Synthesized From FeVO 4 Precursor and Al 2 O 3 Modified Cathode Material Na 4 FeV(PO 4 ) 3 for Sodium Ion Batteries

The NASICON‐type cathode Na 4 FeV(PO 4 ) 3 (NFVP) has attracted interest for its open framework and high theoretical capacity. However, current synthesis methods often introduce impurity phases, degrading electrochemical performance. Moreover, the highly oxidizing V 5+ tends to react with the electrolyte at high voltages, causing structural distortion and irreversibility of the V 4+ /V 5+ redox couple. Therefore, suppressing impurity formation and the side reactions between V 5+ and the electrolyte is key to improving high‐voltage performance. To address these issues, this study proposes a precursor‐based synthesis approach to obtain high‐purity NFVP with excellent electrochemical performance, combined with an Al 2 O 3 protective layer on the NFVP surface. The Al 2 O 3 coating minimizes direct contact between the active material and the electrolyte, effectively suppressing side reactions while enhancing structural stability. As a result, the modified cathode shows greatly improved cycling stability and rate capability in the 2.0–4.3 V range. Specifically, after 200 cycles at 1 C, capacity retention increases from 46.4% to 70%, and the specific discharge capacity at 20 C improves from 17.4 to 53.9 mAh g −1 . This work offers a feasible strategy to enhance the high‐voltage performance of NASICON‐type cathodes for sodium‐ion batteries.