Hierarchical activated carbon microfiber (ACM) electrodes for rechargeable Li–O2 batteries (original) (raw)
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The Journal of Physical Chemistry Letters, 2016
Fundamental understanding of growth mechanisms of Li 2 O 2 in Li−O 2 cells is critical for implementing batteries with high gravimetric energies. Li 2 O 2 growth can occur first by 1e − transfer to O 2 , forming Li + −O 2 − and then either chemical disproportionation of Li + −O 2 − , or a second electron transfer to Li + −O 2 −. We demonstrate that Li 2 O 2 growth is governed primarily by disproportionation of Li + −O 2 − at low overpotential, and surface-mediated electron transfer at high overpotential. We obtain evidence supporting this trend using the rotating ring disk electrode (RRDE) technique, which shows that the fraction of oxygen reduction reaction charge attributable to soluble Li + −O 2 −-based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measurements of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li 2 O 2 morphologies observed at different discharge overpotentials: formation of large (∼250 nm−1 μm) toroids, and conformal coatings (<50 nm) at higher overpotentials. These results highlight that RRDE and EQCM can be used as complementary tools to gain new insights into the role of soluble and solid reaction intermediates in the growth of reaction products in metal−O 2 batteries.
A Study of the Influence of Lithium Salt Anions on Oxygen Reduction Reactions in Li-Air Batteries
Journal of the Electrochemical Society
The influence of lithium salts on O 2 reduction reactions (ORR) in 1, 2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) has been investigated. Microelectrode studies in a series of tetrabutylammonium salt (TBA salt)/DME-based electrolytes showed that O 2 solubility and diffusion coefficient are not significantly affected by the electrolyte anion. The ORR voltammograms on microelectrodes in these electrolytes exhibited steady-state limiting current behavior. In contrast, peak-shaped voltammograms were observed in Li +-conducting electrolytes suggesting a reduction of the effective electrode area by passivating ORR products as well as migration-diffusion control of the reactants at the microelectrode. FT-IR spectra have revealed that Li + ions are solvated to form solvent separated ion pairs of the type Li + (DME) n PF 6 − and Li + (TEGDME)PF 6 − in LiPF 6-based electrolytes. On the other hand, the contact ion pairs (DME) m Li + (CF 3 SO 3 −) and(TEGDME)Li + (CF 3 SO 3 −) appear to form in LiSO 3 CF 3containing electrolytes. In the LiSO 3 CF 3-based electrolytes the initial ORR product, superoxide (O 2 −), is stabilized in solution by forming [(DME) m-1 (O 2 −)]Li + (CF 3 SO 3 −) and [(TEGDME)(O 2 −)]Li + (CF 3 SO 3 −) complexes. These soluble superoxide complexes are able to diffuse away from the electrode surface reaction sites to the bulk electrolyte in the electrode pores where they decompose to form Li 2 O 2. This explains the higher capacity obtained in Li/O 2 cells utilizing LiCF 3 SO 3 /TEGDME electrolytes.
The Journal of Physical Chemistry Letters, 2013
Polyether solvents are considered interesting and important candidates for Li−O 2 battery systems. Discharge of Li−O 2 battery systems forms Li oxides. Their mechanism of formation is complex. The stability of most relevant polar aprotic solvents toward these Li oxides is questionable. Specially high surface area carbon electrodes were developed for the present work. In this study, several spectroscopic tools and in situ measurements using electrochemical quartz crystal microbalance (EQCM) were employed to explore the discharge−charge processes and related side reactions in Li−O 2 battery systems containing electrolyte solutions based on triglyme/lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte solutions. The systematic mechanism of lithium oxides formation was monitored. A combination of Fourier transform infrared (FTIR), NMR, and matrix-assisted laser desorption/ionization (MALDI) measurements in conjunction with electrochemical studies demonstrated the intrinsic instability and incompatibility of polyether solvents for Li−air batteries. SECTION: Energy Conversion and Storage; Energy and Charge Transport
Key scientific challenges in current rechargeable non-aqueous Li–O2 batteries: experiment and theory
Rechargeable Li-air (henceforth referred to as Li-O 2 ) batteries provide theoretical capacities that are ten times higher than that of current Li-ion batteries, which could enable the driving range of an electric vehicle to be comparable to that of gasoline vehicles. These high energy densities in Li-O 2 batteries result from the atypical battery architecture which consists of an air (O 2 ) cathode and a pure lithium metal anode. However, hurdles to their widespread use abound with issues at the cathode (relating to electrocatalysis and cathode decomposition), lithium metal anode (high reactivity towards moisture) and due to electrolyte decomposition. This review focuses on the key scientific challenges in the development of rechargeable non-aqueous Li-O 2 batteries from both experimental and theoretical findings. This dual approach allows insight into future research directions to be provided and highlights the importance of combining theoretical and experimental approaches in the optimization of Li-O 2 battery systems.
Chemistry of Materials, 2012
The high capacity of the layered Li−excess oxide cathode is always accompanied by extraction of a significant amount of oxygen from the structure. The effects of oxygen on the electrochemical cycling are not well understood. Here, the detailed reaction scheme following oxygen evolution was established using real-time gas analysis and ex situ chemical analysis of the surface of the electrodes. A series of electrochemical/chemical reactions involving oxygen radicals constantly produced and decomposed lithium carbonate during cell operation. Moreover, byproducts, including water, affected the cycle life and rate capability: hydrolysis of the electrolyte salt formed hydrofluoric acid that attacked the surface of the electrode. This finding implies that protection of the electrode surface from damage, for example, by a coating or removal of oxygen radicals by scavengers, will be critical to widespread usage of Li−excess transition metal oxides in rechargeable lithium batteries.
Implications of 4 e– Oxygen Reduction via Iodide Redox Mediation in Li–O2 Batteries
ACS Energy Letters, 2016
The nonaqueous lithium−oxygen (Li−O 2) 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−O 2 battery cathode electrochemistry is the formation (discharge) and decomposition (charge) of lithium peroxide (Li 2 O 2). 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 Li 2 O 2 formation. We provide quantitative analysis of the influence of LiI and H 2 O on the electrochemistry in a common Li−O 2 battery employing an ether-based electrolyte and a carbon cathode. We confirm, through numerous quantitative techniques, that the addition of LiI and H 2 O promotes efficient 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 O 2 on charge, where instead a complicated mix of redox shuttling and side reactions is observed.
Influence of Li2O2 morphology on oxygen reduction and evolution kinetics in Li–O2 batteries
Energy & Environmental Science, 2013
Supporting Information Experimental Details Electrode fabrication / catalyst preparation. Catalyst Preparation: Si wafers (6" diameter, n-type doping, 140 nm thick thermal SiO 2) were prepared for catalyst deposition by cleaning using a Piranha solution (3:1 H 2 O 2 :HSO 4). Next, sequential layers of Al 2 O 3 (30 nm) and Fe (1 nm) were deposited using electron beam evaporation without breaking vacuum between layer depositions. The catalyst wafers were then cleaved into small samples (~1 x 1 cm) in preparation for CNT growth. Chemical Vapor Deposition: Multi-walled carbon nanotubes (CNTs) were synthesized in a twofurnace hot-walled thermal chemical vapor deposition system. The two furnaces are connected with a single quartz tube (OD 1"). The upstream furnace (preheater) is used to preheat the carbon precursor to promote gas decomposition and results in improved carbon nanotube growth. The downstream furnace (growth furnace) is where catalyst samples are located and CNTs are grown. The CNTs used in this study were synthesized using the process detailed in the main text. After the post-growth anneal the growth furnace was opened and the tube was rapidly cooled to room temperature under He flow. After cooling, the CNTs were removed from the furnace and then removed from the substrate by gently prying the corners of the monolithic carpet until the carpet uniformly delaminated from the growth substrate. TEM sample preparation. Small electrode fragments were prepared for TEM imaging by sandwiching them inside foldable TEM grids (Ted Pella, Stratatek) in order to minimize damage from sample preparation (i.e. samples were not sonicated). Sample preparation was performed inside an argon-filled glovebox and prepared grids were transported to the TEM in argon-filled airtight containers. Bright field zero-loss imaging was performed at an accelerating voltage of 120 kV in an energy-filtered Zeiss Libra 120 TEM. XRD crystallite size analysis. Average crystallite sizes of Li 2 O 2 in discharged electrodes were determined by peak broadening analysis on raw XRD data (Fig. 3a and additional data) using the Scherrer equation assuming spherical crystallites 1 :
Reaction chemistry in rechargeable Li-O2 batteries
Chemical Society reviews, 2017
The seemingly simple reaction of Li-O2 batteries involving lithium and oxygen makes this chemistry attractive for high-energy-density storage systems; however, achieving this reaction in practical rechargeable Li-O2 batteries has proven difficult. The reaction paths leading to the final Li2O2 discharge products can be greatly affected by the operating conditions or environment, which often results in major side reactions. Recent research findings have begun to reveal how the reaction paths may be affected by the surrounding conditions and to uncover the factors contributing to the difficulty in achieving the reactions of lithium and oxygen. This progress report describes the current state of understanding of the electrode reaction mechanisms in Li-O2 batteries; the factors that affect reaction pathways; and the effect of cell components such as solvents, salts, additives, and catalysts on the discharge product and its decomposition during charging. This comprehensive review of the r...
The effect of O2 concentration on the reaction mechanism in Li-O2 batteries
Journal of Electroanalytical Chemistry, 2017
The promising lithium-oxygen battery chemistry presents a set of challenges that need to be solved if commercialization is ever to be realized. This study focuses on how the O 2 reaction path is effected by the O 2 concentration in the electrolyte. An electrochemical quartz crystal microbalance system was used to measure current, potential, and change in