The Magnesium Insertion Effects into P2‐Type Na_2/3Ni_1/3Mn_2/3O₂

Abstract A Mg‐cell with P2‐Na_2/3Ni_1/3Mn_2/3O₂ layered oxide cathode provides novel reaction mechanism not observed in Na‐cells. The sodium/vacancy ordering and Jahn–Teller effects are suppressed with the insertion of magnesium ion. The Mg‐cell exhibits different features when operating between 4.5 and 0.15 V and 3.9 and 0.15 V versus Mg²⁺/Mg. To analyze the structural and chemical changes during Mg insertion, the cathode is first charged to obtain the Na_1/3Ni_1/3Mn_2/3O₂ compound, which is formally accompanied by an oxidation from Ni²⁺ to Ni³⁺. As structure models Mg_1/6Na_1/3Ni_1/3Mn_2/3O₂ and Mg_1/12Na_1/2Ni_1/3Mn_2/3O₂ are utilized with a large × supercell. On discharge, the Mg‐cell exhibits a multistep profile which reaches ≈100 mA h g⁻¹ with the valence change from Ni³⁺ to Ni²⁺. Such profile is quite different from its sodium counterpart (230 mA h g⁻¹) which exhibits the sodium/vacancy ordering and deleterious presence of Mn³⁺. Depending on how the two interlayer spacings are filled by Na and Mg the “staged,” “intermediated,” and “average” models are analyzed for Mg_yNa₈Ni₈Mn₁₆O₄₈ supercell. This fact suggests differences in the cell performance when Mg is used as counter electrode providing some tips to improve the structure engineering on cathode materials.