Protocols for Evaluating and Reporting Li–O2 Cell Performance - The

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Protocols for Evaluating and Reporting Li−O2 Cell Performance that we believe may facilitate overall progress by the community, and (3), at the very least, to draw attention and spark debate on the question of what parameters are essential to report. These goals are targeted specifically at the significant attention paid by the community to Li−O2 battery metrics and claims, where performanceparticularly capacity and cycling behavioris presented and judged as a primary rationale for publication and where we observe that too often the parameters and experimental description are insufficient to enable comparison with other reports. This is the problem we seek to address. Many works in the field have focused on addressing challenges surrounding the rechargeable Li−O2 system by performing mechanistic studies of oxygen reduction and evolution reactions (ORR/OER), carefully isolating and exploring the effect of the solvent or salt chemistry on the deposited reduced LiOx species, or by investigating the evolving surface chemistry at the cathode/electrolyte or anode/electrolyte interfaces.2−10 This type of fundamental study facilitated noticeable progress toward the realization of Li−O2 batteries, through the demonstration of electrolyte composition optimization, the identification of operating regimes in which electrode materials are stable, showing the utility of heterogeneous and homogeneous catalysts as well as redox mediators, and exploration of modes of controlling the composition and morphology of the deposited species. While these works are extremely valuable to the field, they do not rely heavily on performance metrics and, as such, are somewhat outside of the main scope of this Viewpoint. To demonstrate how insufficient measurement or reporting practices can lead to ambiguity in gauging true performance, we have intentionally designed and are presenting empirical examples using the Li−O2 platform in our lab with Rufunctionalized carbon nanotube cathodes, a 0.1 M LiClO4−DMSO electrolyte, and lithium metal anodes.11 Analogous configurations are commonly used in studies of Li−O2 cells.12−16 Cathode Mass Loading. While reports with nanostructured Li−O2 cathodes displaying hundreds of reversible (though typically capacity-limited) cycles are not uncommon in the literature, the experimental details often reveal extremely low mass loading of the nanostructured cathode material (sometimes less than 15 μg), resulting in very low cell currents (sometimes less than 5 μA). Due to the tiny masses, one then infers very impressive specific capacity and cyclability metrics. However, with such small loadings of cathode materials, good cycle stability is much more readily obtained than that if higher, realistic loadings were used. Furthermore, the small charge transfer involved relaxes the demand on electrode (particularly anode) stability, whether through lower current densities or shorter cycle times. Finally, small loadings are simply impractical for the proposed applications of Li−O2 batteries. A similar phenomena has been noted in the lithium−sulfur

Motivation. The lithium oxygen system, if fully harnessed in the form of a rechargeable battery, offers a tantalizing ideal energy density of 3445 W h L−1, more than doubling state-of-the-art Li ion technology.1 An impressive amount of recent research has attacked this system, and despite its complexity, there now exists a menagerie of different rechargeable Li−O2 battery chemistries and electrode architectures exhibiting some level of promise. However, the system still presents a multitude of unsolved fundamental issues, mostly related to the electrochemical stability of the cathode, electrolyte, and Li metal anode in the exceptionally harsh cell environment, which are often specific to the exact combination of materials utilized. A natural consequence is that drawing comparisons between different Li−O2 battery configurations is intrinsically very difficult. Here we highlight what we consider a significant additional impediment to progress in Li−O2 battery research, the lack of rigorous, accepted protocols for reporting cell performance within the research community. The Li−O2 system is unusual in that the lithium storage reactions are surface-mediated, catalytic processes at a three-phase boundary, where dissolved O2 and Li salt in liquid electrolyte produce solid Li oxides attached to surfaces upon discharge and must produce the reverse reaction upon charge. This raises critical questions regarding which materials are electrochemically active and therefore which masses should be taken into account in reporting gravimetric, volumetric, and areal capacities. It is not even clear that mass is the most relevant metric to use given the surface-mediated nature of the chemistry, though we acknowledge that the electrode surface area is more difficult to measure accurately. In addition, while ostensibly the purpose of cycling data is to demonstrate progress toward a realistic and usable Li−O2 battery, the operating conditions (most frequently the cathode loading) are often so outside of practical conditions that the relevance of the data is questionable. Most critically, reports on Li−O2 cells and half-cells frequently lack a comprehensive description of the cell conditions and cycling parameters used, rendering any comparison to analogous systems extremely difficult or impossible. Ambiguous methods for reporting current and capacity are also a major issue in the field. These metrics are sometimes normalized (i.e., mA h g−1 or mA g−1) without a specific description of the mass used for normalization (scaffold material, catalyst, or both). Under these circumstances, the field is in dire need of increased care from authors in reporting device performance and rigorous evaluation by reviewers and the scientific community in comparing and evaluating reported results. While there may be debate on how to normalize electrochemical efficiencies to material mass, area, and volume, we believe and assert that progress in the field will, at the very least, benefit from reporting some standard set of experimental parameters so that meaningful comparisons between published reports can be made. Our goals here are (1) to illustrate why limited reporting of parameters confounds interpretation and progress in the field, (2) to suggest a set of parameters/metrics © 2016 American Chemical Society

Published: January 21, 2016 211

DOI: 10.1021/acs.jpclett.5b02613 J. Phys. Chem. Lett. 2016, 7, 211−215

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The Journal of Physical Chemistry Letters system, which has led to calls for increased care by researchers in reporting the true mass loading of sulfur in their studies.17,18 To demonstrate the effect of cathode mass loading on cell performance metrics, we cycled cells under the same specific current (100 mA g−1electrode) and specific capacity (1000 mA h g−1electrode) using different mass loadings for the same nanostructured cathode. The actual mass of cathode material has a dramatic effect on the cycling stability of the cell, as seen in Figure 1. More specifically, decreasing the mass loading of

the scaffold interface and dissolution of oxygen for sustaining healthy ORR will become a mass-transportlimited process, resulting in a drop of the cell potential and a lower specific capacity during discharge.19,20 4. Given a limited (or finite) solubility of intermediates or reduced oxygen species (solvent- and salt-dependent),4,5 the use of low cathode mass for a constant volume of electrolyte will produce a lower concentration of these species and preserve healthier ORR/OER longer with a low current density due to a lower concentration of dissolved species (2 orders of magnitude lower concentration if using 10 μg versus 1 mg), which make the system’s cycle stability look better compared to the case of high cathode mass loading. This example suggests that research reports should include actual mass and current values to allow the reviewer and reader to properly understand whether or not the results can be compared to systems with significantly different loadings. Furthermore, it underscores the importance of cathode mass loading as a critical parameter controlling overall cell performance. Current Collector Structure. Though often overlooked, the current collector in Li−O2 batteries can have a major impact on the reported cell performance. Carbon-based gas diffusion layers (GDLs) are frequently used as the current collector for nanostructured cathodes. They have been shown to actively participate in the ORR during discharge by (1) reducing the effective current density on the O2 cathode scaffold and (2) introducing additional discharge capacity as compared to electrochemically inactive current collectors, but their contribution to cycling capacity under the actual cycling conditions is rarely disclosed.21 As an example, we galvanostatically cycled two of the same nanostructured CNT@Ru cathodes with the same loading amount (0.100 mg of electrode) under the same specific capacity (1000 mA h g−1electrode) and specific current (100 mA g−1electrode) to compare the effect of two different commonly used current collectors (stainless steel (SS) mesh and a Toray carbon paper GDL (TGP-H-030)). The performance difference between the GDL and SS current collectors is clearly demonstrated in Figure 2, where the Li−O2 cell with the GDL current collector exhibits extended cycling stability compared to

Figure 1. Discharge−charge profile with the CNT@Ru cathode to the same specific capacity (1000 mA h g−1electrode) and under the same specific current of 100 mA g−1electrode with cathodes that weigh 1.17 (blue) and 0.092 mg (red).

the cathode by ∼10× improved the cycle life by ∼8×. This effect can be explained in terms of several factors: 1. As recently demonstrated, the cycling longevity of a full cell may be limited by the chemical stability of the anode−electrolyte interface.11 It follows that a low mass loading of the cathode reduces the current density on the anode surface, reducing the magnitude of parasitic side reactions at the anode−electrolyte interface (due to lower overpotential during plating and an SEI stable enough to accommodate low currents), and promoting longer term stability. 2. Under conditions for which the total cell current is low (small mass loading on the cathode), the inherent parasitic reaction currents may occupy a larger portion of the actual current. Thus, only a relatively small portion of current from healthy ORR and OER will be required to satisfy electrochemical testing conditions, leading to the appearance of a more stable system. This concern is particularly relevant when a mediating agent with electrochemical activity at voltages similar to those of ORR/OER is used in excess in the cell. 3. The ratio between the amount of dissolved oxygen in the electrolyte and the active surface of the cathode scaffold is dictated by the electrolyte loading volume and the mass of the scaffold. By significantly lowering the mass of the scaffold and incorporating a large excess of electrolyte, both the absolute quantity of dissolved oxygen and its ability to access the entire cathode surface are improved. However, in the case of higher cathode mass loading, diffusion of unreacted oxygen to

Figure 2. Cycling profile of the CNT@Ru cathode with two different current collectors (GDL in red and SS mesh in blue). 212

DOI: 10.1021/acs.jpclett.5b02613 J. Phys. Chem. Lett. 2016, 7, 211−215

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The Journal of Physical Chemistry Letters

values of apparent Coulombic efficiency. However, this method can require high charging voltages, resulting in electrolyte decomposition and corrosion of the cathode scaffold, as previously demonstrated.7 Some reports address these side reactions by setting a voltage window (i.e., enforcing voltage limits) for cycling based on the electrochemical stability of the cell components. Under these conditions, the observed behavior is a much more accurate portrayal of the level of healthy OER/ORR reversibility. However, such cycling plots can still be misleading with regards to ORR/OER reversibility as a result of the mechanisms illustrated above, ambiguous cathode scaffold utilization when cycling at low capacities, and use of a current collector with a huge excess of background capacity. For example, in a given experimental setup with a 100 μg cathode and a combined (cathode plus current collector) discharge capacity of 4 mA h, the normalized first full discharge capacity is 40 000 mA h g−1cathode. If the fixed capacity for cycling is set to 400 mA h g−1cathode, the cell should conceivably achieve 100 cycles even if the discharge process is completely irreversible. If the mass of the cathode is 10 μg, more than 500 cycles without real reversibility can be obtained. In many cases, when apparent reversibility is achieved, it becomes extremely challenging to differentiate between demonstration of healthy cycling behavior and segmented partial cathode scaffold utilization per “cycle”. Recommendations. Appropriate cycling of Li−O2 cells can take months of operation, and collection of results and the number of cells available for testing may become a barrier for Li−O2 researchers. We suggest that one method of addressing this issue is to include a full discharge with the lower potential limit at current rates, consistent with regular long-term cycling. With the known total discharge capacity, an appropriate longterm cycling capacity can be selected, and more accurate statements can be made about reversibility without concerns of measuring segmented cathode discharge capacity, rather than real reversibility. We believe that the lack of an accepted methodology for reporting critical parameters and measurements in Li−O2 batteries continues to be an impediment to progress. Accordingly, we suggest the following guidelines for researchers and reviewers alike, to be taken as a starting point for an evolving consensus in the Li−O2 research community.

SS. As shown in Figure 3, when we discharged a GDL current collector without active material at a cell current of −10 μA, we

Figure 3. Cycling profile of two different current collectors. The SS mesh (blue) was cycled 100 times in the displayed duration, and the Toray carbon paper discharged continuously for over 12 days under the same conditions. SEM images show the evolution of the reduction product morphology on the carbon paper surface discharged to 1.6 mA h.

obtained a surprisingly large capacity contribution (∼4.25 mA h!) from the GDL, showing that it plays a profound role in contributing to overall capacity. Furthermore, pristine and postmortem SEM images in Figure 3 reveal clearly the formation of reduction products on the surface of the GDL. It is clear that the use of a GDL as a current collector for low masses of nanoscale cathode materials can contribute a significant background capacity, dramatically misleading the interpretation of cell performance. The impact of the GDL current collector on cell performance can be attributed to additional active reduction sites available for ORR on its surface. This effect is particularly evident at lower total cell currents. In fact, the mass of the GDL is typically ∼5 mg/cm2, and when used as current collector for nanostructured cathodes, it may well exceed the mass of the nanomaterial by 10×−100×. The actual thickness of deposited Li2O2, under currents normalized to the nanostructured cathode mass, will hence be significantly smaller and will lead to higher reversibility,19,20 potentially due to the use of active current collector and not necessarily as a result of a highly reversible cathode scaffold. On the basis of these considerations, the use of GDLs and other carbon-based current collectors introduces significant ambiguity with respect to discerning the electrochemical performance of different cell components. This highlights the importance of reporting the composition and structural configuration of the current collector, along with its electrochemical activity under relevant testing conditions. Electrochemical Test Methodology. Different techniques for electrochemical testing of Li−O2 cells can also lead to ambiguity. Galvanostatic cycling is one of the most common testing methods to gauge long-term cycling stability and reversibility of new cathode scaffolds, but such evaluations depend on the cycling conditions chosen for discharge and charge. One commonly used approach is cycling to a fixed discharge/charge capacity with an open voltage window, which enables complete recovery of the discharge capacity and high

1. Always report the actual values of the parameters used, not only normalized values, which include: • the actual cathode mass and the mass of catalyst loading if relevant (in grams, not normalized to area or other parameters) • actual currents used in the testing (not normalized to the carbon mass or to surface area) • the actual cycle duration (relevant for stability of the electrode/electrolyte interfaces) • the actual electrolyte volume • if possible, the surface area of the electrode through simple electrical double layer capacity in an Ar atmosphere and the same Li salt. 2. Avoid the use of inappropriate cell components: • Avoid the use of carbon-based current collectors or others that may be electrochemically active. • If using an electrochemically active current collector, include a discharge capacity measurement with no active material at the relevant current that you use for testing your system (actual current used for cycling). 213

DOI: 10.1021/acs.jpclett.5b02613 J. Phys. Chem. Lett. 2016, 7, 211−215

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The Journal of Physical Chemistry Letters

(3) Freunberger, S. A.; Chen, Y.; Drewett, N. E.; Hardwick, L. J.; Bardé, F.; Bruce, P. G. The Lithium-Oxygen Battery with Ether-Based Electrolytes. Angew. Chem., Int. Ed. 2011, 50 (37), 8609−8613. (4) Gittleson, F. S.; Sekol, R. C.; Doubek, G.; Linardi, M.; Taylor, A. D. Catalyst and Electrolyte Synergy in Li-O2 Batteries. Phys. Chem. Chem. Phys. 2014, 16 (7), 3230−3237. (5) Johnson, L.; Li, C.; Liu, Z.; Chen, Y.; Freunberger, S. a.; Ashok, P. C.; Praveen, B. B.; Dholakia, K.; Tarascon, J.-M.; Bruce, P. G. The Role of LiO2 Solubility in O2 Reduction in Aprotic Solvents and Its Consequences for Li−O2 Batteries. Nat. Chem. 2014, 6, 1091−1099. (6) Lu, Y.-C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y. Lithium-Oxygen Batteries: Bridging Mechanistic Understanding and Battery Performance. Energy Environ. Sci. 2013, 6 (3), 750−768. (7) Ottakam Thotiyl, M. M.; Freunberger, S. a.; Peng, Z.; Bruce, P. G. The Carbon Electrode in Nonaqueous Li-O2 Cells. J. Am. Chem. Soc. 2013, 135 (1), 494−500. (8) Sharon, D.; Etacheri, V.; Garsuch, A.; Afri, M.; Frimer, A. a.; Aurbach, D. On the Challenge of Electrolyte Solutions for Li-Air Batteries: Monitoring Oxygen Reduction and Related Reactions in Polyether Solutions by Spectroscopy and EQCM. J. Phys. Chem. Lett. 2013, 4 (1), 127−131. (9) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar, G.; Luntz, a. C. Solvents Critical Role in Nonaqueous Lithium-Oxygen Battery Electrochemistry. J. Phys. Chem. Lett. 2011, 2 (10), 1161− 1166. (10) Lu, Y. C.; Gasteiger, H. a.; Shao-Horn, Y. Catalytic Activity Trends of Oxygen Reduction Reaction for Nonaqueous Li-Air Batteries. J. Am. Chem. Soc. 2011, 133 (47), 19048−19051. (11) Schroeder, M. A.; Pearse, A. J.; Kozen, A. C.; Chen, X.; Gregorczyk, K.; Han, X.; Cao, A.; Hu, L.; Lee, S. B.; Rubloff, G. W.; et al. Investigation of the Cathode−Catalyst−Electrolyte Interface in Aprotic Li−O 2 Batteries. Chem. Mater. 2015, 27 (15), 5305−5313. (12) Sun, B.; Munroe, P.; Wang, G. Ruthenium Nanocrystals as Cathode Catalysts for Lithium-Oxygen Batteries with a Superior Performance. Sci. Rep. 2013, 3, 2247. (13) Jung, H. G.; Jeong, Y. S.; Park, J. B.; Sun, Y. K.; Scrosati, B.; Lee, Y. J. Ruthenium-Based Electrocatalysts Supported on Reduced Graphene Oxide for Lithium-Air Batteries. ACS Nano 2013, 7 (4), 3532−3539. (14) Yilmaz, E.; Yogi, C.; Yamanaka, K.; Ohta, T.; Byon, H. R. Promoting Formation of Noncrystalline Li2O2 in the Li-O2 Battery with RuO2 Nanoparticles. Nano Lett. 2013, 13 (10), 4679−4684. (15) Jian, Z.; Liu, P.; Li, F.; He, P.; Guo, X.; Chen, M.; Zhou, H. Core-Shell-Structured CNT@RuO2 Composite as a High-Performance Cathode Catalyst for Rechargeable Li-O2 Batteries. Angew. Chem., Int. Ed. 2014, 53 (2), 442−446. (16) Nasybulin, E. N.; Xu, W.; Mehdi, B. L.; Thomsen, E.; Engelhard, M. H.; Massé, R. C.; Gu, M.; Bennett, W.; Nie, Z.; Wang, C.; et al. Formation of Interfacial Layer and Long-Term Cyclability of Li-O2 Batteries Formation of Interfacial Layer and Long-Term Cyclability of Li-O2 Batteries. ACS Appl. Mater. Interfaces 2014, 6 (16), 14141− 14151. (17) McCloskey, B. D. Attainable Gravimetric and Volumetric Energy Density of Li-S and Li Ion Battery Cells with Solid SeparatorProtected Li Metal Anodes. J. Phys. Chem. Lett. 2015, 6, 4581. (18) Hagen, M.; Hanselmann, D.; Ahlbrecht, K.; Maça, R.; Gerber, D.; Tübke, J. Lithium-Sulfur Cells: The Gap between the State-of-theArt and the Requirements for High Energy Battery Cells. Adv. Energy Mater. 2015, DOI: 10.1002/aenm.201401986. (19) Griffith, L. D.; Sleightholme, A. E. S.; Mansfield, J. F.; Siegel, D. J.; Monroe, C. W. Correlating Li/O 2 Cell Capacity and Product Morphology with Discharge Current. ACS Appl. Mater. Interfaces 2015, 7 (14), 7670−7678. (20) Gallant, B. M.; Kwabi, D. G.; Mitchell, R. R.; Zhou, J.; Thompson, C. V.; Shao-Horn, Y. Influence of Li2O2Morphology on Oxygen Reduction and Evolution Kinetics in Li−O2 Batteries. Energy Environ. Sci. 2013, 6 (8), 2518.

3. Always report and show the voltage profile of full discharge until the cell voltage drop, followed by subsequent charge to an appropriate fixed voltage below the anodic oxidation potential of the electrolyte. It is recommended to show the cycle life of the cell under these conditions until loss of 70% of the discharge capacity. Always report the first full discharge capacity of the electrode under the lowest actual currents that you used with the same current collectors. 4. Voltage profiles should be presented (V versus t) for each rate and capacity reported in the paper. This is particularly important in the case of high surface area carbons to illustrate the contribution of double layer capacitance (can be as high as 30−50% for low mass loading cycled at low capacities). 5. GV cycling under Ar with the same current values used for cycling should be clearly presented, perhaps as a supplement. This will provide a clear presentation of the double layer capacity at low currents and the electrochemical contribution of background reactions such as decomposition of cell components from processes unrelated to oxygen.

Malachi Noked*,§,⊥ Marshall A. Schroeder†,⊥ Alexander J. Pearse† Gary W. Rubloff† Sang Bok Lee*,§ §

Department of Chemistry and Biochemistry and Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States †

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.B.L.). *E-mail: [email protected] (M.N.). Author Contributions ⊥

M.N. and M.A.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DESC0001160. We acknowledge the support of the Maryland Nanocenter and its AIMLab. M.A.S. acknowledges a graduate fellowship through the John and Maureen Hendricks Charitable Foundation. We acknowledge Nolan Ballew for his valuable contribution in design and fabrication of the electrochemical cells.

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REFERENCES

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The Journal of Physical Chemistry Letters (21) Geaney, H.; O’Connell, J.; Holmes, J. D.; O’Dwyer, C. On the Use of Gas Diffusion Layers as Current Collectors in Li-O2 Battery Cathodes. J. Electrochem. Soc. 2014, 161 (14), A1964−A1968.

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