Ionothermal Synthesis of High-Voltage Alluaudite Na2+2xFe2‑x(SO4)3 Sodium Insertion Compound: Structural, Electronic, and Magnetic Insights

• Published in: ACS applied materials & interfaces 2016-03, Vol.8 (11), p.6982-6991
• Main Authors: Dwibedi, Debasmita, Ling, Chris D, Araujo, Rafael B, Chakraborty, Sudip, Duraisamy, Shanmughasundaram, Munichandraiah, Nookala, Ahuja, Rajeev, Barpanda, Prabeer
• Format: Article
• Language: English
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.5b11302

Exploring future cathode materials for sodium-ion batteries, alluaudite class of Na2FeII 2(SO4)3 has been recently unveiled as a 3.8 V positive insertion candidate (Barpanda et al. Nat. Commun. 2014, 5, 4358). It forms an Fe-based polyanionic compound delivering the highest Fe-redox potential along with excellent rate kinetics and reversibility. However, like all known SO4-based insertion materials, its synthesis is cumbersome that warrants careful processing avoiding any aqueous exposure. Here, an alternate low temperature ionothermal synthesis has been described to produce the alluaudite Na2+2xFeII 2‑x(SO4)3. It marks the first demonstration of solvothermal synthesis of alluaudite Na2+2xMII 2‑x(SO4)3 (M = 3d metals) family of cathodes. Unlike classical solid-state route, this solvothermal route favors sustainable synthesis of homogeneous nanostructured alluaudite products at only 300 °C, the lowest temperature value until date. The current work reports the synthetic aspects of pristine and modified ionothermal synthesis of Na2+2xFeII 2‑x(SO4)3 having tunable size (300 nm ∼5 μm) and morphology. It shows antiferromagnetic ordering below 12 K. A reversible capacity in excess of 80 mAh/g was obtained with good rate kinetics and cycling stability over 50 cycles. Using a synergistic approach combining experimental and ab initio DFT analysis, the structural, magnetic, electronic, and electrochemical properties and the structural limitation to extract full capacity have been described.