Effect of Water and Alkali-Ion Content on the Structure of Manganese(II) Hexacyanoferrate(II) by a Joint Operando X-ray Absorption Spectroscopy and Chemometric Approach

Manganese hexacyanoferrate (MnHCF) is made of abundant elements via a safe and easy synthesis. The material features higher specific capacity at higher potential when compared to other Prussian blue analogs (PBAs). However, the effect of hydration is critical in determining the electrochemical performance, since both the electrochemical behavior and the reaction dynamics are affected by interstitial/structural water and adsorbed water. Here we investigate the electrochemical activity of MnHCF by varying the interstitial ion content through a joint operando XAS (x-ray absorption spectroscopy) - chemometric approach, with the intent to assess the structural and electronic modifications occurring during sodium release and lithium insertion as well as the overall dynamic evolution of the system. In MnHCF both the Fe and Mn centers are electrochemically active, featuring a reversible oxidation during the interstitial ion extraction (Fe2+/Fe3+ and Mn2+/Mn3+). Adsorbed water results into irreversible capacity during charge, but only on the Fe site as suggested by the chemometric analysis. The Mn local environment experiences a substantial yet reversible Jahn-Teller effect upon interstitial ion removal due to the formation of trivalent Mn, which is associated to the equatorial Mn-N distances shrinking of 10%