Na<sub>5/6</sub>[Ni<sub>1/3</sub>Mn<sub>1/6</sub>Fe<sub>1/6</sub>Ti<sub>1/3</sub>]O<sub>2</sub> as an Optimized O3-Type Layered\nOxide Positive Electrode Material for Sodium-Ion

Layered oxides, such as Na<sub><i>x</i></sub>MeO<sub>2</sub> (Me = transition metal, <i>x</i> = 0–1),\nare believed to be the most promising positive electrode materials\nfor Na-ion batteries because of their high true density, large capacities,\nhigh working potentials, and reversibility. This study identified\nNa<sub>5/6</sub>[Ni<sub>1/3</sub>Mn<sub>1/6</sub>Fe<sub>1/6</sub>Ti<sub>1/3</sub>]O<sub>2</sub> as an optimal composition for use as an O3-type\npositive electrode material in Na-ion batteries on the basis of a\ncomprehensive phase diagram, where the end members of the triangular\nphase diagram were Na<sub>2/3</sub>[Ni<sub>1/3</sub><sup>2+</sup>Mn<sub>2/3</sub><sup>4+</sup>]O<sub>2</sub>, Na<sub>2/3</sub>[Ni<sub>1/3</sub><sup>2+</sup>Ti<sub>2/3</sub><sup>4+</sup>]O<sub>2</sub>, and the\nhypothetical composition Na<sub>4/3</sub>[Ni<sub>1/3</sub><sup>2+</sup>Fe<sub>2/3</sub><sup>3+</sup>]O<sub>2</sub>. By investigating the\neffects of the partial substitution of Mn<sup>4+</sup> with Fe<sup>3+</sup> and Ti<sup>4+</sup> within the Na<sub>(2/3+<i>x</i>)</sub>[Ni<sub>1/3</sub>Mn<sub>(2/3–<i>x</i>–<i>y</i>)</sub>Fe<sub><i>x</i></sub>Ti<sub><i>y</i></sub>]O<sub>2</sub> system, we optimized the capacity, working potential,\nand cycle performance. Substitution with Fe enhanced the discharge\ncapacity due to the increased Na<sup>+</sup> content in the initial\ncomposition, although it also led to a reduced cycling stability derived\nfrom irreversible Fe migration to the Na layers. In contrast, substitution\nwith Ti improved the working potential and cycling stability, although\nan excessive Ti content caused capacity degradation with cycling.\nWe found that the O3-type Na<sub>5/6</sub>[Ni<sub>1/3</sub>Mn<sub>1/6</sub>Fe<sub>1/6</sub>Ti<sub>1/3</sub>]O<sub>2</sub> demonstrated\nan excellent cycle stability with minimal capacity loss over 250 cycles,\nwhich was attributed to the suppression of irreversible transition\nmetal migration.