Li x MnO2 (x =0.302) was synthesized by chemical lithiation, using a formaldehyde reducing agent and a LiOH lithium source in an aqueous media. The electrochemical properties and structural stability of the product were characterized by X-ray diffraction and charge–discharge measurements. The chemically lithiated Li x MnO2 had a first charge and discharge capacity of 86.2 and 265mAh/g, respectively, with good cycling behavior. Based on the electrochemical results of the first charge, a two-step mechanism of Li x MnO2 lithiation is proposed. γ-MnO2 is first oxidized by formaldehyde, and then Li diffuses into the γ-MnO2 lattice.