Decoupling Degradation at the Electrode Interfaces in Prussian White Full Cells

Abstract Prussian blue analogues for sodium‐ion battery cathodes are growing in popularity as next‐generation energy storage devices. Prussian White (PW) with formula Na _x Fe[Fe(CN) ₆ ] _y • n H ₂ O is leading the trend, having already been commercialized. However, capacity fade (PW/electrolyte degradation) and safety concerns (cyanide/cyanogen release) still raise concerns. Online electrochemical mass spectrometry, supported by both operando Fourier transform infrared spectroscopy and Mössbauer spectroscopy, is herein used to analyze degradation processes in PW‐based Na‐ion full cells. Apart from the typical cell formation reactions, hydrogen is observed to evolve during cell discharge and evidenced to stem from oxidation of NaH, accumulated upon charge. Over‐oxidation of PW after full desodiation releases CN, which not only forms (CN) ₂ but also degrades the electrolyte. Loss of CN likely results in a nanometric (≈4 nm) surface‐reconstructed passivation layer on PW, thus inhibiting further degradation. Fundamental understanding of degradation reactions in PW full‐cells, as gathered herein, shows that the aforementioned capacity fade and safety concerns are wholly addressable and hence guides the further development of Na‐ion batteries for wider ranges of applications.