Non-aqueous synthesis of stoichiometric alluaudite-type Na2Fe2(SO4)3 for Na-ion cathode

In the pursuit of ever better materials for the efficient storage of energy, the alluaudite-type Na2Fe2(SO4)3 material was recently investigated and turned out to be a promising candidate for positive electrodes in sodium-ion... [ view full abstract ]

In the pursuit of ever better materials for the efficient storage of energy, the alluaudite-type Na₂Fe₂(SO₄)₃ material was recently investigated and turned out to be a promising candidate for positive electrodes in sodium-ion batteries. This is due to both its high potential (around 3.8 V vs Na⁺/Na, the highest potential reported for the Fe³⁺/Fe²⁺ redox couple) as well as its fast kinetics, thanks to large channels with a low barrier of diffusion energy for the sodium ions.

In the literature, this structure has always been synthesized with an excess in sodium (mainly via solid-state synthesis), leading to the off-stoichiometric compound Na_2+2xFe_2-x(SO₄)₃. Attempts to synthesize the stoichiometric compound resulted in the formation of secondary phases which are not electrochemically active. This suggests that the stoichiometric phase is metastable.

Here, we propose a liquid route synthesis to enable a better homogeneity of the reagents, which allows us to synthesize the stoichiometric Na₂Fe₂(SO₄)₃ phase without any secondary phase, while reducing the annealing time from 24h to 2h. To prevent the formation of the well-known, stable and redox inactive Na₂Fe(SO₄)₂∙4H₂O phase, this synthesis takes place in a non-aqueous medium.

We have characterized the structural and electrochemical differences between the stoichiometric and off-stoichiometric phases synthesized via our method. Their compositions were assessed by ICP-OES. Their structures were studied by Mössbauer spectroscopy, scanning electron microscopy, Raman spectroscopy and powder XRD (ex situ and operando) along with Rietveld refinements. Their electrochemical behaviors were characterized by galvanostatic cycling and cyclic voltammetry. Our results appear to confirm the metastability of the stoichiometric phase, which easily decomposes into a mixture of the off-stoichiometric phase and iron sulfate.