Ni and Al can be substituted for Co in Li[Ni1/3Mn1/3Co1/3]O-2 by increasing y and z in x LiNi1/3Mn1/3Co1/3O2-y LiNi2/3Mn1/3O2-z LiNi1/3Mn1/3Al1/3O2 (x+y+z=1) pseudoternary solid solution systems. Additions of Al improve the thermal stability of the charged positive electrode material, at a cost of capacity reduction, which can be at least partially recovered by the addition of Ni. The composition Li[Ni(1/3+0.07)Mn1/3Co(1/3-0.2)Al0.13]O-2 (x=0.4, y=0.2, z=0.4) shows excellent thermal stability compared to Li[Ni1/3Mn1/3Co1/3]O-2, but similar specific capacity. The samples were synthesized via a coprecipitation-based synthetic method to prepare a mixed hydroxide precursor, followed by sintering with a Li salt. Such materials may find application in large-size Li-ion batteries targeted for vehicle applications.