The effects of acid treatment on the electrochemical properties of 0.5 Li2MnO3•0.5 LiNi0.44Co0.25Mn0.31O2 electrodes in lithium cells

The electrochemical properties of 0.5 Li2MnO3 center dot 0.5 LiNi0.44Co0.25Mn0.31O2 electrodes, when preconditioned and activated with acid for 2-24 h, have been studied in lithium cells. Powder X-ray diffraction data and electrochemical measurements provide supporting evidence for an intergrown, composite electrode structure from which Li2O can be leached from the Li2MnO3 (Li2O center dot MnO2) component with acid, thereby mimicking the electrochemical charge process at high potentials (<4.5 V). The MnO2-rich domains generated by acid treatment are reduced during electrochemical discharge at a lower potential than electrochemically generated MnO2-rich domains. With prolonged cycling between 4.6 and 2.0 V, dQ/dV plots of untreated and acid-treated electrodes develop similar, but not identical, character, suggesting a coalescence and redox interaction of the manganese ions in MnO2-rich and Ni0.44Co0.25Mn0.31O2 regions of the structure. Acid treatment eliminates the first-cycle capacity loss of the electrodes, consistent with earlier reports for related systems, but it damages their cycling stability and rate capability. (C) 2006 The Electrochemical Society.