Unveiling the Intercalation Mechanism in Fe₂(MoO₄)₃ as an Electrode Material for Na-Ion Batteries by Structural Determination

Abstract

Monoclinic Fe(MoO) (FMO) shows distinct structural and electrochemical differences in the intercalation mechanism, depending on the guest ion. (1,2) FMO undergoes a single-phase reaction in a Na-ion cell, but a two-phase reaction in a Li-ion cell. Attempts to understand the difference in the mechanisms have been hindered by a lack of structural information on the fully sodiated phase NaFe(MoO) due to its structural complexity and the unavailability of a single crystal. In this work, we have solved and refined the crystal structure of NaFe(MoO) for the first time, using the technique of ab initio structure determination from powder diffraction data. Along with electrochemical and structural characterization, 3D bond valence sum difference map calculations enabled us to ascertain the decisive factors that determine such differences, in terms of the interatomic distance and coordination environment of a guest ion. In the case of Na insertion, only a slight expansion of the structure makes the cavity sites of FMO suitable for Na ions, with adequate distances and coordination with surrounding oxygen atoms, resulting in a solid-solution-type single-phase reaction. In the case of Li insertion, the cavity sites are so large for a Li ion that a significant structural change involving tilting of the FeO and MoO polyhedra is required to accommodate the Li ion in a suitable local environment, which does not allow a continuous structural change but results in a two-phase reaction.

Bilayered Ca0.28V2O5·H2O: High-Capacity Cathode Material for Rechargeable Ca-Ion Batteries and Its Charge Storage Mechanism

“…To understand the mechanism of the observed unusual calcium (de)­insertion reactions, an ab initio structure determination technique using the powder X-ray data was conducted to solve the charged superstructure, using the technique previously reported by us. ,, Figure b shows the comparison of the charged superstructure with the pristine structure. The a and b lattice parameters were almost the same between the pristine and the charged material (11.68 and 3.65 Å vs 11.63 and 3.64 Å, respectively).…”

“…A Rigaku SmartLab X-ray diffractometer with a Cu Kα 1 X-ray tube (λ = 1.5406 Å, 45 kV, and 200 mA) was used to measure the XRD data in the angular range of 3° ≤ 2θ ≤ 110°. The superstructure was solved using GSAS, combined with the single-crystal structure refinement software, CRSYTALS, and MCE for a view of the Fourier electron density maps, using the techniques previously reported by us. − The structure determination was performed as detailed below:…”

Rhombohedral Potassium–Zinc Hexacyanoferrate as a Cathode Material for Nonaqueous Potassium-Ion Batteries

“…The d Q /d V curve of the initial charge–discharge profiles for KZnHCF is presented in Figure b, showing two distinct redox couples: cathodic peaks at 3.76 and 3.98 V and the corresponding anodic peaks at 3.80 and 3.99 V, respectively. In contrast, three sets of redox peaks are observed in the case of sodium intercalation, suggesting that the size difference of the inserted cations can influence the intercalation mechanism for the same host material. , The detailed intercalation mechanism for the two redox couples is not understood clearly at present, requiring elaborate atomic-scale structural determination of the intermediate phases as a further study.…”

“…In contrast, three sets of redox peaks are observed in the case of sodium intercalation, 61 suggesting that the size difference of the inserted cations can influence the intercalation mechanism for the same host material. 62,63 The detailed intercalation mechanism for the two redox couples is not understood clearly The theoretical maximum x value is 2 when the available crystallographic sites are fully occupied. However, reversible K ions are only ∼80% of the maximum value with a range of 0 ≤ x ≤ 1.61.…”

Facile Green Synthesis of Pseudocapacitance-Contributed Ultrahigh Capacity Fe2(MoO4)3 as an Anode for Lithium-Ion Batteries

“…Iron­(III) molybdate, Fe 2 (MoO 4 ) 3 , a Na-super ionic conductor structure with corner-shared FeO 6 octahedra and MoO 4 tetrahedra, which construct a three dimensional (3D) open framework, could provide abundant interstitial empty sites, forming a favorable channel for the diffusion of guest ions such as lithium, sodium, and potassium ions. − Furthermore, the use of this compound is also attractive because it is nontoxic, contains earth-abundant elements, and is environmentally friendly. Additionally, this material could be applied as a possible alternative anode for LIBs that could deliver a theoretical capacity up to 1087 mA h g –1 because of constituent high oxidation state elements Fe 3+ and Mo 6+ species .…”