Alluaudite chemistry: From Intercalation to Conversion
Authors: Barman Pubali , Jha Pawan Kumar , Barpanda Prabeer 
Published: December 2024

Alluaudites, a naturally occurring mineral, are best known as the intercalation-based cathode materials in the field of sodium-ion batteries.1 The 3D tunnel like structure of alluaudite facilitates easy alkali-ion migration making it (de)insertion host. The structural formula can be expressed as A(1)A(2)M(1)M(2)2(XO4)3, where A and M sites are alkali and transition metal ions respectively and XO4 denotes the polyanionic moieties (SO4, PO4, AsO4, MoO4, WO4 and so on). Depending on the inductive effect of the electronegative element, the potential of the materials can be tuned. In 2014, Yamada group reported alluaudite-structured Na2Fe2(SO4)3 as 3.8 V cathode material which benchmarked the highest potential ever observed by Fe2+/Fe3+ redox couple because of the presence of electronegative SO4 moiety.2 In 2017, the first molybdate-based alluaudite material, Na2.67Mn1.67(MoO4)3 was reported as a 3.45 V cathode by probing Mn2+/Mn3+ redox.3 Except Mn, Mo can also serve as another redox centre by using its range of oxidation states from +6 to 0. However, there is an open question remains whether we can access these redox states of Mo to impart extra capacity in these materials.
To address the above issue, two case-studies will be discussed.

Molybdate alluaudite: A series of molybdate-based alluaudites (TM= Co, Ni) were synthesized by using wet-chemical solution-combustion route. The Co-based alluaudite, Na36Co1.32(MoO4)3 was found to act as a high-voltage cathode (4 V vs Na/Na+ and 4.1 V vs Li/Li+) involving Co3+/Co2+ redox center while cycling between 3.0 V to 4.3 V.4 An intercalation-type of mechanism was proved by ex-situ studies combined with first principle calculations. All the above-mentioned alluaudite consists of Mo species, which can be redox-active at lower voltage. When cycled in a low-voltage window (0.01 V to 3.0 V), Na3.36Co1.32(MoO4)3 was found to act as an anode in Na-ion batteries. High capacity (ca. 400-500 mAh/g) was obtained with a central potential ~0.6 V involving conversion and (de)insertion reaction mechanism. The Ni-analogue, Na3.6Ni1.2(MoO4)3 alluaudite, was studied as an anode material in both Li-ion and Na-ion batteries involving conversion and (de)intercalation redox mechanisms like the Co analogue. The underlying redox mechanism will be described for both cases involving post-mortem diffraction, electron microscopy, and spectroscopic tools along with density functional theory (DFT) calculations.

Tungstate alluaudite: The first-ever tungstate alluaudite, Na4Mn(WO4)3, prepared by a solid-state route, was found to work as a 0.4 V anode (vs. Na+/Na) material while cycled in the voltage window of 0.01 V to 3.0 V. Similar to molybdenum, W can also show multiple redox chemistries. A conversion- type mechanism was probed with the help of in-situ XRD, ex-situ TEM, XAS and XPS techniques.

References:

[1] D. Dwibedi, P. Barpanda, A. Yamada, Small Methods 4 (2020) 2000051.
[2] P. Barpanda, G. Oyama, S. Nishimura, S.-C. Chung, A. Yamada, Nat. Commun. 5 (2014) 4358.
[3] J. Gao, P. Zhao, K. Feng, Chem. Mater. 29 (2017) 940-944.
[4] P. Barman, P.K. Jha, A. Chaupatnaik et al, P. Barpanda, Mater. Today Chem. 27 (2023) 101316.