xLi2MnO3·(1 − x)LiMnO2 (x = 0.39, 0.48, 0.57, 0.74) composites with a layered structure of C2/m symmetry are firstly synthesized with the assistance of the pyrolysis of in situ formed lithium stearate, which can partially reduce the electrochemically inactive Li2MnO3. The obtained composites used as cathode material of Li-ion battery exhibit excellent electrochemical property such as high reversible capacity (>210 mAh g−1) and good rate performance with an initial charge/discharge profile similar to the Li-rich solid solution materials in the range of 2.0–4.8 V. Among those obtained composites, 0.57Li2MnO3·0.43LiMnO2 has the best cyclic and rate performance, because it contains enough Li2MnO3 to stabilize the structure of LiMnO2 and has a suitable average valence state of Mn to balance the reversible capacity and Jahn–Teller effect. The present findings indicate that the pyrolysis of in situ formed lithium stearate on Li2MnO3 particle surfaces is a simple and effective way to obtain Mn-based layered Li-rich composite cathode materials of Li-ion battery with good cyclic and rate performance.