In this work, a series of Li2MnO3 samples are synthesized by hydrothermal method, towards understanding the proton substitution, the crystallite morphology and electrochemical properties as a function of precursor concentration, reaction time and temperature. The stoichiometry of Li:Mn ratio and the proton substitution of as-synthesized Li2-xHxMnO3 samples are dependent on the hydrothermal conditions. In sample 180C12h, a maximum charge capacity reaches to 470 mAh g(-1), corresponding to the almost total removal of Li out of Li[Li1/3Mn2/3]O-2, and maximum discharge capacity 256.4 mAh g(-1) is obtained. The remained crystal structure of Li2MnO3 is unstable after more Li+ are extracted, which results in less Li+ to be reinserted and poor cycleability demonstrated. When the sample is charged to lower cut-off voltage 4.5 V, an increased discharge capacity and well cycleability are appeared. For improving the discharge capacity and cycleability, the nanocomposites of Li2-xHxMnO3 treated by LiMnO2 are synthesized by two steps hydrothermal methods. The nanocomposite presents very well electrochemical performance at the cut-off voltage of 2.0-4.4 V than that in pure Li2-xHxMnO3 and LiMnO2. The nanocomposite with weight ratio (Li2-xHxMnO3: LiMnO2) of 1.40:1 presents the discharge capacity of 230 mAh g(-1) at the 20th and of 220 mAh g(-1) at the 40th cycle. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3619786] All rights reserved.