Stoichiometry in the alluaudite-type Na2+2xFe2-x(SO4)3 cathode material for Na-ion batteries

Renewable energy sources, such as solar and wind sources, require solutions to store the energy produced, because of the intermittent nature of these sources. In this context, research on battery technology has seen a shift towards Na-ion batteries and environmentally friendly materials, containing metals such as iron and anions such as phosphates and sulfates.
Alluaudite-type materials offer fast kinetics in batteries thanks to large ion channels, associated with a low barrier of diffusion energy. Sulfate alluaudites form a new type of alluaudites described recently in the literature, with a high potential: around 3.8 V vs Na+/Na, the highest potential reported for the Fe3+/Fe2+ redox couple.1 This was achieved thanks to the larger inductive effect of the sulfates,2 as compared to the usual phosphate groups in this structure. This makes Na2Fe2(SO4)3 a promising candidate for positive electrodes in sodium-ion batteries.
However, the phase reported in the literature is the off-stoichiometric Na-rich Na2+2xFe2-x(SO4)3 phase. Because of the lower content in redox-active Fe ions, it offers a lower theoretical capacity (105 mAh/g for the typical x = 0.25 composition) than the stoichiometric Na2Fe2(SO4)3 phase (120 mAh/g). However, attempts to synthesize the stoichiometric compound used to result in the formation of a mixture of the Na-rich compound with electrochemically inactive secondary phases.3
In this work, we have designed a synthesis route that allows the formation of the stoichiometric phase without any secondary phase, through a precipitation method. To prevent the formation of the well-known, stable and redox inactive Na2Fe(SO4)2∙4H2O phase,4 this synthesis takes place in a non-aqueous medium. The higher homogeneity of the precursors allowed us to reduce the heat treatment from 2h to 24h, which was found to be necessary to obtain the stoichiometric phase, confirming that it is metastable, as suggested previously3.
Quantitative analysis (ICP OES) as well as X-ray diffraction and Mössbauer spectroscopy confirm the stoichiometry of this phase and its metastable nature and provide insight on the structure of the phase. This allowed a structural comparison between the stoichiometric and the Na-rich phases. The electrochemical activity of the stoichiometric phase was demonstrated by operando X-ray diffraction.

1 P. Barpanda, G. Oyama, S. Nishimura, S.-C. Chung and A. Yamada, Nat. Commun., 2014, 5, 4358.
2 G. Oyama, H. Kiuchi, S. C. Chung, Y. Harada and A. Yamada, J. Phys. Chem. C, 2016, 120, 23323–23328.
3 G. Oyama, S. Nishimura, Y. Suzuki, M. Okubo and A. Yamada, ChemElectroChem, 2015, 2, 1019–1023.
4 Y. Meng, T. Yu, S. Zhang and C. Deng, J. Mater. Chem. A, 2016, 4, 1624–1631.