Influence of Hydrocarbon and CO2 on the Reversibility of Li-O2 Chemistry: An in Situ Ambient Pressure X-Ray Photoelectron Spectroscopy Study

The lithium-oxygen (Li-O₂) battery is an attractive energy storage concept because its projected gravimetric energy density is three to five times larger than Li-ion batteries.^{1, 2} Unfortunately, Li-O₂ batteries are still in the nascent stages of research and development. Among its challenges, Li-O₂ batteries suffer from low round-trip efficiency,^3-5 and limited cycle life.^{1, 2} Addressing these challenges is being hampered by the lack of fundamental understanding of the Li-O₂ reaction mechanism and coupled parasitic reactions between aprotic electrolyte, carbon, binder and Li-O₂ reaction intermediates/products.^{2, 5-8} In this study, we report unique evidence for additional parasitic reactions between common impurities in air and Li-O₂ reaction products using solid-state cells free of carbon, binder and aprotic electrolyte.⁹

Results and Discussion

The reactivity between Li₂O₂/Li₂O and surface-adsorbed hydrocarbon species as well as O₂:CO₂ was examined on Li_xV₂O₅ thin-film surface using solid-state batteries via in situ electrochemical ambient pressure X-ray photoelectron spectroscopy (APXPS) measurements.⁹ Figure 1 shows the O 1s, V 2p, and C 1s  spectra as a function of potential applied across the LLTO/LiPON/Li_xV₂O₅ cell (V_cell) during discharge and charge in p(O₂) = 5 x 10^-4 atm. The growth of Li₂O₂/Li₂O on discharge was accompanied with a moderate increase in the carboxylate (O-C=O, 289.0 eV)¹⁰ and carbonate (CO₃^2-, 290 eV)¹⁰ species and the decrease in the hydrocarbon peak (285.0 eV). The formation of carboxylate/carbonate species can be attributed to the reaction between electrochemically formed Li₂O₂/Li₂O and surface hydrocarbon species.  Possible reaction mechanisms include Li₂O₂ + C₂H₂ + O₂ -> 2(HCOOLi), Li₂O + C₂H₂ + 3/2O₂ -> 2(HCOOLi), Li₂O₂ + C₄H₆ + 2O₂ -> 2(CH₃OCO₂Li), and Li₂O + C₄H₆ + 5/2O₂ -> 2(CH₃OCO₂Li). This result highlights that carbon-containing parasitic reaction products could form from adventitious hydrocarbon and may contribute to irreversibility in Li-air batteries, which represents a fundamental challenge for designing Li-O₂batteries with long cycle life and high round-trip efficiency.

Upon initial charging, the intensity of carboxylate and carbonate components (in C 1s) were found to significantly increase by ~200 % from −0.4 to 1.6 V_cell (~1.1 to 3.1 V_Li), which was accompanied with the reduction of Li₂O₂ and Li₂O intensities (in O 1s) and significant increase of Li_xV₂O₅ intensities in the V 2p spectra. Upon further charging to 3 V_cell (~4.5 V_Li), considerable amounts of carboxylate/carbonate species remained. Our findings highlight the difficulties in removing carboxylate/carbonate species upon charging, which can explain the accumulation of carbonates during the cycling of Li-O₂ cells with aprotic electrolytes. The influence of CO₂ containments and materials design that mitigates the reactivity between Li-O₂reaction products and common impurities will be discussed.

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Figure 1. In situ APXPS data of O 1s, V 2p, and C 1s collected from the top cell surface (consisting of Li_xV₂O₅ and LiPON) of an LLTO/LiPON/Li_xV₂O₅ cell upon discharge and charge under p(O₂) = 5 x 10^-4 atm.⁹

Acknowledgments

   This work was supported in part by the MRSEC Program of the National Science Foundation under award number DMR- 0819762, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U. S. Department of Energy under con-tract no. DE-AC03-76SF00098 with the Lawrence Berkeley National Laboratory, and the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract no. DE-AC02-05CH11231. Research conducted at ORNL was supported by the U.S. Department of Energy’s Office of Basic Energy Science, Division of Materials Sciences and Engineering, under contract with UT-Battelle, LLC. E.J.C. is grateful for the financial support from the ALS Postdoctoral Fellowship Program.