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

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).4H(2)O 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 (Delta H degrees(f)) of alluaudite and bloedite are exothermic. Hydrated bloedite (Delta H degrees(f) = -117.16 +/- 1.10 kJ/mol) was found to be significantly more energetically stable than anhydrous alluaudite (Delta H degrees(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 similar to 80 mAh/g. Spray drying can be extended for rapid economic synthesis of sulfate class of battery materials.

Automated summary

In pursuit of high-energy density sodium insertion materials, researchers designed polyanionic frameworks with tuneable high-voltage operation. In this study, alluaudite sodium insertion material was synthesized using scalable aqueous-based spray drying, and a reversible phase transition from alluaudite to bloedite was observed for the first time. Calorimetric data supported the synthesis and transformation pathways, and spray drying showed promising performance as a method for synthesizing sulfate battery materials.