Identifying Electrochemical Processes in the Lithium-Oxygen Battery by Solid State NMR Spectroscopy Michal Leskes(1), Amy J. Moore(1), Gillian R. Goward(2) and Clare P. Grey(1) (1) Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK (2) Department of Chemistry, McMaster University, 1280 Main St. W. Hamilton, Ontario, Canada , L8S 4M1 The Lithium-Oxygen battery Library of possible electrochemical products The lithium-oxygen battery is, in principle, a promising candidate for use as an energy storage system. Theoretically, it can store 3,505Whkg-1 (approaching an order of magnitude more than a conventional lithium ion battery) based on the reaction (in a nonaqueous electrolyte) of Li and O2 to form lithium peroxide (Li2O2) and including the weight of the reactants1. Detecting the 17O spectral signature of various lithiumoxygen compounds at high magnetic fields allows us to identify them when they are formed in the battery . Fitting the second-order quadrupole line shape we can determine the NMR parameters and simulate the spectra at the same conditions (magnetic field and magic angle spinning frequency). The various species are clearly distinguishable by their 17O spectra. eCharge O2 electrolyte Discharge Electrolyte decomposition products composite carbon cathode In practice the development of the battery is still at initial stages with operating cells falling short of their promising potential2. Among the challenges to be addressed are the identification of stable electrolyte systems, inert and porous cathode materials and efficient catalytic species. These can only be achieved with a careful analysis of the electrochemical products formed during the operation of the cell. Here we employ a multi-nuclear solid state NMR spectroscopy which enables us to monitor the evolution of these products during electrochemical cycling and gain insight into processes affecting capacity fading. Characterization by Solid state NMR We have recently demonstrated how solid state NMR (ssNMR) spectroscopy, in particular of the 17O nucleus, is a powerful tool in the investigation of the lithium-air battery as it allows a clear distinction between the main products formed in the cell – lithium peroxide and lithium carbonate3. The advantages of solid state NMR are: Li2O2 Allows a clear distinction between the main discharge products. Detects products formed in the bulk of the cathode as well as on the surface. Detects both crystalline and amorphous materials. carboxylic acids amino acids gas phase organic mol. 17O 17O NMR of cycled cathodes enrichment of the products Cycling the battery with 17O enriched oxygen atmosphere results in isotope enrichment of the products which can be identified and monitored during the cycle. 77% discharge Li+ Li2O2 6% 14% 3% vacuum line 84% 6% 2% partial charge lithium metal anode Li(s) + O2(g) 17O 8% 51% 22% 8% inorganic mol. Taking into account the relaxation of the different species in order to get more quantitative analysis (I=5/2) quadrupole coupling constant, Cq, is a sensitive probe to its chemical environment and can be used to uniquely identify the peroxide species. Li-O2 cell the cell is evacuated to ~0.8bar and cooled to~5C. Then filled back to ~1bar with enriched gas 18% 17O enriched O2 gas 1H ssNMR of cycled cathodes * * *pvdf 1 P.G. Bruce, S. A. Freunberger, L. J. Hardwick, J-M Tarascon, Nature Materials 2012, 11, 19. G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, The Journal of Physical Chemistry Letters 2010, 1, 2193. L. J. Hardwick, P.G. Bruce, Current Opinion in Solid State and Materials Science 2012, 16, 178. 3M. Leskes, N.E. Drewett, L.J. Hardwick, P. G. Bruce, G. R. Goward, C. P. Grey, Angewandte Chemie International Edition 2012, 24, 2880. 2 Cell design and electrochemistry 13C = (1 − ) ssNMR of cycled isotope enriched cathodes Top case (with holes) The cathode reacts with lithium peroxide upon charging forming lithium carbonate. From the second cycle carbonate accumulates on the surface. Stainless steel mesh cathode (40% C) separator +electrolyte lithium current collector spring coin cell Li213CO3 Bottom case O2 out O2 in Battery stopped at various stages, cathode extracted, washed, dried and packed in the NMR rotor - 13C cathode 1H spectra are used to monitor the evolution of lithium hydroxide and formate. 1H-6Li 2D correlations aid in filtering the pvdf signal and identifying a fragment of the DME formed at initial discharge. Indicating proximity in space between the cathode and the carbonate D1Ah/g LiOH 1M LiTFSI, DME, 70mA/g + 1H LiOCH2CH2OLi cycling HCO2Li 13C-13C 2D correlation (RFDR mixing, 20ms at 10kHz MAS) discharge charge 1H 17O 6Li Conclusions • Lithium peroxide is the main discharge product in the initial cycle in DME accompanied by non-negligible electrolyte decomposition forming lithium hydroxide, carbonate and formate. • Upon charge significant amounts of lithium peroxide decompose below 4.5V. • While the hydroxide decomposes upon charging, formate accumulates on the cathode surface. • The carbon cathode, though inert during the first discharge, is unstable in the presence of peroxide at higher voltages forming a layer of carbonate that blocks the surface. • Limiting the capacity to 1000mAh/g results in similar distribution of products with a slight decrease in the charge potential, possibly due to a thinner insulating layer of products. • We have demonstrated that a multinuclear solid state NMR approach is a powerful method for directly detecting product formation and decomposition within the cathode, a necessary step in the evaluation of new electrolytes, catalysts and cathode materials for the development of a viable lithium-air battery. Acknowledgments The UK 850 MHz Solid-State NMR Facility used in this research was funded by EPSRC and BBSRC, as well as by the University of Warwick with partial support through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF). We also thank the National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada) for access to the 900 MHz NMR spectrometer. We are grateful for Prof. Dominic Wright for his help and advice on synthesis and Johnson Matthey for discussions and partial funding of AJM. ML is an awardee of the Weizmann Institute of Science—National Postdoctoral Award Program for Advancing Women in Science and thanks the EU FP7 Marie Curie actions for a intra-European fellowship.
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