Aprotic Li-O 2 cells have attracted considerable research interest due to its outstandingly high theoretical specific capacity. However, published discharge capacities vary considerably among different researchers despite only minor differences in the tested cell components. Some research groups observe low discharge capacities and formation of passivating layers of Li 2 O 2 on the electronically conducting cathode support, while other groups report large capacities and toroidal Li 2 O 2 crystals as discharge product. In this study we show that these differences may be due to water and protons, both possible impurities in Li-O 2 cells, having a large effect on discharge capacity and Li 2 O 2 morphology. As evidenced by XRD, FTIR and UV-visible analysis, Li 2 O 2 is still the main discharge product in Li-O 2 cells containing water, and moreover the Li 2 O 2 yield increases with the concentration of water in the electrolyte. On-line electrochemical mass spectrometry was employed to understand the differences in the discharge-charge behavior due to the addition of water and protons. While water seems to get oxidized at high potentials during charge, protons are consumed at the beginning of the discharge leading to a variety of reactive oxygen species and thus to degradation of cell components.

We present a study of the structure of Tetra-PEG model networks, using proton multiple-quantum NMR at low field in combination with computer simulations. Tetra-PEG is a novel high-performance hydrogel designed by combination of two symmetric tetra-arm macromonomers. In contrast to conventional hydrogels, which are highly heterogeneous due to fixed concentration fluctuations, Tetra-PEG exhibits a much less heterogeneous microstructure as indicated by previous light and small-angle neutron scattering studies. Here, the local-scale connectivity inhomogeneities, i.e., the sol and dangling polymer chains as well as the typical network connectivity defects resulting from the AÀB reaction of four-arm macromonomers, are quantified experimentally for the first time, studying as-prepared Tetra-PEG hydrogels, cross-linked at different polymer concentrations and stoichiometries. To this end, we developed a novel approach for the analysis of double-quantum buildup curves consisting of well-distinguishable components with different segmental dynamic order parameters, benefitting from the superb large-scale homogeneity of the samples. As a model for each component we suggest different connectivity modes between the macromonomers-mainly regular single links and double links between individual stars as well as other network defects with lower order parameters. To support the model, we report results from computer simulations with the bond-fluctuation model, which confirm the concentrationdependent trends of the network and double-link fractions.

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.

Glyme-based electrolytes were studied for the use in lithium-air batteries because of their greater stability towards oxygen reduction reaction intermediates (e.g., superoxide anion radicals (O2˙(-))) produced upon discharge at the cathode compared to previously employed carbonate-based electrolytes. However, contradictory results of glyme stability tests employing KO2 as an O2˙(-) source were reported in the literature. For clarification, we investigated the reaction of KO2 with glymes of various chain lengths qualitatively using (1)H NMR and FTIR spectroscopy as well as more quantitatively using UV-Vis spectroscopy. During our experiments we found a huge impact of small quantities of impurities on the stability of the solvents. Therefore, we studied further the influence of impurities in the glymes on the cycling behavior of Li-O2 cells, demonstrating the large effect of electrolyte impurities on Li-O2 cell performance.

Additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are commonly added to lithium-ion battery electrolytes in order to form a solid electrolyte interphase (SEI) on the anode, suppressing continuous solvent reduction. In this work, we directly compare VC and FEC by analyzing the SEI with FTIR and XPS, and the evolved gases with on-line electrochemical mass spectrometry (OEMS) in different model systems. Since both additives evolve mainly CO 2 during formation, the effect of CO 2 as an additive is compared to the addition of VC and FEC. While Li 2 CO 3 is as expected the main SEI compound found due to the added CO 2 , surprisingly no CO was detected in the gas phase of such cells. Based on FTIR, NMR and OEMS analyses of cells filled with 13 C labeled CO 2 , we suggest a mechanism explaining the beneficial effects of CO 2 and hence also of CO 2 evolving additives in lithium-ion battery cells. While the generation of polycarbonate from FEC or VC reduction is observed, the generation of Li 2 CO 3 may be as important as the generation of polycarbonate.