Understanding Ion-Exchange Reaction Mechanisms in Layered Oxide Cathodes for Beyond Li-Ion Batteries

Abstract

Ion-exchange reactions are frequently used to develop novel metastable electrode materials for alkali-ion batteries that cannot be synthesized using direct chemical reactions. In fact, the ion-exchange reaction has been proven as an effective approach to develop novel transition metal oxide cathodes for lithium, sodium, and potassium ion batteries.[1-4] However, the solid-state ion-exchange mechanism and the resulting phase evolution are barely understood. In this work, we investigated the electrochemical ‘K-to-Na’ and ‘Na-to-K’ ion-exchange reaction mechanisms in layered oxide cathodes as it occurs using operando and ex situ structure characterization techniques.[5-6] Our study demonstrates that the ion-exchange occurs not by a simple Na/K interdiffusion in a given host structure. Instead, we discovered intriguing multi-phase evolutions according to the relative Na/K content, indicating that the ion-exchange systems may be more complex to interpret than the previously understood. We found that not only Na (or K)-layered oxide host interaction but also Na-K interaction affect the ion-exchange reactions. In this presentation, we will discuss how and why those parameters determine the ion-exchange reactions and resulting phase evolutions.