Olivine-Type Fe₂GeX₄ (X = S, Se, and Te): A Novel Class of Anode Materials for Exceptional Sodium Storage Performance

The introduction of abundant metals to form ternary germanium-based chalcogenides can dilute the high price and effectively buffer the volume variation of germanium. Herein, olivine-structured Fe₂GeX₄ (X = S, Se, and Te) are synthesized by a chemical vapor transport method to compare their sodium storage properties. A series of in situ and ex situ measurements validate a combined intercalation-conversion-alloying reaction mechanism of Fe₂GeX₄. Fe₂GeS₄ exhibits a high capacity of 477.9 mA h g^-1 after 2660 cycles at 8 A g^-1, and excellent rate capability. Furthermore, the Na₃V₂(PO₄)₃//Fe₂GeS₄ full cell delivers a capacity of 375.5 mA h g^-1 at 0.5 A g^-1, which is more than three times that of commercial hard carbon, with a high initial Coulombic efficiency of 93.23%. Capacity-contribution and kinetic analyses reveal that the alloying reaction significantly contributes to the overall capacity and serves as the rate-determining step within the reaction for both Fe₂GeS₄ and Fe₂GeSe₄. Upon reaching a specific cycle threshold, the assessment of the kinetic properties of Fe₂GeX₄ primarily relies on the ion diffusion process that occurs during charging. This work demonstrates that Fe₂GeX₄ possesses promising practical potential to outperform hard carbon, offering valuable insights and impetus for the advancement of ternary germanium-based anodes.