The present invention provides a potassium-doped mixed metal oxide cathode material formed by advantageously alloying MnO₂ with potassium and lithium to provide a new mixed metal oxide cathode material as the positive electrode in rechargeable lithium and lithium ion electrochemical cells. By alloying MnO2 with potassium and lithium in a Li_xK_yMn₂O₄ compound, the cathode materials of the present invention afford overcharge protection that allows the cathode to be fully reversible. Manganese dioxide doped with potassium was initially examined as a cathode material for rechargeable lithium and lithium-ion batteries in order to provide a new mixed metal oxide cathode material as the positive electrode in rechargeable lithium and lithium ion electrochemical cells. The Li_xK_yMn₂O₄ material is incorporated into an electrochemical cell with either a lithium metal or lithium ion anode and an organic electrolyte. In one embodiment, the cathode of the present invention comprises a compound with the general formula Li_xK_yMn₂O₄, where y0.5, x+y1.0 and the reversible region for x is between 0.0x1.0-y to provide the required overcharge protection and a high voltage cutoff on charge. In the preferred embodiment, a cathode comprising Li_0.8K_0.1Mn₂O₄ is provided. The cathode materials, cathodes, lithium electrochemical device and methods of this invention answer the long-felt need for a reversible cathode for rechargeable lithium batteries without suffering from the shortcomings, limitations and disadvantages of, rigid stoichiometry electronic control and loss of reversibility.