Hollow Carbon Nanoballs Coupled with Ultrafine TiO2

Subscriber access provided by Karolinska Institutet, University Library

Materials and Interfaces

Hollow Carbon Nanoballs Coupled with Ultrafine TiO2 Nanoparticles as Efficient Sulfur Host for Lithium-Sulfur Batteries Hangyu Gu, Hongkang Wang, Rong Zhang, Tianhao Yao, Ting Liu, Jinkai wang, Xiaogang Han, and Yonghong Cheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03393 • Publication Date (Web): 06 Sep 2019 Downloaded from pubs.acs.org on September 6, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Hollow Carbon Nanoballs Coupled with Ultrafine TiO2 Nanoparticles as Efficient Sulfur Host for Lithium-Sulfur Batteries

Hangyu Gu, Hongkang Wang,* Rong Zhang, Tianhao Yao, Ting Liu, Jinkai Wang, Xiaogang Han, Yonghong Cheng

State Key Lab of Electrical Insulation and Power Equipment, Center of Nanomaterials for Renewable Energy (CNRE), School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, 710049, PR China *Corresponding author. E-mail: [email protected]

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Lithium-sulfur (Li-S) battery as one of the most promising energy storage devices possesses high theoretical capacity and energy density, but suffers from the polysulfide dissolution which hinders its practical application. Herein, we present a facile synthesis of hollow carbon nanoballs (HCNBs) anchored with ultrafine TiO2 nanoparticles (TiO2@HCNBs), in which the HCNBs with hollow interiors and graphitic crystallization accommodate the volume expansion of sulfur upon lithiation and improve the cathode conductivity, while the TiO2 nanoparticles effectively trap the polysulfides through strong chemical bonding. Notably, the S@TiO2@HCNBs with a high sulfur loading of ~73 wt.% demonstrate superior cycle performance at 0.5C over 600 cycles with a slow capacity decay, and deliver high specific capacities of 1217, 1013, 893 and 773 mAh/g at 0.1C, 0.2C, 0.5C and 1C, respectively. Adsorption test clearly reveals the efficient suppress of the polysulfide shuttling owing to the spatial and chemical confinement of the TiO2@HCNBs host.

Keywords: lithium-sulfur batteries; hollow carbon nanoballs; ultrafine TiO2 nanoparticle; spatial confinement;chemical adsorption

1. Introduction Lithium-sulfur (Li-S) battery has been considered as a promising energy storage device because of its high theoretical energy density and specific capacity (about 2600 Wh/kg and 1675 mAh/g, respectively), low cost and environmental friendliness as compared with traditional lithium ion batteries.1-6 However, the commercialization of Li-S batteries is greatly hindered by the following factors: 1) the poor electronic conductivity of sulfur (normally 5×10-30 S·cm-1 at 25 °C) and the intermediate polysulfides; 2) the large volume expansion of sulfur (~80%) during the lithiation/delithiation processes which results in the pulverization of the electrodes; and 3) the “polysulfides shuttle effect” arising from the dissolution of lithium polysulfides in the electrolyte, which leads to fast capacity fading upon cycling as well as low Coulombic efficiency.6-9 With the aim to solve these problems, great efforts have been devoted to improving the electrochemical properties of Li-S batteries through rationally designing the suitable sulfur host materials, the functional interlayers, new electrolytes, as well as the innovative binders and so on.10-22 Among these methods, encapsulating sulfur into functionalized hollow/porous carbon nanostructures is considered as an effective way, owing to the good conductivity and strong sulfur affinity of the carbon-based materials.11, 12, 23 In the last few years, various carbon nanostructures have been successfully designed and applied as sulfur host for Li-S batteries, including graphene, carbon nanotubes/nanowires, and porous carbon spheres and so on.2, 3, 10, 15, 24-28 Although both the capacity and cycle life of Li-S batteries have been greatly improved by using various sulfur/carbon


Page 2 of 18

Page 3 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


composite cathodes, the dissolution of lithium polysulfides has still not been well solved. One important reason is that the binding energy between nonpolar carbon host materials and polar polysulfides is relatively weak, thus bare carbon host cannot efficiently restrain the lithium polysulfide dissolution, which is a major cause of the capacity fading.29-31 Recently, it has been demonstrated that polar metal oxides/sulfides have stronger chemical absorbability on the lithium polysulfides.32-37 However, many metal oxides (e.g., TiO2) usually exhibit intrinsically poor electrical conductivity (note that there are also metal oxides with good electrical conductivity, e.g., Ti4O736 and RuO237), which results in the relatively lower sulfur utilization and require a large amount of conductive carbon agents in the fabrication of electrodes. Integrating carbon-based nanostructures with metal oxides have be suggested as the efficient strategy to spatially and chemically suppressing the shuttle of the polysulfides with simultaneously addressing the conductivity problem.38-41 Generally, efficient carbon-based sulfur host materials should possess the following characteristics: 1) carbon structures should been reasonably designed to maximize the electrolyte permeation and the utilization of sulfur, and to improve the conductivity as well as the volumetric energy densities; 2) heteroatom doping, polar functional groups/surface and metal oxides/sulfides should been employed to increase the chemical interaction between polysulfides and the host materials while maintaining the good electrical conductivity; 3) facile fabrication and cheapness are necessary to facilitate the commercial applications. Herein, we demonstrated a facile method to fabricate hollow carbon nanoballs (HCNBs) anchored with ultrafine TiO2 nanoparticles. Serving as a sulfur host for Li-S batteries, the TiO2@HCNBs effectively restricted the diffusion of lithium polysulfides both physically and chemically. The as-prepared host structure with large internal space and external holes made it easy to load more sulfur and accommodate the large volumetric expansion during the lithiation processes. The graphitic carbon structure efficiently enhanced the cathode conductivity, and TiO2 nanoparticles played an important role in limiting the dissolution of polysulfides into the electrolyte through chemisorption. As a result, the S@TiO2@HCNBs cathode with a high sulfur loading of ~73 wt.% demonstrated superior electrochemical properties. 2. Experimental Section 2.1 Materials synthesis Synthesis of hollow carbon nanoballs (HCNBs). The HCNBs were synthesized by the combined chemical vapor deposition (CVD) and template synthesis. In a typical synthesis, 5.0g CaCO3 nanoparticles (Aladdin, 99.5%, average size of 0.8μm) were annealed at 1100 °C for 30 min with a heating rate of 10 °C /min under the Ar/CH4 flow (200 sccm Ar and 20 sccm CH4). After cooling down to room temperature naturally, the C@CaO particles with conformal morphology



were obtained, which were then etched with the concentrated HCl aqueous solution to remove the CaO template. After washing with deionized water thoroughly, the HCNBs was finally obtained and then dried at 60°C overnight. Synthesis of TiO2@HCNBs. In a typical synthesis, 30mg HCNBs were firstly mixed with 600μL of TiCl4 ethanol solution (TiCl4/ethanol, 1:9 by volume) under ultrasonic irradiation for 1 h, in order to achieve uniform mixing. After the complete evaporation of ethanol, the mixture was then annealed at 350°C for 2 h in an electric oven with a temperature ramping rate of 10 °C/min in air. The mass ratio of TiO2 in TiO2@HCNBs can be readily controlled by adjusting the ratio of TiCl4/ HCNBs. Synthesis of S@TiO2@HCNBs. Commercial sulfur powder (Sigma-Aldrich) was mixed with TiO2@HCNBs by grinding at a weight ratio of 45 : 10, and the composite was then sealed in a Teflon reaction kettle (10 mL) in glovebox under Ar atmosphere, which was heated at 155 °C for 12 h at a heating rate of 1 °C/min, thus S@TiO2@HCNBs was obtained. Synthesis of Li2S6 solution. Li2S6 solution was prepared by the comproportionation reaction between commercial Li2S (Alfa, 99.9%) and S powders in 1, 2-dimethoxyethane (DME, 99.5%, Maclin) / 1, 3-dioxalane (DOL, 99.5%, Maclin) solution. Typically, Li2S6 solution (5 mM) was prepared by reacting Li2S (4.6 mg) and S (9.6 mg) in 20 mL mixture solution of DME and DOL (1:1 in volume), which was heated at 80 °C under constant stirring in Ar-filled glovebox for 20h. The as-obtained solution was used for the adsorption test, in order to demonstrate the trapping effect of the HCNBs and TiO2@HCNBs on the polysulfides. 2.2 Materials characterization The phase structure and evolution were recorded by using X-ray diffraction (XRD) patterns on a Bruker D2 Phaser X-ray diffractometer under Cu Kα radiation (λ=1.5418 Å, at 30 kV and 10 mA). The morphological structures were investigated by scanning electron microscope (SEM, Quanta 250F FEI) and transmission electron microscope (TEM, JEOL-2100). The elemental analysis was performed using energy-dispersive X-ray spectroscopy (EDS) on JEM-F200 instrument. Thermogravimetric analysis (TGA) was conducted by a Mettler thermal analysis TGA/DSC system in Ar or air atmosphere. Raman spectroscopy was performed using a Renishaw Raman RE01 scope at 514 nm (Ar excitation laser). N2 sorption isotherms were recorded using a Quantachrome Surface Area Analyzer (Autosorb iQ-MP) at 77 K, according to the Brunauere-Emmette-Teller (BET) method. The ultraviolet/visible light (UV-Vis) adsorption spectra was measured using a V-670 spectrophotometer (JASCO, ISN-723). 2.3 Electrochemical measurement CR2025 coin-type cells were assembled in Ar-filled glovebox (both H2O and O2 contents less than 1.0 ppm). Slurry was first fabricated by mixing the active materials (S@PCBNs or


Page 4 of 18

Page 5 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S@TiO2@HCNBs), Ketjenblack and polyvinylidene fluoride (PVDF) with weight ratio of 82: 8: 10, in the solvent of N-methyl-2-pyrrolidone (NMP). Afterwards, the slurry was coated on the current collector of aluminum foil, which was dried at 60 °C under vacuum for 12 h. The average sulfur loading in each working electrode was around 2.0 mg/cm2. The electrolyte was 1 M bis (trifluoromethane) sulfonimide lithium (LiTFSI) in the mixture solvent of 1, 3-dioxolane (DOL) and 1, 2-dimethoxyethane (DME) (DOL/DME, 1:1 in volume) with addition of 1 wt.% LiNO3. Lithium foil and microporous membrane (Celgard 2400) were used as the counter electrode and the separator, respectively. Galvanostatic discharge-charge profiles were obtained on a battery test system (Neware BTS) in 1.5-2.8 V (vs. Li/Li+) at room temperature. Cyclic voltammetry was measured using Autolab PGSTAT 302N electrochemical station at 0.1 mV/s. The specific capacities were calculated on the basis of the sulfur weight. The electrochemical impedance spectroscopy (EIS) was measured with voltage amplitude of 10 mV in the frequency range of 10 MHz to 0.01 Hz. 3. Results and discussion

Figure 1. (a) Schematic illustration of the syntheses of HCNBs, TiO2@HCNBs and S@TiO2@HCNBs. (b-g) SEM images of (b) HCNBs, (c, d) TiO2@HCNBs, (e) S@HCNBs and (f, g) S@TiO2@HCNBs. Figure 1a schematically illustrates the syntheses of HCNBs, TiO2@HCNBs and S@TiO2@HCNBs. A strategy combining chemical vapor deposition (CVD) and template synthesis was applied to synthesize the HCNBs. Commercial CaCO3 particles with average size of about 800 nm were used as template and CH4 was used as carbon source. With increasing the



reaction temperature, CaCO3 first decomposed into CaO with conformal morphology at high temperature (over 800 °C, CaCO3 → CaO + CO2↑), and the CaO served as a support for the deposition of carbon shells. When the temperature is high enough (over 1000°C), CH4 started to decompose into vapor carbon (CH4 → C + 2H2↑) which then uniformly deposited on the surface of CaO. The CaO templates can be readily removed via HCl etching, thus leaving the porous graphitic hollow carbon nanoballs (HCNBs). As shown in the SEM image (Figure 1b), the HCNBs with an average size of around 500~800 nm are well dispersed and show hollow interiors. For the deposition of TiO2 on the HCNBs, hydrolysis and decomposition of TiCl4 within HCNBs was applied, followed by annealing at a low temperature in air. The content of TiO2 can be adjusted by changing the weight ratio of TiCl4/HCNBs. In order to optimize the performance of the composite structure, the TiO2 contents were tuned in the range of 15 to 50 wt.%, and the TiO2@HCNBs host achieves better comprehensive performance when the TiO2 content was around 30 wt.% (Figure S1). Notably, the morphology of the TiO2@HCNBs is well maintained, as revealed by Figure 1c-d. In addition, sulfur was loaded within the HCNBs and TiO2@HCNBs host by a modified melting method in an inert atmosphere. Liquid sulfur has good fluidity at 155°C and can easily diffuse into the micro- and mesoporous materials because of the capillarity, allowing the full filling of the inner void space of HCNBs and TiO2@HCNBs. Hence, the sulfur can be uniformly distributed in the HCNBs (Figure 1e) and TiO2@HCNBs (Figure 1f-g).

Figure 2. (a) TEM and (b, c) HRTEM images of S@TiO2@HCNBs. (d) TEM image of S@TiO2@HCNBs with corresponding EDS maps of C, Ti, O and S.

Figure 2a shows the TEM image of the S@TiO2@HCNBs, which apparently shows that the HCNBs are open and interconnected to some extent, thus allowing the facile encapsulation of sulfur into the internal spaces. This distinctive structure not only provides spatial control of sulfur, but also accommodates the volume changes of sulfur during the lithiation/delithiation processes.


Page 6 of 18

Page 7 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Moreover, the HRTEM image taken at the shell region clearly shows the well-defined lattice fringes with d spacing of 0.35 nm (Figure 2b), which can be related to the (002) plane of hexagonal graphitic carbon,5, 42 indicating the high crystallization that is beneficial to the electrical conductivity of the composite structure.5,

43

Meanwhile, the clear lattice fringes with an

inter-planar spacing of about 0.24 nm shown in HRTEM image in Figure 2c are correlated to the (110) planes of anatase TiO2,33 and the particle size is extremely small with size less than 5 nm. Furthermore, elemental EDS mapping of S@TiO2@HCNBs was taken in the region as shown in Figure 2d, revealing the both S and TiO2 are well distributed within the HCNBs.

Figure 3. (a) XRD patterns of HCNBs, TiO2@HCNBs and S@TiO2@HCNBs, (b) TGA curves, (c) Raman spectra and (d) nitrogen sorption isotherms of the HCNBs and TiO2@HCNBs. Inset of (d) shows the corresponding pore size distribution.

The phases of the products were investigated by XRD analysis, as shown in Figure 3a. For the HCNBs, two prominent peaks locate at around 26o and 43o which can be indexed to the (002) and (101) planes of crystalline graphite, respectively. In the XRD pattern of the TiO2@HCNBs composite, five apparent diffraction peaks can be indexed to the (101), (004), (200), (105) and (204) planes of the anatase TiO2 phase (JCPDS No. 71-1166).44 It’s noteworthy to mention that the broad diffraction peaks for TiO2 indicate the small nanocrystallites according to the Scherrer equation. After filling the TiO2@HCNBs with sulfur, all the diffraction peaks in the



S@TiO2@HCNBs can be indexed to the crystalline sulfur, and the peaks for HCNBs and TiO2 become invisible owing to the high content of sulfur. In order to accurately determine the content of sulfur and TiO2, thermogravimetric analysis (TGA) was carried out and shown in Figure 3b. The TGA curves for both S@TiO2@HCNBs and S@HCNBs reveal the sulfur content is almost the same and is approximately 73 wt. %. For the TiO2@HCNBs, the weight loss in the range of 400-600 °C can be corresponded to the combustion of carbon (HCNBs), thus the content of TiO2 is around 30%. Raman spectra were also measured in order to evaluate the graphitic structure of HCNBs and their evolution after loading TiO2 (Figure 3c). In the HCNBs, two prominent peaks at around 1317 and 1579 cm-1 are related to the defects/disorders in the graphitic carbon layers (D-band) and the vibration of sp2 carbon atoms in the hexagonal lattice (G-band),42 respectively. The ID/IG intensity ratio indicates the defect degree in the carbon-based materials, and the ID/IG ratios for the HCNBs and TiO2@HCNBs are 1.01 and 1.29, respectively, indicating the deposition of TiO2 affects the graphitic structure of HCNBs to some extent. Besides, four prominent Raman bands appear in the TiO2@HCNBs, locating at 151 cm−1 (Eg), 399 cm−1 (B1g), 518 cm−1(A1g/B1g) and 641 cm−1 (Eg), which are corresponding to the anatase TiO2 phase.45, 46 The surface and pore characteristics for both HCNBs and TiO2@HCNBs were further investigated by Brunauere-Emmette-Teller (BET) measurements (Figure 3d). The specific surface area for the bare HCNBs was calculated to be ~155 m2/g, and the total pore volume is as high as 1.37 cm3/g, owing to the presence of macro/mesopores in the hollow voids and the carbon shells. After loading TiO2 nanoparticles, the specific surface area for the TiO2@HCNBs is slightly decreased to ~124 m2/g. As shown in the pore size distribution plot (inset of Figure 3d), the mesopore size in the HCNBs becomes enlarged after TiO2 loading in the TiO2@HCNBs, which may be attributed to the partial combustion at the interfacial region near the mesopores when annealing in air. There is no doubt that the porous HCNBs with vast macroporous cavities and mesopores within the carbon shells greatly facilitate the sulfur diffusion into the inner space of TiO2@HCNBs and the suppression of the shuttling polysulfides. The electrochemical performances of both S@HCNBs and S@TiO2@HCNBs cathodes were characterized by using CR2025 coin-type cells. Figure 4a shows the CV curves of S@HCNBs cathode from 1st to 5th cycle at a scan rate of 0.1 mV/s. There are two obvious reduction peaks at 2.30 and 2.05 V, respectively. The peak at 2.30 V corresponds to the conversion of elemental sulfur (S8) into long-chain lithium polysulfides Li2Sx (6 < x ≤ 8) and the peak at 2.05V represents the further reduction of the long chain lithium polysulfides to short chain lithium polysulfides Li2Sx (2 < x ≤ 6) and then to Li2S2 and Li2S.47-49 In contrast, there is only one large oxidation peak at around 2.45 V in the anodic scans, corresponding the conversion of Li2S/Li2S2 to S8. The CV


Page 8 of 18

Page 9 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


curves since the 3rd cycle onward exhibit similar shapes but with large different from the first two curves, indicating the structure reorganization. The galvanostatic discharge-charge curves of S@HCNBs cathode are shown in Figure 4b. The discharge curve at 0.1C (1C = 1675 mA/g) show two obvious discharge platforms at 2.30 and 2.05 V which are ascribed to the conversion of S8 into long-chain lithium polysulfides and Li2S2/Li2S, respectively. Meanwhile, there is only one well-defined platform in the charge curve, indicating the multi-step reaction, which is well consistent with oxidation peaks in the 1st CV curve. The weak tail at around 1.7 V in the 1st CV cathodic scan and the discharge curve can be assigned to the reduction of LiNO3.15 Similar to the S@HCNBs electrode, the CV curves and discharge-charge curves of the S@TiO2@HCNBs cathode are shown in Figure 4c-d. Differently, the five CV curves are well overlapped since the 2nd one onward, indicating the improved reversibility of the electrochemical reactions. By comparing Figures 4b vs. 4d, the S@TiO2@HCNBs also show similar discharge/charge profiles, but which display much higher capacity than that of the S@HCNBs, suggesting the much better redox reaction kinetics and more efficient utilization of the active sulfur material in TiO2@HCNBs host.

Figure 4. Electrochemical properties of the S@HCNBs and S@TiO2@HCNBs. (a) CV curves and (b) voltage profiles of S@HCNBs at different current densities. (c) CV curves and (d) voltage profiles of S@TiO2@HCNBs at different current densities. (e) Rate and (f) cycle performances of the S@HCNBs and S@TiO2@HCNBs electrodes. (g) Long-term cycle performance together with the corresponding Coulombic efficiencies for the S@TiO2@HCNBs electrode at 0.5C. Note that the initial 3 cycles were performed at 0.1C (see Figure S2). (h) Nyquist plots for the fresh S@HCNBs and S@TiO2@HCNBs electrodes.



Figure 4e compares the rate performances of the S@HCNBs and S@TiO2@HCNBs cathodes at different current densities ranging from 0.1 to 1C. The S@HCNBs electrode delivers reversible discharge capacities of 1189, 803, 730 and 615 mA h/g each after 10 cycles at 0.1C, 0.2C, 0.5C and 1C, respectively. When recovering the current density to 0.1C, a reversible capacity of 749 mA h/g is achieved, which is slightly lower than the capacity in the 10th cycle at 0.1C (853 mA h/g). Apparently, S@TiO2@HCNBs cathode delivers even higher discharge capacities of 1217, 1013, 893 and 773 mA h/g at various current densities from 0.1C to 1C. When the current density is back to 0.1C, the discharge capacity of S@TiO2@HCNBs is as high as 1015 mA h/g in the 50th cycle. Figure 4f displays the cycle performances of the S@HCNBs and S@TiO2@HCNBs electrodes at 0.5C, and both of them display superior cycling stability with approximately 90% capacity retention after 50 cycles. In addition, the S@TiO2@HCNBs delivers apparently higher reversible capacity (773 mA h/g in the 50th cycle) than that of the S@HCNBs (653 mA h/g in the 50th cycle). To further demonstrate the superior long-term cycling performance, the S@TiO2@HCNBs electrode was tested at 0.5 C for 600 cycles (Figure 4g). In order to better activate the electrode materials, the S@TiO2@HCNBs was first cycled at 0.1C for 3 cycles (Figure S2). A high specific capacity of 866 mA h/g is initially reached at 0.5C and can be maintained at 508 mA h/g after 600 cycles, with a small average capacity decay rate of 0.069% per cycle (Figure S3). This finding demonstrates the excellent cycling stability of the Li-S batteries using TiO2@HCNBs as host materials. The charge transfer characteristics were further investigated via electrochemical impedance spectroscopy (EIS), and Figure 4h shows the Nyquist plots. It can be observed that both S@HCNBs and S@TiO2@HCNBs electrodes show very small semicircle in high-frequency region, exhibiting low charge transfer resistances (Rct) of approximately 67 and 88 Ω, respectively. The relatively larger Rct of S@TiO2@HCNBs may be attributed to the presence of TiO2 and the relatively low carbon content in the TiO2@HCNBs, as the electrical conductivity of TiO2 is quite poor. Apparently, it’s observed that the use of HCNBs as host for insulating sulfur greatly enhances the electrode conductivity. Besides, the intimate contact between sulfur and host, and the efficiently spatial trapping effect of the hollow carbon on sulfur, which contributes to the superior electrochemical performance of both S@HCNBs and S@TiO2@HCNBs electrodes. Moreover, the introduction of TiO2 can further improve the rate and cycle performances of S@TiO2@HCNBs, as the polar adsorption between TiO2 and the polysulfides can efficiently suppress the shuttle effect of soluble polysulfides in S@TiO2@HCNBs cathode.17, 33, 41


Page 10 of 18

Page 11 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) Photographs of the color changes in Li2S6 solution after adding absorbants (HCNBs or TiO2@HCNBs) and standing for 2 h. (b) UV-Vis spectra of Li2S6 solution before and after mixing with HCNBs and TiO2@HCNBs. (c) Schematic illustration of the lithiation process of sulfur cathode and the diffusion of polysulfides in electrolytes in Li-S batteries.

To better understand the enhanced electrochemical performance of S@TiO2@HCNBs, we examined the adsorption capability of HCNBs and TiO2@HCNBs on the lithium polysulfides (Li2S6), as shown in Figure 5a. Firstly, 10mg of HCNBs and 10mg of TiO2@HCNBs was mixed with 2 mL of Li2S6 solution (10 mmol/L, in the solvent of DOL and DME with volume ratio of 1:1), respectively. After standing at room temperature for 2 hours, the color of the Li2S6 solution with HCNBs becomes light yellow, while the solution with TiO2@HCNBs almost becomes colorless. This phenomenon reveals that HCNBs can absorb parts of lithium polysulfides through physical adsorption effect and spatial confinement, while TiO2@HCNBs can absorb the lithium polysulfides more effectively, which was further confirmed by the UV−vis adsorption spectra (Figure 5b). The Li2S6 solution shows two obvious adsorption peaks at 250 and 300 nm, respectively, which can be both ascribed to the S62- species.50 There is also a slightly characteristic peak at around 400nm, which can be ascribed to the S42- species. Note that the curve shows a sudden drop at around 350 nm, which is due to the switching of the light source. In the Li2S6 solution with HCNBs, the UV-Vis adsorption intensity decreases obviously, which proves the HCNBs can adsorb the lithium polysulfides but with limited capability. However, the adsorption intensity in the Li2S6 solution with TiO2@HCNBs is significantly lower than that with only HCNBs, indicating that TiO2 shows strong adsorption ability to the lithium polysulfides. It has been demonstrated by previous reports that lithium polysulfides are intrinsically polar species with the terminal sulfur bearing most of the negative charge.51, 52 However, non-polar carbon materials



can only provide weak physical confinement of lithium polysulfides, which cause the common diffusion of lithium polysulfides in electrolyte and eventually migrate to the anode. The effective absorption capacity of TiO2@HCNBs is mainly because of the polar-polar interaction between TiO2 and lithium polysulfides. Previous research revealed that metal chalcogenides (oxides and sulfides) such as TiO2 and SnO2 possess intrinsic network polarity, where the surface metal or chalcogen ions synergistically interact with the Sx2- and Li+ ions.17, 53, 54 Therefore, the addition of polar TiO2 nanoparticles to the carbon-sulfur composites results in the reversible storage/release of lithium polysulfides in/from the pores of HCNBs during energy exchange process. Figure 5c schematically illustrates the lithiation process of sulfur cathode and the dissolution-diffusion of polysulfides in electrolytes of Li-S batteries. When elemental sulfur is directly used as cathode material (the upper row), the large volume expansion of sulfur particles upon lithiation will result in the pulverization of the cathode, and the dissolution of lithium polysulfides (LiPs, marked in Figure 5c) would result in the contact loss of the active materials, the accumulation of insulating layers on the anode and finally the capacity fading. When introducing HCNBs as sulfur host (the middle row), the host can accommodate the volume expansion of sulfur effectively because of the flexible external shell and the large internal space, and the dissolution of lithium polysulfides can be inhibited through physical confinement to some extent. In this case, Li-S batteries may show satisfied short-term cycling performance. When elemental sulfur is confined within the TiO2@HCNBs host, the porous structure and proper internal space of TiO2@HCNBs can solve the problem of volume expansion of sulfur. More importantly, the well-distributed TiO2 particles can effectively trap the lithium polysulfides and alleviate their dissolution-diffusion in the electrolyte. What’s more, the highly conductive composite cathode can facilitate the transport of the electrons generated during the redox of the polysulfides. Benefitting from the unique structural and compositional advantages, the S@TiO2@HCNB electrode sufficiently utilizes the active materials and ensures the smooth progress of electrochemical reactions, exhibiting enhanced reversible capacity and long-term cycling stability. 4. Conclusions In summary, we demonstrated the synthesis of hollow carbon nanoballs (HCNBs) via CVD route using commercial CaCO3 particles as conformal templates, and the loading of ultrafine TiO2 nanoparticles onto the HCNBs (TiO2@HCNBs), which served as efficient sulfur hosts. When applied as cathode for Li-S batteries, the S@TiO2@HCNBs electrode exhibited a high reversible capacity of 1217, 1013, 893 and 773 mA h/g at various current densities ranging from 0.1C to 1C, and displayed a long-term cycling stability over 600 cycles with a small average capacity decay rate of 0.069% per cycle at 0.5C. The superior electrochemical performance can be attributed to


Page 12 of 18

Page 13 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the novel structure characteristics of TiO2@HCNBs: 1) the good flexibility, high conductivity, porous structure and large internal spaces of HCNBs, which can not only accommodate the volume expansion of sulfur upon lithiation but also facilitate to improve the electrode conductivity as well as the structure integrity; 2) the strong interaction between polar TiO2 and lithium polysulfides allowed the effective trapping of the lithium polysulfides near the HCNBs, thus resulting in the sufficient utilization of active materials and the suppress of the shuttling of the lithium polysulfides. More importantly, we provided a simplified and low-cost route for the construction of cathode materials for high-performance Li-S batteries, and the as-prepared HCNBs can also be used as a general host for high-capacity electrode materials.

Supporting Information. Effect of TiO2 content in the TiO2@HCNBs on Li-S battery performance; Cycle performance of S@TiO2@HCNBs at 0.5C with 3 initial discharge cycles at 0.1C; Comparison of the cycle performances of S@HCNBs and S@TiO2@HCNBs in the initial 200 cycles.

Acknowledgements. This work was supported by the Natural Science Basis Research Plan in Shaanxi Province of China (No. 2018JM5085) and the Fundamental Research Funds for the Central Universities in China. H.W. appreciates the support of the Tang Scholar Program from the Cyrus Tang Foundation. We thank Dr. Chao Li from the Instrument Analysis Center of Xi’an Jiaotong University for the TEM/EDS measurements.

Conflict of Interest The authors declare no competing financial interest.



References (1) Ji, X.; Lee, K. T.; Nazar, L. F. A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries. Nat. Mater. 2009, 8, 500. (2) Jayaprakash, N.; Shen, J.; Moganty, S. S.; Corona, A.; Archer, L. A. Porous hollow carbon@sulfur composites for high-power lithium-sulfur batteries. Angew. Chem. Int. Ed. 2011, 50, 5904. (3) Guo, J.; Xu, Y.; Wang, C. Sulfur-impregnated disordered carbon nanotubes cathode for lithium-sulfur batteries. Nano Lett. 2011, 11, 4288. (4) Fan, Z.; Wang, B.; Xi, Y.; Xu, X.; Li, M.; Li, J.; Coxon, P.; Cheng, S.; Gao, G.; Xiao, C.; Yang, G.; Xi, K.; Ding, S.; Kumar, R. V. A NiCo2O4 nanosheet-mesoporous carbon composite electrode for enhanced reversible lithium storage. Carbon 2016, 99, 633. (5) Wang, H.; Wang, J.; Cao, D.; Gu, H.; Li, B.; Lu, X.; Han, X.; Rogach, A. L.; Niu, C. Honeycomb-like carbon nanoflakes as a host for SnO2 nanoparticles allowing enhanced lithium storage performance. J. Mater. Chem. A 2017, 5, 6817. (6) Xiao, Z.; Yang, Z.; Zhang, L.; Pan, H.; Wang, R., Sandwich-Type NbS2@S@I-Doped Graphene for High-Sulfur-Loaded, Ultrahigh-Rate, and Long-Life Lithium–Sulfur Batteries. ACS Nano 2017, 11, 8488. (7) Xiao, Z.; Yang, Z.; Li, Z.; Li, P.; Wang, R. Synchronous Gains of Areal and Volumetric Capacities in Lithium–Sulfur Batteries Promised by Flower-like Porous Ti3C2Tx Matrix. ACS Nano 2019, 13, 3404. (8) Barchasz, C.; Leprêtre, J. C.; Alloin, F.; Patoux, S. New insights into the limiting parameters of the Li/S rechargeable cell. J. Power Sources 2012, 199, 322. (9) Mikhaylik, Y. V.; Akridge, J. R. Polysulfide Shuttle Study in the Li/S Battery System. J. Electrochem. Soc. 2004, 151, A1969. (10) Schuster, J.; He, G.; Mandlmeier, B.; Yim, T.; Lee, K. T.; Bein, T.; Nazar, L. F. Spherical ordered mesoporous carbon nanoparticles with high porosity for lithium-sulfur batteries. Angew. Chem. Int. Ed. 2012, 51, 3591. (11) Zhang, K.; Zhao, Q.; Tao, Z.; Chen, J. Composite of sulfur impregnated in porous hollow carbon spheres as the cathode of Li-S batteries with high performance. Nano Research 2012, 6, 38. (12) He, G.; Evers, S.; Liang, X.; Cuisinier, M.; Garsuch, A.; Nazar, L. F. Tailoring porosity in carbon nanospheres for lithium-sulfur battery cathodes. ACS Nano 2013, 7, 10920. (13) Liu, J.; Yang, T.; Wang, D.-W.; Lu, G. Q.; Zhao, D.; Qiao, S. Z. A facile soft-template synthesis of mesoporous polymeric and carbonaceous nanospheres. Nat. Commun. 2013, 4, 2798. (14) Wei Seh, Z.; Li, W.; Cha, J. J.; Zheng, G.; Yang, Y.; McDowell, M. T.; Hsu, P. C.; Cui, Y. Sulphur-TiO2 yolk-shell nanoarchitecture with internal void space for long-cycle lithium-sulphur batteries. Nat.Commun. 2013, 4, 1331. (15) Gu, H.; Zhang, R.; Wang, P.; Xie, S.; Niu, C.; Wang, H. Construction of three-dimensional ordered porous carbon bulk networks for high performance lithium-sulfur batteries. J. Colloid Interf. Sci. 2019, 533, 445. (16) Peng, H.-J.; Hou, T.-Z.; Zhang, Q.; Huang, J.-Q.; Cheng, X.-B.; Guo, M.-Q.; Yuan, Z.; He, L.-Y.; Wei, F. Strongly Coupled Interfaces between a Heterogeneous Carbon Host and a Sulfur-Containing Guest for Highly Stable Lithium-Sulfur Batteries: Mechanistic Insight into Capacity Degradation. Adv. Mater. Interf. 2014, 1, 1400227. (17) Xiao, Z.; Yang, Z.; Wang, L.; Nie, H.; Zhong, M.; Lai, Q.; Xu, X.; Zhang, L.; Huang, S. A


Page 14 of 18

Page 15 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Lightweight TiO2/Graphene Interlayer, Applied as a Highly Effective Polysulfide Absorbent for Fast, Long-Life Lithium-Sulfur Batteries. Adv. Mater. 2015, 27, 2891. (18) Tao, T.; Lu, S.; Fan, Y.; Lei, W.; Huang, S.; Chen, Y. Anode Improvement in Rechargeable Lithium-Sulfur Batteries. Adv. Mater. 2017, 29, 1700542. (19) Xu, G.; Kushima, A.; Yuan, J.; Dou, H.; Xue, W.; Zhang, X.; Yan, X.; Li, J. Ad hoc solid electrolyte on acidized carbon nanotube paper improves cycle life of lithium–sulfur batteries. Energy Environ. Sci. 2017, 10, 2544. (20) Jie, L.; Galpaya, D. G. D.; Yan, L.; Su, M. M.; Zhang, S. Exploiting Robust Biopolymer Network Binder for Ultrahigh-Areal-Capacity Li-S Battery. Energy Environ. Sci. 2017, 10, 750. (21) Zhang, Z.; Wang, Y.; Jian, L.; Sun, D.; Ma, X.; Jin, Y.; Cui, Y. A multifunctional graphene oxide-Zn(II)-triazole complex for improved performance of lithium-sulfur battery at low temperature. Electrochim. Acta 2018, 271, 58. (22) Wang, L.; Yang, Z.; Nie, H.; Gu, C.; Hua, W.; Xu, X.; Chen, X.; Chen, Y.; Huang, S. A lightweight multifunctional interlayer of sulfur–nitrogen dual-doped graphene for ultrafast, long-life lithium–sulfur batteries. J. Mater. Chem. A 2016, 4, 15343. (23) Pei, F.; An, T.; Zang, J.; Zhao, X.; Fang, X.; Zheng, M.; Dong, Q.; Zheng, N. From Hollow Carbon Spheres to N-Doped Hollow Porous Carbon Bowls: Rational Design of Hollow Carbon Host for Li-S Batteries. Adv. Energy Mater. 2016, 6, 1502539. (24) Tang, C.; Li, B.-Q.; Zhang, Q.; Zhu, L.; Wang, H.-F.; Shi, J.-L.; Wei, F. CaO-Templated Growth of Hierarchical Porous Graphene for High-Power Lithium-Sulfur Battery Applications. Adv. Funct. Mater. 2016, 26, 577. (25) Zheng, Z.; Guo, H.; Pei, F.; Zhang, X.; Chen, X.; Fang, X.; Wang, T.; Zheng, N. High Sulfur Loading in Hierarchical Porous Carbon Rods Constructed by Vertically Oriented Porous Graphene-Like Nanosheets for Li-S Batteries. Adv. Funct. Mater. 2016, 26, 8952. (26) Xi, K.; Cao, S.; Peng, X.; Ducati, C.; Kumar, R. V.; Cheetham, A. K. Carbon with hierarchical pores from carbonized metal-organic frameworks for lithium sulphur batteries. Chem. Commun. 2013, 49, 2192. (27) Li, Z.; Wu, H. B.; Lou, X. W. Rational designs and engineering of hollow micro-/nanostructures as sulfur hosts for advanced lithium–sulfur batteries. Energy Environ. Sci. 2016, 9, 3061. (28) Zhong, Y.; Wang, S.; Sha, Y.; Liu, M.; Cai, R.; Li, L.; Shao, Z. Trapping sulfur in hierarchically porous, hollow indented carbon spheres: a high-performance cathode for lithium–sulfur batteries. J. Mater. Chem. A 2016, 4, 9526. (29) Hu, G.; Sun, Z.; Shi, C.; Fang, R.; Chen, J.; Hou, P.; Liu, C.; Cheng, H. M.; Li, F. A Sulfur-Rich Copolymer@CNT Hybrid Cathode with Dual-Confinement of Polysulfides for High-Performance Lithium-Sulfur Batteries. Adv. Mater. 2017, 29, 1603835. (30) Su, D.; Cortie, M.; Fan, H.; Wang, G. Prussian Blue Nanocubes with an Open Framework Structure Coated with PEDOT as High-Capacity Cathodes for Lithium-Sulfur Batteries. Adv. Mater. 2017, 29, 1700587. (31) Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F. Advances in lithium–sulfur batteries based on multifunctional cathodes and electrolytes. Nat. Energy 2016, 1, 16132. (32) Hwang, J.-Y.; Kim, H. M.; Lee, S.-K.; Lee, J.-H.; Abouimrane, A.; Khaleel, M. A.; Belharouak, I.; Manthiram, A.; Sun, Y.-K. High-Energy, High-Rate, Lithium-Sulfur Batteries: Synergetic Effect of Hollow TiO2-Webbed Carbon Nanotubes and a Dual Functional Carbon-Paper Interlayer. Adv. Energy Mater. 2016, 6, 1501480.



(33) Li, Z.; Zhang, J.; Guan, B.; Wang, D.; Liu, L. M.; Lou, X. W. A sulfur host based on titanium monoxide@carbon hollow spheres for advanced lithium-sulfur batteries. Nat. Commun. 2016, 7, 13065. (34) Liu, J.; Yuan, L.; Yuan, K.; Li, Z.; Hao, Z.; Xiang, J.; Huang, Y. SnO2 as a high-efficiency polysulfide trap in lithium-sulfur batteries. Nanoscale 2016, 8, 13638. (35) Cao, B.; Li; Hou, B.; Mo, Y.; Yin, L.; Chen, Y. Synthesis of Double-Shell SnO2@C Hollow Nanospheres as Sulfur/Sulfide Cages for Lithium-Sulfur Batteries. ACS Appl. Mater. Interf. 2016, 8, 27795. (36) Pang, Q.; Kundu, D.; Cuisinier, M.; Nazar, L. F. Surface-enhanced redox chemistry of polysulphides on a metallic and polar host for lithium-sulphur batteries. Nat. Commun. 2014, 5, 4759. (37) Lu, Q.; Sun, Y.; Liao, K.; Zou, X.; Hamada, I.; Zhou, W.; Ni, M.; Shao, Z. Enhancing the cycle life of Li-S batteries by designing a free-standing cathode with excellent flexible, conductive, and catalytic properties. Electrochim. Acta 2019, 298, 421. (38) Choi, S.; Seo, D. H.; Kaiser, M. R.; Zhang, C.; van der laan, T.; Han, Z. J.; Bendavid, A.; Guo, X.; Yick, S.; Murdock, A. T.; Su, D.; Lee, B. R.; Du, A.; Dou, S. X.; Wang, G. WO3 nanolayer coated 3D-graphene/sulfur composites for high performance lithium/sulfur batteries. J. Mater. Chem. A 2019, 7, 4596. (39) Zhong, Y. J.; Xu, X. M.; Liu, Y.; Wang, W.; Shao, Z. P. Recent progress in metal-organic frameworks for lithium-sulfur batteries. Polyhedron 2018, 155, 464. (40) 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. (41) Lu, Q.; Zhong, Y.; Zhou, W.; Liao, K.; Shao, Z. Dodecylamine-Induced Synthesis of a Nitrogen-Doped Carbon Comb for Advanced Lithium–Sulfur Battery Cathodes. Adv. Mater. Interf. 2018, 5, 1701659. (42) Wang, H.; Wang, J.; Xie, S.; Liu, W.; Niu, C. Template synthesis of graphitic hollow carbon nanoballs as supports for SnOx nanoparticles towards enhanced lithium storage performance. Nanoscale 2018, 10, 6159. (43) Gu, H.; Cao, D.; Wang, J.; Lu, X.; Li, Z.; Niu, C.; Wang, H. Micro-CaCO3 conformal template synthesis of hierarchical porous carbon bricks: As a host for SnO2 nanoparticles with superior lithium storage performance. Mater. Today Energy 2017, 4, 75. (44) Wang, H.; Shao, W.; Gu, F.; Zhang, L.; Lu, M.; Li, C. Synthesis of Anatase TiO2 Nanoshuttles by Self-Sacrificing of Titanate Nanowires. Inorg. Chem. 2009, 48, 9732. (45) Hearne, G. R.; Zhao, J.; Dawe, A. M.; Pischedda, V.; Maaza, M.; Nieuwoudt, M. K.; Kibasomba, P.; Nemraoui, O.; Comins, J. D.; Witcomb, M. J. Effect of grain size on structural transitions in anataseTiO2: A Raman spectroscopy study at high pressure. Phys. Rev. B 2004, 70, 134102. (46) Orendorz, A.; Brodyanski, A.; Lösch, J.; Bai, L. H.; Chen, Z. H.; Le, Y. K.; Ziegler, C.; Gnaser, H. Phase transformation and particle growth in nanocrystalline anatase TiO2 films analyzed by X-ray diffraction and Raman spectroscopy. Surf. Sci. 2007, 601, 4390. (47) Manthiram, A.; Fu, Y.; Chung, S. H.; Zu, C.; Su, Y. S. Rechargeable lithium-sulfur batteries. Chem. Rev. 2014, 114, 11751. (48) Yang, Y.; Zheng, G.; Cui, Y. Nanostructured sulfur cathodes. Chem. Soc. Rev. 2013, 42, 3018. (49) Hua, W.; Yang, Z.; Nie, H.; Li, Z.; Yang, J.; Guo, Z.; Ruan, C.; Chen, X.; Huang, S. Polysulfide-Scission Reagents for the Suppression of the Shuttle Effect in Lithium–Sulfur Batteries. ACS Nano 2017, 11, 2209.


Page 16 of 18

Page 17 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(50) Barchasz, C.; Molton, F.; Duboc, C.; Lepretre, J. C.; Patoux, S.; Alloin, F. Lithium/sulfur cell discharge mechanism: an original approach for intermediate species identification. Anal. Chem. 2012, 84, 3973. (51) Wang, L.; Zhang, T.; Yang, S.; Cheng, F.; Liang, J.; Chen, J. A quantum-chemical study on the discharge reaction mechanism of lithium-sulfur batteries. J. Energy Chem. 2013, 22, 72. (52) Pascal, T. A.; Wujcik, K. H.; Velasco-Velez, J.; Wu, C.; Teran, A. A.; Kapilashrami, M.; Cabana, J.; Guo, J.; Salmeron, M.; Balsara, N. X-ray Absorption Spectra of Dissolved Polysulfides in Lithium-Sulfur Batteries from First-Principles. J. Phys. Chem. Lett. 2014, 5, 1547. (53) Evers, S.; Yim, T.; Nazar, L. F. Understanding the Nature of Absorption/Adsorption in Nanoporous Polysulfide Sorbents for the Li–S Battery. J. Phys. Chem. C 2012, 116, 19653. (54) Zhou, G.; Zhao, Y.; Zu, C.; Manthiram, A. Free-standing TiO2 nanowire-embedded graphene hybrid membrane for advanced Li/dissolved polysulfide batteries. Nano Energy 2015, 12, 240.



For Table of Contents Only


Page 18 of 18

Hollow Carbon Nanoballs Coupled with Ultrafine TiO2

Recommend Documents