Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6531−6540
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Three-Dimensional Nanofibrous Air Electrode Assembled With Carbon Nanotubes-Bridged Hollow Fe2O3 Nanoparticles for HighPerformance Lithium−Oxygen Batteries Ji-Won Jung,† Ji-Soo Jang,† Tae Gwang Yun, Ki Ro Yoon, and Il-Doo Kim* Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: Lithium−oxygen batteries have been considered as one of the most viable energy source options for electric vehicles due to their high energy density. However, they are still faced with technical challenges, such as low round-trip efficiency and short cycle life, which mainly originate from the cathode part of the battery. In this work, we designed a three-dimensional nanofibrous air electrode consisted of hierarchically structured carbon nanotube-bridged hollow Fe2O3 nanoparticles (H-Fe2O3/CNT NFs). Composite nanofibers consisted of hollow Fe2O3 NPs anchored by multiple CNTs offered enhanced catalytic sites (interconnected hollow Fe2O3 NPs) and fast charge-transport highway (bridged CNTs) for facile formation and decomposition of Li2O2, leading to outstanding cell performance: (1) Swagelok cell exhibited highly reversible cycling characteristics for 250 cycles with a fixed capacity of 1000 mAh g−1 at a current density of 500 mA g−1. (2) A module composed of two pouch-type cells stably powered an light-emitting diode lamp operated at 5.0 V. KEYWORDS: lithium−air batteries, iron oxide, carbon nanotubes, hierarchical porous structure, cathode
1. INTRODUCTION Lithium−oxygen (Li−O2) batteries (LOBs) have received great attention because of their exceptionally high energy density (3505 W h kg−1) far exceeding that of the state-of-art Li-ion batteries.1,2 However, LOBs still suffer from technical problems, such as short cycle life and high overpotentials, resulting in low round-trip efficiency; these are primarily determined by O2breathing porous cathode, where Li2O2 is formed and decomposed.3,4 Although discharge−charge reactions are ongoing, pores of cathode can be easily clogged and deactivated by insulating Li2O2 that is not sufficiently removed, limiting electron transfer and O2 diffusion in porous cathode. This induces rapid increases of overpotentials, especially for charge process (called oxygen evolution reaction, OER).5 So far, a number of efficient catalysts have been suggested for improved OER, which can lead to facile decomposition of Li2O2. Among them, conductive metals, such as cobalt (Co), ruthenium (Ru), and gold (Au), have been introduced as efficient and stable catalysts.6−8 However, these catalysts are expensive; thus, their use might be limited in practical level of Li−O2 cell fabrication. As one of the alternatives to these, iron (Fe)-based catalysts have been employed as an O2 cathode material because they are cost-effective and abundant in earth’s crust.9,10 Lv et al. synthesized macelike Fe-based oxide and used it as porous cathode material for LOBs.11 However, intrinsic electronic conductivity of iron-based oxide is very low, leading © 2018 American Chemical Society
to considerable loss of energy density in LOBs; therefore, it should be mixed with inexpensive conducting carbon materials (e.g., carbon nanotubes (CNTs), graphene, and carbon nanofibers (CNFs)).12−14 For examples, Wu et al. synthesized Fe2O3 nanoparticles (NPs) supported by Vulcan XC-72 carbon as LOB cathode and suggested that Fe2O3 coupled with conductive carbon can effectively reduce charge overpotential.9 Feng and co-workers reported that a multilayered Fe2O3/ graphene composite, which was produced through thermal casting approach, offers a lot of catalytically active sites and alleviates unwanted parasitic side reactions on graphene.13 Although these results indicated that Fe2O3 is a prospective catalyst, it has rarely been studied, particularly in long cycling cathodes for LOBs. Moreover, technical challenges remain to realize desirable architecture with an optimized hybridization of Fe2O3 catalyst with conductive carbon for highly reversible formation and decomposition of Li2O2. Ideally combined Fe2O3 catalyst and conductive carbon can remarkably reduce overpotentials and maximize discharge capacity of LOBs. In this work, we synthesized hierarchically structured composite nanofibers consisting of hollow Fe2O3 NPs anchored by multiple CNTs (hereafter, H-Fe2O3/CNT NFs) as an air Received: October 11, 2017 Accepted: January 30, 2018 Published: January 30, 2018 6531
DOI: 10.1021/acsami.7b15421 ACS Appl. Mater. Interfaces 2018, 10, 6531−6540
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration of the synthesis of porous H-Fe2O3/CNT NFs via simple electrospinning and two-step heat treatments for in situ growth of CNT and hollow Fe2O3 NPs. Halenda (BJH) method. The crystal structures of the prepared samples were analyzed by powder X-ray diffraction (PXRD, D/MAXRC 12 kW, Rigaku) with Cu Kα radiation (λ = 1.54 Å). The chemical bonding state and elements of the prepared samples were evaluated by X-ray photoelectron spectroscopy (XPS, K-α, Thermo Scientific) and Raman spectrometer equipped with a 514 nm laser (FT-Raman, Bruker). Elemental analysis was conducted using an element analyzer (Flash 2000, Thermo Scientific). 2.4. Electrochemical Measurements. For evaluation of the Li− O2 cells with H-Fe2O3/CNT NFs as air cathode, we utilized Swageloktype cells according to our previous work. Prior to cell assembly, cathode was prepared as follows: 60 mg of H-Fe2O3/CNT NFs, 30 mg of KB, and 10 mg of poly(vinylidene difluoride) (PVDF) (4 wt %) were dissolved in N-methyl-2-pyrrolidone solvent and mixed together using an agate mortar for slurry; the KB electrode without catalyst was composed of KB and PVDF binder in the weight ratio of 9:1. The slurry was cast on a Ni mesh (pore diameter = 100 μm, thickness = 50 μm) and a carbon cloth with micron-sized diameter (HCP330, WizMAC), after which they were dried in a vacuum oven overnight. The mass loading of H-Fe2O3/CNT NFs (or only KB) was approximately 0.5−0.6 mg cm−2. All of the Li−O2 cells were assembled in an Ar-filled glovebox (water content
