Lithium-rich, cobalt-free oxides are promising potential positive electrode materials for lithium-ion batteries because of their high energy density, lower cost, and reduced environmental and ethical concerns. However, their commercial breakthrough is hindered because of their subpar electrochemical stability. This work studies the effect of aluminum doping on Li_1.26Ni_0.15Mn_0.61O₂ as a lithium-rich, cobalt-free layered oxide. Al doping suppresses voltage fade and improves the capacity retention from 46% for Li_1.26Ni_0.15Mn_0.61O₂ to 67% for Li_1.26Ni_0.15Mn_0.56Al_0.05O₂ after 250 cycles at 0.2 C. The undoped material has a monoclinic Li₂MnO₃-type structure with spinel on the particle edges. In contrast, Al-doped materials (Li_1.26Ni_0.15Mn_0.61-xAl_xO₂) consist of a more stable rhombohedral phase at the particle edges, with a monoclinic phase core. For this core-shell structure, the formation of Mn³⁺ is suppressed along with the material's decomposition to a disordered spinel, and the amount of the rhombohedral phase content increases during galvanostatic cycling. Whereas previous studies generally provided qualitative insight into the degradation mechanisms during electrochemical cycling, this work provides quantitative information on the stabilizing effect of the rhombohedral shell in the doped sample. As such, this study provides fundamental insight into the mechanisms through which Al doping increases the electrochemical stability of lithium-rich cobalt-free layered oxides.