Stabilizing nickel-rich layered oxide cathodes by magnesium doping for rechargeable lithium-ion batteries

Nickel-rich layered transition metal oxides are attractive cathode materials for rechargeable lithium-ion batteries but suffer from inherent structural and thermal instabilities that limit the deliverable capacity and cycling performance on charging to a cutoff voltage above 4.3 V. Here we report LiNi_0.90Co_0.07Mg_0.03O₂ as a stable cathode material. The obtained LiNi_0.90Co_0.07Mg_0.03O₂ microspheres exhibit high capacity (228.3 mA h g^-1 at 0.1C) and remarkable cyclability (84.3% capacity retention after 300 cycles). Combined X-ray diffraction and Cs-corrected microscopy reveal that Mg doping stabilizes the layered structure by suppressing Li/Ni cation mixing and Ni migration to interlayer Li slabs. Because of the pillar effect of Mg in Li sites, LiNi_0.90Co_0.07Mg_0.03O₂ shows decent thermal stability and small lattice variation until it is charged to 4.7 V, undergoing a H1-H2 phase transition without discernible formation of an unstable H3 phase. The results indicate that moderate Mg doping is a facile yet effective strategy to develop high-performance Ni-rich cathode materials.