Aqueous spray-drying synthesis of alluaudite Na_2+2xFe_2−x(SO₄)₃ sodium insertion material: studies of electrochemical activity, thermodynamic stability, and humidity-induced phase transition

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 Na_2+2xFe_2−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.