Dual Activation of Distinct Iron Redox Centers in Prussian Blue Analogues via Synergistic Electronic Tuning

The simultaneous activation of multiple, electronically distinct redox centers is a grand challenge in designing advanced electrode materials. In iron-based Prussian blue analogues (PBAs), the high-spin (FeHS) and low-spin (FeLS) sites present a classic case of this problem, with the former suffering from thermodynamic instability and the latter from incomplete activity. This study reveals how a single dopant atom can orchestrate a sophisticated, dual-pathway activation of both iron sites. We provide definitive spectroscopic and electrochemical evidence that copper (Cu) doping functions not as a simple structural stabilizer, but as a precise electronic modulator. It tailors the FeHS crystal field environment to lower the redox energy barrier, conquering its thermodynamic limitations. Simultaneously, it transmits an electronic signal through the cyanide bridge to weaken the FeLS–C bond covalency, thereby awakening its dormant capacity. This discovery, where crystal field and covalency are concurrently tuned, leads to a PBA cathode with outstanding electrochemical performance. As a result, the optimized PBA–Cu cathode delivers a high reversible capacity of 109.6 mAh·g–1 at 1C, improved rate performance (69.0 mAh·g–1 at 10C), and robust cycling stability. This work deciphers a novel mechanism for the targeted electronic tuning of distinct redox centers and offers a powerful design principle for developing advanced PBA cathodes, which may also provide a blueprint for how to leverage targeted electronic perturbations to unlock the full potential of multiredox materials.