Effects of Intra-Structural Interactions of Indium Hexacyanoferrate on the Li+ and K+ Intercalation Potential

Abstract

Due to the lithium reserves for the growing sector of lithium-ion batteries (LIBs), there has been an increased interest in the development of new technologies based on the reactions with other ions, as Sodium (NIBs) and Potassium (KIBs) ion Batteries. Particularly, the intercalation potentials of NIBs are lower than LIBs, limiting their application, for this reason the KIBs are a promising option. Since lithium has a higher polarizing power than potassium, it is expected that the cation-structure interactions influence the intercalation potential. So, in this work we carefully evaluated the structural changes induced by the interactions of Li or K ions with an open framework structure (indium hexacyanoferrate) without water. The lack of water within the structure is important since new intra-interaction phenomena arise, which have not been previously analyzed, providing deeper insights of the importance of the various interactions in this kind of system. The intercalation potentials were evaluated by CV and galvanostatic experiments in a non-aqueous system using a mix of solvent, demonstrating a higher potassium intercalation voltage of 3.9 V vs K+/K in comparison to Lithium 3.3 V vs Li+/Li. The nature of such high potential is evaluated around the electronic structure alterations arising from internal structural modifications and intra structural interactions leading to changes in the way that the ion is interacting with the framework. The lowest formal potential shown in presence of lithium as intercalation ion, can be attributed to a noticeable anisotropy in the charge density on the Fe–(CN)6 octahedron provoking distortions of the molecular block FeCN6, as was confirmed by infrared analysis and Density Functional Theory calculations.