context: Interestingly, in the presence of Ti-ions as a partial substitute for Mn-ions (as in the case of ‘0.2 Mn/0.3 Ti’ Na-TM-oxide), Mn was found to be completely in the +4 oxidation state (i.e., Mn4+ and not Mn3+) in the pristine electrode, unlike for the ‘0.5 Mn/0 Ti’ counterpart (see ESI Fig. S7d†). Such observations suggest that the presence of Ti-ions suppresses the reduction of Mn4+ to the deleterious Mn3+ during electrode preparation and, thus, improves the stability to some extent. However, similar to the case with ‘0.5 Mn/0 Ti’, Mn3+ did form and the content progressively increased during electrochemical cycling (as inferred from post-cycling XPS data, ESI Fig. S4e†), thus, not totally addressing the problem concerning cycling (in)stability. Of course, in the total absence of Mn-ions, as for the completely Ti-substituted ‘0 Mn/0.5 Ti’ Na-TM-oxide, the oxidation state of Ti-ions remained only as +4 in the as-prepared electrodes, as well as in those cycled for even 100 electrochemical cycles, as can be seen from Fig. 5c and d. In contrast to those observed for the ‘control’ ‘0.5 Mn/0 Ti’ Na-TM-oxide cathode, neither any notable variation of Vdischarge and Vcharge with cycling could be seen, nor any change in colour of the separator could be noted in the case of the fully Ti-substituted (for Mn-ion) Na-TM-oxide cathode, which indicates non-occurrence of TM-dissolution. Not invoking here, the possible additional contribution towards capacity fading from Jahn–Teller distortion of Mn3+ ( ) during electrochemical cycling, the oxidation states of TM-ions at the surface of the Na-TM-oxides, in particular the presence/formation of Mn3+ and its likely dissolution into the electrolyte, presents a strong justification for the capacity fading in the presence of Mn-ions and mitigation of the same upon Ti-substitution. In this case, electrode impedance is also likely to be considerably affected by the replacement of Mn-ions by Ti-ions, as discussed in the following sections.
answer: Ti-substituted (for Mn-ion) Na-TM-oxid