Manifold consumption of lithium resources for Li-ion batteries has led to concern over their paucity and high cost. It has triggered global research on alternative sodium-ion batteries due to the abundance, economy, non-toxicity and uniform geographical distribution of Na resources. Driven by economy and sustainability, suites of oxide and polyanionic cathode materials have been reported for sodium-ion batteries. The materials economy can be enhanced by using Fe-containing compounds as Fe is earth-abundant. With this spirit, Barpanda et al discovered alluaudite-type Na₂Fe₂(SO₄)₃ showing the highest Fe³⁺/Fe²⁺ redox activity (ca. 3.8 V vs. Na) along with good electrochemical capacity (ca. 100 mAh/g), reversibility and rate kinetics.

Based on inductive effect principle related to electronegative SO₄^2- based system like Na₂Fe₂(SO₄)₃ can deliver high redox potential. Nevertheless, synthesis of sulphate class of compounds is little tricky as they are prone to water dissolution and thermal decomposition. Hence, all known reports on this SO₄-based compound rule out any possible aqueous synthetic routes (e.g. sol-gel, hydrothermal) due to inherent dissolution of SO₄-based compounds in water. Thus, a cumbersome solid-state synthesis is employed involving (a) preparation of anhydrous FeSO₄ from commercial FeSO_4.7H₂O, (b) prolonged mechanical mixing of FeSO₄ with Na₂SO₄ and (c) extended annealing (350 °C, 24 h under Ar). It calls for alternate energy-savvy synthesis protocols.

Exploring aqueous solvothermal synthesis as economic and scalable method to produce SO₄-based alluaudite cathodes, we have implemented two aqueous syntheses for the first time: (1) spray drying and (2) Pechini method. Solution-assisted synthesis has advantage over solid-state synthesis by reducing the annealing temperature and/or duration, since ionic diffusion is faster in liquid than solid media. The current work will demonstrate (i) various aspects of scalable spray drying and Pechini syntheses to form alluaudite cathode materials, (ii) synchrotron diffraction confirming phase-purity, (iii) energy-savvy attributes by benchmarking the lowest annealing temperature (200 °C) and time (6 h), (iv) discovery of two novel alluaudite phases and (v) the electrochemical performance of resulting alluaudite cathodes. These aqueous routes can work as high-throughput synthesis of SO₄-alluaudites as well as their solid-solution phases.

Aqueous solvothermal route can be used for rapid screening of alluaudite materials. Using these routes, we have produced various solid-solution Na₂(Fe_1-xM_x)₂(SO₄)₃ (M = Mn, Co, Ni) phases as well as stoichiometrically controlled Na_2+2xFe_2-x(SO₄)₃ (x = 0-0.22). We will describe the structure, magnetic properties and electrochemical performance performances of this alluaudite family of compounds correlating their structure with electrochemical performance, Na⁺ diffusion rate kinetics.

References

[1] P. Barpanda, G. Oyama et al, A. Yamada, Nature Commun., 5 (2014), 4358.

[2] L. L. Wong et al., S. Adams, Phys. Chem. Chem. Phys., 17 (2015), 9186.

[3] D. Dwibedi et al, P. Barpanda, ACS Appl. Mater. Interfaces, 8 (2016), 6982.

[4] D. Dwibedi et al, P. Barpanda, J. Mater. Chem. A., 3 (2015), 18564.

[5] D. Dwibedi et al, P. Barpanda, Dalton Trans., 46 (2017) 55.