Stable form of LiMnO.sub.2 as cathode in lithium cell

A secondary lithium ion electrochemical cell, comprising a lithium intercalation anode comprising petroleum coke, a non-aqueous electrolyte comprising IM LiN(CF.sub.3 SO.sub.2).sub.2 in a 50/50 mixture by volume of propylene carbonate and dimethoxyethane, a cathode and a separator between the anode and cathode, wherein the initial cathode material comprises a dried slurry of orthorhombic LiMnO.sub.2 characterized by the specific orthorhombic crystal structure described by the space group Pmnm and the unit cell dimensions a=4.572 .ANG., b=5.757 .ANG.and c=2.805 .ANG., and by the X-ray diffraction pattern shown in FIGS. 1 and 2 as active material, two weight percent of ethylenepropylenediene monomer and ten weight percent of carbon black, in cyclohexane.