Role of Cation Ordering
and Surface Segregation in
High-Voltage Spinel LiMn<sub>1.5</sub>Ni<sub>0.5–<i>x</i></sub>M<sub><i>x</i></sub>O<sub>4</sub> (M = Cr, Fe, and
Ga) Cathodes for Lithium-Ion Batteries
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Abstract
The high-voltage doped spinel oxides LiMn<sub>1.5</sub>Ni<sub>0.5–<i>x</i></sub>M<sub><i>x</i></sub>O<sub>4</sub> (M =
Cr, Fe, and Ga; 0 ≤ <i>x</i> ≤ 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 LiMn<sub>1.5</sub>Ni<sub>0.5</sub>O<sub>4</sub> 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