Effects of Al-doping on the stabilization of monoclinic LiMnO2

Electronic structures of monoclinic LiMnO₂ and LiMn_0.9375Al_0.0625O₂ with ferromagnetic (FM) and antiferromagnetic (AF) ordering have been investigated by ab initio calculation within spin-polarized generalized gradient approximation method. An Al-doping induced complicated AF configuration has been calculated to be the ground state, which suggests a robust Al-doping effect on the magnetic and electronic structures of the monoclinic LiMnO₂. The calculated Mulliken population analyses and partial density of states of Mn-3d and O-2p reveal that a single Al dopant stabilizes its six nearest-neighbor Mn ions in their respective octahedral sites, thereby hindering the migration of Mn ions into the interlayer Li sites during the Li intercalation-deintercalation and therefore improving both the structural stability and the electrochemical performance of the material. Additionally, it is found out that the Al-doping can decrease the JT effect and increase the intercalation voltage. The Al-doping-induced negative formation energy indicates that 6.25% Mn ions in monoclinic LiMnO₂ can be substituted stably by Al ions, to which the equilibrium but not metastable phase of monoclinic LiMn_0.9375Al_0.0625O₂ can be attributed.