The Magnesium Insertion Effects into P2-Type Na 2/3 Ni 1/3 Mn 2/3 O 2 .

A Mg-cell with P2-Na _2/3 Ni _1/3 Mn _2/3 O ₂ 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 Mg ²⁺ /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 O ₂ compound, which is formally accompanied by an oxidation from Ni ²⁺ to Ni ³⁺ . As structure models Mg _1/6 Na _1/3 Ni _1/3 Mn _2/3 O ₂ and Mg _1/12 Na _1/2 Ni _1/3 Mn _2/3 O ₂ are utilized with a large 2 3 a $2\sqrt 3 a$ × 2 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 Ni ³⁺ to Ni ²⁺ . Such profile is quite different from its sodium counterpart (230 mA h g ^-1 ) which exhibits the sodium/vacancy ordering and deleterious presence of Mn ³⁺ . Depending on how the two interlayer spacings are filled by Na and Mg the "staged," "intermediated," and "average" models are analyzed for Mg _y Na ₈ Ni ₈ Mn ₁₆ O ₄₈ 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.
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