Unlocking Reversibility of LiOH-Based Li-Air Batteries

In this issue of Joule, Clare Grey and colleagues report a nonaqueous electrolyte for Li-air batteries capable of achieving reversible reduction and evolution of oxygen to/from lithium hydroxide in the presence of a redox mediator and added water. Key to unlocking this reversibility is inclusion of ionic liquid, which solvates the redox mediator, modulating its oxidizing power toward LiOH and making O2 formation favorable while disfavoring side reactions. A record faradic efficiency of 99.5% is achieved over the first cycle. In this issue of Joule, Clare Grey and colleagues report a nonaqueous electrolyte for Li-air batteries capable of achieving reversible reduction and evolution of oxygen to/from lithium hydroxide in the presence of a redox mediator and added water. Key to unlocking this reversibility is inclusion of ionic liquid, which solvates the redox mediator, modulating its oxidizing power toward LiOH and making O2 formation favorable while disfavoring side reactions. A record faradic efficiency of 99.5% is achieved over the first cycle. The path toward reversibility in lithium-air (Li-air) batteries has been fraught with challenges. Considered attractive for their potential to reach gravimetric energies three to five times higher than Li-ion at the active materials level,1Lu 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: 750-768Crossref Scopus (678) Google Scholar the Li-air battery became one of the most intensively researched beyond-Li-ion chemistries over the past two decades.2Kwak W.J. Rosy Sharon D. Xia C. Kim H. Johnson L.R. Bruce P.G. Nazar L.F. Sun Y.K. Frimer A.A. et al.Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future.Chem. Rev. 2020; 120: 6626-6683Crossref PubMed Scopus (170) Google Scholar The prototypical Li-air cell is deceptively simple: it consists of a Li metal anode, a nonaqueous electrolyte, and a dissolved-O2 gas cathode. During discharge, O2 is reduced by a two-electron process (O2 + 2e− + 2Li+ → Li2O2) to solid lithium peroxide, which grows within the pores of a cathode scaffolding, typically carbon. As researchers now understand,3Liu T. Vivek J.P. Zhao E.W. Lei J. Garcia-Araez N. Grey C.P. Current Challenges and Routes Forward for Nonaqueous Lithium-Air Batteries.Chem. Rev. 2020; 120: 6558-6625Crossref PubMed Scopus (96) Google Scholar along the way, numerous competitive pathways exist which manifest in degrees of problematic side reactions: formation of unstable, highly reactive reduced-oxygen intermediates such as superoxide (O2−) or singlet oxygen, and even side reactions between Li2O2 and the electrolyte or cathode material. All of these lead to irreversibilities upon charge. The reversibility of O2/Li2O2 electrochemistry is also negatively impacted by H2O and even CO2 from ambient air, requiring an ultrapure source of O2 that significantly erodes the attainable energy. The field is still searching for a truly air-breathing cell; in this context, moving the dial toward reversible electrochemistry that is, at minimum, moisture tolerant is attractive. The above challenges, intrinsic to O2 electrochemistry in organic electrolytes, motivated a willingness to reconsider the basic electrochemical processes underpinning the Li-air cell in search of improved reversibility.4Gao H. Gallant B.M. Advances in the chemistry and applications of alkali-metal–gas batteries.Nat. Rev. Chem. 2020; 4: 566-583Crossref Scopus (15) Google Scholar One emergent idea has focused on Li-O2 redox that forms lithium hydroxide (LiOH) rather than Li2O2. Such a reaction was first reported feasible by Grey et al. in 2015,5Liu T. Leskes M. Yu W. Moore A.J. Zhou L. Bayley P.M. Kim G. Grey C.P. Cycling Li-O2 batteries via LiOH formation and decomposition.Science. 2015; 350: 530-533Crossref PubMed Scopus (444) Google Scholar using cells containing glyme electrolyte, trace or added water, and a redox mediator, lithium iodide (LiI), on reduced graphene oxide (rGO) cathodes. The nominal pathway corresponds to a four-electron process by the reaction O2 + 4e− + 4Li+ +2H2O → 4LiOH; the iodide/triiodide couple (I−/I3−) facilitates the formation of LiOH on discharge and its decomposition upon charge but is not consumed. The cells were reported to have high round-trip efficiencies with charging possible at exceptionally low voltages of ~3 V versus Li/Li+, pinned by the redox potential of the mediator (Li-O2 batteries without mediators can charge as high as 4 V).2Kwak W.J. Rosy Sharon D. Xia C. Kim H. Johnson L.R. Bruce P.G. Nazar L.F. Sun Y.K. Frimer A.A. et al.Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future.Chem. Rev. 2020; 120: 6626-6683Crossref PubMed Scopus (170) Google Scholar The original study posited that the mediator was responsible for decomposing LiOH with a suggested end product of O2. Additional studies have since elucidated the discharge mechanism operative in these cells, confirming the four-electron process and highlighting a functional role of the iodide in forming LiOH by aiding the decomposition of the intermediate species H2O2.6Liu T. Kim G. Jonsson E. Castillo-Martinez E. Temprano I. Shao Y.L. Carretero-Gonzalez J. Kerber R.N. Grey C.P. Understanding LiOH Formation in a Li-O2 Battery with LiI and H2O Additives.ACS Catal. 2019; 9: 66-77Crossref Scopus (26) Google Scholar However, the charging reaction has been disputed: a subsequent study revealed that LiOH did not evolve significant O2 upon charge.7Burke C.M. Black R. Kochetkov I.R. Giordani V. Addison D. Nazar L.F. McCloskey B.D. Implications of 4 e(-) Oxygen Reduction via Iodide Redox Mediation in Li-O2 Batteries.ACS Energy Lett. 2016; 1: 747-756Crossref Scopus (101) Google Scholar Central to the discussion has been the puzzling fact of the high standard thermodynamic potential of LiOH oxidation, 4LiOH → 4e− + 4Li+ + O2 + 2H2O, which, at 3.34 V versus Li/Li+, is uphill from that of the iodide oxidation reaction (3I− → I3− + 2e−, ~3 V in glyme-based electrolytes with water),5Liu T. Leskes M. Yu W. Moore A.J. Zhou L. Bayley P.M. Kim G. Grey C.P. Cycling Li-O2 batteries via LiOH formation and decomposition.Science. 2015; 350: 530-533Crossref PubMed Scopus (444) Google Scholar thus rendering it energetically inaccessible.8Viswanathan V. Pande V. Abraham K.M. Luntz A.C. McCloskey B.D. Addison D. Comment on “Cycling Li-O2 batteries via LiOH formation and decomposition”.Science. 2016; 352https://doi.org/10.1126/science.aad8689Crossref Scopus (31) Google Scholar Indeed, it was suggested that products other than O2 formed instead,7Burke C.M. Black R. Kochetkov I.R. Giordani V. Addison D. Nazar L.F. McCloskey B.D. Implications of 4 e(-) Oxygen Reduction via Iodide Redox Mediation in Li-O2 Batteries.ACS Energy Lett. 2016; 1: 747-756Crossref Scopus (101) Google Scholar such as iodates (IO−, IO3−) that reflect a reacted byproduct of the redox mediator having trapped, rather than liberated, oxygen. Now writing in Joule, Temprano, Grey, and colleagues report that true electrochemical reversibility of LiOH—forming O2(g) back upon charge—can be unlocked in Li-air cells through judicious design of the electrolyte.9Temprano I. Liu T. Petrucco E. Ellison J.H.J. Kim G. Jonsson E. Grey C.P. Toward Reversible and Moisture-Tolerant Aprotic Lithium-Air Batteries.Joule. 2020; 4 (this issue): 2501-2520Abstract Full Text Full Text PDF Scopus (5) Google Scholar While it is well known that the I−/I3− couple is solvent dependent to a degree, raising it high enough (i.e., above that of LiOH) has been challenging. The authors report that inclusion of an ionic liquid, 1-butyl-1-methyl-pyrrolidinium-bis(trifluoromethanesulfonyl)imide (Pyr14TFSI), in the electrolyte can achieve this aim. In electrolytes containing tetraglyme(G4)/0.7 M LiTFSI, LiI (50 mM), and H2O (5,000 ppm) without Pyr14TFSI, a 4 e−/O2 process was confirmed upon discharge at ~2.6 V from online electrochemical mass spectrometry (OEMS), yet negligible O2 evolved during charge. Upon adding 900 mM Pyr14TFSI, however, while the discharge process was unchanged, an O2 evolution reaction (OER) commenced upon charge at potentials as low as 3.1 V and was sustained during continued charging at ~3.4 V for a total cycling capacity of 0.5 mAh/cm2 at 50 μA/cm2. OEMS confirmed 4 e−/O2, and proportional disappearance of LiOH was quantified by chemical titration of extracted cathodes at different states of charge. A remarkable 99.5% of O2 consumed during discharge was evolved upon charge. Further spectroscopic characterization lends support to a modified reaction pathway on charge. Without the ionic liquid, UV-visible spectroscopy indicated formation of the oxygen-trapping iodate species IO3− and IO− in addition to the oxidized I3−; inclusion of the ionic liquid suppressed these side products, with only I3−, but no iodates, detectable. Overall, this led to confirmation of the desired OER pathway occurring in two steps:6I− → 2I3− + 4e− (Step 1)4LiOH + 2I3− → 4Li+ + 6I− + 2H2O + O2 (Step 2)Overall: 4LiOH → 4e− + 4Li+ + 2H2O + O2 Key to this success was identifying an electrolyte that could tailor the solvation environment around the redox mediator such that the thermodynamic landscape of the I−/I3− couple becomes favorable for oxidizing LiOH (Figure 1). Examining the local bonding environment of H2O by 1H NMR, the authors found that increasing the concentration of Pyr14TFSI from 0 to 900 mM led to a disruption of H2O in the solvation sphere and increased solvation of I− and H2O by Pyr14TFSI, confirmed further by molecular dynamics simulations. These solvation changes correlated with an increase in the redox potential of the redox mediator, rising proportionally from 3.07 V at 0 mM to 3.24 V with 900 mM Pyr14TFSI. Conducting a detailed thermodynamic analysis, the authors estimated that the Gibbs free energy of reaction corresponding to the desired OER from LiOH would cross from positive (unfavorable) to negative (favorable) at an I−/I3− potential consistent with that in 900 mM ionic liquid and 5,000 ppm of H2O; i.e., 3.24 V. However, raising the oxidizing power of I3− versus LiOH itself is not sufficient; it is also critical to assess the presence of competing pathways that form undesired iodates. Aided by DFT calculations with implicit solvation, the authors estimated the solvation free energy for I− along with that of IO− and IO3−, where IO− is an intermediate along the pathway to IO3− during oxidation of LiOH. Crucially, IO− formation becomes disfavored in the presence of ionic liquid, in spite of the fact that IO3− is still the thermodynamically favored product, providing a quantitative rationale for the shift toward the observed generation of O2. Through this work, the authors shed clarifying light on a complex reaction landscape whose mastery permits an important step forward toward reversible O2/LiOH electrochemistry in moderately hydrated nonaqueous environments. Central to this emergent understanding is an elaborate consideration of the electrochemical thermodynamics, which necessitates a complex accounting for each reactant and product state down to the most fundamental details of solvation. Such an approach inherently relies upon reported, assumed, and/or modeled parameters such as solvation free energies, which have associated uncertainties; thus, the framework developed is best considered a supporting rationale than a proof of the operative pathways, which can instead be found largely in the experimental results that unambiguously show evolution of O2. Some questions remain to be explored, such as the evolution in charging voltage observed upon sustained charge (~3.4 V) throughout O2 evolution compared to the redox potential of iodide (~3.24 V) with 900 mM ionic liquid, which would otherwise be expected to pin the voltage. As the authors themselves point out,9Temprano I. Liu T. Petrucco E. Ellison J.H.J. Kim G. Jonsson E. Grey C.P. Toward Reversible and Moisture-Tolerant Aprotic Lithium-Air Batteries.Joule. 2020; 4 (this issue): 2501-2520Abstract Full Text Full Text PDF Scopus (5) Google Scholar,10Liu T. Kim G. Carretero-Gonzalez J. Castillo-Martinez E. Bayley P.M. Liu Z.G. Grey C.P. Response to Comment on “Cycling Li-O2 batteries via LiOH formation and decomposition”.Science. 2016; 352https://doi.org/10.1126/science.aad8843Crossref Scopus (20) Google Scholar existing thermodynamic calculations are only approximate, particularly when the system is operating in a non-equilibrium state such as seen during charge. The work also brings attention to future hurdles that the O2/LiOH system will need to clear as work continues to study these complex reactions and evaluate their feasibility for practical Li-air batteries. While the authors show that evolution of O2 is possible under a particular set of conditions, the initially high faradic efficiency for O2 evolution diminished significantly within seven cycles to 80%. This indicates that side reactions are occurring in spite of the fact that the authors utilized a Li+ conducting ceramic to separate the anode and cathode, which should block the most egregious issues due to internal shuttling of iodide. The underlying loss of reversibility needs to be understood to evaluate whether intrinsic instability issues, akin to the O2/Li2O2 system, are truly avoided in the LiOH-forming battery. The tolerance of the electrochemistry to varying amounts of water also needs further assessment to evaluate the prospects of a merely moisture-tolerant versus a truly air-breathing cell. Finally, it is interesting to contextualize this scientific progress with that of the field as a whole—from an initially elegant and rather simple paradigm to one demanding increased complexity to master, yet reflecting the remarkable progress that has been made in manipulating increasingly intricate reactions. The above questions will undoubtedly be explored in future work as researchers continue to probe the LiI-mediated, water-tolerant O2/LiOH system, illuminating the electrolyte formulations needed to tilt the scales yet further toward reversibility. Toward Reversible and Moisture-Tolerant Aprotic Lithium-Air BatteriesTemprano et al.JouleOctober 9, 2020In BriefLi-air batteries have attracted significant attention due to their very high energy density, comparable to that of fossil fuels. For their development, a stable discharge product that can be reversibly decomposed during charge needs to be developed. LiOH is a promising candidate, but questions concerning the reversibility of the charge process have been raised. Here, we report for the first time Li-O2 cells that can reversibly cycle via LiOH by using water and an ionic liquid as additives in the electrolyte. Full-Text PDF Open Archive