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 degrees C have been investigated systematically before and after postannealing at 700 degrees C. Neutron diffraction studies reveal that the cation-ordered domain size tends to increase upon annealing at 700 degrees 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 degrees 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 degrees 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 degrees C, resulting in degradation in the rate capability due to an increase in the charge-transfer resistance and polarization loss.