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In Situ Packaging FeFx into Sack-like Carbon Nanoreactors: A Smart Way To Make Soluble Fluorides Applicable to Aqueous Batteries Jian Jiang,*,†,§,¶ Linpo Li,†,§,¶ Maowen Xu,†,§ Jianhui Zhu,*,‡ and Chang Ming Li†,§ †
Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, P.R. China ‡ School of Physical Science and Technology, Southwest University, Chongqing 400715, P.R. China § Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, P.R. China S Supporting Information *
ABSTRACT: Ferruginous materials have long attracted great interest in aqueous batteries since Fe is an earth-abundant and low toxic element. However, their practical application is severely hindered by their poor structural stability during deep cycling. To maximize their cyclability, we herein propose a simple and effective method, by in situ packaging Fe-based materials into carbon nanosacks via a facile CVD approach. To verify our strategy, we purposely choose water-soluble Fe2F5 as a study paradigm. The in situ formed Fe2F5@C nanosacks product exhibits prominent anodic performance with high electrochemical activity and capacity, obviously prolonged cyclic lifetime, and outstanding rate capabilities. Besides, by pairing with the cathode of α-Co(OH)2 nanowire arrays@carbon cloth, a full device of rechargeable aqueous batteries has been developed, capable to deliver both high specific energy and power densities (Max. values reaching up to ∼163 Wh kg−1 and ∼14.2 kW kg−1), which shows great potential in practical usage. Our present work may not only demonstrate the feasibility of using soluble fluorides as anodes for aqueous batteries but also provide a smart way to upgrade cyclic behaviors of Fe-based anodes. KEYWORDS: water soluble, Fe2F5, carbon nanosacks, α-Co(OH)2 nanowire arrays, full cell, aqueous batteries
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effect.12−15 Besides, a large theoretical capacity can be achieved (∼346.5 mAh g−1, taking Fe3O4, for example) according to valence state variations of Fe (Fe3+ ↔ Fe0) in alkaline solution. However, such a high theoretical value can only be realized upon multiple and deep redox reactions in a wide negative potential window. Under that condition, the multiphase changes of ferruginous anodes would inevitably cause huge volumetric expansions and structural deformation/dissolution, resulting in drastic cell capacity fading even after few tens or hundreds of cycles.15 In this regard, it is highly desirable to look for effective and reliable approaches to ensure the electrochemical stability of Fe-based materials, extend the cyclic lifetime, and upgrade their comprehensive performance. As one significant category of ferruginous materials, Fe-based fluorides (e.g., FeF2, FeF3, etc.) have been proved as promising electrode candidates for energy-storage usage.16−18 On one hand, both Fe and F elements are very abundant in nature and readily available (thus quite convenient and affordable to use).19 On the other hand, fluorides often possess greater
INTRODUCTION Nowadays, lithium-ion batteries (LIBs) have almost dominated the rechargeable cells market. They are widely used in tiny electronic consumables, modern electric vehicles, and largescale electrical grids due to their higher energy density and superior long-term cyclic stability over other energy-storage counterparts.1−4 Despite these notable merits, current LIBs still have to face formidable challenges/issues that mainly arise from volatile and flammable organic electrolytes.5 Once LIBs are subjected to extremely high-power charging/discharging or short circuits, they are likely to get combusted or even explosive. To avoid such undesired safety issues, varieties of rechargeable power sources based on aqueous electrolytes (e.g., lithium salt/alkaline solutions) have drawn tremendous attention recently, since water itself is the safest electrolyte and moreover abundant/friendly to the environment.6,7 Many types of aqueous battery (AB) systems have been proposed to date, involving LiMn2O4/VO2, LixV2O5/LiMn2O4, TiO2/Ni(OH)2, zinc/nickel (Zn/Ni), cobalt/nickel (Co/Ni), iron/ nickel (Fe/Ni), etc.6−13 Among them, ABs using ferruginous materials (e.g., Fe3O4, Fe2O3, etc.) have triggered great interest since Fe is an earth-abundant element (the fourth richest element in earth’s crust) and shows a low toxic or corrosive © 2016 American Chemical Society
Received: November 8, 2015 Accepted: January 25, 2016 Published: January 25, 2016 3874
DOI: 10.1021/acsami.5b10737 ACS Appl. Mater. Interfaces 2016, 8, 3874−3882
Research Article
ACS Applied Materials & Interfaces
parameters (in our case: ∼ 13 cm). Before heating, the quartz-tube reactor was sealed and flushed by Ar gas (200 sccm) for 10 min. The furnace was then heated to 500 °C at a heating rate of ∼15 °C/min under a constant Ar flow of 50 sccm, held for 30 min, and allowed to cool down to room temperature naturally. Synthesis of α-Co(OH)2 NWs@carbon Cloth. For details, a 50 mL homogeneous solution containing 0.8 g of Co(NO3)2·6H2O, 0.35 g of NH4F, and 0.8 g of CO(NH2)2 in distilled water was transferred into a 100 mL Teflon autoclave.21 After that, a piece of carbon cloth (4 × 1 × 0.35 cm3; its conductivity is ca. 0.12 Ω−1) fixed on a glass slide (4.5 × 2 × 0.4 cm3) was immersed into the mixed solution and placed against the liner wall. Then, the autoclave was sealed and heated at 130 °C in an electric oven for 6 h. After the autoclave cooled down naturally, the product of α-Co(OH)2 NWs@carbon cloth was fetched out and washed with distilled water several times. Characterization Techniques and Electrochemical Testing. The morphology and the crystalline structure of as-made products were characterized with a JEOL JSM-7800F field emission scanning electron microscope (FE-SEM) with energy dispersive X-ray spectroscopy (EDS) and a JEM 2010F high-resolution transmission electron microscope (HR-TEM). X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu Kα radiation. Raman spectroscopy (Witech. CRM200, 532 nm), Fourier transform infrared (FTIR) spectroscopy (PerkinElmer spectrophotometer), and XPS (Thermo Electron, VG ESCALAB 250 spectrometer) were used to characterize the products. The mass of electrode materials was measured on a microbalance with an accuracy of 0.01 mg (A&D Company N92, Japan). Electrochemical measurements were all performed on an electrochemical workstation (CHI 760D, CH Instruments Inc., USA). The working electrode of Fe2F5@ C nanosacks was fabricated by the convention slurry-coating method. In detail, Fe2F5@C nanosack powders, poly(vinylidene fluoride) (PVDF) binder, and acetylene black were mixed in a mass ratio of 80:10:10 and dispersed/homogenized in N-methyl-2-pyrrolidone (NMP) to form slurries. The homogeneous slurry was then pasted onto a Ni foam (thickness: 1.5 mm) and dried at 100 °C for 10 h under vacuum. The mass loading on each current collector was controlled to be 2.5−4.0 mg/cm2. α-Co(OH)2 NWs grown on carbon cloth scaffold (electrode area: ∼1 × 3.5 cm2) were directly employed as the working electrode. The loading amount of α-Co(OH)2 was determined by weight differences of carbon cloth before and after hydrothermal synthesis. For individual working electrode testing, the performance was evaluated in a three-electrode system, with a Pt foil as counter electrode and an Ag/AgCl as reference electrode in 6 M KOH. Prior to testing, all electrodes were immersed into electrolyte for 15 min. The full ABs device was constructed with activated Fe2F5@ C nanosacks anode and α-Co(OH)2 NWs@carbon cloth cathode in opposition to each other in 6 M KOH aqueous electrolyte. To balance the charge storage between electrodes, the cathode/anode mass ratio is eventually determined to be ∼1.1:1 by referring to rate behaviors of both electrodes. Note that, before electrochemical measurements on rate performance, all electrodes were preactivated by continuous cyclic voltammetry (CV) scans (∼50−60 cycles at a rate of 10 mV s−1). Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an AC voltage with 5 mV amplitude in a frequency range from 0.1 Hz to 100 kHz at open circuit potential. The specific capacities were calculated from galvanostatic charge/discharge curves by using
discharge voltage due to their highly ionic metal−ligand (Fe− F) bonds and superior specific capacity/energy in comparison with sulfide and oxide cathodes.20 For example, a single FeF3 molecule with good thermal stability can theoretically accommodate three Li+ ions in LIBs showing a large theoretical capacity of 712 mAh g−1 and energy density of 1950 Wh kg−1, both of which are far higher than values of other mainstream cathodes (e.g., lithiated transition metal oxides and olivine compounds;
