Influence of Aluminum Substitution on Anionic Redox\nActivation and Stabilization in P′2-Type Na<sub>2/3</sub>MnO<sub>2</sub> for Na/Li Battery Applications

Lithium-ion batteries\nare now successfully developed with higher\nenergy densities than classical aqueous-based battery technologies\nand are used as power sources for electric vehicles and other electric\nstorage applications. Sodium-ion batteries are another emerging battery\ntechnology, but their energy density is not high compared to that\nof their Li counterparts. Anionic redox reactions have attracted attention\ndue to their potential to enhance the reversible capacity and operating\nvoltage of positive electrode materials in Na/Li batteries. To understand\nthe influence of Al substitution on the activation mechanism of anionic\nredox reactions in Na-/Li-containing Mn-based layered oxides, the\nP2-type Al-substituted Na<sub>2/3</sub>Al<sub>0.1</sub>Mn<sub>0.9</sub>O<sub>2</sub> is designed to explore the difference in the anionic\nredox behavior. Li<sub><i>y</i></sub>Al<sub>0.1</sub>Mn<sub>0.9</sub>O<sub>2</sub> is also prepared by electrochemical ion exchange,\nand the anionic redox behavior was compared for both samples. The\nAl-substituted Na<sub>2/3</sub>Al<sub>0.1</sub>Mn<sub>0.9</sub>O<sub>2</sub> provides better cycling performance with partial activation\nof anionic redox, which is not observed for P′2-type Na<sub>2/3</sub>MnO<sub>2</sub>. Good reversibility for Na<sub>2/3</sub>Al<sub>0.1</sub>Mn<sub>0.9</sub>O<sub>2</sub> and Li<sub><i>y</i></sub>Al<sub>0.1</sub>Mn<sub>0.9</sub>O<sub>2</sub> with\nanionic redox is achieved, and this improvement originates from the\nnonexcessive use of anionic redox in electrode materials. This finding\nopens the possibility to develop high-energy sodium/lithium insertion\nmaterials with reversible anionic redox.