Conductive and Polar Titanium Boride as a Sulfur Host for Advanced

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Conductive and Polar Titanium Boride as a Sulfur Host for Advanced Lithium-Sulfur Batteries Chuanchuan Li, Xiaobiao Liu, Lin Zhu, Renzhi Huang, Mingwen Zhao, Liqiang Xu, and Yitai Qian Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01352 • Publication Date (Web): 23 Sep 2018 Downloaded from http://pubs.acs.org on September 23, 2018

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Chemistry of Materials

Conductive and Polar Titanium Boride as a Sulfur

Host

for

Advanced

Lithium-Sulfur

Batteries Chuanchuan Li†, Xiaobiao Liu‡, Lin Zhu†, Renzhi Huang†, Mingwen Zhao*,‡, Liqiang Xu*,†, Yitai Qian† †

Key Laboratory of Colloid & Interface Chemistry (Shandong University),

Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. E-mail: [email protected]. ‡

School of Physics, Shandong University, Jinan, 250100, P. R. China

ACS Paragon Plus Environment

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Abstract Lithium-sulfur batteries are the most promising candidates for advanced electrochemical energy storage systems benefiting from their high energy density and low cost of sulfur. Improving the conductivity of sulfur cathode and stabilizing the polysulfide shuttle are the key factors for obtaining high-performance lithium-sulfur batteries. Herein, metallic and polar TiB2 nanomaterials are applied for the first time as sulfur hosts. The 70S/TiB2 composite exhibits a long-term cycling stability up to 500 cycles at the current density of 1 C. It is worth noting that even when the sulfur areal mass loading is up to 3.9 mg cm-2, a stable capacity of 837 mAh g-1 can be still maintained after 100 cycles. The outstanding electrochemical performance can be attributed to the strong anchoring effect of TiB2 to lithium polysulfides, which is confirmed by the XPS analyses and theoretical calculations with a favorable surface passivated chemistry. The study presented here will shed a new light for metal borides as hosts to improve the cycling life of lithium-sulfur batteries and provide a deep comprehension of the instinct interaction evolution at molecular level, which is invaluable in the material rational fabrication for the future highperformance Li-S batteries. Introduction In searching for high-energy-density and inexpensive rechargeable battery systems, lithiumsulfur batteries (Li-S batteries) have attracted intriguing attentions because of their maximum theoretical capacity (1675 mAh g-1), low cost and eco-friendliness of S cathode. However, lithium-sulfur batteries have regrettably suffered from the low utilization of electrochemically inactive sulfur, dramatic capacity decay during cycling, poor Coulombic efficiency and severe self-discharge. These unfavorable issues finally result in battery failure, impeding the practical


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application. The underlying reason for those obstacles is mainly the notorious shuttle phenomenon, a process of highly soluble intermediate polysulfides dissolving into electrolyte, migrating through separator, poisoning lithium anode. In addition, the insulating nature of both sulfur and the ultimate discharge products lithium (di)sulfide could lead to the high overpotentials and the sluggish charge-transfer kinetics during the redox.1-4 Extensive research efforts have been engaged in the discovery and development of promising inorganic polar and nonpolar materials as sulfur hosts and exciting research progresses have been made in the he past few years. For instance, conductive non polar carbonaceous micro/nanostructured sulfur hosts such as mesoporous carbon, hollow carbon spheres, graphene, carbon nanotubes etc. have been successfully explored in order to tackle the aforementioned problems.5-9 These carbon materials can dramatically improve the conductivity of sulfur cathode and mitigate the dissolution of lithium polysulfides through physical confinement. However, the weak interaction between polar polysulfides and nonpolar carbon materials leads to an easy leakage of polysulfides from the electrode, triggering the shuttle. Thereafter, managing the intrinsic polysulfide shuttle has become a virtual kernel in improving the cyclability and capacity retention for Li-S batteries. Hence, chemical binding and trapping of sulfur species by polar sulfur hosts have been proposed with the merit of their much stronger affinity to the polar polysulfides. Various polar materials including metal oxides,10,11 metal organic framework,12 metal sulfides,13,14 metal hydroxides,15 metal carbides,16,17 metal nitrides18-20 have been applied to achieve the remarkable polysulfides’ confinement. The strong chemical binding of polar host-guest is based on Lewis acid-base interactions or surface redox of intermediate polysulfides.21-24 However, in consideration of the relative low conductivity of most of these compounds, a high proportion of carbon are usually introduced to


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further improve its conductivity. But the adsorbed polysulfide species easily diffused to the carbon substrates to undergo redox process, which produce an extra energy barrier to be overcome. Therefore, the utilization of polar and high conductive compounds has been proved to be effective ways for the fast development of Li–S battery research. It is worth noting that various titanium based materials ranging from semi-conductors to conductors have special advantages in conquering the shuttle effect in Li–S batteries. The pioneering work by Yi Cui and co-workers constructed S-TiO2 yolk-shell architecture to achieve an ultralong cycle life.10 After that, more conductive Ti4O7,25-27 TiC,16,17 TiO,28 TiN29-31 have been investigated for confinement of polysulfides successively. This indeed encourages us to seek for undiscovered Ti-based inorganics especially with superb conductivity. Titanium diboride (TiB2), a well-known earth-abundant base material for ceramic, with high electronic conductivity and attractive chemical stability,32,33 is identified as a candidate host for lithium-sulfur batteries. To the best of our knowledge, there is no report on employing TiB2 or other metal borides as cathode host materials for Li-S batteries. In this study, a handleable TiB2/S cathode avoiding sophisticated manufacturing processes has been developed for the first time with the following five-fold advantages: (1) the outstanding theoretic conductivity (~ 106 S cm-1) and nanosize can both facilitate the electron transport and the sulfur utilization; (2) the polar nature of TiB2 immobilizes lithium polysulfides within the cathode to alleviate the shuttle effect; (3) the highly chemical affinity to polysulfides enables a uniform precipitation of Li2S; (4) both the absent porosity of the inert ingredient and the high tap density of TiB2 promote a high volume energy density; (5) the low cost, environmental friendliness, simple preparation process as well as easily scaled-up procedure of TiB2 propel a further development for lithium-sulfur batteries. With a sulfur content of 70% in the TiB2/S composite, the cathodes demonstrate


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improved cycling capacity, superior long-term life span, excellent Coulombic efficiency, and remarkable electrochemical performance with a high sulfur loading of 3.9 mg cm-2. Experimental Section Preparation of TiB2: The TiB2 was synthesized by a metal-hydrolysis-assisted process reported by our group previously.34 In a typical synthesis, Mg, B, TiO2 were hand milled for several minutes. Subsequently, the mixture was transferred into a 30 mL stainless steel autoclave with 2.7 mL water. The autoclave was heated to 150 °C and was kept at that temperature for 2 hours. Then the product was obtained after washing in turn with 2 M HCl, distilled water, ethanol, and drying in a vacuum oven at 60 °C overnight. Preparation of TiB2-Sulfur Composite: The sulfur was loaded onto the TiB2 through a meltingdiffusion method. The mixture of TiB2-sulfur with a weight ratio of 7:3 was sealed into a weighing bottle and heated at 155 °C for 15 h to obtain the final composite. Material Characterization: The XRD patterns were acquired on a Bruker D8 Advance X-ray diffractometer using Cu kα radiation (λ=1.5406 Å). The morphology and energy dispersive Xray analysis were examined by the transmission electron microscopy (TEM, JEM-1011, Japan) and Scanning electron microscopy (SEM, JSM-7600F, Japan). HRTEM and mapping characterization were performed on the JEOL-2011 instrument at an operating voltage of 200 kV. The specific area and pore size determained by Brunauer–Emmett–Teller (BET) approach were collected using a QuadraSorb SI surface area analyzer. The conductivity was conducted on RTS8 resistivity tester. TGA of the composite was carried out on Mettler Toledo TGA/SDTA851 system from 30 °C to 500 °C at a heating rate of 5 °C min−1 under N2 atmosphere. Laser Raman spectra were conducted on a NEXUS 670 Raman instrument, using an excitation wavelength of 632.8 nm at room temperature. Chemical valence states and electron transfers on the surface of


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materials were investigated using X-ray photoelectron (ESCALAB 250, spectrometer, PerkinElmer) (binding energy was calibrated by C 1s peak at 284.7 eV). Adsorption Test: To evaluate the chemical adsoption of polysulfide by TiB2, the Li2S4 solution was prepared. Elemental sulfur and Li2S with a stoichiometric amount were dissolved in DOL/DME solvents (volume ratio 1:1). The solution was stirred at 60 °C in a sealed bottle under argon protection for several days to finally obtain the brown yellow Li2S4 solution. To test the interaction between TiB2 and polysulfide, the solution was diluted to 10 mM, and 40 mg TiB2 was immersed into the 4 mL of Li2S4 solution. Electrochemical Measurements: A conventional slurry-coating procedure was employed to farbricted the cathode. A mixture of 80% TiB2/S, 10% super P and 10% styrene butadiene rubber (SBR) were homogenized in water to make a slurry. The slurry was coated on the Al foil and dried in a vacuum oven at 60 °C overnight. The electrodes were punched into the small disks with the diameter of 12 mm. The typical sulfur loading of each electrode is controlled to be 1.11.4 mg cm-2. For the preparation of high sulfur areal loading, the mixture was 70% TiB2/S, 20% super P and 10% SBR. The electrochemical properies were performed using CR2032 coin cells with lithium discs as anodes, Celgard 2400 membranes as separator and 1 M LiTFSI in DOL/DME (v/v = 1:1) containing 2% LiNO3 as electrolyte. The cells were assembled in an Arfilled glovebox (Mikrouna, Super 1220/750/900). For the low sulfur mass loading, the electrolyte was 20 µL for each cell, while for the high mass loading of 3.9 mg cm-2, the electrolyte/sulfur ratio was controlled at 13 µL/mgsulfur. The galvanostatic performances were tested in Land CT2001A battery system within a voltage window of 1.7-2.8 V. The specific capacity is based on the mass of sulfur only. Cyclic voltammetric measurements were conducted on CHI 760E Electrochemical Workstation at a scanning rate of 0.1 mV s-1. Electrochemical impedance


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spectroscopy (EIS) measurements were carried out in a frequency range of 0.01 Hz to 100K Hz by a Materials Maters 510 instrument. All the electrochemical measurements were carried out at room temperature. Calculation of Shuttle Factor: The relationship between Coulombic efficiency and the shuttle factor could be expressed using the definition: Ceff =

2+(ln(1+f))⁄f 2-(ln(1-f))⁄f

(1)

Theoretical Calculations: Our first-principles calculations were performed using the Vienna ab initio simulation package known as the VASP code. The electronic-ion interaction was described by a projector augmented wave method (PAW). The energy cutoff of the plane waves was set to 350 eV. The electron exchange–correlation function was treated using a generalized gradient approximation (GGA) in the form proposed by Perdew, Burke, and Ernzerhof (PBE). For the bulk TiB2, both atomic positions and lattice vectors were fully optimized using the conjugate gradient (CG) algorithm until the maximum atomic forces were less than 0.01 eV/Å with an energy precision of 10-5 eV. While in the adsorbate-substrate model the top three layers of TiB2 and the polysulfide were full relaxed while the other layers of TiB2 was fixed for simulating the bulk part. The equilibrium lattice constants for TiB2 are a=b=3.031 Å and c=3.226 Å. In the adsorbate-substrate model, a large supercell with the lattice constants larger than 12 Å along the x- and y-directions and vacuum region of about 15 Å along the z-direction was adopted. The Brillouin zone (BZ) integration was sampled by using a 3×3×1 k-mesh according to the Monkhorst-Pack method. The van der Waals interaction was including by using the DFT-D3 method of Grimme. The decomposition of S8 molecular on the clean (001) and (111) TiB2 surfaces were confirmed by the decomposing energy (ED) calculated using the following definition:


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ED =(ES-TiB2 -ETiB2 -n*µS )/NS

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(2)

where ES-TiB2 and ETiB2 are the total energies of TiB2 with and without S8 molecular. µS is the chemical potential of S atom in the S8 molecular and NS is the number of S atoms. The bind energies (Eb) for S8 and Li2 Sn polysulfides on the sulfidized (001) and (111) surfaces were evaluated using the definition. Eb =ETotal -ESurf -ELi2 Sn (S8 )

(3)

where ETotal and ESurf are the total energies of the sulfidized TiB2 with and without polysulfides (S8 and Li2 Sn ), ELi2 Sn (S8 ) is the total energy of the polysulfides. Results and Discussion The typical hexagonal structure of TiB2 crystal is shown in Figure 1a, in which B atoms insert in the interstice of Ti atoms layers, forming a covalently bonded hexagonal framework. As the (B-)n honeycomb lattice is isoelectronic to graphite, such a structure can easily offer fast paths for electrons and ions. Its crystal structure and atomic bonding also result in the high hardness and structural elasticity of TiB2,33 which is essential to accommodate the volume expansion of the sulfur. Figure 1b shows the exposed terminal titanium atoms on TiB2 (001) surface. The coordination number of surface Ti atoms decreases from 12 to 6. Such a high density of coordinatively unsaturated Ti atoms on (001) surface can aid in forming external chemical bonds, which ensures high surface anchoring sites. Considering these structure character, TiB2 is proposed to chemically bond sulfur and polysulfides to control the shuttle effect. In order to demonstrate the chemical confinement of polysulfide for stabilizing Li-S batteries, nonporous TiB2 nanoparticles with negligible physical confinement of polysulfides have been prepared by a metal-hydrolysis-assisted approach with TiO2 and B powder as starting materials,34 when heated to 150 °C, the metal Mg hydrolysis reaction occurred, releasing a great deal of heat.


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With the presence of Mg and B, the TiO2 can be easily transformed into the TiB2. Figure 2a shows the morphology of TiB2 observed under the transmission electron microscopy (TEM). The obtained TiB2 were nano-sized particles with the diameter less than 300 nm. The lattice-resolved high resolution TEM (HRTEM) image (Figure 2b) depicts the clear lattice fringes with spacing of 0.264 nm, which could be well assigned to the (100) plane of hexagonal TiB2.34,35 The specific surface area and total pore volume are 11.6 m2 g-1 and 0.032 cm-3 g-1, respectively, as determined by the nitrogen adsorption/desorption isotherms (Figure S1, Supporting Information). The pore volume is negligible. As a consequence, the physical confinement has played negligible role in trapping polysulfides, while this work mainly aims at illustrating the inherent chemical interaction for enhanced lithium-sulfur batteries rather than the physical structure effect. The typical XRD pattern of the obtained TiB2 are presented in Figure S2 (Supporting Information). All the diffraction peaks can be indexed to the highly crystalline hexagonal-phase TiB2 (JCPDS No. 85-2083).36 The corresponding energy-dispersive X-ray spectroscopy (EDS) indicates the presence of elemental Ti and B, with an atomic ratio of ~1:2, which is approaching to the ratio of stoichiometric TiB2 (Figure S3, Supporting Information). XPS result of TiB2 bulk (Figure S4, Supporting Information) further validates the surface chemical composites including Ti, B and O elements. The presence of O can be attributed to the unavoidable pollution by the air, which is further confirmed by the XPS result of the commercial TiB2 with a purity of 99.99% (Figure S5). The conductivity of TiB2 measured by the four-point probe approach is 400 S m-1, almost 10 times larger than that of commercial acetylene black (40.2 S m-1), which is essential to facilitate the electron/ ion transport. It is known that high sulfur mass loading is indispensable to guarantee Li-S batteries with both high volumetric and gravimetric energy densities. Here, sulfur has been incorporated onto TiB2


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substrate by a conventional melt-diffusion method. Figure 2c presents the typical TEM image of TiB2/S composite. And there is no obvious morphological difference observed compared to the individual TiB2. HRTEM image taken from the edge of TiB2/S composite (Figure 2d and Figure S6) indicates the present of a sulfur coating layer on TiB2 matrix, and this sulfur layer is sensitive to the electron beam during the HRTEM operation.37 The unchanged lattice fringes in Figure 2e verify the chemical stability of TiB2 after loading sulfur. EDS mapping under TEM observation (Figure 2f) reveals a homogeneous distribution of S, Ti and B, assuring a successful dispersion of sulfur in TiB2 matrix. The sulfur mass content is evidenced to be 70% by thermogravimetric analysis (TGA) under N2 atmosphere (Figure 2g). The obvious signals of sulfur could also be traced in the EDS spectrum (Figure S7), confirming a high sulfur content in the composite. Further evidence stems from the XRD patterns (Figure 2h), wherein the peaks of sublimed sulfur are clearly identified after sulfur loading. Meanwhile, all the reflections of TiB2 descended with TiB2/S and TiB2/Li2S4 composites could be indexed to the aforementioned TiB2 phase, demonstrating a good chemical inertness when employed as sulfur hosts.36,38 Such a result could also be confirmed by the Raman spectrum (Figure 2i), in which the spectrum of TiB2/S composite is simply overlapped of TiB2 and sulfur. Electrochemical performances are investigated using coin cells with Li metal as anode. Figure 3a shows the typical cyclic voltammetry (CV) curves of initial 5 cycles at a scan rate of 0.1 mV s-1. In the first cathodic scan, two reduction peaks centered at 2.27 and 1.93 V can be attributed to the transformation of cyclo-S8 to long-chain lithium polysulfides at the high potential and further reduction to the final product Li2S2/Li2S at the low potential, respectively. In the anodic scan, two oxidization peaks located at 2.43 and 2.53 V are associated to the reverse reaction from the short-chain polysulfides to sulfur. Figure S8 shows a galvanostatic discharge/charge voltage


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profile for the different cycles under a current density of 0.2 C, which exhibit two discharge plateaus and two charge plateaus, which are in agreement with CV tests. The discharge capacity of high potential plateau (QH) is usually employed to evaluate the polysulfide trapping capability.39,40 After 150 cycles, the QH is 69% of that of first cycle, demonstrating an alleviating dissolution of polysulfides into electrolyte. The low potential at 2.05 V is relevant with the further nucleation to Li2S precipitation, which mainly depends on the conductivity. It is notable that plateaus are almost unchanged for 150 cycles, suggesting an excellent capacity reversibility. Such a stability can be attributed to an outstanding conductivity as well as high-efficiency polysulfdes’ trapping ability of TiB2. The matching cycling performance is presented in Figure 3b, in which an initial discharge capacity of 1232 mAh g-1 (reaching ~ 73.6% of theoretic capacity) is accomplished. After 150 cycles, the cathode still retains a discharge capacity of 842.3 mAh g-1, approaching a capacity retention of 68.4%. The cycling performance of TiB2/S cathode at a relatively low rate is comparable to those of the recent works, as confirmed in Table S1. In contrast, the QH of S/TiB2 cathode is obvious higher than that of the cathode using commercial sulfur/Super P (Figure S9a), further indicating an efficient confinement of polysulfides by TiB2. Due to the continuous dissolution of polysulfides, the cathode uses the Super P as sulfur hosts shows a fast capacity decay for 100 cycles under the same condition (Figure S9b). Meanwhile, without the LiNO3 additive to passivate the anode metallic lithium, the Coulombic efficiency is slightly decreased for TiB2/S cathode, but a high capacity retention is still can be obtained (Figure S10). It is verified that the capacity contribution of pure TiB2 is negligible to the lithium-sulfur batteries, as shown in Figure S11. The excellent anchoring effect could be also reflected on the long cycling stability. Figure 3f shows the prolonged cycling performance of 500 cycles at 1 C. A discharge capacity of 534 mAh g-1 can be maintained over


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500 cycles, corresponding to a fade rate of 0.058% per cycle. The average Coulombic efficiency is calculated to be 98.9%. Because the cycling process at 1 C current density is very fast and the dissolution of polysulfides is suppressed to some degree within limited time, which also benefits the Coulombic efficiency. Based on the prior reports, the shuttle factor f is employed to estimate the degree of shuttle effect.41 In this study, the f is calculated to be 0.032, which is much smaller than that of carbon materials in previous studies (f > 0.3).42,43 It is apparent that the remediation of polysulfide shuttle could be offered by the surface active sites of TiB2. To evaluate the chemical stability of the TiB2/S composite, the cells were disassembled and the electrode is examined by XRD. As shown in Figure S12, after 500 cycles, the TiB2 still maintained the highly crystalline hexagonal-phase, demonstrating the highly chemical stability of TiB2 in the electrochemical environment of Li-S batteries. The stability of the electrode is also verified by the EIS data with a slight decrease of the electrode impedance after 100 cycles (Figure S13). Due to the powerful inhibition to LiPSs, the TiB2/S also exhibits a stable cycling performance at a high temperature of 45 °C, as shown in Figure S14. Moreover, the performance of electrode with a practically higher sulfur content of 75% in the composite is evaluated. As is shown in Figure S15, the electrode exhibits the stable capacity of ~830 mAh g-1 at 0.2 C after 100 cycles. To meet the requirement of high energy, we further increase the areal sulfur mass loading to 3.9 mg cm-2. As the low electrolyte volume/sulfur mass (E/S) ratio is important for high energy density, the E/S ratio is fixed at 13 µL/mgsulfur in TiB2/S based cells, and this relatively low ratio is comparable to those of the curretly reported excellent works (Table S2). At a current density of 0.2 C (1.3 mA cm-2), TiB2/S composite exhibits an initial discharge capacity of 965 mAh g-1 (3.76 mAh cm-2). After 100 cycles, a stable capacity of 837 mAh g-1 is obtained, corresponding to a desirable areal capacity of 3.3 mAh cm-2. The lower Coulombic efficiency is in agreement


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with the more serious polysulfide shuttling at a high sulfur mass condition. The areal capacity of 3.9 mg cm-2 sulfur loading is remarkable among the previous sulfur cathodes based on the titanium materials, as depicted in Figure 3e. The excellent electrochemical performance could be attributed to the high conductivity and effective polysulfide confinement provided by the metallic polar TiB2. Evaluating the interactions between polysulfides and hosts is significant in strategically fabricating better materials for Li-S batteries. To confirm the inhibition of the polysulfide shuttle, 4 mL of 10 mM Li2S4 dissolved in 1,3-dioxolane (DOL)/dimethyl ether (DME) solvents was taken as the representative polysulfide. Although this experimental condition is not comparable to the requirement for Li-S batteries with a standard of 3 mL of electrolyte per gram of sulfur, it could provide a visual result to confirm that TiB2 has a high affinity for lithium polysulfides. After adding 40 mg TiB2 powder to the solution for 8 h, the yellow-brown could be decolored completely, as shown in Figure S16a, indicating a pronounced chemical adsorption of TiB2. However, this simulation still would not suffice to revel the real environment of Li-S batteries. Figure S16b shows the digital photographs of the solutions by soaking the electrode films in DOL/DME solvents when the batteries were cycled to 2.05 V. The solution containing TiB2/S cathode film is almost colorless compared to that containing super P/S electrode. Such a distinct contrast provides another visual evidence that polysulfides could be immobilized on the surface of TiB2. With these results in mind, a model involving surface-chemical entrapments of polysulfides is proposed, as illustrated in Figure 4a. To fully clarify the interaction mechanism, the binding features were studied by first-principles calculations with the density-functional theory (DFT). We firstly considered a cyclo-S8 molecule on a clean (001) surface of TiB2. It was found that the


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(001) surface is quite active to the cyclo-S8 molecule due to the dangling bands of the Ti atoms on the surface. The strong interaction between cyclo-S8 molecule and TiB2 (001) surface leads to the decomposition of the cyclo-S8 molecule, forming a S monolayer on the surface, as confirmed by the Figure S17. The average decomposition energies of S8 molecule on the (001) and (111) surfaces (Figure 4b and Figure S18) are -2.82 and -2.91 eV/atom, respectively. The negative decomposition energies imply that the decomposition reaction is exothermic and thus highly plausible, which is confirmed by the molecular dynamics simulations (Video S1). It should be notable that only a rather small amount of sulfur is fragmented and takes part in the formation of sulfidized surface, and the majority of sulfur would be reduced to the final product of Li2S during discharging process. These results suggest that the TiB2 surfaces are easy to be sulfidized in sulfurous environment, further confirmed by the S XPS spectrum. After sulfur loading, an obvious downward shift for S 2p (from 164 eV in the elemental S to 163.65 eV in the TiB2/S composite) evidences the polarization of electrons away from Ti to the S atoms, as shown in Figure 4c and Figure S19. In addition, a small contribution of 12% for S at 162 eV was monitored, indicating the existence of sulfur atoms with the same chemical environment in TiS2 (162.1 eV).44 The conclusion seems clear: the unprecedentedly ultrastrong Ti-S bond could even cause a spontaneous fragmentation of ring S8, forming monolayer S atom passivation layer on the surface, which plays a virtually impactful role in trapping and binding sulfur/polysulfides by chemical interactions.45 The chemical interaction between the obtained sulfur host and polysulfides was further probed. High resolution of Ti 2p spectrum of TiB2, TiB2/S and TiB2/Li2S4 in Figure 4d show two predominant Ti 2p3/2 components, which are corroborated to Ti-B at the low binding energy and Ti-O at the high binding energy, respectively. The latter is an unavoidable partly oxidation when


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TiB2 was exposed to the air.36,46 The peak located at binding energy of 464.7 eV is ascribed to Ti 2p1/2 component. The peak located at binding energy of 464.7 eV is ascribed to Ti 2p1/2 component. In the subsequent discussion about XPS analysis of Ti (Ti 2p3/2 and Ti 2p1/2) to probe the interaction, the binding energy variation of Ti 2p3/2 is taken as an example, following convention. After sulfur being loaded, a slight shift of +0.2 eV to the high binding energy was observed, which implies a diminished electron density around the metal center. Notably, the content of Ti-B bond decreases rapidly. This can be accounted for the fact that S atoms are bounded on the surface to form Ti-S bonds, which is further confirmed by the decreased ratio of B atoms to Ti atoms (1.3:1 compared to the raw 4.5:1). On the other hand, since the sulfur in Li2S4 bears the negative charge, once it contacts with TiB2, the electron is donated to the unoccupied orbitals of Ti atoms, leading to a negative shift of binding energy for Ti 2p. This have been verified as a typical Lewis acid-base interaction.16 The overreaction between sulfur and TiB2 can also feed back to the B atoms. As is shown in Figure 4e, the XPS B 1s spectrum of pristine TiB2 exhibits two peaks that could be ascribed to the B-Ti at 187.65 eV and B-O at 192.3 eV.46 After sulfur loading, a large decrease of the B-Ti bonds validates a sulfidized surface of TiB2, which matches well with the pertinent Ti 2p spectrum. Owing to the occurrence of electron transfer from Ti to S and the lower electronegativity of B (2.04) against S (2.58), the B is induced with an obvious shift to the higher binding energy. A small redshift of B 2p is also detected in TiB2/Li2S4 composite. This can be attributed to the fact that B atoms obtain extra electrons through Ti-B bond due to an increased electron density of Ti atoms. Upon surface sulfidation, the chemical reactivity of the TiB2 is suppressed, since the dangling bonds of the Ti atoms on the surfaces are saturated, leading to moderate interaction with cyclo-S8 molecules. The majority of previous works have always concentrated on the individually sulfide


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species for imitating, especially on the middle/short chain Li2Sn (n=1, 2, 4). However, such simulations cannot model the entire lithiation process, leading to a lack of fundamental comprehension of initial lithiation behavior. Thus, a systematic simulation computation, covering all the lithiation spots, is required to gain a better understand of the interaction. As shown in Figure 5a, the interactions between the sulfidized surfaces and the sulfur species including the S8 and Li2Sn (n=8, 6, 4 and 2) are investigated. The charge difference could offer a visualized bonding information, with an increased electron density around Li atoms and decreased electron density around S atoms of polysulfides. We have considered two typical surfaces (001) and (111) of TiB2 crystal and obtained similar results, except that the binding energy of (001) surface is slightly higher than that of (111) surface (Figure 5b and Figure S20). It was found that the cyclo-S8 molecule can stably adsorbed on the sulfidized surfaces without any tendency of decomposition, in contrast to the clean surfaces. Once lithiation emerges, the binding energy between Li2Sn and the sulfidized surfaces increases significantly with the increase of Li concentration, as shown in Figure 5c. In initial lithiation process, the binding strength is mainly dominated by the weak van der Waals interaction. Due to the shorter sulfur chain, the binding strength for Li2S6 is weaker than that of Li2S8.21 As the lithiation proceed, the S-S and S-Li bonds are formed between Li2Sn and substrate, leading to increasing binding strength. After full lithiation, wherein the maximum binding energy is acquired, the resultant Li2S remains stable. We also evaluate the binding energy at various lithiation stages on B-terminated sulfidized TiB2 (001) surface. As is shown in Figure S21, the rather low binding energies compared to the Titerminated surface indicate the Ti atoms play a more important role in binding polysulfides. The high stability and the strong binding strength with TiB2 surfaces of Li2Sn molecules imply that TiB2 is quite promising for restricting the polysulfide shuttling.


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Conclusion In summary, metallic and polar TiB2 was utilized for the first time in lithium-sulfur batteries with a demonstrated binding mechanism. As a result, 500-cycle long lifespan of the 70S/TiB2 cathode and high areal capacity with the sulfur loading of 3.9 mg cm-2 have been achieved. Noteworthily, since there is no physical confinement in stabilizing polysulfide shuttle in lithiumsulfur batteries, such a stable electrochemical performance can be attributed to the remarkable chemical entrapment. The underlying nature and strength of chemical interactions between Li2Sn and TiB2 are investigated by combining XPS analysis and DFT calculations. The surfacesulfidized modification is unraveled to be vital in binding polysulfides, giving rise to dual chemical S-S and S-Li interactions. We believe that our work will open up a novel prospect and provide guidance for some other metal boride materials as sulfur hosts for high-performance lithium-sulfur batteries.

Figure 1. (a) Crystal structure of titanium diboride TiB2. (b) Schematic of TiB2 (001) surface.


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Figure 2. (a) TEM image of as-prepared TiB2. (b) Lattice-resolved HRTEM image TiB2. (c) TEM image of 70S/TiB2 composite. (d) HRTEM image of 70S/TiB2 composite along (100) direction. (e) corresponding HRTEM image from marked area in (d). (f) STEM image of a single nanocomposite and corresponding EDS mapping. (g) TGA curve. (h) XRD patterns. (i) Raman spectra.


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Figure 3. (a) CV curve of initial cycle. (b) Cycling performance and corresponding Coulombic efficiency for 150 cycles at 0.2 C. (c) Long-term cycling performance over 500 cycles at 1 C. (d) Cycling performance at 0.2 C under high areal sulfur loading of 3.9 mg cm-2. (e) Areal capacity compared with previously reported Ti-based materials for Li-S batteries in the literatures.


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Figure 4. (a) The schematic illustration of surface-chemical entrapment of polysulfides. (b) Side view of sulfur passivated TiB2 on (001) surface. (c) S 2p spectrum of TiB2/S composite. (d) Ti 2p spectra of the TiB2, TiB2/S and TiB2/Li2S4. (e) B 2p spectra of the TiB2, TiB2/S and TiB2/Li2S4.


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Figure 5. (a) Binding conformations of sulfur species and charge difference for lithium polysulfides at various lithiation stages on sulfidized TiB2 (001) surface. (b), (c) Binding energies for lithium polysulfides at various lithiation stages on sulfidized TiB2 (001) and (111) surface.

ASSOCIATED CONTENT Supporting Information. The Supporting Information following files are available free of charge. Nitrogen adsorption/desorption isotherms, XRD pattern, EDS spectrum, XPS spectrum, characterization of commercial TiB2, HRTEM image, EDS spectrum, galvanostatic charge– discharge profiles at 0.2 C, the comparision of galvanostatic charge–discharge curves, cycling


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performance without LiNO3, electrochemical performance of pure TiB2, XRD patterns, Nyquist plots after cycles, high temperature performance, cycling performance of TiB2/75S cathode, adsorption test, sulfidized surface, S 2p spectrum of the elemental sulfur, binding conformations and the video of molecular dynamics simulations. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]. ORCID Chuanchuan Li: 0000-0001-5891-6335 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research is financial supported by the National Nature Science Foundation of China (No. 21471091), Academy of Sciences large apparatus United Fund (No. U1764258), Guangdong Province Science and Technology Plan Project for Public Welfare Fund and Ability Construction Project (2017A010104003), Shenzhen Science and Technology Research and Development Funds (JCYJ20170818104441521), the Fundamental Research Funds of Shandong University (No. 2018JC022) and the Taishan Scholar Project of Shandong Province (No. ts201511004). REFERENCES


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Conductive and Polar Titanium Boride as a Sulfur Host for Advanced

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