Reactant Concentration and Aging-Time-Regulated Potassium Manganese Hexacyanoferrate as a Superior Cathode for Sodium-Ion Batteries

In this study, the sodium citrate assisted coprecipitation method was adopted to prepare high-quality potassium manganese hexacyanoferrate (KMHCF) by adjusting the reactant concentration and aging time. The influences of reactant concentration and aging time on the microstructures and electrochemical performance were investigated systematically. With the variations in preparation conditions, the as-prepared samples showed similar monoclinic crystal structure but different morphologies as well as electrochemical behaviors. The results proved that the lower reactant concentration is more beneficial to the formation of large particles with fewer defects of Fe(CN)6 vacancies and interstitial and coordinated water in the crystal structure. As a result, better cycling stability was obtained for the KMHCF1 samples prepared with a lower reactant concentration when compared to the KMHCF3 samples prepared with a higher reactant concentration. However, the small particles (KMHCF3) benefited the electrochemical kinetics, resulting in a larger specific capacity when compared with the large particles (KMHCF1). Similarly, the aging time also exhibited significant impacts on the microstructure and electrochemical performance of the as-prepared samples. The 24 h was confirmed to be the optimal aging time for the KMHCF samples. Among all samples, the KMHCF2_24h exhibited the best electrochemical performance with a large initial specific capacity of 140.3 mA h g-1 and a high capacity retention of 80.61% after 100 cycles at 20 mA g-1. A very high capacity retention of 85.02% after 500 cycles was achieved even at a high current density of 200 mA g-1. This should be ascribed to the regular cubic morphology of particles with the optimal size (1 μm) and less defects resulted from the optimal reactant concentration and aging time. This study provides a valuable and practical guidance for the synthesis of high-performance Prussian blue analogue cathode materials for sodium-ion batteries.