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Black Anatase Titania with Ultrafast Sodium-Storage Performances Stimulated by Oxygen Vacancies Jun Chen,† Zhiying Ding,† Chao Wang,‡ Hongshuai Hou,† Yan Zhang,† Chiwei Wang,§ Guoqiang Zou,† and Xiaobo Ji*,† †
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China § Tianjin EV Energies Company Limited, Tianjin 300380, China ‡
S Supporting Information *
ABSTRACT: Nanostructured black anatase titania with oxygen vacancies (OVs) is efficiently obtained and employed as an anode in sodium-ion batteries (SIBs) for the first time. The incorporation of OVs into TiO2 is demonstrated to render considerably enhanced-rate performances, higher initial capacities, and an accelerated electrochemical activation process during cycling, derived from the boosted intrinsic electric conductivity and improved kinetics of Na uptake. Bestowed with the integrated merits of OVs and shortened Na ion diffusion length in the nanostructure, black titania delivers a reversible specific capacity of 207.6 mAh g−1 at 0.2 C, retains 99.1% over 500 cycles at 1 C stably, and still maintains 91.2 mAh g−1 even at the high rate of 20 C. Density functional theory (DFT) calculations suggest that the lower sodiation energy barrier of anatase with OVs enables a more favorable Na intercalation into black anatase. Thus, it is of great significance to introduce OVs into TiO2 to stimulate ultrafast and durable sodium-storage properties, which also offers a potential strategy to project more superior electrodes, utilizing internal defects. KEYWORDS: black anatase, oxygen vacancies, anode, sodium-ion batteries, rate performances
1. INTRODUCTION On account of the natural abundance of sodium reserves, lower production cost and comparable electrochemical performances compared to lithium-ion batteries (LIBs), current years have witnessed an exceedingly blooming trend in the research of sodium-ion batteries (SIBs), which are hopeful substitutes for LIBs in certain areas, especially for large-scale applications.1−3 Accordingly, exploiting suitable electrode materials for sodium storages becomes highly crucial. In regard to the anode, hard carbonaceous materials have been proven to possess reasonable sodium storage capabilities, whose inherent physical properties and random structure, however, could seriously hamper Na insertion at high rates. Alternatively, the alloy-type-based materials such as Sn,4 Sb5, and P6 together with the conversion-type-based anodes like sulfides7 have been demonstrated to exhibit high initial capacity, whereas suffer from poor cyclability mostly caused by the huge volume change. As a type of intercalation-based anode materials, TiO2 of various polymorphs (including anatase,8−10 rutile,11−13 and TiO2 (B)14,15 as well as amorphous titania16,17) have recently aroused increasing scientific enthusiasm for sodium storage owing to their superior qualities of being nonpoisononous, having natural abundance and low cost, and especially having © XXXX American Chemical Society
an excellent cycling stability, conferring with their plentiful investigations in LIBs systems.18,19 Among polyphase titania, anatase has been considered to be more electrochemically active for SIBs than rutile or amorphous titania; the suitable diffusion pathways of two dimensions and more plentiful accommodation sites for Na+ intercalation could be provided by the anatase crystal structure,9 which is also supported by the theoretical study revealing a relatively low activation barrier of sodium insertion into the anatase lattice.20−22 However, the reversible Na+ intercalation into anatase is still kinetically hindered, especially at high rates,23 which could be reflected from the activation phenomenon in the discharge−charge process.24,25 Moreover, the quite poor electronic conductivity (∼10−13 S cm−1) makes TiO2 a semiconductor derived from the huge band gap of ∼3.2 eV,26 which severely leads to its inferior rate capabilities. Targeting these main inherent shortcomings, relevant research efforts have been devoted. One typical solution is to improve the diffusion of Na ions by means of nanostructure Received: January 28, 2016 Accepted: March 23, 2016
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DOI: 10.1021/acsami.6b01183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces construction,25,27−29 and the other is aimed at the amelioration of electron transfer through conductive carbon supporting30−33 and doping with alien elements,33−35 among which the doping one is considerably appealing because the electronic conductivity of TiO2 itself could be advanced; in addition, the structure defects would hopefully provide more open channels for ion transport, as has been shown in LIBs.36 Nevertheless, it should be noted that the thermal stability might be declined by heterogeneous defects of impurity.37 Another type of homogeneous defects, oxygen vacancies, which can alter the electronic structures as well as the energetics of ions and electrons transportation in metal oxides without sacrificing crystal stability, have been proven to effectively enable better MoO3−x sodium-ion battery performances, as shown by Xu et al. very recently.38 Triggered by that, the oxygen vacancies induced Ti3+ self-doping, which usually endows TiO2 with a black color, should be attached enough attention to enhance the sodium storage performances of anatase TiO2, while to the best of our knowledge, relevant research is currently absent. As reported anteriorly, the band gap of black titania could be remarkably narrowed to ∼1.5 eV and therefore boost the intrinsic electronic conductivity greatly,39−41 which is promising for the improvement of durable rate capabilities referring to our previous work on Ti3+ self-doped titania for LIBs.42 Furthermore, the introduction of oxygen vacancies can slightly tailor the anatase TiO2 host structure, which might reasonably generate certain effects on Na-intercalation properties. Nonetheless, most of the methods to produce oxygen vacancies require harsh reducing conditions like annealing in H243−45 and CO46 or a vacuum,47 which might limit its practicality. Also considering that reducing the crystalline size of TiO2 could lead to higher reversible sodiation capacity,30 it would be significant and desirable to design a nanosized black anatase titania utilizing an easily feasible approach for application in high-rate SIBs and to explore the effects of oxygen vacancies on its sodium storage properties. Herein, we investigate, for the first time, the enhanced sodium storage performances of black anatase titania, which is constructed as a nanostructure via an efficient reducing route to integrate the benefits of improved electronic conductivity and facilitated Na-ion transport. This black anatase with oxygen vacancies in both bulk structure and surface presents remarkable high rate capabilities and long cycling life when utilized as anode for SIBs. The considerably positive effects of oxygen vacancies on the properties of sodium storage in black anatase structure are deeply probed by combining with detailed experiments and density functional theory (DFT) calculations. It is believed that this study would not only provide insights into the enhanced sodium storage of oxygen-deficient black titania but also be highly informative for structure designing and the optimization of electrode materials for energy storage.
Ti(OH)2C2O4 (HTO) was obtained by the reaction between TBOT and OA.48 To prepare black titania, we first calcined the precursor HTO in air at 350 °C for 2 h, followed by being ground together uniformly with NaBH4 (2:1 in mass) as reductant and then heated in Ar atmosphere at 350 °C for 2 h. The resulting completely black sample was washed by 0.1 mol L−1 HCl and deionized water repeatedly and collected by centrifuge. After being dried at 70 °C in a vacuum oven for 12 h, the black TiO2 powder was obtained and named B-TO for brevity. For comparison, the nanocrystalline white TiO2 was prepared via calcining HTO in air at 350 °C for 4 h and named W-TO. 2.2. Materials Characterization. Powder X-ray diffraction (XRD) was carried out utilizing a Bruker D8 diffractometer with monochromatic Cu Kα radiation (the wavelength of 1.5406 Å) with a scan rate of 2°/min. Scanning electron microscopy (SEM, using FEI Quanta 200) together with transmission electron microscopy (TEM, using JEM-2100F) were employed in the characterization of asprepared samples morphology. Raman spectra were tested using Renishaw InVia with wire 4.2. Brunauere-Emmette-Teller (BET, BELSORP-Mini II) surface area was obtained from the N2 adsorption/ desorption isotherm. X-ray photoelectron spectroscopy (XPS, using KAlpha 1063) was performed to detect the chemical composition of the samples’ surfaces. The DC electrical conductivities were tested on discshaped samples by direct four-point probe conductivity measurements (SDY-40, Guangdong, China). Electron spin resonance (ESR, using JES-FA200) measurements were conducted by utilizing the micro frequency of 9057.904 MHz for the white TiO2 sample and 9059.228 MHz for the black TiO2 sample. 2.3. Electrochemical Tests. The assembly of sodium half-cells was conducted in a MBraun glovebox full of argon atmosphere to investigate the properties of as-prepared samples in sodium storage. The powder of obtained active material (70%) was mixed with 15% super P as the conductive agent and 15% carboxymethyl cellulose as the binder in deionized water, and the formed homogeneous slurry was painted on a copper foil, which was subsequently dried for 12 h at 100 °C in a vacuum oven. The copper foil loaded by active materials were pressed and cut as the electrode slices. Metallic Na was employed as the counter electrode, and Celgard 2400 was used as the separator. The electrolyte was composed of 1 M NaClO4 dissolved in propylene carbonate and added by 5% fluoroethylene carbonate (FEC). Cyclic voltammetric (CV) were measured on Solartron Analytical to examine the electrochemical reaction using the assembled cell in the voltage range of 3−0.01 V (versus Na/Na+). Galvanostatic charge−discharge measurements were carried out on the Arbin battery cycler (BT2000) in 3−0.01 V (versus Na/Na+). The electrochemical impedance measurements (EIS) were performed on Solartron Analytical at an AC voltage of 5 mV amplitude in the frequency range of 100 kHz− 0.01 Hz after 10 discharge−charge cycles. All electrochemical tests were conducted at room temperature. 2.4. Calculation Methods. Density functional theory (DFT) calculations were carried out on the basis of generalized gradient approximation (GGA) exchange-correlation energy function utilizing ultrasoft pseudopotential (USPP) formalism,49 and the cutoff of the kinetic energy was set to 300 eV for all calculations in reciprocal space. The k-points were sampled on Monkhorst−Pack grid of 6 × 6 × 7 for the unit cell, and the maximum self-consistent field convergence tolerance was
