Insights into the Phase Purity and Storage Mechanism of Nonstoichiometric Na_3.4Fe_2.4(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 Na_3.4Co_2.4(PO₄)_1.4P₂O₇ are evidenced to be the pure phase. The model Na_3.4Fe_2.4(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.