Electrochemical lithium intercalation in disordered manganese oxides

Four highly disordered manganese oxides were prepared by reduction of sodium permanganate by chloride, iodide, hydrogen peroxide or oxalate in aqueous medium containing a large excess of Li+ ions, yielding hydrated oxides with Mn valence in the range 3.80-3.92. Thermogravimetric studies showed that the iodide-reduced oxide can be dehydrated to 92% at 240 degreesC, while the other three ones retain water at temperatures up to ca. 400 degreesC, where crystallization is significant. The electrochemical behaviour was studied potentiostatically and galvanostatically in lithium cells on samples dried at 240 degreesC. All samples give a unique, broad, reversible oxidation-reduction peak in the range of 2.0-3.6 V. Cycling capacities vary in decreasing order iodide > hydrogen peroxide > chloride > oxalate. The best compound, Li0.60Na0.16MnO2.33Iapproximate to0.05, has an initial capacity of > 170 Ah/kg, slowly decreasing on cycling to reach 140 Ah/kg after 70 cycles. These performances, which are far superior to those of manganese spinels, are compared to those of recently reported amorphous manganese oxi-iodides prepared in anhydrous conditions. (C) 2001 Elsevier Science B.V. All rights reserved.