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Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue On the incompatibility of lithium–O2 battery technology with CO † Cite this: Chem. Sci.,2017,8,6117 2 Shiyu Zhang, ‡a Matthew J. Nava,‡a Gary K. Chow,b Nazario Lopez, §a Gang Wu, c David R. Britt,b Daniel G. Nocera *d and Christopher C. Cummins *a When solubilized in a hexacarboxamide cryptand anion receptor, the peroxide dianion reacts rapidly with À CO2 in polar aprotic organic media to produce hydroperoxycarbonate (HOOCO2 ) and À À peroxydicarbonate ( O2COOCO2 ). Peroxydicarbonate is subject to thermal fragmentation into two equivalents of the highly reactive carbonate radical anion, which promotes hydrogen atom abstraction reactions responsible for the oxidative degradation of organic solvents. The activation and conversion of the peroxide dianion by CO2 is general. Exposure of solid lithium peroxide (Li2O2)toCO2 in polar aprotic Received 17th March 2017 organic media results in aggressive oxidation. These findings indicate that CO must not be introduced Accepted 19th June 2017 2 À in conditions relevant to typical lithium–O2 cell configurations, as production of HOOCO2 and DOI: 10.1039/c7sc01230f À À O2COOCO2 during lithium–O2 cell cycling will lead to cell degradation via oxidation of organic Creative Commons Attribution 3.0 Unported Licence. rsc.li/chemical-science electrolytes and other vulnerable cell components. – Introduction carbonate-derived CO2 during the recharge cycle of lithium O2 batteries2 prompted us to consider the possibility that The two-electron reduction of molecular oxygen to the peroxide carbonate formation may be a consequence of peroxide dianion is an attractive cathode redox couple for developing combination with carbon dioxide; this would likely confer – 1 rechargeable lithium O2 batteries. Lithium carbonate (Li2CO3) increased solubility and yield powerful oxidizers. To address formation is deleterious to battery performance because it this topic, we utilized an anion-receptor solubilized form of the This article is licensed under a passivates electrodes and causes a drastic reduction in the peroxide dianion12 to elucidate the molecular level details of its round trip efficiency of discharge–charge cycles.2,3 Carbonate reaction with carbon dioxide. As reported herein, we observed formation is typically ascribed to oxidative degradation of the formation of strongly oxidizing peroxy(di)-carbonate inter- Open Access Article. Published on 20 June 2017. Downloaded 9/29/2021 8:51:38 PM. organic electrolytes4–6 and carbon electrodes7 by superoxide8,9 mediates and studied their reaction with organic solvents to and singlet oxygen.10 Although peroxide is oen considered to produce carbonate. In a complementary line of investigation, 2À be a strong oxidant in aqueous media, salts of its dianion (O2 ) we showed that carbon dioxide activation of insoluble Li2O2 are poor oxidizers in organic media due to their extremely low similarly engenders solvent oxidation with the concomitant solubility and so, for this reason, the possible role of peroxide in production of carbonate. Our ndings shed light on the identity furnishing carbonate is underappreciated.11 The presence of and behavior of the hot oxidants generated upon the facile and 2À quantitative combination of O2 with CO2 via direct spectroscopic detection and exploratory reaction chemistry. aDepartment of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA. E-mail: [email protected]; Tel: +1 617 253 5332 Results and discussion bDepartment of Chemistry, University of California, Davis, One Shields Avenue, Davis, 2À CA 95616, USA Reaction of O2 with CO2 using an anion receptor c ’ Department of Chemistry, Queen s University, 90 Bader Lane, Kingston, Ontario ff K7L3N6, Canada Despite the drastic and deleterious e ect that CO2 has upon the – dDepartment of Chemistry and Chemical Biology, Harvard University, 12 Oxford performance of a cycling lithium oxygen battery, our under- Street, Cambridge, MA 02138, USA standing of the chemical entities responsible for this effect is † Electronic supplementary information (ESI) available: Full experimental, poor and based primarily upon computational studies or crystallographic and spectroscopic data. CCDC 1512972. For ESI and observation of terminal reaction products.2,8 To examine the crystallographic data in CIF or other electronic format see DOI: effect of CO2 on the oxidative power of peroxide, an anion 10.1039/c7sc01230f 13 3 ‡ receptor complex of the peroxide dianion, [O2 mBDCA-5t- These authors contributed equally. À H ]2 (1, Fig. 1),12 was employed as a soluble source of peroxide § Current address: Centro de Investigaciones Qu´ımicas, IICBA, Universidad 6 Autonoma´ del Estado de Morelos, Avenida Universidad 1001, Colonia dianion. The anion receptor mBDCA-5t-H6 encapsulates the Chamilpa, C.P. 62209, Cuernavaca, Morelos, Mexico. peroxide dianion via six N–H/O hydrogen bonds. Since its This journal is © The Royal Society of Chemistry 2017 Chem. Sci.,2017,8,6117–6122 | 6117 View Article Online Chemical Science Edge Article Fig. 3 Variable temperature 13C NMR (left) and 17O NMR (right) analysis 13 Fig. 1 The reaction scheme of peroxide cryptate 1 with CO2 and a line of the reaction between CO2 and 1. 2À 2À drawing of [O23mBDCA-5t-H6] and [CO33mBDCA-5t-H6] . as candidates to correspond to the observed 13C NMR discovery, this cryptate has enabled exploration of the reactivity signals, since hydroperoxycarbonate is known to be active 16,17 13 of the peroxide dianion with small molecules in polar organic for sulde oxidation. The salt [PPN][HOO CO2] (PPN ¼ media without the complicating inuence of acidic protons.12,14 bis(triphenylphosphine)iminium), which was generated in situ 13 18–20 Despite being a simple molecule, the peroxide dianion has from H2O2 and bicarbonate [PPN][H CO3](d ¼ 160.0 ppm), yielded rich and previously unknown chemistry, including showed a single 13C resonance at d ¼ 157.5 ppm, conrming the À metal-free oxidation of carbon monoxide (CO) generating identity of the minor intermediate as HOOCO2 . Creative Commons Attribution 3.0 Unported Licence. carbonate, which is encapsulated by the anion receptor as Moreover, 13C Gauge-Independent Atomic Orbital (GIAO) 3 2À 14 [CO3 mBDCA-5t-H6] (2, Fig. 1). NMR calculations of the chemical shis of potential candidates While the conversion of 1 to 2 under CO (1 atm, 40 C) takes were performed.15 From a range of potential chemical species À À two hours to go to completion, exposing a dimethylformamide- (Fig. 4), symmetric peroxydicarbonate ( O2COOCO2 ) emerged d7 (DMF-d7) solution of 1 to CO2 (1 atm, 25 C) resulted in the as the most plausible assignment for the major product at d ¼ essentially instantaneous formation of carbonate cryptate 156.9 ppm, having the best match between the observed and À 3 2 1 13 15 [CO3 mBDCA-5t-H6] as indicated by H NMR spectroscopy. calculated C NMR chemical shi. In an effort to indepen- À À Formation of O2 gas was not observed by gas chromatography dently generate O2COOCO2 , an experiment was carried out in 15 13 (GC) analysis of the reactor headspace gases, suggesting the which excess CO2 was added to a frozen mixture of potassium This article is licensed under a possibility of oxygen incorporation into the solvent molecules. tert-butoxide and bis(trimethylsilyl) peroxide giving rise to To probe the fate of the “missing oxygen atom” according to the a single new 13C NMR resonance at d ¼ 155.5 ppm (À40 C), equation at the top of Fig. 1, the reaction of CO2 and 1 was next tentatively supporting our identication of the major 1 +CO2 Open Access Article. Published on 20 June 2017. Downloaded 9/29/2021 8:51:38 PM. performed in the presence of oxygen atom acceptors. While 1 on product as symmetric peroxydicarbonate. Differences in the its own is unreactive towards PPh3 and methoxythioanisole at medium and reaction conditions may account for the observed 25 C, exposing a mixture of 1 and an organic oxygen-atom chemical shi difference (155.5 ppm here versus 156.9 ppm, cÀ acceptor to CO2 (1 atm, 25 C) resulted in the rapid formation above). Similarly, superoxide (O2 ) has been documented to of triphenylphosphine oxide (90%, Fig. 2) or 1-(methylsul nyl)- absorb two equivalents of CO2, generating unsymmetrical per- 4-methoxybenzene (61%, Fig. 2), respectively. oxydicarbonate (Fig. 4) as a precipitate.21 In our hands, the low Aiming to establish the chemical identity of the oxidant(s) generated upon exposure of peroxide cryptate 1 to CO2, we fol- lowed the reaction by variable temperature 13C NMR spectroscopy. A strong new signal at d ¼ 156.9 ppm, together with one minor species resonating at d ¼ 157.4 ppm, was observed at À50 C (Fig. 3). We rst considered peroxycarbonate À À À ( OOCO2 ,Fig. 3) and hydroperoxycarbonate (HOOCO2 , Fig. 3) Fig. 4 Possible intermediates during the conversion of 1 and CO2 to 2 Fig. 2 Addition of CO2 to 1 in the presence of an oxygen-atom (top) and formation of symmetric and unsymmetric peroxydicarbonate acceptor. (bottom). 6118 | Chem. Sci.,2017,8, 6117–6122 This journal is © The Royal Society of Chemistry 2017 View Article Online Edge Article Chemical Science solubility of this unsymmetrical peroxydicarbonate material which contains two chemically inequivalent 17O atoms (d ¼ 13 À À precluded its characterization by solution C NMR studies 278.7 ppm for HOOCO2 and 264.0 ppm for HOOCO2 ), in À À under conditions we employed successfully for in situ charac- contrast to the situation for O2COOCO2 in which the peroxy À À À terization of O2COOCO2 and HOOCO2 . This establishes that oxygen atoms are related by symmetry. The appearance of the 17 À different oxidants are generated upon addition of CO2 to relatively sharp O NMR signals assigned to HOOCO2 coupled superoxide as compared with the peroxide dianion (Fig. 4). with the concurrent observation of monodeprotonated cryptand À À À 14 1 Further support for the formation of HOOCO2 and O2- ([mBDCA-5t-H5] ) by H NMR spectroscopy strongly suggests À À COOCO2 upon interaction of CO2 with peroxide sources was that HOOCO2 is not strongly sequestered inside the anion provided by variable temperature 17O NMR spectroscopy.

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