Unraveling the Degradation Mechanism of Sodium Iron Hexacyanoferrate Cathode in Sodium Ion Batteries

Abstract

Sodium iron hexacyanoferrates (Na2FeFe(CN)6) are considered among the most promising cathode materials for sodium ion batteries due to their high theoretical energy density and low cost. However, structural Fe(CN)64- vacancies seriously impair structure stability and deteriorate electrochemical performance. So far, the mechanisms by which Fe(CN)64- vacancies cause performance degradation and ultimately result in material failure remain unclear, leading to persistent controversies in this field. Herein, we systematically investigate the degradation mechanisms induced by Fe(CN)64- vacancies from experimental and theoretical perspectives. Defective Na2FeFe(CN)6 cathode exhibits more hysteretic low-spin iron reaction kinetics, especially during charge transfer and ion diffusion. Cryo-electron microscopy reveals that interfacial side reactions triggered by Fe(CN)64- vacancies during electrochemical cycling produce excessive Na2CO3 and NaF byproducts, which deplete electrochemically active Na+ within defective structure, causing electrochemical failure of high-spin Fe-N interactions and ultimately leading to poor structural stability. Importantly, pouch full cells (71% retention after 650 cycles) and all-solid-state batteries (82% retention after 500 cycles) fabricated from industrial-scale low-defect Na2FeFe(CN)6 cathode exhibit excellent cycling stability. This work offers valuable mechanistic insights into vacancy-induced degradation of Na2FeFe(CN)6 cathode and contribute to the advancement of practical sodium storage cathodes materials.