A Solvent-Controlled Oxidation Mechanism of Li2O2 in Lithium-Oxygen Batteries

Abstract

•High-donicity solvent promotes soluble LiO2 generation upon Li2O2 oxidation•Preferential formation of soluble LiO2 leads to poor stability of Li-O2 battery•Bypassing the generation of soluble LiO2 improves the stability of Li-O2 batteries Rechargeable lithium-oxygen (Li-O2) batteries promise to provide energy density of 3–5 times that of state-of-the-art Li-ion batteries. However, unsolved discrepancies on charging intermediates and oxygen evolution reaction mechanism prevent rational electrolyte design for efficient and long-life Li-O2 batteries. In this study, we reveal a solvent-controlled oxidation mechanism of Li2O2 and that preferential formation of soluble LiO2 in the high-donicity solvents leads to poor cycling stability. Our work offers new insights into resolving the discrepancy of Li-O2 charging mechanism and design strategies to achieve efficient and stable Li-O2 batteries. Long-unresolved discrepancies in the oxidation mechanism of lithium peroxide (Li2O2) impede electrode/electrolyte development for Li-O2 batteries. In this study, we report direct evidence of the formation of soluble LiO2 upon the oxidation of Li2O2 and reveal a strong solvent-controlled Li2O2-oxidation reaction mechanism. We exploit a thin-film rotating ring-disk electrode to show that soluble LiO2 is generated when oxidizing Li2O2 in high-donicity solvent but is absent in low-donicity glyme-based solvent. Synchrotron-based X-ray absorption near-edge structure spectroscopy further proved the presence of LiO2 upon Li2O2 oxidation in high-donicity solvent but not in low-donicity solvent. We show that preferential formation of soluble LiO2 in the high-donicity solvent could account for poor cycling stability, which suggests that strategies that bypass the formation of soluble LiO2 upon Li2O2 oxidation are critical. Our work offers new insights in resolving the discrepancy in Li-O2 charging mechanism and design strategies to achieve efficient and long-life Li-O2 batteries. Long-unresolved discrepancies in the oxidation mechanism of lithium peroxide (Li2O2) impede electrode/electrolyte development for Li-O2 batteries. In this study, we report direct evidence of the formation of soluble LiO2 upon the oxidation of Li2O2 and reveal a strong solvent-controlled Li2O2-oxidation reaction mechanism. We exploit a thin-film rotating ring-disk electrode to show that soluble LiO2 is generated when oxidizing Li2O2 in high-donicity solvent but is absent in low-donicity glyme-based solvent. Synchrotron-based X-ray absorption near-edge structure spectroscopy further proved the presence of LiO2 upon Li2O2 oxidation in high-donicity solvent but not in low-donicity solvent. We show that preferential formation of soluble LiO2 in the high-donicity solvent could account for poor cycling stability, which suggests that strategies that bypass the formation of soluble LiO2 upon Li2O2 oxidation are critical. Our work offers new insights in resolving the discrepancy in Li-O2 charging mechanism and design strategies to achieve efficient and long-life Li-O2 batteries. Non-aqueous lithium-oxygen (Li-O2) batteries have attracted intensive research attention owing to their potential to provide gravimetric energy density 3–5 times that of conventional Li-ion batteries.1Abraham K.M. Jiang Z. A polymer electrolyte-based rechargeable lithium/oxygen battery.J. Electrochem. Soc. 1996; 143: 1-5Crossref Scopus (1769) Google Scholar, 2Black R. Adams B. Nazar L.F. Non-aqueous and hybrid Li-O2 batteries.Adv. Energy Mater. 2012; 2: 801-815Crossref Scopus (431) Google Scholar, 3Bruce P.G. Freunberger S.A. Hardwick L.J. Tarascon J.M. 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Bowden M.E. Engelhard M.H. Zhang J.-G. The mechanisms of oxygen reduction and evolution reactions in nonaqueous lithium-oxygen batteries.ChemSusChem. 2014; 7: 2436-2440Crossref PubMed Scopus (57) Google Scholar and in situ selected-area electron diffraction (SAED)40Luo L. Liu B. Song S. Xu W. Zhang J.-G. Wang C. Revealing the reaction mechanisms of Li-O2 batteries using environmental transmission electron microscopy.Nat. Nanotechnol. 2017; 12: 535Crossref PubMed Scopus (123) Google Scholar have provided direct evidence to support the generation of the intermediate of LiO2 and the formation of Li2O2 during ORR.Step 1: O2 + Li+ + e− → LiO2(ads or sol)(Equation 1) Step 2: LiO2(ads) + Li+ + e− → Li2O2(Equation 2) and: LiO2(sol) + LiO2(sol) → Li2O2 + O2(Equation 3) The confirmation of the soluble discharge intermediate product LiO2(sol)23Trahan M.J. Mukerjee S. Plichta E.J. Hendrickson M.A. Abraham K.M. Studies of Li-air cells utilizing dimethyl sulfoxide-based electrolyte.J. Electrochem. 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Lee R.-C. Pereiro E. Wu N.-L. Tonti D. Spatial distributions of discharged products of lithium-oxygen batteries revealed by synchrotron X-ray transmission microscopy.Nano Lett. 2015; 15: 6932-6938Crossref PubMed Scopus (50) Google Scholar, 32Trahan M.J. Gunasekara I. Mukerjee S. Plichta E.J. Hendrickson M.A. Abraham K.M. Solvent-coupled catalysis of the oxygen electrode reactions in lithium-air batteries.J. Electrochem. Soc. 2014; 161: A1706-A1715Crossref Scopus (35) Google Scholar, 33Gunasekara I. Mukerjee S. Plichta E.J. Hendrickson M.A. Abraham K.M. A study of the influence of lithium salt anions on oxygen reduction reactions in Li-air batteries.J. Electrochem. Soc. 2015; 162: A1055-A1066Crossref Scopus (75) Google Scholar, 34Wang Y. Liang Z. Zou Q. Cong G. Lu Y.-C. Mechanistic insights into catalyst-assisted nonaqueous oxygen evolution reaction in lithium-oxygen batteries.J. Phys. Chem. C. 2016; 120: 6459-6466Crossref Scopus (59) Google Scholar, 35Kwabi D.G. Tułodziecki M. Pour N. Itkis D.M. Thompson C.V. Shao-Horn Y. Controlling solution-mediated reaction mechanisms of oxygen reduction using potential and solvent for aprotic lithium-oxygen batteries.J. Phys. Chem. Lett. 2016; 7: 1204-1212Crossref PubMed Scopus (80) Google Scholar, 36Torres W. Mozhzhukhina N. Tesio A.Y. Calvo E.J. A rotating ring disk electrode study of the oxygen reduction reaction in lithium containing dimethyl sulfoxide electrolyte: role of superoxide.J. Electrochem. Soc. 2014; 161: A2204-A2209Crossref Scopus (38) Google Scholar, 37Amin H.M.A. Molls C. Bawol P.P. Baltruschat H. The impact of solvent properties on the performance of oxygen reduction and evolution in mixed tetraglyme-dimethyl sulfoxide electrolytes for Li-O2 batteries: mechanism and stability.Electrochim. Acta. 2017; 245: 967-980Crossref Scopus (17) Google Scholar, 38Herranz J. Garsuch A. Gasteiger H.A. Using rotating ring disc electrode voltammetry to quantify the superoxide radical stability of aprotic Li-air battery electrolytes.J. Phys. Chem. C. 2012; 116: 19084-19094Crossref Scopus (148) Google Scholar, 39Cao R. Walter E.D. Xu W. Nasybulin E.N. Bhattacharya P. Bowden M.E. Engelhard M.H. Zhang J.-G. The mechanisms of oxygen reduction and evolution reactions in nonaqueous lithium-oxygen batteries.ChemSusChem. 2014; 7: 2436-2440Crossref PubMed Scopus (57) Google Scholar asserts the importance of controlling and manipulating the solvation of the discharge intermediates LiO2(sol) for Li-O2 batteries. This interplay has been widely studied and understood via the Hard Soft Acid Base theory19Laoire C.O. Mukerjee S. Abraham K.M. Plichta E.J. Hendrickson M.A. Elucidating the mechanism of oxygen reduction for lithium-air battery applications.J. Phys. Chem. C. 2009; 113: 20127-20134Crossref Scopus (583) Google Scholar, 20Laoire C.O. Mukerjee S. Abraham K.M. Plichta E.J. Hendrickson M.A. Influence of nonaqueous solvents on the electrochemistry of oxygen in the rechargeable lithium-air battery.J. Phys. Chem. C. 2010; 114: 9178-9186Crossref Scopus (800) Google Scholar, 23Trahan M.J. Mukerjee S. Plichta E.J. Hendrickson M.A. Abraham K.M. Studies of Li-air cells utilizing dimethyl sulfoxide-based electrolyte.J. Electrochem. Soc. 2013; 160: A259-A267Crossref Scopus (225) Google Scholar, 24Abraham K.M. Electrolyte-directed reactions of the oxygen electrode in lithium-air batteries.J. Electrochem. Soc. 2015; 162: A3021-A3031Crossref Scopus (111) Google Scholar, 25Johnson 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: 1091-1099Crossref PubMed Scopus (763) Google Scholar, 41Kwabi D.G. Bryantsev V.S. Batcho T.P. Itkis D.M. Thompson C.V. Shao-Horn Y. Experimental and computational analysis of the solvent-dependent O2/Li+-O2− redox couple: standard potentials, coupling strength, and implications for lithium-oxygen batteries.Angew. Chem. Int. Ed. 2016; 128: 3181-3186Crossref Google Scholar that the stability of the ORR intermediate, O2− (a soft base) depends on the acidity of the solvated Li+, which is directly influenced by the donicity of the solvent molecule and the coordination with Li+. Solvents with strong donating ability (i.e. high-donicity solvents) give rise to stronger Li+-solvent bond strength, which lowers the hardness of the Li+.19Laoire C.O. Mukerjee S. Abraham K.M. Plichta E.J. Hendrickson M.A. Elucidating the mechanism of oxygen reduction for lithium-air battery applications.J. Phys. Chem. C. 2009; 113: 20127-20134Crossref Scopus (583) Google Scholar, 20Laoire C.O. Mukerjee S. Abraham K.M. Plichta E.J. Hendrickson M.A. Influence of nonaqueous solvents on the electrochemistry of oxygen in the rechargeable lithium-air battery.J. Phys. Chem. C. 2010; 114: 9178-9186Crossref Scopus (800) Google Scholar, 23Trahan M.J. Mukerjee S. Plichta E.J. Hendrickson M.A. Abraham K.M. Studies of Li-air cells utilizing dimethyl sulfoxide-based electrolyte.J. Electrochem. Soc. 2013; 160: A259-A267Crossref Scopus (225) Google Scholar, 24Abraham K.M. Electrolyte-directed reactions of the oxygen electrode in lithium-air batteries.J. Electrochem. Soc. 2015; 162: A3021-A3031Crossref Scopus (111) Google Scholar, 25Johnson 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: 1091-1099Crossref PubMed Scopus (763) Google Scholar, 41Kwabi D.G. Bryantsev V.S. Batcho T.P. Itkis D.M. Thompson C.V. Shao-Horn Y. Experimental and computational analysis of the solvent-dependent O2/Li+-O2− redox couple: standard potentials, coupling strength, and implications for lithium-oxygen batteries.Angew. Chem. Int. Ed. 2016; 128: 3181-3186Crossref Google Scholar Thus the superoxide (O2−), which is a soft base, has an increased affinity for the Li+ ions solvated in solvents with high-donicity forming the complex of Li+(solvent)n---O2− (LiO2(sol)).20Laoire C.O. Mukerjee S. Abraham K.M. Plichta E.J. Hendrickson M.A. Influence of nonaqueous solvents on the electrochemistry of oxygen in the rechargeable lithium-air battery.J. Phys. Chem. C. 2010; 114: 9178-9186Crossref Scopus (800) Google Scholar, 24Abraham K.M. Electrolyte-directed reactions of the oxygen electrode in lithium-air batteries.J. Electrochem. Soc. 2015; 162: A3021-A3031Crossref Scopus (111) Google Scholar Trahan et al.23Trahan M.J. Mukerjee S. Plichta E.J. Hendrickson M.A. Abraham K.M. 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