The introduction of transition metals such as iron in oxides of alloying elements as, for instance, SnO2 has been proven to enable higher capacities and superior charge storage performance when used as lithium‐ion electrode materials. Herein, we report the evaluation of such electrode materials, precisely (carbon‐coated) Sn0.9Fe0.1O2−δ(−C), for sodium‐ion battery applications. The comparison with SnO2 as reference material reveals the beneficial impact of the presence of iron in the tin oxide lattice, enabling higher specific capacities and a greater reversibility of the de‐/sodiation process – just like for lithium‐ion battery applications. The overall achievable capacity, however, remains relatively low with about 300 mAh g−1 and up to more than 400 mAh g−1 for Sn0.9Fe0.1O2‐δ and Sn0.9Fe0.1O2−δ‐C, respectively, compared to the theoretical specific capacity of more than 1,300 mAh g−1 when assuming a completely reversible alloying and conversion reaction. The subsequently performed ex situ/operando XRD and ex situ TEM/EDX analysis unveils that this limited capacity results from an incomplete de‐/sodiation reaction, thus, providing valuable insights towards an enhanced understanding of alternative reaction mechanisms for sodium‐ion anode material candidates.

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