Sodium Intercalation Mechanism of 3.8 V Class Alluaudite Sodium Iron Sulfate

Abstract

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.

Recent Progress in Iron‐Based Electrode Materials for Grid‐Scale Sodium‐Ion Batteries

“…The electrode showed the highest Fe 2+ /Fe 3+ redox couple voltage (3.8 V) ever reported and was associated with good reversible capacity (102 mAh g −1 at 0.05 C) (Figure b) . The sloping voltage curve over the entire Na composition range suggests a single‐phase (solid solution) reaction mechanism, which was verified by the comparative XRD patterns and Mössbauer spectra . The structure of the Na 2.4 Fe 1.8 (SO 4 ) 3 undergoes a small volume change of ≈3.5% after an initial structural rearrangement during the first charging process, where a small amount of Fe irreversibly migrates from the original site to the Na site .…”

“…The sloping voltage curve over the entire Na composition range suggests a single‐phase (solid solution) reaction mechanism, which was verified by the comparative XRD patterns and Mössbauer spectra . The structure of the Na 2.4 Fe 1.8 (SO 4 ) 3 undergoes a small volume change of ≈3.5% after an initial structural rearrangement during the first charging process, where a small amount of Fe irreversibly migrates from the original site to the Na site . Some high‐performance Na 2.4 Fe 1.8 (SO 4 ) 3 electrodes were successfully constructed with a conductive carbon matrix .…”

Sulfate‐Based Cathode Materials for Li‐ and Na‐Ion Batteries

“…The fully occupied Na1 site does not allow any Na diffusion, however partially occupied Na2 and Na3 sites form 1D diffusion channels along the c ‐axis. During the first charge process, Fe migrates on the Na1 site, which explains the different curve shape of the first charge compared to the following ones . DFT calculations suggest that a full desodiation might be correlated to a volume shrinkage of 8 % and a possibly irreversible phase transition rendering a complete oxidation unfavorable ,.…”

Recent Advances in Sodium-Ion Battery Materials

“…The relatively high work potential may result from the high inductive effect of the SO 4 2− . The sloping voltage curve indicates a single-phase (solid solution) mechanism, which was also proved by XRD patterns and Mossbauer Spectra [104]. Yamada and co-workers have carefully investigated the phase purity of off-stoichiometry Na 2+2x Fe 2−x (SO 4 ) 3 materials, and a more precise phase with the composition of Na 2.4 Fe 1.8 (SO 4 ) 3 was isolated [105].…”