Spectroscopic Characterization of Solid Discharge Products in Li-Air Cells with Aprotic Carbonate Electrolytes

Raman, infrared, and X-ray photoelectron spectroscopies were used to characterize the thick coating of reaction products on carbon and MnO2-coated carbon cathodes produced during discharge of Li-air cells. The results show that neither Li2O2 nor Li2O are major components of the insoluble discharge products; instead, the products are composed primarily of fluorine, lithium, and carbon, with surprisingly little oxygen. The complex reaction chemistry also appears to involve the formation of ethers or alkoxide products at the expense of the carbonate solvent molecules (ethylene carbonate and dimethyl carbonate). The irreversible discharge reaction is likely electrochemically promoted with Li+, anions, and dissolved oxygen. Exactly how the molecular O-2 participates in the reaction is unclear and requires further study. The addition of a conformal coating of MnO2 on the carbon lowers the cell's operating voltage but does not alter the overall discharge chemistry.