The instability of currently used electrolyte solutions and of the carbon support during charge-discharge in non-aqueous lithium-oxygen cells can lead to discharge products other than the desired Li2O2, such as Li2CO3, which is believed to reduce cycle-life. Similarly, discharge in an O2 atmosphere which contains H2O and CO2 impurities would lead to LiOH and Li2CO3 discharge products. In this work we therefore investigate the rechargeability of model cathodes pre-filled with four possible Li-air cell discharge products, namely Li2O2, Li2CO3, LiOH, and Li2O. Using Online Electrochemical Mass Spectrometry (OEMS), we determined the charge voltages and the gases evolved upon charge of pre-filled electrodes, thus determining the reversibility of the formation/electrooxidation reactions. We show that Li2O2 is the only reversible discharge product in ether-based electrolyte solutions, and that the formation of Li2CO3, LiOH, or Li2O is either irreversible and/or reacts with the electrolyte solution or the carbon during its oxidation.

Lithium-ion coin cells containing electrolytes with and without 1,3-propane sultone (PS) and vinylene carbonate (VC) were prepared and investigated. The electrochemical performance of the cells is correlated with ex situ surface analysis of the electrodes conducted by Fourier transform infrared and X-ray photoelectron spectroscopies and in situ gas analysis by online electrochemical mass spectrometry (OEMS). The results suggest that incorporation of both PS and VC results in improved capacity retention upon cycling at 55 °C and lower impedance. Ex situ surface analysis and OEMS confirm that incorporation of PS and VC alter the reduction reactions on the anode inhibiting ethylene generation and changing the structure of the solid electrolyte interface. Incorporation of VC results in CO2 evolution, formation of poly­(VC), and inhibition of ethylene generation. Incorporation of PS results in generation of lithium alkylsulfonate (RSO2Li) and inhibition of ethylene generation. The combination of PS and VC reduces the ethylene gassing during formation by more than 60%.

In this work we present a novel on-line electrochemical mass spectrometer design, which enables quantitative gas evolution analysis with a sealed battery design, applied to the study of the charging processes in a Li-O2 battery. Successive charge/discharge cycles were performed using Vulcan-carbon based positive electrodes in electrolytes composed of 0.2 M LiTFSI and two different solvents: i) propylene carbonate (PC), and, ii) bis(2-methoxyethyl) ether (diglyme). Results on the PC based electrolyte reveal a strong potential dependence of the evolved gaseous products which is maintained throughout subsequent cycles, consisting predominantly of O2 below 3.7 V and of predominantly CO2 above 3.7 V. The observed capacity fading is most likely caused by the gradual accumulation of discharge products which can only be oxidized at high anodic potentials. With diglyme electrolyte, the predominant gas during charging is O2. However, while the number of electrons/O2 closely corresponds to the oxidation of Li2O2 at the beginning of each charging cycle (2 e−/O2), it increases with potential and with the number of cycles, suggesting the gradual formation of other oxygen-containing discharge products which can only be oxidized at high potential with the parallel formation of CO2.

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The development of Li2S electrodes is a crucial step toward industrial manufacturing of Li-S batteries, a promising alternative to Li-ion batteries due to their projected two times higher specific capacity. However, the high voltages needed to activate Li2S electrodes, and the consequent electrolyte solution degradation, represent the main challenge. We present a novel concept that could make feasible the widespread application of Li2S electrodes for Li-S cell assembly. In this concept, the addition of redox mediators as additives to the standard electrolyte solution allows us to recover most of Li2S theoretical capacity in the activation cycle at potentials as low as 2.9 VLi, substantially lower than the typical potentials >4 VLi needed with standard electrolyte solution. Those novel additives permit us to preserve the electrolyte solution from being degraded, allowing us to achieve capacity as high as 500 mAhg(-1)Li2S after 150 cycles with no major structural optimization of the electrodes.