context: A very rapid increase in separation between the ‘charge-averaged’ discharge and charge voltages, within the first few cycles (i.e., ∼10 cycles), can be observed for the ‘no’ Ti-containing Na-TM-oxide (i.e., ‘0.5 Mn/0 Ti’) (see Fig. 4a and b). This is a direct reflection of the rapid increment in corresponding voltage hysteresis (see Fig. 3a), and also ties-up with the rapid capacity fading observed in these very first 10 cycles (see Fig. 3d). By contrast, the changes in ‘charge-averaged’ voltages in case of the fully Ti-substituted Na-TM-oxide are insignificant, even up to the entire duration of galvanostatic cycling (viz., 100 cycles). Interestingly, despite the notable difference in behavior between the ‘0.5 Mn/0 Ti’ and ‘0 Mn/0.5 Ti’ Na-TM-oxide cathodes in the context of variations of the ‘charge-averaged’ discharge and charge voltages during cycling, the corresponding net-average voltages (viz., average of Vdischarge and Vcharge) do not appear to change during cycling in both the cases, with the net-average voltage for the fully Ti-substituted counterpart being consistently greater than that for the ‘control’ Na-TM-oxide cathode by ∼0.25 V. A decrease and increase in Vdischarge and Vcharge, respectively, during electrochemical cycling, are primarily manifestations of build-up of electrode/cell impedance (as also suggested by Jung et al.), whereas any notable variation of the net-average voltage during cycling (as not observed here) is more likely to be a manifestation of irreversible change in the structure (even if just at the surface) of the concerned electrode material. Accordingly, the above observations (viz., Fig. 4) indicate that replacement of Mn-ions by Ti-ions in the Na-TM-oxide cathode considerably suppresses the build-up of electrode/cell impedance upon electrochemical cycling. These will be discussed in more detail in the next two sections.
answer: ‘0.5 Mn/0 Ti’ and ‘0 Mn/0.5 Ti’ Na-TM-oxide