Nickel-rich layered oxides are promising positive electrode materials for lithium-ion batteries due to their high capacity and decreased cobalt content. The application of surface coatings is a common approach to slowing or potentially stopping deleterious reactions at the electrode-electrolyte interface of lower-Ni content layered oxides. However, their efficacy on Ni-rich LiNi0.8Mn0.1Co0.1O2 (NMC811) is less certain and knowledge on how to design effective coatings with favorable properties is sparse. In this work, we develop a convenient solution-based deposition method for the synthesis of aluminum oxide coatings NMC811 secondary particles and we study the effects of annealing temperature on their structure and electrochemical lifetime in lithium-ion batteries. Using energy dispersive X-ray spectroscopy (EDS) and X-ray fluorescence spectroscopy (XRF), we quantify the amount and distribution of aluminum oxide on the cathode particles. Changes in the coating phase and composition as a function of annealing temperature are tracked with solid-state nuclear magnetic resonance (SS-NMR) and X-ray photoelectron spectroscopy (XPS). 27Al NMR spectroscopy at very high field (23.5 T) provides direct evidence that after annealing up to 400 °C, 4-, 5- and 6-coordinate aluminum is present, here assigned to an amorphous alumina coating, but after annealing to 600 ºC, a γ-LiAlO2-like coating is observed. We further differentiate between Al in the bulk and surface phase and identify, for the first time, the critical temperature at which doping occurs in NMC811. Surface/bulk doping starts to occur in the range 500 – 600 °C, with considerable bulk doping being found at 800 °C. The onset of Al diffusion coincides with the decrease in capacity retention, contradicting previous studies and giving new insight into the relationship between lifetime and lithium-ion conductivity.