Reaching the Energy Density Limit of Layered O3‐NaNi_0.5Mn_0.5O₂ Electrodes via Dual Cu and Ti Substitution

<jats:title>Abstract</jats:title><jats:p>Although being less competitive energy density‐wise, Na‐ion batteries are serious alternatives to Li‐ion ones for applications where cost and sustainability dominate. O3‐type sodium layered oxides could partially overcome the energy limitation, but their practical use is plagued by a reaction process that enlists numerous phase changes and volume variations while additionally being moisture sensitive. Here, it is shown that the double substitution of Ti for Mn and Cu for Ni in O3‐NaNi<jats:sub>0.5−</jats:sub><jats:italic><jats:sub>y</jats:sub></jats:italic>Cu<jats:italic><jats:sub>y</jats:sub></jats:italic>Mn<jats:sub>0.5−</jats:sub><jats:italic><jats:sub> z</jats:sub></jats:italic>Ti<jats:italic><jats:sub>z</jats:sub></jats:italic>O<jats:sub>2</jats:sub> can alleviate most of these issues. Among this series, electrodes with specific compositions are identified that can reversibly release and uptake ≈0.9 sodium per formula unit via a smooth voltage‐composition profile enlisting minor lattice volume changes upon cycling as opposed to Δ<jats:italic>V</jats:italic>/<jats:italic>V</jats:italic>≈23% in the parent NaNi<jats:sub>0.5</jats:sub>Mn<jats:sub>0.5</jats:sub>O<jats:sub>2</jats:sub> while showing a greater resistance against moisture. The positive attributes of substitution are rationalized by structure considerations supported by density functional theory (DFT) calculations. Electrodes with sustained capacities of ≈180 mAh g<jats:sup>−1</jats:sup> are successfully implemented into 18 650 Na‐ion cells having greater performances, energy density‐wise (≈250 Wh L<jats:sup>−1</jats:sup>), than today's Na<jats:sub>3</jats:sub>V<jats:sub>2</jats:sub>(PO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>F<jats:sub>3</jats:sub>/HC Na‐ion technology which excels in rate capabilities. These results constitute a step forward in increasing the practicality of Na‐ion technology with additional opportunities for applications in which energy density prevails over rate capability.</jats:p>