Controlling Sulfur Vacancies in TiS2âx Cathode Insertion Hosts via the

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Controlling Sulfur Vacancies in TiS2-x Cathode Insertion Hosts via the Conversion of TiS3 Nanobelts for Energy Storage Applications Casey Hawkins, and Luisa Whittaker-Brooks ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00266 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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ACS Applied Nano Materials

Controlling Sulfur Vacancies in TiS2-x Cathode Insertion Hosts via the Conversion of TiS3 Nanobelts for Energy Storage Applications Casey G. Hawkins and Luisa Whittaker-Brooks *,ǁ ǁ

Department of Chemistry, University of Utah, 315 South 1400 East, Rm 2020, Salt Lake City,

Utah, 84112, USA

ABSTRACT

The electronic properties of titanium (IV) sulfide (TiS2) have been scrutinized for many decades due to its strong tendency towards non-stoichiometry with either titanium excess or sulfur deficiency in its crystal structure. Here, the systematic solid-state transformation of TiS3 to TiS2-x nanobelts as a means to control the non-stoichiometry of TiS2-x nanostructures is reported. Careful structural, optical, and electronic studies were performed to elucidate the real nature of TiS2 (i.e., semimetal or semiconductor).

Experimental evidence gathered by diffraction,

spectroscopy, and electrical measurements for TiS2-x as a function of sulfur deficiencies indicates it behaves as a semimetal even at non-stoichiometry ranges as low as x = 0.15.

Optical

characterization shows a decrease in the bandgap of TiS2-x nanobelts with increasing nonstoichiometry deviations. Electrical transport measurements suggest an increase in the electrical

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conductivity of TiS2-x nanobelts with increasing sulfur vacancies. Furthermore, we also report the influence of non-stoichiometries on the electrochemical performance of lithium-ion batteries based on TiS2-x nanobelt-assembled film cathodes. Our results demonstrate that cathodes based on sulfur deficient TiS2-x nanobelts deliver efficient Li+ intercalation/insertion activity, excellent cycling life, enhanced specific capacity, and excellent rate capability pointing to the importance of carefully controlling defects and stoichiometries in materials as a way to favorably tune their electronic properties.

KEYWORDS: titanium sulfide, nanostructured cathode materials, electrical properties, electrochemical performance, energy storage

INTRODUCTION 2D materials and van der Waals solids have sparked research interest across multiple disciplines due to their remarkable electrical, optical, chemical, and thermal properties.1-3 Although 2D materials have been historically investigated for many decades, it was only until the (re)discovery and successful isolation of graphene that non-carbon 2D materials started to rise from the ashes.4, 5 To date, graphene has been the starring protagonist of all research performed on 2D materials. However, the lack of an intrinsic band gap has limited graphene’s potential for optoelectronic device applications.

Conveniently, a plethora of 2D inorganic materials such as,

transition metal dichalcogenides (MX2, M = Mo, Nb, Ti, Ta and X= S, Se, Te) can be cleverly engineered to selectively tune their band gaps and electron affinities. Tailoring their electronic structure can be readily achieved through careful selection of the transition metal and chalcogen comprising their versatile crystal structure.2, 6-8 Among all transition metal dichalcogenides, TiS2


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has been extensively studied due to its variety of potential applications in electrochemical intercalation,9-11 hydrogen storage,12 and thermoelectrics.13-16 Moreover, TiS2 is an exciting 2D material exhibiting interesting and complex behavior as a result of the interplay between electron-electron and electron-lattice interactions.17

In the context of energy storage applications, TiS2 has been extensively studied as an intercalating cathode material for lithium-ion batteries due to its high electrical conductivity (≈ 104 S m-1), a higher theoretical specific capacity (239 mAh g-1) than that of LiCoO2 (130 mAh g1

), and a low discharge voltage (2.1 V vs. Li/Li+).10, 18-21 However, the inability to control sulfur

vacancies in TiS2 has constrained its use in lithium-ion battery technologies. As such, there is a need to develop fabrication methods that allow for the control of the stoichiometry in TiS2 as a means to increase its potential as a cathode insertion host in lithium-ion batteries.

Architecturally, the 2D nature of TiS2 arises from its crystal structure featuring neutral and covalently held single-atom-thick or polyhedral-thick layer of atoms forming interlayers that are weakly bonded via van der Waals interactions. The relatively low energy required to break these van der Waals forces (40-70 meV) allows the formation of single- to few-layer-thick 2D TiS2 nanostructures. Several approaches have been reported for synthesizing TiS2 nanostructures including the archetypal micromechanical cleavage “scotch tape” method involving the repeated exfoliation of TiS2 mesas; however, this method gives a very low yield of nanostructured flakes and is thus unsuitable for precise positioning of TiS2 structures within device architectures and for scaling to practical quantities. To mitigate these challenges, various alternative synthetic approaches such as chemical exfoliation by dispersing in a solvent having the appropriate surface


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tension, colloidal synthesis, chemical transport, and solid state synthesis from its elemental precursors have been recently reported.22-24 Although various synthetic procedures have been reported in the literature (vide supra), the reliable fabrication of stoichiometric TiS2 still remains a challenge. Factors such as pressure, temperature, reaction time, and synthesis conditions have been demonstrated to affect sulfur volatilization, thus generating non-stoichiometric TiS2.13 In a similar vein, it is well documented and accepted that during the synthesis of TiS2, excess Ti atoms intercalate into the van der Waals gap which subsequently donate electrons into the TiS2 conduction band.25, 26 Furthermore, sulfur volatilization and Ti intercalation are speculated to be responsible for discrepancies in the observed electronic properties of the purported TiS2. Since the structural, physical, and chemical understanding of this ubiquitous compound is for the most uncertain, two conflicting results as to whether TiS2 is a semimetal or a semiconductor have generated a wealth of theoretical and experimental evidence that support both scenarios. For example, studies on the optical absorption,27 Hall effect under high-pressure,28, 29 angle-resolved photoemission,30, 31 and transport properties31 have all endorsed TiS2 as a semiconductor with a band gap ranging from 0.02 to 2.5 eV.

Alternatively, resistivity measurements,32-34 x-ray

emission and absorption band spectrum,35 infrared reflectance,36 and combined x-ray absorption spectroscopy and electron-energy-loss spectroscopy37 have all argued that TiS2 is a semimetal with an indirect p/d band overlap ranging from 0.2 to 1.5 eV. We report here the systematic control of non-stoichiometries in TiS2-x by the non-destructive removal of S from TiS3 sacrificial nanobelts. Such control allows for the elucidation of how defects –particularly, sulfur vacanciesaffect the electronic properties of TiS2. Hall Effect measurements, optical absorption, and dipole selection rules operational in x-ray photoelectron spectroscopy yield detailed insight into the optical properties and nature of the electronic structure in non-stoichiometric TiS2-x nanobelts.


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Similarly, we report how non-stoichiometries in TiS2-x nanobelts affect their electrochemical performance when used as cathode insertion host materials in energy storage devices.

EXPERIMENTAL SECTION Synthesis Ti powder (Alfa Aesar 99%, ~325 mesh) and S powder (Alfa Aesar 99.5%, ~100 mesh) were used as received. All work was performed using Schlenk-line techniques or performed in a nitrogen-filled glovebox to limit oxygen exposure. To synthesize TiS3 nanobelts, elemental Ti and elemental S were ground together using an agate mortar and pestle in a 1:3 ratio. The reactants were then placed into a quartz ampule and evacuated to ≈10-3 Torr using a vacuum line prior to sealing. The sealed quartz tube (Figure 1A) was heated in a furnace to 450 oC for 20 h. For the conversion of TiS3 to TiS2-x nanobelts, as-synthesized TiS3 was immediately placed in a new quartz ampule. A glass wool plug was added into the tube as a constriction to prevent the precursors from coming into physical contact during handling. Subsequently, 2 molar equivalents of elemental Ti powder were added while being kept separated from the TiS3 precursor. This was performed to allow Ti to react with volatilized sulfur obtained during the pyrolysis process thus removing any volatilized sulfur. The quartz ampule was evacuated and then flame-sealed under vacuum (Figure 1B). The new reaction ampule was placed in a furnace and heated to 450 oC for 24-144 h. After the reaction was completed, a dense formation of nanobelts was observed and further characterized.


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Figure 1. Schematic depiction of the arrangement of the sealed quartz ampoule for the synthesis of TiS3 (A) and TiS2-x (B) nanobelts.

Characterization The morphologies of the resulting products were examined with a scanning electron microscope (SEM) equipped with a field-emission gun (FEI Nova Nano, FE-SEM 630) operated at an accelerating voltage of 10 keV. Phase identification and purity of the as-synthesized TiS3 and TiS2 nanobelts were obtained by x-ray diffraction (XRD) on a Bruker D8 Advance, at a scanning rate of 1.2o min-1 in the 2θ range between 5o and 70o using monochromated CuKα radiation (λ = 1.5406 Å). The operating voltage and current were kept at 40 kV and 40 mA, respectively. The dimensions and crystallinity of the as-synthesized nanobelts were examined with a JEOL JEM 2800 field-emission gun transmission electron microscope (TEM) operated at 200 keV. Highresolution TEM images and selected area electron diffraction (SAED) patterns were independently acquired and provided insights into the crystal growth habits and crystallinity of


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the TiS3 and TiS2-x nanobelts, respectively. To prepare the samples for TEM/SAED analysis, the nanobelts were dispersed in isopropanol via sonication and then deposited onto a 400-mesh holey carbon-coated copper grid. For spectroscopy measurements, all films fabricated were handled in an inert N2 atmosphere and never exposed to oxygen and water levels above 5 ppm before introduction into the ultra-high vacuum (UHV) system for X-ray photoelectron (XPS) and ultraviolet photoelectron (UPS) spectroscopy measurements.

XPS and UPS spectra were

acquired in a custom-built UHV chamber equipped with a cylindrical mirror electron analyzer and operated at a base pressure of 10-10 Torr. UPS was performed by using a He I (21.22 eV) excitation line of a He plasma in a discharge lamp. Spectra were taken at a pass energy of 5 eV for a nominal experimental resolution smaller than 150 meV. XPS was performed on an additional sample series, using the Al Kα line (1487.6 eV) with a resolution of 0.8 eV. A pass energy of 100 eV was used for survey scans, while a 40 eV pass energy was used for detailed scans. All measurements were carried out at normal take-off angles. The acquired spectra were calibrated against an adventitious carbon peak at 284.6 eV. Curve fitting was carried out using CasaXPS software with a Gaussian-Lorentzian product function and a non-linear Shirley background. The precise Ti/S stoichiometries were determined by inductively coupled plasma atomic emission spectroscopy (ICP-OES) analyses. The solid samples were digested using a 2.5 : 2.5 : 1 HNO3 : HF : H2O solution heated to 85oC for 12 h before injection into the ICP-MS column. An error of 1% is expected when determining the stoichiometry of our samples via ICPMS. Optical band gaps for the as-synthesized products were determined by acquiring optical diffuse reflectance measurements at room temperature using a Hitachi U-4100 UV-Vis-NIR spectrometer equipped with an integrating sphere. The electrical properties of the as-synthesized samples were measured using an Ecopia 7000 Photonic Hall measurement system. Four-point


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probe conductivity measurements were acquired for ≈ 150-175 nm-thick thin films on a quartz substrate.

Electrochemical studies

Cathodes were fabricated by mechanically mixing 80 wt % TiS2-x nanobelts (active material), 10 wt % polyvinylidene fluoride, PVDF (binder), and 10 wt % acetylene black (conductive additive) in N-methyl-2-pyrrolidone.

The resulting slurry was cast onto a carbon coated

aluminum (Al) foil current collector (MTI Corporation) and dried at 60 oC for 24 h under vacuum.

The cathode material was punched into ½” diameter discs.

Each disc had

approximately 7 mg of TiS2-x in the dried slurry cast. CR2032 coin cells were assembled in an argon-filled glovebox using the cathode prepared above and Li foil as the anode.

Celgard

(2400) was used as the separator. A 1.0 M LiPF6 in ethylene carbonate/diethyl carbonate (50/50 v/v) was used as the electrolyte.

Galvanostatic measurements were conducted at room

temperature using a Neware BT-4008 battery testing system. Cells were cycled between 1.0 V and 3.0 V versus lithium at 230 mA g-1.

RESULTS AND DISCUSSION

Recent advances in techniques to synthesize and isolate 2D graphene-like materials that were previously considered to be thermodynamically unstable have led to paradigm-shifting discoveries in solid-state physics. Here, we developed a synthesis approach which allows us to elucidate the effects that variations on the Ti/S stoichiometries have on the optical and electrical


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properties of TiS2 nanostructures. TiS2 is known to crystallize in a hexagonal crystal symmetry related to the layered CdI2-type. As shown in Figure 2A, TiS2 crystal structure is comprised of 2D slabs of edge-shared TiS6 octahedra that form infinite layers perpendicular to the c-axis. In the layers, strong covalently held TiS6 octahedra are stacked on top of each other and separated by a van der Waals’ gap.20, 38 This structural arrangement allows for the fabrication of 1D/2D layered TiS2 nanostructures such as nanobelts and nanowires. The controlled conversion of TiS2x

nanostructures is performed according to the following chemical reaction:

Equation 1

2 Ti + 6S 2 TiS 2TiS

First, stoichiometric TiS3 is synthesized from its elemental powders as the sacrificial template. As depicted in Figure 2B, the TiS3 monoclinic crystal structure can be described as a stack of parallel layers with each layer composed of 1D chains of trigonal prismatic TiS6 units which are held by S-S van der Waals interactions within the gaps.39,

40

Similarly to TiS2, TiS3 tends to

crystallize as anisotropic structures due to its crystal structure having two easy cleavage planes, i.e., one parallel to the crystallographic bc plane and the second parallel to the ab plane both of which ease the reduction of bulk TiS3 into belt-like nanostructures.


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Figure 2. Schematic depiction of A) hexagonal TiS2 and B) monoclinic TiS3 crystal structures.

Subsequently and crucial to the fabrication of TiS2-x nanostructures, is the balance of two competing reaction dynamics, i.e., the high temperature removal of S from TiS3 and the possible disruption of the nanostructure morphologies due to these high temperatures. Upon heating, sulfur bleaches out from the TiS3 sacrificial template. Here, a titanium source is introduced to avoid the reabsorption of sulfur by the TiS3 sacrificial template. A reaction temperature of 450 o

C enabled the conversion of TiS3 to TiS2-x while preserving the nanostructure morphology.


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Furthermore, the Ti/S ratio is carefully controlled by allowing sulfur to bleach out of the TiS3 sacrificial nanostructure over a time range between 24-144 h.

Low- and high-resolution SEM micrographs of the as-synthesized TiS3 sacrificial template are presented in Figures 3A and 3B, respectively. In all cases, we note the formation of highly facetted nanobelts with a width distribution centered at 255 ± 5 nm (based on statistical distributions obtained for 500 nanobelts, Figure S1). Also, these nanobelts show thicknesses centered at 88 ± 3 nm (from tapping mode atomic force microscopy measurements), much smaller than the average width or length. The nanobelts are clearly electron-transparent as shown in the SEM image in Figure 3B, further corroborating their thin cross-sections. Figures 3C and 3D show SEM images of the products obtained upon desulfurization of the TiS3 sacrificial template. Upon completion of the reaction, the morphology of the products is still dominated by the formation of nanobelts. From statistical distributions, these nanobelts are 253 ± 10 nm in width and 90 ± 2 nm in thickness, suggesting that there are no substantial differences in nanobelt sizes upon desulfurization.


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Figure 3. SEM images of TiS3 (A, B) and TiS2-x (C, D) nanobelts.

The single-crystalline nature of the as-synthesized TiS3 and its conversion to TiS2 are verified by the lattice-resolved HRTEM and SAED patterns presented in Figure 4. The lattice fringes observed in Figure 4A are 4.93, 2.23 and 2.04 Å, which can be indexed to the interplanar spacing of the (100), (004), and (210) planes of the TiS3 crystal structure, respectively. Figure 4B shows the SAED pattern for a TiS3 nanobelt. The SAED pattern does not change along the length of the nanobelt, underscoring its single-crystalline nature. Furthermore, the observed diffraction spots are associated with reflection planes from the TiS3 monoclinic crystal structure. The desulfurization of TiS3 to TiS2-x also yields a crystalline product as evidenced in the latticeresolved HRTEM and SAED patterns presented in Figure 4C and 4D. The fringes observed in Figure 4C are 3.45 and 1.75 Å, which can be indexed to the lattice spacing between (010) and (110) planes of the TiS2 crystal structure, respectively. The indexed SAED pattern in Figure 4D illustrates that the (110) and (010) crystallographic planes are oriented with respect to each other


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at an angle of ≈ 60o, which is consistent with a six-fold symmetry corresponding to the TiS2 hexagonal crystal structure.20

Based on the HRTEM studies, both TiS3 and TiS2-x have a

preferred growth direction along their c-axis which strongly suggest the formation of layered structures in the out-of-plane direction. Most importantly, the crystallographic changes presented in Figure 4 further corroborate the complete conversion of TiS3 to TiS2 due to the systematic control of the desulfurization process.

Figure 4. HR-TEM images of TiS3 and TiS2-x (x = 0.02) single-nanobelts. (A) Lattice-resolved HR-TEM image of an individual TiS3 nanobelt. (B) Indexed SAED pattern for TiS3. (C) Lattice fringes of TiS1.98. (D) Indexed SAED pattern for TiS1.98.

The composition, purity, and chemical binding environment of as-synthesized TiS3 and TiS2 nanobelts were further evaluated by means of XPS analyses. As shown in Figure 5A, two strong peaks at 455.9 and 462.0 eV are observed for pristine TiS3 nanobelts. These two peaks are


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assigned to Ti 2p3/2 and Ti 2p1/2, respectively, with a spin-orbit doublet splitting of 6.1 eV, matching closely the characteristic Ti4+ peaks in TiS3.41 The S 2p XPS spectrum presented in Figure 5B is less straightforward. Here, the XPS spectrum is comprised of three symmetric overlapped peaks at 161.2, 162.5, and 163.7 eV. These three peaks are assigned to S 2p1/2 of the disulfide (S22-) pairs, a S 2p3/2 and S 2p1/2 doublet of the S22- and sulfide S2- unit, and a S 2p3/2 of the S2- atom. The fitted S22-/ S2- peak intensity ratio is 2:1 which infers that TiS3 is composed of one S22- and one S2- units.41, 42 Figure 5C shows the Ti 2p XPS spectrum obtained for a TiS3 sacrificial template converted to TiS2-x. Evident changes are observed in the Ti 2p and S 2p XPS spectra upon desulfurization of TiS3 to TiS2-x. In addition to the main characteristic Ti4+ peaks, two additional small contributions at 458.0 and 464.0 eV are observed. These contributions may be attributed to the slight formation of a TiOx layer at the surface of the nanobelts upon limited air exposure during transfer of sample into the XPS chamber.43 Furthermore, the XPS spectrum presented in Figure 5D shows that the conversion of TiS3 to TiS2-x yields a well-resolved S 2p3/21/2

doublet composed of two peaks centered at 160.8 and 161.9 eV, with a spin-orbit doublet

splitting of 1.2 eV. No binding energy peaks associate with S22- are detected thus indicating the full conversion of TiS3 to TiS2-x.


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Figure 5. XPS spectra of TiS3 and TiS2-x (x = 0.02) nanobelts. (A-B) Ti 2p and S 2p regions, respectively, for TiS3. (C-D) Ti 2p and S 2p regions, respectively, for TiS1.98.

During the conversion, TiS2 seeds –due to sulfur volatilization- are formed at the surface of the TiS3 nanostructures and are propagated inward at a rate that is strongly dependent on the reaction time.

Figure 6 depicts the XRD patterns of the TiS3 sacrificial precursor upon annealing at 450

o

C for various reaction times. All experimental reflections can be assigned either to TiS3 (Joint

Committee of Powder Diffraction Standards (JCPDS) No. 15-0783) or TiS2 (JCPDS No. 150853). As shown in Figure 6, upon annealing for 24 h, the desulfurization of TiS3 yields mixed phases comprising the precursor (monoclinic TiS3) and hexagonal TiS2. After 48 h, TiS3 is fully converted to a phase that is closer to that of stoichiometric TiS2. Although our XRD studies demonstrate full conversion of TiS3 to TiS2-x, the technique is not capable of elucidating subtle


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non-stoichiometry variations in TiS2 due to vacancies and defects that occurred during sulfur volatilization. To obtain the Ti/S ratio of our samples, ICP-OES measurements as a function of annealing (desulfurization) time were performed. The Ti/S stoichiometries obtained by ICPOES measurements are also presented in Figure 6. Here, the fact that the desulfurization of TiS3 does not yield stoichiometric TiS2 is evident. This result is in close agreement to early reports by Jeannin et.al, which demonstrate the challenge of synthesizing stoichiometric TiS2.44,

45

Remarkably, the Ti/S stoichiometry does not systematically vary with desulfurization time. By desulfurizing TiS3 multiple times, we concluded that at longer desulfurization times, there is the evolution of a secondary reaction involving the desulfurized TiS3 nanobelt and the TiSx formed during the reaction of Ti powder (used as a sink to prevent sulfur from reacting with the TiS3 sacrificial template) with volatilized sulfur obtained during the pyrolysis process.

This

secondary reaction coexists with the actual formation of TiS2-x. Just as the target material (TiS2x)

could lose sulfur at these temperatures, it is also possible that the TiSx formed from the Ti

powder could lose sulfur. During the extended pyrolysis process, sulfur is rapidly lost from the TiS3 sacrificial template. Moreover, as the desulfurization of TiS3 begins to slow down, it is most likely that the sulfur adsorbed by the Ti sink is released back into the ampoule and readsorbed by the TiS3 sacrificial template causing the sulfur vacancies in the final product (TiS2-x) to fill up. However, the desulfurization approach presented in this work allows the systematic investigation of the electronic properties of TiS2 as a function of sulfur vacancies (vide infra).


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Figure 6. XRD spectra of TiS3 and various TiS2-x nanobelts as a function of desulfurization time. Calculated stoichiometries via ICP-OES analyses are presented for each desulfurization time.

To further investigate the effects that sulfur vacancies have on the electronic properties of our TiS2-x nanobelts, their band energies were probed via ultraviolet photoemission (UPS) spectroscopy studies. UPS probes occupied states at the top of the valence band and yields insights into work function (Φ) variations due to non-stoichiometries in TiS2-x.

Figure 7

displays the direct photoemission spectra of TiS3 and TiS2-x nanobelts with different sulfur


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vacancy schemes. Here, all energy levels are referenced to a common Fermi Level (0 eV). The Φ can be calculated using the following equation:

Φ (eV) = hv – (Ecutoff – EF)

(1)

Where, hv is the He I energy of 21.2 eV, Ecutoff is the half height of the secondary electron cutoff, and EF is the Fermi edge.

Calibration of the UPS measurements was conducted via the

acquisition of the UPS spectrum of a silver reference. From Figure 7A, a change in the Φ upon conversion of TiS3 to TiS2-x is observed as reflected by the variation in the Ecutoff region of the UPS spectra. Here, the Φ of TiS3 is found to be 4.70 ± 0.01 eV, while that of TiS2-x is 4.55 ± 0.01 eV. It is believed that the lower Φ value stems from the creation of sulfur defect states (i.e. sulfur vacancies) below the conduction band of TiS2. These sulfur defect states trigger the formation of donor energy states below the conduction band and shallow acceptor states near the valence band. Both states are responsible for raising the Fermi level and thus lowering the Φ. However, as shown in Figure 7A, no significant shift in the Φ of TiS2-x upon desulfurization was observed. Interestingly, the evolution of a shoulder at lower binding energies (≈ 0.94 eV) around the Fermi edge is observed (Figure 7B). This shoulder is attributed to Ti3+ 3d band defect states due to sulfur vacancies. As depicted in Figure 7B, TiS3 shows the lowest shoulder intensity suggesting that it possesses the least amount of defect states in comparison to the as-synthesized TiS2-x nanobelts. Also, it is important to note that in the case of the UPS spectra for our TiS2-x nanobelts, the shoulder intensity increases with increasing non-stoichiometry deviations. This trend has been previously reported in UPS studies of oxygen deficient TiOx structures.46,

47

However, the possibility that the feature at ≈ 0.94 eV emerges as a result of the samples being exposed to UV light during the acquisition of the UPS spectra cannot be neglected.


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Figure 7. UPS spectra of TiS3 and TiS2-x nanobelts. (A) Secondary electron cutoff for work function determination. (B) Magnification of the UPS spectra close to the Fermi level for the determination of Ti3+ 3d band defect states.

The changes in the optical bandgap as a function of sulfur vacancies were investigated via diffuse reflectance UV-vis spectroscopy studies.

The reflectance spectra of the sacrificial

template (TiS3) and different non-stoichiometric TiS2-x nanobelts are shown in Figure 8.

The

optical bandgap is calculated from the corresponding modified Kubelka-Munk function [F(R)hν]n.48 The intercept of the tangent of the inflexion point of [F(R)hν]n vs photon energy


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with the x-axis on the Tauc plots was used to estimate the bandgaps. Here, n = 2 is used to determine the bandgap for a material having a direct interband transition. As presented in Figure 8A, TiS3 nanobelts show two direct transitions at 1.22 ± 0.01 eV and 1.73 ± 0.01 eV. These direct transitions correspond to first and second excitonic states, respectively.49, 50 Figure 8B illustrates the optical bandgap of various non-stoichiometric TiS2-x nanobelts upon desulfurization.

Remarkably, the optical bandgap is very sensitive to non-stoichiometry

variations. As shown in Figure 8B, the optical bandgap is narrowed from 0.86 ± 0.01 eV to 0.65 ± 0.01 eV as more sulfur vacancies are present in the nanobelts.

Figure 8. Kubelka-Munk function versus photon energy for the UV-vis-NIR diffuse reflectance spectra of (A) TiS3 and (B) TiS2-x nanobelts.


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It is well documented that excess Ti atoms tend to migrate into the van der Waals gap of TiS2 leading to a partial filling of the Ti (3d) conduction band and to a degenerate extrinsic electrical behavior.51 Notwithstanding, little attention has been devoted to elucidating sulfur defect effects in TiS2 due to the challenges associated with addressing the nature of the extrinsic carriers and stoichiometries responsible for either its metallic or semiconducting behavior. Due to the control of the non-stoichiometry levels in TiS2-x upon desulfurization of TiS3 nanobelts, the electrical properties of TiS2-x nanobelts as a function of sulfur vacancies were effectively determined. Figure 9 shows the electrical properties of TiS3 and TiS2-x nanobelts. Figure 9A depicts the Hall electrical conductivity of the sacrificial template, TiS3. As observed, the electrical conductivity increases with increasing temperature thus corroborating the semiconducting nature of TiS3. The Hall Effect studies reveal n-type conductivity at all temperatures with a maximum electrical conductivity value of 0.27 S cm-1 at 500 K. Likewise, the quasi-linear dependence of the electrical conductivity on temperature indicates that the conduction in TiS3 is mainly dominated by a thermally activated process that can be explained by an Arrhenius-type behavior. As per the Arrhenius equation,48 an activation energy (Ea) of 1.3 eV is obtained for TiS3 nanobelts which is in agreement with the experimental Eg value of 1.22 eV obtained by diffuse reflectance experiments.

Figure 9B shows the electrical conductivity as a function of temperature for non-stoichiometric TiS2-x nanobelts. Similarly to TiS3, the Hall coefficients for TiS2-x at all temperatures are negative suggesting n-type conduction. As depicted in Figure 9B, the electrical conductivity decreases with increasing temperature indicating a metallic behavior for the TiS2-x nanobelts. Analysis shows that the electrical conductivity increases gradually with increasing sulfur


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vacancies with a maximum electrical conductivity value of 3.5 S cm-1 at 150 K for TiS1.85 nanobelts. The fact that the electrical conductivity increases with increasing non-stoichiometries is not surprising since higher density of sulfur vacancies in the TiS2-x nanobelts should lead to an increase in the n-type doping level in accordance to density functional theory (DFT) calculations performed on titanium sulfide compounds.52 The introduction of sulfur defect states close to the Fermi level is likely responsible for the increase in the metallic character of TiS2-x defect systems.

Figure 9. Ensemble electrical transport measurements for (A) TiS3 and (B) TiS2-x nanobelts.


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Motivated by the fact that we can control sulfur vacancies in TiS2-x nanobelts as well as the fabrication of a sufficiently highly conductive cathode insertion host material for lithium-ion batteries, CR2032 coin cells were assembled and a galvanostatic charge-discharge technique was employed to evaluate the electrochemical properties of TiS2-x nanobelts having two different sulfur stoichiometries (i.e., TiS1.85 and TiS1.98).

Figure 10 shows the electrochemical

performance for TiS1.85 and TiS1.98 nanobelts in a half-cell configuration (Li/TiS2-x). Figure 10A shows charge-discharge voltage profiles for a coin cell comprising TiS1.98 as the cathode active material. The coin cell was ran for 100 cycles at a current density of 230 mA g-1 between 3.0 and 1.0 V. The TiS1.98 nanobelt electrode demonstrates a specific capacity of ≈ 192 mAh g-1 during its first charge/discharge cycle.

Moreover, we observe a very monotonic charge-

discharge curve suggesting a single phase reaction between Li and TiS1.98 to yield LixTiS1.98 solid solutions. Theoretical calculations for the formation of LiTiS2 indicate that a capacity of 238 mAh/g should be obtained. However, in our study, we only observed a capacity of 192 mAh/g. This is 80.6% of the theoretical maximum. This would indicate that only 0.8 Li atom per TiS2 is present in the discharged cathode. The cause could be sulfur vacancies that make the Ti3+ atom no longer electrochemically active or diffusion limitations as the cathode nears saturation. However, at this current rate, ≈ 0.8 moles of Li is inserted into the TiS1.85 lattice. We also observe an irreversible capacity loss of ≈ 4.5 mAh g-1 in the first cycle. This initial capacity loss is commonly observed in intercalation type materials due to the formation of a solid-electrolyte interphase layer. From the cycling performance profile (Figure 10B), it can be seen that the TiS1.98 nanobelt electrode maintain a high capacity after 100 cycles (183 mAh g-1, 95 % of the initial capacity) with almost no fading which suggest that this insertion host is extremely stable upon repeated charge-discharge cycles. Furthermore, between the voltage windows of 1-3 V, the


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coulombic efficiency remains close to 100% during the charge-discharge cycling tests, suggesting good reversibility for the lithiation/de-lithiation process.

To investigate the

correlation between morphology changes in TiS1.98 cathodes and cell performance, cells operated after 100 cycles were disassembled and their morphologies were investigated via scanning electron microscopy (Figure S2). Based on the SEM micrographs acquired before and after 100 charge/discharge cycles, we do not observe any significant change in the morphology of our cathodes. This is a clear evidence of the morphological stability of the cathodes upon cycling. Conversely, if coin cell batteries are assembled using a more sulfur deficient TiS2-x cathode insertion host (ca. TiS1.85), a specific capacity of ≈ 225 mAh g-1 during the first charge/discharge cycle is obtained (Figure 10C). This represents a 17 % increase in the specific capacity of TiS2 which we believe might stem from increasing the electrical properties of TiS2-x upon the incorporation of sulfur vacancies. As shown in Figure 10D, the capacity retention is still over 90 % after 100 cycles. This corresponds to a very small capacity fade of 0.1 % per cycle with a Coulombic efficiency above 99 %.

Besides increasing the electrical properties of TiS2-x

nanobelts by incorporating sulfur vacancies, we believe the very small volume change (≈ 2 %, obtained via in operando XRD measurements, Figure S3) during the lithiation and de-lithiation processes also aids in improving the cycling performance of the coin cell battery.


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Figure 10. Electrochemical performance for coin cell batteries comprising TiS1.85 and TiS1.98 as cathode materials. A) Galvanostatic charge and discharge curves for TiS1.98 electrodes. B) Cycling performance and Coulombic efficiency for TiS1.95 electrodes charge/discharged at 230 mA g-1. C) Galvanostatic charge and discharge curves for TiS1.85 electrodes.

D) Cycling

performance and Coulombic efficiency for TiS1.85 electrodes charge/discharged at 230 mA g-1


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CONCLUSIONS This work demonstrates the synthesis, characterization, and electronic properties of TiS2-x nanobelts by the controlled nondestructive removal of sulfur from TiS3 sacrificial templates. The desulfurization approach provided here allows for the systematic control of sulfur deficiencies in TiS2 nanostructures with almost no detrimental effect on the 1D morphological nature of the synthesized nanobelts. Optical studies reveal the narrowing of TiS2-x bandgap with increasing sulfur vacancies. Transport measurements confirm the metallic nature of the converted TiS2-x nanobelts regardless of the stoichiometry. A more pronounced metallic behavior is obtained with increasing sulfur vacancies in TiS2-x nanobelts.

Although these studies validate the

challenges associated with the fabrication of stoichiometric TiS2, we do believe nonstoichiometric TiS2-x appears to remain semi-metallic even at high sulfur vacancy concentrations. Additionally, due to its low charge transfer resistance (high electrical conductivity), high Li-ion diffusivity, and excellent structural stability, non-stoichiometric TiS2-x nanobelts possess the following electrochemical properties, (i) a high specific capacity of 225 mAh g-1, which is close to the theoretical specific capacity for TiS2 (239 mAh g-1); (ii) excellent capacity retention; and (iii) stable cycle performance at 230 mA g-1 over 100 cycles. Our findings provide fundamental insights for guiding the rational design of other non-stoichiometric metal chalcogenide cathode materials that could pave the realization of high power, stable, and long cycling lithium-ion batteries.


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SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI:xx.xxxx/acsami.xxxxxx. Width distributions observed in TiS3 and TiS1.98 nanobelts; SEM images of the cathode layer before and after 100 charge/discharge cycles; In operando XRD measurements performed on fully a lithiated/de-lithiated TiS1.85 coin cell battery. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the NSF MRSEC program at the University of Utah under grant # DMR 1121252.

CGH acknowledges funding from the Utah Governor’s Office of Energy

Development. LWB would also like to acknowledge the financial support from the Marion Milligan Mason Award for Women in the Chemical Sciences administered by the American Association for the Advancement of Science (AAAS).


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51. Amzallag, E.; Baraille, I.; Martinez, H.; Rérat, M.; Loudet, M.; Gonbeau, D., Electronic and Structural Properties of Ti Vacancies on the (001) Surface of TiS2: Theoretical Scanning Tunneling Microscopy Images. J. Chem. Phys. 2007, 126, 074703. 52. Iyikanat, F.; Sahin, H.; Senger, R. T.; Peeters, F. M., Vacancy Formation and Oxidation Characteristics of Single Layer TiS3. J. Phys. Chem. C 2015, 119, 10709-10715.

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