Sodium-ion batteries (SIB) are said to be a more sustainable battery technology compared to conventional lithium-ion batteries (LIB) due to their use of less critical raw materials. The rising demand for batteries has led to an increased interest in SIBs in the past years. One promising class of active materials are Prussian blue analogues (PBAs), generalized as AxM′[M″(CN)6]y ⋅ zH2O (A are intercalated alkali metal cations, M′ and M″ are transition metals).

Syntheses and characterization of many PBA materials have already been described in the literature. But while it is known that highly toxic hydrogen cyanide (HCN) can form during some specific syntheses that are based on a decomposition mechanism, quantifications have not been reported consistently, although being important for upscaling the production. So, in this work, we investigated three different synthesis routes for iron-based PBAs – NaxFe[Fe(CN)6] (FeHCF), called Prussian White in its sodiated form – in terms of electrochemical performance of the product and with regard to their safety and scalability.

We have measured that the HCN formation upon the standard co-precipitation is small but not negligible. This is due to the slightly acidic environment created by the hydrolysis of transition metal salts in aqueous solutions. Scale-up of these synthesis routes would also release concerning amounts of HCN. HCN formation in an acid-facilitated reaction is, as expected, extremely high. A modified co-precipitation route includes the chelating agent sodium citrate. It not only improves the crystal growth conditions but also neutralizes the reactor environment, and no HCN is produced. This material outperforms the other tested materials, reaching over 160 mAh/g with a long cycle life. We conclude that pH-neutrality is a critical requirement for a safely scaled production of FeHCF as cathode material.