Alluaudites: From Sulfates to Tungstates
Authors: Barman Pubali , Dwibedi Debasmita , Barpanda Prabeer 
Published: February 2024

Alluaudites are essentially naturally occurring mixed metal (Mn and Fe) phosphate-based minerals [1] consisting of an open framework structure to support fast alkali migration. The general formula for alluaudite can be expressed by A(1)A(2)M(1)M(2)2(XO4)3 where A and M sites are alkali ion and transition metals respectively and X can be S, P, As, Mo, W, V and so on [2]. The presence of polyanionic moieties (XO4) assures structural stability and high-voltage operation arising from the inductive effect [3]. Till date, a variety of phosphate and sulfate-based alluaudites have been reported as cathodes for Li-ion and Na-ion batteries [4,5]. In 2014, Yamada group reported a Fe-based sulfate alluaudite, Na2Fe2(SO4)3, acting as a 3.8 V SIB cathode material with excellent rate capability and energy density. It ushered the discovery of various high-voltage sulfate alluaudites materials by optimizing different synthesis routes. While they offer high-voltage performance, the presence of SO4 species makes them susceptible to thermal decomposition and moisture-sensitive. We have observed a clear phase transition from alluaudite to hydrated bloedite with exposure to the humid atmosphere [6]. As compared to sulfates, phosphate alluaudites are moisture stable but suffer from lower voltage operation due to the lower inductive effect of P. On the other hand, they had shown excellent electrocatalytic activity for both ORR and OER mechanism [7]. Recently, Gao et al. reported the first molybdate-based alluaudite Na2.67Mn1.67(MoO4)3 working as a 3.45 V Mn-based insertion cathode for SIBs [8]. Inspired by this work, we have extended the molybdate alluaudite chemistry towards its Co, Ni and Cu- analogue, Na3.36Co1.32(MoO4)3, Na2.4Ni0.8(MoO4)2 and Na4Cu(MoO4)3. The Co- molybdate was found to act as 4.0 V cathode material against Na/Na+. All three molybdates have been found to show excellent activity while cycling within the lower voltage region by exploiting its Mo- redox which has been probed by different ex-situ post-mortem analysis. Moreover, both Co- and Ni- analogues had shown excellent activity in ORR and OER reaction as an add-on to its battery application. We have detected the mechanism by using in-situ Raman spectroscopy, TEM and XPS analysis. Finally, for the first time, we have synthesized Na4Mn(WO4)3, a tungstate alluaudite whose structure and electrochemistry are not reported anywhere to the best of our knowledge. Using inductive effect principle, various alluaudite systems will be discussed.
References:
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