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

Abstract

Mn³⁺ (d⁴) in manganese-based Prussian blue analogues (MnPBA) exhibits intrinsic orbital degeneracy upon sodiation/desodiation, resulting in severe Jahn-Teller distortion, which usually causes rapid capacity decay and sluggish kinetics. Unfortunately, traditional modification strategies are insufficient for electronic tuning of Mn³⁺ to mitigate 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. IVCT drives electron distribution from localized to delocalized, achieves electronic coupling, and mitigates Jahn-Teller by transferring a single-electron of Mn³⁺ e_g orbital. Notably, the reported Na_1.2V_0.63Mn_0.58Fe(CN)₆ cathode demonstrates excellent 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 mechanism of IVCT and quasi-zero-strain reaction mechanism 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.