Enhancing the Reversibility and Kinetics of Heterovalent Ion-Substituted Mn-Based Prussian Blue Analogue Cathodes via Intervalence Charge Transfer.

Mn3+ (d4) in manganese-based Prussian blue analogues (MnPBA) exhibits intrinsic orbital degeneracy upon sodiation/desodiation, resulting in severe Jahn-Teller distortion, which causes rapid capacity decay and sluggish kinetics. Unfortunately, traditional modification strategies are deficient for electronic tuning of Mn3+ to address these issues. Herein, Intervalence Charge Transfer (IVCT) of manganese and iron to vanadium ions is unraveled in a series of novel V3+-substituted MnPBA to enhance electrochemical reaction reversibility and kinetics. Precisely, IVCT drives electron distribution from localized to delocalized, achieves electronic coupling and mitigates Jahn-Teller by transferring a single-electron of Mn3+ eg orbital. Notably, the reported Na1.2V0.63Mn0.58Fe(CN)6 cathode demonstrates incredible rate capability (136.9 mAh g-1 at 20 mA g-1 and 94.9 mAh g-1 at 20 A g-1), remarkable long-cycle stability (91.6% capacity retention after 300 cycles at 20 mA g-1 and 90.7% after 2000 cycles at 2 A g-1), and robust performance across a wide temperature range (98.59% capacity after 300 cycles at -30°C and 50 mA g-1), surpassing the majority of reported sodium-ion cathodes. The intrinsic functioning mechanisms of IVCT and quasi-zero-strain reaction mechanisms were adequately understood through systematic in-situ/ex-situ characterizations. This study further develops electron-tuning of PBA, opening a new avenue toward advanced sodium-ion battery cathode materials.