Rechargeable lithium batteries have ushered the wireless revolution over last two decades and are now matured to enable green automobiles. However, the growing concern on scarcity and large-scale applications of lithium resources have steered effort to realize sustainable sodium-ion batteries, Na and Fe being abundant and low-cost charge carrier and redox centre, respectively. However, their performance is limited owing to low operating voltage and sluggish kinetics. Here we report a hitherto-unknown material with entirely new composition and structure with the first alluaudite-type sulphate framework, Na2Fe2(SO4)3, registering the highest-ever Fe3+/Fe2+ redox potential at 3.8 V (versus Na, and hence 4.1 V versus Li) along with fast rate kinetics. Rare-metal-free Na-ion rechargeable battery system compatible with the present Li-ion battery is now in realistic scope without sacrificing high energy density and high power, and paves way for discovery of new earth-abundant sustainable cathodes for large-scale batteries.

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.

Vying for newer sodium-ion chemistry for rechargeable batteries, Na2FeP2O7 pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na2FeP2O7 as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP2O7. High-temperature analyses of the desodiated state NaFeP2O7 show an irreversible phase transition from triclinic (P1̅) to the ground state monoclinic (P21/c) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P2O7)4– anion, which offers better safety than other phosphate-based cathodes. It establishes Na2FeP2O7 as a safe cathode candidate for large-scale economic sodium-ion battery applications.

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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.

future. [1][2][3] To avoid this scenario, Na-ion batteries can play a vital role. Contrary to Li, Na has abundant natural resources with even geographic distribution. In addition to being the fifth-most abundant element in earth's crust, the Na charge carrier is also the second lightest alkali element in the periodic table. In this context, mammoth effort has been geared to build efficient Na-ion batteries. 2D layered oxides are the most important category of cathode materials, which has been proven by their dominance in commercial Li-ion batteries. Although the only difference is the intercalation ion, the electrochemical behavior and structure transformation of Na analogs are very different to that in Li-ion batteries. These are mainly caused by the larger size of Na, which can stabilize the layered structure (empirical stability criterion for ABO 2 : r B /r A <0.86 for α-NaFeO 2 type structure) and thus prevent Na from occupying tetrahedral sites and promote Na cation ordering. [31] NaMO 2 compounds can retain the layered structure for all 3d transition elements M, whereas Co and Ni are the exclusive 3d transition metals that can offer ground state stability of layered structure of LiMO 2 . Thereby, intensive survey for a variety of NaMO 2 compounds was performed. [4][5][6]32] However, even with the inherently stable structural nature of NaMO 2 compounds, they usually suffer from a slopy voltage profile distributed over a wide potential range at lower average voltage (at least 0.3 V and in many cases over 0.5 V) as compared to the Li analog, identifying only a few complicated compounds that can generate >3.5 V versus Na/Na + .Development of high voltage cathode materials is of paramount importance for Na batteries, bearing in mind the relative difference in anode potential of Li/Li + : −3.03 V and Na/ Na + : −2.71 V versus NHE. With an essential lack of high-voltage layered cathode material, polyanion compounds form a major stream toward the stable cathode materials operating over 3.5 V in Na batteries. Light and small polyanion units such as (SO 4 ) 2− , (PO 4 ) 3− , (BO 3 ) 3− , (SiO 4 ) 4− , which have also been widely explored as major components in Li-ion battery cathodes, offer two major functions; i) raising the redox potential largely as compared to the simple oxides with identical redox couple, and ii) providing inherent safety to the battery system. These two very important features led olivine LiFePO 4 to a great commercial success in the Li battery market over a decade ago, [35,37,38,63] proving that additional atoms X (X = P in this case), other than Efficient energy storage is a driving factor propelling myriads of mobile electronics, electric vehicles and stationary electric grid storage. Li-ion batteries have realized these goals in a commercially viable manner with ever increasing penetration to different technology sectors across the globe. While these electronic devices are more evident and appealing to consumers, there has been a growing concern for micro-to-mega grid storage systems. Ove...

The quest to explore and to discover novel cathode materials is key to sustaining the progress of Li-ion rechargeable batteries. It has encouraged materials' chemists to design and develop a variety of polyanionic compounds. This paper reports and summarizes recent progress in the development of alkali metal pyrophosphate based cathode materials for battery applications. It points out that the pyrophosphate-based polyanionic-framework materials offer a rich crystal chemistry, ease of synthesis and robust structure offering good alkali-ion mobility. Thus, they can form a promising class of cathode materials both for lithium-and sodium-ion secondary batteries. In the Perspectives section, it is speculated that keen study of the myriad of pyrophosphate phases, selective screening and design of key pyrophosphate compounds with open frameworks and optimizing those compounds with nanosizing and carbon coating can lead to breakthroughs in the realization of commercially viable high-voltage Li-ion and Na-ion pyrophosphate cathodes.

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.