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The emergence of sodium-ion batteries (SIBs) employing cathodes based on earth abundant sodium and iron is expected to be ideal for large-scale electrical energy storage systems, for which the cost factor is of primary importance. However, these iron-based layered oxides still show unsatisfactory cycle performance, and the redox of the fleeting Fe3+/Fe4+ couple needs to be better understood. In this study, we examine the quasi-reversibility of the layered α-NaFeO2 cathode in sodium-ion cells. A NaFeO2 powder sample that has the O3 -type layered structure was synthesized via a solid-state synthesis method. The changes in Fe oxidation states and crystallographic structures were examined during the electrochemical sodium cycling of the NaFeO2 electrodes. Ex situ Mössbauer spectroscopy analysis revealed the chemical instability of Fe4+ in a battery cell environment: more than 20% of Fe4+ species that was generated in the desodiated Na1–x FeO2 electrode was spontaneously reduced back to Fe3+ states during open circuit storage of the charged cell. The in situ synchrotron X-ray diffraction further revealed the nonequilibrium phase transition behavior of the NaFeO2 cathode. A new layered phase (denoted as O″3 ) was observed in the course of sodium deintercalation, and an asymmetric structural behavior during cycling was identified. These findings explain the quasi-reversibility of α-NaFeO2 in the sodium cell and provide guidance for the future development of iron-based cathode materials for sodium-ion batteries.

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The nature of light element(s) in the core holds key to our understanding of Earth's history of accretion and differentiation, but the core composition remains poorly constrained. Carbon has been proposed to be a major constituent of the inner core, with broad implications for the global carbon cycle, the budget of volatiles in the Earth and origin of carbon‐based life in the Solar System. However, existing estimates of the inner core's carbon content remain highly controversial because of poor constraints on the behavior of compressed iron carbides. Here we investigated the structure, elasticity, and magnetism of Eckstrom‐Adcock carbide Fe7C3up to core pressures, using synchrotron‐based single‐crystal X‐ray diffraction and Mössbauer spectroscopy techniques. We detected two discontinuities in the compression curve up to 167 gigapascals (GPa), the first of which corresponds to a magnetic collapse between 5.5 and 7.5 GPa and is attributed to a ferromagnetic to paramagnetic transition. At the second discontinuity near 53 GPa, Fe7C3softens and exhibits Invar behavior, presumably caused by a high‐spin to low‐spin transition. Considering the magneto‐elastic coupling effects, an Fe7C3‐dominant composition can match the density of the inner core, making the core potentially the largest reservoir of carbon in Earth.

[1] We have carried out nuclear resonant scattering measurements on 57 Fe-enriched Fe 3 C between 1 bar and 50 GPa at 300 K. Synchrotron Mössbauer spectra reveal a pressure-induced magnetic transition in Fe 3 C between 4.3 and 6.5 GPa. On the basis of our nuclear resonant inelastic X-ray scattering spectra and existing equation-of-state data, we have derived the compressional wave velocity V P and shear wave velocity V S for the high-pressure nonmagnetic phase, which can be expressed as functions of density (r):) and V S (km/s) = 1.45 + 0.24r(g/cm 3