FeNb2O6 as a High-Performance Anode for Sodium-Ion Batteries Enabled by Structural Amorphization Coupled with NbO6 Local Ordering.

Pseudocapacitance-type transition metal oxides have been extensively investigated as anodes for lithium-ion batteries (LIBs). Currently, they are also gaining attention for sodium-ion batteries (SIBs) due to their low volume change and safety. However, their performance in sodium storage remains limited, primarily due to the larger Na⁺ ion radius. Here, for the first time, an iron niobate is reported with a columbite structure as a high-performance sodium storage anode. The presence of iron triggers the loss of long-range order through disorder of the FeO₆ octahedra local structure, subsequently allowing reversible sodium storage in an amorphous phase. Simultaneously, the formation of short-range ordered zigzag-chain structures within the NbO₆ planes creates a "skeleton" that offers abundant active sites for pseudocapacitive ion storage and enhanced ion diffusion pathways. These characteristics of FeNb₂O₆ make it an effective intercalation host, offering high capacity along with fast Na⁺ kinetics, as demonstrated through operando and ex situ characterizations. It leads to an applicable reversible capacity (>300 mAh g^-1) with a favorable average voltage of ≈0.6 V and excellent rate capability (180.4 mAh g^-1 at a current density of 2 A g^-1). This study provides insights into the development of intrinsically active transition metal oxides for Na⁺-ion intercalation.