Role of Cation Ordering and Surface Segregation in High-Voltage Spinel LiMn1.5Ni0.5–xMxO4 (M = Cr, Fe, and Ga) Cathodes for Lithium-Ion Batteries

The high-voltage doped spinel oxides LiMn1.5Ni0.5–xMxO4 (M = Cr, Fe, and Ga; 0 ≤ x ≤ 0.08) synthesized at 900 °C have been investigated systematically before and after postannealing at 700 °C. Neutron diffraction studies reveal that the cation-ordered domain size tends to increase upon annealing at 700 °C. Time-of-flight secondary-ion mass spectroscopy data reveal that the dopant cations M = Cr, Fe, and Ga segregate preferentially to the surface, resulting in a more stable cathode–electrolyte interface and superior cyclability at both room temperature and 55 °C with conventional electrolytes. The doping with Cr and Fe stabilizes the structure with a significant disordering of the cations in the 16d sites even after postannealing at 700 °C, resulting in high rate capability due to low charge-transfer resistance and polarization loss. In contrast, the Ga-doped and undoped LiMn1.5Ni0.5O4 samples experience an increase in cation ordering upon postannealing at 700 °C, resulting in degradation in the rate capability due to an increase in the charge-transfer resistance and polarization loss.