Letter pubs.acs.org/NanoLett
A New Design Strategy for Observing Lithium Oxide GrowthEvolution Interactions Using Geometric Catalyst Positioning Won-Hee Ryu,†,‡ Forrest S. Gittleson,†,§ Jinyang Li,† Xiao Tong,∥ and André D. Taylor*,† †
Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06511, United States ‡ Department of Chemical and Biological Engineering, Sookmyung Women’s University, 100 Cheongpa-ro 47-gil, Yongsan-gu, Seoul, 04310, Republic of Korea § Sandia National Laboratories, 7011 East Avenue, Livermore, California 94550, United States ∥ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *
ABSTRACT: Understanding the catalyzed formation and evolution of lithium-oxide products in Li−O2 batteries is central to the development of next-generation energy storage technology. Catalytic sites, while effective in lowering reaction barriers, often become deactivated when placed on the surface of an oxygen electrode due to passivation by solid products. Here we investigate a mechanism for alleviating catalyst deactivation by dispersing Pd catalytic sites away from the oxygen electrode surface in a well-structured anodic aluminum oxide (AAO) porous membrane interlayer. We observe the cross-sectional product growth and evolution in Li−O2 cells by characterizing products that grow from the electrode surface. Morphological and structural details of the products in both catalyzed and uncatalyzed cells are investigated independently from the influence of the oxygen electrode. We find that the geometric decoration of catalysts far from the conductive electrode surface significantly improves the reaction reversibility by chemically facilitating the oxidation reaction through local coordination with PdO surfaces. The influence of the catalyst position on product composition is further verified by ex situ X-ray photoelectron spectroscopy and Raman spectroscopy in addition to morphological studies. KEYWORDS: Lithium−oxygen batteries, catalytic membrane, product morphology, nanoparticles, oxygen evolving catalyst
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catalysts on the oxygen electrode are easily deactivated as solids collect and passivate their surfaces.13−27 In practice, a common approach to improve cell cyclability has involved restricting the capacity to limit the amount of products formed during discharge.14,28 Thinner product layers are more easily evolved at catalytic sites on the oxygen electrode when products are located in close proximity, enabling more efficient electron transfer. Although capacity limitation improves cycle life, discharge products continue to accumulate over many cycles, leading to complete catalyst deactivation and eventually cell death. Understanding the mechanisms of discharge product formation and evolution is necessary to guide future cell design. While the direct observation of discharge products has been reported at the oxygen electrode surface,29−31 the morphological and compositional features of products greater
eveloping an efficient and sustainable next-generation battery chemistry is necessary to meet the growing demand for energy storage systems.1,2 The lithium−oxygen (Li−O2) battery promises a considerably higher theoretical energy density than typical Li-ion systems and is ideal for use in large scale applications such as electric vehicles and uninterruptible power supplies, essential for carbon neutrality.3−5 Unlike conventional Li-ion batteries containing heavy metal-based cathode materials (i.e., LiCoO2, LiFePO4), Li−O2 batteries utilize lightweight oxygen gas as a ubiquitous cathode material, enabling 2−3 times greater practical capacity per mass.6−8 The operation of Li−O2 batteries involves numerous surface reactions in which solid lithium-oxide products are formed and evolved at the porous oxygen electrode (2Li+ + O2 + 2e− ↔ Li2O2, Eo = 2.96 V vs Li/Li+).9,10 However, the sluggish kinetics associated with oxygen evolution remains an obstacle to achieving better efficiency and cyclability of these cells.11,12 Recent progress has been made in the development of diverse solid catalysts (i.e., noble metals, metal oxides) to facilitate the oxidation of solid products (LiO2, Li2O2), yet © XXXX American Chemical Society
Received: February 26, 2016 Revised: June 17, 2016
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DOI: 10.1021/acs.nanolett.6b00856 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Cross-sectional image of (a,b) pristine AAO membrane and (c,d) Pd-AAO catalytic membrane. Inset images of panel (a) and (c) present photographs of pristine AAO membrane and Pd-AAO membrane, respectively. (e) Schematic illustration of a Li−O2 cell structure employing the AAO membrane or Pd-AAO membrane (catalytic membrane) and cross-sectional views for discharge.
Results and Discussion. To better understand the crosssectional growth and evolution of products in the Li−O2 cells, we selected a commercial AAO template as the insulating membrane backbone due to (i) the good stability of alumina (Al2O3) against unexpected side-reactions, (ii) its brittleness in the cutting process, and (iii) its straight and vertically aligned pore channels. We present a cross-sectional view of AAO and Pd-decorated AAO (Pd-AAO) membranes (Figure 1). The thickness and uniform pore diameter of the AAO template are 55 μm and 200 nm, respectively (Figure 1a,b). We utilized Pd nanoparticles synthesized with a previously reported solutionbased polyol method as catalytic sites (Figure S1).32,35 To fabricate the catalytic membrane with geometric configuration of catalyst particles, Pd nanoparticles (NPs) with a diameter of 20−30 nm were discretely functionalized on the AAO pore walls by simple immersion of the AAO template in Pd nanoparticle ink followed by drying (Figure 1c). The Pd NP clusters are well distributed without significant particle agglomeration (Figure 1d). While the pristine AAO membrane is typically white in color, the Pd-AAO membrane is uniformly brown in color, corresponding to a uniform dispersion of the Pd NPs on the AAO scaffold (insets of Figure 1a,c). Using energy dispersive X-ray spectroscopy (EDS) analysis for different spots, the particles on the wall were confirmed to be composed of Pd (Figure S2). AAO or Pd-AAO membranes were added between an oxygen electrode and a porous separator as an interlayer in a typical Li−O2 cell. Cells were electrochemically cycled and the AAO interlayers were removed for cross-sectional imaging at different discharge or charge states (Figure 1e). We confirm that the AAO itself does not affect cell function through a comparison of charge/ discharge profiles for cells with and without AAO interlayers (Figure S3). The electrically insulating, stable AAO membrane only provides pore space to capture the growing discharge products for cross-sectional observation. Most previous studies of the morphological evolution of Li− O2 products have been conducted by observing the oxygen
than a few nanometers from the electrode are difficult to investigate. Therefore, encapsulating products into a porous membrane and examining their cross-sectional features ex situ could be a useful way to further understand the mechanisms belying Li−O2 cell operation. In a previous report, we demonstrated that dispersing catalyst sites in an insulating membrane over the oxygen electrode helps to maintain their function by reducing passivation of the active surfaces.32 A benefit of this approach is that the direct electrochemical reaction (lithium-oxide formation) is prevented on the isolated catalysts and the catalytic sites are continuously available to facilitate oxygen evolution, even very far from the electrode. Because of the benefits that the catalytic membrane architecture offers, it is necessary to investigate the nature of far-away products and elucidate the function of electrically disconnected catalytic sites. In this work, we directly observe lithium-oxide product formation and evolution both in the presence of catalysts and without to elucidate the significance of geometric catalyst decoration. Anodic aluminum oxide (AAO) is employed as an insulating, porous backbone of the catalytic membrane and Pd nanoparticles dispersed throughout the AAO are used as the catalytic sites. The brittleness of the AAO membranes allows them to be cross-sectioned without destroying the well-defined pore structure, thus preserving the morphology of the integrated lithium-oxide products. By adopting AAO rather than the previously reported PAN (polyacrylonitrile) polymer membrane, possible side-reactions between the lithium-oxide products and the polymers can also be eliminated.33,34 Here we introduce discrete Pd nanoparticles, uniformly distributed on the AAO nanopore walls, to demonstrate the effect of catalyst position on mechanistic function. Improvements in cell function with a catalytic membrane are reflected in reduced cell impedances and lower oxidation overpotentials. Ex situ characterization by Raman and X-ray photoelectron spectroscopy elucidates the composition of discharge products and the catalytic function of the isolated Pd sites. B
DOI: 10.1021/acs.nanolett.6b00856 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 2. Cross-sectional images of AAO membrane at different electrochemical states with different magnifications: (a,b) discharged for 1 h, (c,d) discharged for 5 h, (e,f) discharged for 10 h, (g,h) discharged for 10 h and charged for 1 h, (i,j) discharged for 10 h and charged for 5 h, and (k,l) discharged for 10 h and charged for 10 h. Pink dotted lines indicate the cross-sectional boundary of the AAO membrane. Up and down arrows are correspond to electrode side and separator side, respectively. Yellow dotted lines relate the discharge product interface. A current density of 100 mA/ gcarbon is applied for discharge and charge.
Figure 3. Cross-sectional images of Pd-AAO membrane at different electrochemical states with different magnifications: (a,b) discharged for 1 h, (c,d) discharged for 5 h, (e,f) discharged for 10 h, (g,h) discharged for 10 h and charged for 1 h, (i,j) discharged for 10 h and charged for 5 h, (k,l) discharged for 10 h and charged for 10 h. Pink dotted lines indicate the cross-sectional boundary of the Pd-AAO membrane. Up and down arrows are correspond to electrode side and separator side, respectively. Yellow dotted lines relate the discharge product interface. A current density of 100 mA/gcarbon is applied for discharge and charge. The Pd particles are distinguished by spot EDS analysis during SEM observation.
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DOI: 10.1021/acs.nanolett.6b00856 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 4. (a) Morphological evolution schematic of products for discharge and charge in the AAO and Pd-AAO membrane. Each cross-section schematic corresponds to an electrochemical state in the charge/discharge profiles. Pink dots and blue regions indicate the Pd catalyst and discharge products, respectively. (b) Electrochemical impedance spectroscopy (EIS) spectra of Li−O2 cells employing AAO membrane and Pd-AAO membrane before and after the 1, 5, and 10 h discharge. (c) galvanostatic intermittent titration curves of Li−O2 cells employing AAO and Pd-AAO membranes, which were acquired with a current density of 50 mA/g for 24 min and a 120 min time interval during the first charging.
electrode surface.36−39 Such reports have proposed a growth mechanism for
