Structural Modulation of Low-Cost Cu-Mn-Fe Prussian Blue Analogs for Long-Calendar-Life Sodium Ion Batteries

Abstract High electrochemical-performance and cost-efficient cathodes are crucial for the development of grid-scale sodium-ion batteries (SIBs). Prussian blue analogs (PBAs) are generally regarded as promising cathode candidates for SIBs, although their practical application has been limited by low capacity or poor cycling ability caused by their poor crystallinity and reversibility. Herein, a series of low-cost and high-quality ternary PBAs are prepared by structural regulation to simultaneously achieve high capacity, stable cyclability, and wide temperature suitability. The prepared CuHCF-3 sample has delivered a high specific capacity of 132.4 mAh g-1 with 73.3% capacity retention over 1000 cycles when applied as a cathode material for SIBs. A highly reversible three phase (monoclinic to cubic to tetragonal) sodium-ion storage mechanism is revealed via multiple in-situ techniques. Density functional theory calculations indicate that the key for achieving high capacity and long lifespan lies in the synergistic effect of Mn and Cu in the crystal structure of PBAs. The presence of Mn could enhance electronic conductivity, improve the operating voltage, and provide more possible redox centers while the presence of Cu-ions can restrain the Jahn-Teller distortions and buffer huge volume expansion during cycling. Furthermore, the wide-temperature suitability of these materials and Na-ion full cells are investigated based on kilogram-scale production as a demonstration for their practical application. We hope that this work can provide new insights into designing high-performance and low-cost electrode materials for practical grid-scale energy storage systems.