Article pubs.acs.org/Langmuir
Bioinspired Hierarchical Nanofibrous Silver-Nanoparticle/Anatase− Rutile-Titania Composite as an Anode Material for Lithium-Ion Batteries Yan Luo,†,‡ Jiao Li,† and Jianguo Huang*,† †
Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China Shaoxing Test Institute of Quality and Technical Supervision, Shaoxing, Zhejiang 312071, China
‡
S Supporting Information *
ABSTRACT: A new bioinspired hierarchical nanofibrous silver-nanoparticle/ anatase−rutile-titania (Ag-NP/A−R-titania) composite was fabricated by employing a natural cellulose substance (e.g., commercial laboratory cellulose filter paper) as the structural scaffold template, which was composed of anatase-phase titania (A-titania) nanotubes with rutile-phase titania (R-titania) nanoneedles grown on the surfaces and further silver nanoparticles (AgNPs) immobilized thereon. As it was employed as an anode material for lithium-ion batteries (LIBs), high reversible capacity, enhanced rate performance, and excellent cycling stability were achieved as compared with those of the corresponding cellulose-substancederived nanotubular A-titania, R-titania, heterogeneous anatase/rutile titania (A− R-titania) composite, and commercial P25 powder. This benefited from its unique porous cross-linked three-dimensional structure inherited from the initial cellulose substance scaffold, which enhances the sufficient electrode/electrolyte contact, relieves the severe volume change upon cycling, and improves the amount of lithium-ion storage; moreover, the high loading content of the silver component in the composite improves the electrical conductivity of the electrode. The structural integrity of the composite was maintained upon long-term charge/ discharge cycling, indicating its significant stability.
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INTRODUCTION Nanoarchitectured porous materials fabricated through template synthetic approaches, in particular metal oxides, are demonstrating significant potential in multitudinous fields including energy-related applications.1 Titanium dioxide (TiO2)-based functional materials have been extensively investigated for various functions because of their outstanding chemicophysical properties.2 It has been recently demonstrated that titania can be employed as a promising anode material for rechargeable lithium-ion batteries (LIBs) because of its high capacity retention, low self-discharge rate, excellent cycling stability, and high safety.3 The diffusion path and the electronic conductivity are the two main factors influencing the rate performances of the anodes; hence, the reduction of the size of the titania matter is an effective choice to diminish the diffusion length for both electronic and ionic transport, resulting in enhanced lithium-ion intercalation activity and cycling performance.4 Various nanostructured titania matter have thus been developed, aiming at decreasing the lithium-ion diffusion path length and increasing the electrode−electrolyte contact area, such as anatase titania nanospheres,5 nanocages,6 nanosheets,7 nanorods,8 nanotubes,9 and nanoparticles,10 which achieved improved electrochemical activities. In particular, nanotubular structured titania shows significantly high capacity and stable reversible capacity retention because of the short diffusion path © XXXX American Chemical Society
of lithium ions and sufficient surface area for the electrode/ electrolyte contact.11 Moreover, the nanotubular structure plays an essential role in accommodating the expansion and contraction of the titania nanotubes during the lithium-ion insertion/extraction processes.12 Titania is known to exist in different crystallographic phases, for example, anatase, rutile, brookite, and TiO2 (B), where anatase-phase titania (A-titania) is thought to be favorable for fast lithium-ion insertion/ extraction without obvious volume expansion because of its tetragonal structure.13 On the other hand, the heterogeneous anatase/rutile bi-phase titania composite possesses unique properties because of the synergistic effect between the two phases.14 It was reported that anatase/rutile bi-phase titania nanoparticles showed better cyclic performance at both low and high current rates as compared with the anatase phase one when employed as an anode material for LIBs.15 Nevertheless, the low electrical conductivity (ca. 10−13 S/cm)16 of titania limits its practical application in lithium storage because a high overpotential is generally used during the cycling process,17 Special Issue: Tribute to Toyoki Kunitake, Pioneer in Molecular Assembly Received: April 23, 2016 Revised: June 11, 2016
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DOI: 10.1021/acs.langmuir.6b01556 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
silver mirror reaction process. In a typical procedure, 2.50 mL of 5.0 wt % fresh NaOH aqueous solution was added into 50.0 mL of 3 wt % AgNO3 solution, forming a dark-brown precipitate of AgOH, and then, concentrated NH3·H2O was added into the mixture dropwise until the precipitate redissolved. Subsequently, 15.0 mL of 10 wt % D-glucose solution was added into the prepared Tollen’s reagent with swift and careful agitation to ensure mixing. The solution was then immediately poured into a Petri dish where 30.0 mg of the A−R-titania composite was placed. After immersion for about 5 min, the reacted solution was poured out and the obtained Ag-NP/A−R-titania composite was thoroughly washed with water and was dried in a vacuum at 30 °C for 24 h. The details of the structural and spectral characterization of the samples are provided in the Supporting Information. Electrochemical Measurements. For electrochemical performance tests, the working electrodes were prepared by mixing 70 wt % active material, 20 wt % conductive additive (acetylene black), and 10 wt % poly(vinyl difluoride) (PVDF), followed by coating the mixture on the nickel foam. The electrolyte used was 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 v/v). Lithium metal foil was used as the counter electrode, and a Celgard 2300 film was used as the separator. The standard CR2025-type coin cells were assembled in an argon-filled glovebox (O2 and H2O levels
