Sodium iron hexacyanoferrate (NaFeHCF) has been considered as a potential cathode for sodium-ion batteries owing to its low-cost and easily prepared procedure. However, it is still challenging to achieve long cyclic stability and superior rate capability, and the sodium storage mechanism of sodium-rich NaFeHCF is still elusive. Herein, a sodium-rich NaFeHCF with rhombohedral structure is presented with excellent electrochemical performances within 2.0–4.2 ​V. The specific capacity of ~115 ​mA ​h ​g⁻¹ is obtained by utilizing two plateaus around 2.9 and 4.06 ​V, respectively. Remarkable rate performance from 10 to 4000 ​mA ​g⁻¹ and 1000 cycles with high capacity retention is achieved as well. Synchrotron powder X-ray diffraction (PXRD) and structural refinement reveals that sodium-ions occupy three different sites (interstitial, face and edge) in rhombohedral unit cell, which contribute different capacities on different plateaus during Na⁺ extractions. Moreover, the rhombohedral structure is well-maintained after long-term Na⁺ extractions/insertions and reversible phase transitions with small volume variation are observed through in-situ synchrotron PXRD. The kinetic properties of Na⁺ in rhombohedral unit cell are identified by ab-initio molecular dynamics method and density functional theory calculations, which indicate that Na⁺ transport on three-dimensional diffusion paths, thus enabling the outstanding rate performance of NaFeHCF.

六氰合铁酸铁钠（NaFeHCF）由于其低成本且易于制备的过程而被认为是钠离子电池的潜在阴极。然而，实现长周期稳定性和优异的倍率能力仍然具有挑战性，并且富钠的NaFeHCF的钠存储机制仍然难以捉摸。本文介绍了具有菱形结构的富含钠的NaFeHCF，在2.0-4.2 V范围内具有出色的电化学性能。通过分别利用2.9和4.06 V附近的两个平稳区获得〜115 mA h g ^-1的比容量。从10到4000 mA g ^-1的卓越速率性能并实现了1000个具有高容量保持率的循环。同步加速器粉末X射线衍射（PXRD）和结构细化显示，钠离子占据菱面体晶胞中的三个不同位置（间隙，面和边缘），在Na ⁺提取过程中，它们在不同的高原上贡献不同的容量。此外，在长期Na ⁺提取/插入后，通过原位同步加速器PXRD观察到菱形体的结构得到了很好的维护，并且观察到了体积变化小的可逆相变。通过从头算分子动力学方法和密度泛函理论计算，确定了菱形体单元格中Na ⁺的动力学性质，这表明Na ⁺ 在三维扩散路径上迁移，从而使NaFeHCF具有出色的速率性能。