State-of-the-art Li-ion batteries in long-range electric vehicles increasingly rely on LiNi 1−x−y Mn x Co y O 2 (NMC) cathodes with a trend toward higher Ni content. While high-Ni compositions can increase capacity, they also decrease the material's stability and therefore battery lifetime. Though aluminum has been substituted into a few NMC compositions, to date, no systematic study has been performed. In this study, the impact of different levels of Al substitution was explored across the full range of NMC compositions by preparing a total of 320 materials with varying aluminum content from 0 to 15%. Both Xray diffraction (XRD) and cyclic voltammetry were performed on all samples. The single-phase layered oxide region is quite large when no Al is present and decreases slightly in size as aluminum content increases. The nickel-rich region (1 − x − y > 0.8) was entirely single phase for 0 and 5% Al levels, while at higher Al content small amounts of Li−Al−O phases were found as contaminants detrimental to the electrochemistry, indicating an optimal Al level in high-Ni materials. Key battery metrics including discharge capacity, average voltage, and diffusion coefficients were obtained for all materials and correlated to structural changes seen in the XRD. It was found that Al suppressed the detrimental hexagonal phase transitions at high voltage (∼4.3 V) in Ni-rich compositions. Increased Al content also improved the diffusion coefficients in the Ni-rich materials. Overall, this study provides the first generalized view of the effects of Al substitution on NMC cathode materials. This work can serve as a base for further exploration into Al substitution on less-studied NMC compositions.