Insights into the Phase Purity and Storage Mechanism of Nonstoichiometric Na 3.4 Fe 2.4 (PO 4 ) 1.4 P 2 O 7 Cathode for High‐Mass‐Loading and High‐Power‐Density Sodium‐Ion Batteries

Mixed‐anion‐group Fe‐based phosphate materials, such as Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 , have emerged as promising cathode materials for sodium‐ion batteries (SIBs). However, the synthesis of pure‐phase material has remained a challenge, and the phase evolution during sodium (de)intercalation is debating as well. Herein, a solid‐solution strategy is proposed to partition Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 into 2NaFePO 4 ⋅ Na 2 FeP 2 O 7 from the angle of molecular composition. Via regulating the starting ratio of NaFePO 4 and Na 2 FeP 2 O 7 during the synthesis process, the nonstoichiometric pure‐phase material could be successfully synthesized within a narrow NaFePO 4 content between 1.6 and 1.2. Furthermore, the proposed synthesis strategy demonstrates strong applicability that helps to address the impurity issue of Na 4 Co 3 (PO 4 ) 2 P 2 O 7 and nonstoichiometric Na 3.4 Co 2.4 (PO 4 ) 1.4 P 2 O 7 are evidenced to be the pure phase. The model Na 3.4 Fe 2.4 (PO 4 ) 1.4 P 2 O 7 cathode (the content of NaFePO 4 equals 1.4) demonstrates exceptional sodium storage performances, including ultrahigh rate capability under 100 C and ultralong cycle life over 14000 cycles. Furthermore, combined measurements of ex situ nuclear magnetic resonance, in situ synchrotron radiation diffraction and X‐ray absorption spectroscopy clearly reveal a two‐phase transition during Na + extraction/insertion, which provides a new insight into the ionic storage process for such kind of mixed‐anion‐group Fe‐based phosphate materials and pave the way for the development of high‐power sodium‐ion batteries.