Lithium ion batteries have been widely used as a main energy storage system for electronic mobile devices. The demand for the longer battery running time is growing as the new functionalities are added to the small devices. To meet such demand, the lithium ion capacities of cathode and anode materials must be improved. The present cathode materials are the layered transition metal oxides such as LiCoO2, LiNi1/3Co1/3Mn1/3O2. Many experiments have reported that the higher the Ni content is, the larger the specific capacity is: the initial discharge capacity of LiNi0.85Co0.075Mn0.075O2 was estimated to be 206 mAh/g and that of LiNi0.8Co0.15Al0.05O2 (NCA) reached 226 mAh/g which is about 39% greater than that of LiNi1/3Co1/3Mn1/3O2. However, the high nickel content has critical issues in the thermal stability, which is believed to be related to the metal ions mixing in the lithium layers as the delithiation proceeds. In order to resolve this problem, many attempts have been devoted where the composition of the alloy has been changed by a small fraction. In this work, we have investigated the local structure of NCA using density functional calculations and cluster expansion method. From the calculation results, we find that the aluminum atoms do not like other aluminum atoms to be located at the nearest positions in the same layer. Instead, aluminum atoms prefer being situated in different layers. AlO6 has the distorted octahedral structure in which the two bonds of six Al-O bonds have the larger bond length and the oxygen ions attached to the Al atoms have more negative charges but the charges also have assigned inequivalently to those six oxygen ions. The combination of the charge inequivalence and the relative positions of the aluminum atoms makes the differences in the electrostatic potential energy that determine the preferable local structure of NCA.

Clarification of the local structure of LiNi0.8Co0.15Al0.05O2 using density functional theory calculations and cluster expansion method

248th National Meeting (2014)

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