Therefore, the differences of the intercalation behavior in Regime 1, 2, and 3 is due to the limit of Li + mass transfer of both solution and solid diffusion. For Regime 3, large quantities of Na + will be inserted into the host structure due to both ion exchange and electrochemical intercalation, which squeeze the Li phase and force the quick formation of Li0.94CoO2 and Na0.48CoO2 phase equilibrium which end up on the equilibrium line (Supplementary Fig. ). After that, the process will be similar to that from Na0.48CoO2 to Li0.94CoO2 as described above (a transition to red path lines on the phase diagram). The intercalation curve looks like Na + discharge, and ion exchange is needed after each intercalation step to drive the host back to two-phase equilibrium. The ion pathway of Regime 2 is the combination of Regime 1 and Regime 3 that at the beginning during the plateau of ~0.45 V, Li + intercalation is dominate. From ~ 0.3 V, twophase equilibrium was established and the host switch to Na + intercalation and requires ion exchange to increase the Li + content in the structure. This switch in intercalation behavior could be caused by solid diffusion limitation that the emergent Li0.94CoO2 cannot merge with the original Li phase fast enough and the original Li phase was consumed by ion exchange with solution Na + . Additionally, the higher currents in Regime 2 than Regime 1 promotes more Na phase growth during the beginning of the intercalation process (Supplementary Fig. ). The three types of ion pathways can also be clearly explained on the CoO2-LiCoO2-NaCoO2 phase diagram as shown using the blue lines (Fig. ).