A Matter of Safety: Factors affecting the gas evolution of Prussian white (PW) cathode material for sodium-ion batteries are evaluated. H₂ is the main gas detected, especially in hydrated PW and during overcharge, while the evolution of CO₂ and (CN)₂ strongly depends on the electrolyte conductive salt. The use of oxidative NaClO₄ as a conductive salt causes evolution of (CN)₂ and HCN during regular cycling, and especially so during overcharge.

Abstract

As global energy storage demand increases, sodium-ion batteries are often considered as an alternative to lithium-ion batteries. Hexacyanoferrate cathodes, commonly referred to as Prussian blue analogues (PBAs), are of particular interest due their low-cost synthesis and promising electrochemical response. However, because they consist of ~50 wt% cyanide anions, a possible release of highly toxic cyanide gases poses a significant safety risk. Previously, we observed the evolution of (CN)₂ during cycling via differential electrochemical mass spectrometry (DEMS), but were unable to determine a root cause or mechanism. In this work, we present a systematical investigation of the gas evolution of Prussian white (PW) with different water content via DEMS. While H₂ is the main gas detected, especially in hydrated PW and during overcharge (4.6 V vs. Na⁺/Na), the evolution of CO₂ and (CN)₂ depends on the electrolyte conductive salt. The use of oxidative NaClO₄ instead of NaPF₆ is the leading cause for the formation of (CN)₂. Mass spectrometric evidence of trace amounts of HCN is also found, but to a much lower extent than (CN)₂, which is the dominant safety risk when using NaClO₄-containing electrolyte, which despite being a good model salt, is not a viable option for commercial applications.