Recently, the mixed transition metal oxides of the form Li (Ni_1-y-zCo_yMn_z)O₂, havebecome the center of attention as promising candidates for novel battery material. These materials have also revealed very interesting magnetic properties due to the alternate stacking of planes of metal oxides on a 2D triangular lattice and the Li-layers. The title compound, Li (Ni_0.8Co_0.1Mn_0.1) O₂, has been investigated by both magnetometry and measurements and μ⁺SR. We find the evolution of localized magnetic moments with decreasing temperature below 70 K. The magnetic ground state (T = 2 K) is, however, shown to be a frustrated system in 3D, followed by a transition into a possible 2D spin-glass above 22 K.With further increasing temperature the compound show the presence of remaining correlations with increasing effective dimensionality all the way up to the ferrimagnetic transition at T_C = 70 K.