Na 2 Fe 2 Se 2 O: a double anti-perovskite with prevalence of anionic redox activity in Na-ion batteries.

Na 2 Fe 2 Se 2 O with the I 4/ mmm space group was studied in sodium-ion batteries, delivering a reversible specific capacity of more than 140 mA h g -1 . Operando XRD and XAS studies disclosed bifunctional redox behaviour with the prevalence of anionic electrochemical activity and a likely partial decomposition of the material, which, however, does not influence the electrochemical behaviour of the system.