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Promoting Solution Discharge of Li-O2 batteries with Immobilized Redox Mediators Zhenjie Liu, Lipo Ma, Limin Guo, and Zhangquan Peng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02798 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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The Journal of Physical Chemistry Letters

Promoting Solution Discharge of Li-O2 batteries with Immobilized Redox Mediators Zhenjie Liu,†,‡ Lipo Ma,† Limin Guo,† and Zhangquan Peng*,† †State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, China ‡University of Science and Technology of China, Hefei 230026, China

Corresponding Author * E-mail address: [email protected]

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ABSTRACT For years the aprotic Li-O2 battery suffered from a severe capacity-current trade-off that would be unacceptable for a beyond Li-ion battery. Recent fundamental study of Li-O2 electrochemistry revealed that this dilemma is caused by the growth of Li2O2 on the cathode surface and can be solved by discharging Li2O2 in the electrolyte solution. Among the strategies that can promote solution growth of Li2O2, redox mediators (i.e., soluble catalysts) demonstrate prominent performance. However, soluble redox mediators may shuttle from the cathode to the lithium anode and decompose thereon causing severe deterioration of the lithium anode and degradation of the mediators’ functionality. Here, we report that immobilized redox mediators (e.g., anthraquinone, AQ) in the form of a thin conductive polymer film (PAQ) on the cathode can effectively promote solution growth of Li2O2 even in weakly solvating electrolyte solutions that would otherwise lead to surface film growth and early cell death. The PAQ-catalyzed Li-O2 battery can deliver a discharge capacity that is up to ~ 50 times what its pristine counterpart does at the same current densities and is comparable to the capacity realized by soluble AQ-catalyzed Li-O2 batteries. Most importantly, the adverse “cross-talk” between the lithium anode and the redox mediators immobilized on the cathode has been completely eliminated. TOC GRAPHICS

KEYWORDS Li-O2 battery, capacity-current trade-off, oxygen reduction reaction, immobilized redox mediator


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The aprotic Li-air (O2) battery is a heavily touted energy storage device because of its unrivaled theoretical specific energy among all known battery chemistries.1-4 Typically, a Li-O2 battery consists of a lithium anode separated from a porous oxygen cathode by a Li+ conducting electrolyte, and its operation replies on oxygen reduction reaction (ORR) producing insulating solid Li2O2 on discharge and the reverse Li2O2 oxidation on recharge.5,6 However, current Li-O2 batteries are encountering many formidable scientific and technical challenges, among which the severe capacity-current trade-off, viz., the discharge capacity decreases drastically upon the increase in the discharge current density, is one of the most vexing issues of Li-O2 batteries and will not be acceptable for a beyond Li-ion battery.7,8 Recent fundamental study of Li-O2 electrochemistry indicated that the capacity-current trade-off is resulted from the passivation of the cathode surface by the discharge product, i.e., insulating and solid Li2O2,9,10 and could be addressed by discharging Li2O2 in the electrolyte solution because in this case the electrode surface would not be readily passivated by Li2O2 generated in solution.11,12 So far, it has been reported that solution growth of Li2O2 can be realized by discharging Li-O2 batteries at low current densities,8,13 employing high donor-number (DN) electrolyte solvents12,14 and salts,15 adding protic additives such as water16-18 and phenol,19 and most effectively introducing soluble redox mediators such as viologens,20 iron phthalocyanine,21 organic quinones,22,23 etc.24 However, these approaches, as described above, either bring about new problems or degrade quickly. For instance, low discharge current density will not make a high-power Li-O2 battery;7,8 high DN electrolyte solvents are usually susceptible to nucleophilic attack or proton abstraction by ORR intermediates and therefore less stable;11,25 protic additives obviously will react with lithium anode producing hazardous H2;16 soluble redox mediators may shuttle from


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the cathode to the lithium anode and decompose thereon causing severe deterioration of the lithium anode and degradation of the mediator’s functionality.26-28 In consequence, the grand challenge is to realize solution discharge of Li-O2 batteries containing more stable low DN electrolyte solvents without bringing any side effects to the lithium anode. Here, we report that immobilized redox mediators (e.g., anthraquinone AQ) in the form of a thin conductive polymer film (PAQ, prepared by electro-grafting of the diazonium salt of AQ on the cathode) have the ability to promote solution discharge of Li-O2 batteries containing low DN electrolyte solvents (e.g., acetonitrile MeCN and dimethoxyethane DME) without bringing any side effects to the lithium anode. It comes to our notice that pioneering studies, which employ redox polymers (e.g., polyethylenedioxythiophene) to manipulate ORR in Li-O2 batteries, have been reported,

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particularly, an organic catalyst immobilized on a polymer backbone is

designed to reduce the ORR overpotential during discharge.30 Although adopting the similar route, the present study is focused on enhancing the discharge capacity, rather than the discharge overpotential (although they are related), and further understanding how immobilized AQ dictates the discharge pathway in a low DN electrolyte. We find that on PAQ-coated electrode surface the ORR kinetics of the redox couples of O2/O2- and O2/Li2O2 are significantly enhanced. Moreover, the PAQ-catalyzed Li-O2 battery can deliver a discharge capacity that is up to ~ 50 times what its pristine counterpart does at the same current densities, and is comparable to the capacity realized by soluble AQ-catalyzed Li-O2 batteries. A range of complementary research techniques including scanning electron microscopy (SEM), x-ray diffraction (XRD), Fourier transformation infrared (FTIR) and Raman spectroscopy and differential electrochemical mass spectrometry (DEMS) have been employed to understand the solution discharge of Li-O2 batteries promoted by the immobilized redox mediators.


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Electroreduction of diazonium salt is a well-documented electrochemical reaction and has been widely used to functionalize various conductor and semi-conductor surfaces with thin polymeric films.32 Here, immobilization of the redox mediators of AQ is realized by electroreduction of the diazonium salt of AQ (i.e., 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate AQN2+BF4‾) on electrode surfaces, according to a previously described procedure.33 The detailed synthesis and characterization of AQN2+BF4‾ can be found in Supporting Information (Materials and Figure S1). The immobilized redox mediators of AQ are in the form of a thin conductive polymer film (PAQ) that preserves the redox activities of AQs and has a surface charge density of 2.63-5.56 mC cm-2areal (Figure 1a blue curve) and an ellipsometric thickness of ~ 10-20 nm, which are consistent with the results reported by other worker.33

Figure 1. CVs of the redox of (a) O2/O2- in 0.1 M TBAP MeCN and (b) O2/Li2O2 in 0.1 M LiClO4 MeCN on pristine Au and PAQ-coated Au. CVs of the PAQ-coated Au electrode under Ar are presented as blue curves in (a). The scan rate is 100 mV s-1.

The PAQ-coated Au electrode has first been examined by cyclic voltammetry (CV) to assess their electro-catalytic activity toward the redox reactions of O2/O2‾ in 0.1 M TBAClO4 MeCN and O2/Li2O2 in 0.1 M LiClO4 MeCN, respectively. For comparison CVs have also been


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conducted on a pristine Au electrode.34 For the O2/O2‾ couple, increased ORR current and decreased reduction-oxidation peak separation (Figure 1a black curve), compared with the results on the pristine electrode (Figure 1a green curve), have been observed on PAQ-coated Au electrodes, suggesting that the PAQ film can effectively catalyze, via its own redox reaction (Figure 1a blue curve), the electro-reduction of O2 to O2‾. For the redox of O2/Li2O2, much higher ORR currents have been observed on the PAQ-coated Au electrode (Figure 1b black curve) than on the pristine electrode (Figure 1b green curve). The enhanced ORR currents suggest that more Li2O2 have been produced on PAQ-coated Au electrode, which is very different to the observation on the pristine electrode showing decreased reduction current and rapid blocking of the electrode surface due to the surface-mediated growth of Li2O2.35 We tentatively assign the enhanced ORR current to the formation of Li2O2 via a solution-mediated pathway and will provide more compelling experimental evidences in the following paragraphs. Similar CV results have also been obtained on PAQ-coated electrodes in 0.1 M LiClO4 DME electrolyte solutions (Supporting Information, Figure S2). Moreover, galvanostatic discharging of the pristine and PAQ-coated Au electrode in MeCN and DME electrolytes provides direct evidence that the PAQ film can catalyze redox of O2/Li2O2 and significantly enhance the discharge capacities on planar electrodes (Supporting Information, Figure S3). Going beyond the model electrode of planar Au for the basic study of the oxygen electrochemistry, we proceed to examine the electrochemical performance of the PAQ film attached to the gas diffusion layer (GDL) electrodes that have higher surface area and are more pertinent to Li-O2 batteries. The electro-grafting of PAQ on GDL has been realized by the similar procedure as for planar electrodes, and the formation of the PAQ film on GDL has been confirmed by CV and FTIR (Supporting Information, Figure S4 and S5).


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Li-O2 cells, comprised of a lithium or protected lithium anode, pristine GDL or PAQ-coated GDL cathode, and 0.1 M LiClO4 MeCN or DME electrolyte, have been assembled and operated under 1 atm of O2. In MeCN electrolyte, the pristine Li-O2 cell (Figure 2a and b green curves) shows very limited discharge capacity and poor rate capability (e.g., capacities of 7.4, 3.8 and 1.1 mAh m-2BET at current densities of 0.1, 0.2 and 0.5 mA cm-2areal, respectively). In contrast, the PAQ-catalyzed Li-O2 cell demonstrates significantly enhanced electrochemical performance (Figure 2a black curves). For instance, the PAQ-catalyzed Li-O2 cell has delivered discharge capacities of 315.7, 178.9 and 53.4 mAh m-2BET at current densities of 0.1, 0.2 and 0.5 mA cm2

areal,

respectively, which are ~ 40-50 times higher than those obtained with pristine GDL-based

Li-O2 cells operated under the same conditions. It is notable that the discharged capacities realized by the thin PAQ films are comparable to those achieved by Li-O2 cells containing soluble redox mediators of AQs (Figure 2a blue curves). That means the discharge process by the PAQ films could be a solution phase mechanism, which has been confirmed in subsequent experiments. Similar results have also been obtained for PAQ-catalyzed Li-O2 batteries containing 0.1 M LiClO4 DME electrolyte solutions (Supporting Information, Figure S6).


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Figure 2. (a) Discharge curves of Li-O2 batteries with (green) pristine, (blue) soluble AQ-, and (black) PAQcatalyzed GDL cathodes in 0.1 M LiClO4 MeCN at various current densities. (b) An enlarged view of the discharge curves of the Li-O2 cells containing pristine GDL.

To understand the enhancement in the discharge capacity of the PAQ-catalyzed Li-O2 batteries, a range of complementary research techniques have been employed to characterize the pristine, soluble AQ-, and PAQ-catalyzed Li-O2 batteries prior to and at the end of discharge. Figure 3a and b show the SEM images of the pristine GDL electrode. After electro-grafting, the PAQ-catalyzed GDL surface becomes roughened, as can be seen from Figure 3c and d. At the end of discharge of Li-O2 cells with pristine GDL cathode, products with morphologies of conformal film (major) and tiny nano-sheet (minor) have been identified (Figure 3e and f), which are typical for the surface-mediated growth of Li2O2 in low DN electrolyte solvents.11 In sharp contrast, densely-packed, toroid-shaped, micron-sized Li2O2 particles have been observed on PAQ-catalyzed GDL at the end of discharge (Figure 3g and h). The appearance of large toroidal


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Li2O2 particles is often attributed to a solution-mediated Li2O2 growth process in Li-O2 batteries.8,11 For soluble AQ-catalyzed Li-O2 batteries, large Li2O2 particles with a toroidal morphology have been obtained at the end of discharge as expected (Figure 3i and j). Similar SEM results have been observed for PAQ-catalyzed Li-O2 cells containing 0.1 M LiClO4 DME electrolyte solutions (Supporting Information, Figure S7).


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Figure 3. SEM images of the (a, b) pristine GDL, (c, d) PAQ-coated GDL, (e, f) GDL discharged in the absence of soluble AQ, (g, h) PAQ-coated GDL at the end of discharge, and (i, j) GDL discharged in the presence of soluble AQ at a current density of 0.1 mA cm-2 in 0.1 M LiClO4 MeCN.

The above discharged Li-O2 batteries have also been examined with PXRD for crystalline products (Figure 4a), where Li2O2 has been identified as the dominant product in all the cathodes examined. The dominance of Li2O2 in the PAQ- catalyzed GDL electrode has further been confirmed by FTIR (Figure 4b) and Raman (Figure 4c) spectroscopy, in which only minor side reaction product (e.g., Li2CO3) has been identified. Another compelling evidence for the dominant Li2O2 formation in PAQ-catalyzed Li-O2 battery has been provided by an in situ DEMS study (Figure 4d), in which the O2 consumption rate has been recorded in the course of ORR driven by a linear potential scan from open circuit potential (2.8 V vs. Li|Li+) to 2.0 V. By integrating the charge passed during the potential scan and the amount of O2 consumed, the charge-to-mass ratio has been quantified to be 2.03 e‾/O2, which is close to the theoretical value of 2.00 e‾/O2 for the ideal O2 reduction to Li2O2. The combined SEM, FTIR, Raman and DEMS results conclude that discharge of the PAQ-catalyzed Li-O2 battery is overwhelmingly dominated by the solution-mediated Li2O2 formation with negligible side reactions.


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Figure 4. (a) PXRD pattern, (b) FTIR, and (c) Raman spectra of the pristine, AQ-catalyzed, and PAQcatalyzed GDL electrodes prior to and at the end of discharge. (d) DEMS study of a PAQ-catalyzed Li-O2 battery containing 0.1 M LiClO4 MeCN discharged by linear potential scan at 0.5 mV s-1 from 2.8 V to 2.0 V under 1 atm of O2.

It is interesting that a thin PAQ film on the cathode has the ability to drive the solution discharge of Li-O2 batteries containing low DN electrolyte solvents (MeCN and DME) that would otherwise lead to surface-mediated growth of Li2O2, limited discharge capacity and poor rate capability. Compared with the soluble AQ-catalyzed Li-O2 battery, in which solutionmediated growth of Li2O2 has been understood both experimentally and theoretically,23,36 the only difference of the PAQ-catalyzed Li-O2 battery is that the AQs are immobilized and in the form of a conductive polymer film tethered to the cathode surface. The solution-mediated growth of Li2O2 in PAQ-catalyzed Li-O2 battery can be ascribed to (i) the solvation of the PAQ film by electrolyte solvent providing a surface-confined reaction layer similar to the diffusive layer containing AQ- produced in soluble AQ-catalyzed Li-O2 batteries, and (ii) a decreased van der


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Waals interaction energy between the formed Li2O2 particles and the GDL surface due to the PAQ film in between. Previous studies have showed that the PAQ film is both ionically and electronically conductive.33,37 As a result, when the PAQ film is electro-reduced, the AQmoieties within the PAQ film will reduce O2 in the presence Li+ ions producing LiAQO2 intermediates that will transform to Li2O2 and regenerate AQ by a disproportionation process.23,36 The PAQ film provides a surface-confined reaction layer similar to, albeit with a much smaller thickness, the diffusive layer containing the AQ‾ anions produced in AQ-catalyzed Li-O2 batteries, and the newly formed Li2O2 is still away from the underlying GDL surface, i.e., within or on top of the solvated PAQ film. In addition, because of the presence of the PAQ film, the van der Waals interaction energy between the Li2O2 particle and GDL surface is greatly decreased. Typically, the van der Waals interaction energy of a particle-surface pair can be qualitatively represented by W = -AR/6D, where A is the Hamaker constant, R is the radius of the particles, and D is the distance between the particle and the surface.38 Obviously, an intervening film of PAQ will decrease the van der Waals interaction energy between Li2O2 particle and GDL, which is consistent the ubiquitous observation that thin organic films have been frequently used to assign the anti-fouling (or adsorption) properties to the underlying solid surfaces.39 The immobilized redox mediators of AQ not only promote discharge of Li-O2 batteries, just as soluble AQ does, but also eliminate the adverse shuttle effects of their soluble siblings of AQs, because the chemical bonding nature between the electrode surface and the films from the electro-reduction of diazonium salt. The x-ray photoelectron spectroscopic study (Supporting Information, Figure S8) of the lithium anode of a Li-O2 cell containing soluble AQ shows the existence of -C=O, –CO, -COO and -CO3 moieties, indicating that AQ has shuttled from the


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cathode to the anode and decomposed thereon. While for the lithium anode of a PAQ-catalyzed Li-O2 battery, no signal of redox mediators (characterized by -C=O) has been identified, indicating that the shuttle of redox mediators has been completely avoided. In summary, we have reported that immobilized redox mediators of AQ in the form of a thin conductive polymer film (PAQ) have the ability to address the severe capacity-current tradeoff of Li-O2 batteries containing weakly solvating electrolyte solutions without bringing any side effects to the lithium anode. The immobilized redox mediators of AQ not only facilitate ORR kinetics but also promote solution discharge in the PAQ-catalyzed Li-O2 batteries. A range of complementary research techniques has been employed to understand the solution-mediated growth of Li2O2. The work reported here provides a new avenue to address both the capacitycurrent trade-off and the vexing “cross-talk” between the lithium anode and the redox mediator in current Li-O2 batteries, and also suggests that surface engineering of electrode materials could play an important role toward unlocking the energy capabilities of Li-O2 batteries. However, the presented study is a proof of concept in this direction and reports the discharge performance only. Future efforts are needed to examine the potential of immobilized redox mediators for LiO2 batteries. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs. jpclett. please add manuscript number.


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Materials, experimental details, additional cyclic voltammetry, HMR and XPS data (PDF) AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Z.P. is indebted to the National Natural Science Foundation of China (Grant No. 91545129, 21575135, 21733012, 21633008 and 21605136), the “Strategic Priority Research Program” of the CAS (Grant No. XDA09010401) and the Ministry of Science and Technology of China (Grant No. 2016YBF0100100) REFERENCES (1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li–O2 and Li–S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29. (2) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. A Reversible and Higher-Rate Li-O2 Battery. Science 2012, 337, 563-566. (3) Luntz, A. .C; McCloskey, B. D. Nonaqueous Li-Air Batteries: A Status Report. Chem. Rev. 2014, 114, 11721-11750. (4) Lu, J.; Li, L.; Park, J.-B.; Sun, Y.-K.; Wu, F.; Amine, K. Aprotic and Aqueous Li-O2 Batteries. Chem. Rev. 2014, 114, 5611-5640. (5) Abraham, K. M.; Jiang, Z. A Polymer Electrolyte-Based Rechargeable Lithium/oxygen Battery. J. Electrochem. Soc. 1996, 143, 1-5.


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