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

A Mg-cell with P2-Na2/3Ni1/3Mn2/3O2 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 Na1/3Ni1/3Mn2/3O2 compound, which is formally accompanied by an oxidation from Ni2+ to Ni3+. As structure models Mg1/6Na1/3Ni1/3Mn2/3O2 and Mg1/12Na1/2Ni1/3Mn2/3O2 are utilized with a large 2 root 3a x 2 root 3a supercell. On discharge, the Mg-cell exhibits a multistep profile which reaches approximate to 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 MgyNa8Ni8Mn16O48 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.