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.

中文翻译：

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扩展焦磷酸钠铁的固溶体范围：非化学计量的 Na3Fe2.5(P2O7)2 作为钠离子电池的新型阴极

铁基焦磷酸盐由于其大骨架、成本效益和高能量密度而成为钠离子电池有吸引力的阴极。然而，对晶体结构的了解很少，迄今为止只报道了有限的候选者。在这项工作中，我们首次发现 中心阳离子相互取代可以得到连续固溶体Na _{4− α} Fe _{2+ α /2} (P ₂ O ₇ ) ₂ (0 ≤  α ≤ 1) -对称的Na3和Na4位点，同时保持阴离子P ₂ O ₇的晶体结构单元不变。特别是，因此提出了一种新型的非化学计量的Na ₃ Fe _2.5 (P ₂ O ₇ ) ₂，及其结构、储能机制。和电化学性能进行了广泛的研究，以揭示所制备的非化学计量电极具有令人瞩目的性能，可逆放电容量为 83 mAh g ^-1，工作电压为 2.9 V（相对于 Na ⁺ /） 。 Na)，500次循环后仍保留了89.2%的初始容量，并且在1600 mA g ^-1的电流密度下增强了51 mAh g ^-1的倍率性能。这项研究表明，焦磷酸铁钠可以形成扩展的固溶体成分。而有前景的相隐藏在Na _{4− α} Fe _{2+ α /2} (P ₂ O ₇ ) ₂范围内，为探索用于构建高性能SIB的新型正极材料提供了更多机会。