Ion exchanged LCO, sLCO, and NCO materials are then intercalated in the 1:1000 Li: Na solution to cutoff voltages of 0.1 V or -0.2 V vs Ag/Ag + and given sufficient time for ion exchange. The ion exchange OCP curves reveal substantial difference in the time to equilibrium. Na0.32Li0.35CoO2 (from LCO) (Fig. ) could not ion exchange to reach OCP of ~ 0.45 V (twophase equilibrium) after intercalation to both voltage cutoff of 0.1 and -0.2 V. The XRD spectra showed co-existence of a Li0.94CoO2 phase and a Na0.67CoO2 phase for the -0.2 V cutoff and a (001)Na phase peak between Na0.67CoO2 and Na0.48CoO2 for the 0.1 V cutoff indicating deviation from two-phase equilibrium (Supplementary Fig. ). Both Na0.27Li0.40CoO2 (from sLCO) and Na0.27Li0.40CoO2 (from NCO) reached equilibrium with intercalation at a C/10 rate, which were shown by OCP curves and confirmed by ICP-MS and XRD (Fig. and Supplementary Fig. ). It is worth mentioning that the ion exchange process resembles pseudo Na1-vCoO2 charging, which can be enhanced to less than 2 hours by using a more dilute slurry with about half the mass loading for electrodes (Supplementary 7) . Furthermore, the ion exchange for NCO retains faster rates (time to reach the final plateau) if the Na1-vCoO2 phase retains a high vacancy level (Fig. and Supplementary Table ). The rate becomes much slower when v < 0.33 after intercalation, which highlights the importance of Na phase vacancy, therefore cutoff voltage, on the ion exchange kinetics. From the SEM images of the three particle types after intercalation to -0.2 V, the smaller sLCO and NCO particles show no cracking unlike the larger LCO, which may be due to the increasing energy required for fracture with decreasing particle size (Supplementary Fig. ) . The smaller particle sizes can manage the larger stress from both the Li phase and Na phase separation in a single particle. The large mechanical stress in big particles could be a key hindrance to the ion exchange. Based on the ion exchange kinetics and particle fracture resilience, NCO particles were selected for further investigations and a cutoff voltage of -0.2 V were selected to ensure fast ion exchange to maintain two-phase equilibrium and large intercalation capacity.