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 secondary 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 organic solvents, ethers such as tetrahydrofuran and dimethoxyethane were readily oxidized to produce acids whereas carbonates (ethylene carbonate, propylene carbonate, diethylcarbonate) were relatively inert. Consequently, when a spinel-loaded composite cathode was charge/discharge cycled in the potential range of 3.6 to 4.3 V (vs. Li/Li+), both the acid concentration and the extent of spinel dissolution was much higher in the ether-containing electrolytes as compared to the carbonates. The results, obtained from the chemical analysis on acid-attacked spinel powders and from the open-circuit potential measurement of composite cathodes, indicated that Li and Mn ion extraction is dominant in the earlier stage of acid attack. As the spinel dissolution further continues, 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 solvent-derived acid generation was not significant in those electrolytes containing fluorinated salts (LiPF6, LiBF4, and LiAsF6), yet the spinel dissolution in these electrolytes was still appreciable because acids were generated via another pathway; a reaction between the F-containing anions and impurity water.