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The nature of Li2O2 oxidation in a Li-O2 battery revealed by operando X-ray diffraction Swapna Ganapathy, Brian D. Adams, Georgiana Stenou, Maria S. Anastasaki, Kees Goubitz, Xue-Fei Miao, Linda F. Nazar, and Marnix Wagemaker J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja508794r • Publication Date (Web): 23 Oct 2014 Downloaded from http://pubs.acs.org on October 26, 2014
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Journal of the American Chemical Society
The nature of Li2O2 oxidation in a Li-O2 battery revealed by operando Xray diffraction
Swapna Ganapathy1, Brian D. Adams2, Georgiana Stenou1, Maria S. Anastasaki1, Kees Goubitz1, Xue-Fei Miao1, Linda F. Nazar*2 and Marnix Wagemaker*1
1
Fundamental Aspects of Materials and Energy,
Department of Radiation Science and
Technology, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands 2
Department of Chemistry and the Waterloo Institute for Nanotechnology, University of
Waterloo, Waterloo, Ontario, N2L 3G1, Canada
*
[email protected];
[email protected] 1 ACS Paragon Plus Environment
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Abstract Fundamental research into the Li-O2 battery system has gone into high gear, gaining momentum due its very high theoretical specific energy. Much progress has been made towards understanding the discharge mechanism, but the mechanism of the oxygen evolution reaction (OER) on charge (i.e., oxidation) remains less understood. Here, using operando X-ray diffraction we show that oxidation of electrochemically generated Li2O2 occurs in two stages, but in one step for bulk crystalline (commercial) Li2O2, revealing a fundamental difference in the OER process depending on the nature of the peroxide. For electrochemically generated Li2O2, oxidation proceeds first through a non-crystalline lithium peroxide component; followed at higher potential by the crystalline peroxide via a Li deficient solid solution (Li2-xO2) phase. Anisotropic broadening of the X-ray Li2O2 reflections confirms a platelet crystallite shape. Based on the evolution of the broadening during charge we speculate that the toroid particles are deconstructed one platelet at a time, starting with the smallest sizes that expose more peroxide surface. In the case of in-situ charged bulk crystalline Li2O2, the Li vacancies preferentially form on the interlayer position (Li1), which is supported by first-principle calculations and consistent with their lower energy compared to those located next to oxygen (Li2). The small actively oxidizing fraction results in a gradual reduction of the Li2O2 crystallites. The fundamental insight gained in the OER charge mechanism and its relation to the nature of the Li2O2 particles is essential for the design of future electrodes with lower over-potentials, one of the key challenges for high performance Li-air batteries.
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Introduction Research into non-aqueous Li-air/Li-O21,2 batteries has exploded over the past few years.3-5 This is primarily due to their high theoretical specific energy in the charged state (~11,500 Wh/kg),6 making them extremely attractive for electric automobiles. The electrochemical processes that
→ Li2O2 . Several drive this battery are represented by the total chemical reaction: 2Li + O2 ← bottlenecks that impede the functioning of this battery system need to be addressed before it can become viable. These include the high (dis)charge overpotential resulting in a lower round trip efficiency,2,7,8 slow kinetics, electrolyte instability (side product formation)9-12 leading to poor cyclability, and the requirement of high purity O2. Of fundamental importance is the understanding of the mechanism of lithium peroxide formation and oxidation and the governing factors. Over the past years there has been significant progress in the understanding of the Li2O2 formation process during discharge. Clear correlations have been established between the solvent donor number,13 discharge voltage, current density14,15 and composition of the gas diffusion electrode on the morphology of Li2O2 formed and the mechanism of their formation, be it via solution14 or on the electrode surface.10 But one of the many challenges of the Li-O2 system includes the mechanism of Li2O2 oxidation which remains less well understood, with the elusive LiO2 superoxide intermediate remaining experimentally very difficult to observe. There are only a few inconclusive reports of superoxide observation both in- and ex-situ.15-17 Modeling approaches suggest a Li deficiency driven solid solution reaction resulting in the formation of Li2xO2
18
or that smaller crystallites would decompose first at lower potentials.19 Experimentally, it
has been suggested that amorphous Li2O2 decomposes first at lower potentials20 whereas a solid solution reaction has not been observed.
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In this article, we combine operando X-ray diffraction (XRD), Rietveld refinement, calculations, and on-line electrochemical mass spectrometry (OEMS) to elucidate the mechanism of the oxidation evolution reaction, comparing electrochemically generated Li2O2 (E-Li2O2) and bulk crystalline (commercial) Li2O2 (C-Li2O2) during the charge reaction in a Li-O2 cell. A clear difference is observed between the oxidation of E-Li2O2 and C-Li2O2 which can be explained by the difference in the nature of the particles and crystallites. The OER mechanism, however, appears similar for both E-Li2O2 and C-Li2O2 as Rietveld refinement of the operando data reveals Li deficiency in both cases indicating that the OER takes place via a solid solution reaction. Materials and Methods Operando XRD cell: A Li-O2 cell that allows X-ray diffraction measurements to be conducted in reflection mode during battery cycling was designed and custom built. The top half of the cell is comprised of a Kapton® window with a diameter of 20 mm enclosed in a stainless steel casing with a small inlet to allow the intake of O2 gas. An Al mesh current collector was integrated into this top half. The bottom half consisted of a stainless steel coin attached to a spring, also enclosed in stainless steel, akin to that of a typical Swagelok® cell design. This formed the current collector on the anode side. The top and bottom halves of the cell were separated with a Teflon® spacer to prevent a short circuit, after which they were clamped together from the outside with a non-conductive clamp. The battery stack was assembled within, with the cathode on the side of the Kapton® window. The schematic is shown in the supporting information section (Figure S1). Electrochemistry: Gas diffusion electrodes (cathodes) were fabricated by casting a mixture of activated carbon (Kuraray Chemical) and a lithiated Nafion® binder14,21 on carbon paper (Spectracarb). The cathodes were dried at 100°C for 24 hours to remove all surface adsorbed
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water, after which discs of 16 mm were punched from the sheets. The preloaded Li2O2 cathodes were made by combining Vulcan XC72 carbon (Cabot Corp.), Li2O2 (Sigma-Aldrich, 90%) and PTFE with a weight ratio of 4:1:1 in 2-propanol and casting the mixture on Toray carbon paper (TGP-H-030, Fuel Cell Store). This was carried out in an argon filled glove box (H2O and O2 content < 1 ppm). These preloaded Li2O2 electrodes were dried under vacuum at room temperature to remove the 2-propanol solvent. The electrolyte consisted of a solution of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Aldrich) dissolved in dried and distilled tetraethylene glycol dimethyl ether (TEGDME,
