Na3Fe2(SO4)2(SO3N) as a potential high capacity cathode material

• The introduction of nitrogen in NaxFe2(SO4)3 forms NaxFe2(SO4)2(SO3N).
• Increase in the theoretical energy density: from 480 W h kg−1 to 650 W h kg−1.
• N allows further oxidation even when all Fe sites are in 3+ configurations.
• No evidence of Fe4+ involvement was found.
• Formation of nitrogen-nitrogen bonds enabling further energy storage.

A novel sulfate material, Na2Fe2(SO4)3, was recently proposed as a high-density cathode material for sodium-based batteries (Barpanda et al., 2014). This study presents ab initio simulations describing the effect of partial oxygen-to-nitrogen substitution in this material with the aim of increasing the sodium capacity of the structure as well as its energy density. The considered structure with the most likely substitution configuration increases the theoretical energy density to 650 W h kg−1, improving over the theoretical value of 480 W h kg−1 for the original, nitrogen-free structure.

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