Research Article www.acsami.org
In Situ Engineering Toward Core Regions: A Smart Way to Make Applicable FeF3@Carbon Nanoreactor Cathodes for Li-Ion Batteries Linpo Li,†,§,∥ Jianhui Zhu,‡,∥ Maowen Xu,†,§ Jian Jiang,*,†,§ and Chang Ming Li*,†,§ †
Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, and ‡School of Physical Science and Technology, Southwest University, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, People’s Republic of China § Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, No. 2 Tiansheng Road, BeiBei District, Chongqing 400715, People’s Republic of China S Supporting Information *
ABSTRACT: Iron fluorides (FeFx) have attracted great interest in Li-ion batteries due to their high theoretical capacity, low cost, and preferable cell safety. However, their practical utilization is severely impeded by inferior electrode kinetics, leading to poor electrode cyclability and rate capabilities. The major bottleneck should be lack of any effective engineering techniques to make reliable encapsulation and conducting matrix on soluble FeFx species. Herein, we propose an applicable synthetic strategy where the massive production of FeF3@carbon nanoreactors (total size: ∼60 nm) can be easily achieved by in situ engineering toward the core regions in hybrids, with the iron rust wastes and common solvents as raw materials. Such functionalized configurations can well make up for the shortcomings of FeF3 species, enabling them with outstanding cathode behaviors involving excellent reversible capacity retention (∼270% higher than that of a bare FeF3 electrode after 600 cycles) and drastically enhanced rate performance. This paradigm work provides a facile and scalable method to make superior and sustainable cathodes and, moreover, offers a feasible engineering protocol to make water-soluble species encapsulated into carbon matrix, not merely for batteries but also for other wide range of fields like catalysis, nanomedicine, etc. KEYWORDS: FeF3@carbon nanoreactor, in situ engineering, smart core−shell evolution, long-life cathode, Li-ion batteries
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INTRODUCTION Building rechargeable energy-supply systems with both high energy and power densities has triggered worldwide interest to fulfill our ever-growing demands for either portable electronics or electrical vehicles.1−3 To develop reliable power sources, Liion batteries (LIBs) have long been the research focus since they possess overwhelming advantages over other cell counterparts (e.g., Ni-MH and lead−acid cells, etc.).4 Though the currently used LIBs can meet essential criteria in long-term cyclic stability and lifespan, they still have to face some formidable challenges.5,6 One major issue is ascribed to the use of cathode materials based on lithiated transition metal oxides and olivine compounds whose maximal practical capacity is lower than ∼200 mAh g−1 (far less than the value of carbon anodes at ∼372 mAh g−1).7−10 Another notable constraint for commercial concern should be their high cost and poor sustainability, which is led by cathode fabrications with rare and expensive Ni/Co salts.4,5 Thereby, for continual and further progress of LIBs industries, future development of cathodes with more abundant and cheaper elements should be the most rational way to overcome all current issues.9,12−14 Metal fluoride species (covering BiF2, CuF2, NiF2, FeF2, CoF3, FeF3, etc.) have recently been studied as alternative cathode materials for LIBs application.4,5,15,16 Among fluoridebased cathodes, iron fluorides (FeFx) have attracted particular © 2017 American Chemical Society
attention since they are most promising economical and highcapacity (e.g., a single FeF3 molecule can theoretically react with three Li+, contributing a specific capacity as large as ∼712 mAh g−1) candidates to replace the commercial lithium cobalt oxide (LiCoO2).2,17 Both Fe and F are low toxicity, readily available, and earth-abundant elements when compared to other counterparts like Co, V, Ti, Mn, P, and Ni, etc.4,5,18,19 Despite these significant merits, deep researches on FeFx cathodes have to be held back unfortunately, with major reasons including (1) their intrinsically slow ionic diffusion and insulating properties and (2) structural decomposition/ reconstruction during repeated conversion reactions. These adverse properties may result in inferior rate capabilities, extremely low active utilization efficiency, and rather poor capacity retention.9,16,20 To overcome these obstacles, downsizing FeFx materials into the nanoscale has been performed by means of high-energy mechanical ball-milling, pulsed laser deposition, and ionic-liquid-assisted methods, etc.11,20,21 Though using these strategies may well circumvent the above shortcomings and improve electrode kinetics because of accelerated Li+ diffusion in FeFx and increased active reaction Received: March 26, 2017 Accepted: May 10, 2017 Published: May 10, 2017 17992
DOI: 10.1021/acsami.7b04256 ACS Appl. Mater. Interfaces 2017, 9, 17992−18000
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) General schematics displaying the entire evolution of the FeF3@C hybrid matrix. SEM observations toward samples at distinct evolution stages: (b and c) Fe2O3 nanospheres; (d and e) Fe3O4@C intermediates; (f and g) FeF3@C hybrids.
after 600 cycles (for single FeF3, only ∼70 mAh g−1 remains after 170 cycles), with prominent rate capability and good longterm cyclic endurance (negligible capacity decay in total 600 cycles). Our work may not only offer an economical way to realize the encapsulation of FeFx within C matrix for outstanding cathode behaviors, but also set a feasible and interesting platform to fabricate other functional hybrids hardly made via traditional techniques.
sites, their cathode performance in LIBs is still hard to meet the practical application criteria, which is mainly induced by detrimental electrode pulverizations and agglomerations in lithium-storage processes.22 Immobilization of FeFx nanocrystals into functionalized carbon (C) matrix may be a rational and reliable way to promote the electrode cyclic endurance.23 However, the hybrid fabrication of FeF3@C matrix is difficult to realize since most of fluorides are polar molecules and tend to be soluble in water. Thereby, all traditional carbonization methods via soaking treatments in organics (e.g., sucrose, glucose, polymers)-containing solution are inapplicable.4 Other catalytic approaches at high-temperature conditions are also scarcely used to fabricate FeF3@C hybrid matrix (wherein the stoichiometric ratio of Fe/F is 1:3) due to unstable thermal properties of metal fluorides.4 To upgrade the comprehensive cathodic behaviors, a smart, affordable, and scalable synthesis of FeF3@C core−shell hybrids is urgently desired. We herein develop a simple and effective evolution strategy for large-scale and low-cost production of FeF3@C nanoreactors for LIBs cathode. Note that the raw materials merely involve the ethylene glycol and Fe2O3 nanospheres made from the useless iron rust.24−26 After a quick chemical vapor deposition (CVD) process, the crystallized carbon shells (10−20 layers) are in situ catalytically grown on the nanosphere surface, giving rise to the generation of core− shell Fe3O4@C hybrids.27 Such intermediates are further transformed into FeF3@C nanoreactors (size: ∼60 nm) when adequately exposed in HF atmosphere followed by an annealing treatment (all this operation proceeds in a sealed and safe container).9 The as-formed FeF3@C hybrids exhibit far superior cathode performance when compared to bare FeF3, capable of retaining a specific capacity of ∼260.78 mAh g−1
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EXPERIMENTAL SECTION
All involved chemicals were purchased from Sigma-Aldrich Reagent Co. The reagents and solvents were of analytical grade and used without any extra purification. Synthesis of Fe3O4@C Hybrids. The initiating materials of Fe2O3 nanospheres were fabricated using a facile hydrothermal treatment toward the iron rust.28 For details, ∼0.2 g of bulky iron rust was ground into powder and then dissolved into ∼30 mL of diluted HNO3 (mass concentration: 50%) at 50 °C under vigorous stirring. Next, the resulting solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, which was sealed and kept still in an electric oven at 220 °C for 30 min. Then, the red samples were collected by vacuum filtration, washed with deionized water several times, and dried at 60 °C. The hybrid synthesis of Fe3O4@C was carried out in a horizontal, quartz tube furnace system. Approximately 1 g of Fe2O3 nanopowders was evenly distributed in the center of the quartz tube while 5 mL of ethylene glycol (EG) loaded in an alumina boat was put at the upstream zone (in our case, the distance from the alumina boat to quartz tube center: ∼14 cm). Prior to heating, the tube reactor was sealed and flushed with N2 gas (200 sccm) for 30 min. Then, the furnace was heated to 550 °C (heating rate: ∼10 °C min−1) under a constant N2 flow of 50 sccm, kept for 30 min, and allowed to cool down to room temperature naturally. Synthesis of FeF3@C Nanoreactors. Approximately 0.2 g of Fe3O4@C hybrid intermediates packaged by an absorbent paper were 17993
DOI: 10.1021/acsami.7b04256 ACS Appl. Mater. Interfaces 2017, 9, 17992−18000
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) XRD patterns of samples at distinct evolution stages. (b−e) TEM observations, (f) EDS detecting, and (g−k) elemental mappings on the ultimate hybrids of FeF3@C nanoreactors. and reference electrode. A solution of 1 M LiPF6 dissolved in a 1:1 (v/ v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte. Galvanostatic charge/discharge tests were conducted by a professional battery tester (NEWARE, Shenzhen), while the electrochemical impedance spectroscopy and cyclic voltammetry (CV) scans were measured on an electrochemical workstation (CorrTest CS310). Before battery testing, all cells were aged for 8 h.
transferred to a 30 mL Teflon container. Afterward, the whole container was put into a 100 mL Teflon-lined autoclave wherein ∼30 mL of HF acid (48 wt %) was added beforehand. Notice that there is no direct contact between samples and HF solution. In the following step, the autoclave was carefully sealed and kept at 95 °C for 8 h. When naturally cooled down to room temperature, the gray powder samples were taken out, dried at 80 °C for 6 h in a vacuum oven, and annealed at 170 °C for 4 h in N2 atmospheres to remove the crystal/ chemically adsorbed water. Synthesis of FeF3 Nanoparticles. For comparative study, bare FeF3 nanoparticles were made via the same synthetic procedure for the case of FeF 3@C hybrids, by directly exposing ∼0.2 g Fe2O3 nanospheres into HF atmospheres (acid volume: ∼30 mL) at 95 °C for 8 h. Afterward, the powder samples were collected, dried at 80 °C for 6 h in vacuum oven, and annealed at 170 °C in N2 atmospheres for 4 h. Characterization Techniques and Battery Testing. The morphological and crystalline structures of samples were characterized by 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 (HRTEM). X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS; Thermo Electron, VG ESCALAB 250 spectrometer) was also used to characterize the products. Thermogravimetric analysis (TGA) was performed on an SDT600 apparatus under a heating rate of ∼5 K min−1 in N2 atmosphere. The mass of electrode materials was measured on a microbalance with an accuracy of 0.01 mg (A&D Company N92, Japan). The working electrode of FeF3@C was fabricated by the conventional slurry-coating method. In details, FeF3@C active nanopowders, poly(vinylidene fluoride) (PVDF) binder, and carbon 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 an Al film (thickness: 0.4 mm) and dried at 120 °C for 12 h under a vacuum condition. The mass loading on each current collector was controlled at the level of 1.0−1.5 mg/cm2. The electrode testing was performed using CR-2032 coin-type cells in a potential range of 1.5−4.5 V. Cells were assembled in an argon-filled glovebox (DELLIX LS800S; H2O
