Prussian Blue Analogues for Potassium-Ion Batteries: Application of Complementary Operando X-Ray Techniques

Abstract

In the last 5 years, potassium-ion batteries (PIBs) have been considered as a promising post-lithium-ion battery technology, much due to cost effectiveness and a variety of electrode materials available [1]. Prussian Blue Analogues (PBAs), with advantages such as simple synthesis, high capacity, and eco-friendliness, have gained a huge interest [2–4] and here we present a detailed study on the electrochemical mechanism of K x Mn2/3Fe1/3[Fe(CN)6] y .zH2O. We do this by combining complementary operando techniques: X-ray diffraction and X-ray absorption spectroscopy, supported by ex situ 57Fe Mössbauer spectroscopy. Chemometric analysis of the operando data allows us to follow the physico-chemical and structural evolution during electrochemical cycling, providing deeper understanding. From this, a reversible phase transition between monoclinic and cubic crystal structure was observed during the oxidation/reduction of Mn species. The local geometry information on Fe and especially Mn, as obtained from the EXAFS data analysis, serves to explain the volume expansion of the Fe–CN–M (M = Mn, Fe) framework during the extraction of potassium. This paves the way for optimizing the PBA composition for PIB application.