The Fe^4+/3+ Redox Mechanism in NaFeO₂: A Simultaneous Operando Nuclear Resonance and X‐ray Scattering Study

Abstract

Simultaneous operando Nuclear Forward Scattering and transmission X-ray diffraction and 57 Fe Mössbauer spectroscopy measurements were carried out in order to investigate the electrochemical mechanism of NaFeO 2 vs. Na metal using a specifically designed in situ cell. The obtained data were analysed using an alternative and innovative data analysis approach based on chemometric tools such as Principal Component Analysis (PCA) and Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS). This approach, which allows the unbiased extraction of all possible information from the operando data, enabled the stepwise reconstruction of the independent "real" components permitting the description of the desodiation mechanism of NaFeO 2. This wealth of information allows a clear description of the electrochemical reaction at the redox-active iron centres, and thus an improved comprehension of the cycling mechanisms of this material vs. sodium.

Direct Mechanochemical Synthesis, Phase Stability, and Electrochemical Performance of α-NaFeO2

“…The principal component in all Mössbauer spectra, NaFeO 2 A, exhibits an isomer shift of ∼0.36 mm/s and quadrupole splitting of 0.46 mm/s. These parameters are in excellent agreement with recent reports. ,, The second broader component NaFeO 2 B has a slightly smaller isomer shift, ∼0.34 mm/s, and larger quadrupole splitting of ∼0.8 mm/s; this component is also significantly broader, indicating the disorder or distribution of the local environment. A similar increase in quadrupole splitting for part of Fe­(III) is observed in operando measurements and is typically accompanied by the appearance of Fe­(IV) ,, but is also observed before the appearance of Fe­(IV) at V < 3.8 V vs Li/Li + in Li­(Ni,Fe,Al)­O 2 .…”

“…44,73,74 The second broader component NaFeO 2 B has a slightly smaller isomer shift, ∼0.34 mm/s, and larger quadrupole splitting of ∼0.8 mm/s; this component is also significantly broader, indicating the disorder or distribution of the local environment. A similar increase in quadrupole splitting for part of Fe(III) is observed in operando measurements and is typically accompanied by the appearance of Fe(IV) 44,73,74 but is also observed before the appearance of Fe(IV) at V < 3.8 V vs Li/Li + in Li(Ni,Fe,Al)O 2 . 75 The likely origin of the B component is thus vacancies nearby the Fe site.…”

Experimental Considerations for Operando Metal‐Ion Battery Monitoring using X‐ray Techniques

“…Although they have higher initial cost than the pouch and coin cells, they can be reused and mounted on‐site for experiments at large facilities. Moreover, they are a versatile option for multi‐technique experiments [40,41] . This is the type of cell used in the present study, allowing both reflection and transmission geometries (See Figure 2 for examples of the different cell environments that will be discussed in this work).…”

“…Moreover, they are a versatile option for multi-technique experiments. [40,41] This is the type of cell used in the present study, allowing both reflection and transmission geometries (See Figure 2 for examples of the different cell environments that will be discussed in this work).…”

Polymorphism in Weberite Na2Fe2F7 and its Effects on Electrochemical Properties as a Na-Ion Cathode

“…In addition, the predicted profiles align well with experimental results. Below x = 1, large overpotentials are observed in the GITT data, which our DFT results suggest is due to the high potentials (>5.75 V) required for further Na extraction, well outside the electrochemical stability window of conventional carbonate electrolytes. − The high predicted potentials at the top of charge arise from a rather unstable fully deintercalated structure, which is likely partly due to the difficulty of oxidizing Fe past Fe 3+ . , In fact, Fe 4+ has never been observed in fluoride materials. Large overpotentials are also observed in the GITT data at the end of discharge ( x > 2), although our calculations suggest that Na should be able to intercalate up to x = 3 within the potential range used in our experiments.…”