Understanding the Electrochemical Ion Exchange Process in Layered Oxide Cathodes

Ion intercalation is a versatile process that involves ion extraction and reinsertion, accompanied by redox processes and structural changes in the bulk of a material. Transition metal oxides with a layered structure have received significant attention as secondary battery electrodes thanks to their ability to accommodate a wide range of mobile cations with varying radii and charges (e.g., H + , Li + , Na + , K + , Zn 2+ , Ca 2+ ). Numerous transition metal oxides (A x TMO 2 , A is the mobile cation, and TM is the transition metal) with varying layer stacking sequences have been developed by modulating both the A and TM chemistry, making this materials class highly versatile with tunable intercalation properties. The ability to accommodate various mobile ions within a layered structure also opens opportunities for synthesizing novel metastable materials, and, for examining the links between electro-chemo-mechanics and phase behavior. Cation exchange through (electro)chemical methods is an effective approach to synthesize metastable compounds with new crystal structures, chemical compositions, and a tunable intercalation chemistry. By utilizing Li + / Na + or Na + / K + exchange reactions, researchers have successfully obtained unique structures that are otherwise impossible to synthesize through traditional methods, thereby allowing for the modulation of their properties as battery electrode materials. However, it is unclear how ion exchange evolves as a function of cycling and, at the material level, how it impacts the (de)intercalation properties over time. The spatial distribution of exchanged ions within the electrode structure remains unclear, particularly as a function of state of charge. Therefore, in the presentation, we will discuss the evolution of the long-range and local structure, and the ion (de)intercalation properties of the Ni-rich cathode during electrochemical Li / Na ion exchange using a variety of operando and ex situ characterizations, including diffraction, spectroscopy, and imaging techniques.