Investigation on the charging process of Li2O2-based air electrodes in Li-O2 batteries with organic carbonate electrolytes

The charging process of Li₂O₂-based air electrodes in Li-O₂ batteries with organic carbonate electrolytes was investigated using in situ gas chromatography/mass spectroscopy (GC/MS) to analyze gas evolution. A mixture of Li₂O₂/Fe₃O₄/Super P carbon/polyvinylidene fluoride (PVDF) was used as the starting air electrode material, and 1-M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in carbonate-based solvents was used as the electrolyte. We found that Li₂O₂ was actively reactive to 1-methyl-2-pyrrolidinone and PVDF that were used to prepare the electrode. During the first charging (up to 4.6 V), O₂ was the main component in the gases released. The amount of O₂ measured by GC/MS was consistent with the amount of Li₂O₂ that decomposed during the electrochemical process as measured by the charge capacity, which is indicative of the good chargeability of Li₂O₂. However, after the cell was discharged to 2.0 V in an O₂ atmosphere and then recharged to ∼4.6 V, CO₂ was dominant in the released gases. Further analysis of the discharged air electrodes by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy indicated that lithium-containing carbonate species (lithium alkyl carbonates and/or Li₂CO₃) were the main discharge products. Therefore, compatible electrolytes and electrodes, as well as the electrode-preparation procedures, need to be developed for rechargeable Li-air batteries for long term operation.