Identifying the distribution of Al³⁺ in LiNi_0.8Co_0.15Al_0.05O₂

The doping of Al into layered Li transition metal (TM) oxide cathode materials, LiTMO₂, is known to improve the structural and thermal stability, although the origin of the enhanced properties is not well understood. The effect of aluminum doping on layer stabilization has been investigated using a combination of techniques to measure the aluminum distribution in layered LiNi_0.8Co_0.15Al_0.05O₂ (NCA) over multiple length scales with ²⁷Al and ⁷Li MAS NMR, local electrode atom probe (APT) tomography, X-ray and neutron diffraction, DFT, and SQUID magnetic susceptibility measurements. APT ion maps show a homogeneous distribution of Ni, Co, Al, and O₂ throughout the structure at the single particle level in agreement with the high-temperature phase diagram. ⁷Li and ²⁷Al NMR indicates that the Ni³⁺ ions undergo a dynamic Jahn-Teller (JT) distortion. ²⁷Al NMR spectra indicate that the Al reduces the strain associated with the JT distortion, by preferential electronic ordering of the JT lengthened bonds directed toward the Al³⁺ ion. The ability to understand the complex atomic and orbital ordering around Al³⁺ demonstrated in the current method will be useful for studying the local environment of Al³⁺ in a range of transition metal oxide battery materials.