Li2MnO3 is chemically activated by the pyrolysis process of an in situ formed lithium stearate derived from the chemically adsorbed stearic acid on the Li2MnO3 particle surfaces. It is found that different Mn-containing compounds are formed depending on the pyrolysis temperature under N2 atmosphere. When the pyrolysis reaction temperature is 340°C, the layered structure of Li2MnO3 can be maintained and the average valence of Mn is reduced from +4.00 to +3.48 associated with 23% Li ions losing; while the pyrolysis reactions at 400–600°C lead to the formation of orthorhombic LiMnO2 and/or cubic MnO. As cathode material for Li-ion battery, the activated Li2MnO3 derived from the pyrolysis process at 340°C exhibits an initial charge/discharge profile similar to the Li-rich solid solution materials but with a much larger reversible discharge capacity (255mAhg−1) and a dramatically improved rate performance as compared to the pristine Li2MnO3. The present findings indicate that the pyrolysis of in situ formed lithium stearate is a simple and effective way to chemically activate Li2MnO3 so as to improve the cyclic capacity and rate performance.