The layered transition metal oxides emerge as a new family of host materials for Li + extraction owing to their thermodynamic favorability of Li + to Na + and the ability to form heterostructures to stabilize Li + and Na + co-intercalation . In particular, Li1-vCoO2 (v represents vacancy level) has been shown to undergo spontaneous, non-Faradaic ion exchange at room temperature in Na + containing aqueous electrolyte when 1-v < 0.94 to form phase equilibrium between Na0.51CoO2 and Li0.94CoO2 . This leads to (NaLi)1-vCoO2 heterostructures that have separations in the spatial distribution of alkali ions and cobalt valence states (Co 3+ / 4+ ), and variations of layer spacing throughout the particle. For Na1-vCoO2 parent material with 0.48<1-v<0.94, the phase equilibrium is between Na0.48CoO2 and Li0.94CoO2. The slight difference in the Na phase could be due to particle property difference. When biphasic equilibrium occurs between Na0.48CoO2 and Li0.94CoO2, it indicates that the mobile ions will be swinging between Na + and Li + with host vacancy level v changing between 0.06 and 0.52 (Fig. ). The vacancy dependent composition indicates theoretically high Li + to Na + selectivity since the Li + contents increase as the vacancy level decreases during intercalation. Record high Li + to Na + selectivity (more than four orders of magnitude) has been validated for (NaLi)1-vCoO2 in our previous work . However, the design of paired intercalation and recovery processes to enable reversible cycling has not been addressed. Fundamental understanding of how (NaLi)1-vCoO2 respond to (de)intercalation at different reaction rates in solutions with different concentrations and ratios of Li + and Na + is needed to design the system to finish the loop for Li + extraction and recovery.