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.