It becomes clear that when the cutoff voltage is set for NCO particles, the intercalation Crate is critical to determine the capacity per intercalation, the ion exchange time, the final depth of intercalation, and Li + selectivity. To understand the phase transformation during the repetitive intercalation and ion exchange, in situ synchrotron XRD was conducted during initial ion exchange and (de)intercalation from starting particles of Na0.67CoO2. A 1:100 1 M Li:Na solution was used to avoid Li transport issues in the in situ cell. During the ion exchange (Fig. ), the (001)Na peak (1.14 Å -1 ) and (002)Na peak (2.29 Å -1 ) of the Na0.67CoO2 parent material is left shifting to lower Q values indicating the formation of Na phase with larger layer spacings and vacancy. At 0.0 V, a small peak at 1.32 Å -1 emerges that continually right shifts and grows in intensity until reaching 1.34 Å -1 . This indicates the formation of Li0.94CoO2 phase and suggests that it is facilitated by the formation of high vacancy Li phase. The appearance of the (003)Li peak is also coupled with abrupt emergence and increase in intensity of the Li phase peaks at ~0.05 V and decrease of the Na phase peaks that indicates a major change in the lattice to a predominant R-3m phase from the C2/m phase despite only a small amount of Li + present in the material. The (001)Na and (002)Na peaks continue to left shift until reaching the position of Na0.48CoO2 at 0.5 V. During the ion exchange process, the (001)Na peak intensity continues to diminish while the (003)Li peak intensity grows, and two-phase equilibrium was established. During the following intercalation, the electrochemical response resembles the Na1-vCoO2 discharging (Na + intercalation) behavior. The (001)Na and (002)Na peaks right shift while the Li phase peaks maintain their position. Additionally, the Li phase continues to grow in intensity while the Na phase decreases in intensity, which suggests that during the Na phase transformation, ion exchange with solution Li + occurs resulting in an increase of the volume fraction of Li phase. The (003)Li peak is the strongest at the end of intercalation (Supplementary Fig. ).