How simple are the models of Na intercalation in aqueous media?

Abstract

Intercalation is among interesting phenomena changing everyday life with a remarkable speed. Numerous generations of modern batteries use it and markedly contribute to the portfolio of energy materials for future sustainable energy provision. Na-ion batteries operating in aqueous media, being presumably the “number two” compared to other alternatives for portable and automotive applications, are probably among potential leaders in the case of future larger-scale energy storage schemes, specifically because of safety and sustainability issues. However, there is a certain lack of detailed understanding of such systems. In this work, using model electrodes we further emphasise that it is not only the electrode materials themselves that determine the performance of intercalation electrodes. Surprisingly, comparatively slight changes in the electrolyte composition and solvation effects can very radically control intercalation of alkali metal cations. Therefore, it would be stimulating to adjust existing strategies in designing future aqueous battery systems. Several examples are given using one of the state-of-the-art cathode materials, Na₂Ni[Fe(CN)₆], with model electrodes prepared as quasi-uniform thin films.