Three regimes with different reaction rates are defined (Fig. ). Regime 1 is defined as Cinter < CpseudoIX (i.e. Cinter = C/50 and CpseudoIX = C/12), and its key features are the largest intercalation capacity (1-v changes from 0.50 to 0.92), highest intercalation potential, and no Na + intercalation feature on the electrochemical curve. Regime 2 is defined as Cinter ~ CpseudoIX, and its key features are smaller intercalation capacity, a more slopped plateau that occurs at a lower potential than Regime 1, which could describe the same reaction. For Regime 2, there is a clear transition to Na + intercalation behavior below 0.3 V which could be due to larger Na phase domains in Regime 2 than Regime 1. Finally, Regime 3 is defined as Cinter > CpseudoIX, and its key features are a stretched Na + intercalation curve and the smallest capacity, which suggests the Na + intercalation dominates. The regimes were found to exist in different Li: Na molar ratio solutions (Supplementary Fig. ). These regimes were found to be general not only to the NCO based particles but also for other synthesized LiCoO2 over a range of sizes (Supplementary Fig. ). It is extremely interesting to see that in Regime 1 single step of intercalation can achieve the targeted depth of intercalation (v~ 0.06) with minimal ion exchange, which is confirmed by ICP-MS (Na + content decreases from 0.07 to 0.04 from -0.2 V to 0.1 V). To explain the significant differences among the three regimes, the reaction for the long plateau in Regime 1 needs to be identified and the reaction pathways for all regimes need to be unveiled. Regime 2 intercalation process and phase evolution were observed using in situ synchrotron XRD (Fig. ). The sample was prepared by 1:1000 solution at C/50 intercalation outside the flow cell followed by deintercalation in Li + solution inside the flow cell (Supplementary Fig. ). The in situ intercalation was carried out after switching solution to 1:100 Li: Na at C/10 starting from Li1-vCoO2. The electrochemical curve resembles the Regime 2 extraction process. The increased potential of the plateau could be due to higher shift in potential with 1:100 solution compared to 1:1000 solution (Supplementary Fig. ). At the beginning of intercalation on the plateau at ~0.55 V, peaks of Li phase shift towards the Li0.94CoO2 phase positions and no Na phase was observed (Fig. ). As the (003)Li peak shifts to 1.32 Å -1 corresponding to the approximately the Li0.66CoO2 phase, Na0.48CoO2 phase emerged as indicated by the (001)Na peak (Supplementary Fig. ). The emergence of Na phase could be due to either Na + intercalation or concurrent ion exchange. Once the Li phase finally transforms into the Li0.94CoO2 phase, the electrochemical curve transitions to a Na1-vCoO2 discharging type and the (001)Na peak began to right shift until the potential cutoff. The following second deintercalation showed the same behavior as the first deintercalation (Supplementary Fig. ). High C-rates promote larger Na phases than low C-rates during this process indicated by the appearance of the Na phase peak with faster C-rates. (Supplementary Fig. ). The in situ XRD provides critical information that the plateau ~0.55 V (~0.40 V in 1:1000 solution, Supplementary Fig. ) is Li + intercalation. With only a slight ~50 mV difference between the two plateaus in Regime 1 and 2, the long plateau in Regime 1 could also correspond to Li + intercalation into Li phase.