ELECTROLYTE EFFECTS ON SPINEL DISSOLUTION AND CATHODIC CAPACITY LOSSES IN 4 V LI LIXMN2O4 RECHARGEABLE CELLS/

Spinel dissolution and cathodic capacity losses in 4 V Li/LixMn2O4 sec ondary cells were examined in various electrolyte solutions comprising different solvents and Li salts. It was found that spinel dissolution is induced by acids that are generated as a result of electrochemical oxidation of solvent molecules on composite cathodes. Among various o rganic solvents, ethers such as tetrahydrofuran and dimethoxyethane we re readily oxidized to produce acids whereas carbonates (ethylene carb onate, propylene carbonate, diethylcarbonate) were relatively inert. C onsequently, when a spinel-loaded composite cathode was charge/dischar ge cycled in the potential range of 3.6 to 4.3 V (vs. Li/Li+), both th e acid concentration and the extent of spinel dissolution was much hig her in the ether-containing electrolytes as compared to the carbonates . The results, obtained from the chemical analysis on acid-attacked sp inel powders and from the open-circuit potential measurement of compos ite cathodes, indicated that Li and Mn ion extraction is dominant in t he earlier stage of acid attack. As the spinel dissolution further con tinues, however, oxygen losses from the lattice become more important. The combined feature of solvent oxidation and spinel dissolution was also affected by the nature of lithium salts added. Generally, the sol vent-derived acid generation was not significant in those electrolytes containing fluorinated salts (LiPF6, LiBF4, and LiAsF6), yet the spin el dissolution in these electrolytes was still appreciable because aci ds were generated via another pathway; a reaction between the F-contai ning anions and impurity water.