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.

Research highlights▶ Use in situ GC/MS to investigate gas evolution during charging process. ▶ Li2O2 is chargeable to release O2 in high conversion. ▶ Li2O2 reacts with NMP and PVDF, but seems not to react with carbonate solvents. ▶ Carbonate species are formed during discharge in carbonate electrolytes. ▶ Nearly no Li2O2 is formed during discharge in carbonate electrolytes.