An Investigation of Lithium Ion Insertion into Spinel Structure Li‐Mn‐O Compounds

Two kinds of spinel structure lithium manganese oxides obtained by a melt‐impregnation method were examined in a lithium nonaqueous cell. The first type shows a voltage profile of a typical spinel electrode with the characteristic two‐step process, which delivers a larger capacity, but fading capacity on cycling (type I). The other type delivers a slightly lower capacity and has ideal rechargeability with a quasi‐one‐step process (type II). The mechanism of the lithium ion insertion into the spinel compound was studied by several physical and chemical methods (differential chronopotentiometric curves, cyclic voltammetry, and open‐circuit voltage measurement). X‐ray diffraction was also used to investigate the structural changes for both types of compounds at different oxidation depths at first charge. The results show that a two‐phase reaction (cubic a 0 = 8.154 Å and a 0 = 8.072 Å) occurred in the range of 0.1 < x <0.45, and a one‐phase reaction cubic a 0 = 8.163 to 8.247 Å occurred in the range of 0.45 < x < 1.0 for the first charge for the type I spinel. A one‐phase reaction (cubic a 0 = 8.105 to 8.223 Å) was observed over the entire intercalated range of 0.25 < x < 1.04 for the type II spinel compound. The capacity fading on cycling for the type I spinel occurs only in the range of 0.1 < x < 0.45, where the shape of the charge and discharge curves in this range was gradually changed to an S‐shape from and L‐shape during cycling. This is attributed to the fact that the two‐phase structure is transformed to more stable one‐phase structure.