Co -free Li -rich Mn-based layered oxides are promising candidates for next-generation lithium -ion batteries (LIBs) due to their high specific capacity, high voltage, and low cost. However, their commercialization is hindered by limited cycle life and poor rate performance. Herein, an in -situ simple and low-cost strategy with a nanoscale double -layer architecture of lithium polyphosphate (LIPP) and spinel phase covered on top of the bulk layered phase, is developed for Li1.2Mn0.5NI0.2O2 (LMNO) using Li*-conductor LiPP (denoted as LMNO@S-LiPP). With such a double -layer covered architecture, the half-cell of LMNO@S-LiPP delivers an extremely high capacity of 202.5 mAh"g(-1) at 1 A.g(-1) and retains 85.3% of the initial capacity after 300 cycles, so far, the best high rate electrochemical performance of all the previously reported LMNOs. The energy density of the full -cell assembled with commercial graphite reaches 620.9 Wb" kr (based on total weight of active materials in cathode and anode). Mechanism studies indicate that the superior electrochemical performance of LMNO@S-LiPP is originated from such a nanoscale double -layer covered architecture, which accelerates Li -ion diffusion, restrains oxygen release, inhibits interfacial side reactions, and suppresses structural degradation during cycling. Moreover, this strategy is applicable for other high -energy-density cathodes, such as LiNi0.8Co0.1Mn0.1O2, Li1.2Ni0.13Co0.13Mn0.54O2, and LiCoO2. Hence, this work presents a simple, cost-effective, and scalable strategy for the development of high-performance cathode materials.