Sodium Metal Sulphate Alluaudite Class of High Voltage Battery Insertion Materials

Electrochemical energy storage has recently seen an exponential demand in the large-scale (power) grid storage sector. Earth abundant sodium-ion batteries are competent to enable this goal with economic viability. In a recent report in sodium-ion battery research, alluaudite framework Na2Fe2(SO4)3 has been reported with the highest Fe3+/Fe2+ redox potential (ca. 3.8 V, P. Barpanda, G. Oyama, S. Nishimura, S. C. Chung, and A. Yamada., Nature Commun. 5: 4358, 2014) with energy density comparable to the state-of-the-art Li-ion batteries. Material discovery is as essential as optimization of the existing materials to yield better performance for efficient energy storage. In a goal to optimize the synthesis of the reported alluaudite, this work first time reports the aqueous based Pechini synthesis for sodium metal sulphate alluaudite. It is a two-step method, where complexing agent plays a crucial role in holding the metal ions reserving their oxidation states. In the 2nd step, this complexing agent leaves the product with porous morphology. Taking advantage of its porous as well as 3D conductive framework, the complex attains fast electron/ion transport and sodium intercalation. Moreover, the single-phase reaction mechanism during sodium intercalation is reflected in its cycling property. It performs as a desirable cathode with operating potential as high as 3.7 V. While pursuing the synthesis, we observed an excess amount of sodium sulphate in the precursor mixture is needed to reduce the amount of impurities. To optimize the composition of the alluaudite phase and to explore novel compounds, we have carefully surveyed the Na2SO4-FeSO4 binary system. This work explores the possible compositional and structural flexibility in the Pechini synthesized alluaudites. A comparative study between compositional and redox activity in these samples will further inspire improvement of the alluaudite-type sodium metal sulphates for advanced sodium-ion batteries.