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

Abstract

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 FeSO4.7H2O, (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(Fe1-xMx)2(SO4)3 (M = Mn, Co, Ni) phases as well as stoichiometrically controlled Na2+2xFe2-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.