Water Effect on the Specific Capacity of Aprotic Li-O2 Cells in a Sealed Two-Compartment Cell
Authors: Schwenke K. Uta , Metzger Michael , Restle Tassilo , Gasteiger Hubert A. 
Published: February 2020

Lithium-air batteries are one of the most promising future electric energy storage systems to power long range full electric vehicles due to the very large theoretical capacity of the oxygen cathode. Assuming Li2O2 as the main discharge product, a theoretical specific energy density of 2450 Wh per kg discharged cathode material can be gained compared to 620 Wh per kg for the standard lithium ion cathode material LiCoO2[1].
Though, the real discharge capacity of the oxygen cathode strongly depends on the morphology of the electrically conductive support material on which the discharge product Li2O2 precipitates. Hence, we found, that the discharge capacity of the first cycle is directly proportional to the external surface area and limited to 1-2 monolayers of Li2O2 [2]. Only if we are using electrolytes based on substances which are reactive towards oxygen reduction reaction intermediates (e.g., superoxide anion radicals) like propylene carbonate [3], as-received tetraglyme, which contains reactive impurities from the production, or anodically degraded glymes [4], we exceed this limit. Further, it seems to be possible to achieve large discharge capacities without reaction and destruction of cell components if the chosen discharge rate is extremely low [5], which is however also not a solution for real application.
In our group we could obtain very large specific capacities at high discharge rates if we were using water saturated oxygen [3]. As Li2O2 reacts with water to H2O2 and LiOH we expected LiOH to be a major discharge product in these cells. However, LiOH was neither detected by infrared spectroscopy nor by x-ray diffraction but huge amounts of Li2O2. Thus, we believe that water can be used to enhance the solubility of Li2O2 in aprotic solvents so that Li2O2 can recrystallize on the conductive support which increases the discharge capacity of Li-O2 cells also at high rates.
To further study this water effect on the specific capacity of aprotic Li-O2 cells, interference by the reactivity of the lithium anode with water has to be excluded. Therefore, we developed a sealed two-compartment-cell which allows us to add water in the catholyte but not in the anolyte. Using in-situ On-line Electrochemical Mass Spectrometry (OEMS) [6] the evolution rates of discharge and charge products and their correlation with the potential in the cell is studied. Analyzing the e-/O2 ratio during discharge and charge allows identification of the major cell reactions (formation of Li2O2 2e-/O2, formation of LiOH 4e-/O2) and the effect of water on the specific capacity. The OEMS results are further correlated to quantitative Li2O2 analysis and FTIR characterization of the electrodes.

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Acknowledgments:

Support of BASF SE in the framework of its scientific network on electrochemistry and batteries is acknowledged by TUM.