Highly crystalline Li[Ni1-x-yCoxMny]O-2 (x+y <= 0.5) (Li[Ni0.6Co0.2Mn0.2]O-2, Li[Ni0.55Co0.15Mn0.3]O-2, and Li[Ni0.5Co0.25Mn0.25]O-2) were synthesized through a coprecipitation method. The capacities of the prepared samples were proportional to the amount of Ni in the host structure. The thermogravimetric analysis (TGA) and in situ high-temperature-X-ray diffraction (HT-XRD) analysis revealed that changes in the amount of manganese ions in the host structure profoundly affect the structural stability of the samples with x+y <= 0.5. Li[Ni0.55Co0.15Mn0.3]O-2, containing the highest manganese content (y=0.3), showed the most stable structural integrity among the samples as confirmed by in situ HT-XRD. The electrochemical performances of the samples in Ni amount (0.5 <= 1-x-y <= 0.6) with the variation of Co (0.15 <= x <= 0.25) did not significantly vary under the test conditions (3.0-4.3 V). The small increase of Mn ions plays an important role in preservation of its initial structural symmetry during the high-temperature heating as well as electrochemical cycling. Furthermore, the structural stability has a relationship with the thermal stability and the electrochemical stability, especially at an elevated temperature (55 degrees C). On the basis of the differential scanning calorimetry and TGA results, the Li[Ni0.55Co0.15Mn0.3]O-2 sample demonstrated improved thermal stability compared to the other samples. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2988729] All rights reserved.