context: The carefully designed combination of TM-ions and non-TM-ions suppressed the occurrences of phase transformations during electrochemical cycling in Na ‘half’ cells. More importantly, in addition to air-cum-water stability, Ti-substitution for Mn-ions was found to drastically improve the cyclic stability, with the completely Ti-substituted Na-TM-oxide exhibiting an excellent first cycle reversible capacity of ∼140 mA h g−1 (within 2.0–4.0 V, at C/5), negligible voltage hysteresis and a capacity retention of ∼80% after 100 cycles. In the presence of Mn-ions, formation of Mn3+ at the particle surfaces, concomitant dissolution of Mn-ions (upon disproportionation) in electrolyte, steep rise in cathode/cell impedance and associated rapid increase in voltage hysteresis upon cycling were found to be the major causes for rapid capacity fading, which were suppressed/eliminated upon partial/complete Ti-substitution. The incorporation of functionalized MWCNTs, via simple physical mixing with the cathode-active particles, further improved the capacity retentions to ∼87% after 100 cycles and ∼75% after 200 cycles, along with bestowing excellent rate capability, viz., capacities at 2C and 5C being ∼65% and ∼50% of capacity at C/10, respectively.
answer: MWCNTs