Abstract
In this work we prepared Li1.2Ni0.2Mn0.6O2 (LNMO)
using a hydroxide co-precipitation method and investigated the effect of
co-modification with NH4F and Al2O3. After surface co-modification, the
first cycle Coulombic efficiency of Li1.2Ni0.2Mn0.6O2 improved from
82.7% to 87.5%, and the reversible discharge capacity improved from 253
to 287 mAh g−1 at C/20. Moreover, the rate capability also increased
significantly. A combination of neutron diffraction (ND), high-resolution
transmission electron microscopy (HRTEM), aberration-corrected
scanning transmission electron microscopy (a-STEM)/electron energy
loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS)
revealed the changes of surface structure and chemistry after NH4F and
Al2O3 surface co-modification while the bulk properties showed relatively
no changes. These complex changes on the material’s surface include the
formation of an amorphous Al2O3 coating, the transformation of layered
material to a spinel-like phase on the surface, the formation of nanoislands of active material, and the partial chemical reduction
of surface Mn4+. Such enhanced discharge capacity of the modified material can be primarily assigned to three aspects: decreased
irreversible oxygen loss, the activation of cathode material facilitated with preactivated Mn3+ on the surface, and stabilization of
the Ni-redox pair. These insights will provide guidance for the surface modification in high-voltage-cathode battery materials of
the future.
