Unraveling the Origins of C‐Rate Dependent Capacity Decay in Fe‐Mn‐Based Prussian Blue Cathodes for Sodium Ion Batteries

ABSTRACT Prussian blue analogues (PBAs), especially low‐cost Mn‐doped Fe‐based PBAs, demonstrate significant potential for power‐type sodium‐ion batteries (SIBs), owing to their open framework and high theoretical capacity (∼170 mAh g −1 ). However, their capacity decay mechanisms, particularly under rapid charge/discharge conditions, remain insufficiently understood, hindering their practical implementation in high‐power energy storage systems. Here, we systematically investigate the failure mechanisms of Mn‐doped Fe‐based PBAs through multiple in/ex situ characterizations of Mn‐doped Fe‐PBA cathodes cycled under various current densities. Our analysis reveals that there are several pathways induce electrode degradation at high rate: (1) transition metal dissolution that disrupts redox‐active sites, (2) mechanical degradation that impairs electrode kinetics under high‐rate conditions, (3) severe and rapid Na dendrite growth, and (4) interfacial side reactions that generate a thick cathode‐electrolyte interphase layer. Rather than viewing transition metal dissolution and mechanical degradation as isolated degradation pathways, this study reveals that capacity collapse at 10 C is fundamentally driven by the intrinsic kinetic mismatch and mechanical failure, offering key insights for the design of durable, high‐power PBA‐based SIBs.