To confirm this and resolve the ion pathway of Regime 1 in 1:1000 solution, electrode composition from starting Li0.50CoO2 was characterized in situ by measuring the change of solution ion concentration for Regime 1 intercalation. The electrode composition changed within the first minute of intercalation that the Na + composition increased from 0.02 to 0.11 and reached its maximum of 0.26 by the first change in the intercalation curve slop at ~0.60 V (Supplementary Fig. ). A second electrode revealed the same trend but with less ion exchange. The variability in ion exchange may be due to differences in contact time without current (intercalation start time) as slow C-rates suppress ion exchange compared to no current (Supplementary Fig. ). This composition change provides important information about the ion pathway. During the first minute, there is barely any increase of the total cation content in the host material, so that the Na + content increase should come from ion exchange. Based on our previous work, ion exchange prefers to establish equal potential phase equilibrium, therefore, for charge conservation, the vacancy level of Li phase will decrease, and a high vacancy Na phase (v>0.52) will be preferred to form . This matches the fact that the Na + content kept increasing during intercalation from 0.8 V to 0.6 V. This first segment of the electrochemical curve resembles the transformation of Na0.37CoO2 to Na0.48CoO2 . The Na + composition decreased after passing 0.60 V and was 0.15 (first attempt) and 0.05 (second attempt) by the time the major plateau at 0.50 V was reached.