In Situ FTIR-Assisted Synthesis of Nickel Hexacyanoferrate Cathodes for Long-Life Sodium-Ion Batteries

Prussian blue analogs (PBAs) with stable framework structures are ideal cathodes for rechargeable sodium-ion batteries. The chelating agent-assisted coprecipitate method is an effective way to obtain low-defect PBAs that can limit the appearance of too many vacancies and water molecules and achieve an optimized Na-storage performance. However, for this method, the mechanism of chelating agent-assisted synthesis is still unclear. Herein, the synthesis process of nickel hexacyanoferrate (NiHCF) has been investigated by in situ infrared spectroscopy detection. The results show that the chelating agent oxalate slows down the nucleation process and effectively inhibits the formation of the Fe–C≡N–Ni frame in the aging process, producing highly crystallized and low-defect NiHCF samples. High-quality NiHCF presents a high specific capacity of 86.3 mAh g–1 (a theoretical value of ∼85 mAh g–1), an ultrastable cyclic retention of 90% over 800 cycles, and a remarkable high capacity retention of 74.6% at a current density of 4250 mA g–1 (50C). Particularly, the NiHCF//hard carbon full cell presents a high specific energy density of over 210 Wh kg–1 and an outstanding cyclic stability without obvious capacity attenuation over 1000 cycles.