The evidence of the decrease of Na + content is critical for resolving the Regime 1 pathway. This indicates that even though at high vacancy level (0.52<v<0.06) Na0.48CoO2 is the preferred phase based on the two-phase equilibrium, it is suppressed during intercalation in Regime 1. Moreover, the transformation of high vacancy Na phase (formed before intercalation started via ion exchange) to Na0.48CoO2 (0.8 V to 0.6 V) should prefer the formation of the equal potential Li0.94CoO2 phase based on the two-phase equilibrium. If so, ion exchange of solution Li + with Na phase could occur and result in the decrease of Na + content as the Li0.94CoO2 phase is formed while maintaining the existing high vacancy Li phase . But the high Li + content phase (Li0.94CoO2) could disappear via merging with the existing Li phase. Therefore, the long plateau could be an equivalent Li + intercalation plateau with complicated Li + and Na + interactions involving vacancy Na phase formation and transition to Na0.48CoO2 that then leads to Li0.94CoO2 phase formation and merging with existing Li phase. During this phase evolution, elastic forces are expected to be strongly influential . In phase separating Li + electrodes, the C-rate can strongly influence the phase separation dynamics and resulting phase distribution . Elastic forces begin to dominate the phase behavior and distribution at very slow C-rates as the characteristic time to reorganize the phase domains to minimize strain is faster than the characteristic times of the solidstate diffusion and charge transfer. Finally, this Regime 1 ion pathway can be achieved since high vacancy Li phase have available sites for Li + intercalation (exclusive site effects), and adequate time is given for Li + diffusion both from the solution and in the solid (Supplementary Fig. ) . For the commercial and synthesized Li0.67CoO2 without Na ion exchange before intercalation, the pure Li + intercalation plateau could be accessed. Since the starting LCO and sLCO have reduced vacancy level of 33%, therefore, slower ion exchange kinetics, Na phase formation was reduced before intercalation current started (Supplementary Fig. ). This enabled Li + intercalation behavior followed by better post intercalation ion exchange kinetics.