Dissolution of Spinel Oxides and Capacity Losses in 4 V Li / Li x Mn2 O 4 Cells

Dissolution of spinel manganese oxides and the concomitant cathodic capacity losses were examined in 4 V Li/PC + \nDME + LiClO4/LiMn2O4 cells where PC is propylene carbonate and DME is dimethoxyethane. Dissolved Mn2 contents \nin the electrolytes were analyzed as a function of cathode potential and carbon contents in the composite cathodes. \nCharacteristically, manganese dissolution was notably high at the charged state (at >4.1 V vs. Li/Lit), in which potential \nrange an electrochemical oxidation of the solvent molecules was also prominent. From this and another observation \nwhereby the Mn dissolution increased with increasing carbon content in the composite cathodes, it was proposed that, at \nthe charged state of the cathode the solvent molecules are electrochemically oxidized on carbon surfaces and an as-generated \nspecies promotes the manganese dissolution. Results of an ac impedance study revealed that Mn dissolution brings \nabout an increase in contact resistances at the Mn-depleted spinel/carbon interface, and also in the electrode reaction \nresistances for Li intercalation/deintercalation. Thus, the Mn dissolution causes capacity losses in two different pathways; \nmaterial loss of the loaded spinel and polarization loss due to a cell resistance increment. The former prevailed \nwhen cathodes contained excess amounts of carbon, while the latter became more of a problem as the carbon contents \ndecreased.