Research Article pubs.acs.org/journal/ascecg
Correlating Titania Nanostructured Morphologies with Performance as Anode Materials for Lithium-Ion Batteries Crystal S. Lewis,† Yue Ru Li,† Lei Wang,† Jing Li,‡ Eric A. Stach,§ Kenneth J. Takeuchi,†,‡ Amy C. Marschilok,†,‡ Esther S. Takeuchi,†,‡,∥ and Stanislaus S. Wong*,†,⊥ †
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, United States Department of Materials Sciences and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275, United States § Center for Functional Nanomaterials, Building 735, Brookhaven National Laboratory, Upton, New York 11973, United States ∥ Energy Sciences Directorate, Interdisciplinary Sciences Building, Building 734, Brookhaven National Laboratory, Upton, New York 11973, United States ⊥ Condensed Matter Physics and Materials Sciences Division, Building 480, Brookhaven National Laboratory, Upton, New York 11973, United States ‡
S Supporting Information *
ABSTRACT: Titanium oxide is a ubiquitous and commonly used material in the environment. Herein, we have systematically probed the use of various hydrothermally derived titania (TiO2) architectures including zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanowires, and three-dimensional (3D) urchin-like motifs as anode materials for lithiumion batteries. The structure and morphology of these nanomaterials were characterized using electron microscopy. The surface areas of these materials were quantitatively analyzed through Brunauer−Emmett−Teller (BET) adsorption measurements and were found to be relatively similar for both 1D and 3D samples with a slightly higher surface area associated with the 0D nanoparticles. Hence, to normalize for the surface area effect, readily available 0D commercial nanoparticles (Degussa P25), which possessed a similar surface area to that of as-prepared 1D and 3D materials, were also analyzed. Electrochemical analysis revealed a superior performance of hydrothermally derived 3D urchin-like motifs as compared with both as-prepared 0D and 1D samples as well as commercial Degussa P25. Our studies suggest the greater overall importance of morphology as opposed to surface area in dictating the efficiency of the Li ion diffusion process. Specifically, the 3D urchins yielded consistent rate capabilities, delivering 214, 167, 120, 99, and 52 mAh/g under corresponding discharge rates of 0.1, 1, 10, 20, and 50 C, respectively. Moreover, these 3D motifs gave rise to a stable cycling performance, exhibiting a capacity retention of ∼90% in cycles 1−100 under a discharge rate of 1 C. Furthermore, the rate capability and cycling performance of our 3D hierarchical motifs were (i) comparable to those of anatase TiO2/TiO2-(B) hybrid structures even with little if any electrochemically promising bronze (B) phase herein and (ii) clearly enhanced as compared with previous results using similar anatase 3D microspheres. KEYWORDS: Titanium dioxide, Metal oxide, Morphology dependence, Electrochemical performance, Anode materials, Lithium-ion batteries
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(0.1 V versus Li/Li+). While providing for high energy density due to this low potential, the low lithiation potential can nonetheless also be problematic, particularly under high rate charge conditions, where lithium metal deposition on the anode can occur and thereby result in an internal short circuit and thermal runaway.4 Another drawback associated with these carbon-based materials involves operational volume expansion and contraction, which can lead to a loss of electrical contact
INTRODUCTION Lithium-ion batteries (LIBs) have gained widespread interest for use in applications as diverse as electric vehicles and power storage for electronics as a result of their high rate capability, energy density, and long cycle life.1,2 Nevertheless, electrochemical performance is inherently dependent upon characteristics such as electrode properties and the selection of electrode materials, specifically for the anode and the cathode. Currently, conventional LIBs use carbon-based anode materials, associated with advantages of low cost, high abundance, and outstanding kinetics.1,3 For example, graphite is currently very widely used as an anode material due in part to its low lithiation potential © XXXX American Chemical Society
Received: April 13, 2016 Revised: August 30, 2016
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DOI: 10.1021/acssuschemeng.6b00763 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
causes a lattice mismatch which can lead to strains between the coexisting phases and thereby engender decreased Li ion diffusion.28,29 By comparison with rutile, the anatase phase of TiO2 appears to be more electrochemically active due to its 3D network tetragonal crystal structure formed by the stacking of 1D zigzag chains consisting of disordered edge-sharing TiO2 octahedra.21 This geometry can potentially accommodate 0.5 Li ions per TiO2 unit and correspondingly provides for a voltage profile at 1.7 V versus Li+/Li; the higher total lithium insertion for anatase likely arises because of its 3D Li ion diffusion pathways.30−32 Similar in behavior to the rutile analogue, decreasing the particle size of anatase TiO2 to the nanoscale regime notably enhances the electrochemical performance with a reversible capacity as large as ∼200 mAh/g.1 Yet another pristine polymorph has gained attention, since it was first discovered in 2005 as an electrochemically active material, namely TiO2-bronze (B).33 This material is a metastable polymorph formed by the dehydration of layered tunnel-structure hydrogen titanate.34 Its crystalline phase maintains a monoclinic structure running along the [010] orientation and has been shown to favor incorporation, intercalation, and diffusion of Li ions, both theoretically and experimentally. As such, the “bronze” phase denotes a potentially excellent host for lithium ion insertion and deinsertion due to its open channel structure.35 Moreover, its theoretical capacity of ∼337 mAh/g should once again be tunable by decreasing the sizes of these materials to the nanoscale regime.1,4,36,37 Therefore, our studies are significant, because we are among the first to pursue a systematic correlation between (i) the morphologies of crystalline 0D, 1D, and 3D anatase TiO2 materials, by comparison with commercial 0D TiO2, which incorporates a mixture of anatase and rutile phases, with (ii) electrochemical activity. We have found that the superior electrochemical performance of 3D TiO2 illustrates the highest rate capabilities measured of all of the samples we have tested, despite their small surface area. By using a similar synthetic methodology for all of these materials, direct correlations between structure and performance have been drawn. Many studies have analyzed and compared electrochemical performance as a function of size. However, few groups have performed morphology-dependent studies for LIBs. In one such work, Yu et al. evaluated the electrochemical performance of ordered arrangements of organized, mesoporous, rod-shaped carbons (OMC) of varying lengths. From these studies, it was determined that the shortest lengths of rods (i.e., ∼500−800 nm) noticeably outperformed the longer lengths (i.e., ∼1.2 and 1.9 μm) with the highest reversible discharge capacity measured at current densities of 100 to 1000 mA/g.38 The enhanced performance of the shorter-length carbon rods observed was attributed to the increased active surface area created by the corresponding rise in both mesoporous volume and pore size.38 The closest experiments to date in terms of intellectual scope and scientific objectives, relating to a similar morphologydependent study for electrochemical analysis, are associated with the work of Leng et al., who compared the electrochemical behavior of (i) 0D nanoparticles (raw material), (ii) as prepared, surfactant-assisted 1D nanotubes, and, ultimately, (iii) 2D nanosheets of anatase TiO2. Although these nanosheets were not entirely crystalline, they demonstrated the highest capacity of ∼82.8 mAh/g at the 630th cycle at a current density of 2000 mA/g. 39 By analogy with studies on CuO
between these materials with their adjoining current collectors, thereby decreasing the battery’s capacity.3,5 As a result, metal oxides have become an attractive option for battery electrode materials due to their low cost, wide abundance, and relative environmental compatibility. Furthermore, as compared with commercialized graphitic anodes, these metal oxides demonstrate advantages such as high capacity and improved stability.6 Specifically, tin dioxide (SnO2) and iron oxide (Fe3O4) offer high theoretical lithium storage capacities of ∼790 mAh/g and ∼926 mAh/g, respectively.7−10 However, one significant drawback of these particular metal oxides is their large volume expansion and contraction upon cycling, which leads to a steep decline in capacity.11 Another metal oxide anode material, which possesses distinctive electrochemical behaviors from either SnO2 or Fe3O4, is titania (TiO2). TiO2 is a stable, nontoxic, inexpensive, and abundant material.12,13 It has been a familiar and commonly used material for decades, because of its broad applications in the context of solar cells, catalysis, and environmental applications,14−16 and more specifically for its use as a component of paint, sunscreens, perfumes, food coloring, and photocatalysts.17,18 Generally, in the context of this study, TiO2 is a promising candidate as a potential anode material due to its ample availability, low toxicity, and overall safety of handling.1 Especially when compared with graphite, Fe3O4, and SnO2, TiO2 maintains a higher lithiation potential (i.e., ∼1.6 V versus Li/Li+) and provides for low volume change during Li insertion/deinsertion (∼3−4%), a finding which is conducive to enhancing cycle life and which reinforces its promise as a candidate for an efficient anode battery material.19,20 Moreover, decreasing TiO2 to the nanoscale should increase its overall surface area, thereby reducing the lithium ion diffusion length and ultimately contributing to an enhanced lithium ion flux at the interface between active material and the electrolyte.13,21,22 The redox reaction has been studied by both X-ray photoelectron spectroscopy experiments23,24 and theoretical calculations.25,26 Overall, the lithium reaction associated with the TiO2 polymorphs can be expressed via eq 1 as x Li+ + TiO2 + x e− ↔ LixTiO2
(1)
Though different polymorphs of TiO2 (i.e., rutile, brookite, and anatase) yield comparatively dissimilar electrochemical behaviors, it should be possible to finely tune TiO2 as a function of crystalline structure, size, and morphology.1 Depending upon the inherent TiO2 crystal structure, increased Li ion diffusion may be potentially achievable, thereby improving the measured capacity and cycling performance.1 Such a strategy should provide for a facile method for understanding the precise role of each parameter in terms of fostering enhanced electrochemical performance. Moreover, the crystal structure of each polymorph maintains its own specific advantages and disadvantages. For example, rutile TiO2 is known to be a very stable phase, though in its bulk crystalline form, it can only accommodate for negligible amounts of Li (
