Some polymorphs of MnO2, i.e., alpha-, beta-, gamma-MnO2 (the latter in the form of two electrochemical MnO2 and a chemical MnO2), and ramsdellite (R-MnO2), have been submitted to electrochemical potential spectroscopy (EPS) tests. This quasiequilibrium technique affords a better deconvolution of the various redox steps, thus giving more information with respect to previous tests based on cyclic voltammetries. The electrochemical behavior has been correlated to the type of channels present 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 87%) are related to the structure stabilization induced by spinel-like domains in the lithiated pristine frameworks. A simple in situ lithiation 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 was gradually inserted in the cathode upon electrolyte addition, so to form Li-1R-MnO2. Li cells with LixR-MnO2 (x similar to 0.3 or similar to 1) show an excellent behavior especially when a galvanostatic-potentiostatic charging regime is applied. Stable capacities of 0.15 Ah/g may be reached, this producing specific energies above 400 Wh/kg, a value which compares fairly well with that of a 4 V, LiMn2O4-based cell. The in situ full lithiation of a Li-0.3R-MnO2 cathode permits fabrication of 3 V Li-ion cells with this efficient material.