Résumé
Sodium layered oxides NaxMO2 (x ≤ 1 and M = transition metal ions) gain interest as sodium ion battery (NIB) cathodes due to their high energy density and cost-effectiveness. The nature of transition metal ions (M) defines the material properties, and the substitution of M with redox inactive Ti4+ is often seen as beneficial in reducing phase transitions during cycling and thus improving the cycle life. In this respect, our present study focuses on understanding the origin of this improvement by studying the highly substituted P2 Na0.67Ni0.30Zn0.03Mn0.67-yTiyO2 (0 ≤ y ≤ 0.67) phases, based on their electrochemical performance combined with structural analyses and DFT calculations. The results indicate that Ti4+, by increasing the M-O bond ionicity, disrupts the Na+-vacancy ordering at lower voltages (< 4V, until ~60% SOC), reduces the participation of O 2p in the redox process, thereby suppressing Na-removal and the extent of P2-O2 phase transition at high voltages. We show that this effect becomes maximum for y=0.52 (P2 Na0.67Ni0.30Zn0.03Mn0.15Ti0.52O2) and beyond, for which we observe a nearly solid-solution like behavior of P2-type structure. However, the d0 Ti4+ is prone to cation migration leading to poor structural reversibility as observed from operando XRD analyses, making the highly Ti4+-substituted material less suitable for practical applications. An optimum ratio of y= 0.3 (Na0.67Ni0.3Zn0.03Mn0.37Ti0.3O2) is beneficial for cycle life as well as rate capability and the study points the importance of carefully selecting transition metal combinations in finest ratio to achieve the best performing sodium layered oxide electrode materials.