Hierarchically Designed Cathodes Composed of Vanadium Hexacyanoferrate@Copper Hexacyanoferrate with Enhanced Cycling Stability

Prussian blue analogues (PBAs) have been highlighted as electrode materials for aqueous rechargeable batteries (ARBs) because of their favorable crystal structure and electrochemical activity. However, dissolution of the transition-metal ions during cycling degrades the materials and hinders the development of long-life-span batteries. To overcome this limitation, a strategy to revive the capacity degradation of PBA-based cathodes was developed herein based on designing all-PBA-based core@shell materials, while specific reduction upon introducing the shell layers was minimized. The core@shell materials were constructed using a V/Fe PBA (high capacity) core and a Cu/Fe PBA (high cycling stability) shell via a two-step co-precipitation method. The electrochemical performances including specific capacity, cycling stability, and rate capability as a function of the Cu/Fe PBA shell thickness were explored. At the optimal Cu/Fe PBA thickness, improved capacity retention after 200 cycles of >90% (72% for the core only) was attained with negligible capacity reductions from 94 (core only) to 90 (core@shell) mA h g-1, arising from the high electrochemical activity and stability of the Cu/Fe PBA shell and stabilized interfaces due to the crystallographic coherence between the core and shell materials. In addition, the power performance of the core@shell materials was significantly improved, e.g., C38.4C/C0.6C for a core@shell of 80% and core only of 62%, arising from the unique chemical coordination and facile ion diffusion kinetics of the Cu/Fe PBA shell. The newly developed V/Fe@Cu/Fe PBA-based cathodes offer an effective strategy for fabricating sustainable and low-cost ARBs.