Insights into the Phase Purity and Storage Mechanism of Nonstoichiometric Na3.4Fe2.4(PO4)1.4P2O7 Cathode for High‐Mass‐Loading and High‐Power‐Density Sodium‐Ion Batteries

Mixed‐anion‐group Fe‐based phosphate materials, such as Na4Fe3(PO4)2P2O7, 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 Na4Fe3(PO4)2P2O7 into 2NaFePO4 ⋅ Na2FeP2O7 from the angle of molecular composition. Via regulating the starting ratio of NaFePO4 and Na2FeP2O7 during the synthesis process, the nonstoichiometric pure‐phase material could be successfully synthesized within a narrow NaFePO4 content between 1.6 and 1.2. Furthermore, the proposed synthesis strategy demonstrates strong applicability that helps to address the impurity issue of Na4Co3(PO4)2P2O7 and nonstoichiometric Na3.4Co2.4(PO4)1.4P2O7 are evidenced to be the pure phase. The model Na3.4Fe2.4(PO4)1.4P2O7 cathode (the content of NaFePO4 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. A solid‐solution strategy is proposed to eliminate the impurity in Na4Fe3(PO4)2P2O7 materials. The as‐prepared pure‐phase Na3.4Fe2.4(PO4)1.4P2O7 exhibits enhanced specific capacity and cycling performance up to 14000 cycles. And a two‐phase transition during Na+ extraction/insertion is distinctly revealed, thus providing new insights into both synthesis strategy and ion‐storage mechanism of the promising Fe‐based mixed‐anion‐group compound.