Unraveling the degradation mechanism of sodium iron hexacyanoferrate cathodes 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 have remained 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. A 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 structures, 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 cathodes exhibit excellent cycling stability. This work offers valuable mechanistic insights into vacancy-induced degradation of Na₂FeFe(CN)₆ cathodes and contributes to the advancement of practical sodium storage cathode materials.