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A viewpoint on heterogeneous electrocatalysis and redox mediation in nonaqueous Li-O batteries 2
Bryan D. McCloskey, and Dan Addison ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02866 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016
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A viewpoint on heterogeneous electrocatalysis and redox mediation in nonaqueous Li-O2 batteries Bryan D. McCloskey1,2,*, Dan Addison3 Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA, 94720 Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 3 Liox Power Inc., 129 N. Hill Ave., Pasadena, CA 91106, United States. 1 2
*
[email protected] Keywords: Li-air battery, redox mediator, parasitic decomposition, Li-O2 mechanism, quantitative analysis Introduction. Li-air battery research faces many challenges in pursuit of attaining a substantial fraction of the battery’s large theoretical energy density. Many reviews have now been published outlining our understanding of these issues, as well as potential directions of research to address them.1-9 In this viewpoint, we wish to specifically address two strategies that have been presented to improve the performance of Li-air batteries, namely heterogeneous electrocatalysis (new electrode materials) and redox mediation (new redox-active electrolyte additives) at the positive electrode. These methods have been reported to improve energy efficiency through lowering of discharge and charge overpotentials, as well as to improve the discharge capacity of the system through enhanced solution-phase charge shuttling.10-13 While there is certainly a substantial amount of work still left to completely understand the appropriate design of cathode materials and electrolyte additives, we hope to convey to the reader two critical conclusions about appropriate characterization of these systems: 1) given the highly reactive nature of the intermediates and products in a Li-O2 cell, particularly when compared to a Li-ion cell, multiple quantitative analyses are required to support any conclusions concerning the efficacy of new strategies to improve battery performance, 2) heterogeneous electrocatalysts and redox mediators can induce side reactions that can be easily misconstrued as improved battery performance, making quantitative measurements all the more important. We believe that many useful directions of research in these areas would benefit our understanding of how to control the highly reactive environment in a Li-air battery, but that they should only be pursued if using appropriate measurement techniques that provide a clear interpretation of these strategies’ effect on the performance of the cell. In order to appropriately discuss these issues, we will first briefly introduce our understanding of the electrochemical reactions occurring in the cell.
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The nonaqueous Li-O2 electrochemistry. Although a few important exceptions exist,14-15 nearly all studies on Li-O2 cells have reported 2 e- oxygen reduction to form lithium peroxide, Li2O2, as the primary reaction occurring at the Li-O2 battery cathode via reaction 1:3 2 (Li+ + e-) + O2 Li2O2 Eo=2.96 V vs. Li/Li+ (1) Most challenges facing the Li-O2 cell can be traced to the unique properties of lithium peroxide. For example, Li2O2 is both a wide band gap electronic insulator and insoluble in all organic electrolytes typically employed in Li-O2 cells.16-21 It therefore deposits on the cathode surface as it is formed, eventually resulting in electronic passivation that causes a precipitous decrease in cell voltage that signifies the end of discharge, always at a value well below the theoretical capacity of the cell.16, 2223
Furthermore, Li2O2 and its transient intermediates, O2- and LiO2, are highly reactive and can
induce parasitic degradation of both the electrolyte and electrode.1,
24-26
We believe that the
identification of electrolyte and cathode compositions that provide stable Li-O2 cycling presents the most important direction of research in the field today. Of importance to our discussion, the mechanism by which Li2O2 forms during discharge directly impacts the total attainable cell capacity and the stability of the battery components, as well as the cell behavior during charge. Two general mechanisms have been recently identified, as outlined in Fig 1.27-32 In the first, Li2O2 is formed through a two-step sequential charge transfer, where the transient intermediate, lithium superoxide (LiO2), remains bound to the cathode surface: Li+ + e- + O2 → LiO2*
(2)
LiO2* + e- + Li+ → Li2O2*
(3)
where ‘*’ denotes a surface bound species. This Li2O2 formation mechanism, which is typically called the ‘surface mechanism,’ occurs in poorly Lewis acidic or basic electrolytes, where no LiO2 solubility is observed.32 When this mechanism dominates the Li2O2 formation process, Li2O2 is observed to conformally coat the electrode surface, which results in relatively fast passivation after only a few nanometer thick film is formed.16, 30, 32-34
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A second mechanism, the so-called ‘solution mechanism’ (Figure 1), occurs when LiO2 has finite solubility in the electrolyte, thereby allowing it, or its solvated radical anion (O2-), to diffuse away from the cathode site at which it is originally formed and deposit as Li2O2 through a disproportionation reaction at a nucleated Li2O2 crystal:30, 32 Li+ + e- + O2 → LiO2*
(4)
LiO2* → Li+ (sol) + O2- (sol)
(5)
O2- diffusion
(6)
2Li+ + 2O2- → Li2O2 (s) + O2
(7)
This mechanism results in the formation of large (micron-sized) Li2O2 toroids typically found on the cathode surface, although deposition of toroids on the separator adjacent to the cathode is also possible. LiO2/O2- diffusion away from their initial site of formation allows the electronically conductive cathode surface to remain accessible to both O2 and Li+ for longer periods, thereby resulting in substantially larger discharge capacities compared to cases where the surface mechanism dominates. To induce LiO2 solubility, a variety of different strategies have been employed, all typically involving the increase of the electrolyte’s Lewis acidity or basicity.27, 30-32 Unfortunately, typical Lewis basic or acidic electrolyte compositions are more prone to parasitic attack by reduced oxygen species than their low Lewis basic/acidic counterparts.35 Identifying electrolytes that enable both high capacity and stability is a critical challenge currently facing Li-O2 batteries. It is worth noting that even in electrolytes where LiO2 solubility is induced, its solution-phase lifetime is still short (likely no longer than a few seconds, although not accurately measured to the best of our knowledge) before it disproportionates to Li2O2, which is insoluble even in highly Lewis acidic/basic electrolytes.3 Therefore, shuttling of reduced oxygen species to the Li metal anode is less of a concern than direct O2 crossover and reaction with Li metal, which, along with the possibility of employing redox mediators (as is discussed later), likely necessitates Li metal protection strategies. Understanding these mechanisms of Li2O2 formation, as well as the solubility/insolubility of LiO2/Li2O2, is important when considering the rational design of heterogeneous electrocatalysts and redox mediators. Typical reports usually emphasize the use of heterogeneous catalysis to improve the kinetics of the oxygen reduction reaction (ORR, forward eqn. 1) on discharge and oxygen evolution reaction (OER, reverse eqn. 1) on charge, or the use of redox mediators to provide a
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solution-based pathway for charge transport to circumvent the insulative nature of solid Li2O2. By employing these strategies, the overpotentials typically observed during constant current discharge/charge cycles (particularly the charge cycle) have been shown to be suppressed in many studies, and the attainable discharge capacities have been improved. It is important to understand that a simple reduction in these overpotentials is not necessarily an indicator of improved ORR or OER efficiency, as the lower overpotentials could be related to the promotion of other reactions instead. Any claims of lower OER or ORR overpotentials need to be supported by quantifying oxygen consumption/evolution, combined with quantifying solid product formation through titrations or calibrated spectroscopy. It is also important to note that, while being useful as part of a larger study involving quantitative analyses, qualitative spectroscopic techniques, such as raman, infrared spectroscopy, and x-ray diffraction, many times do not provide sufficient sensitivity to identify side products that may be formed during battery operation. As an example, x-ray diffraction patterns of discharged cathodes extracted from cells in which substantial parasitic reactions occur (those which employ carbonate-based electrolytes) showed no product reflections after discharge, even though other techniques clearly observed the formation of these products.36 Below, we provide our thoughts on heterogeneous electrocatalysis to improve ORR and OER kinetics, followed by discussions on redox mediation to improve charge transport during charge and discharge in a Li-O2 cell. Heterogeneous Electrocatalysis. Porous carbon-based cathodes, those in which a high surface area carbon black powder is bound to a metal mesh using a polymer binder, are commonly employed in Li-O2 cells.24, 37-40 Many studies have also incorporated materials into these cathode compositions that have been shown to serve as heterogeneous electrocatalysts for aqueous oxygen reduction or evolution, including Mn, Co, Ru, Pt, Pd, Ag, Au, and N-doped carbon materials, in an attempt to improve the kinetics of the oxygen reduction and evolution reactions.10, 41-48 However, given the mechanisms discussed above and outlined in Fig. 1, this direction of research is hindered by a few important properties of the discharge/charge processes, which are outlined in Figure 2. If Li2O2 forms through the surface mechanism, Li2O2 will quickly coat the catalyst surface during discharge (noting again the largely insoluble nature of Li2O2 in all studied organic electrolytes), rendering the catalyst surface inaccessible to solution reactants (Li+ and O2) and necessarily resulting in Li2O2 growth at Li2O2 surfaces rather than catalyst surfaces (Figure 2a-I).34 On the other hand, if the solution mechanism is dominant, the catalyst sites may remain accessible during long periods of
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discharge. However, the kinetics of oxygen reduction in Li+-bearing nonaqueous electrolytes are quite fast even on carbon surfaces (Figure 2a-II and 2c), particularly when compared to aqueous oxygen reduction, raising questions about the need for improved catalysis over simple, cost-effective carbon electrodes.23 Instead of originating from poor ORR kinetics, most polarization losses on discharge appear to be related to overall cell impedance and transport limitations, for example, Liion transfer through the Li metal solid-electrolyte interface, charge transport resistance through deposited Li2O2, and at high rates (>1 mA/cm2), O2 diffusion within the porous cathode.23, 49 These limitations are obviously not addressed by improved heterogeneous electrocatalysis. Also of note, employing electrocatalysts which tend to strongly bind O2 or LiO2, and thereby selectively activate a 4 e- ORR reaction,50 can result in substantial increases in parasitic side reactions in the nonaqueous Li-O2 cell, where O2 ideally never dissociates during Li2O2 formation/oxidation. As a result, these catalysts (e.g., Pt, Au, MnO2) increase electrolyte degradation during cell operation (Figure 3), and in some instances, at overpotentials that are quite low compared to similar charging overpotentials at carbon surfaces.48, 51-52 Of course, increased electrolyte degradation results in poor oxygen evolution efficiency on charge compared to purely carbon electrodes, thereby further limiting cell rechargeability.25 Observed overpotentials are particularly high during charge of a typical Li-O2 cell, and hence most early work on heterogeneous electrocatalysis in Li-O2 cells targeted the reduction of the charge overpotential.53-56 However, noting again that Li2O2 forms as a solid, insoluble crystal, it is difficult to envision how Li2O2 deposited away from active catalytic sites would be able to diffuse back to catalytic sites in typical electrolyte compositions (Figure 2b-I). Furthermore, the inherent oxidation overpotential of peroxide has been calculated and shown experimentally (Fig. 2c) to be quite low, once again raising questions about the need for OER electrocatalysis during charge.57 In support of this concept, the highest rate of oxygen evolution from Li2O2 during charge is experimentally observed to occur in the initial stages of charge, where the overpotential is no more than ~100 mV (Figure 3).24,
48
The cause of the ever increasing charge overpotential is in fact related to the
formation of highly stable carbonate degradation products at the oxygen evolving Li2O2/electrolyte interface, which blocks O2 evolving surfaces and results in the ever-increasing overpotential observed in Fig. 3 when carbon electrodes are employed.24 If Li2O2 toroids are formed, charge transfer resistance through the toroids likely also result in large overpotentials.2
Strategies to
improve conductivity in bulk Li2O2, such as doping during the deposition process, have been
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suggested and provide a potentially useful route to reduce overpotentials related to poor charge transport during discharge and charge.58-59 Nevertheless, instabilities of porous carbon cathodes during cell cycling have also been reported, and other materials should be explored as potential cathode alternatives. Therefore, it is our opinion that research on cathode materials should focus on the identification of stable materials that exhibit poor electrocatalytic activity towards electrolyte degradation and that improved ORR/OER electrocatalysis is not necessary in studies employing organic-based electrolytes. Redox mediators for Li-O2 cell charge. Given the insulating nature of Li2O2, adding small molecules (redox mediators) to the electrolyte that reversibly accept and donate electrons at a known potential is an interesting concept to allow charge transport to occur through the solution phase during Li2O2 formation and oxidation.12-13 An idealized example of redox mediated Li2O2 oxidation is shown schematically in Fig. 4a, where a redox mediator is oxidized at the electrode surface, diffuses through the electrolyte to the surface of a large Li2O2 crystal, and is then reversibly reduced by Li2O2, which itself is oxidized to evolve O2.
In the context of Li2O2 oxidation, an appropriate mediator would
be selected to have a reversible redox potential slightly above the equilibrium potential of Li2O2 formation/oxidation.60 Therefore, any rise in the charge overpotential above this reversible redox potential will result in mediator oxidation and initiate the shuttling effect presented in Fig. 4a, such that the battery will charge at the redox potential of the mediator rather than the high potential that results from product blocking of the O2-evolving surface (Fig. 4b). Numerous oxidation mediators have been explored,12-13, 61-66 and future development of mediators should focus on those that are stable over many cycles and can operate slightly above the open circuit potential of the cell while still actively oxidizing Li2O2 to O2. Redox mediator development, and related strategies to improve charge transport over length scales greater than a few nanometers, will be important for high capacity Li-O2 cells in which Li2O2 is formed through the solution mechanism on discharge. Here, large Li2O2 toroids (~1 µm) are formed, such that a large percentage of Li2O2 is not electronically accessible to the electrode surface on charge (noting again that Li2O2 is insoluble in organic electrolytes and cannot diffuse back to the electrode surface through the electrolyte, as is highlighted in Fig. 2b-I).30-31 Therefore, shuttling charge to the periphery of the Li2O2 toroid, as shown in Fig. 4a, provides a route to Li2O2 toroid
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oxidation. As a side note, redox mediation in cells in which Li2O2 is conformally formed on the electrode surface through the ‘surface mechanism’ (Fig. 1) is likely unnecessary. Charge transport through a conformal Li2O2 thin film is governed by hole tunneling, such that discharge sudden death occurs once the film grows sufficiently thick to disallow tunneling.18 However, during charge, the alignment of the Li2O2 valence band maximum and the electrode Fermi level under a positive voltage bias substantially reduces the barrier to hole tunneling and increases polaronic hole transport concentration. This substantially improves charge transport through the thin film,18 likely rendering redox mediation during charge unnecessary in cells where the surface mechanism of Li2O2 formation dominates. Of course, the surface process to form Li2O2 results in low capacity cells given that hole tunneling is disallowed once the Li2O2 thickness reaches only a few nanometers.16 Practical Li-O2 cells will likely need to rely on the ‘solution mechanism’ (Fig. 1) to achieve energy densities that are competitive with Li-ion batteries. There are two important considerations for the characterization of redox mediators for Li2O2 oxidation. First, mediators are small organic molecules that are governed by the same chemical stability requirements as organic electrolyte solvents. In other words, during a full battery discharge/charge cycle, mediators can be attacked by reactive reduced oxygen products, such as O2-, LiO2, and Li2O2.1, 25, 67-68 Nucleophilic attack at heteroatom centers, as well as hydrogen abstraction at α and β sites in molecules containing heteroatoms (as is typical for organic redox mediators) are both critical considerations for the molecular design of mediators.1 Furthermore, some redox mediators, e.g., I-, are nucleophiles that can induce parasitic electrolyte degradation reactions.65 As a result, we emphasize once again that identifying the reversible reactions in a Li-O2 battery through appropriate quantitative measures of stability (O2 consumption/evolution, Li2O2 formation and oxidation) while characterizing the influence of a redox mediator on the electrochemistry is a requirement. Second, once a redox mediator has been oxidized, there is no guarantee that it will diffuse to a Li2O2 particle and react with it. Once oxidized, redox mediators can diffuse out of the cathode and to the Li metal anode, where they can be reduced, effectively creating an electron shuttle between the cathode and anode.69 This shuttling strategy has in fact been developed for overcharge protection in rechargeable Li-ion batteries.70-71 In Li-O2 batteries, this shuttling is not desired, as it does not contribute to Li2O2 oxidation, thereby reducing the Faradaic efficiency of the charging process. An
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example of this shuttling is shown in Fig. 4c,69 where a Li-O2 cell that was not previously discharged and contains LiI (3I- → I3- + 2e- occurs at ~3V vs. Li/Li+) is charged, and a ~3V potential plateau is sustained at modest (
