Improved Reversible Capacity and Cycling Stability by Linear (N=O) Anions in Fe[Fe(CN)₅NO] as Sodium-Ion Battery Cathode

Prussian blue analogues (PBAs) are promising cathode materials for sodium-ion batteries (SIBs) due to their tunable chemistry, open channel structure, and low cost. However, excessive crystal water and volume expansion can negatively impact the lifetime of actual SIBs. In this study, a novel iron nitroprusside: Fe[Fe(CN)₅NO] (PBN) was synthesized to effectively eliminate the detrimental effects of crystal water on the reversible capacity and cycling stability of PBA materials. Experiments and DFT calculations demonstrated that PBN has lower crystal water and volume expansion compared to Fe[Fe(CN)₆] (PB). Also, the N=O bond in PBN significantly reduces the diffusion potential of Na⁺ in the skeleton. Without any modification, the cathode material exhibited a capacity of up to 148.6 mAh g⁻¹ at 50 mA g⁻¹ as well as maintained 102.9 mAh g⁻¹ after 200 cycles. This work expands our knowledge of the crystal structure of PBA cathode materials and facilitates the rational design of high-quality PBA cathodes for SIBs.