Capacity fading mechanisms and origin of the capacity above 4.5 V of spinel lithium manganese oxides
Lithium ion batteries have become attractive for portable devices due to their high operating voltage, high energy density, low self-discharge, and long cycle life. Lithium ion cells currently use the layered LiCoO2 cathode, but cobalt is expensive and relatively toxic. In this regard, spinel LiMn2O4 has become appealing since manganese is inexpensive and environmentally benign. However, LiMn2O4 exhibits severe capacity fade. This dissertation explores the electrochemical performances of spinel oxides derived from LiMn2O4 in a wide voltage range of 3.5 to 5.2 V to understand the capacity fading mechanisms and the origin of the high voltage capacity above 4.5 V. From a systematic investigation of a series of singly substituted LiMn2-yMyO4 and doubly substituted LiMn2-y-zMyLizO4 (M = Li, Al, or transition metal) oxides, the doubly substituted LiMn2-y-zNiyLizO4 oxides are found to show superior cyclability, low irreversible capacity (IRC), high rate capability, low electrode resistance, and excellent storage performance compared to LiMn2O4 despite a similar manganese dissolution. For example, LiMn1.85Ni0.075Li0.075O4 retains more than 99 and 97 % of its initial capacity after 50 cycles at, respectively, 25 and 60 o C. Its excellent electrochemical performances are found to be due to the small lattice parameter difference between the two cubic phases formed in the two-phase region during cycling and the consequent low microstrain. This concept of minimizing the lattice parameter difference and microstrain could successfully enable the use of spinel lithium manganese oxides for electric vehicle applications. In contrast to the literature suggestions that other transition metal ions or excess oxygen are essential to impart the high voltage (> 4.5 V) capacity, spinel lithium manganese oxides free from other transition metal ions are found to show capacity above 4.5 V even with a significant amount of oxygen deficiency if there are extractable Li+ ions in the 8a tetrahedral sites after the oxidation state of manganese reaches 4+. In the presence of other transition metals, the reversibility of the high voltage capacity depends on the element replacing Mn3+, and it decreases in the order LiMn2-yNiyO4 > LiMn2-yCoyO4 ≈ LiMn2-y-zCoyLizO4 > LiMn2-y-zNiyLizO4. Based on the results, the capacity above 4.5 V in manganesebased spinel oxides could be understood to arise from a participation of the O2-:2p band or a hybridized band involving M3+/4+:3d (or M2+/3+:3d) and O2-:2p.