Abstract:

New energy storage materials comprising Prussian blue and its analogues exhibit promising application potential as positive electrode materials for sodium-ion batteries because of their open frame structure, which is beneficial for the storage and rapid migration of sodium ions. Among these, Fe-based Prussian blue has attracted considerable attention due to its abundant resources, simple preparation, and high specific capacity. The electrochemical performance of Prussian blue is strongly correlated with its lattice structure, especially the sodium content, vacancy concentration, and crystal water. The lattice structure of Prussian blue can be tuned by controlling the processing parameters. Herein, two types of Na-enriched Fe-based Prussian blue cathode materials were synthesized via a facile coprecipitation method with and without a chelating agent, i.e., sodium citrate. The effects of sodium citrate on the crystal structure formation and sodium storage performances were investigated using X-ray diffraction （XRD）, scanning electron microscopy, thermogravimetric analysis （TGA）, and electrochemical characterization techniques. The results denoted that the Fe-based Prussian blue material prepared using the chelating agent exhibited a monoclinic structure. The material exhibited a well-defined cubic particle morphology with an average size of approximately 400 nm and good crystallinity as well as particle dispersibility. However, the Prussian blue sample prepared without the chelating agent displayed a cubic lattice structure. The particles exhibited a spherical morphology with a size of approximately 150 nm and presented significant particle agglomeration. XRD and TGA revealed that the sample with a monoclinic structure had a high sodium content but a low water content per formula unit when compared to the cubic structure. As a sodium-ion battery cathode, the sample with the monoclinic structure exhibited a high reversible capacity of 129.9 mA·h/g, a high first-cycle coulombic efficiency of 99.5%, and a remarkable cycling performance with a capacity retention of 75.7% after 100 cycles at a current density of 30 mA/g. This electrode exhibited a good rate performance, and the reversible specific capacities were 129.3, 121.7, 116.7, 110.7, 87.8, 63.6, and 45.3 mA·h/g at a current density of 30, 50, 100, 200, 400, 600, and 800 mA/g, respectively. When the current density returned to 30 mA/g, the reversible specific capacity was restored to 117.3 mA·h/g. The cyclic voltammetry and electrochemical impedance tests indicated that the monoclinic Prussian blue electrode exhibited lower electrode polarization and charge transfer resistance when compared with that exhibited by the cubic sample, resulting in faster electrode reaction kinetics and an excellent electrochemical performance. The role of the chelating agent, i.e., sodium citrate, in the Prussian blue particle nucleation and growth processes is also discussed.

Key words: sodium ion battery, cathode material, Fe-based Prussian blue, chelating agent