The intricate roles of Al2O3 on the structure and electrochemical performances of LiNi0.5Co0.2Mn0.3O2 cathode materials

Severe capacity fading of in NCM layered oxide cathode materials has hampered their practical adoption. This problem can be effectively alleviated by the introduction of Al3+. Previous studies have shown that when Al3+ is introduced into the lattice to partially replace the transition metals, it acts as a structural stabilizer and inter-Al3+ does not participate in the redox reactions. However, the reaction of an aluminum source with lithium salts has not attracted enough attention in the synthesis of Al3+doped cathode materials. In this paper, Al2O3-modified LiNi0.5Co0.2Mn0.3O2 cathode materials are revisited by the solid-state reaction methods. Combined with XRD results, SEM and mapping analysis, it is found that during the heattreatment process, a part of Al3+ ions are doped into the lattice of the cathode material, and it is proved that most parts of Al2O3 react with LiOH⋅H2O to form α-LiAlO2, which is coated on the surface of the cathode material. Considering that lithium will be consumed because of the reaction between Al2O3 and LiOH⋅H2O, we systematically adjusted the lithium amount. It is found that because LiOH reacts preferentially with Al2O3, the amount of lithium reacting with the Ni0.5Co0.2Mn0.3(OH)2 precursor is reduced, which will reduce the electrochemical performance of the cathode material. The deterioration of the electrochemical properties of LiNi0.5Co0.2Mn0.3O2 cathode materials with insufficient lithium is mainly due to the transition of the material structure from a layered structure to a spinel structure. Moreover, the lithium supplemented samples show higher initial charge and discharge capacities, rate capabilities and cycling performances than the lithium un-supplemented samples. •LiOH would preferentially react with Al2O3 rather than NCM precursor under the heat treatment to form LiAlO2.•The amount of lithium reacting with the precursor is reduced, which will reduce the electrochemical performance of the cathode material.•Insufficient lithium will lead to the transition of the material structure from a layered structure to a spinel structure.•The lithium amount should be carefully adjusted.