ELECTROCHEMISTRY OF MANGANESE-DIOXIDE IN LITHIUM NONAQUEOUS CELLS .4. JAHN-TELLER DEFORMATION OF MNO6-OCTAHEDRON IN LIXMNO2

X-ray diffractional (XRD) and electrochemical studies of Li(y)MnO2 prepared from heat-treated electrolytic manganese dioxide (250-degrees-C, 7 days) were carried out in the rechargeable region [MnO1.86(1)-MnO1.57(2)] of this material in lithium non-aqueous cells. The shape in the midpoint voltage curve between charge and discharge curves was observed to be quite different between the ranges MnO1.86(1)-MnO1.75 and MnO1.75-MnO1.57(2). XRD data indicated that the reaction proceeded in a homogeneous phase throughout the whole region. However, a discontinuous change in the orthorhombic unit cell parameters vs. x in MnO(x) plots was observed at ca. MnO1.75. In order to explain such discontinuity in electrochemical and structural data, the Jahn-Teller deformation of a MnO6 octahedron in Li(y)MnO2 was considered assuming a possible MnO6 octahedral linkage to form an orthorhombic unit cell. Our results indicated that the Li(y)MnO2 phase in the region MnO1.86(1)-MnO1.75 is not the same as that in the region MnO1.75-MnO1.57(2). The discontinuity observed in the electrochemical and structural data are ascribed to a phase transition at ca. MnO1.75. The effect of the Jahn-Teller distortion of MnO6 octahedra upon the electrochemical behavior of several MnO2s is discussed.