Structural and electrochemical behaviors of metastable Li2/3[Ni1/3Mn2/3]O2 modified by metal element substitution

Layered metastable lithium manganese oxides, Li_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ precursors. The Al and Co doping produced the T^#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni_1/3−xMn_2/3−yM_x+yO₂ slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T^#2-Li_2/3[Ni_1/3Mn_2/3]O₂ were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g⁻¹ in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g⁻¹ along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T^#2 into the O2 structure was observed during the initial lithium deintercalation.