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 dimet hylsulfone at 150-degrees-C. Two redox steps are evidenced: in the com position 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 2.9 5 V. Comparison of the chronopotentiometric curves for gamma-MnO2, ram sdellite and rutile forms prove that these two insertion processes can not be assigned to lithium accommodation into ramsdellite and pyrolusi te units. At this temperature, the lithium insertion process into gamm a-MnO2 is found to be highly reversible on the whole lithium concentra tion range (0<x<0.95) and characterized by a high kinetics of lithium transport. D(Li) evaluated from potentiometric and voltammetric measur ements is in the range 5x10(-9) cm2 s-1. During extended galvanostatic experiments performed at a discharge/charge rate of C, a high efficie ncy is recovered since, from an initial capacity of 300 A h kg-1, abou t 180 A h kg-1 are still available after the 20th cycle.