The Li and Na phase element distribution was characterized on a single particle level for Regimes 1 and 3 (Supplementary Fig. ). For Regime 3, the particle selected was intercalated to (NaLi)0.80CoO2. The electrode was pulled out of the solution immediately without ion exchange. The scanning transmission electron microscopy (STEM) images with electron energy-loss spectroscopy (EELS) mapping of the Li K edge reveals nonuniformity for Regime 3 (Fig. and Supplementary Fig. ). Furthermore, the energy dispersive X-ray spectroscopy (EDS) signal was used to quantify the Na + content in each of the particle which confirms the large particle to particle heterogeneity again. Phase separation heterogeneity has been well characterized for insertion materials from the single particle level to the electrode level . The effects of particle characteristics and processing conditions can affect the phase separation morphology and ability of phase domains to coarsen during intercalation . For Regime 3, the heterogeneity could be caused by the fast intercalation rate. The sample selected in Regime 1 was in the middle of the plateau at v=0.3 (Fig. and Supplementary Fig. ). Based on our assignment of ion pathway in Regime 1, the particle should have minimal level of Na + content with uniform Li + distribution. Based on the EELS and EDS mapping, this is true for some of the particles, but not for all. There is still large heterogeneity existed for the Na + content and elemental distribution. But this could be caused by the ion exchange after the current stops as mentioned in the ex situ XRD characterization, which cannot be avoided for ex situ samples.