Effects of carbon additives on spinel dissolution and capacity losses in 4 V Li/LixMn2O4 rechargeable cells

Abstract

Carbon additives of different surface area were incorporated in LixMn2O4 composite cathodes and their effects on the extent of solvent oxidation, spinel dissolution and capacity losses in 4 V cells were investigated. The used electrolyte was 1 M LiClO4/PC:DME mixed solvent (1:1 volume ratio), where PC is propylene carbonate and DME is dimethoxyethane. The solvent molecules were electrochemically oxidized during the charging period (>4.1 V vs Li/Li+) and the solvent oxidation currents were roughly proportional to the carbon surface area. A similar proportionality was found between the carbon surface area and the extent of spinel dissolution which was assessed by analyzing Mn2+ ions in the intermittently sampled electrolyte solutions while Li/LixMn2O4 cells were repeatedly cycled in the potential range of 3.6–4.3 V (vs Li/Li+). These surface area dependent features were explained based on the previous finding that solvent molecules are oxidized on carbon surface and the spinel dissolution is facilitated by acids that are generated after solvent oxidation. Results obtained from an ac impedance study on composite cathodes suggested that the contact resistance developed at the spinel/carbon interface and the electrode reaction resistances for Li+ intercalation/deintercalation steadily increase as spinel particles are corroded by the solvent-derived acid attack. The observed spinel capacity losses were thus accounted for by two factors; material losses arising from spinel dissolution and polarization losses caused by an increment in cell resistances. The relative importance between the two seems to be determined by carbon surface area: The material loss is dominant with larger surface area carbons since spinel dissolution is severe in this case, while the polarization losses are more of a problem as carbon surface area becomes smaller.