Article pubs.acs.org/IECR
Scalable Template-Free Synthesis of Na2Ti3O7/Na2Ti6O13 Nanorods with Composition Tunable for Synergistic Performance in SodiumIon Batteries Ching-Kit Ho, Chi-Ying Vanessa Li,* and Kwong-Yu Chan Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong S Supporting Information *
ABSTRACT: Solid-state reactions are a simple and scalable synthetic method, but they lack controlled nanostructures at present. Here, we report an energy-efficient solid-state synthesis, via the addition of carbon, to mass-produce uniform, singlecrystalline, one-dimensional metal oxide nanorods with tunable composition according to performance demands. The carbon added in the solid-state reaction provides an alternative lowtemperature route for the metal oxide formation due to the extra local heat generation and CO2/CO release from carbon oxidation. To demonstrate the methodology, a series of singlecrystalline Na2Ti3O7/Na2Ti6O13 nanorods with tunable composition are synthesized and applied in sodium-ion batteries. The local heat generated from carbon allows formation of Na2Ti3O7 at 450 °C, a reaction temperature much lower than that of conventional solid-state methods (750−1000 °C), and Na2CO3 is regenerated to be recycled in the synthesis. The high theoretical capacity of Na2Ti3O7 and low volume expansion of Na2Ti6O13 upon charge−discharge are synergistically exploited to achieve high electrochemical performance and stability.
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INTRODUCTION Sodium-ion batteries (NIBs) are regarded as potential candidates to replace lithium-ion batteries (LIBs) in largescale electric energy storage applications such as stationary storage for renewable energy production in the near future.1 To improve battery performance, nanostructuring one-dimensional (1D) electrode materials is an effective strategy. Onedimensional nanostructured electrode materials result in enhanced electrochemical performance due to shorter ion and electron pathways, greater mechanical flexibility, and higher stress tolerance.2−5 However, current research and development on the fabrication of high-performance nanostructured electrode materials mainly relies on unscalable, high energyconsuming, and tedious synthetic procedures. The technical gap in the manufacturing process between research and industry hinders the material commercialization, which in turn limits the practicability and utilization of NIBs. Na2Ti3O7, previously used as photocatalyst and humidity sensor,6 has recently been applied as anode material in sodiumion batteries.7−12 Irregular Na2Ti3O7 particles synthesized by a solid-state method can reach a discharge capacity of 177 mA h g−1, close to the theoretical value (178 mAh/g), but only at low discharge rate of 0.1C. Another member of sodium titanates, Na2Ti6O13, demonstrates a minimum unit cell volume expansion of 1% upon Na+ insertion−extraction, rendering superior cycling performance but a much lower theoretical capacity than that of Na2Ti3O7, at 50 mA h g−1. A mixture of © XXXX American Chemical Society
Na2Ti6O13/carbon black is reported to retain 78% of its discharge capacity over 3000 cycles at a high rate of 20C.13 It would be desirable to synthesize nanostructures with complementary advantages of the two sodium titanates. Sodium titanates deliver relatively low working potential (vs Na+/Na), which can mitigate the intrinsic drawback of low cell voltage of NIBs relative to that of LIBs. Conventional solid-state synthesis is simple and scalable and is generally used for the large-scale production of the materials. A high annealing temperature is required for solid-state synthesis to produce materials with sufficient yield and crystallinity; this inevitably results in production of micrometer-sized irregular particles due to sintering.7,8,10 Therefore, the use of conventional solid-state synthesis for large-scale production of nanostructured materials remains a great challenge. In this paper, we extend a solid-state synthesis, via the addition of low-cost carbon, to achieve mass-production of 1D single-crystalline nanorods with tunable composition. To demonstrate the methodology, a series of uniform, singlecrystalline Na2Ti3O7/Na2Ti6O13 1D nanorods are synthesized and applied in sodium-ion batteries. The role of carbon in the synthesis is discussed in detail. The electrochemical performReceived: May 18, 2016 Revised: August 23, 2016 Accepted: September 2, 2016
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DOI: 10.1021/acs.iecr.6b01867 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
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RESULTS AND DISCUSSION Structural Characterization by XRD. Successful synthesis of Na2Ti3O7/Na2Ti6O13 nanorods with tuned composition by carbon-assisted solid-state method is confirmed by the XRD and their XRD patterns shown in Figure 1. The sample
ance of the synthesized sodium titanated nanorods is also related to the synthesis to understand the complementary advantages of Na2Ti3O7/Na2Ti6O13.
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EXPERIMENTAL SECTION Material Synthesis. Initially, anatase TiO2 and anhydrous Na2CO3, corresponding to 10% excess of Na2CO3 in molar stoichiometry (Na2CO3:TiO2 = 1:3) are thoroughly mixed and ground in an agate mortar for 15 min. The anatase TiO2 (product number 637254) is in the form of irregular particles, as shown in Figure S1. Then, Vulcan carbon is added at different weight percent for different product composition, and the mixture is dispersed and stirred in ethanol to form a slurry and dried at 40 °C. Finally, the mixture is heated in a tube furnace at 800 °C for 20 h in ambient air and cooled to room temperature. The as-synthesized product is denoted as NTO-X, where X indicates the weight percentage of Vulcan carbon added in the synthesis. The weight percent of Vulcan carbon is calculated based on Cm/Nam + Tim + Cm, where Nam, Tim and Cm represent the mass of Na2CO3, anatase TiO2, and Vulcan carbon, respectively. For control comparison, Na 2Ti 3O 7 irregular particles using the same method are prepared without the addition of Vulcan carbon. All the chemicals were purchased from Sigma-Aldrich and used without further purification. Characterization. The composition, structure, and morphology of the samples are characterized by X-ray diffraction [XRD, Bruker AXS D8 diffractometer using Cu Kα radiation (λ = 0.1541 nm) at a sweep rate of 0.02 deg s−1], field emission scanning electron microscopy (FE-SEM, Hitachi S-4800), and transmission electron microscopy (TEM, Philips TECNAI 20 at 200 kV). The Brunauer−Emmett−Teller (BET) surface area is measured by a Micromeritics ASAP 2020 analyzer at 77 K. CHNS elemental analysis of the Vulcan carbon was performed on a Flash EA 1112 elemental analyzer. Differential scanning calorimetry (DSC) is performed using a TA Instruments Q2000 instrument under 20 mL/min air flow at a heating rate of 3 °C/min. The gas evolution of the synthesis is monitored by a Hiden CATLAB integrated microreactor/mass spectrometer system (CATLAB-MS) under atmospheric pressure. Electrochemical Tests. The electrochemical tests were carried out by galvanostatic cycling in a two-electrode coin cell CR2032 at room temperature. The working electrode was prepared by a slurry casting procedure: 70 wt % active material and 20 wt % Super P carbon were mixed with 10 wt % polyvinylidene (PVDF) in N-methyl pyrrolidinine (NMP) to form a slurry. The slurry was then evenly spread on a copper foil as current collector by a doctor blade and dried under vacuum at 120 °C for 12 h to remove NMP. CR2032 coin cells were assembled in an argon-filled glovebox (O2 < 0.5 ppm and H2O < 0.1 ppm) using Na foil as the counter and reference electrode and Whatman glass microfiber filter (Grade GF/F) as separator. A 1 M NaClO4 solution in propylene carbonate (PC) was used as electrolyte. The cells were galvanostatically charged and discharged using a Biologic VMP3 multichannel electrochemical station under different discharge rates in which 1C = 178 mA g−1 (based on the theoretical capacity of Na2Ti3O7) with a cutoff voltage window of 0.01−2.5 V (vs Na+/Na). Cyclic voltammetry (CV) studies were carried out between cutoff voltages of 0.01 and 2.5 V (vs Na+/Na) at a scan rate of 0.2 mV s−1.
Figure 1. X-ray diffraction patterns of the sodium titanate samples synthesized by various addition of Vulcan carbon (0−90 wt %) in template-free, solid-state reaction. The content of the sodium titanates is calculated according to the semiquantitative method.14−16
Table 1. Summary of the Phase Constitution and Morphology of the Samples product composition (wt %)
sample notation
Vulcan carbon added (wt %)
Na2Ti6O13
TiO2
morphology
NTO-90
90
0
85
15
NTO-84
84
30
70
0
NTO-72 NTO-34 NTO-25 NTO-15 NTO-0
72 34 25 15 0
46 87 95 100 100
54 13 5 0 0
0 0 0 0 0
microspheres aggregated from nanorods microspheres aggregated from nanorods nanorods nanorods nanorods nanorods irregular particles
Na2Ti3O7
composition summarized in Table 1 is calculated according to the ratio between the major peaks of Na2Ti3O7 (100) at 10.5° and Na2Ti6O13 (200) at 11.8°, with the exclusion of Na2SO4.14−16 No trace of carbon is observed in the final products. With carbon addition up to 15% in the synthesis, the diffraction peaks for NTO-0 and NTO-15 can be indexed to a pure monoclinic phase of Na2Ti3O7 (JCPDS 72-0148). When the concentration of carbon is increased to 25 wt %, i.e., NTO25, new peaks corresponding to a monoclinic phase of Na2Ti6O13 (JCPDS 73-1398) begin to emerge, in addition to the Na2Ti3O7 peaks. Na2Ti6O13 becomes the dominant phase when 72 wt % carbon is added, i.e., NTO-72. When the carbon B
DOI: 10.1021/acs.iecr.6b01867 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
nanorod growth is facilitated by the anisotropic bonding in the crystal structures of Na2Ti3O715 and Na2Ti6O13.16 The nanorods are single-crystalline as characterized by TEM and shown in Figure 3. Concise and orderly lattice fringes of
content is as high as 90 wt %, i.e., NTO-90, the Na2Ti3O7 peaks completely disappear, and small peaks of rutile TiO2 appear because of the partial decomposition of Na2Ti6O13 into rutile TiO2 at 800 °C.17,18 No major TiO2 peaks can be observed in NTO-72 and NTO-84 (Figure S2). Several trends correlating to the addition of carbon in the synthesis are observed from XRD results as follows: (i) The higher the weight percent of carbon introduced, the higher the weight percent of Na2Ti6O13 obtained. Thus, high carbon content favors the formation of Na2Ti6O13. (ii) High weight percent of carbon results in higher crystallinity of the product. (iii) When the carbon is added beyond 72 wt %, tiny peaks corresponding to Na2SO4 (thenardite) begin to appear, due to the trace amount of sulfur-containing species in the Vulcan carbon (
