Highly Conductive Porous Transition Metal Dichalcogenides via Water

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Highly Conductive Porous Transition Metal Dichalcogenides via Water Steam Etching for High-Performance Lithium−Sulfur Batteries Zhubing Xiao,† Zhi Yang,*,‡ Liujiang Zhou,† Linjie Zhang,† and Ruihu Wang*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ Nanomaterials & Chemistry Key Laboratory, Wenzhou University, Wenzhou 325027, China S Supporting Information *

ABSTRACT: Lithium−sulfur (Li−S) batteries show significant advantages for nextgeneration energy storage systems owing to their high energy density and cost effectiveness. The main challenge in the development of long-life and high-performance Li−S batteries is to simultaneously facilitate the redox kinetics of sulfur species and suppress the shuttle effect of polysulfides. In this contribution, we present a general and green water-steam-etched approach for the fabrication of H- and O-incorporated porous TiS2 (HOPT). The conductivity, porosity, chemisorptive capability, and electrocatalytic activity of HOPT are enhanced significantly when compared with those of raw TiS2. The synthetic method can be expanded to the fabrication of other highly conductive transition metal dichalcogenides such as porous NbS2 and CoS2. The asobtained HOPT can serve as both a substitute of conductive agents and an additive of interlayer materials. The optimal electrode delivers discharge capacities of 950 mA h g−1 after 300 cycles at 0.5 C and 374 mA h g−1 after 1000 cycles at 10 C. Impressively, an unprecedented reversible capacity of 172 mA h g−1 is achieved after 2500 cycles at 30 C, and the average capacity fading rate per cycle is as low as 0.015%. Importantly, four half-cells based on this electrode in series could drive 60 light-emitting diode indicator modules (the nominal power 3 W) after 20 s of charging. The instantaneous current and power of this device on reaching 275 A g−1 and 2611 W g−1, respectively, indicate outstanding high-power discharge performance and potential applications in electric vehicles and other large-scale energy storage systems. KEYWORDS: lithium−sulfur batteries, polysulfides, shuttle effect, ultrahigh rate, titanium disulfide

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INTRODUCTION The ever-increasing demand for electric vehicles, portable electronics, smart grids, and energy storage systems has dramatically promoted the development of high-energy-density rechargeable batteries with good cycling performance.1,2 Lithium−sulfur (Li−S) batteries are considered to be one of the most promising candidates owing to their high theoretical specific capacity of 1675 mA h g−1.3,4 Additionally, sulfur has the advantages of low cost, natural abundance, environmental benignity, and nontoxicity. However, the insulating nature of solid sulfur compounds and the notorious shuttle effect of soluble polysulfide (PS) intermediates usually result in low utilization of sulfur, poor cycling life, and severe self-discharge; these technical challenges greatly impede the practical application of Li−S batteries.5−8 Various strategies have been exploited to circumvent the above-mentioned issues. One of the recently emerging approaches is the integration of conductive carbon materials [graphene, carbon nanotubes (CNTs), porous carbon, etc.] and polar sulphilic materials (metal oxides, metal chalcogenides, metal/covalent organic frameworks, etc.) to localize sulfur.9−19 Carbon materials in these composites serve as sulfur hosts to enhance the electrical conductivity and accommodate active sulfur species. The incorporation of polar materials can © 2017 American Chemical Society

synergetically trap PS species via strong chemical binding, thus greatly mitigating the dissolution and shuttle effect of soluble PS.20 Despite remarkable advances, the high-power performance of these Li−S batteries is still far from satisfactory; most Li−S batteries suffer from significant capacity fading after long-term cycling (>500 cycles) at high rates. Therefore, it is an urgent task to seek new polar materials for improving the overall performance of Li−S batteries at high rates, especially beyond 10 C. As a popular class of polar materials with a graphene analogue structure, transition metal dichalcogenides (TMDs) have attracted increasing interest from the academia and industry because of their unique physical properties, high structural stability, and rich transition metal d-electrons.21−23 Attractively, their incorporation into nanoelectronic devices and energy storage devices has been reported to bring out new fascinating performance that can be competitive with and even superior to that of graphene.23 Among the layered TMDs, titanium disulfide (TiS2) has the lightest weight, low cost, high stability, and good binding affinity to PS.24−27 Recently, the use Received: March 24, 2017 Accepted: May 19, 2017 Published: May 19, 2017 18845

DOI: 10.1021/acsami.7b04232 ACS Appl. Mater. Interfaces 2017, 9, 18845−18855

Research Article

ACS Applied Materials & Interfaces of TiS2 as either porous reservoirs for sulfur compounds28 or encapsulation shells for Li2S29 has been reported, but their rate performance is still unsatisfactory. The main reason for the issues may be rooted in the relatively low electrical conductivity of TiS2 and the insufficient ability to suppress the shuttle effect of PS, which motivates us to explore new technologies that will synchronously improve the conductivity and sulfiphilicity of TiS2 and other TMDs. As far as tailoring the properties and enhancing the performance of these materials is concerned, surface functionalization by chemical approaches is one of the most important and effective means. Similar to graphene modification, the electrical conductivity and textural properties of TiS2 can be mediated through surface oxidation, edge passivation, and molecular functionalization.30−32 The optimization of porosity and enhancement of electrical conductivity are beneficial for both adsorption of sulfur species and fast electronic/ionic transport in Li−S batteries. Inspired by these facts, herein, we present a facile and environment-benign strategy for the modification of commercially available TiS2, based on watersteam-etched technology. This method possesses excellent generality and can be extended to other TMDs such as CoS2 and NbS2. Particularly, the use of as-obtained H- and Oincorporated porous TiS2 (HOPT) in Li−S batteries shows attractive advantages: (1) the enhanced electrical and ionic conductivities of HOPT can greatly improve the electrochemical kinetics of the Li−S redox reaction; (2) the enhanced porosity of HOPT can provide more tunnels for fast ion transport and electrolyte accessibility, which allows for high-rate performances; (3) the enhanced polarity and the heteropolar surface active sites in HOPT increase its chemical binding affinity for PS, which can alleviate the PS shuttle effect without disturbing Li+ ion transfer; and (4) as an electrochemically active component, HOPT not only is capable of undergoing reversible lithium insertion within the working voltage of sulfur but also can directly replace conductive agents, thus increasing the overall energy density of Li−S batteries. By virtue of the aforementioned preponderances, we choose HOPT as both a partial substitute of conductive agents and an additive of interlayer materials and present a promising cell configuration involving a CNTs−S/HOPT cathode coupled with a lightweight graphene/HOPT interlayer. The optimal electrode shows high rate capacities and excellent cycling stability; it delivers a superb reversible capacity of 172 mA h g−1 over 2500 cycles at 30 C with an average capacity decay rate of 0.015%. The remarkable rate performance at an ultrahigh rate of 30 C is superior to that reported in Li−S batteries.

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homogeneous slurry. The slurry was cast onto an aluminum foil with a spreader and dried in a vacuum oven at 60 °C overnight. The slurry-coated aluminum foils were pressed with a roller machine; circular pieces with a diameter of 14.0 mm were then punched out to generate HOPT electrodes. Synthesis of CNTs−S Composite. The CNTs−S composite was prepared by following a melt−diffusion method. In a typical procedure, a mixture of CNTs and sulfur was ground and heated at 160 °C for 12 h, followed by heating at 180 °C for another 12 h. CNTs−S composite was obtained after the resultant mixture was cooled to room temperature. Synthesis of CNTs−S and CNTs−S/300HOPT Electrodes. CNTs−S (80%), 10% acetylene black, and 10% PVDF were dispersed in NMP to form a homogeneous slurry. The slurry was pasted onto an aluminum foil with a spreader and dried in a vacuum oven at 60 °C overnight. The slurry-coated aluminum foils were pressed with a roller machine; circular pieces with a diameter of 14.0 mm were then punched out to generate CNTs−S electrode. The CNTs−S/ 300HOPT electrode was prepared using the same procedure, except that 5% part of acetylene black was replaced by 300HOPT. Preparation of CNTs−S/300HOPT@G/300HOPT and CNTs− S/300HOPT@G Electrodes. A double slurry-coating method was used according to the modified literature methods.18 Typically, graphene was ultrasonically dispersed in NMP, and 300HOPT was added under vigorous stirring to form a graphene/300HOPT slurry. The slurry was pasted onto the surface of CNTs−S/300HOPT and dried in a vacuum oven at 60 °C overnight. The slurry-coated aluminum foils were pressed with a roller machine; circular pieces with a diameter of 14.0 mm were then punched out to generate CNTs−S/ 300HOPT@G/300HOPT electrode, where the areal weight of the G/ 300HOPT interlayer is 0.08 mg cm−2. CNTs−S/300HOPT@G was prepared using the same procedure in the absence of 300HOPT in the G/300HOPT slurry. Electrochemistry Measurements. Electrochemical experiments were performed via CR2025 coin-type test cells assembled with the lithium metal as the counter and reference electrodes in an argon-filled glovebox, where both moisture and oxygen levels were kept below 1.0 ppm. Celgard 2400 membrane was used as the separator to isolate the electrons. The electrolyte was 1 mol L − 1 lithium bis(trifluoromethane)sulfonimide (LiTFSI) with 1% anhydrous LiNO3 dissolved in 1,3-dioxolane/dimethoxyethane (DOL/DME, 1:1 by volume). The electrolyte/sulfur ratio was about 10 mL g−1. The discharge−charge measurements were conducted at a voltage interval of 1.5−3.0 V using a Neware battery test system (Neware Technology Co.). Before testing, the cells were aged for 24 h. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI604E electrochemical workstation at a scan rate of 0.1 mV s−1. The direct current voltage was kept at opencircuit voltage, and an alternating current voltage of 5 mV in amplitude was applied with a frequency of 200 kHz to 20 mHz. Visualized Li2S6 Adsorption Test. An Li2S6 stock solution of 10 mmol L−1 was first prepared through dissolving Li2S and sulfur in a molar ratio of 1:5 in DOL/DME (1:1 by volume) in an argon-filled glovebox. Graphene, CNTs, TiS2, 300HOPT, and graphene/ 300HOPT composite with the same surface area were added into 15.0 mL of Li2S6 solution separately. The resultant mixture was stirred at room temperature for 30 min to ensure thorough adsorption. The adsorption ability was visually detected by the color change of the resultant solution, and blank Li2S6 solution in DME/DOL was used as the reference. Symmetrical Cell Assembly and Measurements. The electrodes for symmetrical cells were fabricated in the absence of sulfur according to the reported method.13 Typically, each electrode material (commercial graphene, CNTs, TiS2, 300HOPT, and commercial graphene/300HOPT composite) and PVDF binder at a weight ratio of 4:1 were dispersed in NMP. The mixture was stirred at room temperature to form a homogeneous slurry. The slurry was coated on aluminum foils with a spreader and dried in a vacuum oven at 60 °C overnight. Electrode disks with a diameter of 14.0 mm were punched out of slurry-coated aluminum foils. These disks were used as identical

EXPERIMENTAL SECTION

Preparation of HOPT. Five hundred milligrams of commercially available TiS2 (Alfa Aesar, 99.9%) and 50 agate balls of 6 mm diameter were placed into a 100 mL agate vial in a planetary ball mill. After ball milling under argon for 1 h, the powder was collected and placed into a quartz tube without further modification. A mist of droplets from ultrapure water was passed into a quartz tube containing the ballmilled TiS2 using an Ar-carrying gas at different temperatures. After water steam etching for appropriate time and cooling to room temperature under argon flow, the powders of HOPT were collected and denoted as 100HOPT, 200HOPT, 300HOPT, 400HOPT, and 500HOPT when TiS2 was etched at 100, 200, 300, 400, and 500 °C, respectively. Preparation of HOPT Electrodes. HOPT (80%), 10% conductive carbon, and 10% poly(vinylidene difluoride) (PVDF) were dispersed in N-methylpyrrolidinone (NMP) to form a 18846

DOI: 10.1021/acsami.7b04232 ACS Appl. Mater. Interfaces 2017, 9, 18845−18855

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ACS Applied Materials & Interfaces working and counter electrodes, and 40 μL of the electrolyte containing 0.5 mol L−1 Li2S6 and 1.0 mol L−1 of LiTFSI was added. A control test was also performed using the same electrolyte without Li2S6.

with the increase of etching temperature from 100 to 500 °C, in accordance with the progressive increment of their pore sizes (Figure 2b). The specific surface area (SSA) and pore volume (PV) of HOPT also increase with the increase in the etching temperature and range from 12.7 to 47 m2 g−1 and from 0.075 to 0.25 cm3 g−1, respectively (Table S1). These values are much higher than the corresponding 9.6 m2 g−1 and 0.037 cm3 g−1 of TiS2. These results reveal that water steam etching is a feasible method to improve the porosities of HOPT. The enhanced SSA and PV can provide fast ion transport channels in the processes of charge and discharge, thus giving rise to the possibility of high rate performance. Notably, the modification and functionalization of TiS2 could also be tuned by varying the etching time. When the etching time at 300 °C is elongated from 30 to 45 and 60 min, obvious increments of SSA and PV are achieved. The chemical reaction between water steam and TiS2 is further confirmed by the weight loss profiles of TiS2 (Figure S1). In comparison with that of ball-milled TiS2, remarkably high weight losses are displayed after TiS2 etched by water steam in the temperature range from 100 to 500 °C and then heated at the same temperature. The large weight loss is mainly attributed to the substitution of sulfur atoms in TiS2 by oxygen atoms or hydroxyl groups. The perforation of TiS2 during water steam etching was confirmed by the transmission electron microscopy (TEM) analysis of the representative 300HOPT. In sharp contrast with the continuous and smooth surface of TiS2 (Figure 2c), rugged caves and trenches can be identified clearly in the TEM images of 300HOPT (Figures 2d and S2); these defects are distributed heterogeneously on the surface and edge of 300HOPT. In addition, the high-resolution TEM images of TiS2 and the corresponding SAED patterns show long-range ordered lattice fringes and a perfect hexagonal crystallographic lattice, respectively (Figure 2e), whereas 300HOPT possesses a large amount of lattice defects, and its SAED pattern also evolves into discontinuous rings with scattered spots (Figure 2f). Water steam etching also exerts important effects on the chemical composition and oxidation state of TiS2. X-ray photoelectron spectroscopy (XPS) survey spectra of HOPTs

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RESULTS AND DISCUSSION The synthetic procedures for HOPT are shown in Figure 1. Mechanical milling of commercially available TiS2 under argon

Figure 1. Schematic illustration for the synthesis of HOPTs.

gave rise to particle size-reduced TiS2, and the subsequent chemical reaction between ball-milled TiS2 and nebulized water steam under dynamic argon gas in a relatively low temperature range from 100 to 500 °C for 30 min generated HOPTs. When the reaction of water steam etching was performed at 100, 200, 300, 400, and 500 °C, the resultant HOPTs were denoted as 100HOPT, 200HOPT, 300HOPT, 400HOPT, and 500HOPT, respectively. Time and temperature of water steam etching play pivotal roles in modulating the porosity of HOPT. As shown in Figure 2a, the nitrogen sorption isotherms of these HOPTs show a typical IV type behavior with hysteresis at relatively high pressures.33 The hysteresis loops exhibit an increasing trend

Figure 2. (a) Nitrogen sorption isotherms at 77 K and (b) pore size distribution profiles of TiS2 and HOPTs. TEM images and selected area electron diffraction (SAED) analyses (inset) for (c,e) TiS2 and (d,f) 300HOPT. 18847

DOI: 10.1021/acsami.7b04232 ACS Appl. Mater. Interfaces 2017, 9, 18845−18855

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Figure 3. (a) Ti 2p XPS spectra, (b) Fourier transform infrared (FT-IR) spectra, (c) X-ray diffraction (XRD) patterns, and (d) Raman spectra of TiS2 and HOPT. (e) Side-viewed atomic structure of mimic HOPT crystal. Brown, yellow, red, and gray spheres represent Ti, S, O, and H atoms, respectively. (f) Density of states (DOS) of pristine (gray), oxygen-substituted (blue), and electron-doped TiS2 nanosheets with an electron doping concentration of 6.25 × 1013 cm−2, calculated using density functional theory (DFT) + U with Fermi level set to zero.

2850 and 2920 cm−1, the new peaks at 3440 and 2498 cm−1 in the spectra of HOPTs validate the formation of S−H and O−H bonds, respectively.30,35 Notably, 300HOPT shows the strongest S−H vibration peak. When the etching temperatures are further increased from 300 to 400 and 500 °C, the S−H vibration peak attenuated gradually, suggesting a correlative decrement in the S−H concentration in HOPTs at higher temperatures. The enhanced conversion from Ti−OH to Ti−O at 400 and 500 °C is probably responsible for the higher oxygen content and less hydrogen atoms in 400HOPT and 500HOPT. Although hydrogen and oxygen atoms are introduced into the S−Ti−S framework during water steam etching, the structural framework of TiS2 is maintained, as supported by the powder XRD patterns and Raman spectra of HOPTs. The XRD pattern of TiS2 can be indexed to the 1T-TiS2 phase (space group P3̅m1; JCPDS # 88-1967) (Figure 3c). The diffraction peaks of TiS2 can be observed clearly in the XRD pattern of HOPTs. In addition, 300HOPT, 400HOPT, and 500HOPT provide two new diffraction peaks at 25.3° and 38.3°, which are assigned to the (101) and (004) lattice planes of anatase TiO2, respectively.36,37 The characteristic peak of the TiO2 phase also occurs at 147 cm−1 in the Raman spectra of 300HOPT, 400HOPT, and 500HOPT (Figure 3d).36 Two prominent peaks corresponding to the in-plane Eg (234 cm−1) and out-plane A1g (330 cm−1) modes of 1T-TiS2 can be observed clearly in HOPTs.30 Furthermore, such two peaks between HOPTs and TiS2 have no noticeable differences, further indicating that water steam etching has not destroyed the structural framework of TiS2. Benefiting from the structural stability after the injection of extra electrons, the conductivity of TiS2 could be regulated by changing the time and temperature of water steam etching, thereby providing a promising method for enhancing the electrical conductivity of TiS2. On the basis of the aforementioned observations, a plausible mechanism for water steam etching of TiS2 is proposed (Figure 4). First, the nucleophilic attack of water molecules occurs on the surface of TiS2 owing to a strong affinity between Ti4+ sites and the lone pair electrons of oxygen in water. One hydrogen atom of each water molecule gradually transfers to a

show a visible O 1s peak in comparison with that of TiS2 (Figure S3). The atomic percentages of oxygen in 100HOPT, 200HOPT, 300HOPT, 400HOPT, and 500HOPT are 2.2, 4.8, 7.0, 8.4, and 11.1% (Table S1), respectively, indicating a monotonic increment in the oxygen content along with the etching temperature. The high-resolution Ti 2p XPS spectra of HOPTs consist of two pairs of doublet peaks, suggesting a mixed valence state of titanium (Figure 3a). As for 100HOPT and 200HOPT, the binding energy peaks at 464.5 and 459.0 eV correspond to Ti4+ 2p1/2 and Ti4+ 2p3/2, respectively,30,34 which are identical with those of TiS2. Apart from these known peaks, there are two additional weak peaks at 462.1 and 455.8 eV assignable to Ti3+ 2p1/2 and Ti3+ 2p3/2, respectively, indicating that exotic electrons are injected into the S−Ti−S framework during water steam etching. Notably, Ti3+ 2p3/2 and Ti4+ 2p3/2 peaks in the spectra of 300HOPT, 400HOPT, and 500HOPT are shifted toward a lower binding energy region owing to their high oxygen contents. It is known that the titanium element only has two valence states of Ti3+ and Ti4+, and Ti4+ is readily reduced into Ti3+ through a simple electron transfer process. In our case, the total valence of HOPTs is compensated by the introduction of thiol and hydroxyl groups. Because the precise quantitative determination of hydrogen atoms is still practically difficult, the content of hydrogen atoms in HOPTs can be considered to be associated with the relative amount of Ti3+. The ratios of Ti4+/Ti3+ in 100HOPT, 200HOPT, 300HOPT, 400HOPT, and 500HOPT are calculated to be 52.34, 18.24, 11.85, 22.41, and 25.39, respectively. The lowest ratio of Ti4+/ Ti3+ in 300HOPT indicates that it has the highest hydrogen content. The presence of hydrogen atoms after water steam etching is also validated by the broad signals in the 1H solidstate magic-angle spinning nuclear magnetic resonance (MASNMR) spectra of HOPTs (Figure S4). Moreover, a slight position shift toward a low field is observed with the increase in the etching temperature, which is mainly attributed to higher etching temperatures leading to the introduction of more oxygen atoms in the S−Ti−S framework. This is further evidenced by the FT-IR spectra of HOPTs and TiS2 (Figure 3b). Apart from the peaks of the S−Ti stretching vibration at 18848

DOI: 10.1021/acsami.7b04232 ACS Appl. Mater. Interfaces 2017, 9, 18845−18855

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the effect of water steam etching on the electrical conductivity of HOPTs, the electrical resistivity properties of HOPTs were examined. As shown in Table S1, the experimental electrical resistivities of HOPTs are in the range of 9.10 × 10−4 to 3.12 × 10−5 Ω·m at room temperature, which are lower than that of TiS2 (9.55 × 10−4 Ω·m) and acetylene black (1.20 × 10−4 Ω· m). Among them, 300HOPT possesses the lowest electrical resistivity of 3.12 × 10 −5 Ω·m, suggesting that the conductivities of HOPTs are closely related to the degree of water steam etching. When the etching temperature of TiS2 increases from 100 to 300 °C, more exotic electrons are introduced into the S−Ti−S framework, resulting in an obvious enhancement in the electron concentrations and subsequent electrical conductivity. However, when the etching temperature is higher than 300 °C, severe oxidation results in the conversion of Ti−OH to Ti−O and corresponding decrement of Ti3+, thus bringing out low electrical conductivities in 400HOPT and 500HOPT. The presence of Ti3+ induces an enhanced electrical conductivity that guarantees a high-speed electron transformation and high sulfur utilization in the whole electrode. Moreover, Ti3+ is derived from the H incorporation during water steam etching of TiS2. The hydroxyl and thiol groups in HOPTs are favorable for adsorbing the PS species, suppressing the shuttle effect and improving the cycle stability. To better understand the effects of the simultaneous introduction of hydrogen and oxygen in the S−Ti−S framework on the conductivity of HOPTs, DFT calculations were performed on pristine TiS2 and oxygen-substituted TiS2 monolayers (see Supporting Information for details). The effect of hydrogen incorporation was evaluated in the electron doping model. As shown in Figure 3e,f, the substitution of each of the 32 sulfur atoms with one oxygen atom in TiS2 slightly decreases the DOS. After doping with an electron concentration of 6.25 × 1013 cm−2, the electron concentrations in the conduction bands of pristine TiS2 and oxygen-substituted TiS2 increase, which confirms that the incorporation of hydrogen can enhance the conductivity of HOPTs. The electrical resistance was also

Figure 4. Plausible mechanism for water steam etching of TiS2.

neighboring sulfur atom, synchronously forming one S−H bond and one O−H bond on Ti4+ sites. Subsequent rearrangement of hydrogen atoms through hydrogen transfer from the OH to SH group induces the formation of the Ti−O bond and the cleavage of the S−Ti bond with concomitant release of the H2S molecule. In this step, high temperature can result in severe water oxidation of TiS2, as evidenced by the weight loss profiles of TiS2 (Figure S1) and the high oxygen content at higher etching temperatures (Table S1), which can be mainly attributed to the intermediates containing the S−H group being metastable at higher etching temperatures as the energy of such configurations is much higher than that of the pristine S−Ti−S framework.22,38 The consecutive oxygenation by water induces structural evolution from TiS2 to TiO2, during which sulfur atoms are replaced gradually by iso-electronic oxygen atoms from H2O. This is supported by the XRD patterns (Figure 3c) and Raman spectra of TiS2 and HOPTs (Figure 3d). Water steam etching of TiS2 donates extra electrons into the S−Ti−S framework; these exotic electrons can induce the reinforcement of electron−electron correlations, resulting in a consequent improvement of electrical conductivity. To validate

Figure 5. (a) CV profiles of CNTs−S/300HOPT@G/300HOPT for the initial four cycles. (b) CV profiles, (c) galvanostatic charge−discharge profiles at 0.2 C, and (d) rate performance profiles of CNTs−S, CNTs−S/TiS2, CNTs−S/300HOPT, CNTs−S/300HOPT@G, and CNTs−S/ 300HOPT@G/300HOPT electrodes. 18849

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Figure 6. (a) Cycling performance of CNTs−S, CNTs−S/TiS2, CNTs−S/300HOPT, CNTs−S/300HOPT@G, and CNTs−S/300HOPT@G/ 300HOPT electrodes at 10 C. (b) Cycling performance of the CNTs−S/300HOPT@G/300HOPT electrode at 30 C. (c) Digital photographs of 60 white, blue, green, and red indicators composed of 2835 LED modules powered by four lithium batteries in series.

300HOPT. The sulfur loading in the CNTs−S composite is about 63 wt % (Figure S6a). The cross-sectional SEM image of CNTs−S/300HOPT@G/300HOPT shows a typical threelayered structure (Figure S6b), and 300HOPT flakes anchored on the interconnected graphene sheets can be visualized directly in the top-view SEM images and the corresponding elemental mappings (Figure S7). The measured thicknesses of the G/300HOPT interlayer and the CNTs−S/300HOPT cocathode are approximately 2 and 35 μm, respectively. The weight of the interlayer is determined by weighing the electrode with and without the double slurry-coating.18 The G/ 300HOPT interlayer occupies about 8 wt % in the integrated electrode after excluding the weight of the aluminum foil. Notably, such weight percentage is lighter than those in most of the reported free-standing interlayers.39−42 As control experiments, we also fabricated CNTs−S, CNTs−S/TiS2, CNTs−S/ 300HOPT, and CNTs−S/300HOPT@G electrodes (Figure S8). The electrochemical performance of CNTs−S/300HOPT@ G/300HOPT was initially evaluated using CV in the voltage range from 1.5 to 3.0 V at a scan rate of 0.1 mV s−1. As shown in Figure 5a, two main reduction peaks at 2.32 and 2.08 V are presented during the first cathodic scan, which corresponds to the reduction of element sulfur to soluble PS and the ultimate transformation to solid-state Li2S2/Li2S, respectively. In the

calculated directly from these values (Table S2). The electrical resistivities of pristine TiS2 and oxygen-substituted TiS2 monolayers are 0.28 and 2.20 Ω·m, respectively. After electron doping, their electrical resistivities are deceased markedly to 5.50 × 10−6 and 5.60 × 10−6 Ω·m, respectively, almost 5 orders of magnitude lower than those of pristine and oxygensubstituted TiS2 nanosheets. When the electron doping concentration is increased from 6.25 × 1013 to 9.95 × 1014 cm−2, the electrical resistivity of pristine TiS2 further drops to as low as 2.80 × 10−6 Ω·m. These theoretical electrical resistivities are in agreement with the experimental results, and this reveals that the improvement of electrical conductivity in HOPTs results from the incorporation of hydrogen and oxygen during water steam etching. Moreover, the electrical conductivity can be modulated through variation of the etching time and temperatures. To explore the practical application of HOPTs in Li−S batteries, HOPTs were used as both substitute of acetylene black and additive of interlayer materials. A cell containing a CNTs−S/300HOPT cocathode coupled with a graphene (G)/ 300HOPT hybrid interlayer was fabricated with a lithium foil as the counter electrode (Figure S5). Typically, G/300HOPT interlayer with 5 wt % 300HOPT was overlaid on the CNTs− S/300HOPT cocathode via a double slurry-coating method.35 The electrode is referred as CNTs−S/300HOPT@G/ 18850

DOI: 10.1021/acsami.7b04232 ACS Appl. Mater. Interfaces 2017, 9, 18845−18855

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However, CNTs−S and CNTs−S/300HOPT electrodes show a serious polarization (Figure S9b,c), and the discharge plateaus diminish when the current is higher than 10 C. These observations reveal that CNTs−S/300HOPT@G/300HOPT possesses enhanced electrochemical kinetics and good electrolyte accessibility for charge−discharge products at high rates. The remarkable cycle stability of CNTs−S/300HOPT@G/ 300HOPT is illustrated in Figures 6a and S10. When charge− discharge processes are performed over 300 cycles at 0.5 C and over 750 cycles at 1 C, the discharge capacities of the electrode remain at 935 and 710 mA h g−1 (Figure S10), respectively, and the average capacity degradation rates per cycle are only 0.04 and 0.02%, respectively. Impressively, a discharge capacity of 374 mA h g−1 is well-maintained above 1000 cycles at 10 C with an average degradation rate of 0.016% per cycle. In sharp contrast, CNTs−S and CNTs−S/TiS2 electrodes almost stagnate at 390 and 610 cycles, respectively, and the reversible capacities of CNTs−S/300HOPT and CNTs−S/300HOPT@ G after 1000 cycles rapidly fade to 221 and 368 mA h g−1, respectively (Figure 6a). The attractive reversible capacities and the low degradation rates in our work are comparable with those of recently reported Li−S batteries and ranks CNTs−S/ 300HOPT@G/300HOPT among the state-of-the-art cathode materials (Table S4). Most impressively, upon further increasing the current rate to 30 C, the discharge capacity of 172 mA h g−1 is still retained in the 2500th cycle with an average capacity degradation rate of only 0.015% per cycle (Figure 6b). Moreover, a stable Coulombic efficiency of over 98% is preserved during 2500 cycles. As far as we know, such high reversible capacity and low capacity degradation rate at 30 C have rarely been reported, suggesting that CNTs−S/ 300HOPT@G/300HOPT holds great promise for long-term storage applications with high power output. The promising rate performance of CNTs−S/300HOPT@ G/300HOPT encourages us to investigate its high-power application. Four half-cells containing 4.8 mg of S were assembled in series. With an open-circuit voltage of 9.5 V, the device can efficiently drive 60 white, blue, green, or red lightemitting diode (LED) indicator modules (nominal voltage and power are 12 V and 3 W, respectively) after only 20 s of charging (Figure 6c), suggesting that the instantaneous current and power of the device are as high as 275 A g−1 and 2611 W g−1, respectively. These results vividly reveal an outstanding high-power performance of the CNTs−S/300HOPT@G/ 300HOPT electrode, which ranks it among state-of-the-art energy storage devices. It should be mentioned that the presence of a small amount of 300HOPT (∼5 wt % based on the substitution of acetylene black) exerts considerable effects on the electrochemical behavior of the electrodes and greatly improves their rate performance. To reveal the intrinsic reasons for this enhancement, we first investigated coin cells based on bare HOPTs without sulfur loading. The CV and charge−discharge profiles of HOPTs (Figure S11a,b) confirm Li+ insertion−desertion behavior in the voltage range from 1.5 to 3.0 V, which coincides with that of sulfur cathodes. Notably, 300HOPT exhibits the best rate performance and the highest discharge capacity among HOPTs (Figure S11c), which can be ascribed to the highest electrical conductivity and abundant pores in 300HOPT. The high porosity and outstanding conductivity endow 300HOPT with good electrolyte infiltration and fast charge transport, thus overcoming the barrier of electrical insulation for solid sulfur and Li2S2/Li2S in Li−S batteries. The contents of 300HOPT in

subsequent anodic scan, the appearance of the strong oxidation peak around 2.35 V is related to the coupled conversion from Li2S2/Li2S to element sulfur via the formation of PS intermediates.7,8 Interestingly, both cathodic and anodic peaks are intensified in subsequent cycles, which probably results from the rearrangement of active sulfur from original positions to more energetically stable sites.18,43 In addition, two weak anodic peaks assignable to the delithiation processes of TiS2 are observed around 1.96 and 2.52 V, which are probably attributed to TiS2 being capable of working in the midvoltage range (1.5− 3.0 V vs Li+/Li) that coincides with the working voltage of pure sulfur in Li−S batteries.27,28 In comparison with those in CNTs−S, CNTs−S/TiS2, CNTs−S/300HOPT, and CNTs− S/300HOPT@G electrodes (Figure 5b and Table S3), the onset potentials13 and peak positions of the reduction peaks slightly increase in the CNTs−S/300HOPT@G/300HOPT electrode, whereas a significant decrement is observed in those of the oxidation peak, which are significant indications for efficient electrochemical systems because they reflect a low polarization during cell operation. This upshift in the reduction peaks and downshift in the oxidation peaks stem from the enhanced entrapping ability for PS and accelerated solid− liquid−solid electrochemical conversion of sulfur species induced by 300HOPT.44,45 The CNTs−S/300HOPT@G/ 300HOPT electrode also presents the lowest voltage hysteresis (ΔV) in the galvanostatic charge−discharge profiles (Figure 5c), which suggests a highly facile electrochemical redox reaction and low resistance, echoing the CV results (Figure 5b). Additionally, the collection coefficients (the ratio of the peak area associated with the formation of Li2S to that for the formation of PS)18 of the CNTs−S, CNTs−S/TiS2, CNTs−S/ 300HOPT, CNTs−S/300HOPT@G, and CNTs−S/ 300HOPT@G/300HOPT electrodes are 2.3, 2.4, 2.7, 3.0, and 3.5, respectively (Figure 5c). These results conclude that the presence of 300HOPT in the cathode and in the interlayer simultaneously suppresses the shuttle effect of PS and accelerates the redox reaction of sulfur species. The rate capabilities of these electrodes were tested by increasing the charge−discharge current density every five cycles from 0.2 to 30 C. In comparison with CNTs−S, CNTs− S/TiS2, CNTs−S/300HOPT, and CNTs−S/300HOPT@G, CNTs−S/300HOPT@G/300HOPT shows remarkably improved rate capabilities at various current rates (Figure 5d). When the current densities are increased from 0.2 to 0.5, 1, 3, and 5 C, the discharge capacities are 1118, 995, 812, 633, and 530 mA h g−1 (determined based on the total mass of S and 300HOPT), respectively (Figure 5d). Impressively, the CNTs− S/300HOPT@G/300HOPT electrode still delivers considerable discharge capacities of 452, 368, and 152 mA h g−1 at high rates of 10, 20, and 30 C, respectively. After undergoing an ultrahigh rate up to 30 C, the capacity of CNTs−S/ 300HOPT@G/300HOPT recovers to 915 mA h g−1 when the current is abruptly switched back to 0.5 C, which is very close to the initial value (995 mA h g−1), suggesting excellent rate capability and good stability of the electrode. The galvanostatic charge−discharge profiles of CNTs−S/ 300HOPT@G/300HOPT and CNTs−S and CNTs−S/ 300HOPT at various rates are depicted in Figure S9. As the rates increase progressively, the polarization degree of the electrode enhances slightly, but the typical characteristic of two plateaus can be identified clearly at 5 C, indicative of low polarization. Notably, sloped upper and lower discharge plateaus are also discerned at ultrahigh rates of 20 and 30 C. 18851

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Figure 7. (a) Digital photographs of Li2S6 adsorption on G, CNTs, TiS2, and 300HOPTs. (b) Digital photographs of cathode pieces and electrolytes for cycled CNTs−S/300HOPT@G/300HOPT (1), CNTs−S/300HOPT@G (2), and CNTs−S (3). (c) Polarization curves of symmetrical Li2S6− Li2S6 cells.

color of the solution from tawny to yellow. The sharp contrast confirms that polar 300HOPT have a much stronger Li2S6 adsorption capability than graphene and CNTs. The excellent entrapment ability of 300HOPT toward PS is also corroborated by the SEM images of the CNTs−S/300HOPT@G/300HOPT electrode after 2500 cycles at 30 C. The top-view SEM images and the corresponding elemental mappings show richer sulfur distribution around 300HOPT (Figure S13a−c) than that before cycling (Figure S7), suggesting that the dissolved active sulfur species are anchored on the surface of 300HOPT.9 Strong carbon signals are indicative of the unblocked electron pathways after long-term cycling, whereas distinguishable oxygen and fluorine signals reveal the penetration of proper electrolytes into the electrode material (Figure S13d,e).46 This balance among electrons, electrolytes, and the trapped active materials allows for successive reutilization and better dispersion of sulfur-containing species to achieve a long cell lifespan.27,46 The electrodes were disassembled after the charge−discharge processes were performed for 2500 cycles at 30 C, and cycled cathode pieces were soaked in a mixed solution of DOL/DME (1:1, v/v). As shown in Figure 7b, there is no detectable color change for the solution containing cycled CNTs−S/300HOPT@G/300HOPT, whereas the solutions containing cycled CNTs−S/300HOPT@G and CNTs−S electrodes turn into faintly yellow and yellow, respectively, which suggests that the presence of 300HOPT in the cathode and in the interlayer mitigates the loss of soluble sulfur in the whole Li−S system. In addition, the distortion and deformation of electrode pieces caused by the corrosion of the aluminum foil and exfoliation of the active materials are found clearly for the cycled CNTs−S/300HOPT@G cathode piece, and even more serious damage is detected for the cycled CNTs−S (Figure 7b). By contrast, the cycled CNTs−S/300HOPT@G/ 300HOPT electrode remains intact, which is mainly attributed to the favorable electrochemical reaction kinetics of the cell. Ultraviolet−visible (UV−vis) spectra of the solution containing cycled cathode pieces further reveal that 300HOPTs can

the graphene/300HOPT interlayer also have an important effect on the electrochemical performance of Li−S batteries. It can be observed clearly in Figure S12a that the rate performance at various current rates increases as the weight ratio of 300HOPT increases from 0 to 5%, whereas further increment from 5 to 7% leads to a sharp drop in the rate performance. Obviously, their rate performance is superior to that of the interlayer-free electrode. Among them, the highest discharge capacity and the best capacity recovery are achieved by the G/300HOPT interlayer with 5% 300HOPT. The promising electrochemical performance is further supported by EIS measurements. The depressed semicircles in the highfrequency region of the Nyquist plots correspond to the charge transfer resistance between the electrode and PS. CNTs−S/ 300HOPT@G/5% 300HOPT possesses the lowest charge transfer resistance (Figure S12b). Therefore, the presence of optimal amounts of 300HOPT not only effectively overcomes the kinetic barriers of charge transformation but also significantly improves the rate capability of the electrodes. Notably, 300HOPT in the G/300HOPT interlayer only occupies 4 wt ‰ (8% × 5%) weight of CNTs−S/ 300HOPT@G/5% 300HOPT; thus, the capacity contribution of 300HOPT in the G/300HOPT interlayer is negligible. Another characteristic of the incorporated 300HOPT is the enhancement of its chemical interactions with the sulfur species, which improves the cycling performance through effectively suppressing the shuttle effect of PS. To confirm the strong chemical interaction between HOPTs and PS, the adsorption test of PS was carried out. Here, 10 mmol L−1 Li2S6 in DOL/DME (1:1 by volume) was used as a representative adsorbate to model the static adsorption of PS. Graphene, CNTs, ball-milled TiS2, 300HOPT, and graphene/300HOPT composite with the same total specific surface area were used as adsorbents. Digital photographs taken after the adsorption for 30 min are presented in Figure 7a. 300HOPT and G/ 300HOPT visibly decolor the Li2S6 solution, whereas graphene shows negligible color variation, and CNTs only change the 18852

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Figure 8. High-resolution TEM images of (a) CoS2 and (b) PCS. (c) Rate performance profiles of CNTs−S/CoS2, CNTs−S/PCS, CNTs−S/ PCS@G, and CNTs−S/PCS@G/PCS electrodes. High-resolution TEM images of (d) NbS2 and (e) PNS. (f) Rate performance profiles of CNTs− S, CNTs−S/NbS2, CNTs−S/PNS, CNTs−S/PNS@G, and CNTs−S/PNS@G/PNS electrodes.

thickened obviously (Figure S16b). Prominently, it exhibits a high initial discharge capacity of 800 mA h g−1 in the first cycle at 0.2 C (3.7 mA cm−2) and good stability with a capacity retention of more than 81% at the 200th cycle (Figure S16c), overwhelmingly outperforming that of the CNTs−S electrode. Considering high sulfur loading and low content of conductive agents in the electrodes, the excellent cycle performance can be ascribed to the successful coupling of the CNTs−S/300HOPT cocathode and the graphene/300HOPT interlayer. The significant promotion of the electrochemical performances of TiS2 by water steam etching has excellent generality. Other typical TMDs, such as CoS2 and NbS2, were etched under the same conditions as HOPTs. Taking CoS2 as an exemplary case, as shown by TEM images in Figure 8a,c, abundant cavity and lattice defects are distributed heterogeneously on the surface of etched CoS2, when compared with the regular and smooth surface of raw CoS2, showing the successful application of the present water-steam-etched approach in the fabrication of porous CoS2 (denoted as PCS). As expected, when PCS was used as a partial substitute of acetylene black and an additive of the graphene interlayer in the sulfur cathode system, the resultant CNTs−S/PCS@G/ PCS electrode exhibits much higher rate capacities than CNTs−S, CNTs−S/CoS2, CNTs−S/PCS, and CNTs−S/ PCS@G (Figure 8c). CNTs−S/PCS@G/PCS also displays an excellent cycling performance even at a high current density of 10 C (Figure S17), and a reversible discharge capacity as high as 302 mA h g−1 can be achieved after 600 consecutive cycles, which overwhelmingly surpasses those in CNTs−S, CNTs−S/CoS2, CNTs−S/PCS, and CNTs−S/PCS@G. The outstanding rate performance and long-term stability indicate that PCS can suppress the shuttle effect of PS and improve the redox kinetics of sulfur species during charge and discharge. Likewise, enhanced electrochemical performances are also achieved for porous NbS2 (denoted as PNS) (Figures 8d−f and S18). These results are strong evidence that water steam etching is a highly effective approach for the fabrication of porous TMDs with enhanced conductivity and sulfiphilicity.

effectively suppress the shuttle effect of PS during cell operation.47−49 The intensity of the absorption peaks follows the following order: CNTs−S > CNTs−S/300HOPT@G > CNTs−S/300HOPT@G/300HOPT (Figure S14) because of the strong adsorption capability of 300HOPT toward PS in the cathode and/or the interlayer of the electrode. To further validate the strong chemical interaction between 300HOPT and PS, symmetrical cells were assembled by sandwiching Li2S6-containing electrolytes between two identical electrodes.13 CV tests of the designed symmetrical cells were performed in a voltage window from −1.0 to 1.0 V (Figure 7c). The current density of 300HOPT in the Li2S6-containing electrolyte is much higher than those in graphene, CNTs, and TiS2. By contrast, only a minor capacitive current is provided when Li2S6-free electrolyte is used. These results suggest that the capacitive current is mainly originated from the redox current of Li2S6, and only a minor contribution is from 300HOPT itself. Thus, 300HOPT not only absorbs PS on its surface through strong chemical interactions but also provides accessibility for electric charge transfer to trigger redox reactions of PS. This substantially facilitates charge transfer that is further verified by EIS measurements of the symmetrical cells. 300HOPT has the lowest charge transfer resistance (Figure S15), further suggesting that the charge transfer of PS on the surface of 300HOPT is much faster than those on the surfaces of graphene, CNTs, and TiS2; thus, the redox kinetics of PS is promoted by the addition of highly sulfiphilic 300HOPT. In this context, 300HOPT acts as a catalytic redox mediator;50−52 strong guest−host interactions between 300HOPT and sulfur species and the enhanced electrical conductivity of 300HOPT cooperatively propel the reversible redox of solution-phase PS on the electrode surface and the subsequent liquid−solid nucleation of Li2S. High sulfur loading is another crucial factor for high area capacity and volumetric energy density. To improve the energy density of CNTs−S/300HOPT@G/300HOPT, an electrode with higher area sulfur loading of 2.8 mg cm−2 was fabricated through increasing the sulfur content (Figure S16a). The crosssectional SEM images show that the cocathode material is 18853

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Lithium−Sulfur Batteries. ACS Appl. Mater. Interfaces 2014, 6, 8789− 8795. (5) Huang, J. Q.; Zhuang, T. Z.; Zhang, Q.; Peng, H. J.; Chen, C. M.; Wei, F. Permselective Graphene Oxide Membrane for Highly Stable and Anti-Self-Discharge Lithium Sulfur Batteries. ACS Nano 2015, 9, 3002−3011. (6) Yang, C.-P.; Yin, Y.-X.; Guo, Y.-G.; Wan, L.-J. Electrochemical (de) Lithiation of 1D Sulfur Chains in Li−S Batteries: A Model System Study. J. Am. Chem. Soc. 2015, 137, 2215−2218. (7) Carter, R.; Oakes, L.; Muralidharan, N.; Cohn, A. P.; Douglas, A.; Pint, C. L. Polysulfide Anchoring Mechanism Revealed by Atomic Layer Deposition of V2O5 and Sulfur-Filled Carbon Nanotubes for Lithium−Sulfur Batteries. ACS Appl. Mater. Interfaces 2017, 9, 7185− 7192. (8) Xu, N.; Qian, T.; Liu, X.; Liu, J.; Chen, Y.; Yan, C. Greatly Suppressed Shuttle Effect for Improved Lithium Sulfur Battery Performance through Short Chain Intermediates. Nano Lett. 2017, 17, 538−543. (9) Sun, Z.; Zhang, J.; Yin, L.; Hu, G.; Fang, R.; Cheng, H.-M.; Li, F. Conductive Porous Vanadium Nitride/Graphene Composite as Chemical Anchor of Polysulfides for Lithium-Sulfur Batteries. Nat. Commun. 2017, 8, 14627. (10) Yao, H.; Zheng, G.; Hsu, P.-C.; Kong, D.; Cha, J. J.; Li, W.; Seh, Z. W.; McDowell, M. T.; Yan, K.; Liang, Z.; Narasimhan, V. K.; Cui, Y. Improving Lithium−Sulphur Batteries through Spatial Control of Sulphur Species Deposition on a Hybrid Electrode Surface. Nat. Commun. 2014, 5, 3943. (11) Du, W.-C.; Yin, Y.-X.; Zeng, X.-X.; Shi, J.-L.; Zhang, S.-F.; Wan, L.-J.; Guo, Y.-G. Wet Chemistry Synthesis of Multidimensional Nanocarbon−Sulfur Hybrid Materials with Ultrahigh Sulfur Loading for Lithium−Sulfur Batteries. ACS Appl. Mater. Interfaces 2016, 8, 3584−3590. (12) Fan, Q.; Liu, W.; Weng, Z.; Sun, Y.; Wang, H. Ternary Hybrid Material for High-Performance Lithium−Sulfur Battery. J. Am. Chem. Soc. 2015, 137, 12946−12953. (13) Yuan, Z.; Peng, H.-J.; Hou, T.-Z.; Huang, J.-Q.; Chen, C.-M.; Wang, D.-W.; Cheng, X.-B.; Wei, F.; Zhang, Q. Powering Lithium− Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts. Nano Lett. 2016, 16, 519−527. (14) Zhang, J.; Yang, C.-P.; Yin, Y.-X.; Wan, L.-J.; Guo, Y.-G. Sulfur Encapsulated in Graphitic Carbon Nanocages for High-Rate and Long-Cycle Lithium-Sulfur Batteries. Adv. Mater. 2016, 28, 9539− 9544. (15) Li, Z.; Li, X.; Ge, X.; Ma, J.; Zhang, Z.; Li, Q.; Wang, C.; Yin, L. Reduced Graphene Oxide Wrapped MOFs-Derived Cobalt-Doped Porous Carbon Polyhedrons as Sulfur Immobilizers as Cathodes for High Performance Lithium Sulfur Batteries. Nano Energy 2016, 23, 15−26. (16) Mi, K.; Jiang, Y.; Feng, J.; Qian, Y.; Xiong, S. Hierarchical Carbon Nanotubes with a Thick Microporous Wall and Inner Channel as Efficient Scaffolds for Lithium-Sulfur Batteries. Adv. Funct. Mater. 2016, 26, 1571−1579. (17) Wu, F.; Lee, J. T.; Zhao, E.; Zhang, B.; Yushin, G. Graphene− Li2S−Carbon Nanocomposite for Lithium−Sulfur Batteries. ACS Nano 2016, 10, 1333−1340. (18) Xiao, Z.; Yang, Z.; Wang, L.; Nie, H.; Zhong, M.; Lai, Q.; Xu, X.; Zhang, L.; Huang, S. A Lightweight TiO2/Graphene Interlayer, Applied as a Highly Effective Polysulfide Absorbent for Fast, Long-Life Lithium-Sulfur Batteries. Adv. Mater. 2015, 27, 2891−2898. (19) Wang, L.; Yang, Z.; Nie, H.; Gu, C.; Hua, W.; Xu, X.; Chen, X.; Chen, Y.; Huang, S. Lightweight Multifunctional Interlayer of Sulfur− Nitrogen Dual-Doped Graphene for Ultrafast, Longlife Lithium− Sulfur Batteries. J. Mater. Chem. A 2016, 4, 15343−15352. (20) Jin, C.; Zhang, W.; Zhuang, Z.; Wang, J.; Huang, H.; Gan, Y.; Xia, Y.; Liang, C.; Zhang, J.; Tao, X. Enhanced Sulfide Chemisorption Using Boron and Oxygen Dually Doped Multi-walled Carbon Nanotubes for Advanced Lithium−Sulfur Batteries. J. Mater. Chem. A 2017, 5, 632−640.

CONCLUSIONS We have successfully fabricated HOPTs with enhanced electrical conductivity, sulfiphilicity, and porosity through water steam etching of TiS2 at relatively low temperatures; the layered structure of TiS2 remains intact during the chemical modification. Using HOPTs as substitutes of both conductive agents and additive of interlayer materials, a new cell configuration containing a CNTs−S/300HOPT cocathode coupled with a graphene/300HOPT hybrid interlayer was prepared via an industrially accepted double slurry-coating process. The electrode possesses excellent high-power cycling stability and delivers a reversible capacity of 172 mA h g−1 after 2500 cycles at 30 C. Such excellent performances with respect to specific capacity, rate capability, and cycling stability are verified to originate from the special characteristics of HOPTs, such as excellent structural stability, outstanding electrical conductivity, strong adsorption ability toward PS, and high electrocatalytic activity. These encouraging properties of HOPTs might enable Li−S batteries to be used in electric vehicles and other large-scale electrochemical energy storage systems. In summary, this study provides a new approach to enhance the electrical conductivity and sulfiphilicity of TMDs, which is an inspiration to develop high-power Li−S batteries with long-lasting cycling performances.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04232. Theoretical calculation, additional morphological and structural characteristics, and electrochemical performance of the material (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Y.). *E-mail: [email protected] (R.W.). ORCID

Zhi Yang: 0000-0002-9265-5041 Liujiang Zhou: 0000-0001-5814-4486 Ruihu Wang: 0000-0002-6209-9822 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21601191 and 51572197) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

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REFERENCES

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DOI: 10.1021/acsami.7b04232 ACS Appl. Mater. Interfaces 2017, 9, 18845−18855