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

A Mg-cell with P2-Na_2/3 Ni_1/3 Mn_2/3 O₂ 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_1/3 Ni_1/3 Mn_2/3 O₂ compound, which is formally accompanied by an oxidation from Ni²⁺ to Ni³⁺ . As structure models Mg_1/6 Na_1/3 Ni_1/3 Mn_2/3 O₂ and Mg_1/12 Na_1/2 Ni_1/3 Mn_2/3 O₂ are utilized with a large $2\sqrt{3}a$ × $2\sqrt{3}a$ 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_y Na₈ Ni₈ Mn₁₆ O₄₈ 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.