na)-ion batteries that meet these requirements. We begin by reviewing the current state of the art of insertion electrodes and highlighting the intrinsic material properties of electrodes that must be re-engineered for extension to larger-scale applications. We then present a detailed discussion of the relevant criteria for the conceptual design and appropriate selection of new electrode chemical compositions.We describe how the open-circuit voltage of Li-ion batteries can be manipulated and optimized through structural and compositional tuning by exploiting differences in the electronegativity among possible electrode materials. We then discuss which modern synthetic techniques are most sustainable, allowing the creation of new materials via environmentally responsible reactions that minimize the use of energy and toxic solvents. Finally, we present a case study showing how we successfully employed these approaches to develop a large number of new, useful electrode materials within the recently discovered family of transition metal fluorosulfates. This family has attracted interest as a possible source of improved Li-ion batteries in larger scale

The tavorite structured LiFeSO4F forms a potential cathode with an attractive mix of high redox voltage (ca 3.6 V) and reversible capacity (∼140 mAh/g). It exists as a metastable phase capable of showing polymorphism that can be thermodynamically or chemically triggered. Fluorosulphate (LiMSO4F) belongs to a structurally diverse family of compounds. Here, we demonstrate the effect of Mn and Zn substitution on the structural and electrochemical properties of the parent LiFeSO4F. Mn and Zn substitution into LiFeSO4F stabilizes the triplite and sillimanite phases respectively, tuning the Fe2+/Fe3+redox activity in the voltage range of 3.6∼3.9 V, respectively. Notably, the polymorphism in LiFeSO4F leads to the realization of cathode material with the highest Fe2+/Fe3+redox potential close to 4 V. Copyright 2013 by The Electrochemical Society.

We report on the preparation and electrochemical characterization of Li 2M(SO 4) 2 (M = Co, Fe), where the Fe-based phase was previously unknown. Both compounds can be made at low temperatures (< 320 °C) and the Fe-based phase displays an open circuit voltage of 3.83 V vs. Li +/Li 0 for the Fe 3+/Fe 2+ redox potential. This corresponds to the highest voltage ever obtained for the Fe 3+/Fe 2+ redox couple in an iron-based, fluorine-free compound and is only matched by the triplite phase of LiFeSO 4F. While the theoretical capacity of 102 mA h g - 1 will likely limit the commercial prospects for this phase, knowledge of the structure will provide useful insight into the structural origin of these increased potentials. © 2012 Elsevier B.V. All rights reserved.

Isothermal acid solution calorimetry was employed to investigate the relative thermochemical stabilities of two polymorphs of the LiFe 1-xMnxSO4F (0 ≤ x ≤ 1) solid solution series: triplite and tavorite. These compounds have shown promise as lithium-ion battery cathodes, and a fuller understanding of their thermodynamics will aid in synthesis and their practical application. The linear energetic trends among triplites and tavorites indicate greater stabilization of each of these structures from the binary components with increase in manganese content and suggest a negligible heat of mixing of Fe and Mn ions. The tavorite phase, formed for x < 0.2, appears energetically more stable than the triplite. The formation of the disordered triplite structure appears to be entropy driven, and the factors that increase the disorder of the system (e.g. rapid phase formation) favor the triplite structure. Further, the free energy change associated with the tavorite to triplite transformation obtained by calculating configurational entropies using measured enthalpies was almost zero (-1.3 ± 0.8 kJ mol-1) at ambient temperature but becomes exothermic at 500 °C (-4.3 ± 0.8 to -6.8 ± 0.8 kJ mol-1). This suggests that both tavorite and triplite (with random cation distribution) are equally stable at ambient temperature but the tavorite to triplite transformation is thermodynamically favored at high temperatures because of entropy. © The Royal Society of Chemistry 2012.

Polyanionic electrode materials offer an attractive 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, KFeSO4F, which, with removal of K, leads to a new polymorph of FeSO4F crystallizing in the high-temperature structure of KTiOPO4. 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 host structures for insertion reactions. © 2012 American Chemical Society.

Our work in metal fluorosulphate chemistry, which was triggered by the discovery of the tavorite-phase of LiFeSO 4F, has unveiled many novel Li- and Na-based phases with desirable electrochemical and/or transport properties. Further exploring this rich crystal chemistry, we have synthesized the Na-based magnesium, copper and zinc fluorosulphates, which crystallise in the maxwellite (tavorite-like framework) structure just as their Fe and Co counterparts, which were previously reported. These phases show ionic conductivities in the range of ∼10 -7 S cm -1 or ∼10 -11 S cm -1 depending upon their synthesis process and no reversible electrochemical activity versus Na. © 2011 Elsevier Masson SAS. All rights reserved.

Transition-metal fluorosulfates are currently under intense investigation for their use as anodes in Li-ion batteries. The substitution of Zn into LiFeSO4F has been found to trigger a transition from the tavorite structure to two separate polymorphs which crystallize in the sillimanite and triplite structures (see picture). These new phases show Fe2+/Fe 3+ redox potentials of 3.6 and 3.9 V versus Li, respectively. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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 Fe 2+/Fe 3+ 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)SO 4 F. Not only is this the highest voltage reported so far for the Fe 2+/Fe 3+ redox couple, exceeding that of LiFePO 4 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 LiFePO 4 and LiFeSO 4F respectively), to give a capacity of ∼125 mA h g -1. © 2011 Macmillan Publishers Limited. All rights reserved.

Using magnetic susceptibility and low-temperature neutron diffraction experiments we present a thorough characterization of the magnetic properties of LiFeSO 4F and FeSO 4F. Temperature dependent magnetic susceptibility measurements show a transition to long-range antiferromagnetic order at 100 K in FeSO 4F whereas the ordering temperature in LiFeSO 4F is 25 K. We attribute the decreased ordering temperature to a structural change which decreases the strength of the magnetic interactions along the length of the chains and to the difference of supersuper-exchange interactions between Fe 2+ and Fe 3+ ions. Powder neutron diffraction experiments were used to determine the magnetic structures of both compounds, which are discussed in terms of exchange interactions and the anisotropy of Fe 2+. The iron magnetic moments are antiparallel within the chains, whereas the coupling between the chains is different in the lithiated compound compared to the delithiated one, resulting in different spin arrangements. © 2011 American Chemical Society.

A novel hydrated fluoroselenate NaCoSeO 4F•2H 2O has been synthesized, and its structure determined. Like its sulfate homologue, NaCoSO 4F•2H 2O, the structure contains one-dimensional chains of corner-sharing MO 4F 2 octahedra linked together through F atoms sitting in a trans configuration with respect to each other. The magnetic properties of the two phases have been investigated using powder neutron diffraction and susceptibility measurements which indicate antiferromagnetic ordering along the length of the chains and result in a G-type antiferromagnetic ground state. Both compounds exhibit a Néel temperature near 4 K, and undergo a field-induced magnetic phase transition in fields greater than 3 kOe. © 2011 American Chemical Society.