Unraveling Water–Defect Coupled Degradation via Deuterium Isotope Labeling in Prussian Blue Analogue Cathodes for Long‐Life Sodium‐Ion Batteries

ABSTRACT Sodium‐based Prussian blue analogues (NaPBA) are promising cathodes for sodium‐ion batteries owing to their high capacity and low cost. Nonetheless, rapid capacity fading during prolonged cycling remains a critical challenge, as the underlying degradation mechanisms are not fully understood. Here, we employ deuterium isotope labeling to unveil a dual‐coupled degradation pathway in NaPBA cathodes, where crystalline water‐induced lattice distortion is coupled with defect‐triggered electrolyte decomposition. Density functional theory screening identifies In(OTf) 3 as a multifunctional electrolyte additive, which suppresses both degradation processes via synergistic cation–anion effects. Molecular dynamics simulations and isotope‐ratio mass spectrometry (IRMS) reveal that In 3+ strongly coordinates with crystalline water, suppressing its repeated insertion/extraction and preventing framework collapse, while OTf ‐− anions passivate [Fe(CN) 6 ] 4− vacancies, reducing solvent adsorption and inhibiting electrolyte decomposition at defect sites. Consequently, the NaPBA||Na cells incorporating In(OTf) 3 exhibit significantly improved electrochemical performance, demonstrating 80.63% capacity retention after 1000 cycles at 1C and nearly threefold higher discharge capacity at 20C compared to baseline cells, outperforming the most reported PBA‐based batteries. This cation–anion modulation strategy provides a general design principle for electrolyte engineering in PBA‐based energy‐storage systems.