The iron-based mixed polyanionic compound Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) with high theoretical capacity and excellent structural stability has attracted comprehensive attention as a promising cathode material for sodium-ion batteries (SIBs). Nevertheless, the constrained conductivity and restricted diffusion kinetics during sodium storage pose an unprecedented challenge to rate capability and cycling stability. Herein, an oxygen vacancy strategy by dual-anionic doping (F⁻ and SiO₄⁴⁻) is first proposed and the corresponding function mechanism in structural and electronic optimization is unraveled. Experimental analyses and theoretical calculations reveal that the combination of F⁻ with higher electro-negativity and the SiO₄⁴⁻ group with a larger ionic radius can efficiently expand the ion diffusion channels and induce more oxygen vacancies, thus boosting electronic conductivity, as well as accelerating sodium ion transfer kinetics. Benefitting from the synergy of F/Si dual doping and abundant oxygen vacancies, NFPP-0.1F/0.05Si outperforms both the undoped and single-anionic-doped NFPP samples, and delivers a remarkably high capacity of 119.6 mA h g⁻¹ at 0.1C, a conspicuous rate performance of 67.7 mA h g⁻¹ at 10C, and good cycling stability with a capacity retention of 80.86% over 4000 cycles at 50C. More encouragingly, the full cells (NFPP-0.1F/0.05Si‖hard carbon) also exhibit outstanding long-term cycling performance at 50 mA g⁻¹, and demonstrate a reversible capacity of 77.8 mA h g⁻¹ and still retain 78.3% of their initial capacity after 200 cycles, manifesting great practical potential.

中文翻译：

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Na4Fe3(PO4)2P2O7 阴极中 F 和 Si 双掺杂诱导的氧空位能够提高钠存储的电化学性能


铁基混合聚阴离子化合物Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ (NFPP)具有高理论容量和优异的结构稳定性，作为一种有前途的钠离子电池(SIBs)正极材料引起了广泛的关注。然而，钠储存过程中受限的电导率和受限的扩散动力学对倍率性能和循环稳定性提出了前所未有的挑战。在此，首次提出了双阴离子掺杂（F ^-和SiO ₄ ⁴⁻ ）的氧空位策略，并揭示了相应的结构和电子优化的作用机制。实验分析和理论计算表明，具有较高电负性的F ^-与具有较大离子半径的SiO ₄ ⁴⁻基团结合可以有效扩大离子扩散通道并诱导更多的氧空位，从而提高电子电导率，以及加速钠离子转移动力学。受益于F/Si双掺杂和丰富的氧空位的协同作用，NFPP-0.1F/0.05Si的性能优于未掺杂和单阴离子掺杂的NFPP样品，并在0.1C下提供了119.6 mA hg ^-1的极高容量， 67 的显着速率性能。10C下为7mA hg ^-1 ，循环稳定性良好，50C下4000次循环后容量保持率为80.86%。更令人鼓舞的是，全电池（NFPP-0.1F/0.05Si‖硬碳）在50 mA g ⁻¹下也表现出出色的长期循环性能，并表现出77.8 mA hg ⁻¹的可逆容量，并且仍然保留了78.3%的容量。其在200次循环后的初始容量，表现出巨大的实用潜力。