RECHARGEABLE GAMMA-MNO2 FOR LITHIUM BATTERIES USING A SULFONE-BASED ELECTROLYTE AT 150-DEGREES-C

The electrochemical lithium intercalation in gamma-MnO2 as well as in the ramsdellite and pyrolusite phases has been studied in molten dimethylsulfone at 150-degrees-C. Two redox steps are evidenced: in the composition range 0<x<0.35-0.4, a slow decrease of the potential (3.8-3V) is observed while for 0.35-0.4<x<0.9 a voltage plateau appears at 2.95 V. Comparison of the chronopotentiometric curves for gamma-MnO2, ramsdellite and rutile forms prove that these two insertion processes cannot be assigned to lithium accommodation into ramsdellite and pyrolusite units. At this temperature, the lithium insertion process into gamma-MnO2 is found to be highly reversible on the whole lithium concentration range (0<x<0.95) and characterized by a high kinetics of lithium transport. D(Li) evaluated from potentiometric and voltammetric measurements is in the range 5x10(-9) cm2 s-1. During extended galvanostatic experiments performed at a discharge/charge rate of C, a high efficiency is recovered since, from an initial capacity of 300 A h kg-1, about 180 A h kg-1 are still available after the 20th cycle.