Subscriber access provided by ORTA DOGU TEKNIK UNIVERSITESI KUTUPHANESI
Article 2
An Electrochemical Impedance Study of the Capacity Limitations in Na-O Cells Kristian Bastholm Knudsen, Jessica E. Nichols, Tejs Vegge, Alan C. Luntz, Bryan D. McCloskey, and Johan Hjelm J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02788 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 8, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 28
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
An Electrochemical Impedance Study of the Capacity Limitations in Na-O2 Cells Kristian B. Knudsena,b, Jessica E. Nicholsb,c, Tejs Veggea, Alan C. Luntzd,e, Bryan D. McCloskeyb,c,*, Johan Hjelma,* a
Department of Energy Conversion and Storage, Technical University of Denmark, 4000 Roskilde, Denmark b
Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States
c
Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States d
SUNCAT Center for interface Science and Catalysis, Department of Chemical Engineering, Stanford University, 443 Via Ortega Stanford, California 94305-5025, United States
e
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States *
Corresponding authors: Bryan D. McCloskey,
[email protected], +1 510-642-2295; and Johan Hjelm,
[email protected], +45 46 77 58 87
Page 1 of 21 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 28
0.0 Abstract Electrochemical impedance spectroscopy, pressure change measurements, and scanning electron microscopy were used to investigate the non-aqueous Na-O2 cell potential decrease and rise (sudden deaths) on discharge and charge, respectively. In order to fit the impedance spectra from operating cells, an equivalent circuit model was used that takes into account the porous nature of the positive electrode and is able to distinguish between the electrolyte resistance in the pores and the charge-transfer resistance of the pore walls. The results obtained indicate that sudden death on discharge is caused by, depending on the current density, either accumulation of large NaO2 crystals that eventually block the electrode surface, and/or by a thin film of NaO2 forming on the cathode surface at the end of discharge, which limits chargetransfer. The commonly observed sudden rise in potential towards the end of charge may be caused by a concentration depletion of NaO2 dissolved in the electrolyte near the cathode surface and/or an accumulation of degradation products on the cathode surface.
1.0 Introduction Metal-O2 batteries have in recent years received a great deal of attention due to their high theoretical energy densities. However, Li-O2 and Na-O2 both suffer from many technical limitations. For example, in galvanostatic discharge the cell potential suddenly drops well before their theoretical energy densities (3456 Wh/kg for Li-O2 and 1105 Wh/kg for Na-O2) are achieved1. This phenomenon is generally referred to as sudden death and in Li-O2 batteries is a result of the blocking of charge transport2–5 through the main discharge product, lithium peroxide (Li2O2). Li2O2 is insoluble in most stable organic electrolytes (e.g. lithium trifluorosulfonylimide (LiTFSI) in 1,2-dimethoxyethane (DME)) and deposits as a conformal passivating film on the cathode6–9. For other Lewis acidic or basic electrolytes, e.g. dimethylsulfoxide (DMSO) or electrolytes with some H2O content, the discharge mechanism consists of both an electrochemical process forming Li2O2 conformal coatings and a (Li+ + O2-) solution mediated mechanism that forms larger toroid shaped particles6,9. Even in electrolytes in which Li2O2 deposits primarily as large toroids, ultimately poisoning of
Page 2 of 21 ACS Paragon Plus Environment
Page 3 of 28
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
the toroid growth occurs and charge transport limitations through conformally deposited Li2O2 eventually cause sudden death6,9. In Na-O2, a sudden death behavior is observed during both discharge and charge, with the charge sudden death resulting in a dramatic increase in cell potential near the end of charge. The overall Na-O2 cell chemistry has, in e.g. diglyme1,10,11 and DME12, been shown to follow Na+(solv) + O2(solv) ⇄ NaO2(s), with the forward (or backward) arrow describing discharge (or charge)1,10–12. In these systems, very large (10-50 μm) cubic crystals of NaO2 are observed upon discharge and since NaO2 also is an electronic insulator11,13, it is suggested that the large NaO2 crystals are formed by a solution mechanism11. The sudden death on discharge has been suggested to be due to O2 transport limitations due to a buildup of the large Na-O2 crystals1 or due to a blocking of the electrochemistry at the cathode/electrolyte interface due to a buildup of the insulating NaO2 crystals on the electrochemically active cathode surface10. In this paper, we investigate the charge-transfer resistance and surface capacitances in the Na-O2 battery by applying electrochemical impedance spectroscopy (EIS) to operating cells. We combine these results with pressure change measurements and scanning electron microcopy (SEM) of discharged cathodes to understand the mechanisms of sudden death that occurs on both discharge and charge.
2.0 Experimental 2.1 Materials and Cell Assembly 1.2-dimethoxyethane (BASF, DME) and sodium triflouromethanesulfonate (Sigma-Aldrich, NaOtf) were used as received. The prepared electrolytes, 0.5 M NaOtf in DME, had
