Grant Hill1,Chong Liu1
University of Chicago1
Insertion materials are key for electrochemical energy storage, but also have important applications in the areas of electrochemical ion separations, desalination, and catalysis. For ion separations, electrochemical extraction with layered cobalt oxides is a promising method for the separation of Li from Na-rich resources. However, the resulting composition and phase of the host material from co-insertion of cations and its relationship to Li separation performance has not been explored. Here, we developed a core-shell structured (NaLi)<sub>1-x</sub>CoO<sub>2</sub> via non-Faradaic ion-exchange using parent Li<sub>1-x</sub>CoO<sub>2</sub> with Na ions. Using synchrotron X-rays and scanning transmission electron microscopy, we reveal the phase separation of low vacancy Li-phase (core), high-vacancy Na-phase (shell), and intermediate phase. The spatial distribution and chemical composition of these phases play critical roles to the Li selectivity. Additionally, we investigate the relationship between alkali cation vacancy on the extent of ion-exchange. Using electrochemical intercalation, this material successfully extracted Li from 1:20,000 Li to Na solution to 7.6:1 Li to Na, which is a Li selectivity of 1.5 x 10<sup>5</sup> over Na. Therefore, this material has potential application in Li extraction from seawater and other Li containing brines. Beyond application, heterogeneity characterization of ion-exchange and co-insertion processes allows for a deeper understanding of phase transformation and ion transport phenomenon in insertion materials. These insights may guide development of new materials for electrochemical ion separations, energy storage, and other applications.