ROS-Resistant Redox Mediator in Lithium-Oxygen Batteries

ROS-Resistant Redox Mediator in Lithium-Oxygen Batteries | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article ROS-Resistant Redox Mediator in Lithium-Oxygen Batteries Won-Jin Kwak, Hyun-Wook Lee, Jiwon Hwang, Ja-Yeong Kim, Gabriel Morais, and 9 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4370577/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The utilization of redox mediators (RMs) in lithium-oxygen batteries (LOBs) have underscored their utility in addressing the challenge of the elevated overpotential during the charging process. Nonetheless, the generation of highly electrophilic singlet oxygen ( 1 O 2 ) throughout the battery cycles leads to adverse reactions with RMs, thereby impeding their effectiveness. In the quest for enhanced RM durability, this study unveils a novel RM, 7,7'-bi-7-azabicyclo[2.2.1]heptane (BAC), incorporating N–N interconnected aza-bicycles, and assesses its efficacy and robustness relative to those of other N–N non-bicyclic RMs. Unlike non-bicyclic RMs, which exhibit diminished O 2 evolution after exposure to 1 O 2 , BAC maintains consistent O 2 profiles during charging, indicating its superior 1 O 2 resistance and steady redox-catalyst performance in LOBs. Theoretical analyses align with the experimental findings that the bicyclic structural design of BAC confers a superior ability to resist oxidative degradation through cleavage of the C–H bonds adjacent to nitrogen atoms. Physical sciences/Energy science and technology/Energy storage/Batteries Physical sciences/Materials science/Materials for energy and catalysis/Batteries Physical sciences/Chemistry/Electrochemistry/Batteries Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction The high theoretical capacity of lithium-oxygen batteries (LOBs) is a major attraction for potentially replacing common battery systems. However, large charging overpotential in LOBs triggers various side reactions, and redox mediators (RMs) have been adopted to mitigate charging overpotential. Since the initial introduction of RMs to LOBs, a range of RMs has been developed to minimize energy loss and increase cyclability of LOBs 1−2345 . The primary role of RMs is promoting the decomposition of lithium peroxide (Li 2 O 2 ) at lower charging potential, preventing the detrimental reactions occurring at typical charging potential. A range of studies have been conducted to elucidate the diverse interactions between RMs and Li 2 O 2 and the wide array of mechanistic pathways they induce, with a particular emphasis on reaction kinetics 6−7 8 9 10 . The decomposition of Li 2 O 2 is characterized by a two-electron transfer process wherein oxidized RMs engage in a series of reactions with Li 2 O 2 , resulting in the liberation of O 2, as described in the following equation; Li 2 O 2 + 2RM + → 2Li + + O 2 + 2RM. The decomposition of Li 2 O 2 driven by RMs involves an outer-sphere electron transfer between RM and Li 2 O 2 , and the redox potentials of RMs can be tuned in accordance with Marcus theory for more favorable decomposition kinetics 11−1213 . The evolution of highly electrophilic 1 O 2 throughout the operational cycles of LOBs holds paramount importance 6 − 7 8 , as 1 O 2 generation is known to cause various side reactions that impact the lifespan of LOBs. Most RMs are electron-rich and therefore tend to undergo irreversible reactions with electrophilic 1 O 2 9 , 10 ,14 (Fig. 1a). Such reactions result in gradual degradation in the capacity of RMs to decompose Li 2 O 2 throughout cycles, leading to deteriorating electrochemical performance 2 ,15 . The redox potentials of RMs were found to be correlated to 1 O 2 evolution, since recent research indicated RMs with redox potential below the theoretical 1 O 2 evolution threshold of 3.54 V exhibit attenuated 1 O 2 generation through Li 2 O 2 decomposition during charging processes 16,17 (Fig. 1b). To mitigate the challenges associated with controlling redox potential and reactivity towards 1 O 2 in existing RMs, we adopted a rational design approach based on structural analysis. The first step of this approach involved categorizing existing RMs, especially those featuring structures with nitrogen atoms, given that most RMs contain one or more nitrogen atoms. According to molecular architecture, we classified precedented N-containing organic RMs which are employed in LOBs into three categories: arylamines, nitroxyl radicals, and alkylamines/alkylhydrazines (Fig. 1c). Each scaffold contains site(s) that are susceptible to either interaction with or generation of 1 O 2 . In the category of arylamine RMs such as 10-methylphenothiazine (MPT), 5,10-dimethylphenazine (DMPZ), tris[4-(diethylamino)phenyl]amine (TDPA), and N,N,N',N' -tetramethyl- p -phenylenediamine (TMPD) (Fig. 1c, left side), unsaturated hydrocarbon backbones show vulnerability to unwanted cycloadditions with electrophilic 1 O 2 due to the high electron density associated with the aromatic nature of arylamines. A previous study revealed that DMPZ, a widely recognized arylamine-type RM, undergoes deactivation by 1 O 2 in LOBs leading to the generation of an endoperoxide-type intermediate through a [4+2] cycloaddition reaction 9 . The same literature also suggests, despite its absence from Fig. 1b, DMPZ is considered susceptible to an alternative degradation pathway known as formal [2+2] cycloaddition resulting in a dioxetane-type intermediate. Nitroxyl radical RMs (Fig. 1c, center) such as 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) and 1-methyl-2-azaadamantane- N -oxyl (MAZO) are characterized by a bond order of 1.5 (bond dissociation energy, 100 kcal/mol) 18,19 and benefit from steric protection due to the presence of four α-methyl groups. This structural configuration enables their persistence as stable radicals, but their stability coupled with a tendency to resist oxidation and readily undergo reduction, resulting in relatively a higher redox potential than other types of RMs. Nitroxyl radical-based RMs predominantly exhibit redox potentials above 3.54 V (vs Li + /Li), increasing the ratio of 1 O 2 to triplet oxygen ( 3 O 2 ) production during the operation of LOBs. A notable compound in this category is TEMPO, with a redox potential of 3.74 V (vs Li + /Li), which generates significant amount of 1 O 2 16 ,20 . Alkylamine and alkylhydrazine RMs (Fig. 1c, right side) possess low redox potentials similar to those of arylamine-type RMs. Notably, within the alkylhydrazine classification, recent introductions include RMs such as 1,1'-bipyrrolidine (BP55), 1-(pyrrolidin-1-yl)piperidine (BP56), and 1,1'-bipiperidine (BP66). Apart from BP66, these RMs are characterized by low redox potentials and have been demonstrated to possess a high kinetic rate for Li 2 O 2 decomposition along with an efficient quenching capacity for 1 O 2 , markedly diminishing the yield of 1 O 2 16 . Nevertheless, the presence of protons at carbons neighboring nitrogen atoms predisposes them to facile oxidation through the formation of iminiums or C-centered radicals. These reactive intermediates in turn facilitate further degradation through reactions with nucleophiles or somophiles, including water vapor 21 present in LOBs, to eventually impact the overall integrity of LOB systems. Advancing the development of a new RM scaffold that circumvents the chemical and electrochemical shortcomings inherent to the existing three RM classes, a stepwise strategy was employed to explore one structural variation at a time (Fig. 2, left side). Beginning with the TEMPO structure as a basis, attempts were made to decrease the redox potential by removing the oxygen atom from the nitroxyl group and introducing N–H or N–alkyl bonds, leading to the formulation of 2,2,6,6-tetramethylpiperidine (TEMP) and alkyl-substituted tertiary amines. However, prior theoretical analysis suggested that these alternatives would still be prone to oxidation, resulting in their exclusion from further consideration 22– . The strategy then shifted towards substituting the oxygen atom with nitrogen, but the N , N' -bipyridyl dication (pyridine dimer) was assessed to be excessively unstable due to the repulsion between the nitrogen cations, with a theoretical predisposition towards N–N bond cleavage. An approach to remove olefins from the N , N' -bipyridyl dication to decrease the possibility of cycloaddition reactions with 1 O 2 theoretically leads to BP66 structure, which, however, remains prone to Cα–H oxidation. A further modification involved the full substitution of α carbons with four methyl groups on each piperidine side, akin to the TEMP–TEMP (N–N coupled dimer). Computational evaluations predicted significant steric congestion, rendering the synthesis unattainable. Even if synthesis were achievable, the anticipated steric hindrance could limit the accessibility to Li 2 O 2 , thereby impeding decomposition within LOBs. These series of experimental and theoretical attempts to innovate within the framework of N-containing RMs like TEMPO and BP66 (or BP55) ultimately resulted in a continuous loop of stagnation, unable to break free from the existing paradigms. Using BP66 as a starting point and applying Bredt’s rule, we envisioned that the bicyclic 7,7'-bi-7-azabicyclo[2.2.1]heptane (BAC) with its nitrogen-bearing carbons placed at the bridgehead positions could function as an RM that would be resistant to parasitic oxidation processes (Fig. 2, right side). Bredt’s rule, postulated by Julius Bredt in 1924, asserts that the formation of π bonds between the bridgehead atom and adjacent atoms is inherently forbidden due to the orthogonal alignment of their p orbitals, leading to poor p orbital overlap 26 . Despite the protective premise of Bredt’s rule, other potential oxidative degradation pathways involving reactive oxygen species (ROS) such as 1 O 2 and superoxide (O 2 •− ) are still possible. To address this, we conducted a thorough density functional theory (DFT) evaluation of both BAC and BP RMs to ensure that BAC would be resistant to all possible oxidation pathways. Subsequently, the synthesis of BAC was realized, and its resistance to ROS under LOB operational conditions was experimentally examined by utilizing various chemical and electrochemical techniques such as nuclear magnetic resonance (NMR), cyclic voltammetry (CV), differential electrochemical mass spectroscopy (DEMS) analyses. Results Computational evaluation of RM resistance to oxidation by 1 O 2 . To confirm that the bridged BAC molecule we envisioned would be resistant to parasitic oxidation processes, we first performed DFT calculations on BAC, BP55 and BP66 to assess the activation barriers for their oxidative degradation through Cα hydrogen abstraction by 1 O 2 (Fig. 3). Cα hydrogen abstraction by 1 O 2 of alkylamines can proceed on either the closed-shell singlet (CSS) or the open-shell singlet (OSS) surface (Fig. 3a). After the formation of an initial encounter complex I , the CSS process (via TS-1 ) produces an ion pair consisting of an iminium and the OOH anion, which then combine in a barrierless or near-barrierless fashion to generate the zwitterionic structure III . The OSS process (via TS-2 ) produces a carbon-centered radical and the OOH radical, which can either recombine, undergo a radical-polar crossover to the CSS surface ( vide infra ), or dissociate to react with other chemical species present in the system. Fig. 3b shows the free energy evolution along the two possible Cα hydrogen abstraction pathways for BAC, BP55 and BP66. Our calculations showed that for all three RMs surveyed, the OSS Cα hydrogen abstraction transition state TS-2 is more favorable than the CSS transition state TS-1 . BAC was also found to have much higher Cα hydrogen abstraction barriers than either BP55 or BP66. (It is worth noting that for all three RMs, the CSS and OSS surfaces intersect after TS-2 ; it is therefore plausible that a radical-polar crossover event will follow TS-2 , leading to intermediate III .) The lower energy of TS-2a compared to TS-1a can be rationalized by noting that TS-1a momentarily produces an iminium ion with the C=N double bond at a bridgehead position, which is highly strained according to Bredt’s rule. In contrast, TS-2a produces a carbon-centered radical at the bridgehead position instead of forming a full double bond, lessening the strain. Despite being the more favorable pathway of the two, TS-2a still has a prohibitively high activation barrier of 37.9 kcal/mol, which lends support to our expectation that the bridged structure of BAC could confer exceptional stability under oxidative conditions. In contrast, the OSS Cα hydrogen abstraction barriers for BP55 and BP66 were only 20.4 and 22.6 kcal/mol, respectively (Fig. 3c), indicating that these two RM species are susceptible to oxidative degradation by 1 O 2 at room temperature. We next considered possible degradation of the RM molecules by 1 O 2 through concerted H 2 abstraction processes on the CSS surface (Fig. 4). This process leads to the generation of H 2 O 2 and an enamine (Pathway A) or alkene (Pathway B) depending on the location of H 2 abstraction (Fig. 4a). Fig. 4b shows the free energy evolution along the possible concerted H 2 abstraction pathways for BAC, BP55 and BP66. For both BP55 and BP66, the enamine-generating Pathway A was found to be more favorable, consistent with prior literature findings that neighboring amine substituents accelerate oxidative degradation through concerted H 2 abstraction 27 . For BAC, on the other hand, Pathway A would lead to a highly strained "bridgehead" double bond, resulting in an extremely high 52.2 kcal/mol barrier. TS-4a , which avoids placing the double bond in the bridgehead position, has a much lower barrier of 35.4 kcal/mol (Fig. 4c), albeit still prohibitively high for room-temperature conditions. These calculations further established that the bridged bicyclic structure of BAC would render it resistant to oxidative degradation by 1 O 2 through multiple possible mechanisms. A direct comparison of OSS (radical) oxidation of the three RMs by 1 O 2 is shown in Fig. 5a. The calculated energies indicate that BAC is dramatically more resistant to OSS oxidation by 1 O 2 than BP55 or BP66. The unusually high resistance to OSS oxidation for BAC can be attributed to the geometry of its radical form IVa after hydrogen atom transfer. In the BP55- and BP66-derived radicals IVb and IVc , the carbon radical centers are able to attain mostly planar geometries, as would be preferred in unstrained model systems (see Supplementary Fig. 1 for comparison). In the BAC-derived radical IVa , however, planarization is energetically extremely costly for the bridgehead carbon, leading to the carbon radical center being much more nonplanar (Fig. 5b). This leads to poor delocalization of the unpaired electron and higher energies as a result. Similarly, the high energy cost of planarizing a bridgehead carbon also renders CSS oxidation of BAC (through TS-1a or TS-3a ) extremely unfavorable. In addition, O 2 •− degradation of BAC was also found to be energetically all the way uphill. Overall, our computational analyses confirmed the soundness of our Bredt’s-rule-based design principle and predicted that BAC would be exceptionally stable to oxidative degradation by 1 O 2 at room temperature. Investigation of reactivity of RMs with 1 O 2 . To substantiate the computational findings regarding the stability of BP55, BP66, and BAC against 1 O 2 , each RM was synthesized 16 ,28 , and a method involving 1 O 2 -enriched environment is employed for degradation monitoring; upon exposure to 525 nm LED, tetraphenylporphyrin (TPP) is initially excited to its singlet state and undergoes intersystem crossing (ISC) to the triplet state, which, in the presence of proximal 3 O 2 , facilitates energy transfer, leading to the formation of the highly reactive 1 O 2 species 29,30 (Fig. 6a). In the 1 O 2 experiment, NMR analysis was concurrently performed to monitor the degradation of RMs, with Fig. 6b distinctly emphasizing the C–H bonds designated for tracing across RMs. The 1 H NMR spectra, displayed in black and bluish purple in the first and second rows (Figs. 6c–e and Figs. 6f–h), depict the conditions of each RM prior to exposure to 1 O 2 , without and with the electrolytes and photocatalyst, respectively; the elevated, dense peaks in chemical shift ranging from 3.3 to 3.7 represent the proton patterns of tetraethyleneglycol dimethylether (TEGDME) as one of the electrolyte molecules. The analytes containing the RMs, the electrolytes, and the photocatalyst were exposed to 1 O 2 environment, and the corresponding spectra are obtained and shown in Figs. 6i–k. Characteristic proton peaks—eight of each H α and H β for BP55 and eight of each H α and H β with four of H γ for BP66—completely disappeared while new peaks, belonging to an unidentified byproduct(s), were detected in the spectra (Figs. 6i,k). These results were consistent with our computational prediction that oxidative degradation through the OSS pathway would be feasible at room temperature for both BP55 (20.4 kcal/mol barrier, corresponding to the difference between II and TS-2 ) and BP66 (22.6 kcal/mol). However, BAC demonstrated chemical stability without unintended deterioration, following exposure to 1 O 2 as shown in Fig. 6k. Electrochemical behavior of RM s . The durability of RMs towards ROS was evaluated via CV by comparing the electrochemical activity of RMs before and after exposure to ROS. All examined RMs demonstrated a redox potential under 3.54 V, conforming to the criteria for diminishing 1 O 2 evolution during charging process (Figs. 7a–c). Nevertheless, both BP55 and BP66 were found to be devoid of redox-active properties after exposure to 1 O 2 , indicating a considerable loss of electrochemical activity due to the oxidative effects of 1 O 2 (Figs. 7d,e). This observation reveals the irreversible reaction triggered by 1 O 2 resulting in the formation of byproducts lacking in redox activity, which is characteristic of trap-type-RM behavior 10 . One notable is that no discernible changes in CV profiles of the RMs upon exposure to O 2 •− were shown implying the resilience of BP55 and BP66 to O 2 •− , compared to their sensitivity to 1 O 2 (Supplementary Figs. 12a,b). Remarkably, BAC maintains reversible CV profiles even after exposure to either 1 O 2 or O 2 •− , sustaining redox activity without any potential shift (Figs. 7c,f and Supplementary Fig. 12c). This persistence is attributed to the chemical robustness of BAC, as proven by NMR analysis, which also underlines high durability of BAC in maintaining electrochemical activity. The stability of RMs during galvanostatic cycling test was verified in LOB configuration (Supplementary Fig. 13), with the initial charging potential of BP66 approaching 3.50 V and a longer plateau than BP55. In general, the relationship between the redox potentials of RMs and kinetic of Li 2 O 2 decomposition adheres to an inverted parabola shape in accordance with Marcus theory 16 . Given that the kinetic of Li 2 O 2 decomposition approaches saturation around 3.70 V 10 , 31 , it is reasonable to deduce that BP66, with its higher redox potential compared to BP55, exhibits enhanced kinetics for Li 2 O 2 decomposition, resulting in a longer charging voltage plateau. However, the terminal charging voltage for both BP55 and BP66 had progressively increased by the third cycle, reaching the typical charging voltage of LOBs without a redox catalyst. For both BP55 and BP66, a decline in the retention of the charging voltage was observed, indicative of the RMs undergoing degradation concurrently with the cycling process. Moreover, amount of evolved 1 O 2 accumulated over cycles, which affects the capacity of both BP55 and BP66 to sustain the low charging voltages. Differing from BP55 and BP66, BAC retained a stable redox potential before and after exposure to 1 O 2 . The chemical stability of BAC, anchored in its resistance to 1 O 2 , further delineates the proficiency of BAC in oxidizing Li 2 O 2 . To confirm the resilience of BAC against 1 O 2 , relying solely on electrochemical measurements proves insufficient. Hence, to validate the continuous performance of BAC as an RM post 1 O 2 exposure, the species of gases evolved were analyzed by DEMS during the initial step of the charging process in the presence of three RMs: BP55, BP66, and BAC. Fig. 8a shows the voltage and gas evolution profiles comparing BP55 before and after 1 O 2 exposure. Before 1 O 2 exposure, BP55 exhibited charging plateau at 3.25 V (vs Li + /Li) producing certain amount of O 2 with suppressed H 2 and O 2 gas. However, 1 O 2 exposed BP55 only gave concentrated O 2 evolution during the first hour of charging and nearly no O 2 evolved. Large amount of CO 2 rather evolved after indicating extensive side reactions occurred at high charging potential 32–33 34 and BP55 lost function as RM by 1 O 2 . BP66 demonstrated results analogous to those of BP55 (Fig. 8b). BP66 before 1 O 2 exposure exhibited higher charging potential than BP55, which is in accord with Fig.6. A little increased amount of O 2 than that with BP55 was detected and it was due to the higher charging kinetics of BP66 with higher charging potential. After 1 O 2 exposure, BP66 also reacted with 1 O 2 and lost catalytic function, therefore the O 2 evolution profile changed to unstable and large amount of CO 2 was detected. That is, BP55 and BP66 has trap-type RMs characteristics and 1 O 2 critically affects to their function as RM eventually deactivating them. Fig. 7c shows the behavior of BAC which contrasts to BP55 or BP66. In both before and after 1 O 2 exposure, BAC gave catalytic O 2 evolution profiles implying highly 1 O 2 resistive characteristic of BAC (Fig. 8c). Particularly, CO 2 evolution was suppressed even after 1 O 2 exposure which is indicative of well-preserved functionality of BAC as RM. Based on the gas evolution profiles in Fig. 8, accumulated gas evolution rate was recalculated (Supplementary Fig. 15 and Supplementary Table 1). Proportion of O 2 before 1 O 2 exposure was comparable for BP55 and BP66, 47 and 43% respectively, but still lower than BAC suggesting comparatively low Li 2 O 2 decomposition kinetic of BP55 and BP66. The solution of BP55 and BP66, following exposure to 1 O 2 , gave a depressed ratio of O 2 , with CO 2 levels rising from 42 to 74% and from 40 to 67%, respectively. Increment of CO 2 demonstrates that BP55 and BP66 lose function as RM by the aggressive attack of 1 O 2 and side reactions occurred intensively at high charging potential. In contrast, BAC showed highly catalytic behavior with the ratio of evolved O 2 even after 1 O 2 exposure comparable to before 1 O 2 exposure (79 and 82%, respectively). The solution of BAC, after exposure to 1 O 2 , exhibited an increased H 2 ratio compared to its pre-exposure state. However, the total ratios of H 2 and CO 2 in the solution of BAC before and after exposure were nearly identical, suggesting that minimal side reactions occurred at similar rates in both environments. Electrochemical measurements consistently point out that BP55 and BP66 have no durability against 1 O 2 , thereby both are significantly fragile to 1 O 2 -involved oxidation. More importantly, such 1 O 2 reaction makes BP55 and BP66 lose their electrochemical activity. BAC, on the other hand, has highly 1 O 2 durable molecular structure and therefore promise preservation of electrochemical activity even under the continuously 1 O 2 evolving environment, the LOBs. Conclusion Despite the high energy density of LOBs, addressing high charging overpotentials and controlling 1 O 2 evolution remains essential. Current redox mediators often exceed a 3.54 V redox potential, initiating 1 O 2 production and/or deteriorating upon exposure to 1 O 2 . This complicates achieving both low redox potential and robust 1 O 2 resistance. BP55 and BP66, for example, achieved the necessary redox potential to curb 1 O 2 evolution but undergo chemical alterations from its oxidation by 1 O 2 , rendering them ineffective. Conversely, the new mediator BAC, designed in compliance with Bredt’s rule, retains its chemical and electrochemical properties after 1 O 2 exposure, demonstrating high electrochemical reversibility and efficient oxygen release during charging. These findings underscore the importance of the stability of RMs towards 1 O 2 for maintaining chemical and electrochemical functionality. Molecular design, supported by computational studies that calculated the energy profiles for plausible oxidative side reactions of RMs and 1 O 2 , highlights the bicyclic compound BAC as particularly robust. This theoretical robustness of BAC was further confirmed through experimental validations using NMR, CV, and DEMS analyses, establishing a promising strategy for developing 1 O 2 -stable materials essential for enhancing LOBs. Nevertheless, while challenges such as the kinetics of RMs, byproduct accumulation, and electrode design are crucial for advancing LOBs, the development of BAC, which robustly withstands 1 O 2 challenges, is pivotal for addressing these specific issues. Moreover, the rational design process employed in the development of BAC is expected to enhance LOB technology while also yielding valuable insights into the development of mediators and catalysts for diverse organocatalytic reactions. Methods Computational methods. All DFT computations were performed using Gaussian 16, Revision C.01 35 . Unless otherwise specified, molecular geometries were optimized using the long-range-corrected ωB97X-D 36 functional and the def2-SVP basis set. Frequency calculations were performed at the same level of theory as that used for geometry optimization to characterize the stationary points as either minima (no imaginary frequencies) or first-order saddle points (one imaginary frequency) on the potential energy surface. Intrinsic Reaction Coordinate (IRC) calculations were performed to confirm the first-order saddle points as real transition states connecting the expected reactants and products. Thermal contributions to Gibbs free energies were calculated from vibrational frequencies using the quasi-rigid rotor-harmonic oscillator (RRHO) approach of Grimme 37 implemented through Paton’s GoodVibes 38 Python script. Single-point energies were calculated with the ωB97X-D functional and the triple-zeta-quality def2-TZVPP. Solvation effects were incorporated in single-point energy calculations using Truhlar’s SMD 39 model using the keyword scrf=(smd,solvent=generic,read) and specifying eps=7.8 for TEGDME. Because the isolated 1 Δ g singlet oxygen molecule is multi-determinant and cannot be modeled accurately by DFT methods, 1 Δ g singlet oxygen was instead modeled by adding 22.5 kcal/mol to the free energy of triplet ( 3 Σ g ) oxygen (22.5 kcal/mol being the experimentally determined energy gap between 1 Δ g singlet oxygen and 3 Σ g triplet oxygen), a method employed successfully by Mullinax et al. for modeling singlet oxygen reactions 40 ,41 . Visualizations of molecular structures were obtained using CYLview 42 . Reiterative Monte Carlo conformational searches were performed with the Merck molecular force field (MMFF) implemented in Spartan '20 43 . All conformers within 10.0 kcal/mol of the lowest-energy conformer through Monte Carlo searches were reoptimized in Gaussian using the DFT methods described above. Synthesis of 7,7'-Bi-7-azabicyclo[2.2.1]heptane (BAC). A 40 mL vial was charged with a magnetic stir bar, 7-chloro-7-azabicyclo[2.2.1]heptane (467 mg, 3.55 mmol) and THF (15.1 mL) under nitrogen gas and was cooled to –78 ℃. To the solution, a 1.66 M tert -butyllithium (2.15 mL) was added dropwise. After being stirred for an hour at –78 ℃, the reaction mixture was allowed to warm to 23 ℃. After 22 hours, the resulting mixture was treated with water (15 mL) and extracted with diethyl ether (15 mL x 3). The organic layer was washed with brine solution (20 mL x 3), dried over sodium sulfate, filtered, and concentrated. The crude mixture was purified by flash column chromatography (ethyl acetate: hexane = 1:8) to give 205 mg (1.07 mmol) of BAC. After the purification process stated above, over 98.5% purity of BAC was observed. To increase the purity eliminating minor impurities such as grease and unknown compounds, preparative HPLC was necessary. 1 H NMR spectrum of BAC was matched with the literature 28 . Note: The preparation of 7-chloro-7-azabicyclo[2.2.1]heptane is included in Supplementary Information. Isolated Yield: 60%; Physical Property: White solid.; 1 H NMR (600 MHz, CDCl 3 containing 0.03% (v/v) TMS): δ 3.22 (td, J = 3.1, 1.5 Hz, 4H), 2.08–2.02 (m, 4H), 1.61–1.55 (m, 4H), 1.21 (dd, J = 6.9, 2.5 Hz, 4H), 1.14 (dd, J = 7.0, 2.5 Hz, 4H). Electrochemical measureme nt. Electrochemical experiments were conducted through galvanostat/potentiostat (WBSCS 3000Ls, WonATech, Korea) under 25 ℃. For CV, three electrodes system was chosen utilizing platinum wire, glassy carbon (3 mm) and RE-7 (0.01 M Ag/Ag + in acetonitrile) as counter, working and reference electrode, respectively. Cycle tests were conducted assembling Swagelok-type cell with Li metal (12 mm), GF/B (12.5 mm) and GDL (12 mm). All Li foils were pretreated to coat protective layer following previously reported method 44 . Electrolyte consists of 1 M LiTFSI, 50 mM additives and 0.5 mM TPP in TEGDME or same composition after 1 O 2 or O 2 •− exposure were used for CV and cycle tests were conducted with the above electrolytes substituting TEGDME to DMAc. In-situ DEMS analysis. Generation of H 2 (m/z = 2), O 2 (m/z = 32) and CO 2 (m/z = 44) gases were monitored with in-situ DEMS analysis. A custom-built cell containing 2032 coin cell was utilized to analyze gas evolution during the initial charging process with as-prepared electrolyte. Each charge-discharge procedure was conducted at 200 μA rate for five hours in 25 °C. The in-situ DEMS cells were assembled within an argon-filled glovebox and the gas evolution detection were set at five-minute intervals. ROS exposure treatment . To expose RM to 1 O 2 , electrolytes containing 1 M LiTFSI, 50 mM additives and 0.5 mM TPP were exposed to light source which has maximum wavelength at 525 nm (525PF, HepatoChem) over set time. O 2 was continuously purged at 30 cc min – 1 . In O 2 •− exposure test, 0.6 mM KO 2 and 1.2 mM 18-crown-6 ether were used to generate O 2 •− and both were added to each electrolyte. As-prepared electrolytes were stirred for an hour to generate O 2 •− enough. Declarations Data availability The data that support the findings of this study are accessible within the Article and its Supplementary Information, or can be provided by the corresponding authors upon reasonable request. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1F1A1061297), the Core Research Institute Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2021R1A6A1A10044950), and the H2KOREA funded by the Ministry of Education (2022Hydrogen fuel cell-002, Innovative Human Resources Development Project for Hydrogen Fuel Cells). This research was also supported by Learning & Academic research institution for Master 's·PhD students, and Postdocs (LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS-2023-00285390). S.C. is grateful to Oberlin College for financial support. DFT calculations were performed using the SCIURus, the Oberlin College HPC cluster (NSF MRI 1427949), as well as computing resources through allocation CHE210088 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program (NSF #2138259, #2138286, #2138307, #2137603, and #2138296). Author contributions S.-E.S. and W.-J.K. collaboratively designed and supervised the project, managing its administration. The project was jointly led by H.-W.L., J.H., and J.-Y.K., who conducted the experiments with assistance from M.C., H.C., and H.-B.Y. Assisted by G.N.M. and K.S.T., G.N.M and K.S.T conducted the DFT calculations under the guidance of S.C.. S.-T.K. undertook the preliminary DFT studies. J.H. and S.J.K. conducted the differential electrochemical mass spectroscopy analyses. Competing interests A patent related to the use of BAC in lithium air secondary battery: Kwak, W.-J.; Lee, H.-W.; Suh, S.-E.; Hwang, J.; Choi, M. "Electrolyte for Lithium Air Secondary Battery and Lithium Air Secondary Battery Including the Same" Korean Patent Application No. 10-2024-0058602, Filing Date: 05/02/2024. The remaining authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/xxxx Correspondence and requests for materials should be addressed to Shuming Chen, Sung-Eun Suh or Won-Jin Kwak. References Bi, X. et al. Understanding the Role of Lithium Iodide in Lithium–Oxygen Batteries. Adv. Mater. 34 , 2106148 (2022). Kundu, D., Black, R., Adams, B. & Nazar, L. F. 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A dendrite- and oxygen-proof protective layer for lithium metal in lithium–oxygen batteries J. Mater. Chem. A 7 , 3857-3862 (2019). Additional Declarations There is NO Competing Interest. Supplementary Files 24NCATLOBBACRMSI.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4370577","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":305424513,"identity":"2bdb39a6-c183-426c-bc62-bcf80853bb63","order_by":0,"name":"Won-Jin 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University","correspondingAuthor":false,"prefix":"","firstName":"Sung-Eun","middleName":"","lastName":"Suh","suffix":""}],"badges":[],"createdAt":"2024-05-05 06:55:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4370577/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4370577/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57061072,"identity":"42e431b2-aabb-4d1b-a98e-e103df869ad4","added_by":"auto","created_at":"2024-05-24 06:08:00","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":443902,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEngineering N-containing redox mediators overcoming pitfalls to enhance performance in lithium oxygen batteries (LOB). a\u003c/strong\u003e, Reversibility of RMs affected by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e evolved during disproportionation (DISP) of LiO\u003csub\u003e2\u003c/sub\u003e under controlled charging potential and phenomena observed in LOBs. \u003cstrong\u003eb\u003c/strong\u003e, Known N-containing RMs with their redox potentials (V vs Li\u003csup\u003e+\u003c/sup\u003e/Li). \u003cstrong\u003ec\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eProblematic sites in three N-containing RM scaffolds linked to undesired oxidation events with singlet oxygen involvement. Abbreviation:\u0026nbsp;RM, redox mediator; degrd, degraded; MPT, 10-methylphenothiazine; DMPZ, 5,10-dimethylphenazine; TDPA, tris[4-(diethylamino)phenyl]amine; TMPD, \u003cem\u003eN,N,N',N'\u003c/em\u003e-tetramethyl-\u003cem\u003ep\u003c/em\u003e-phenylenediamine; TEMPO, 2,2,6,6-tetramethyl-1-piperidinyloxyl; MAZO, 1-methyl-2-azaadamantane-\u003cem\u003eN\u003c/em\u003e-oxyl; BP55, 1,1'-bipyrrolidine; BP56, 1-(pyrrolidin-1-yl)piperidine; BP66, 1,1'-bipiperidine; E\u003csub\u003ered\u003c/sub\u003e\u003csup\u003e°\u003c/sup\u003e, standard redox potential; Cα–H, α carbon–hydrogen bond.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/efb0d0f4ac8ca2a697d8feb3.png"},{"id":57061073,"identity":"118cb408-e882-4776-8e58-bfc320801a8f","added_by":"auto","created_at":"2024-05-24 06:08:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":359096,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructure-performance relationship for development of ROS-resistant redox mediator. \u003c/strong\u003eAbbreviation: TEMP, 2,2,6,6-tetramethylpiperidine; NMR, nuclear magnetic resonance; CV, cyclic voltammetry; DEMS, differential electrochemical mass spectroscopy; CSS, closed-shell singlet; OSS, open-shell singlet; BAC, 7,7'-bi-7-azabicyclo[2.2.1]heptane.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/8f125d9178c0eba84e4b8726.png"},{"id":57061574,"identity":"8d1fabf1-279d-4edf-908a-cd2b240b2fb6","added_by":"auto","created_at":"2024-05-24 06:16:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":585175,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDFT computational evaluation of Cα hydrogen abstraction oxidation pathways\u003c/strong\u003e.\u003cstrong\u003e a\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eTwo plausible mechanisms for the oxidative degradation of alkylamine and alkylhydrazine RMs through Cα hydrogen abstraction. \u003cstrong\u003eb\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eCalculated free energy diagram for the oxidation of three RMs, i.e. BP55, BP66 and BAC, by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e through two Cα hydrogen abstraction pathways at the ωB97X-D/def2-TZVPP, SMD(TEGDME)//ωB97X-D/def2-SVP level of theory. \u003cstrong\u003ec\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eCalculated structures of Cα hydrogen abstraction transition states and activation free energies (kcal/mol). Interatomic distances are in Å.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/660dd3c26289754f8df42bdc.png"},{"id":57061076,"identity":"f5468464-76d3-495b-8fa5-5994f49fdedf","added_by":"auto","created_at":"2024-05-24 06:08:00","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":644920,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDFT computational evaluation of concerted H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e abstraction oxidation pathways\u003c/strong\u003e. \u003cstrong\u003ea\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eTwo plausible mechanisms for the oxidative degradation of alkylamine and alkylhydrazine RMs through concerted H\u003csub\u003e2\u003c/sub\u003e abstraction. \u003cstrong\u003eb\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eCalculated free energy diagram for the oxidation of three RMs, i.e. BP55, BP66 and BAC, by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e through two concerted H\u003csub\u003e2\u003c/sub\u003e abstraction pathways at the ωB97X-D/def2-TZVPP, SMD(TEGDME)//ωB97X-D/def2-SVP level of theory.\u003cstrong\u003e c\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eCalculated structures of concerted H\u003csub\u003e2\u003c/sub\u003e abstraction transition states and activation free energies (kcal/mol). Interatomic distances are in Å.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/71444b489a2744fe91a8cafb.png"},{"id":57061078,"identity":"14a8b084-2ddf-486a-976a-bfb85ba5a0ce","added_by":"auto","created_at":"2024-05-24 06:08:00","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":372030,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDFT-calculated energies and structures\u003c/strong\u003e. \u003cstrong\u003ea\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eCalculated free energy evolution along the OSS oxidation pathway for the three RMs, i.e. BAC, BP55, and BP66.\u003cstrong\u003e b\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eCalculated geometries of carbon-centered radicals after Cα hydrogen abstraction.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/20f7ccda2e9a07f9415944de.png"},{"id":57061575,"identity":"e7746f2b-fb8f-4168-b273-32cdd8bbf8a6","added_by":"auto","created_at":"2024-05-24 06:16:00","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":474755,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEvaluation of resistance to \u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e in RMs through NMR spectroscopy. a\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eSchematic representation of RMs reacting with \u003cem\u003ein situ\u003c/em\u003e generated \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003cstrong\u003e \u003c/strong\u003eThe generation of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e is facilitated in the solution containing 0.50 mM tetraphenylporphyrin (TPP) and 50 mM of each RM within an ether-based electrolyte solution composed of 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME). The reaction mixture undergoes sustained oxygenation and agitation, subsequently exposed to UV/Visible light (λ\u003csub\u003emax\u003c/sub\u003e = 525 nm) irradiation for 48 hours. \u003cstrong\u003eb\u003c/strong\u003e–\u003cstrong\u003ek\u003c/strong\u003e, C–H bonds within the structure of RMs marked for tracking through NMR analysis following exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e. \u003cstrong\u003ec\u003c/strong\u003e–\u003cstrong\u003ee\u003c/strong\u003e,\u003csup\u003e 1\u003c/sup\u003eH spectra of RMs without electrolyte and photocatalyst. \u003cstrong\u003ef\u003c/strong\u003e–\u003cstrong\u003eh\u003c/strong\u003e, before exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e. \u003cstrong\u003ei\u003c/strong\u003e–\u003cstrong\u003ek\u003c/strong\u003e, after exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e. The full \u003csup\u003e1\u003c/sup\u003eH NMR spectra (600 MHz, 294 K, chloroform-\u003cem\u003ed\u003c/em\u003e)\u003cstrong\u003e \u003c/strong\u003ecan be found in Supporting Information (Supplementary Figs. 8–11, 17, 18, 24). Abbreviation: ISC, intersystem crossing; PC, photocatalyst; EL, electrolyte; H\u003csub\u003eα\u003c/sub\u003e/H\u003csub\u003eβ\u003c/sub\u003e/H\u003csub\u003eγ\u003c/sub\u003e, protons on α/β/γ carbons, respectively; H\u003csub\u003eendo\u003c/sub\u003e/H\u003csub\u003eexo\u003c/sub\u003e/H\u003csub\u003ebridgehead\u003c/sub\u003e, endo/exo/bridgehead protons of BAC, respectively.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/965da7f339fc282abde173e5.png"},{"id":57061079,"identity":"113a3fbe-4966-4d9c-bd2c-244b39ef861f","added_by":"auto","created_at":"2024-05-24 06:08:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":319910,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComparison of electrochemical activity changes of each RMs exposed to \u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e. a–c\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003ecyclic voltammetry (CV) curves of RMs at scan rates of 10, 50, and 100 mV s\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003ed–f\u003c/strong\u003e, CV curves of RMs following 48-hour exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e at scan rates of 10, 50, and 100 mV s\u003csup\u003e-1\u003c/sup\u003e. Electrolyte contains 1 M of LiTFSI, 50 mM of each RM and 0.5 mM of TPP.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/33a474297373008699e11962.png"},{"id":57061080,"identity":"6593ad78-9d51-4d05-b1b5-6d226038648f","added_by":"auto","created_at":"2024-05-24 06:08:00","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":620380,"visible":true,"origin":"","legend":"\u003cp\u003eDifferential electrochemical mass spectroscopy (DEMS) analysis of gases during the charging of LOB with each RM. Voltage profiles and gas evolution profiles of a, BP55\u003csup\u003e \u003c/sup\u003ebefore after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, b, BP66 before and after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, and c, BAC before and after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, and corresponding gas evolution profiles at first charging at 0.2 mA for a duration of four hours. The cathode was used after discharging at 0.2 mA for five hours in an electrolyte solution containing 50 mM of each RM and data were collected at five-minute intervals.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/31d20a96f731361c4ec20f73.png"},{"id":66607239,"identity":"36289ecf-c3a9-414d-aa11-028892fc5933","added_by":"auto","created_at":"2024-10-14 18:44:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4247277,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/fe56d9e1-e520-4f30-bfe4-f5df72477639.pdf"},{"id":57061081,"identity":"e37b29b9-ad2a-4b25-a5a8-ea23c0c21917","added_by":"auto","created_at":"2024-05-24 06:08:00","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":49182700,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"24NCATLOBBACRMSI.docx","url":"https://assets-eu.researchsquare.com/files/rs-4370577/v1/4008ad176446dd9afd366943.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"ROS-Resistant Redox Mediator in Lithium-Oxygen Batteries","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe high theoretical capacity of lithium-oxygen batteries (LOBs) is a major attraction for potentially replacing common battery systems. However, large charging overpotential in LOBs triggers various side reactions, and redox mediators (RMs) have been adopted to mitigate charging overpotential. Since the initial introduction of RMs to LOBs, a range of RMs has been developed to minimize energy loss and increase cyclability of LOBs\u003csup\u003e1\u0026minus;2345\u003c/sup\u003e. The primary role of RMs\u0026nbsp;is\u0026nbsp;promoting the decomposition of lithium peroxide (Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) at lower charging potential, preventing the detrimental reactions occurring at\u0026nbsp;typical\u0026nbsp;charging potential. A range of studies have been conducted to elucidate the diverse interactions between RMs and Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and the wide array of mechanistic pathways they induce, with a particular emphasis on reaction kinetics\u003csup\u003e6\u0026minus;7\u003ca href=\"#_edn4\" name=\"_ednref4\" title=\"\"\u003e\u003c/a\u003e8\u003ca href=\"#_edn5\" name=\"_ednref5\" title=\"\"\u003e\u003c/a\u003e9\u003ca href=\"#_edn6\" name=\"_ednref6\" title=\"\"\u003e\u003c/a\u003e10\u003c/sup\u003e. The decomposition of Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e is characterized by a two-electron transfer process wherein oxidized RMs engage in a series of reactions with Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, resulting in the liberation of O\u003csub\u003e2,\u0026nbsp;\u003c/sub\u003eas\u003csub\u003e\u0026nbsp;\u003c/sub\u003edescribed in the following equation; Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e + 2RM\u003csup\u003e+\u003c/sup\u003e \u0026rarr; 2Li\u003csup\u003e+\u003c/sup\u003e + O\u003csub\u003e2\u003c/sub\u003e + 2RM. The decomposition of Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e driven\u003csub\u003e\u0026nbsp;\u003c/sub\u003eby RMs involves an outer-sphere electron transfer between RM and Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, and the redox potentials of RMs can be tuned in accordance with Marcus theory for more favorable decomposition kinetics\u003csup\u003e11\u0026minus;1213\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe evolution of highly electrophilic \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e throughout the operational cycles of LOBs holds paramount importance\u003csup\u003e6\u003c/sup\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003csup\u003e7\u003c/sup\u003e\u003csup\u003e8\u003c/sup\u003e, as \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e generation is known to cause various side reactions that impact the lifespan of LOBs. Most RMs are electron-rich and therefore tend to undergo irreversible reactions with electrophilic \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e9\u003c/sup\u003e\u003csup\u003e,\u003c/sup\u003e\u003csup\u003e10\u003c/sup\u003e\u003csup\u003e,14\u003c/sup\u003e (Fig. 1a). Such reactions result in gradual degradation in the capacity of RMs to decompose Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e throughout cycles, leading to deteriorating electrochemical performance\u003csup\u003e2\u003c/sup\u003e\u003csup\u003e,15\u003c/sup\u003e. The redox potentials of RMs were found to be correlated to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e evolution, since recent research indicated RMs with redox potential below the theoretical \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e evolution threshold of 3.54 V exhibit attenuated \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e generation through Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u0026nbsp;\u003c/sub\u003edecomposition during charging processes\u003csup\u003e16,17\u003c/sup\u003e (Fig. 1b).\u003c/p\u003e\n\u003cp\u003eTo mitigate the challenges associated with controlling redox potential and reactivity towards \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e in existing RMs, we adopted a rational design approach based on structural analysis. The first step of this approach involved categorizing existing RMs, especially those featuring structures with nitrogen atoms, given that most RMs contain one or more nitrogen atoms. According to molecular architecture, we classified precedented N-containing organic RMs which are employed in LOBs into three categories: arylamines, nitroxyl radicals, and alkylamines/alkylhydrazines (Fig. 1c). Each scaffold contains site(s) that are susceptible to either interaction with or generation of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eIn the category of arylamine RMs such as 10-methylphenothiazine (MPT), 5,10-dimethylphenazine (DMPZ), tris[4-(diethylamino)phenyl]amine (TDPA), and \u003cem\u003eN,N,N\u0026apos;,N\u0026apos;\u003c/em\u003e-tetramethyl-\u003cem\u003ep\u003c/em\u003e-phenylenediamine (TMPD) (Fig. 1c, left side), unsaturated hydrocarbon backbones show vulnerability to unwanted cycloadditions with electrophilic \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e due to the high electron density associated with the aromatic nature of arylamines. A previous study revealed that DMPZ, a widely recognized arylamine-type RM, undergoes deactivation by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e in LOBs leading to the generation of an endoperoxide-type intermediate through a [4+2] cycloaddition reaction\u003csup\u003e9\u003c/sup\u003e. The same literature also suggests, despite its absence from Fig. 1b, DMPZ is considered susceptible to an alternative degradation pathway known as formal [2+2] cycloaddition resulting in a dioxetane-type intermediate.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNitroxyl radical RMs\u0026nbsp;(Fig. 1c,\u0026nbsp;center)\u0026nbsp;such as\u0026nbsp;2,2,6,6-tetramethyl-1-piperidinyloxyl\u0026nbsp;(TEMPO) and\u0026nbsp;1-methyl-2-azaadamantane-\u003cem\u003eN\u003c/em\u003e-oxyl\u0026nbsp;(MAZO) are characterized by a bond order of 1.5 (bond dissociation energy, 100 kcal/mol)\u003csup\u003e18,19\u003c/sup\u003e and benefit from steric protection due to the presence of four \u0026alpha;-methyl groups. This structural configuration enables their persistence as stable radicals, but their stability coupled with a tendency to resist oxidation and readily undergo reduction, resulting in relatively a higher redox potential than other types of RMs. Nitroxyl radical-based RMs predominantly exhibit redox potentials above 3.54 V (vs Li\u003csup\u003e+\u003c/sup\u003e/Li), increasing the ratio of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e to triplet oxygen (\u003csup\u003e3\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e) production during the operation of LOBs. A notable compound in this category is TEMPO, with a redox potential of 3.74 V (vs Li\u003csup\u003e+\u003c/sup\u003e/Li), which generates significant amount of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e16\u003c/sup\u003e\u003csup\u003e,20\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAlkylamine and alkylhydrazine RMs (Fig. 1c, right side) possess low redox potentials similar to those of arylamine-type RMs. Notably, within the alkylhydrazine classification, recent introductions include RMs such as 1,1\u0026apos;-bipyrrolidine (BP55), 1-(pyrrolidin-1-yl)piperidine (BP56), and 1,1\u0026apos;-bipiperidine (BP66). Apart from BP66, these RMs are characterized by low redox potentials and have been demonstrated to possess a high kinetic rate for Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition along with an efficient quenching capacity for \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, markedly diminishing the yield of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e16\u003c/sup\u003e. Nevertheless, the presence of protons at carbons neighboring nitrogen atoms predisposes them to facile oxidation through the formation of iminiums or C-centered radicals. These reactive intermediates in turn facilitate further degradation through reactions with nucleophiles or somophiles, including water vapor\u003csup\u003e21\u003c/sup\u003e present in LOBs, to eventually impact the overall integrity of LOB systems.\u003c/p\u003e\n\u003cp\u003eAdvancing the development of a new RM scaffold that circumvents the chemical and electrochemical shortcomings inherent to the existing three RM classes, a stepwise strategy was employed to explore one structural variation at a time (Fig. 2, left side). Beginning with the TEMPO structure as a basis, attempts were made to decrease the redox potential by removing the oxygen atom from the nitroxyl group and introducing N\u0026ndash;H or N\u0026ndash;alkyl bonds, leading to the formulation of 2,2,6,6-tetramethylpiperidine (TEMP) and alkyl-substituted tertiary amines. However, prior theoretical analysis suggested that these alternatives would still be prone to oxidation, resulting in their exclusion from further consideration\u003csup\u003e22\u0026ndash;\u003c/sup\u003e. The strategy then shifted towards substituting the oxygen atom with nitrogen, but the \u003cem\u003eN\u003c/em\u003e,\u003cem\u003eN\u0026apos;\u003c/em\u003e-bipyridyl dication (pyridine dimer) was assessed to be excessively unstable due to the repulsion between the nitrogen cations, with a theoretical predisposition towards N\u0026ndash;N bond cleavage. An approach to remove olefins from the \u003cem\u003eN\u003c/em\u003e,\u003cem\u003eN\u0026apos;\u003c/em\u003e-bipyridyl dication to decrease the possibility of cycloaddition reactions with \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e theoretically leads to BP66 structure, which, however, remains prone to C\u0026alpha;\u0026ndash;H oxidation. A further modification involved the full substitution of \u0026alpha; carbons with four methyl groups on each piperidine side, akin to the TEMP\u0026ndash;TEMP (N\u0026ndash;N coupled dimer). Computational evaluations predicted significant steric congestion, rendering the synthesis unattainable. Even if synthesis were achievable, the anticipated steric hindrance could limit the accessibility to Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, thereby impeding decomposition within LOBs. These series of experimental and theoretical attempts to innovate within the framework of N-containing RMs like TEMPO and BP66 (or BP55) ultimately resulted in a continuous loop of stagnation, unable to break free from the existing paradigms.\u003c/p\u003e\n\u003cp\u003eUsing BP66 as a starting point and applying Bredt\u0026rsquo;s rule, we envisioned that the bicyclic 7,7\u0026apos;-bi-7-azabicyclo[2.2.1]heptane (BAC) with its nitrogen-bearing carbons placed at the bridgehead positions could function as an RM that would be resistant to parasitic oxidation processes (Fig. 2, right side). Bredt\u0026rsquo;s rule, postulated by Julius Bredt in 1924, asserts that the formation of \u0026pi; bonds between the bridgehead atom and adjacent atoms is inherently forbidden due to the orthogonal alignment of their p orbitals, leading to poor p orbital overlap\u003csup\u003e26\u003c/sup\u003e. Despite the protective premise of Bredt\u0026rsquo;s rule, other potential oxidative degradation pathways involving reactive oxygen species (ROS) such as \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e and superoxide (O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e) are still possible. To address this, we conducted a thorough density functional theory (DFT) evaluation of both BAC and BP RMs to ensure that BAC would be resistant to all possible oxidation pathways. Subsequently, the synthesis of BAC was realized, and its resistance to ROS under LOB operational conditions was experimentally examined by utilizing various chemical and electrochemical techniques such as nuclear magnetic resonance (NMR), cyclic voltammetry (CV), differential electrochemical mass spectroscopy (DEMS) analyses.\u003c/p\u003e\n\u003cdiv id=\"edn7\"\u003e\u003cbr\u003e\u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eComputational evaluation of RM resistance to oxidation by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo confirm that the bridged BAC molecule we envisioned would be resistant to parasitic oxidation processes, we first performed DFT calculations on BAC, BP55 and BP66 to assess the activation barriers for their oxidative degradation through C\u0026alpha; hydrogen abstraction by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e (Fig. 3). C\u0026alpha; hydrogen abstraction by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e of alkylamines can proceed on either the closed-shell singlet (CSS) or the open-shell singlet (OSS) surface (Fig. 3a). After the formation of an initial encounter complex \u003cstrong\u003eI\u003c/strong\u003e, the CSS process (via \u003cstrong\u003eTS-1\u003c/strong\u003e) produces an ion pair consisting of an iminium and the OOH anion, which then combine in a barrierless or near-barrierless fashion to generate the zwitterionic structure \u003cstrong\u003eIII\u003c/strong\u003e. The OSS process (via \u003cstrong\u003eTS-2\u003c/strong\u003e) produces a carbon-centered radical and the OOH radical, which can either recombine, undergo a radical-polar crossover to the CSS surface (\u003cem\u003evide infra\u003c/em\u003e), or dissociate to react with other chemical species present in the system.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFig. 3b shows the free energy evolution along the two possible C\u0026alpha; hydrogen abstraction pathways for BAC, BP55 and BP66. Our calculations showed that for all three RMs surveyed, the OSS C\u0026alpha; hydrogen abstraction transition state \u003cstrong\u003eTS-2\u003c/strong\u003e is more favorable than the CSS transition state \u003cstrong\u003eTS-1\u003c/strong\u003e. BAC was also found to have much higher C\u0026alpha; hydrogen abstraction barriers than either BP55 or BP66. (It is worth noting that for all three RMs, the CSS and OSS surfaces intersect after \u003cstrong\u003eTS-2\u003c/strong\u003e; it is therefore plausible that a radical-polar crossover event will follow \u003cstrong\u003eTS-2\u003c/strong\u003e, leading to intermediate \u003cstrong\u003eIII\u003c/strong\u003e.)\u003c/p\u003e\n\u003cp\u003eThe lower energy of \u003cstrong\u003eTS-2a\u003c/strong\u003e compared to \u003cstrong\u003eTS-1a\u003c/strong\u003e can be rationalized by noting that \u003cstrong\u003eTS-1a\u003c/strong\u003e momentarily produces an iminium ion with the C=N double bond at a bridgehead position, which is highly strained according to Bredt\u0026rsquo;s rule. In contrast, \u003cstrong\u003eTS-2a\u003c/strong\u003e produces a carbon-centered radical at the bridgehead position instead of forming a full double bond, lessening the strain. Despite being the more favorable pathway of the two, \u003cstrong\u003eTS-2a\u003c/strong\u003e still has a prohibitively high activation barrier of 37.9 kcal/mol, which lends support to our expectation that the bridged structure of BAC could confer exceptional stability under oxidative conditions. In contrast, the OSS C\u0026alpha; hydrogen abstraction barriers for BP55 and BP66 were only 20.4 and 22.6 kcal/mol, respectively (Fig. 3c), indicating that these two RM species are susceptible to oxidative degradation by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e at room temperature.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe next considered possible degradation of the RM molecules by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e through concerted H\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eabstraction processes on the CSS surface (Fig. 4). This process leads to the generation of H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and an enamine (Pathway A) or alkene (Pathway B) depending on the location of H\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eabstraction (Fig. 4a). Fig. 4b shows the free energy evolution along the possible concerted H\u003csub\u003e2\u003c/sub\u003e abstraction pathways for BAC, BP55 and BP66. For both BP55 and BP66, the enamine-generating Pathway A was found to be more favorable, consistent with prior literature findings that neighboring amine substituents accelerate oxidative degradation through concerted H\u003csub\u003e2\u0026nbsp;\u003c/sub\u003eabstraction\u003csup\u003e27\u003c/sup\u003e. For BAC, on the other hand, Pathway A would lead to a highly strained \u0026quot;bridgehead\u0026quot; double bond, resulting in an extremely high 52.2 kcal/mol barrier. \u003cstrong\u003eTS-4a\u003c/strong\u003e, which avoids placing the double bond in the bridgehead position, has a much lower barrier of 35.4 kcal/mol (Fig. 4c), albeit still prohibitively high for room-temperature conditions. These calculations further established that the bridged bicyclic structure of BAC would render it resistant to oxidative degradation by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e through multiple possible mechanisms.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA direct comparison of OSS (radical) oxidation of the three RMs by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e is shown in Fig. 5a. The calculated energies indicate that BAC is dramatically more resistant to OSS oxidation by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e than BP55 or BP66. The unusually high resistance to OSS oxidation for BAC can be attributed to the geometry of its radical form \u003cstrong\u003eIVa\u003c/strong\u003e after hydrogen atom transfer. In the BP55- and BP66-derived radicals \u003cstrong\u003eIVb\u003c/strong\u003e and \u003cstrong\u003eIVc\u003c/strong\u003e, the carbon radical centers are able to attain mostly planar geometries, as would be preferred in unstrained model systems (see Supplementary Fig. 1 for comparison). In the BAC-derived radical \u003cstrong\u003eIVa\u003c/strong\u003e, however, planarization is energetically extremely costly for the bridgehead carbon, leading to the carbon radical center being much more nonplanar (Fig. 5b). This leads to poor delocalization of the unpaired electron and higher energies as a result. Similarly, the high energy cost of planarizing a bridgehead carbon also renders CSS oxidation of BAC (through \u003cstrong\u003eTS-1a\u003c/strong\u003e or \u003cstrong\u003eTS-3a\u003c/strong\u003e) extremely unfavorable. In addition, O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e degradation of BAC was also found to be energetically all the way uphill. Overall, our computational analyses confirmed the soundness of our Bredt\u0026rsquo;s-rule-based design principle and predicted that BAC would be exceptionally stable to oxidative degradation by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e at room temperature.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInvestigation of reactivity of RMs with \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo substantiate the computational findings regarding the stability of BP55, BP66, and BAC against \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, each RM was synthesized\u003csup\u003e16\u003c/sup\u003e\u003csup\u003e,28\u003c/sup\u003e, and a method involving \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e-enriched environment is employed for degradation monitoring; upon exposure to 525 nm LED, tetraphenylporphyrin (TPP) is initially excited to its singlet state and undergoes intersystem crossing (ISC) to the triplet state, which, in the presence of proximal \u003csup\u003e3\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, facilitates energy transfer, leading to the formation of the highly reactive \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e species\u003csup\u003e29,30\u0026nbsp;\u003c/sup\u003e(Fig. 6a). In the \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e experiment, NMR analysis was concurrently performed to monitor the degradation of RMs, with Fig. 6b distinctly emphasizing the C\u0026ndash;H bonds designated for tracing across RMs. The \u003csup\u003e1\u003c/sup\u003eH NMR spectra, displayed in black and bluish purple in the first and second rows (Figs. 6c\u0026ndash;e and Figs. 6f\u0026ndash;h), depict the conditions of each RM prior to exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, without and with the electrolytes and photocatalyst, respectively; the elevated, dense peaks in chemical shift ranging from 3.3 to 3.7 represent the proton patterns of tetraethyleneglycol dimethylether (TEGDME) as one of the electrolyte molecules. The analytes containing the RMs, the electrolytes, and the photocatalyst were exposed to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e environment, and the corresponding spectra are obtained and shown in Figs. 6i\u0026ndash;k. Characteristic proton peaks\u0026mdash;eight of each H\u003csub\u003e\u0026alpha;\u003c/sub\u003e and H\u003csub\u003e\u0026beta;\u003c/sub\u003e for BP55 and eight of each H\u003csub\u003e\u0026alpha;\u003c/sub\u003e and H\u003csub\u003e\u0026beta;\u003c/sub\u003e with four of H\u003csub\u003e\u0026gamma;\u003c/sub\u003e for BP66\u0026mdash;completely disappeared while new peaks, belonging to an unidentified byproduct(s), were detected in the spectra (Figs. 6i,k). These results were consistent with our computational prediction that oxidative degradation through the OSS pathway would be feasible at room temperature for both BP55 (20.4 kcal/mol barrier, corresponding to the difference between \u003cstrong\u003eII\u003c/strong\u003e and \u003cstrong\u003eTS-2\u003c/strong\u003e) and BP66 (22.6 kcal/mol). However, BAC demonstrated chemical stability without unintended deterioration, following exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e as shown in Fig. 6k.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical behavior of\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eRM\u003c/strong\u003e\u003cstrong\u003es\u003c/strong\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe durability of RMs towards ROS was evaluated via CV by comparing the electrochemical activity of RMs before and after exposure to ROS. All examined RMs demonstrated a redox potential under 3.54 V, conforming to the criteria for diminishing \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e evolution during charging process (Figs. 7a\u0026ndash;c). Nevertheless, both BP55 and BP66 were found to be devoid of redox-active properties after exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, indicating a considerable loss of electrochemical activity due to the oxidative effects of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e (Figs. 7d,e). This observation reveals the irreversible reaction triggered by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e resulting in the formation of byproducts lacking in redox activity, which is characteristic of trap-type-RM behavior\u003csup\u003e10\u003c/sup\u003e.\u0026nbsp;One notable is that\u0026nbsp;no discernible changes in CV profiles of the RMs upon exposure to\u0026nbsp;O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e were shown implying the resilience of BP55 and BP66 to O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e, compared to their sensitivity to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e (Supplementary Figs. 12a,b). Remarkably, BAC maintains reversible CV profiles even after exposure to either \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e or O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e, sustaining redox activity without any potential shift (Figs. 7c,f and Supplementary Fig. 12c). This persistence is attributed to the chemical robustness of BAC, as proven by NMR analysis, which also underlines high durability of BAC in maintaining electrochemical activity.\u003c/p\u003e\n\u003cp\u003eThe stability of RMs during galvanostatic cycling test was verified in LOB configuration (Supplementary Fig.\u0026nbsp;13), with the initial charging potential of BP66 approaching 3.50 V and a longer plateau than BP55. In general, the relationship between the redox potentials of RMs and\u0026nbsp;kinetic of Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition adheres to an inverted parabola shape in accordance with Marcus theory\u003csup\u003e16\u003c/sup\u003e. Given that the kinetic\u0026nbsp;of Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition approaches saturation around 3.70 V\u003csup\u003e10\u003c/sup\u003e\u003csup\u003e,\u0026nbsp;31\u003c/sup\u003e, it is reasonable to deduce that BP66, with its higher redox potential compared to BP55, exhibits enhanced kinetics for Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition, resulting in a longer charging voltage plateau. However, the terminal charging voltage for both BP55 and BP66 had progressively increased by the third cycle, reaching the typical charging voltage of LOBs without a redox catalyst. For both BP55 and BP66, a decline in the retention of the charging voltage was observed, indicative of the RMs undergoing degradation concurrently with the cycling process. Moreover, amount of evolved \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e accumulated over cycles, which affects the capacity of both BP55 and BP66 to sustain the low charging voltages. Differing from BP55 and BP66, BAC retained a stable redox potential before and after exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e. The chemical stability of BAC, anchored in its resistance to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, further delineates the proficiency of BAC in oxidizing Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo confirm the resilience of BAC against \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, relying solely on electrochemical measurements proves insufficient. Hence, to validate the continuous performance of BAC as an RM post \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, the species of gases evolved were analyzed by DEMS during the initial step of the charging process in the presence of three RMs: BP55, BP66, and BAC. Fig. 8a shows the voltage and gas evolution profiles comparing BP55 before and after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure. Before \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, BP55 exhibited charging plateau at 3.25 V (vs Li\u003csup\u003e+\u003c/sup\u003e/Li) producing certain amount of O\u003csub\u003e2\u003c/sub\u003e with suppressed H\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e gas. However, \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposed BP55 only gave concentrated O\u003csub\u003e2\u003c/sub\u003e evolution during the first hour of charging and nearly no O\u003csub\u003e2\u003c/sub\u003e evolved. Large amount of CO\u003csub\u003e2\u003c/sub\u003e rather evolved after indicating extensive side reactions occurred at high charging potential\u003csup\u003e32\u0026ndash;33\u003c/sup\u003e\u003csup\u003e34\u003c/sup\u003e and BP55 lost function as RM by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e. BP66 demonstrated results analogous to those of BP55 (Fig. 8b). BP66 before \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure exhibited higher charging potential than BP55, which is in accord with Fig.6. A little increased amount of O\u003csub\u003e2\u003c/sub\u003e than that with BP55 was detected and it was due to the higher charging kinetics of BP66 with higher charging potential. After \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, BP66 also reacted with \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e and lost catalytic function, therefore the O\u003csub\u003e2\u003c/sub\u003e evolution profile changed to unstable and large amount of CO\u003csub\u003e2\u003c/sub\u003e was detected. That is, BP55 and BP66 has trap-type RMs characteristics and \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e critically affects to their function as RM eventually deactivating them. Fig. 7c shows the behavior of BAC which contrasts to BP55 or BP66. In both before and after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, BAC gave catalytic O\u003csub\u003e2\u003c/sub\u003e evolution profiles implying highly \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e resistive characteristic of BAC (Fig. 8c). Particularly, CO\u003csub\u003e2\u003c/sub\u003e evolution was suppressed even after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure which is indicative of well-preserved functionality of BAC as RM. Based on the gas evolution profiles in Fig. 8, accumulated gas evolution rate was recalculated (Supplementary Fig. 15 and Supplementary Table 1). Proportion of O\u003csub\u003e2\u003c/sub\u003e before \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure was comparable for BP55 and BP66, 47 and 43% respectively, but still lower than BAC suggesting comparatively low Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e decomposition kinetic of BP55 and BP66. The solution of BP55 and BP66, following exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, gave a depressed ratio of O\u003csub\u003e2\u003c/sub\u003e, with CO\u003csub\u003e2\u003c/sub\u003e levels rising from 42 to 74% and from 40 to 67%, respectively. Increment of CO\u003csub\u003e2\u003c/sub\u003e demonstrates that BP55 and BP66 lose function as RM by the aggressive attack of \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e and side reactions occurred intensively at high charging potential. In contrast, BAC showed highly catalytic behavior with the ratio of evolved O\u003csub\u003e2\u003c/sub\u003e even after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure comparable to before \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure (79 and 82%, respectively). The solution of BAC, after exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, exhibited an increased H\u003csub\u003e2\u003c/sub\u003e ratio compared to its pre-exposure state. However, the total ratios of H\u003csub\u003e2\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e in the solution of BAC before and after exposure were nearly identical, suggesting that minimal side reactions occurred at similar rates in both environments. Electrochemical measurements consistently point out that BP55 and BP66 have no durability against \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, thereby both are significantly fragile to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e-involved oxidation. More importantly, such \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e reaction makes BP55 and BP66 lose their electrochemical activity. BAC, on the other hand, has highly \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e durable molecular structure and therefore promise preservation of electrochemical activity even under the continuously \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e evolving environment, the LOBs.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eDespite the high energy density of LOBs, addressing high charging overpotentials and controlling \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e evolution remains essential. Current redox mediators often exceed a 3.54 V redox potential, initiating \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e production and/or deteriorating upon exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e. This complicates achieving both low redox potential and robust \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e resistance. BP55 and BP66, for example, achieved the necessary redox potential to curb \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e evolution but undergo chemical alterations from its oxidation by \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, rendering them ineffective. Conversely, the new mediator BAC, designed in compliance with Bredt\u0026rsquo;s rule, retains its chemical and electrochemical properties after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e exposure, demonstrating high electrochemical reversibility and efficient oxygen release during charging. These findings underscore the importance of the stability of RMs towards \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e for maintaining chemical and electrochemical functionality. Molecular design, supported by computational studies that calculated the energy profiles for plausible oxidative side reactions of RMs and \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, highlights the bicyclic compound BAC as particularly robust. This theoretical robustness of BAC was further confirmed through experimental validations using NMR, CV, and DEMS analyses, establishing a promising strategy for developing \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e-stable materials essential for enhancing LOBs. Nevertheless, while challenges such as the kinetics of RMs, byproduct accumulation, and electrode design are crucial for advancing LOBs, the development of BAC, which robustly withstands \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e challenges, is pivotal for addressing these specific issues. Moreover, the rational design process employed in the development of BAC is expected to enhance LOB technology while also yielding valuable insights into the development of mediators and catalysts for diverse organocatalytic reactions.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eComputational methods.\u0026nbsp;\u003c/strong\u003eAll DFT computations were performed using Gaussian 16, Revision C.01\u003csup\u003e35\u003c/sup\u003e.\u0026nbsp;Unless otherwise specified, molecular geometries were optimized using the long-range-corrected \u0026omega;B97X-D\u003csup\u003e36\u003c/sup\u003e functional and the def2-SVP basis set. Frequency calculations were performed at the same level of theory as that used for geometry optimization to characterize the stationary points as either minima (no imaginary frequencies) or first-order saddle points (one imaginary frequency) on the potential energy surface. Intrinsic Reaction Coordinate (IRC) calculations were performed to confirm the first-order saddle points as real transition states connecting the expected reactants and products. Thermal contributions to Gibbs free energies were calculated from vibrational frequencies using the quasi-rigid rotor-harmonic oscillator (RRHO) approach of Grimme\u003csup\u003e37\u003c/sup\u003e implemented through Paton\u0026rsquo;s GoodVibes\u003csup\u003e38\u003c/sup\u003e Python script. Single-point energies were calculated with the \u0026omega;B97X-D functional and the triple-zeta-quality def2-TZVPP. Solvation effects were incorporated in single-point energy calculations using Truhlar\u0026rsquo;s SMD\u003csup\u003e39\u003c/sup\u003e model using the keyword scrf=(smd,solvent=generic,read) and specifying eps=7.8 for TEGDME. Because the isolated \u003csup\u003e1\u003c/sup\u003e\u0026Delta;\u003csub\u003eg\u003c/sub\u003e singlet oxygen molecule is multi-determinant and cannot be modeled accurately by DFT methods, \u003csup\u003e1\u003c/sup\u003e\u0026Delta;\u003csub\u003eg\u003c/sub\u003e singlet oxygen was instead modeled by adding 22.5 kcal/mol to the free energy of triplet (\u003csup\u003e3\u003c/sup\u003e\u0026Sigma;\u003csub\u003eg\u003c/sub\u003e) oxygen (22.5 kcal/mol being the experimentally determined energy gap between \u003csup\u003e1\u003c/sup\u003e\u0026Delta;\u003csub\u003eg\u003c/sub\u003e singlet oxygen and \u003csup\u003e3\u003c/sup\u003e\u0026Sigma;\u003csub\u003eg\u003c/sub\u003e triplet oxygen), a method employed successfully by Mullinax \u003cem\u003eet al.\u003c/em\u003e for modeling singlet oxygen reactions\u003csup\u003e40\u003c/sup\u003e\u003csup\u003e,41\u003c/sup\u003e. Visualizations of molecular structures were obtained using CYLview\u003csup\u003e42\u003c/sup\u003e. Reiterative Monte Carlo conformational searches were performed with the Merck molecular force field (MMFF) implemented in Spartan \u0026apos;20\u003csup\u003e43\u003c/sup\u003e. All conformers within 10.0 kcal/mol of the lowest-energy conformer through Monte Carlo searches were reoptimized in Gaussian using the DFT methods described above.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSynthesis of\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e7,7\u0026apos;-Bi-7-azabicyclo[2.2.1]heptane\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e(BAC).\u0026nbsp;\u003c/strong\u003eA 40 mL vial was charged with a magnetic stir bar, 7-chloro-7-azabicyclo[2.2.1]heptane (467 mg, 3.55 mmol) and THF (15.1 mL) under nitrogen gas and was cooled to \u0026ndash;78 ℃. To the solution, a 1.66 M \u003cem\u003etert\u003c/em\u003e-butyllithium (2.15 mL) was added dropwise. After being stirred for an hour at \u0026ndash;78 ℃, the reaction mixture was allowed to warm to 23 ℃. After 22 hours, the resulting mixture was treated with water (15 mL) and extracted with diethyl ether (15 mL x 3). The organic layer was washed with brine solution (20 mL x 3), dried over sodium sulfate, filtered, and concentrated. The crude mixture was purified by flash column chromatography (ethyl acetate: hexane = 1:8)\u0026nbsp;to give 205 mg (1.07 mmol) of BAC. After the purification process stated above, over 98.5% purity of BAC was observed. To increase the purity eliminating minor impurities such as grease and unknown compounds, preparative HPLC was necessary. \u003csup\u003e1\u003c/sup\u003eH NMR spectrum of BAC was matched with the literature\u003csup\u003e28\u003c/sup\u003e. Note: The preparation of\u0026nbsp;7-chloro-7-azabicyclo[2.2.1]heptane is included in\u0026nbsp;Supplementary Information.\u0026nbsp;Isolated Yield: 60%;\u0026nbsp;Physical Property: White solid.;\u0026nbsp;\u003csup\u003e1\u003c/sup\u003eH NMR (600 MHz, CDCl\u003csub\u003e3\u0026nbsp;\u003c/sub\u003econtaining 0.03% (v/v) TMS): \u0026delta; 3.22 (td, \u003cem\u003eJ\u003c/em\u003e = 3.1, 1.5 Hz, 4H), 2.08\u0026ndash;2.02 (m, 4H), 1.61\u0026ndash;1.55 (m, 4H), 1.21 (dd, \u003cem\u003eJ\u003c/em\u003e = 6.9, 2.5 Hz, 4H), 1.14 (dd, \u003cem\u003eJ\u003c/em\u003e = 7.0, 2.5 Hz, 4H).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical measureme\u003c/strong\u003e\u003cstrong\u003ent.\u0026nbsp;\u003c/strong\u003eElectrochemical experiments were conducted through galvanostat/potentiostat (WBSCS 3000Ls, WonATech, Korea) under 25\u0026nbsp;℃. For CV, three electrodes system was chosen utilizing platinum wire, glassy carbon (3 mm) and RE-7 (0.01 M Ag/Ag\u003csup\u003e+\u003c/sup\u003e in acetonitrile) as counter, working and reference electrode, respectively. Cycle tests were conducted assembling Swagelok-type cell with Li metal (12 mm), GF/B (12.5 mm) and GDL (12 mm). All Li foils were pretreated to coat protective layer following previously reported method\u003csup\u003e44\u003c/sup\u003e. Electrolyte consists of 1 M LiTFSI, 50 mM additives and 0.5 mM TPP in TEGDME or same composition after \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e or\u0026nbsp;O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e exposure were used for CV and cycle tests were conducted with the above electrolytes substituting TEGDME to DMAc.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eIn-situ DEMS analysis.\u0026nbsp;\u003c/strong\u003eGeneration of\u0026nbsp;H\u003csub\u003e2\u003c/sub\u003e (m/z = 2),\u0026nbsp;O\u003csub\u003e2\u003c/sub\u003e (m/z = 32)\u0026nbsp;and CO\u003csub\u003e2\u003c/sub\u003e (m/z = 44)\u0026nbsp;gases\u0026nbsp;were monitored with in-situ DEMS analysis. A custom-built cell containing 2032 coin cell was utilized to analyze gas evolution during the initial charging process with as-prepared electrolyte. Each charge-discharge procedure was conducted at\u0026nbsp;200 \u0026mu;A\u0026nbsp;rate\u0026nbsp;for five hours in 25 \u0026deg;C.\u0026nbsp;The in-situ DEMS cells were assembled within an argon-filled\u0026nbsp;glovebox\u0026nbsp;and\u0026nbsp;the\u0026nbsp;gas evolution detection\u0026nbsp;were\u0026nbsp;set at five-minute\u0026nbsp;intervals.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eROS exposure treatment\u003c/strong\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eTo expose RM to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, electrolytes containing 1 M LiTFSI, 50 mM additives and 0.5 mM TPP were exposed to light source which has maximum wavelength at 525 nm (525PF, HepatoChem) over set time. O\u003csub\u003e2\u003c/sub\u003e was continuously purged at 30 cc min\u003csup\u003e\u0026ndash;\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e. In\u0026nbsp;O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e exposure test, 0.6 mM KO\u003csub\u003e2\u003c/sub\u003e and 1.2 mM 18-crown-6 ether were used to generate\u0026nbsp;O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e and both were added to each electrolyte. As-prepared electrolytes were stirred for an hour to generate\u0026nbsp;O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026bull;\u0026minus;\u003c/sup\u003e enough.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are accessible within the Article and its Supplementary Information, or can be provided by the corresponding authors upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1F1A1061297), the Core Research Institute Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2021R1A6A1A10044950), and the H2KOREA funded by the Ministry of Education (2022Hydrogen fuel cell-002, Innovative Human Resources Development Project for Hydrogen Fuel Cells). This research was also supported by Learning \u0026amp; Academic research institution for Master \u0026apos;s\u0026middot;PhD students, and Postdocs (LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS-2023-00285390). S.C. is grateful to Oberlin College for financial support. DFT calculations were performed using the SCIURus, the Oberlin College HPC cluster (NSF MRI 1427949), as well as computing resources through allocation CHE210088 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services \u0026amp; Support (ACCESS) program (NSF #2138259, #2138286, #2138307, #2137603, and #2138296).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eS.-E.S. and W.-J.K. collaboratively designed and supervised the project, managing its administration. The project was jointly led by H.-W.L., J.H., and J.-Y.K., who conducted the experiments with assistance from M.C., H.C., and H.-B.Y. Assisted by G.N.M. and K.S.T., G.N.M and K.S.T conducted the DFT calculations under the guidance of S.C.. S.-T.K. undertook the preliminary DFT studies. J.H. and S.J.K. conducted the differential electrochemical mass spectroscopy analyses.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA patent related to the use of BAC in lithium air secondary battery: Kwak, W.-J.; Lee, H.-W.; Suh, S.-E.; Hwang, J.; Choi, M. \u0026quot;Electrolyte for Lithium Air Secondary Battery and Lithium Air Secondary Battery Including the Same\u0026quot; Korean Patent Application No. 10-2024-0058602, Filing Date: 05/02/2024. The remaining authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary information The online version contains supplementary material available at https://doi.org/xxxx\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence and requests for materials\u003c/strong\u003e should be addressed to Shuming Chen, Sung-Eun Suh or Won-Jin Kwak.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBi, X. et al. Understanding the Role of Lithium Iodide in Lithium\u0026ndash;Oxygen Batteries. \u003cem\u003eAdv. Mater.\u003c/em\u003e \u003cstrong\u003e34\u003c/strong\u003e, 2106148 (2022). \u003c/li\u003e\n\u003cli\u003eKundu, D., Black, R., Adams, B. \u0026amp; Nazar, L. F. A Highly Active Low Voltage Redox Mediator for Enhanced Rechargeability of Lithium\u0026ndash;Oxygen Batteries. \u003cem\u003eACS Cent. 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A\u003c/em\u003e\u003cstrong\u003e7\u003c/strong\u003e, 3857-3862 (2019).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4370577/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4370577/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe utilization of redox mediators (RMs) in lithium-oxygen batteries (LOBs) have underscored their utility in addressing the challenge of the elevated overpotential during the charging process. Nonetheless, the generation of highly electrophilic singlet oxygen (\u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e) throughout the battery cycles leads to adverse reactions with RMs, thereby impeding their effectiveness. In the quest for enhanced RM durability, this study unveils a novel RM, 7,7'-bi-7-azabicyclo[2.2.1]heptane (BAC), incorporating N–N interconnected aza-bicycles, and assesses its efficacy and robustness relative to those of other N–N non-bicyclic RMs. Unlike non-bicyclic RMs, which exhibit diminished O\u003csub\u003e2\u003c/sub\u003e evolution after exposure to \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e, BAC maintains consistent O\u003csub\u003e2\u003c/sub\u003e profiles during charging, indicating its superior \u003csup\u003e1\u003c/sup\u003eO\u003csub\u003e2\u003c/sub\u003e resistance and steady redox-catalyst performance in LOBs. Theoretical analyses align with the experimental findings that the bicyclic structural design of BAC confers a superior ability to resist oxidative degradation through cleavage of the C–H bonds adjacent to nitrogen atoms.\u003c/p\u003e","manuscriptTitle":"ROS-Resistant Redox Mediator in Lithium-Oxygen Batteries","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-24 06:07:55","doi":"10.21203/rs.3.rs-4370577/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"194bff54-1c92-4718-8b47-6201ab06f046","owner":[],"postedDate":"May 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":32248917,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"},{"id":32248918,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Batteries"},{"id":32248919,"name":"Physical sciences/Chemistry/Electrochemistry/Batteries"}],"tags":[],"updatedAt":"2024-10-14T18:36:15+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-24 06:07:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4370577","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4370577","identity":"rs-4370577","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}