Li1.59H0.41MnO3, a Li2MnO3-type cathode material, was synthesized by hydrothermally treating an amorphous MnOOH in LiOH and (NH4)(2)S2O8 aqueous solution. Such a rocksalt material was confirmed as proton-substituted Li2MnO3, i.e., Li1.59H0.41MnO3 instead of a composite of Li2MnO3 with other polymorphs of manganese oxides with Mn(IV) such as Li2Mn4O9 and Li4Mn5O12. The Li1.59H0.41MnO3 shows a remarkable difference in structure and electrochemical performance from those proton-substituted Li2MnO3 by acid treatment in previous studies. The Li1.59H0.41MnO3 cathode behaved similarly to that of Li2MnO3 material and delivered a high discharge capacity of 256 mAh g(-1) at 20 mAh g(-1) (4.8-2.0 V). (C) 2008 The Electrochemical Society.