Controlling Reversible Expansion of Li2O2 Formation and Decomposition by Modifying Electrolyte in Li-O2 Batteries

Abstract

•A relatively stable, low-reactive intermediate was formed by a soluble catalyst•The charge overpotential of the Li-O2 battery has been reduced significantly•Li2O2 formation and decomposition have been expanded reversibly•An ultralong cycle life (up to 371 cycles) has been obtained in the Li-O2 battery As a result of the increasing demands for electric vehicles with a long driving range, the Li-O2 battery, which has the highest theoretical energy density, has been widely investigated for many years. However, its development is critically hindered by limited capacity (far less than in theory), large overpotential, and severe side reactions, which can be attributed to the low catalytic efficiency of solid catalyst and high reactivity of the O2− intermediate. Here, we present a Ru(II) polypyridyl complex (RuPC) as a high-efficiency soluble catalyst for Li-O2 batteries to overcome these difficulties. We show that the Ru center has a good interaction with O2− species, which can not only reversibly expand Li2O2 formation and decomposition with a low overpotential but also suppress the side reactions, thereby enabling an excellent performance for the Li-O2 battery. More importantly, it provides an alternative way of designing the soluble organic catalysts for metal-O2 batteries. The aprotic lithium-oxygen (Li-O2) battery has attracted worldwide attention because of its ultrahigh theoretical energy density. However, its practical application is critically hindered by cathode passivation, large polarization, and severe parasitic reactions. Here, we demonstrate an originally designed Ru(II) polypyridyl complex (RuPC) though which the reversible expansion of Li2O2 formation and decomposition can be achieved in Li-O2 batteries. Experimental and theoretical results revealed that the RuPC can not only expand the formation of Li2O2 in electrolyte but also suppress the reactivity of LiO2 intermediate during discharge, thus alleviating the cathode passivation and parasitic reactions significantly. In addition, an initial delithiation pathway can be achieved when charging in turn; thus, the Li2O2 products can be decomposed reversibly with a low overpotential. Consequently, the RuPC-catalyzed Li-O2 batteries exhibited a high discharge capacity, a low charge overpotential, and an ultralong cycle life. This work provides an alternative way of designing the soluble organic catalysts for metal-O2 batteries. The aprotic lithium-oxygen (Li-O2) battery has attracted worldwide attention because of its ultrahigh theoretical energy density. However, its practical application is critically hindered by cathode passivation, large polarization, and severe parasitic reactions. Here, we demonstrate an originally designed Ru(II) polypyridyl complex (RuPC) though which the reversible expansion of Li2O2 formation and decomposition can be achieved in Li-O2 batteries. Experimental and theoretical results revealed that the RuPC can not only expand the formation of Li2O2 in electrolyte but also suppress the reactivity of LiO2 intermediate during discharge, thus alleviating the cathode passivation and parasitic reactions significantly. In addition, an initial delithiation pathway can be achieved when charging in turn; thus, the Li2O2 products can be decomposed reversibly with a low overpotential. Consequently, the RuPC-catalyzed Li-O2 batteries exhibited a high discharge capacity, a low charge overpotential, and an ultralong cycle life. This work provides an alternative way of designing the soluble organic catalysts for metal-O2 batteries. 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When charging in turn, the interaction between the Ru center and the LiO2 intermediate can also promote the initial delithiation of Li2O2 products, which is a more kinetically favorable and highly reversible pathway. Moreover, because of the interaction between the Ru center and the LiO2 intermediate, the RuPC catalyst can serve as a highly mobile trap for the LiO2 intermediate to suppress the superoxide-related parasitic reactions, thus increasing the reversibility of the Li-O2 battery. As a result, the RuPC-catalyzed Li-O2 batteries can deliver a high discharge capacity (∼9,281 mAh g−1) that is two to three times what the RuPC-free ones (∼4,100 mAh g−1) can achieve, an ultralong cycle life (371 cycles) that is ∼31 times what the RuPC-free ones can achieve (12 cycles), and a lower charge overpotential (0.54 V) than that of the RuPC-free ones (1.30 V), as well as reduced side products. Finally, we have revealed the O2 redox mechanisms of the RuPC-catalyzed Li-O2 batteries by combining the experimental and theoretical results. To reversibly expand Li2O2 formation and decomposition with a low overpotential, several key conditions need to be satisfied: (1) the ability to induce LiO2 to dissolve into the electrolyte, (2) the power to suppress the high reactivity of LiO2, and (3) good Li2O2/catalyst contact interface. It is well known that O2− has a relatively large radius and low charge density and can be considered a moderately soft base.37Laoire C.O. Mukerjee S. Abraham K. 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 (802) Google Scholar On the basis of the hard-soft acid-base theory, a substance with the properties of soft acid can interact with O2− species and control its reactivity. In addition, the removable solution-phase catalyst can reach everywhere in the electrolyte and provide a good Li2O2/catalyst contact interface. Thus, taking the above conditions into account, the metal-organic complexes are the primary choice because they have superior catalytic activities and high solubility in organic solvents.38Seo J.S. Whang D. Lee H. Jun S.I. Oh J. Jeon Y.J. Kim K. A homochiral metal-organic porous material for enantioselective separation and catalysis.Nature. 2000; 404: 982-986Crossref PubMed Scopus (3736) Google Scholar, 39Carrettin S. Corma A. Iglesias M. Sánchez F. Stabilization of Au(III) on heterogeneous catalysts and their catalytic similarities with homogeneous Au(III) metal organic complexes.Appl. Catal. A Gen. 2005; 291: 247-252Crossref Scopus (92) Google Scholar, 40Gong L. Wenzel M. Meggers E. Chiral-auxiliary-mediated asymmetric synthesis of ruthenium polypyridyl complexes.Acc. Chem. 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Preparation of novel sulfur and phosphorus containing oxazolines as ligands for asymmetric catalysis.Tetrahedron. 1994; 50: 799-808Crossref Scopus (96) Google Scholar, 42Al-Rawashdeh N.A.F. Chatterjee S. Krause J.A. Connick W.B. Ruthenium bis-diimine complexes with a chelating thioether ligand: delineating 1,10-phenanthrolinyl and 2,2′-bipyridyl ligand substituent effects.Inorg. Chem. 2014; 53: 294-307Crossref PubMed Scopus (30) Google Scholar and then used it as a multifunctional soluble catalyst for Li-O2 batteries. As shown in Figures S2–S5, the structure of RuPC was well established by the 1H nuclear magnetic resonance (NMR), 13C NMR, Fourier transform infrared (FTIR), and high-resolution mass spectrometry (HRMS) spectra. Next, to understand the electrochemical behavior of RuPC and to distinguish it from the redox mediators reported before,16Sun D. Shen Y. Zhang W. Yu L. Yi Z. Yin W. Wang D. Huang Y. Wang J. Wang D. et al.A solution-phase bifunctional catalyst for lithium-oxygen batteries.J. Am. Chem. Soc. 2014; 136: 8941-8946Crossref PubMed Scopus (369) Google Scholar, 17Lim H.-D. Lee B. Zheng Y. Hong J. Kim J. Gwon H. Ko Y. Lee M. Cho K. Kang K. Rational design of redox mediators for advanced Li-O2 batteries.Nat. Energy. 2016; 1: 16066Crossref Scopus (264) Google Scholar, 43Chen Y. Freunberger S.A. Peng Z. Fontaine O. Bruce P.G. Charging a Li-O2 battery using a redox mediator.Nat. Chem. 2013; 5: 489-494Crossref PubMed Scopus (700) Google Scholar, 44Lim H.-D. Song H. Kim J. Gwon H. Bae Y. Park K.-Y. Hong J. Kim H. Kim T. Kim Y.H. et al.Superior rechargeability and efficiency of lithium-oxygen batteries: hierarchical air electrode architecture combined with a soluble catalyst.Angew. Chem. Int. Ed. 2014; 53: 3926-3931Crossref PubMed Scopus (393) Google Scholar we conducted a cyclic voltammetry (CV) experiment in a typical Li-O2 battery environment (Figure S6). Under an argon atmosphere (Figure S6A), both curves of the Li-O2 batteries with (red) and without (black) RuPC show no reaction features in the voltage range between 2.2 V and 4.2 V, whereas the one with RuPC exhibits the Ru2+/Ru3+ redox couple in the potential of ∼4.64 V. As a comparison, measurements in O2 atmosphere were also carried out (Figure S6B). Both the batteries with (red) and without (black) RuPC exhibit a cathodic peak associated with oxygen reduction reaction (ORR) near 2.62 V; however, the cathodic peak current of the battery with RuPC is larger than that of the RuPC-free one, implying the RuPC can promote the ORR. Notably, a significant difference appears in the anodic region. The battery without RuPC shows a sharp anodic peak at ∼4.44 V, which is attributed to Li2O2 decomposition and indicates the sluggish kinetics of the oxygen evolution reaction (OER), whereas the battery with RuPC exhibits a lower anodic peak at ∼3.73 V. These observations demonstrate that the introduction of RuPC in the Li-O2 battery could reduce the OER overpotential significantly. Distinctly, because the Li2O2 oxidation (∼3.73 V) is prior to the RuPC oxidation (∼4.64 V) in RuPC-containing Li-O2 batteries, the mechanism of the OER overpotential improvement in this case is different from that of redox mediators.16Sun D. Shen Y. Zhang W. Yu L. Yi Z. Yin W. Wang D. Huang Y. Wang J. Wang D. et al.A solution-phase bifunctional catalyst for lithium-oxygen batteries.J. Am. Chem. Soc. 2014; 136: 8941-8946Crossref PubMed Scopus (369) Google Scholar, 17Lim H.-D. Lee B. Zheng Y. Hong J. Kim J. Gwon H. Ko Y. Lee M. Cho K. Kang K. Rational design of redox mediators for advanced Li-O2 batteries.Nat. Energy. 2016; 1: 16066Crossref Scopus (264) Google Scholar, 43Chen Y. Freunberger S.A. Peng Z. Fontaine O. Bruce P.G. Charging a Li-O2 battery using a redox mediator.Nat. Chem. 2013; 5: 489-494Crossref PubMed Scopus (700) Google Scholar, 44Lim H.-D. Song H. Kim J. Gwon H. Bae Y. Park K.-Y. Hong J. Kim H. Kim T. Kim Y.H. et al.Superior rechargeability and efficiency of lithium-oxygen batteries: hierarchical air electrode architecture combined with a soluble catalyst.Angew. Chem. Int. Ed. 2014; 53: 3926-3931Crossref PubMed Scopus (393) Google Scholar Hence, the RuPC does not act as a redox mediator here, and the OER overpotential improvement would be due to the catalytic activity of RuPC on Li2O2 decomposition. To confirm that the improved electrochemical performances were caused by RuPC, we chose the carbon black nanoparticles as cathode material without loading any other catalysts. Figure 1B illustrates the full discharge curves of the Li-O2 batteries with and without RuPC in a 0.5 M LiClO4/DMSO system. The RuPC-catalyzed Li-O2 batteries exhibited a high discharge capacity of ∼9,281 mAh g−1 at a current density of 200 mA g−1, which is two to three times higher than that of the RuPC-free Li-O2 batteries (∼4,100 mAh g−1). To identify whether the capacity was derived from an O2 reduction reaction, we discharged the battery with RuPC under Ar atmosphere. As shown in Figure S7, a negligible capacity of 2.7 mAh g−1 was observed, which means the discharge capacity of the RuPC-containing Li-O2 battery was indeed contributed by the O2 reduction. These outcomes demonstrate that the discharge capacity of the Li-O2 battery could be significantly enhanced by RuPC, which is consistent with the results of CV experiment. As for the charge process, the charging overpotential of the Li-O2 battery also obtained a significant improvement in the presence of RuPC (Figures 1B and 1C). Specifically, at a current density of 100 mA g−1 with a cutoff capacity of 1,000 mAh g−1 (Figure 1C), the battery without RuPC exhibited a voltage platform at about 4.26 V in the charge process, whereas that of the RuPC-containing battery was only 3.50 V; moreover, the lower charging voltage could be achieved with the increased RuPC concentration (Figure S8). According to the electrochemical impedance spectrometry (EIS) results (Figure S9 and Table S1), the overpotential reduction is not due to the solution resistance decline. Therefore, such a significant improvement suggests that the kinetics of Li2O2 decomposition were promoted greatly by RuPC, confirming the catalytic role of RuPC. Furthermore, the Li-O2 batteries with and without RuPC were tested at a current density of 400 mA g−1 with a cutoff capacity of 500 mAh g−1. The charge voltage platforms of the battery with RuPC could still be maintained at around 3.75 V even after 300 cycles (Figure S10A), whereas that of the battery without RuPC soon increased to 4.0 V and then remained above 4.2 V (Figure S10B). In addition, the discharge terminal voltage of the battery without RuPC decreased to 2.0 V early at the 12th cycle, and the discharge terminal voltage of the battery with RuPC remained above 2.0 V steadily even after 371 cycles (Figure 1D). Further, when under a cutoff capacity of 1,000 mAh g−1, the RuPC-containing Li-O2 battery could still operate stably over 100 cycles at a current density of 400 mA g−1 (Figure S11). Besides, the RuPC-containing Li-O2 batteries exhibited good rate capability with discharge capacities of ∼11,031, ∼9,281, and ∼7,723 mAh g−1 at current densities of 100, 200, and 300 mA g−1, respectively (Figure S12). Moreover, the voltage attenuation of the discharge or charge process was not obvious as the current density increased. These outcomes fully supported that the electrochemical performance of the Li-O2 battery could be significantly improved by RuPC. Further, the discharged and charged carbon electrodes were extracted and detected by scanning electron microscopy (SEM). In the absence of RuPC, the particles with toroidal morphology and the film-like discharge products were co-existing on the carbon surface after full discharge (Figures 2A, 2B, S13A, and S13B). However, when RuPC was present, only toroidal particles, clearly and compactly, could be found on the carbon surface after full discharge (Figures 2C, 2D, S13C, and S13D). Such an observation also suggests intuitively that