Extending the solid solution range of sodium ferric pyrophosphate: Off-stoichiometric Na₃Fe_2.5(P₂O₇)₂ as a novel cathode for sodium-ion batteries

Iron-based pyrophosphates are attractive cathodes for sodium-ion batteries due to their large framework, cost-effectiveness, and high energy density. However, the understanding of the crystal structure is scarce and only a limited candidates have been reported so far. In this work, we found for the first time that a continuous solid solution, Na_4−αFe_2+α/2(P₂O₇)₂ (0 ≤ α ≤ 1, could be obtained by mutual substitution of cations at center-symmetric Na3 and Na4 sites while keeping the crystal building blocks of anionic P₂O₇ unchanged. In particular, a novel off-stoichiometric Na₃Fe_2.5(P₂O₇)₂ is thus proposed, and its structure, energy storage mechanism, and electrochemical performance are extensively investigated to unveil the structure–function relationship. The as-prepared off-stoichiometric electrode delivers appealing performance with a reversible discharge capacity of 83 mAh g⁻¹, a working voltage of 2.9 V (vs. Na⁺/Na), the retention of 89.2% of the initial capacity after 500 cycles, and enhanced rate capability of 51 mAh g⁻¹ at a current density of 1600 mA g⁻¹. This research shows that sodium ferric pyrophosphate could form extended solid solution composition and promising phase is concealed in the range of Na_4−αFe_2+α/2(P₂O₇)₂, offering more chances for exploration of new cathode materials for the construction of high-performance SIBs.