Understanding Surface Structural Stabilization of the High-Temperature and High-Voltage Cycling Performance of Al³⁺-Modified LiMn₂O₄ Cathode Material

Stabilization of the atomic-level surface structure of LiMn₂O₄ with Al³⁺ ions is shown to be significant in the improvement of cycling performance, particularly at a high temperature (55 °C) and high voltage (5.1 V). Detailed analysis by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy, etc. reveals that Al³⁺ ions diffuse into the spinel to form a layered Li­(Al<i>_x</i>,Mn<i>_y</i>)­O₂ structure in the outmost surface where Al³⁺ concentration is the highest. Other Al³⁺ ions diffuse into the 8a sites of spinel to form a (Mn_3–<i>x</i>Al<i>_x</i>)­O₄ structure and the 16d sites of spinel to form Li­(Mn_2–<i>x</i>Al<i>_x</i>)­O₄. These complicated surface structures, in particular the layered Li­(Al<i>_x</i>,Mn<i>_y</i>)­O₂, are present at the surface throughout cycling and effectively stabilize the surface structure by preventing dissolution of Mn ions and mitigating cathode–electrolyte reactions. With the Al³⁺ ions surface modification, a stable cycle performance (∼78% capacity retention after 150 cycles) and high Coulombic efficiency (∼99%) are achieved at 55 °C. More surprisingly, the surface-stabilized LiMn₂O₄ can be cycled up to 5.1 V without significant degradation, in contrast to the fast capacity degradation found in the unmodified case. Our findings demonstrate the critical role of ions coated on the surface in modifying the structural evolution of the surface of spinel electrode particles and thus will stimulate future efforts to optimize the surface properties of battery electrodes