Lithium batteries for UPS, portable electronics and electrical vehicles rely on high-energy cathodes. Li-rich manganese-rich oxide (xLi₂MnO₃·(1 − x)LiMO₂, M = transition metals) is one of the few materials that might meet such a requirement, but it suffers from poor energy retention due to serious voltage and/or capacity fade, which challenges its applications. Here we show that this challenge can be addressed by optimizing the interactions between the components Li₂MnO₃ and LiMO₂ in the Li-rich oxide (i.e. stabilizing the layered structure through Li₂MnO₃ and controlling Li₂MnO₃ activation through LiMO₂). To realize this synergistic effect, a novel Li₂MnO₃-stabilized Li_1.080Mn_0.503Ni_0.387Co_0.030O₂ was designed and prepared using a hierarchical carbonate precursor obtained by a solvo/hydro-thermal method. This layered oxide is demonstrated to have a high working voltage of 3.9 V and large specific energy of 805 W h kg⁻¹ at 29 °C as well as impressive energy retention of 92% over 100 cycles. Even when exposed to 55 °C, energy retention is still as high as 85% at 200 mA g⁻¹. The attractive performance is most likely the consequence of the balanced stability and specific energy in the present material, which is promisingly applicable to other Li-rich oxide systems. This work sheds light on harnessing Li₂MnO₃ activation and furthermore efficient battery design simply through compositional tuning and temperature regulation.