Optimal Ti-Substitution\nin Layered Oxide Cathodes\nfor Na-Ion Batteries

Sodium\nlayered oxides Na<sub><i>x</i></sub>MO<sub>2</sub> (<i>x</i> ≤ 1 and M = transition metal ions) gain\ninterest as sodium-ion battery (NIB) cathodes due to their high energy\ndensity and cost-effectiveness. The nature of transition metal ions\n(M) defines the material properties, and the substitution of M with\nredox inactive Ti<sup>4+</sup> is often seen as beneficial in reducing\nphase transitions during cycling and thus improving the cycle life.\nIn this respect, our present study focuses on understanding the origin\nof this improvement by studying the highly substituted P2 Na<sub>0.67</sub>Ni<sub>0.30</sub>Zn<sub>0.03</sub>Mn<sub>0.67–<i>y</i></sub>Ti<sub><i>y</i></sub>O<sub>2</sub> (0 ≤ <i>y</i> ≤ 0.67) phases based on their electrochemical performance\ncombined with structural analyses and DFT calculations. The results\nindicate that Ti<sup>4+</sup>, by increasing the M–O bond ionicity,\ndisrupts the Na<sup>+</sup>-vacancy ordering at lower voltages (<4\nV, until ∼60% SOC) and reduces the participation of O 2<i>p</i> in the redox process, thereby suppressing Na-removal and\nthe extent of P2–O2 phase transition at high voltages. We show\nthat this effect becomes maximum for <i>y</i> = 0.52 (P2\nNa<sub>0.67</sub>Ni<sub>0.30</sub>Zn<sub>0.03</sub>Mn<sub>0.15</sub>Ti<sub>0.52</sub>O<sub>2</sub>) and beyond, for which we observe\na nearly solid-solution-like behavior of the P2-type structure. However,\nthe d<sup>0</sup> Ti<sup>4+</sup> is prone to cation migration leading\nto poor structural reversibility as observed from operando XRD analyses,\nmaking the highly Ti<sup>4+</sup>-substituted material less suitable\nfor practical applications. An optimum ratio of <i>y</i> = 0.3 (Na<sub>0.67</sub>Ni<sub>0.3</sub>Zn<sub>0.03</sub>Mn<sub>0.37</sub>Ti<sub>0.3</sub>O<sub>2</sub>) is beneficial for the cycle\nlife as well as rate capability, and the study points to the importance\nof carefully selecting transition metal combinations in the finest\nratio to achieve the best performing sodium layered oxide electrode\nmaterials.