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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Sea-Sponge-like Structure of Nano-Fe3O4 on Skeleton‑C with Long Cycle Life under High Rate for Li-Ion Batteries Shipei Chen,†,∥ Qingnan Wu,†,∥ Ming Wen,*,† Qingsheng Wu,*,† Jiaqi Li,† Yi Cui,*,‡ Nicola Pinna,§ Yafei Fan,§ and Tong Wu‡ †
School of Chemical Science and Engineering, State Key Laboratory of Pollution Control and Resource Reuse, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092, P. R. China ‡ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States § Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany S Supporting Information *
ABSTRACT: To meet the demands of long cycle life under high rate for lithium-ion batteries, the advancement of anode materials with stable structural properties is necessarily demanded. Such promotion needs to design reasonable structure to facilitate the transportation of electron and lithium ions (Li+). Herein, a novel C/Fe3O4 sea-sponge-like structure was synthesized by ultrasonic spray pyrolysis following thermal decomposition process. On the basis of sea-sponge carbon (SSC) excellences in electronic conductivity and short Li+ diffusion pathway, nano-Fe3O4 anchored on stable SSC skeleton can deliver high electrochemical performance with long cycle life under high rate. During electrochemical cycling, well-dispersed nano-Fe3O4 in ∼6 nm not only averts excessive pulverization and is enveloped by solid electrolyte interphase film, but also increases Li+ diffusion efficiency. The much improved electrochemical properties showed a capacity of around 460 mAh g−1 at a high rate of 1.5C with a retention rate of 93%, which is maintained without degradation up to 1000 cycles (1C = 1000 mA g−1). KEYWORDS: sea-sponge skeleton, C/Fe3O4 composite, anode, long cycle life, Li-ion battery
1. INTRODUCTION Lithium-ion batteries (LIBs) have been widely used in the portable consumer electronics market due to its good features of small size, high voltage, high power density, no memory effect, low self-discharge, environmental friendliness, etc.1−5 However, it gradually unsatisfied the requirements of social development because of the low theoretical capacity, especially the commercial graphite anode (372 mAh g−1) materials. Therefore, it is an urgent task to achieve higher capacity with long cycle life under larger rate for LIBs. Fe3O4 is one of the most attractive anode materials whose theoretical capacity is up to 930 mAh g−1, much higher than that of conventional graphite owing to a novel conversion mechanism.6−10 Besides, it has advantages in density (5.22 g cm−3), low cost, and environmental friendliness.10,11 Hence, it has gained extensive attentions and investigations recently. However, Fe3O4 nanoparticles (NPs) suffer from several problems in the charge−discharge process, such as easy aggregation of particles, enormous volume expansion, slow Li+ diffusion, and low electronic conductivity, which result in the quick capacity fading along with poor cycling behavior, these problems have limited its commercialization.8−13 There are two common strategies to solve these problems mentioned above: (i) Improving electron and Li+ transfer. © XXXX American Chemical Society
Employing porous carbon skeleton and nanosized Fe3O4 phase are both good ways to enhance electronic conductivity and Li+ transmission efficiency for high-efficiency anode material; (ii) Enhancing structural stability. Fabricating magnetite−carbon composites is carried out to ensure cycling stability of Fe3O4 relying on the stability of carbon skeleton. The introduced carbon not only increases the electronic conductivity of composites for excellent rate performance, but also buffers the strains of Li+ insertion/extraction, and thus finally endows the anode materials with high capacity and stability over long cycles. These two strategies can be simultaneously addressed by fabricating carbon core−shell structures or integrating carbon skeleton with Fe3O4 NPs to achieve high electrochemical performance and stability. Nevertheless, over hundreds of cycles, the magnetite−carbon composite anode still has obvious capacity fading especially under relative high rate and its low specific volumetric capacity cannot meet the growing commercial requirements. The reasons are speculated that the carbon skeleton is not stable enough and the Fe3O4 NPs pulverization is Received: February 23, 2018 Accepted: May 23, 2018
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DOI: 10.1021/acsami.8b02839 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
2. EXPERIMENTAL SECTION
not avoided effectively for long cycles. Therefore, it is worthwhile to design new magnetite−carbon composite with long cycling life and ideal capacity under high current density. In this work, a sea-sponge structure composite of skeleton-C modified with Fe3O4 NPs was explored for superior Li-ion battery anode. The composite was synthesized through in situ thermal decomposition process to anchor Fe3O4 on a sea-sponge carbon (SSC) skeleton (Figure S1, Supporting Information (SI)) prepared by ultrasonic spray pyrolysis (USP) method.14,15 Such a design has multiple superiorities. (i) 3D conductive SSC skeleton is rich in interconnected pores, which can offer suitable expansion space for active materials and enhance the surface electrochemical reactivity and electron transfer (Figure S1). (ii) Fe3O4 NPs in ∼6 nm will greatly enhance the diffusion efficiency of Li+, and due to its lower degree of cracking,6 the formation of solid electrolyte interphase (SEI) will be reduced and the generated SEI film can work as a stable layer to prevent Fe3O4 NPs from further reactions with electrolyte, which can reduce the irreversible capacity during cycling (Figure 1). (iii) The high
2.1. Synthesis of Sea-Sponge Carbon (SSC) Skeleton. The SSC skeleton was synthesized by ultrasonic spray pyrolysis.14 An ultrasonic atomizer nebulizes a precursor solution (sodium chloroacetate, ClCH2COONa) ultrasonically producing droplets. The droplets were carried by argon into a furnace in which water evaporation and precursor decomposition occurs.14 The product was collected in absolute ethanol bubblers with the generated salt dissolving, leaving behind the SSC skeleton. Later, the products were washed with ethanol (75 vol %), centrifuged, and dried in a vacuum oven for 5 h at 60 °C. 2.2. Synthesis of C/Fe3O4 (6 nm). As-prepared SSC skeleton (100 mg) was added into 100 mL of triethylene glycol (TREG, Sinopharm Chemical Reagent Co. Ltd.) and sonicated for 1 h. Then, 0.2 g of the iron(III) acetylacetonate (Fe(acac)3, Sinopharm Chemical Reagent Co. Ltd.) was added under mechanical stirring. After heating to 278 °C (3 °C min−1 of heating rate) under vigorous mechanical stirring, the resulting mixture was kept at reflux for 1 h. Then, the obtained composites were washed with ethanol (75 vol %), centrifuged, and dried in a vacuum oven for 5 h at 60 °C. Moreover, Fe3O4 NPs were prepared by the same method without SSC skeleton. The mass loading of Fe3O4 NPs is by changing the amount of the Fe(acac)3, so 0.35 or 0.7 g of Fe(acac)3 was added to control the mass loading of Fe3O4 in C/Fe3O4 for comparison. 2.3. Synthesis of C/Fe3O4 (30 and 60 nm). Fe(acac)3 was dissolved in 15 mL of TREG under stirring and then 15 mg of SSC skeleton was added into the mixed solution. After sonicated for 1 h, the mixture was transferred into a 30 mL Teflon-sealed autoclave. The autoclave was heated at 180 °C for 2−5 h and then cooled down to room temperature naturally. The product (C/Fe3O4) was washed with ethanol (75 vol %), centrifuged, and dried overnight in a vacuum oven at 70 °C. The diameter of Fe3O4 NPs was depended on the solvent heat reaction time, for example, the reaction of 2 h (5 h) was used to obtain Fe3O4 NPs with 30 nm (60 nm). 2.4. Synthesis of C@Fe3O4@C. In a typical synthesis,19 50 mg of C/ Fe3O4 (6 nm) powder and 1.8 g of glucose (analytical purity, Sinopharm Chemical Reagent Co., Ltd.) were dispersed in 20 mL of water by ultrasonication for 10 min, which was placed in a Teflon-lined autoclave (30 mL) and heated at 180 °C for 3 h. And then the products were collected by centrifugation, washing (ethanol, 75 vol %), and drying (24 h, 80 °C). The C/Fe3O4@C (6 nm) was obtained by carbonizing the carbon shell at 500 °C for 3 h (20 °C min−1 of heating rate) under an argon atmosphere. 2.5. Product Characterization. The morphology was determined by transmission electron microscopy (TEM, JEOL, JEM-2100EX) and field emission scanning electron microscopy (JEOL, S-4800). Elemental analysis was performed by energy-dispersive spectrometry (EDS, Oxford, TN-5400). The composite was characterized by X-ray diffraction (XRD, Bruker, D8 Advance) with Cu Kα X-ray radiation source (λ = 0.154056 nm). X-ray photoelectron spectroscopy (XPS, PerkinElmer, PHI-5000C ESCA) with Al Kα radiation (hν = 1486.6 eV) was performed to determine the surface elemental. The high-resolution spectrum, the narrow spectrum, and full spectrum (0−1400 eV) were recorded by RBD 147 interface (RBD Enterprises). Binding information was calibrated using the C 1s = 284.6 eV as a reference, and the data analysis was performed using the XPS Peak41. The thermal behavior and composition were tested by thermogravimetric analysis (TGA, Netzsch, STA409PC). The specific surface area was determined by adsorption isotherm of nitrogen (Micromeritics, ASAP2020) at −196 °C based on the Brunauer−Emmett−Teller (BET) equation. Fourier transform infrared (FTIR, Thermo, NEXUS) spectroscopy was performed to determine molecular structure. 2.6. Electrode Preparation and Electrochemical Analyses. The working electrode was fabricated by mixing the C/Fe3O4 micron spheres powders, poly(vinylidene difluoride), and acetylene black at a weight ratio of 80:10:10 in N-methyl-pyrrolidone to obtain a homogeneous slurry. The slurry was casted onto copper foil (9 μm) and dried in a vacuum oven at 100 °C for 12 h. CR2025-type coin cells were fabricated inside a glovebox (filled with Ar gas, O2 and H2O content
