A novel Li[Ni0.67Co0.15Mn0.18]O2 cathode material encapsulated completely within a concentration-gradient shell was successfully synthesized via co-precipitation. The Li[Ni0.67Co0.15Mn0.18]O2 has a core of Li[Ni0.8Co0.15Mn0.05]O2 that is rich in Ni, a concentration-gradient shell having decreasing Ni concentration and increasing Mn concentration toward the particle surface, and a stable outer-layer of Li[Ni0.57Co0.15Mn0.28]O2. The electrochemical and thermal properties of the material were investigated and compared to those of the core Li[Ni0.8Co0.15Mn0.05]O2 material alone. The discharge capacity of the concentration-gradient Li[Ni0.67Co0.15Mn0.18]O2 electrode increased with increasing upper cutoff voltage to 4.5V, and cells with this cathode material delivered a very high capacity, 213mAh/g, with excellent cycling stability even at 55°C. The enhanced thermal and lithium intercalation stability of the Li[Ni0.67Co0.15Mn0.18]O2 was attributed to the gradual increase in tetravalent Mn concentration and decrease in Ni concentration in the concentration-gradient shell layer.