Sustainable Aqueous Synthesis and Electrochemical Insights on High-Voltage Sodium Alluaudite Insertion Materials

The scope of this work is to demonstrate various energy-savvy synthesis routes for alluaudite frameworked high-voltage sodium insertion hosts with generic formula Na(2)Na(1)M(1)M(2)2(XO4)3 (M = 3d metals; X= S/ P/ V/ W/ Mo). Here, we have explored two unconventional aqueous routes, namely Pechini and spray drying methods, for SO4-based alluaudites and solution combustion route PO4-based alluaudites. Using these sustainable routes, we could achieve the lowest ever synthesis temperature (ca. 200 °C) and the lowest synthesis duration (ca. 6 h) for sulfate class of alluaudites. Further, we have discovered a hitherto unknown electrochemically active NaCoFe2(PO4)3 phosphate alluaudite. The electrochemical analyses revealed a reversible (de)sodiation activity providing 80% of net theoretical capacity. Both sulphate and phosphate alluaudites delivered good rate capability and operational stability over 100 cycles maintaining a Coulombic efficiency of 98%.