Oxygen Reduction Via Iodide Redox Mediation in Li-O2 Batteries
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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/308004421 Implications of 4 e- Oxygen Reduction via Iodide Redox Mediation in Li-O2 Batteries Article in ACS Energy Letters · September 2016 DOI: 10.1021/acsenergylett.6b00328 CITATIONS READS 3 206 7 authors, including: Vincent Giordani Dan Addison Liox Power, Inc. Independent Researcher 30 PUBLICATIONS 769 CITATIONS 30 PUBLICATIONS 400 CITATIONS SEE PROFILE SEE PROFILE Linda F. Nazar Bryan Mccloskey University of Waterloo University of California, Berkeley 264 PUBLICATIONS 19,366 CITATIONS 66 PUBLICATIONS 4,941 CITATIONS SEE PROFILE SEE PROFILE All content following this page was uploaded by Vincent Giordani on 15 September 2016. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Letter http://pubs.acs.org/journal/aelccp Implications of 4 e− Oxygen Reduction via − Iodide Redox Mediation in Li O2 Batteries Colin M. Burke,†,‡,# Robert Black,§,∥,# Ivan R. Kochetkov,§,∥ Vincent Giordani,⊥ Dan Addison,*,⊥ Linda F. Nazar,*,§,∥ and Bryan D. McCloskey*,†,‡ † Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States ‡ Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § University of Waterloo, Department of Chemistry, Waterloo, Ontario N2L 3G1, Canada ∥ Waterloo Institute for Nanotechnology, Waterloo, Ontario N2L 3G1, Canada ⊥ Liox Power Inc., 129 North Hill Avenue, Pasadena, California 91106, United States *S Supporting Information − − ABSTRACT: The nonaqueous lithium oxygen (Li O2) electrochemistry has garnered significant attention because of its high theoretical specific energy compared to the state-of-the-art lithium-ion battery. The common − active nonaqueous Li O2 battery cathode electrochemistry is the formation (discharge) and decomposition (charge) of lithium peroxide (Li2O2). Recent reports suggest that the introduction of lithium iodide (LiI) to an ether-based electrolyte containing water at impurity levels induces a 4 e− oxygen reduction reaction forming lithium hydroxide (LiOH) potentially mitigating instability issues related to typical Li2O2 formation. We provide fl quantitative analysis of the in uence of LiI and H2O on the electrochemistry − in a common Li O2 battery employing an ether-based electrolyte and a carbon cathode. We confirm, through numerous quantitative techniques, ffi − that the addition of LiI and H2O promotes e cient 4 e oxygen reduction to LiOH on discharge, which is unexpected given that only 2 e− oxygen reduction is typically observed at undoped carbon electrodes. Unfortunately, LiOH is not reversibly oxidized to O2 on charge, where instead a complicated mix of redox shuttling and side reactions is observed. he ever increasing demands for sustainable, low carbon- scientific challenges inhibit the achievement of reversible, ’ ffi emission energy in today s society requires that we e cient Li2O2 formation and oxidation, with two being develop electrochemical energy storage systems that − T particularly pressing. First, Li2O2 and its intermediates (O2 will exceed those of current technologies. They are necessary, and LiO ) are known to undergo parasitic side reactions with for example, to deliver reliable, low-cost dispatchable power 2 organic electrolytes and carbon cathodes during battery from renewable energy and to power zero CO2-emission 3−15 vehicles. While lithium-ion is currently the battery chemistry of operation. When a realistically small electrolyte volume is − choice to meet these needs for vehicular transport, this used, these reactions limit Li O2 cell rechargeability to only a technology is approaching its theoretical energy storage limits, few cycles. Additionally, the products of these parasitic dictated by intercalation processes at the positive and negative reactions include solid carbonate/carboxylate species, which electrodes. New chemistries are urgently needed in hopes of accumulate at the electrode surface, thereby blocking O2 meeting the electrochemical energy storage needs of future evolution from the Li O −electrolyte interface on charge.16 generations. Because of its high theoretical specific energy, the 2 2 − As a result, the potential on charge necessarily increases to Li air (or O2) battery is currently being explored as a possible 1,2 − values substantially higher (>1 V higher) than the equilibrium alternative to lithium-ion. In typical nonaqueous Li O2 cell potential for O evolution from Li O , creating an energy compositions, lithium peroxide (Li2O2) formation and 2 2 2 oxidation has been identified as the reversible electrochemical inefficient battery. With these parasitic reactions seemingly reaction at the cathode: exacerbated at high potentials, this challenge perpetuates itself. +− 2Li++ O 2e ↔ Li O (1) 222 Received: August 3, 2016 The forward reaction occurs on discharge, and the reverse Accepted: September 9, 2016 reaction occurs on charge. Unfortunately, many unsolved © XXXX American Chemical Society 747 DOI: 10.1021/acsenergylett.6b00328 ACS Energy Lett. 2016, 1, 747−756 ACS Energy Letters Letter Therefore, the avoidance of parasitic reactions is necessary to fully consumed during discharge, the discharge electro- improve both rechargeability and energy efficiency. chemistry reverts back to the more common 2 e− oxygen Second, Li2O2 is a wide bandgap insulator and is insoluble in reduction reaction to form Li2O2. On charge, the concen- all studied organic electrolytes. Therefore, as it deposits on the trations of LiI and H2O and the identities of the discharge cathode during discharge, it passivates the electrode surface, products all have a dramatic influence on the observed − fi limiting Li O2 cell discharge capacity to a minor fraction of electrochemistry and resultant voltage pro les. Isotopic labeling that necessary to compete with the specific energy of Li-ion confirmed that O evolution is observed to occur only from − 2 batteries.17 23 Recent reports have elucidated strategies to Li O , although reactions between Li O and oxidized iodo 2 2 − 2 2 partially circumvent this passivation issue by triggering a species (I3 ) can result in other product formation at low 24−27 ∼ solution-based mechanism for Li2O2 formation. However, potentials ( 3 V). No O2 evolution was observed from this mechanism results in Li2O2 deposition in large toroids and electrochemically formed LiOH. Instead, LiOH reacts with I2, 28 ∼ − perhaps Li2O2 deposition away from the cathode surface. A which electrochemically forms at 3.5 V from I3 oxidation, to fraction of Li2O2 therefore becomes electronically isolated from form H2O and LiIO3, the latter of which can be solubilized in the conductive cathode surface, likely contributing to the water-contaminated 1,2-dimethoxyethane (DME). Thus, while − observed large charge overpotentials. the 4e oxygen reduction activity on carbon electrodes in the − Substantial research into redox mediators is being pursued to presence of H2O impurities and I is intriguing, the addition of − address these charge transport limitations. In the context of Li− LiI and H2OtoaLi O2 battery does not promote a reversible O2 batteries, redox mediators are small, soluble molecules with oxygen electrochemistry in the cell compositions studied. a reversible redox potential slightly above the equilibrium We initially investigated the impact of various H2O and LiI − potential of the Li O2 battery (experimentally observed as electrolyte concentrations on the discharge and charge voltage 29 fi − ∼2.85 V). During charge, the mediator oxidizes at electroni- pro les of the Li O2 cell. The voltage response for a cally accessible sites on the cathode, diffuses to electronically Ketjenblack positive electrode discharged and charged (100 μ 2 2 ff isolated Li2O2, and accepts charge via an outer sphere electron A/cm to a capacity of 1 mAh/cm )withdierent transfer from the Li2O2, allowing the Li2O2 to be oxidized while concentrations of LiI and H2OinDME-basedaprotic regenerating the redox mediator.30 The battery therefore electrolyte is shown in Figure 1. Only small changes are charges near the redox potential of the mediator, which, if appropriately selected, is only slightly above the open-circuit potential of the battery, rather than reaching the high potentials required to presumably drive charge transport through Li2O2. The lower charge potential also provides a route to reduce parasitic processes observed at high potentials. Numerous redox − mediators have now been studied in Li O2 batteries, including tetrathiafulvalene (TTF),31 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO),32 tris[4-(diethylamino)phenyl]amine (TDPA),33 iron phthalocyanine (FePC),34 and LiI.35,36 LiI is a redox mediator of particular interest in a cell containing minor H2O impurities. Recent reports indicated that in a cell containing LiI and H2O, Li2O2 was not the ultimate discharge product, but rather LiOH, possibly providing a route to reducing parasitic reactions typically associated with Li2O2 formation and oxidation.35,36 Furthermore, an incredibly low voltage gap (∼200 mV) between the discharge and charge voltage plateaus was observed, indicating that the cell achieved Figure 1. Voltage profiles of Ketjenblack (KB) electrodes (4 mg of ffi 36 KB) discharged and charged in DME/LiTFSI electrolyte with