Ion Exchange Pathways of Lithium and Sodium in Layered Oxides

Ion exchange in layered materials is one type of special ion transport process, which involves ion insertion and ion extraction simultaneously. This type of bi-directional ion transport requires the rearrangement of interlayer ions and will induce the phase evolution of layered host structures. More importantly, ion exchange is a powerful method to access metastable materials with advanced functionalities for energy storage applications. However, the ion exchange reaction pathways in layered materials remain elusive. Here, using layered oxides as model materials, we tracked the real-time phase evolution during Li-Na ion exchange in layered oxides. An interesting pseudo-charging behavior has been observed. Combining with the chemical composition information, the ion exchange pathway of phase separation between Li-rich and Na-poor phases was revealed. Depending on the chemical potential of exchange ions in the solution side, we identified two different exchange routes, the surface reaction-limited route and the diffusion-limited route. The phase separation behavior accompanied by the charge transfer is general in both ion exchange routes. Both DFT calculation and experiments point to that the co-existence of Li-rich and Na-poor phases is governed by thermodynamics. Besides, we demonstrate that structural vacancy level and lithium preference are critical in determining the feasibility of ion exchange. Guided by this understanding, Na<i>_y</i>CoO₂ was converted from the parent Li<i>_x</i>CoO₂ for the first time and Li_0.94CoO₂ was converted from Na<i>_y</i>CoO₂ at 1-1000 Li-Na (molar ratio) with electrochemical assisted ion exchange.