ELECTROCHEMICAL SODIUM INSERTION INTO THE SOL-GEL BIRNESSITE MANGANESE-DIOXIDE

Electrochemical sodium insertion in the sol-gel lamellar birnessite Mn O1.84, 0.64 H2O having a high preferred orientation is investigated. C hronopotentiometric and voltammetric measurements evidence two inserti on processes: the first (0 < x less-than-or-equal-to 0.35) occurs betw een 4.25 and 2.6 V and is accompanied by a significant decrease of the interlayer spacing while the second appears at 2.5 V (0.35 < x less-t han-or-equal-to 0.8) and induces no variation of the c parameter. The layered structure of the sol-gel compound allows the reversible insert ion of Na+ ions and cycling galvanostatic experiments performed at 40- degrees-C, at a discharge charge rate of C/15 show that a high specifi c capacity (140 Ah kg-1), ie 75% of the initial capacity is achieved a fter the twentieth cycle.