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A Flexible Lithium-Ion Fiber Battery by Regularly Stacking Two-Dimensional Titanium Oxide Nanosheets Hybridized with Reduced Graphene Oxide Tatsumasa Hoshide, Yuanchuan Zheng, Junyu Hou, Zhiqiang Wang, Qingwen Li, Zhigang Zhao, Renzhi Ma, Takayoshi Sasaki, and Fengxia Geng Nano Lett., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017
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A Flexible Lithium-Ion Fiber Battery by Regularly Stacking Two-Dimensional Titanium Oxide Nanosheets Hybridized with Reduced Graphene Oxide Tatsumasa Hoshide,1,2 Yuanchuan Zheng,1 Junyu Hou,1 Zhiqiang Wang,1 Qingwen Li,3 Zhigang Zhao,3,* Renzhi Ma,2 Takayoshi Sasaki,2 Fengxia Geng1,* 1
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China 2
International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
3
Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industry Park, Suzhou 215123, China E-mail:
[email protected];
[email protected] Abstract: Increasing interest has recently been devoted to developing small, rapid, and portable electronic devices; thus, it is becoming critically important to provide matching light and flexible energy-storage systems to power them. To this end, compared with the inevitable drawbacks of being bulky, heavy, and rigid for traditional planar sandwiched structures, linear fiber-shaped lithium-ion batteries (LIB) have become increasingly important owing to their combined superiorities of miniaturization, adaptability, and weavability, the progress of which is heavily dependent on developing new fiber-shaped electrodes. Here, we report a novel fiber battery electrode based on the most widely used LIB material, titanium oxide, which is processed into two-dimensional nanosheets and assembled into a macroscopic fiber by a scalable wet-spinning process. The titania sheets are regularly stacked and conformally hybridized in situ with reduced graphene oxide (rGO), thereby serving as efficient current collectors, which endows the novel fiber electrode with excellent integrated mechanical properties combined with superior battery performances in terms of linear densities, rate capabilities, and cyclic behaviors. The present study clearly demonstrates a new material-design paradigm toward novel fiber electrodes by assembling metal oxide nanosheets into an ordered macroscopic structure, which would represent the most promising solution to advanced flexible energy storage systems.
KEYWORDS: Two-dimensional sheets, titanium oxide, assembly, flexible devices, fiber battery 1 ACS Paragon Plus Environment
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Introduction Portable and wearable electronic devices with high flexibility have achieved increasing popularity and a booming market in recent years for their wide application possibilities, a few notable examples of which include Google Glass, Apple Watch, Nike fit, and the Cute-circuit Galaxy dress.1-4 The most promising technology for meeting the energy demands of these advanced electronic devices is the lithium-ion batteries (LIBs) because of their high energy density, the absence of a memory effect, a long cyclic life, and a high working voltage, among others.5-7 Conventional LIBs, however, are typically heavy, bulky, and rigid, causing serious incompatibility with wearable electronic devices, and therefore there is a high demand for novel battery systems that possess advantageous characteristics of light weight, miniaturization to tiny volumes, and high flexibility, which can deliver power to miniaturized electronic devices in deformed states and can even be woven into textiles.8 Developing LIBs in a fiber shape could be one practical solution to the requirement for bendable structures and adaptability to devices with various shapes, including miniaturized electronic devices, e-textiles, and implantable medical devices. To date, intensive efforts have been made to fabricate fiber battery electrodes of many different materials. Most of these efforts, however, have been directed toward incorporating the active materials, for example, silicon (Si),9 manganese oxide (MnO2),10 titanium oxide (TiO2),11 lithium titanate (Li4Ti5O12), or lithium magnate (LiMn2O4),12,13 etc., on a linear substrate, including metal wires, carbon yarns, or aligned carbon nanotube (CNT)-based fibers as well as conducting material-coated plastic/rubber fibers. These substrates serve as linear supporting backbones as well as charge collectors for the deposited active materials. Unfortunately, to date almost all the fiber electrodes thus formed have suffered from two serious problems. One is that the loading content of active materials is extremely low due to the constraint of limited surfaces, resulting in a practical issue that the fiber electrode cannot provide a large energy storage capacity; the second is that the contact between active materials and substrate is typically weak due to a lack of chemical bonding between the current collector and the coated active materials, which will additionally degrade the energy storage capability. As fiber electrodes play a pivotal role in upgrading both the electrochemical performance and wearable capability of fiber batteries, a revolution in electrode fabrication is urgently desired,6,14 especially in directly employing electrochemically active materials that 2 ACS Paragon Plus Environment
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possess intrinsic energy storage capability as fiber electrodes,15 by which the above-mentioned stubborn hurdles could be overcome. Within the field of battery research and development, titanium-based anode materials have recently attracted widespread attention.16 These materials are abundant and typically operate at a potential above 0.8 V vs Li+/Li, offering a significant safety advantage over conventional graphite anodes. Two-dimensional (2D) sheets of titanium oxide, although largely overlooked, are an unparalleled platform for electrochemical reaction use, which are planar molecular sheets composing of edge-linked TiO6 octahedra in a lepidocrocite-type structure.17 Their high percentage of surface atoms and ultrathin thickness would be helpful to speed up the chemical reactions taking place in the battery by increasing the number of reactive metal centers and shortening the diffusion length of lithium/electron inside particles. In this work, we report a novel fiber electrode based on titania sheets molecularly hybridized with reduced graphene oxide designed for flexible, efficient energy storage with superior cyclic stability, which has promising applicability in widely used wearable electronic products. The constructed prototype battery was flexible and tolerant to various deformations. To the best of our knowledge, this is the first report eliminating the linear substrate backbone and directly utilizing active materials to make high-performance fiber-shaped LIBs. This work provides important hints for devising optimized structures in flexible fiber electrodes and lays a foundation for various emerging applications in wearable electronic devices.
Results and discussion The hybrid fiber was achieved by a scalable approach of wet-spinning the mixture dispersion of two-dimensional titanium oxide sheets and graphene oxide (GO) followed by a reduction treatment in hydroiodic acid (HI), as schematically illustrated in Figure 1. The elementary titania sheets and GO were prepared by previously reported protocols.18,19 The dispersion of GO was top-down synthesized from natural graphite according to an improved Hummer’s method. The GO sheets thus obtained were heavily oxygenated, bearing abundant pendant oxygen-containing functionalities, for instance, carboxylic acid (C(=O)OH), phenolic hydroxyl (-OH), and epoxy groups on sp3-hybridized carbon on the basal plane. The O:C atomic ratio was determined to be 0.79 by elemental analysis. Protonation of the surface pendant groups caused the sheets to naturally exhibit negative charges. Because of their excellent hydrophilicity 3 ACS Paragon Plus Environment
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and the electrostatic repulsion between the negative charges on the GO surface, the resulting colloidal suspension was stable indefinitely, appearing homogenously brown. The tapping-mode atomic force microscopic (AFM) characterization shown in Figure 2a demonstrated that the average lateral dimensions for graphene oxide were approximately 4-6 µm. The thickness was typically 1.1±0.2 nm, greater than the 0.34 nm of pristine graphene sheets, which was due to the displacement of sp3 carbon atoms above and below the original graphene plane and the presence of covalently bound oxygen atoms.20 The monolayer titania sheets possessed structural similarity to graphene. The sheets were built up of edge-linking TiO6 octahedra in a lepidocrocite-type structure, featuring a typical 2D geometrical anisotropy with infinite planar length but an ultrathin molecular thickness of approximately 0.75 nm.21,22 A representative AFM image given in Figure 2b shows that the yielded titania sheets had an average height of 1.1 nm, and the deviation from crystallographic value should be explained by the surface absorption of electrolyte ions or solvent molecules. It should be mentioned that the delaminated titania sheets contained Ti vacancies and had a composition slightly deviating from the stoichiometry of TiO2, which granted the sheets an overall negative charge with a nominal formula of Ti0.87O20.52-. The surface charges enabled a stable aqueous colloidal dispersion by preventing aggregation via Coulombic repulsion (inset in Figure 2b). Since both titania sheets and graphene oxide were two-dimensional in nature and could be considered as disc-like nano-objects with infinitely large aspect ratio, their colloids at sufficiently high concentrations could exhibit unusual properties of stable liquid crystallinity. The insets in Figures 2a and b are digital images for the obtained suspensions, manifesting an obvious appearance of shiny textures, evidence of the presence of quasi-long-range molecular order and formation of a nematic liquid crystalline (LC) mesophase. Under an optical microscope with crossed polarizers and a wave retardation of 530 nm, marble patterns with various interference colors caused by the birefringence of the sample were observed, which is characteristic of LC phases (Supporting Information, S1). Homogenous mixing of titania sheets and GO with excellent dispersibility was realized due to the highly negatively charged surface of both types of sheets, for which there was no appreciable precipitate even after the mixture colloid was allowed to stand still for weeks. In addition, negligible degradation in the LC behavior was noticed, and the nanosheet mixture colloid exhibited LC ordering similar to single-component nanosheet 4 ACS Paragon Plus Environment
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colloids, as shown by the vivid birefringent Schlieren textures observed by polarized optical microscopy (Figure 2c). Analogously to the fabrication of titania fibers with ordered sheet-stacking structure by simply injecting a liquid crystalline suspension of titania sheets into a coagulation bath of chitosan,23 continuous titania/GO fiber could be obtained from the mixture LC colloid via an industrially viable wet-spinning technique. The 2D titania/GO sheets, typically bearing negative charges, acted as rigid platelets, while the long-chain polymer of chitosan, once protonated in acidic media, could serve as an elastic glue to provide electrostatic binding forces between the sheets, maintaining structural integrity. The so-spun wet fiber morphologically resembled a flat ribbon exhibiting a typical width of ∼1200 µm and a thickness of 60 µm (Supporting Information, S2). The unusual belt-like morphology should be due to the ordered sheet-to-sheet stacking along the perpendicular direction, inherited from the intrinsic order in the LC phase. After drying in ambient atmosphere, the fiber was treated in hydroiodic (HI) acid, from which the brown titania/GO hybrid fiber turned black while maintaining its overall structural integrity (Figures 2d and e). The change in color was suggestive of an extension of π- π conjugation according to Huckel’s rule, implying the successful reduction of GO into reduced graphene oxide (rGO).24 As titania played the major role of energy storage while rGO was expected to serve only as charge collector, the GO content in the titania/GO hybrid was kept to a minimum of 20 wt%, and the rGO in titania/rGO following chemical reduction could be accordingly calculated to be approximately 10.8 wt%. An electron diffraction taken on a fiber slice cut at a tilting angle showed clear spots corresponding to in-plane diffractions of titania, further confirming that titania sheets were the major ingredient in the hybrid (Supporting Information, S3). The linear density of the as-prepared hybrid fiber electrode was in a narrow distribution within 0.12–0.19 mg cm−1 (Supporting Information, Figure S4). Among the many documented reducing agents, HI was selected because the product with the highest electrical conductivity to date was produced by HI.25 It should also be noted that HI could work to fragment and dissolve chitosan in the gallery26 in addition to eliminating the oxygenated groups on the graphene basal plane, as schematically illustrated in Figure 1, yielding a hybrid fiber free of any surfactants or polymers. The oxygen-containing groups on both the graphene plane and chitosan that are sandwiched between the layers were etched out; accordingly, the characteristic bands for carboxylic acid, phenol, hydroxyl, etc., all disappeared in the infrared 5 ACS Paragon Plus Environment
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(IR) spectrum (Supporting Information, S5). The removal of chitosan could also be witnessed by the shortened step for chitosan combustion in thermal gravity analysis (Supporting Information, S6). As chitosan is a polymer with very low electrical conductivity, especially in a dry state, the decrease in chitosan content to a nearly negligible level was helpful to improve the electrical features of the hybrid electrode. With such treatment, the electrical conductivity was increased by six orders of magnitude from 5.0 h, and the full battery woven into a textile.
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