Article pubs.acs.org/JPCC
Elucidating the Oxygen Reduction Reaction Kinetics and the Origins of the Anomalous Tafel Behavior at the Lithium−Oxygen Cell Cathode Shrihari Sankarasubramanian,*,† Jeongwook Seo,† Fuminori Mizuno,‡,# Nikhilendra Singh,‡ and Jai Prakash*,† †
Center for Electrochemical Science and Engineering, Department of Chemical and Biological Engineering, Illinois Institute of Technology, 10 West 33rd Street, Chicago, Illinois 60616, United States ‡ Toyota Research Institute of North America, 1555 Woodridge Avenue, Ann Arbor, Michigan 48105, United States S Supporting Information *
ABSTRACT: Development of Li−O2 cells, potentially providing ∼3 times the capacity of Li-ion cells, depends on a fundamental understanding of the oxygen reduction reaction (ORR) at the cathode. The present study investigates the mechanism and kinetics of the oxygen reduction reaction (ORR) on a glassy carbon (GC) electrode in an oxygen saturated solution of 0.1 M lithium bistrifluoromethanesulfonimidate (LiTFSI) in 1,2-dimethoxyethane (DME) using cyclic voltammetery (CV) and the rotating ring-disk electrode (RRDE) technique. A reaction scheme considering disproportionation of LiO2 on both the cathode surface and the electrolyte bulk to form Li2O2 was proposed, and the RRDE measurements, in conjunction with an electrochemical kinetics model, were used to calculate the corresponding rate constants. The surface disproportionation reaction was found to dominate the kinetics of the ORR, and the model could explain experimental observations regarding the cell discharge products. Further, the widely reported anomalous Tafel behavior was observed over the course of these studies. Potentiostatic, point-by-point measurements of the kinetic current were carried out, and a scan rate independent evaluation of the corresponding transfer coefficient from a dimensionless CV was obtained. The measured transfer coefficient was explained invoking Marcus−Hush kinetic theory, and the solvent reorganization energy was proposed as a more comprehensive alternative to the Gutmann donor number to evaluate solvent effects on reaction kinetics. This study provides a comprehensive account of the ORR mechanism, evidence of the surface disproportionation reaction being dominant, and explains the widely reported (and previously unexplained) anomalous Tafel behavior in Li−O2 cells.
1. INTRODUCTION Lithium−oxygen cells with their high theoretical specific energy (3505 Wh kg−1) represent the future of energy storage for electric vehicle (EV) applications.1 This potential is due to the predicted pack level specific energy of Li−O2 cells (∼500−900 Wh kg−1) leading to electric vehicles with a driving range approaching ∼1100 km (680 miles) for a 400 kg pack1 as compared to automotive Li-ion cells (105 Wh kg−1 at the battery pack level) achieving a driving range of about 140 miles for the same pack weight.2 Practical realization of Li−O2 cells is impeded by problems of poor cycle life, high charge and discharge overpotentials resulting in low coulombic efficiency and low power density,3 and side reactions with N2, CO2, and H2O during ambient air operation.4,5 The poor cycle life is believed to be caused by the oxidative instability of the electrolyte6−8,50,51 together with electrode passivation due to irreversible product deposits and high overpotentials for decomposing them.9,10 There exists a consensus in the literature about Li2O2 being the final discharge product of the Li−O2 cell.7−16,28−30,44−47 © XXXX American Chemical Society
The elementary steps involved in the oxygen reduction reaction (ORR) are less understood and subject to debate. Li2O2 could conceivably be formed by the stepwise, two-electron, electrochemical reduction of O2. This mechanism has been proposed by several groups to explain their experimental observations.16,28,45 Another possible mechanism for the formation of Li2O2 is by the solution phase chemical disproportionation of LiO2 (formed by electrochemically reducing O2) yielding a net one-electron ORR. This mechanism has been invoked to explain the effect of the electrolyte solvent donor number on the ORR12,13 and to account for observations using a variety of techniques (for example, X-ray diffraction (XRD),30,47 Raman and surface enhanced Raman spectroscopy (SERS),30,47,48 electrochemical quartz crystal microbalance (EQCM),46 atomic force microscopy (AFM) 46 ). Reports using the same techniques have come to radically different conclusions Received: September 27, 2016 Revised: February 13, 2017
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DOI: 10.1021/acs.jpcc.6b09747 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Electrochemical measurements of the oxygen reduction reaction in 0.1 M LiTFSI in DME. (a) Cyclic voltammograms on a 0.196 cm2 glassy carbon (GC) working electrode at a scan rate of 50 mVs−1: (i) scans in O2 saturated and Ar saturated electrolyte solutions; (ii) scans in an O2 saturated electrolyte solution over various voltage windows. (b) Linear sweep voltammograms on a rotating ring-disk electrode with a glassy carbon disk and platinum ring in O2 saturated electrolyte. Disk currents were recorded at a scan rate of 1 mV s−1. Ring currents recorded with the Pt ring held at 3.5 V vs Li/Li+. All electrode surfaces were polished using a 0.05 μm alumina suspension after each scan to remove any surface deposition.
surface and bulk reactions is quantified. The use of binding energies of intermediates to screen surfaces for ORR activity is examined elsewhere.20,21 Second, identifying the rate-determining step for the ORR, which is key to rational catalyst development, is impeded by the observation by various authors of the varying values for the Tafel slope.12−15 The lack of a definitive understanding of the rate-determining step hinders the development of suitable catalysts that could address the major issue of cycle life in Li− O2 cells. The variation of the Tafel slope due to changes in the transfer coefficient have been previously reported for other systems.22−25 A similar analysis for the Li−O2 cell has been carried out herein and, in conjunction with the mechanism, provides a comprehensive understanding of the cathode kinetics. The identification of the rate-determining step and its proximate energy landscape, along with free energy values on various catalysts (calculated theoretically using density functional theory21), provides an important first step toward rational electrolyte selection and catalyst design and would lead to the practical realization of Li−O2 cells.
regarding the mechanism (as is evident from comparing analyses of data from SERS,45,47 AFM,46,49 or EQCM,28,46 for example). Several of these studies 45,46,49 also employ unstable50,51 DMSO as the electrolyte solvent, and the effect of this choice on the data obtained is to be examined. Thus, the complex interplay of factors such the solvent,12,13,16 salt,52,53 applied overpotential,11 and electrolyte water content5,49 makes elucidation of the Li−O2 ORR mechanism challenging. We believe classical electrochemical techniques in conjunction with the wealth of available spectroscopic data can let us understand the exact underlying reaction kinetics. Mechanisms with multiple possible parallel steps on the electrode surface and steps in the electrolyte bulk call for the application of the rotating ring-disk electrode (RRDE) technique. This has been previously used for kinetic investigations of the H+ ORR on various surfaces such as lead ruthenate pyrochlores,17 ruthenium,18 and cobalt− palladium.19 Our recent work has also demonstrated the application of this technique to examine the superoxide electrochemistry in nonaqueous systems.20 In the case of Li+ ORR, the question of formation of Li2O2 on the disk or by disproportionation in solution is addressed in the present study by examining the number of electrons involved in the reduction reaction on the disk and by measuring the amount of LiO2 that goes into the electrolyte. A novel surface disproportionation reaction is proposed. Further, the rate constants for each of the elementary steps is calculated, and the competition between the
2. MATERIALS AND METHODS The 0.1 M solution of lithium bis-trifluoromethanesulfonimidate (LiTFSI) (Aldrich) in 1,2-dimethoxyethane (DME) (Sigma-Aldrich, ReagentPlus, ≥99%) was prepared in an MBraun argon filled glovebox with H2O and O2 levels
