Lithium-rich layered oxides, xLi2MnO3·(1−x)LiMO2(M = Ni, Mn, Co), have been under intense investigation as high-performance cathode materials for lithium ion batteries due to their high discharge capacity, low cost and environmental benignity. Unfortunately, the commercialized application of these cathode materials have so far been hindered by their severe capacity and voltage fading during high voltage cycling (>4.5 V vs. Li/Li+). In an attempt to overcome these problems, herein, highly crystalline Al2O3 layer from the hydrolysis of aluminum isopropoxide are coated on 0.5Li2MnO3·0.5LiNi0.5Co0.2Mn0.3O2 with controlling the growth of Al2O3 crystals. The coin cell with bare cathode material delivers a high discharge capacity over 268.2 mAh g−1 between 2.0 V and 4.8 V, while the Al2O3 coated cathode material shows the excellent cycling stability corresponding to 98% capacity retention after 100 cycles at 1C. More importantly, the highly crystalline Al2O3 coated cathode materials exhibit a significantly lower discharge voltage decay compared to the bare cathode materials, which could be ascribed to the suppression of the layered-to-spinel transformation by compact and highly crystalline Al2O3 layer. The results here will shed light on developing cathode materials with special structures and superior electrochemical properties for high-performance lithium ion batteries.