Understanding the Role of NH₄F and Al₂O₃ Surface Co-modification on Lithium-Excess Layered Oxide Li_1.2Ni_0.2Mn_0.6O₂

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 Mn(4+). 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 Mn(3+) 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.