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 Na-4 Fe-3 (PO4 )(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 (PO4 )(2) P-2 O-7 into 2NaFePO(4) & sdot; Na-2 FeP2 O-7 from the angle of molecular composition. Via regulating the starting ratio of NaFePO4 and Na-2 FeP2 O-7 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 Na-4 Co-3 (PO4 )(2) P-2 O-7 and nonstoichiometric Na-3.4 Co-2.4 (PO4 )(1.4) P-2 O-7 are evidenced to be the pure phase. The model Na-3.4 Fe-2.4 (PO4 )(1.4) P-2 O-7 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.