The non-Faradaic ion exchange kinetics were tested in a 1:1000 (1mM: 1M) Li: Na acetonitrile (ACN) solution. ACN was chosen due to the fast ion exchange kinetics for NCO that is measurable by the change in the open circuit potential (OCP) curve (Supplementary Fig. ). This OCP curve changes resembles the charging curve of Na0.67CoO2 to Na0.48CoO2 . For comparing the different materials at the same starting state, both LCO and sLCO were deintercalated in pure Li + electrolyte to 33% vacancy to match the vacancy of NCO material and then soaked in 1M NaClO4 ACN solution to reach two-phase equilibrium (Supplementary Fig. ). For NCO particles, ion exchange to establish the two-phase equilibrium was allowed to occur in 1: 1000 Li: Na solution (Supplementary Fig. ). The compositions after ion exchange were Na0.32Li0.35CoO2 for LCO (Li0.94CoO2 and Na0.51CoO2 equilibrium), Na0.27Li0.40CoO2 for sLCO parent (Na0.48CoO2 and Li0.94CoO2 equilibrium), and Na0.27Li0.40CoO2 for NCO (Na0.48CoO2 and Li0.94CoO2 equilibrium), measured by XRD (Fig. and Supplementary Fig. ) and inductively coupled mass spectroscopy (ICP-MS) confirming two-phase equilibrium. The biphasic regime seems to consist of slight variations in the equilibrium, which may be related to variation between electrode properties .