Architecture and Performance of the Novel Sulfur Host Material Based

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Architecture and Performance of the Novel Sulfur Host Material Based on Ti2O3 Microspheres for Lithium-sulfur Batteries Peng Zeng, Manfang Chen, Shouxin Jiang, Yongfang Li, Xin Xie, Hong Liu, Xinyu Hu, Chun Wu, Hongbo Shu, and Xianyou Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05874 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on June 1, 2019

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ACS Applied Materials & Interfaces

Architecture and Performance of the Novel Sulfur Host Material Based on Ti2O3 Microspheres for Lithium-Sulfur Batteries Peng Zenga, Manfang Chena, Shouxin Jianga, Yongfang Lia, Xin Xiea, Hong Liua, Xinyu Hua, Chun Wub, Hongbo Shua, and Xianyou Wanga (a: National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, China b: College of Material Science and Engineering, Changsha University of Science & Technology, Changsha, Hunan 410114, China) ABSTRACT:

Lithium-sulfur

batteries

are

considered

as

the

promising

next-generation green secondary batteries. No matter the enhancement of the cycling stability or the suppression of polysulfide species shuttle, although much progress has recently been achieved, improving the conductivity of host materials and capturing sulfide species as far as possible are still hot topics in the research of lithium-sulfur batteries nowadays. Here we put forward a novel sulfur host architecture based on Ti2O3

microspheres

fabricated

by

magnesiothermic

reduction.

The

Ti2O3

microspheres possess both high electronic conductivity and excellent ability of anchoring lithium polysulfide species. The high electronic conductivity endowed by the narrow bandgap can adequately activate the insulative sulfur and reduce battery

Corresponding

author: Xianyou Wang, Tel: +86 731 58293377; E-mail: [email protected] 1

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resistance so that acquiring high specific capacity and excellent rate capability, while the polar Ti2O3 could afford abundant polar active sites for absorption of polysulfides for high capacity retention. As a result, Ti2O3 microspheres are applied in the research of lithium-sulfur batteries, and the excellent electrochemical performance has been revealed. The initial specific capacity is 1245 mAh g-1 at 0.2 C with the capacity retention of 91.57% after 180 cycles. Even with the high areal loading of 3.6 mg cm-2, it could deliver the initial capacity of 665 mAh g-1 at 0.5 C and good capacity retention of 70.98% after 300 cycles. Apparently, the preparation and application of Ti2O3 microspheres can not only further extend the application field of the Ti-based compound, but also boost the electrochemical performance of lithium-sulfur batteries. KEYWORDS: Lithium-Sulfur batteries, Ti2O3 microspheres, Magnesiothermic reduction, Polysulfide species, Shuttle effect 1. INTRODUCTION Lithium-sulfur batteries are considered one of the most promising energy storage systems based on the ideal theoretical specific capacity (1675 mAh g-1) and energy density (2600 Wh kg-1).1-3 Besides, some conspicuous features of the raw material (sublimed sulfur) including the low price and easy acquisition can be also counted as the vital factors for the bright prospects of lithium-sulfur batteries.4 However, inescapable aspects do exist and thus hinder the actual application. Firstly, from the standpoint of the row material, the electronic conductivity of sublimated sulfur is so poor that the active substance cannot be fully utilized, causing it difficult to reach the theoretical specific capacity.5 Secondly, from the charge and discharge mechanism, 2


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the electrode reaction of sulfur cathode includes multi-step oxidation and reduction reaction, and the polysulfide species with complex phase transition shuttling between electrodes are accompanied, causing so called shuttle effect.6 It is untoward for the lithium-sulfur batteries to make the long-cycle life come true in the presence of the severe shuttle effect.6 Although so much obstruction exists, it is not entirely powerless to explore the effective ways to boost electrochemical performance of lithium-sulfur batteries. As far as it goes, it is generally accepted that the creative endeavors could include but are not limited to be, the host materials of sulfur, the modification of the interlayer and the improvement of the anode.7-9 Among these ways, the most extensive tactics is screening applicable host materials for grappling polysulfide species and thus improving the stability of lithium-sulfur batteries.10 Carbonaceous materials11-16 always have their place in the pantheon of host materials for lithium-sulfur batteries. On one hand, their good conductivity can increase utilization of active substance; on the other hand, they can be designed porous structure to limit the shuttle of lithium polysulfide species. However, there is something wrong over long-life cycle, since the physical interaction provided by porous structure is not so strong that the polysulfide species can still escape from cathode and cause fast capacity fading after a long-life cycle. 1, 8, 10 Taking the polar of lithium polysulfide species into consideration, some polar materials may be fully used as sulfur host in that the polar-polar interaction is relatively stronger compared to physical trapping.4,

8, 17

3


As a consequence, polar


transition metal oxides/sulfide/nitrides have been proposed one after another as sulfur hosts, such as MnO2,18-20 TiO2,21-22 MoS2,23-24 C3N425 and so on. Particularly, the Ti-based metal oxides play vital roles in achieving the goals of high performance lithium-sulfur batteries. Ti-based metal oxides usually consist of transition metal ions and anion of oxygen, and they are always with a strong polar surface, which makes the shuttle of lithium polysulfide species be restrained.26-27 For further increasing the performance of lithium-sulfur batteries, the Ti-based oxides based on their unique physical and chemical properties, have been one of the hottest orientations as host in the past few years. α-TiO2 has been put forward in the early days, and its adsorption to lithium polysulfide species also has been proved by experiment and theoretical calculation.6,

28

Subsequently, the modification and optimization for TiO2 samples

focused mainly on the morphology design and fine adjustment,28-29 which combined physical limitation and chemical absorption to suppress shuttle effect. Unfortunately, the electronic conductivity of TiO2 is so poor that the insulating sulfur cannot be fully utilized, causing relative low specific capacity. For surmounting these problems, the discovery and synthesis of Magnéli phase TinO2n−1 (3 < n < 10, such as Ti4O7, Ti5O9),17, 26, 30-31 a kind of highly conductive oxide with strong interaction to lithium polysulfide species, maybe count as a milestone to Ti-based materials for the application of lithium-sulfur batteries. As the supplement and development, TiO9, 32 has been successfully prepared and applied in lithium-sulfur batteries as the host. Recently, TiO-TiO2 heterostructure/polypyrrole also has been reported as a multifunctional sulfur host for lithium-sulfur batteries, which showed good 4


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electrochemical properties.33 All in all, the Ti-based materials show their superiority and enormous potential as sulfur host in the advanced lithium-sulfur batteries. High conductivity Ti2O3 has rarely been reported in the application of lithium-sulfur batteries, although abundant Ti-based materials including TiO, TiO2, Ti4O7, Ti5O9 and H-TiOx@S/PPy heterostructure are reported before. Ti2O3 belongs to the binary oxide and shows hexagonal crystal structure.34-36 On account of the narrow bandgap, Ti2O3 is highly conductive, and its conductivity in single-crystalline samples can reach ~100 S cm−1.37 As the existence of Ti (III) and oxygen vacancy ,38 Ti2O3 should be potential in a wide field. However, it is difficult to prepare pure product and results in its scant attention. According to available reports, the current preparation technologies for Ti2O3 are based on some harsh production conditions such as high temperature34,

39

(>1300℃), reducing condition40-41 (expensive reactive metal or

hazardous pure hydrogen), which lead to the above method be not the best choice for preparation of Ti2O3. Herein, we put forward to prepare Ti2O3 microspheres (Ti2O3-MS) via magnesium thermally reducing TiO2. On account of the excellent electronic conductivity, Ti2O3 could adequately activate the insulative sulfur and reduce battery resistance so that acquiring high specific capacity and excellent rate capability. Furthermore, visualized adsorption test and X-ray photoelectron spectroscopy (XPS) test proved that Ti2O3 has stronger adsorption of polysulfides than TiO2, and thus could effectively suppress the shuttle effect. As a result, the Ti2O3-MS/S cathode exhibits better electrochemical performance compared with the counterpart. 5



2. EXPERIMENTAL SECTION 2.1 Preparation of the TG. Titanium glycolate (TG) was prepared according to an improved widespread method. Typically, 2 mL of tetrabutyl titanate was added to 50 mL of glycol dropwise, stirring magnetically until a transparent solution was formed. After that, the as-prepared solution was transferred to 200 mL of acetone and kept stirring for 10 minutes. Allowing the resultant mixture to rest for 10 hours, and the white sediment could be collected by suction filtration. Finally, the product was obtained after drying at 80℃ for 10 hours. 2.2 Preparation of the TiO2-MS. 0.5 g of the as-prepared TG powder was annealed at 800℃ for 2 h, and the white TiO2 microspheres (named TiO2-MS) powder could be acquired. 2.3 Preparation of Ti2O3-MS. 0.5 g of the as prepared TiO2-MS powder and 0.08 g of magnesium powder were mixed uniformity in an agate mortar by grinding for 15 minutes. The mixture was annealed at 1000℃ for 2 h in the argon protection environment, and the black powder was acquired. In the end, the pure product (named Ti2O3-MS) was obtained by treating with the black powder using diluted HCl (5 mol L-1) to eliminate the impurity. 2.4 Preparation of the Ti2O3-MS/S composite and the TiO2-MS/S composite. 0.1 g of Ti2O3-MS and 0.3 g of sublimed sulfur powder were mixed uniformity in an agate mortar by grinding for 30 minutes, then, the mixture was transferred to an airtight container and treated at 160℃ for 12 h in the argon protection environment, so the Ti2O3-MS/S composite was obtained. The preparation of the TiO2-MS/S 6


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composite was the same above. 2.5 Lithium polysulfide adsorption tests. The Li2S6 solution was prepared by dissolving the Li2S6 compound into the solvent (DME/DOL =1:1, by volume), magnetic stirring until a uniform solution was formed. The concentration of Li2S6 solution was controlled by 2 mmol L-1. ~20 mg TiO2-MS and Ti2O3-MS were dispersed in 3 mL of the diluted Li2S6 solution, respectively, and the color change of the solutions were observed and photographed. 2.6 Materials Characterization. The structure and morphology of the above materials was identified by scanning electron microscopy (SEM, TM 4000, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). X-ray diffraction (XRD, Model LabX-6000, Shimadzu, Japan) was used to confirm the phase ingredient. Thermogravimetric analyses (TGA, TA Instruments, USA) was carried out to evaluate the sulfur content in a high highly purified N2 atmosphere. To explore valence state of element and function groups of the as-prepared samples, X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi, American) was examined. Brunauer-Emmett-Teller (BET) was used to analyze the specific surface area and the porous structure of the sample. 2.7 Electrochemical Measurements. The preparation of electrode slices was followed by common steps. Firstly, string a slurry with 80 wt% composites (TiO2-MS/S or Ti2O3-MS/S), 10 wt% Super P, and 10 wt% PVDF in NMP. Secondly, coating the slurry on a carbon-coated aluminum. Finally, forced air drying at 60 ℃ for 1 h and vacuum drying at 50℃ for 24 h. The pure S electrode was prepared by 7



modulating a slurry of 70 wt% sublimed sulfur, 20 wt % Super P, and 10 wt % PVDF in NMP, and the other steps were the same above. The common areal sulfur mass loading was controlled by ~1.5 mg cm-2. Moreover, the higher areal sulfur loading (3.6 mg cm-2) was also controlled to evaluate the performance of Ti2O3-MS/S cathode. A lithium wafer was used to serve as the anode of the lithium-sulfur batteries. A polypropylene microporous film was employed as a separator (so-called Celgard separator). A total of 1 M LiTFSI with 2 wt% LiNO3 was dissolved in the solution of 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME, 1:1, v/v) as the electrolyte. The assembled batteries were tested at various current rates from 0.1 C and 2 C (1 C = 1675 mA g-1) on a Neware tester (BTS-XWJ-6.44S-00052 Neware, Shenzhen, China) at the voltage window from 1.7 V to 2.8 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were examined with an electrochemical workstation (CHI 660e, Chenhua, China). The cyclic voltammetry examinations were tested at the potential between 1.7 and 2.8 V with a scan rate of 0.1 mV S−1. The electrochemical impedance spectroscopy (EIS) was controlled by the frequency range of 10−2 to 105 Hz with an amplitude of 5 mV. All the tests were carried at room temperature. 3. RESULTS AND DISCUSSION The preparation of Ti2O3 is an age-old question. It can be found that currently reported technologies need the harsh production condition such as high temperature (>1300℃), expensive reducing agent (expensive reactive metal or hazardous pure hydrogen), which lead to a complex preparation process and high cost of Ti2O3.36,40-41 8


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Hence, a better synthetic process of Ti2O3 is usually expected. As shown in Figure 1, we put forward a simple and facile technology route. At first, TG microspheres are obtained as precursor according to a universal method. Afterwards, the TiO2-MS is prepared by sintering the TG powder at 800℃ for 2 h in the air. The as prepared TiO2-MS powder is mixed with magnesium powder with a molar ratio of 2:1 to form homogeneous mixture. The mixture is annealed at 1000℃ for 2 h in the argon protection environment, and the black powder could be acquired. In the end, the pure product (named Ti2O3-MS) is obtained by treating with the black powder using diluted HCl (5 mol L-1) to eliminate the impurity.

Figure 1. The preparation of TiO2-MS and Ti2O3-MS.

Corresponding chemical reactions related to the preparation process of Ti2O3-MS could be summarized as follows.42 Ti(OBu)4 + 2HOCH2CH2OH → Ti(OCH2CH2O)2 (TG) + 4HOBu

(1)

Ti(OCH2CH2O)2 + 5O2 → TiO2 + 4CO2 + 4H2O

(2)

2TiO2 + Mg → Ti2O3 + MgO

(3)

The equation (1) demonstrates the formation of TG, and the equation (2) reveals the transformation from TG to TiO2. Magnesium powder is introduced as a reductant to despoil segmental oxygen atoms in TiO2 to form Ti2O3, just as shown in equation (3). 9



Structural and morphological features of the above materials are demonstrated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2a-b show the TG microspheres, and their diameter are about ~1.5 μm, together with a good dispersity. Besides, they are well-proportioned and their surface is smooth. The morphology of TG is completely solid sphere, without visible pore structure (Figure 2c-d). The formation of microsphere structure could account for the rapidly nucleating and uniformly growth of titanium alkoxide solution in acetone (containing trace water, 0.3%).43 Figure 2e-f demonstrate the morphology of TiO2-MS. The TiO2-MS still maintains a microsphere structure, with a ~ 1 μm diameter. The dispersity of TiO2-MS is very close to the TG, while the surface of TiO2-MS becomes very rough, which is far different from the surface of TG. Besides, a broken hole can be observed in SEM image (Figure 2f), and the edge of TiO2-MS becomes bright just like showed by TEM pictures (Figure 2g-h). Figure 2i-j describe the Ti2O3-MS microspheres, with a ~1 μm diameter. At high magnification (Figure 2j), it can be realized that the Ti2O3-MS sphere is a lot of primary particle aggregate. As shown in TEM images of Ti2O3-MS (Figure 2k), it becomes more transparent than TiO2-MS. The transparent outer layers of TiO2-MS and Ti2O3-MS are maybe caused by the escape of the small groups.22, 42

10


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Figure 2. (a, b) TG, (e, f) TiO2-MS and (i, j) Ti2O3-MS for SEM images. (c, d) TG, (g, h) TiO2-MS and (k, l) Ti2O3-MS for TEM images (inset: FFT patterns).

To analyze the crystal phase and structure of the products, transmission electron microscopy image and the corresponding fast Fourier transform (FFT) diffraction pattern of Ti2O3-MS are shown in Figure 2l, in which the interplanar distances can be assigned to Ti2O3 (104) with d=0.24 nm.43 Furthermore, the XRD patterns of corresponding compounds are shown in Figure 3a-b. There are no obvious diffraction peaks of TG powder in Figure 3a, hinting the amorphous state of TG. The crystal structure of TiO2-MS transforms into rutile phase during the annealing process at 800℃ in the air. XRD pattern of Ti2O3-MS is shown in Figure 3b, indicating the as-prepared material can be indexed to Ti2O3 crystalline phase (JCPDS No. 71-1056).35 The diffraction peak positions of TiO2-MS/S and Ti2O3-MS/S are matched with the standard card (JCPSD No. 42-1278) in Figure 3c. Based on 11



thermogravimetric analyses (TGA) results in Figure 3d, the sulfur contents in the TiO2-MS/S and Ti2O3-MS/S composites are calculated to be approximately 72 wt% and 70 wt%, respectively. Sulfur distribution in the Ti2O3-MS/S composite is confirmed by SEM and the corresponding EDX elemental mapping was given in Figure S3. The results confirm that Ti, S, and O are homogeneously distributed in the Ti2O3-MS/S composite.

Figure 3. XRD patterns for (a) TG and TiO2-MS, (b) Ti2O3-MS and (c) TiO2-MS/S and Ti2O3-MS/S. (d) TGA curves of TiO2-MS/S and Ti2O3-MS/S.

To evaluate adsorption capacity for polysulfide species of TiO2-MS and Ti2O3-MS, visualized adsorption test is carried out. Dispersing ~20 mg TiO2-MS and Ti2O3-MS in ~3 mL Li2S6 solution, respectively, and the color changes are related to the adsorption capacity for polysulfide species. It can be seen from the inset of Figure 4a that after resting for 2 h, the yellow Li2S6 solution mixed with Ti2O3-MS powder 12


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transfers almost transparent solution, while another solution mixed with TiO2-MS only shows a slight color change, indicating that the chemical interaction between Ti2O3-MS and Li2S6 is stronger than between TiO2-MS and Li2S6. Ultraviolet-visible (UV-vis) absorption tests are also carried out to further confirm the adsorption capacity for polysulfide species of the two samples. There are characteristic peaks within the wave length scope of 200 and 350 nm (Figure 4a), which are related to the high concentration of Li2S6.23 Compared with the sample added with TiO2-MS, the absorbance of the sample added with Ti2O3-MS has a great decrease, which is an apparent symbol that the Ti2O3-MS can adsorb polysulfide species more effectively than TiO2-MS.44 Further, X-ray photoelectron spectroscopy (XPS) has been carried out to analyze the bonding characteristics and explain the adsorption mechanism. According to S 2p spectrum in Figure 4b, strong peaks at ~162.4 eV, ~166.6 eV, ~168.5 eV and ~170 eV are assigned to sulfides maybe from lithium polysulfides of various chain lengths which are caused by the oxidation of Li2S6.23,

31

The peak

around ~165 eV could be assigned to the S-S bond of Li2S6, while the peak at ~163.5 eV is the terminal sulfur (S-Li bond).26 The peak at ~162 eV is ascribed to S-metal bond in metal sulfide (here is Ti-S bond).17 The high-resolution spectrum of Ti 2p is shown in Figure 4c. In the XPS spectrum of pure Ti2O3-MS, it clearly displays three typical Ti-O spin splitting peaks located at ~464.2 eV (Ti 2p1/2), ~458.5 eV (Ti 2p3/2) and ~454.9 eV. After adsorption, there is an energy shift of 0.2 eV toward lower energy direction, which suggests Ti2O3-MS have a strong redox ability of the Ti species and strong chemical interaction with polysulfide species.45 In addition, the 13



small peak at ~456.5 eV and 462.5 eV could be fitted from the data of Ti 2p spectrum, which means the existence of Ti-S bond.33 As shown in Figure 4d, the O 1s spectrum displays three peaks at 531.5 eV, 530.3 eV and 529.3 eV, signifying hydroxyls species (OH-), absorbed oxygen species (O2- and O-) and lattice oxygen (O2-), respectively.46 After absorption, the lattice oxygen component shifted 1.7 eV to higher binding energy resulting from the interaction of Li and O presumably.47 Synchronously, it induces an additional peak at 529.6 eV, which reveals the chemical bonding between O and Li (Li-O bind).46

Figure 4. (a) UV/vis adsorption image of Li2S6 solution with TiO2-MS and Ti2O3-MS (inset: Li2S6 solution and Li2S6 solution with TiO2-MS or Ti2O3-MS after resting for 2 h. (b) Refined XPS spectra of S 2p. Refined XPS spectrum of (c) Ti 2p and (d) O 1s before and after absorption of Li2S6.

To confirm whether the Ti2O3-MS could take part in electrode reaction or not, 14


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the first three CV curves of Ti2O3-MS electrode (with no sulfur) are shown in Figure 5a. There is no obvious redox peak in the potential range of 1.7 to 3.0 V at a scan rate of 0.1 mV s-1, suggesting that the Ti2O3-MS could not react with the Li+ during the electrochemical reaction process. In addition, the reduction peak below 1.7 V (vs. Li/Li+) is attributed to the decompose reaction of LiNO3. The result reveals that Ti2O3-MS host will not donate primary capacity at the potential range of 1.7 to 2.8 V, so it could be concluded that the specific capacity of Ti2O3-MS/S electrode is donated by sublimed sulfur principally.48 The typical cyclic voltammetry (CV) curves of the TiO2-MS/S and Ti2O3-MS/S are shown in Figure 5b-d, and the potential window is from 1.7 V to 2.8 V with a scan rate of 0.1 mV s-1. The two cathodic peaks at ~2.3 and ~2.0 V can be assigned to the reduction of long-chain lithium polysulfide species (Li2Sx, 4 ≤ x ≤ 8) and short-chain insoluble lithium sulfides (Li2S2 or Li2S).23 The two anodic peaks correspond to the successive oxidation of Li2S2/Li2S to Li2S8/S8.20 As shown in Figure 5c-d, the positions and magnitudes of the redox peaks for the Ti2O3-MS/S electrode are almost unchanged from the second to the fifth cycle, which indicate a higher reversibility of the Ti2O3-MS/S electrode than TiO2-MS/S electrode. Compared with TiO2-MS/S electrode, the hysteresis of the corresponding redox peak is smaller for Ti2O3-MS/S electrode, indicating its smaller polarization caused by the intrinsic electronic conductivity.48 Furthermore, the redox peaks for Ti2O3-MS/S are sharper and stronger than those of TiO2-MS/S, which should be due to a faster electrochemical kinetics for Ti2O3-MS/S.49 The faster reaction kinetics is further assessed by Li+ diffusion coefficient. Figure 15



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5e exhibits EIS plots of the TiO2-MS and Ti2O3-MS electrodes after resting for 24 h, in which the slope line signifies the Warburg impedance (Wo). Equivalent circuit diagram is also provided in the insert of Figure 5e, where Rs, Rct, and CPE represent the ohm resistance, the charge transfer resistance and constant phase element. Comparing with TiO2-MS/S electrode, the T2O3-MS/S electrode shows a smaller semicircle, indicating T2O3-MS/S electrode has lower charge transfer resistance (Rct). Furthermore, the smaller ohmic resistance (Rs) in high frequency for T2O3-MS/S electrode can be clearly observed. The Li+ diffusion coefficients of the two cathodes are calculated by equation (4). 50 D Li+ = (R2T2)/(2A2n4F4c2σ2) Where D

+ Li ,

(4)

R, T, A, n, F and c represent the Li+ diffusion coefficient, gas constant,

absolute temperature, surface area of electrode, electron transfer number and Faraday constant and Li+ concentration, respectively. σ stands for the Warburg factor calculated by equation (5).51 Z′ = Rs + Rct + σω-1/2

(5)

The relation between Z′ and the inverse of the root square of the lower angular frequency (ω−1/2) is shown in Figure 5f. The results are listed in Table 1, and the Li+ diffusion coefficients of the two electrodes are 9.3×10-13 cm2 S-1 for Ti2O3-MS/S electrode and 1.6×10-13 cm2 S-1 for TiO2-MS/S electrode, thus Ti2O3-MS/S electrode has better reaction kinetics.

16


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Figure 5. CV curves of (a) Ti2O3-MS electrode (without sulfur), (b) comparison of TiO2-MS/S electrode and Ti2O3-MS/S electrode, (c) TiO2-MS/S electrode and (d) Ti2O3-MS/S electrode. (e) EIS of TiO2-MS/S and Ti2O3-MS/S electrodes after resting for 12 h (inset: equivalent circuit diagram) and (f) dependence of Z′ on ω−1/2 for the two electrodes. Table 1. EIS Arguments of TiO2-MS/S and Ti2O3-MS/S Composites

Sample

Rs (Ω)

Rct (Ω)

Σ (Ω S-1/2)

D Li+ (cm2 S-1)

TiO2-MS/S

9.83

121.42

163.00

1.6×10-13

Ti2O3-MS/S

3.64

52.61

51.46

9.3×10-13

17



Electrochemical performance of the as-prepared materials is further examined. Figure 6a-b show the cycling stability of Ti2O3-MS/S cathode at 0.2 C. It can be seen that the initial specific capacity is 1245 mAh g-1, even after 180 cycles, the specific capacity can still keep as high as 1140 mAh g-1, accompanied by a low fading rate of 0.04% per cycle. For comparison, the first-cycle galvanostatic charge/discharge (GCD) curves of pure S, TiO2-MS/S and Ti2O3-MS/S at 0.5 C (1 C = 1675 mA g-1) are shown in Figure 6c. The initial specific capacities of the three electrodes are 765 mAh g-1, 883 mAh g-1 and 1060 mAh g-1, respectively, and the corresponding utilization of active substance (element sulfur) are 45.67%, 52.72% and 63.28%, respectively. Furthermore, it can be observed that the polarization potentials are 400 mV, 320 mV and 200 mV, respectively, thereby the Ti2O3-MS/S electrode has a least polarization. After 250 cycles, the Ti2O3-MS/S electrode keeps reversible capacity of 860 mAh g−1, and its capacity retention is 81.10% with a low fading rate of 0.08% per cycle. As to the other two cathodes, the capacity retention of the TiO2-MS/S electrode is 68.52% after 110 cycles with fading rate of 0.29% per cycle, not to speak to 250 cycles; and pure S cathode becomes even worse, which is a 57.53% capacity retention after 110 cycles with the fading rate of 0.38% per cycle (Figure 6d). The distribution of polysulfide species on separators can be compared by opening batteries after 10 cycles at 0.5 C and observing the change of the separator color. The results are shown in the inset of Figure 6d. The color of the three separators becomes gradually shallow, which indicates a diminishing polysulfide species. These results reveal the good performance of Ti2O3-MS, and the enhancement order of the capacity retention is 18


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Ti2O3-MS > TiO2-MS > pure S. Figure 6e is the cycle life curve of Ti2O3-MS/S cathode at 1 C. The initial specific capacity of the electrode is 890 mAh g-1, even after 300 cycles the discharge capacity is still as high as 610 mAh g-1 along with a fading rate of 0.10% per cycle. The energy of the battery based on Ti2O3-MS/S cathode has been further tested by a simple experiment. As shown in the insert of Figure 6e, the lithium sulfur battery based on Ti2O3-MS/S cathode can light 26 light emitting diodes (LEDs, nominal voltage of 2.0-2.2 V).

19



Figure 6. (a) Charging and discharging curve of Ti2O3-MS/S electrode at 0.2 C. (b) Cycling stability of Ti2O3-MS/S electrode at 0.2 C. (c) The first cycle charge/discharge profiles of pure S electrode, TiO2-MS/S electrode and Ti2O3-MS/S electrode at 0.5 C. (d) Cycling stability of pure S electrode, TiO2-MS/S electrode and Ti2O3-MS/S electrode at 0.5 C (inset: the opening cells after 10 cycles at 0.5 C). (e) Cycling stability of Ti2O3-MS/S electrode at 1 C (inset: LED logo lit by lithium-sulfur battery based on Ti2O3-MS/S electrode).

Figure 7a-b show the rate capability of the Ti2O3-MS/S and TiO2-MS/S composites at different current rates. The initial specific discharge capacities at 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C are 1400/1250 mAh g−1, 1280/1170 mAh g−1, 1055/880 mAh g−1, 890/600 mAh g−1 and 740/260 mAh g−1, respectively. The high specific capacity of Ti2O3-MS/S cathode can be ascribed to excellent electronic conductivity and thus cause the high utilization of active materials. When the current rate returns to 0.1 C, the cell still holds the high specific capacity of 1350 mAh g−1, indicating good capacity recovery capability from heavy current to small current. Particularly, even at a high current density of 2 C, the GCD curve of Ti2O3-MS/S cathode shows two distinct potential plateaus. For confirming the microsphere structure could keep intact after long-term discharge-charge cycles or not, the SEM images of Ti2O3-MS/S in the fresh electrode and the electrode after long cycle are shown in Figure S2a-b. It can be seen that the morphology of the microsphere is well maintained. In order to better meet the practical application requirements, the performance of high sulfur loading Ti2O3-MS/S cathode is studied. Figure 7c describes the cycle performance based on a high areal sulfur loading (3.6 mg cm-2) Ti2O3-MS/S electrode at 0.5 C, and it still 20


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could deliver a high initial capacity of 665 mAh g-1 and good capacity retention of 70.98% after 300 cycles, which suggests the vast prospect of practical application.

Figure 7 (a) The charge/discharge profiles Ti2O3-MS/S electrode at different rates. (b) Rate capability of TiO2-MS/S electrode and Ti2O3-MS/S electrode. (c) Cycling stability of Ti2O3-MS/S electrode at 0.5 C in a high sulfur areal loading (3.6 mg cm-2).

To further analyze the electrochemical performance of Ti2O3-MS/S cathode in Ti-based materials, corresponding experimental results have been summarized as shown in Table 2. Compared with Ti4O7 electrodes and TiO electrodes, Ti2O3-MS electrodes still keep its superiority in the aspects of both initial specific capacity and cycle stability. Evaluating Ti-based oxides comprehensively, some well-founded conclusion could be concluded. Polar O-Ti-O units in Ti-based oxides can provide abundant polar sites, which could capture polar polysulfide species. Based on its high electronic conductivity, Ti2O3 becomes one of the best choices for host materials of 21



Page 22 of 31

lithium-sulfur batteries to activate insulating sublimed sulfur. Therefore, Ti2O3 will be a great application prospect in advanced lithium-sulfur batteries. Table 2. Ti-based oxides for lithium-sulfur batteries

sample

areal active initial material retention fading rates cycles capacity reference mass loading (%) rate (%) -1 (mAh g ) (mg cm−2) ~ 1.5

0.2

180

1245

91.57

0.04

~ 1.5

0.5

250

1060

81.10

0.08

~ 3.6

0.5

300

665

70.98

0.09

Ti4O7@C/S

1.0-1.2

0.1

300

1411

71.58

0.10

[31]

H-TiOx@S/PPy

0.8-1

0.5

500

1050

56.19

0.09

[33]

Oxygen-deficient TiO2/S

Not mentioned

0.2

100

1472

61.14

0.39

[52]

CNT@TiO2-x/S

~ 2.2

0.2

150

1204

72.01

0.19

[53]

C-Co/TiO2/S

~ 1.5

0.5

200

1209

41.63

0.29

[54]

TinO2n-1@C/S

~ 2.0

0.2

100

1138

70.39

0.30

[55]

TiO2-B NT/CNT/S

~1

0.5

200

1095

76.44

0.12

[56]

TiO2 nanowire/S

~ 1.7

0.5

100

830

66.27

0.33

[57]

Ti2O3-MS/S

This work

4. CONCLUSIONS Ti2O3-MS has been successfully prepared through magnesium thermally reducing TiO2. The excellent electronic conductivity and the strong ability to capture polysulfide species make it an excellent sulfur host material of lithium-sulfur batteries. The batteries based on Ti2O3-MS/S cathodes show good electrochemical performance, for example, an initial capacity of 1060 mAh g-1 at 0.5 C with a fading rate of 0.08% per cycle within 250 cycles. Even at high current density of 2 C, the batteries based on Ti2O3-MS/S electrode still has the capacity of 680 mAh g-1, with two distinct potential plateaus. With the high sulfur areal loading of 3.6 mg cm-2, it still could deliver a high initial capacity of 665 mAh g-1 at 0.5 C and good capacity retention of 70.98% after 300 cycles. The preparation and application of Ti2O3 microspheres as sulfur host in 22


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lithium sulfur battery can not only further extend the application field of the Ti-based compound, but also boost electrochemical performance of lithium-sulfur batteries. AUTHOR INFORMATION Corresponding

author

*E-mail: [email protected] Tel: +86 731 58293377. Fax: +86 731 58292052. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported financially by National Key Research and Development Program of China (2018YFB0104200). REFERENCES (1) Chu, S.; Cui, Y.; Liu, N., The Path Towards Sustainable Energy. Nat. Mater. 2016, 16, 16-22. (2) Peng, H. J.; Huang, J. Q.; Zhang, Q., A Review of Flexible Lithium-Sulfur and Analogous Alkali Metal-Chalcogen Rechargeable Batteries. Chem. Soc. Rev. 2017, 46, 5237-5288. (3) Chung, S.-H.; Manthiram, A., Designing Lithium-Sulfur Cells with Practically Necessary Parameters. Joule 2018, 2, 710-724. (4) Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F., Review—The Importance of Chemical Interactions between Sulfur Host Materials and Lithium Polysulfides for Advanced Lithium-Sulfur Batteries. J. Electrochem. Soc. 2015, 162, A2567-A2576. (5) Zhang, J.; Huang, M.; Xi, B.; Mi, K.; Yuan, A.; Xiong, S., Systematic Study of Effect on 23



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Table of Contents Graphic (TOC)

31


Architecture and Performance of the Novel Sulfur Host Material Based

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