Understanding the Structural Evolution and Lattice Water Movement for Rhombohedral Nickel Hexacyanoferrate upon Sodium Migration

Prussian blue analogues (PBAs) have been regarded as prospective cathode materials for sodium-ion batteries due to tunable chemical composition and structure. Herein, a high-performance rhombohedral nickel hexacyanoferrate is synthesized via a controllable low-temperature reaction process. It can deliver impressive capacity retention of 87.8% after 10 000 cycles at 10C and high rate discharge capacity of 53 mAh g–1 at 40C. According to the structural evolution and lattice water movement, superior electrochemical performance is ascribed to small lattice alteration and high reversibility of rhombohedral–cubic transition upon Na+ insertion/extraction. The environment information of local- and long-range structure evolution is revealed by ex situ X-ray absorption spectroscopy (XAS) and in situ X-ray diffraction (XRD). Importantly, lattice water movement during cycling by Fourier transform infrared (FTIR) measurements offers an experimental validation about Na+ nonlinear migration path, as well as the accumulative lattice distortion effect from large-size Na(OH2)+ unit. The revealed mechanism points out the modified path for PBAs.