Evaluation of Sn${}_{0.9}$Fe${}_{0.1}$O${}_{2‐\delta }$ as Potential Anode Material for Sodium‐Ion Batteries

The introduction of transition metals such as iron in oxides of alloying elements as, for instance, SnO${}_2$ 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) Sn${}_{0.9}$Fe${}_{0.1}$O${}_{2-\delta }$(−C), for sodium-ion battery applications. The comparison with SnO${}_2$ 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 Sn${}_{0.9}$Fe${}_{0.1}$O${}_{2-\delta }$ and Sn${}_{0.9}$Fe${}_{0.1}$O${}_{2-\delta }$-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.