Aqueous spray-drying synthesis of alluaudite Na2+2xFe2−x(SO4)3 sodium insertion material: studies of electrochemical activity, thermodynamic stability, and humidity-induced phase transition

Abstract

In pursuit of high-energy density sodium insertion materials, polyanionic frameworks can be designed with tuneable high-voltage operation stemming from inductive effect. Alluaudite Na2Fe2(SO4)3 polysulfate forms one such earth-abundant compound registering the highest Fe3+/Fe2+ redox potential (ca. 3.8 V vs. Na/Na+). While this SO4-based system exhibits high voltage operation, it is prone to thermal decomposition and moisture attack leading to hydrated derivatives, making its synthesis cumbersome. Also, the Na–Fe–S–O quaternary system is rich with (anhydrous to hydrated) phase transitions. Herein, we demonstrate scalable aqueous-based spray drying synthesis of alluaudite Na2+2xFe2−x(SO4)3 sodium insertion material involving the formation of bloedite Na2Fe(SO4)2·4H2O as an intermediate phase. Moreover, a reversible phase transition from alluaudite to bloedite under controlled conditions of temperature and relative humidity is reported for the first time. Thermochemistry measurements revealed the enthalpies of formation (ΔH°f) of alluaudite and bloedite are exothermic. Hydrated bloedite (ΔH°f = −117.16 ± 1.10 kJ/mol) was found to be significantly more energetically stable than anhydrous alluaudite (ΔH°f = −11.76 ± 1.25 kJ/mol). The calorimetric data support the observed synthesis and transformation (hydration-dehydration) pathways. Spray drying route led to spherical morphology delivering capacity ~80 mAh/g. Spray drying can be extended for rapid economic synthesis of sulfate class of battery materials.

Spherical Shell with CNTs Network Structuring Fe‐Based Alluaudite Na2+2δFe2−δ(SO4)3 Cathode and Novel Phase Transition Mechanism for Sodium‐Ion Battery

“…Owing to the higher working voltage of 3.8 V, the prepared material in the present work still exhibited a decent energy density close to 360 Wh kg −1 , which is relatively higher than other reported materials (Figure 7i and Table S7, Supporting Information). [17][18][22][23][24]32,36] On the other hand, both NFS@C and NFS@C@2% CNTs showed an irreversible capacity loss in the initial charging process, as shown in Figure 8a and Figure S9, Supporting Information, which is the same as the previous reports. [9,10,40] Two anodic peaks at 3.67 and 4.12 V existed in discharging and three corresponding reduction peaks occurred in the first charging process by CV test, and it displayed three pairs of reversible redox peaks in the second and third cycles (Figure 8b and Figure S9, Supporting Information).…”

Accelerating the Phase Formation Kinetics of Alluaudite Sodium Iron Sulfate Cathodes via Ultrafast Thermal Shock

“…Crystal water significantly influences the phase formation of sodium iron sulfate. ,,− Depending on whether the iron source (FeSO 4 ·7H 2 O) in the ingredients is predehydrated, two types of precursors can be obtained during the high-energy ball-milling process. Apparently, the hydrous precursors obtained after ball milling are not simply mixtures of raw reagents.…”

“…For comparison, HF-NFS was also characterized through SEM and TEM. As shown in Figure S8, significant aggregation of the superfine primary particles could be observed on the surface because of long-period sintering, ,, causing the average size of the secondary particles to be larger than that of HS-NFS, reaching ca. 2 μm.…”

Facile synthesis and phase stability of Cu-based Na2Cu(SO4)2·xH2O (x = 0–2) sulfate minerals as conversion type battery electrodes

“…25 In a typical calorimetry test, 6-8 mg of sample was pressed into a pellet and then dropped into 25 g of 5 M HCl placed in the sample chamber of the calorimeter as mentioned elsewhere. 22,36 The sample dissolution causes a heat flow due to the temperature difference and is recorded as a calorimetric signal. The integrated area under the recorded microwatt signal from a linear baseline corresponds to total heat effects, which on conversion into joules with KCl calibration corresponds to the enthalpy of sample dissolution (ΔH ds ).…”