Influence of Al3+ ions on the morphology and structure of layered LiMn1-xAlxO2 cathode materials for the lithium ion battery

Orthorhombic LiMn1-xAlxO2 nanocrystals (Pmmn (59), origin choice 2) with different morphologies were successfully synthesized using Mn2O3 and Al2O3 as Mn- and Al-sources in a 3.5 M LiOH aqueous solution hydrothermally treated at 150 °C. Although Al3+ ions introduced into the starting hydrothermal solution was in a large amount from 0 to 0.33 of Al/Mn molar ratio, noted as Al/Mn(HT) where HT refers to hydrothermal treatment; the ratio was as low as 0.05 in the phase-pure LiMn1-xAlxO2 crystals, noted as Al/Mn(SS) where SS refers to solid solution, while a Al/Mn(HT) ratio higher than 0.5 resulted in the formation of orthorhombic LiAlO2 phase. With an increase of Al/Mn(SS), the average oxidation number was higher than 3 and gradually increased, leading to a monotonous shrinkage of the unit cell along a-axis (the zigzag direction of the metal-oxygen octahedron layers). Furthermore, Al3+ also influenced on the morphology of the crystals: (1) cuboid shaped crystals as (Al/Mn(SS) < 0.02), with the ascendant growth in the direction vertical to the layer; and (2) cubic-like crystals as Al/Mn(SS) > 0.02 can be obtained. Galvanostatic measurement exhibited that the capacity of the electrode composed of Li0.93Mn0.96Al0.04O2 can reach a discharge capacity of 175 mAh/g at a current density of 0.1C and the capacity retention could be 82.9% after 25th cycling, showing that Al3+ can stabilize the layered structure by retarding its transforming to spinel one.