RAMSDELLITE-MNO2 FOR LITHIUM BATTERIES - THE RAMSDELLITE TO SPINEL TRANSFORMATION

A pure and highly crytalline form of ramsdellite-MnO2 has been synthesized by acid treatment of the spinels LiMn2O4 and Li2Mn4O9 at 95-degrees-C. Although the ramsdellite-MnO2 framework remains intact on lithiation at 70-degrees-C, the hexagonally-close-packed oxygen array buckles towards a cubic-close-packed structure to accommodate the inserted lithium ions. The reaction is reversible but the instability of the structure on cycling limits the utility of ramsdellite-MnO2 as a rechargeable electrode in lithium cells. The ramsdellite structure can be stabilized by reaction with LiOH or LiNO3 at 300-400-degrees-C; this reaction, which displaces manganese ions from the MnO2 framework into interstitial octahedral sites generates spinel-related domains that coexist with the lithiated ramsdellite phase. At 300-degrees-C, under vacuum, the lithiated ramsdellite phase Li0.5MnO2 transforms to the spinel LiMn2O4; at 300-400-degrees-C, in air, it oxidizes slowly and transforms to a defect spinel LiMn2O4+delta (0 < delta less-than-or-equal-to 0.5) via an intermediate compound. A mechanism for the ramsdellite-spinel transition is proposed.