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.

The kinetics of a two-phase electrochemical reaction in Li x FePO 4 was investigated by potential-step chronoamperometry under various experimental conditions: amplitude of potential step, direction of potential step, particle size, and thickness of composite electrodes. Only under a small potential step (10 mV) applied to large Li x FePO 4 particles (203 nm), the chronoamperogram showed a momentary current increase, followed by gradual decline, indicating that the nucleation and growth governed the electrode kinetics. In that condition, the chronoamperogram was analyzed with the Kolmogorov−Johnson−Mehl−Avrami (KJMA) model, which describes the kinetics of phase transition. The obtained Avrami exponent of ca. 1.1 indicates that the phase transition proceeds with a one-dimensional phase-boundary movement, which is consistent with the previously reported mechanism. From the temperature dependence of the obtained rate constant, the activation energy of the phase-boundary movement in Li x FePO 4 was estimated to be 42 and 40 kJ mol −1 in cathodic and anodic reactions, respectively.

Structural characterization X-ray diffraction patterns of polycrystalline powder samples were acquired by a Rigaku RINT-TTR III powder diffractometer equipped with a Cu-K radiation source ( 1 = 1.5405 Å,  2 = 1.5443 Å) operating at 50 kV and 300 mA. Typical scans were performed in the 2 range of 8~90 (step size = 0.028.s-1). Synchrotron X-ray diffraction (S-XRD) was conducted on the Powder Diffraction beamline at BL-10 of the Australian Synchrotron (Clayton, Australia) using radiation source of wavelength ( = 0.82550 Å, calibrated against a LaB 6 standard). For S-XRD, the powder samples were loaded inside glass capillaries ( = 0.3 mm) inside an Ar-filled glovebox. Rietveld analysis 1 of the S-XRD data was performed using the GSAS program 2 with EXPGUI front-end 3 , taking into account the zero-shifts, scale factors, background corrections and pseudo-Voigt peak shape parameters. Physical characterization Mössbauer characterization was conducted with a Topologic System Inc. unit having a 57 Co-ray source duly calibrated with an -Fe foil as standard. Around 0.1 g

An alluaudite‐type sodium iron sulfate has recently been discovered as a 3.8 V cathode material for low‐cost, high‐power, and efficient sodium‐ion batteries. To optimize the composition of the alluaudite phase and to explore further compounds, we have carefully surveyed the Na2SO4‐FeSO4 binary system. Solid‐state reactions at a moderate temperature of 623 K produce two stable phases: 1) vanthoffite‐structured Na6Fe(SO4)4 and 2) alluaudite‐type Na2+2xFe2−x(SO4)3 with a certain non‐stoichiometry. The possible compositional and structural flexibilities demonstrated in this work inspire further improvement of the alluaudite‐type sodium metal sulfates for advanced sodium‐ion batteries.

Alluaudite sodium iron sulfate Na2+2x Fe2–x (SO4)3 is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fe3+/Fe2+ redox reactions (3.8 V vs Na/Na+), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), 23Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fe3+/Fe2+ redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions.