LITHIATED MNO2 PHASES AS CATHODES FOR 3-V LI AND LI-ION CELLS

Some polymorphs of MnO2, i.e., alpha-, beta-, gamma-MnO2 (the latter i n the form of two electrochemical MnO2 and a chemical MnO2), and ramsd ellite (R-MnO2), have been submitted to electrochemical potential spec troscopy (EPS) tests. This quasiequilibrium technique affords a better deconvolution of the various redox steps, thus giving more informatio n with respect to previous tests based on cyclic voltammetries. The el ectrochemical behavior has been correlated to the type of channels pre sent in the structures and the rutile content of the polymorphs. R-MnO , is the form showing the highest reversibility (55% up to 3.4 V). EPS tests were also applied to thermally lithiated alpha-, gamma-, and R- MnO2. The higher charge/discharge efficiencies of these forms (up to 8 7%) are related to the structure stabilization induced by spinel-like domains in the lithiated pristine frameworks. A simple in situ lithiat ion technique was applied to a cell having Li-0.3R-MnO2 as a cathode. By interposing a Li sheet between cathode and current collector, Li wa s gradually inserted in the cathode upon electrolyte addition, so to f orm Li-1R-MnO2. Li cells with LixR-MnO2 (x similar to 0.3 or similar t o 1) show an excellent behavior especially when a galvanostatic-potent iostatic charging regime is applied. Stable capacities of 0.15 Ah/g ma y be reached, this producing specific energies above 400 Wh/kg, a valu e which compares fairly well with that of a 4 V, LiMn2O4-based cell. T he in situ full lithiation of a Li-0.3R-MnO2 cathode permits fabricati on of 3 V Li-ion cells with this efficient material.