Electrolyte Effects on Spinel Dissolution and Cathodic Capacity Losses in 4 V Li / Li x Mn2 O 4 Rechargeable Cells

Abstract

Spinel dissolution and cathodic capacity losses in 4 V Li/LiMn204 secondary cells were examined in various electrolyte
\nsolutions comprising different solvents and Li salts. It was found that spinel dissolution is induced by acids that
\nare generated as a result of electrochemical oxidation of solvent molecules on composite cathodes. Among various organic
\nsolvents, ethers such as tetrahydrofuran and dimethoxyethane were readily oxidized to produce acids whereas carbonates
\n(ethylene carbonate, propylene carbonate, diethylcarbonate) were relatively inert. Consequently, when a spinelloaded
\ncomposite cathode was charge/discharge cycled in the potential range of 3.6 to 4.3 V (vs. Li/Li), both the acid
\nconcentration and the extent of spinel dissolution was much higher in the ether-containing electrolytes as compared to
\nthe carbonates. The results, obtained from the chemical analysis on acid-attacked spinel powders and from the open-circuit
\npotential measurement of composite cathodes, indicated that Li and Mn ion extraction is dominant in the earlier
\nstage of acid attack. As the spinel dissolution further continues, however, oxygen losses from the lattice become more
\nimportant. The combined feature of solvent oxidation and spinel dissolution was also affected by the nature of lithium
\nsalts added. Generally, the solvent-derived acid generation was not significant in those electrolytes containing fluorinated
\nsalts (LiPF5, LiBF4, and LiA5F6), yet the spinel dissolution in these electrolytes was still appreciable because acids were
\ngenerated via another pathway; a reaction between the F-containing anions and impurity water.