Unveiling the Key Factors: Mn/Fe Ratio, Distribution Homogeneity, and Electronic Conductivity of Na₄Mn_xFe_3‐x(PO₄)₂P₂O₇ for Practical Sodium‐Ion Batteries

Abstract High Mn content Na 4 Mn x Fe 3‐x (PO 4 ) 2 P 2 O 7 materials are attractive cathodes for sodium‐ion batteries with a higher operating voltage than that of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 . However, the poor intrinsic electronic conductivity and the Jahn–Teller distortion caused by Mn 3+ still make it difficult to obtain satisfactory Na 4 Mn x Fe 3‐x (PO 4 ) 2 P 2 O 7 up to now. In this work, Na 4 Mn x Fe 3‐x (PO 4 ) 2 P 2 O 7 /C‐CNT is successfully synthesized with a uniformly distributed Mn/Fe structure using Mn/Fe oxalate coprecipitates as precursors. The Na 4 Mn 1.7 Fe 1.3 (PO 4 ) 2 P 2 O 7 /C‐CNT sample with optimal Mn/Fe ratio and enhanced electronic conductivity exhibits a high mid‐discharge voltage of 3.70 V versus Na/Na + with a reversible capacity of 111 mAh g −1 at 0.05C, and also excellent rate capability (89.4 mAh g −1 at 20C) and superior cycling stability (90.4% capacity retention after 1000 cycles at 0.5C). An appropriate Mn/Fe ratio and uniform Mn/Fe distribution in the Na 4 Mn 1.7 Fe 1.3 (PO 4 ) 2 P 2 O 7 structure are critical to suppress the Jahn–Teller distortion of Mn 3+ . DFT calculation indicates the necessity of improving its electronic conductivity compared to that of conventional Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 . This work opens up valuable insights and strategies to design novel Na 4 Mn x Fe 3‐x (PO 4 ) 2 P 2 O 7 cathodes for practical sodium‐ion batteries.