In order to improve the performance of the Li[Ni0.5Co0.2Mn0.3]O2, a novel spherical Li[Ni0.5Co0.2Mn0.3]O2–Li2MnO3 core–shell composite has been successfully synthesized by a simple sol–gel deposition method. It is found that the core–shell composite consists of Li[Ni0.5Co0.2Mn0.3]O2 as the core and Li2MnO3 as the outer shell according to examination by scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction (XRD). The core–shell composite delivers a high capacity of 180.4 mAh g−1 at a rate of 0.1 C in the voltage range of 2.0–4.4 V. Most importantly, the core–shell composite exhibits excellent capacity retention of 98.8% after 200 cycles at a rate of 0.5 C, and 93.8% capacity retention after 100 cycles at a rate of 5 C at room temperature. When cycled at an elevated 55 °C, the prepared sample exhibits only 2.3% capacity loss after 100 cycles. Meanwhile, the core–shell composite shows significantly improved thermal stability relative to that of the pristine Li[Ni0.5Co0.2Mn0.3]O2. The improved stability of the core–shell composite is ascribed to the stable Li2MnO3 outer shell, which can prevent the Li[Ni0.5Co0.2Mn0.3]O2 core from direct contact with the electrolyte, and the core material could operate with no risk of structural disruption at high voltages and high temperatures.