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Improvement of Na Ion Electrode Activity of Metal Oxide via Composite Formation with Metal Sulfide Seung Mi Oh, In Young Kim, Sharad Bandu Patil, Boyeon Park, Jang Mee Lee, Kanyaporn Adpakpang, Seen Ae Chae, Oc Hee Han, and Seong-Ju Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11220 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Applied Materials & Interfaces

Improvement of Na Ion Electrode Activity of Metal Oxide via Composite Formation with Metal Sulfide Seung Mi Oh,† In Young Kim,† Sharad B. Patil,† Boyeon Park,† Jang Mee Lee,† Kanyaporn Adpakpang,† Seen Ae Chae,‡ Oc Hee Han,* †,‡,§ and Seong-Ju Hwang*,† †

Department of Chemistry and Nanoscience, College of Natural Sciences, Ewha Womans University, Seoul 03760, Korea ‡

§

Western Seoul Center, Korea Basic Science Institute, Seoul 03759, Korea

Graduate School of Analytical Science and Technology, Chungnam National University, Daejeon 34134, Korea

* To whom all correspondances are addressed. Tel: +82-2-3277-4370; +82-2-6908-6220 Fax: +82-2-3277-3419 E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The composite formation with a conductive metal sulfide domain can provide an effective methodology to improve the Na-ion electrode functionality of metal oxide. The heat-treatment of TiO2(B) under CS2 flow yields an intimately-coupled TiO2(B)−TiS2 nanocomposite with intervened TiS2 domain, since the reaction between metal oxide and CS2 leads to the formation of metal sulfide and CO2.

The negligible change in lattice parameters and significant

enhancement of visible light absorption upon the reaction with CS2 underscore the formation of conductive metal sulfide domains. The resulting TiO2(B)−TiS2 nanocomposites deliver greater discharge capacities with better rate characteristics for electrochemical sodiation−desodiation process than does the pristine TiO2(B).

The

23

Na magic angle spinning nuclear magnetic

resonance analysis clearly demonstrates that the electrode activities of the present nanocomposites rely on the capacitive storage of Na+ ions and the TiS2 domains in TiO2(B)−TiS2 nanocomposites play a role as mediators for Na+ ions to and from TiO2(B) domains. According to the electrochemical impedance spectroscopy, the reaction with CS2 leads to the significant enhancement of charge transfer kinetics, which is responsible for the accompanying improvement in electrode performance.

The present study provides clear evidence for the

usefulness in composite formation between the semiconducting metal oxide and metal sulfide in exploring new efficient NIB electrode materials.

KEYWORDS: Composite materials, Composite formation, Metal sulfide, Na ion batteries, Capacitive storage


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1. Introduction As alternative power sources to Li-ion batteries (LIBs), Na-ion batteries (NIBs) attract increasing research interest because of their remarkable advantages such as low price and rich abundance of sodium element.1 The electrode materials for NIB can operate on the basis of the same working principle as the LIB, i.e. intercalation−deintercalation, alloying−dealloying, and conversion.1,2 However, a less electropositive nature and a lower reactivity of Na than Li make narrower a candidate pool of electrode materials for NIB. Many transition metal oxides can serve as electrode materials for NIB in terms of intercalation and/or conversion reactions.2−6 Among diverse metal oxides, titanium oxides possess many merits as electrode materials for NIB in terms of high stability, low price, rich abundance, low toxicity, etc.7,8 In comparison with many other metal oxides like Fe2O3 with significantly higher theoretical capacity and high natural abundance,9,10 this TiO2 material boasts higher reversibility for the repeated insertion/extraction of Na+ ions. The presence of many TiO2 polymorphs provides additional opportunity to improve the electrochemical activity of titanium oxide via the tailoring of crystal structure.8 These advantages of titanium oxide render them one of the most promising electrode materials for NIB.11,12 However, the wide bandgap nature of titanium oxide with larger bandgap energy of >3.2 eV leads to the low electrical conductivity, which has detrimental effect on the electrode performance of this material especially under high current density.13 To overcome the inferior electrode performance of TiO2, many attempts have been made such as carbon coating, cation doping, and anion doping including sulfur doping.14−21 Taking into account the lower electronegativity of sulfur than oxygen, the (metal−sulfur) bond has less ionic nature than the (metal−oxygen) bond, leading to the smaller bandgap energy and higher electrical conductivity of metal sulfide than metal oxide.13,22 Thus the composite formation of metal oxide−metal sulfide


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would be effective in improving the electrical conductivity and electrochemical activity of metal oxide.23 Actually, several metal sulfides like TiS2 show much higher electrical conductivity than corresponding metal oxide like TiO2, which can act as conducting path. Additionally, there is other class of electrochemically active metal sulfide such as FeS2, SnS2, MoS2, and NiS2 acting as additional electrode component in terms of intercalation/conversion mechanism.24,25 Therefore, the present sulfurization strategy for metal oxide is supposed to be useful as a new methodology to explore high-performance electrode materials for NIB. At the present time of this submission, we are unaware of any report about the synthesis of metal oxide−metal sulfide nanocomposites applicable for NIB electrode via the formation of metal sulfide domain via the heat-treatment under CS2 flow, although there are several attempts to enhance the electrode performance of TiO2 via sulfurization.26−28 Although there have been several studies about the NIB electrode functionality of metal oxide,29,30 no study has been carried out for probing the mechanism for the improvement of the electrode performance of TiO2 upon the introduction of metal sulfide domain. Among many characterization techniques,

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Na magic angle spinning-

nuclear magnetic resonance (MAS-NMR) spectroscopy is highly sensitive to the variation of the chemical environment of Na ions.31,32 This technique can provide critical information about the mechanism responsible for the NIB electrode activity of the TiO2-based material and its evolution upon the composite formation.


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Figure 1. Schematic diagram of the formation of TiS2 domain in the TiO2(B) nanowire. In this study, efficient NIB electrode materials of TiO2(B)−TiS2 nanocomposites are synthesized by the reaction between the pristine TiO2(B) nanowire and CS2 molecule, as illustrated in Figure 1. The incorporation of conductive TiS2 domain into the TiO2(B) matrix is probed by the combination of diffraction, microscopic, and spectroscopic tools. The structural and morphological evolutions of the pristine TiO2(B) nanowire upon the reaction with CS2 are systematically investigated to elucidate the influence of the incorporation of metal sulfide domain on the physicochemical properties of titanium oxide.

The resulting TiO2(B)−TiS2

nanocomposites are applied as electrode materials for NIB to understand the effect of composite formation on the electrochemical activity of titanium oxide. The mechanism responsible for the electrode activities of the present nanocomposites is studied with 23Na MAS NMR spectroscopy for the as-prepared materials and their electrochemically cycled derivatives.

To study the

dependence of the physicochemical properties and electrode activities of the present nanocomposites on the content of TiS2, several reactant ratios of CS2/TiO2(B) are applied.

2. Experimental


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2.1. Synthesis. The pristine TiO2(B) was prepared by the hydrothermal treatment of TiO2 in basic media and the following heat-treatment at elevated temperature.33−35 Typically, anatase TiO2 (3 g) soaked in 15 M NaOH solution (50 mL) was heated at 150 °C for 72 h. The resulting powder was thoroughly washed with 0.01 M HCl solution and distilled water to become the solution pH neutral. Then the obtained materials were heated at 400 °C for 5 h in air to induce a phase transformation into TiO2(B). The thermogravimetric (TG) analysis clearly demonstrates the complete removal of water molecules from the precursor titanium oxide at 400 °C (see Figure S1 of Supporting Information). The incorporation of TiS2 domain into the pristine TiO2(B) was achieved by the reaction of the pristine TiO2(B) with CS2 vapor at 350 °C for 1 h. The CS2 vapour was generated by bubbling of Ar carrier gas into liquid CS2 with the flow rate of 30 mL min−1. The content of metal sulfide was varied by employing different volumes of CS2 liquid 0.3, 0.5, 0.7, and 0.9 mL for 0.3 g of TiO2(B).

The resulting nanocomposites with the

increasing sulfur contents are denoted as TTS1, TTS2, TTS3, and TTS4, respectively. For comparison, another sulfur-doped TiO2(B) material was prepared by heating the mixture of the pristine TiO2(B) (0.1 g) and thiourea (0.2 g) at 400 °C for 2 h, as reported in a previous study.36 Additionally, the physical mixture of TiO2(B) and TiS2 was prepared via mechanical mixing in the same composition of the TTS3 nanocomposite. 2.2. Characterization. The powder X-ray diffraction (XRD) analysis with Cu Kα radiation was done for the structural characterization of the present materials by Rigaku D/Max-200/PC diffractometer. Elemental CHNS analysis and inductively coupled plasma (ICP) spectrometry were utilized for determining the chemical compositions of the present materials. They were carried out by PerkinElmer 2400 Series II and PerkinElmer Optima 8300, respectably. TG analysis was employed to probe the thermal behavior of the precursor titanium oxide. The


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evolution of optical property upon the composition formation with TiS2 was examined with diffuse reflectance UV−vis spectroscopy using a Sinco S-4100 spectrometer. The chemical bonding natures of the present materials were studied by measuring micro-Raman spectra with a Horiba Jobin-Yvon Labram Aramis spectrometer, where an Ar-ion laser with the wavelength of 514 nm was utilized as an excitation source. The Raman image was obtained for 20 µm × 20 µm region. The step size in the scans was fixed to 1 µm. The crystal morphologies of the present materials were characterized with field emission-scanning electron microscopy (FE-SEM; JEOL JSM-6700F) and high-resolution transmission electron microscopy (HR-TEM; JEOL JEM2100F) analyses. The elemental distributions of the TTS materials were probed by energy dispersive spectrometry (EDS)−elemental mapping and scanning TEM (STEM)−EDS line scan analysis. The surface areas of the present materials were determined by measuring N2 adsorption−desorption isotherms at −196 °C using Micromeritics ASAP2020. Prior to the measurement, the materials were degassed at 120 °C for 3 h under vacuum. Ti K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were measured at the beam line 10C of the Pohang Accelerator Laboratory (PAL) in Korea.

All the XANES/EXAFS spectra were collected in a transmission mode at room

temperature. All the measured spectra were energy-calibrated by the simultaneous measurement of the spectrum of Ti metal. The data analysis for the measured XANES/EXAFS data was done by the standard procedure, as reported previously.37,38 The oxidation states and chemical bonds of the present materials were determined by X-ray photoelectron spectroscopy (XPS; PHI5100 Perkin-Elmer spectrometer) and Fourier transformed-infrared (FT-IR; Varian Scimitar Series FTS800) spectroscopy.

The XPS spectra were measured using a PHI5100 Perkin-Elmer

spectrometer. The energy calibration was done with reference to the energy of C 1s peak (284.8


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eV).

Solid-state

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Na MAS NMR spectra were acquired on a Varian INOVA 600 MHz

spectrometer at 14.1 T magnetic field using 2.5-mm rotors. All of the samples except Na2Ti3O7 and the pristine TiO2(B) were packed in the rotors in the glove box under dry nitrogen environment, and the MAS NMR spectra were measured at a radio frequency of 158.706 MHz using 25 kHz spinning rate, 20 s pulse repetition delay time, and an external chemical shift reference of 1 M NaCl aqueous solution. For excitation pulses, 1.3 µs pulse length was used when 90°-pulse-length for 1 M NaCl solution was 3.75 µs. In order to separate Na sites with different quadrupole coupling constants, nutation spectra were acquired with various pulse lengths of 1.3, 2.6, 3.9, 5.2, and 6.5 µs. 2.3. Electrochemical measurements. The electrode activities of the present TTS nanocomposites were tested as an anode for NIB. The galvanostatic charge−discharge cycling was performed in the potential range of 0.05−3.0 V at several current densities using Maccor multichannel galvanostat/potentiostat. The coin-cell of Na/1 M NaClO4 in ethylene carbonate (EC): propylene carbonate (PC) (50:50 in vol%) + fluoroethylene carbonate (FEC) (5vol%)/active material was fabricated for electrochemical cycling tests.

The composite

electrode was obtained via a thorough mixing of the active material (70wt%), Super P (20wt%), and polyvinylidene fluoride (PVDF, 10wt%) in N-methyl-2-pyrrolidene (NMP) and the following deposition of the obtained slurry on Cu foil.

The electrochemical impedance

spectroscopy (EIS) data presented here were collected in the frequency range of 100 kHz to 10 mHz using IVIUM impedance analyzer at open circuit potential.

3. Results and Discussion


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3.1. Powder XRD and micro-Raman spectroscopic analyses. Figure 2A presents powder XRD patterns of the pristine TiO2(B), TTS nanocomposites, and the reference TiS2. All the present TTS nanocomposites display typical Bragg reflections of TiO2(B) phase (PDF #741940) without significant shift of peak positions. As listed in Table S1 of Supporting Information, all the present TTS materials possess nearly identical lattice parameters before and after the reaction with CS2, suggesting the negligible influence of CS2 treatment on the crystal structure of TiO2(B) lattice. In addition to these TiO2-related peaks, the (110) peak of TiS2 phase (PDF #882479) appears at 2θ = 53.7° for the TTS nanocomposites,39,40 suggesting the formation of TiS2 phase upon the reaction with CS2. The present XRD analysis demonstrates that the heat-treatment under CS2 flow results in the incorporation of metal sulfide domains into the pristine TiO2(B). A marked size difference between O2− and S2− anions makes it difficult to substitute oxide ion of metal oxide lattice with sulfide ion.41,42 The phase of metal sulfide formed by the CS2 treatment is also examined with micro-Raman spectroscopy. As presented in Figure 2B, a series of phonon lines of TiO2(B) lattice appear at ~120, ~180, ~195, ~254, ~284, ~287, ~440, ~607, and ~677 cm−1 for the pristine TiO2(B).43−45 After the CS2 treatment, all the TTS nanocomposites have very intense Raman peaks at ~333 and ~236 cm−1, which are assigned as A1g and Eg vibration modes of TiS2 phase, respectively, confirming the formation of TiS2 domain. In comparison with the TiO2-related Raman features, the TiS2-related ones show much higher intensities, since the Raman phonon line of the heavier atom has stronger spectral weight than does that of the lighter atom.46 The composite structure of the present TTS3 nanocomposite is probed by monitoring the spatial distribution of the intensity ratio of the TiS2- and TiO2(B)-related Raman peaks at 195 and 335 cm−1 (I195/I335). As illustrated


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in Figure 2C, there are TiS2 domains embedded in the matrix of the pristine TiO2(B), highlighting the formation of TiS2−TiO2 nanocomposite via the heat-treatment under CS2 flow.

Figure 2. A) Powder XRD patterns and B) micro-Raman spectra of (a) the pristine TiO2(B), the TiO2(B)−TiS2 nanocomposites of (b) TTS1, (c) TTS2, (d) TTS3, and (e) TTS4, and (f) the reference TiS2.

C) Spatial distribution of the TiS2 and TiO2(B) phases in the TTS3

nanocomposite from the relative Raman intensity of I335/I195. D) Diffuse reflectance UV−vis spectra and (inset) photographs of (a) the pristine TiO2(B) (black circles), the TiO2(B)−TiS2 nanocomposites of (b) TTS1 (red squares), (c) TTS2 (green triangles), (d) TTS3 (blue inverse triangles), and (e) TTS4 (pink diamonds), and (f) the reference TiS2 (cyan hexagons). 3.2. Diffuse reflectance UV−vis spectroscopy, photograph, and elemental analysis. The diffuse reflectance UV−vis spectra of the pristine TiO2(B), TTS nanocomposites, and the


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reference TiS2 are illustrated in Figure 2D. A distinct absorption edge is discernible at ~356 nm for the pristine TiO2(B), reflecting its semiconducting nature with the bandgap energy of 3.5 eV. Conversely, the reference TiS2 does not display any absorption edge, which is related to its metallic nature. Like the reference TiS2, all the present TTS nanocomposites including TTS1 with the lowest TiS2 content show strong absorption in the entire region of visible light without any absorption edge, indicating the effective incorporation of highly conductive TiS2 domain into the semiconducting TiO2(B) lattice by the reaction with CS2. This interpretation is further confirmed by the color change of the pristine TiO2(B) from white to dark gray upon the reaction with CS2, see the inset of Figure 2D. The increase of CS2 reactant leads to the gradual darkening of the color of the pristine TiO2(B), verifying the enhanced absorption of visible light caused by the formation of conductive TiS2 domain. Of prime interest is that the color of the precursor becomes remarkably darker even with a small quantity of CS2, underscoring the high efficiency of the composition formation with TiS2 via the present method. It is worthwhile to mention that the present TTS nanocomposites show gray color which is darker than the dark yellow color of reference TiS2. Considering the fact that the physical mixture of TiO2(B) and TiS2 also shows similar gray color (see Figure S2 of Supporting Information), the observed darkening upon the composite formation is attributable to the light scattering effect caused by the uniform mixing of two different shaped crystals. The incorporation of metal sulfide domain is further confirmed by the CHNS elemental analysis. The TTS nanocomposites show the increase of sulfur content with increasing the content of CS2 reactant; the sulfur contents of the present nanocomposites are determined as 4.42, 5.16, 6.02, and 8.16wt% for TTS1, TTS2, TTS3, and TTS4, respectively. corresponding

molar

compositions

are

calculated

to

be

The

0.94TiO2(B)−0.06TiS2,


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0.93TiO2(B)−0.07TiS2, 0.91TiO2(B)−0.09TiS2, and 0.88TiO2(B)−0.12TiS2, for TTS1, TTS2, TTS3, and TTS4, respectively. This result confirms that the increase of CS2 content induces a gradual increase in the TiS2 concentration in the final products. Also, the CHNS analysis demonstrates that all the present TTS nanocomposites commonly contain only a negligible amount of carbon (9 Å−1, the increase of CS2 content leads to the spectral modification to TiS2-like feature, reflecting the formation of TiS2 domains in the TTS nanocomposites. As shown in Figure 4D, the reference TiS2 displays two intense FT peaks corresponding to the coordination shells of (Ti−S) and (Ti−Ti) at ~2.0 and ~2.9 Å (i.e. no phase-shift corrected distance). In comparison, two strong FT peaks are discernible for both the nanocomposites and the pristine TiO2(B) at


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shorter distances of ~1.6 and ~2.6 Å, which are assigned as (Ti−O) and (Ti−Ti) bonding pairs, respectively. In contrast to the pristine TiO2(B), both the TTS3 and TTS4 nanocomposites show additional FT feature corresponding to the (Ti−S) coordination shell of TiS2 phase at ~2.0 Å. As the content of CS2 reactant increases, this peak becomes stronger, providing strong support for the formation of TiS2 domain in the TiO2(B) matrix via the reaction with CS2. 3.5. XPS and FT-IR analyses.

The chemical state of sulfur species in the TTS

nanocomposites is investigated with FT-IR spectroscopy. As presented in Figure 5A, the sulfurdoped TiO2(B) material prepared with thiourea demonstrates intense IR bands corresponding to symmetric stretching vibrations of S=O at ~1126 cm−1, which is related to the surface adsorbed SO42− species.51,52 Conversely, all the TTS nanocomposites as well as the reference TiS2 do not display these sulfate-related IR bands, indicating the absence or very low content of the sulfate species in these materials. The oxidation state of sulfur species in the present nanocomposites is also probed with XPS that is sensitive to surface species. As plotted in S 2p XPS spectra of Figure 5B, the reference TiS2 shows distinct XPS peaks at low binding energies (BEs) of ~162 and ~161 eV corresponding to anionic S2− species, whereas an intense XPS peak corresponding to the S6+ species appears at a higher BE of ~168 eV for the S-doped TiO2(B) material prepared with thiourea,53 as reported for sulfur-doped materials synthesized with thiourea, (NH4)2SO4, and H2SO4.54−56 This is in good agreement with the FT-IR results showing the presence of the sulfate species in these materials. Conversely all the present TTS nanocomposites display intense S 2p peaks corresponding to sulfide/polysulfide species at ~161−163 eV as well as a weaker sulfaterelated peak at ~168 eV, strongly suggesting the predominant formation of anionic sulfur species in these materials and the partial formation of sulfate on the surface.57 In addition, the evolution


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of the chemical bonding character of titanium ion upon the composite formation is studied with Ti 2p XPS technique. As plotted in Figure 5C, the pristine TiO2(B) shows two intense Ti 2p XPS peaks at ~458 and ~464 eV, which are typical spectral features of tetravalent Ti4+ species.58,59 After the reaction with CS2, these Ti 2p peaks of the pristine TiO2(B) become asymmetric with the advent of a shoulder peak at a lower energy side. Taking into account the lower electronegativity of sulfur than oxygen, the appearance of the low energy peak is assigned as the increase of electron density in titanium ions via the formation of more covalent (Ti−S) bond than (Ti−O) one. This can be regarded as another evidence for the formation of titanium sulfide species.60,61

Figure 5. A) FT-IR spectra, B) S 2p XPS spectra, and C) Ti 2p XPS spectra of (a) the pristine TiO2, (b) TTS1, (c) TTS2, (d) TTS3, (e) TTS4, (f) S-doped TiO2 prepared with thiourea, and (g) the reference TiS2. 3.6. Electrochemical measurement. The obtained TTS nanocomposites are applied as anode materials for NIB to study the effect of TiS2 incorporation on the Na ion electrode performance of titanium oxide. As plotted in Figure S4 of Supporting Information, all the present TTS nanocomposites show similar potential profiles for the 1st and 2nd cycles with significant irreversible capacity fading like the pristine TiO2(B). In the 1st discharging step, all the present


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materials show the occurrence of an irreversible reaction related to the decomposition of electrolyte,2,7 which is followed by reversible insertion/extraction of sodium ion in the subsequent cycles. Notable similarities in overall profile shapes and working potentials between the TTS nanocomposites and TiO2(B) strongly suggest that the electrode activities of the present nanocomposite mainly originate from titanium oxide component. Figure 6A plots the potential profiles of the 5th discharge−charge cycle for the pristine TiO2(B) and the TTS nanocomposites. All of the present TTS nanocomposites still show similar potential profiles to the pristine TiO2(B), suggesting the main contribution of the TiO2(B) component to the electrode functionality of the present materials. Like the pristine TiO2(B), all the TTS nanocomposites exhibit gradually decreasing slope below 1.5 V, which is mainly responsible for the electrode activity of these materials. Conversely, the reference TiS2 material displays electrochemical activity at higher working potential of 1.5−2.5 V, which is related to the sodiation−desodiation process of layered TiS2 material.62 As can be seen clearly from Figure 6A, the incorporation of TiS2 domain gives rise to only a weak increase of discharge capacity at above 1.5 V, whereas it induces a significant increase of the discharge capacity at below 1.5 V.

This observation

strongly suggests that the improvement of the electrode activity of TiO2(B) phase upon the composite formation with TiS2 is attributable not to the simple addition of the charge storage by incorporated TiS2 phase but to the synergistic enhancement of the charge storage capacity of semiconducting TiO2(B) material by the intervention of conductive TiS2 domains.

The

maintenance of incorporated TiS2 domain after the electrochemical cycling is obviously confirmed by micro-Raman analysis for the electrochemically-cycled TTS3 material showing Raman spectral feature of TiS2 phase (see Figure S5 of Supporting Information). After several initial cycles, a high Coulombic efficiency (~99%) is commonly observed for all the present


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TTS nanocomposites and the pristine TiO2(B), showing the retention of the high electrochemical stability of titanium oxide upon the introduction of TiS2 domains.

All the present TTS

nanocomposites deliver large discharge capacities of ~90−115 mAhg−1 after the 100th cycle at the current density of 500 mAg−1, which are markedly superior to that of the pristine TiO2(B) (~75 mAhg−1), as plotted in Figure 6B. Among the materials under investigation, the TTS3 nanocomposite has the largest discharge capacity of ~115 mAhg−1 at 500 mAg−1.

Figure 6. A) Potential profiles of the 5th discharge−charge cycle, B) plots of discharge capacity vs. cycle numbers, C) plots of current density-dependent discharge capacity, and D) Nyquist plots (experimental data: circles, calculated data: solid lines) and (inset) equivalent circuit of the pristine TiO2(B) (black) and the TiO2(B)−TiS2 nanocomposites of TTS1 (red), TTS2 (green), TTS3 (blue), and TTS4 (pink). As a reference, the physical mixture of TiO2(B) and TiS2 is prepared in the same composition as the TTS3. The formation of uniform mixture of TiO2(B) and TiS2 is confirmed by powder


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XRD, FE-SEM, EDS−elemental mapping, and FT-IR analyses, as plotted in Figure S6. Even after the mixing, both the TiO2(B) and TiS2 components retain their original crystal structures and morphologies. The physical mixture of TiO2(B) and TiS2 shows an inferior discharge capacity (~78 mAhg−1 at 500 mAg−1, see Figure S7 of Supporting Information) over the TTS3 nanocomposite. This result provides strong evidence for the synergistic effect of the nanoscale mixing between TiO2(B) and conductive TiS2 domains in the same particles. The beneficial effect of the composite formation with TiS2 is more distinct on the electrode performance at higher current density condition (see Figure 6C), confirming the usefulness of the incorporation of conductive TiS2 domain. In comparison with the TTS nanocomposites, the sulfur-doped TiO2(B) material prepared with thiourea has much smaller discharge capacities of ~71 mAhg−1 at 500 mAg−1 (see Figure S7 of Supporting Information). This result clearly demonstrates that, in contrast to the reaction with CS2, the conventional sulfurization method with thiourea is not so effective in improving the NIB electrode activity of the pristine TiO2(B) due to the formation of sulfate domain. Since metal sulfate is mostly an ionic compound with wide bandgap and poor electrical conductivity, the formation of metal sulfate is less useful in improving the electrical conductivity and electrode performance of metal oxide than that of metal sulfide. 3.7. EIS analysis. The evolutions of the charge-transfer behavior and ion diffusion kinetics of the pristine TiO2(B) upon the composite formation with TiS2 are examined with EIS. Figure 6D illustrates the EIS spectra of the TTS nanocomposites and the pristine TiO2(B) subjected to the 100th electrochemical cycles. All the present TTS nanocomposites display smaller diameters of the semi-circle in the high-medium frequency region compared to that of the pristine TiO2(B), confirming the decrease of charge-transfer resistance in the former case. The fitted data are summarized in Table S3 of Supporting Information. In the low frequency region, all the present


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TTS nanocomposites display greater slopes of inclined lines than does the pristine TiO2(B), indicating the improvement of Na ion diffusion in the bulk electrode. From the Zre vs. ω−1/2 plot in the Figure S8 of Supporting Information, the Warburg coefficient (σw) is determined to be 156, 111, 116, 111, and 102 Ω s−1/2 for the pristine TiO2(B), TTS1, TTS2, TTS3, and TTS4, respectively.63 Since σw is inversely proportional to ion diffusivity,64 the greater slope of the pristine TiO2(B) than those of the TTS nanocomposites indicates the higher Na+ diffusivity of the latter.

This result clearly demonstrates the enhancement of ion diffusion upon the

incorporation of conductive TiS2 domains, which is responsible for the improvement of the electrode performance of titanium oxide upon the reaction with CS2. 3.8.

23

Na MAS NMR analysis. The local chemical environments of Na+ ions in the pristine

TiO2(B) and the TTS nanocomposite, as well as their evolutions upon electrochemical cycling, are probed by 23Na MAS NMR spectroscopy to elucidate the mechanism responsible for the NIB electrode activities of the present TiO2-based materials. Figure 7 plots the

23

Na MAS NMR

spectra of the pristine TiO2(B) and TTS3 nanocomposite and their electrochemically cycled derivatives, as well as the reference of layered Na2Ti3O7. A well-defined quadrupole powder pattern centered near −13 ppm accompanied with a sharp peak at 3 ppm appears in the

23

Na

MAS NMR spectrum of layered Na2Ti3O7. The relatively sharp signal comes from highly spherically symmetric Na sites (Na2) with small quadrupole coupling constants and the broad pattern comes from low spherically symmetric Na sites (Na1) with large quadrupole coupling constants in crystalline Na2Ti3O7.32 The population ratio for the two sites estimated using peak areas is 1:1 as expected from crystallographic data.32

The large difference in quadrupole

coupling constants between the two Na sites is confirmed with longer effective 90° pulse length for the sharp peaks (see Figure S9 of Supporting Information).


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Figure 7.

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Na MAS NMR spectra of A) the pristine TiO2(B) and its electrochemically cycled

derivatives to B) 1.0 and C) 1.5 cycles, D) the TTS3 nanocomposite and its electrochemically cycled derivatives to E) 1.0, F) 1.5, and G) 1.0 cycles and aged for 9 days and H) the reference layered Na2Ti3O7. The spectral intensities are normalized with sample weights and acquisition numbers. The sharp peak at 3 ppm in the spectrum in H) is clipped to 1/4 of its original height. On the other hand, the 23Na MAS NMR spectrum for the pristine TiO2(B) has the broad signal at the similar resonance frequencies to that for low symmetric Na1 sites of Na2Ti3O7 but in a smoothed featureless shape. The large signal from the pristine TiO2(B) confirms a significant amount of Na in the pristine TiO2(B) even before any cell cycles.

The signal for highly

symmetric Na2 sites is drastically reduced, indicating a lower population for this highly symmetric Na site in the pristine TiO2(B) than in Na2Ti3O7. After 1.0 and 1.5 electrochemical cycles, this broad signal of the pristine TiO2(B) becomes more symmetric and smoothed


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compared with the spectrum of the as-prepared TiO2(B). However, the resonance peak at −11 ppm observed for the cycled TiO2(B) is still more asymmetric than that reported in the previous literature.65 From the repeated NMR measurements for the cycled materials, we found that the aging of the material makes this resonance peak smoother and more symmetric. Thus, the asymmetric shape of this peak in the present NMR spectrum strongly suggests more fresh nature of the present material with negligible ageing. Actually, this broadened resonance is composed of broadly overlapped signals with a chemical-shift distribution due to the minor inhomogeneity of electronic and chemical environments from site to site even for specific Na sites. This inhomogeneity mainly comes from the inhomogeneous distribution of Na ions in the sample and the symmetry variation of the broadened resonance reflects the population redistribution of Na ions for the broadly overlapped signals. Therefore, the change of the peak shape to more symmetric one upon ageing is probably related to the fact that Na ions move slowly to near Na sites to reach an equilibrium state with the less inhomogeneous distribution of Na ions for a given time. Additionally, the electrochemical cycling leads to the enhancement of the relatively sharp signals near 5 ppm. These signals near 5 ppm are confirmed to have longer effective 90° pulse length (see Figure S9 of Supporting Information). In comparison with the spectrum after 1.0 cycles, that after 1.5 cycles shows more distinct broad component covering from 5 to 40 ppm near the baseline, indicating another signal from the reversible Na+ ions. The present spectra of the electrochemically cycled TiO2(B) derivatives subjected to the 1.0 and 1.5 cycles are not much different from the previously reported spectra for TiO2 nanotubes charged or discharged, strongly suggesting that the capacitive storage of Na ions is responsible for the electrode activity of the pristine TiO2(B).65,66

Even after the composite formation with TiS2, the TTS3


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nanocomposite still shows a similar NMR spectrum to that of the pristine TiO2(B), suggesting that the nanocomposite retains the local symmetries of the Na sites of TiO2(B). However, the electrochemically cycled derivatives of TTS3 have relatively sharp additional peaks at 50 ppm in Figures 7E and 7F, and the Na+ ions for the signals are quite mobile. As the samples are aged, these peaks are decreased and the other peaks for the Na+ ions in the TiO2 domains are increased (see Figure 7G), which looks very similar to that for the electrochemically cycled TiO2(B) in Figure 7B. These peaks at 50 ppm must originate from the Na+ ions in the TiS2 domains of the TTS nanocomposites since these features are similar to the signals observed for NaxTiS2 for x = 0.2.67 Although the present result might reflect the presence of the TiOxSy phase,68 the negligible change of lattice parameters strongly suggest no substitution of oxide ion with much larger sulfide ion to yield the TiOxSy phase. In addition, even if there were TiOxSy phases in our samples, the 23Na NMR signals for the Na ions in the TiOxSy phases are expected to appear close to those for the Na ions in the TiO2(B) domains. Thus, these resonances at 50 ppm is attributable to the Na+ ion in the intervened TiS2 domain. The decreased intensity of these peaks at aging with the accompanying increase of signal intensities for the Na+ ions in the TiO2 domains clearly indicates that the TiS2 domains mediate Na+ ions transport to and from TiO2(B) domains in the nanocomposites. This result clearly demonstrates that most of the electrochemical storage of Na+ ions in the present TTS nanocomposite still originates from the TiO2(B) components. As evidenced by the results of EIS and electrochemical cycling, the intervened TiS2 domains act mainly as ionic paths with easier and faster ion movements rather than as electrochemically active components, leading to the improvement of the electrode activity of the TiO2(B) domains in the nanocomposites.


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In addition to the present sulfurization strategy, the composite formation of TiO2 with conductive carbon is highly effective in improving the NIB electrode performance of titanium oxide.15−17 In one instance, the TiO2(B) encapsulated in porous carbon network shows a large discharge capacity of ~140 mAh g−1 after the 50th cycle at the current density of 42 mA g−1,16 which is comparable to that of the present TTS nanocomposite, confirming the comparable merit of the present synthetic method to the previously reported carbon composite method. Based on this comparison, the application of both the strategies of sulfurization and encapsulation with carbon can lead to the further improvement of the NIB electrode performance of titanium oxide.

4. Conclusion In this work, we are able to develop an efficient methodology to explore high-performance nanocomposite electrode materials for NIB. The Na+ ion storage capacity of TiO2(B) can be significantly improved by the formation of conductive metal sulfide domain via the heattreatment under CS2 flow. The resulting TTS nanocompneosites show much higher electrical conductivity and better anode performance for NIB than does the pristine TiO2(B) phase, indicating the beneficial role of TiS2 incorporation in improving the electrochemical activity of semiconducting metal oxide. The

23

Na MAS NMR analysis demonstrates that the electrode

functionalities of the pristine TiO2(B) and TTS nanocomposites commonly originate from the capacitive storage of Na+ ions mainly on the TiO2(B) component. In addition, the NMR data show that TiS2 domains in TTS nanocomposites facilitate ion movements to and from the TiO2(B) domains by playing a role as ionic paths providing faster kinetics and lower activation energy. The EIS results presented here also provide strong evidence for the enhancement of the charge transport and electrical conductivity of titanium oxide upon the incorporation of


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conductive TiS2 domain, which is mainly responsible for the remarkable improvement of the NIB anode performance of titanium oxide upon the reaction with CS2.

All the present

experimental findings underscore that the incorporation of metal sulfide via the reaction with CS2 provides an effective way to improve the NIB electrode performance of semiconducting metal oxide and also to explore efficient electrode materials for NIB. Currently, we are trying to apply the present method for developing nanocomposite electrode materials for other emerging secondary batteries such as multivalent ion-batteries and Li−S batteries.

Supporting Information. TG curve of the precursor titanium oxide; lattice parameters, elemental concentration, N2 adsorption−desorption isotherms, potential profiles of the 1st and 2nd cycles, parameters obtained from Nyquist plots, Zre vs. ω−1/2 plots of the pristine TiO2(B) and the TTS nanocomposites; discharge capacity plots of the physical mixture of TiO2(B) and TiS2 and Sdoped TiO2; micro-Raman spectra of TTS3 after cycling; photograph, XRD, SEM, EDS mapping, FT-IR spectra of the physical mixture of TiO2(B) and TiS2; 23Na MAS nutation NMR spectra of the pristine TiO2(B) and TTS3 nanocomposite and its electrochemically cycled derivatives. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *,† E-mail: [email protected]. Tel: +82-2-3277-4348.


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*,†,‡,§ E-mail: [email protected]. Tel: +82-2-6908-6220.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2014R1A2A1A10052809) and the National Science and Technology Council in Korea (DRC-14-1-KBSI). The experiments at PAL were supported in part by MOST and POSTECH. Ms. S.H. Moon at KBSI was acknowledged for her technical assistance in NMR experiments.

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