Positive electrode active materials for the rechargeable sodium ion battery are in-situ synthesized during initial charge process in an electrochemical cell from precursor composites of Na2O2 and Mn3O4. Unlike the synthesis of metal oxides, e.g., alpha-NaFeO2, etc., the present materials are subjected to no heat-treatment. When beginning with a precursor composite of which the Na/Mn ratio is 1.0, the electrode delivers 194 mA h g(-1) during the subsequent discharge process. The electrode retains 84% of the initial discharge capacity after 30 cycles. X-ray diffractometry (XRD) and transmission electron microscopy identifies three phases in the precursor composite; i.e., crystalline Mn3O4 and Na2O2 as well as an amorphous phase. Ex-situ XRD reveals that Mn3O4 in the precursor transforms to lambda-MnO2 after the initial charge process as a result of the in-situ synthesis. Moreover, we also found the composite functions when the Na/Mn ratio of the composite is more than 1.0. An extra amount of Na can be provided to cancel the irreversible capacity of the negative electrode when we start from a composite of which the Na/Mn ratio is greater than 1.0, whereby we demonstrate the feasibility of a full-cell. (C) 2016 The Electrochemical Society. All rights reserved.