Tài liệu Improving the electrochemical performance of lithium–sulfur batteries using an nb doped tio2 additive layer for the chemisorption of lithium polysulfides

Electrochimica Acta 285 (2018) 16e22 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Improving the electrochemical performance of LithiumeSulfur batteries using an Nb-Doped TiO2 additive layer for the chemisorption of lithium polysulfides Wan-Ting Tsou, Cheng-Yu Wu, Hao Yang, Jenq-Gong Duh* Department of Materials Science and Engineering, National Tsing-Hua University, 101, Kuang Fu Road, Sec. 2, Hsin-Chu, 300, Taiwan a r t i c l e i n f o a b s t r a c t Article history: Received 20 January 2018 Received in revised form 26 July 2018 Accepted 29 July 2018 Available online 2 August 2018 This study presents a method to suppress the migration of lithium polysulfides in lithiumesulfur batteries by introducing a dual-layer electrode structure. Herein, unlike conventional methods of mixing the polar additives with sulfur/carbon composites, melted sulfur mixed with mesocarbon microbeads are used as the electrode and covered with an additive layer of Nb-doped TiO2/graphite composite via twostep blade coating. By doping TiO2 with Nb, electrical and lithium ion conductivity of TiO2 can be increased, thereby enhancing the redox reaction kinetics. Most importantly, chemisorption of lithium polysulfides to NbeTiO2 can effectively mitigate the shuttle effect, resulting in higher capacity and longer cycle life. The electrode with the NbeTiO2 additive layer results in a 1st and 100th cycle specific capacity of 1883 mAh g
1 and 894 mAh g
1, respectively, at 0.1 C (1 C ¼ 1675 mAh), indicating enhanced electrochemical performance as compared with that of bare lithium-sulfur batteries. X-ray photoelectron spectroscopy (XPS) study was conducted to investigate the interaction between polysulfides and Nb eTiO2. The results indicate that the NbeTiO2elayered electrode efficiently traps polysulfides on the cathode and improves the rate capability, cycle performance, and specific capacity. © 2018 Published by Elsevier Ltd. Keywords: Lithiumesulfur battery Shuttle effect Metaleoxide additives Dual layer Chemisorption 1. Introduction Recently, given the profound environmental issues associated with fossil-fuel-based transportation, replacing traditional cars with electric vehicles or hybrid electric vehicles has become a global effort. The rapidly increasing demand for high-power sources and energy storage systems has turned lithiumesulfur (LieS) batteries into a potentially promising energy storage alternative because the active material, sulfur, is low-cost and eco-friendly. In addition, LieS batteries deliver a high gravimetric capacity of 1675 mAh g
1 and a high theoretical specific energy of 2600 Wh kg
1 [1]. Although LieS batteries have many advantages, several critical problems must be solved prior to wider commercial applications. The primary drawbacks include the insulating nature of sulfur (5  10
30 S cm
1 at 25  C) [2], large volume expansion (~80%) during the formation of Li2S, and dissolution of lithium polysulfides (Li2Sx, 4  x  8) through the reduction of S8 or oxidation of short-chain polysulfides in liquid electrolyte, which * Corresponding author. E-mail address: [email protected] (J.-G. Duh). https://doi.org/10.1016/j.electacta.2018.07.214 0013-4686/© 2018 Published by Elsevier Ltd. ultimately leads to the shuttle effect [3], causing low utilization of sulfur, poor Coulombic efficiency, and rapid LieS battery capacity fading during the chargeedischarge process [4,5]. To overcome these obstacles, mesoporous/microporous carbons [6,7], graphene [8], carbon nanotubes [9], and carbon fiber [10] have been used to encapsulate elemental sulfur. Carbonesulfur composite cathodes enhance the electrical conductivity and the physically confined LiPSs (lithium polysulfides) in the carbon pores or layers. However, polar LiPSs still diffuse out of non-polar carbon after long cycling owing to the lack of chemical interaction between LiPSs and carbon. Hence, incorporating polar metal oxide or sulfides in new cathode designs is a rapidly developing research area [11]. These polar inorganic additives can be classified by the electrical conductivity of conductors (e.g., Ti4O7 [12] and Mxene [13]), semiconductors (e.g., TiO2 [14,15], Nb2O5 [16], and FeS2 [17]), and insulators (e.g., SiO2 [18], Mg0.6Ni0.4O [19], and Al2O3 [20]). These additives provide active sites to absorb LiPSs and enhance surface electrochemical kinetics, effectively reducing the shuttle effect and prolonging the cycle life [21]. Previous studies have reported that Nb-doped TiO2 has many electrochemical applications [22]. NbeTiO2 is selected as the W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22 absorbent to anchor LiPSs owing to relatively high electrical and better lithium ion conductivity as compared to other semiconductors [23,24]. Additionally, NbeTiO2 participates in the electrochemical reaction, thereby providing additional capacity for the LieS battery. Here, instead of mixing metal oxide additives with sulfur and carbon, a dual-layer cathode is developed with an NbeTiO2 coating on an MCMBesulfur layer via the doctor-blading technique. The NbeTiO2 layer acts as a protective layer that physically and chemically prevents LiPSs from dissolving directly into the electrolyte. Moreover, NbeTiO2 layer renders a pathway for lithium ions to diffuse into the cathode. NbeTiO2 was synthesized to fabricate dual-layer LieS cathodes, and the electrochemical behaviors were studied in detail. Furthermore, lithium ion diffusivity was analyzed using AC impedance, and chemisorption was analyzed via X-ray photoelectron spectroscopy (XPS). 17 2.5. Electrochemical measurements 2. Experimental Electrochemical studies were conducted in 2230-type coin cells in an Ar-filled glove box (O2 < 0.1 ppm; H2O < 0.1 ppm) using lithium foil as the counter electrode. The sulfur fraction in MCMB was determined to be 46.8 wt% via thermogravimetric analysis (TGA). The sulfur content in the MCMB/S/NTO15 was calculated using an average pristine electrode weight of 1.2e1.3 mg. The electrolyte was a solution of lithium bis(trifluoromethanesulfonyl) imide (1 M) in 1:1 v/v 1,2-dimethoxyethane (DME) and 1,3dioxolane (DOL) containing LiNO3 (1 wt%). The cells were charged and discharged using an Arbin battery tester, and cycled between 1.6 and 2.6 V (vs. Li/Liþ) at 25  C ± 0.1  C. Electrochemical impedance spectroscopy (EIS) measurements were carried out over the frequency range of 100 to 0.01 kHz at 2.15 V. The ac impedance and CV measurements were obtained using an Ametek 263A electrochemical workstation. 2.1. Preparation of materials 3. Results and discussion The MCMBesulfur composites were prepared by mixing and melting sulfur (95% purity, SHOWA KAKO) and MCMB at 155  C for 5 h in a sealed glass bottle. NbeTiO2 was synthesized according to procedures reported elsewhere [25], Nb0.15Ti0.85O2 (NTO15) was selected for this study. 3.1. Cathode characterization 2.2. Preparation of cathodes The pristine cathodes abbreviated as MCMB/S were prepared by mixing 80 wt% MCMBesulfur composite powder, 10 wt% Super P, and 10 wt% polyethylene oxide/polyvinylpyrrolidone (PEO/PVP) in DI water to form a slurry that was cast on aluminum foil via doctor blading. On the other hand, the slurry of 80 wt% NTO15s, 10 wt% graphite, and 10 wt% PEO/PVP was cast on the MCMB/S cathodes to construct the dual-layer structure. The dual-layer cathodes were abbreviated as MCMB/S/NTO15. All electrodes were dried for 24 h at 40  C in a vacuum oven. 2.3. Preparation of XPS samples To investigate the interaction between NTO15s and LiPSs, Li2S8 was selected as the representative. A 20 mM Li2S8 solution was prepared by dissolving dry sulfur powder and Li2S in a molar ratio of 7:1 in anhydrous tetrahydrofuran (THF) at 55  C in a glove box. After stirring for 48 h, the Li2S8 solution with a redebrown color was obtained. The solvent was dried out in a vacuum oven, and the resulting solid was heated at 130  C in a glove box to remove any residual solvent to produce Li2S8 powder. 50 mg NTO15s were added to 5 mL Li2S8 solution and stirred for 48 h to obtain the NTO15/Li2S8 solution. Then, the precipitated product was dried under vacuum to obtain NTO15/Li2S8 powder for XPS analysis. 2.4. Characterization X-ray diffraction (XRD) patterns were measured on a Bruker D2phaser using Cu-Ka radiation (l ¼ 1.5418 Å) at 30 kV. Sample morphology was analyzed via field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7600F) equipped with energydispersive spectroscopy (EDX). High-resolution XPS (HR-XPS) performed with a ULVAC PHI Quantera SXM spectrometer with a monochromatic Al-Ka X-ray source was used to investigate the interaction between NTO15s and LiPSs. The XPS spectra were fitted using GaussianeLorentzian functions and a Shirley-type background; C 1s peak at 285.0 eV was used to calibrate the spectra. Fig. 1a shows the morphology of NTO15s synthesized via the hydrothermal method, which are hollow spherical nanoparticles with a mesoporous surface. Fig. 1b presents the XRD pattern of NTO15s. As Compared to the spectrum of anatase TiO2, the XRD pattern of NTO15s shifts to lower angles, suggesting that TiO2 is doped with Nb atoms, resulting in lattice expansion (Nb5þ ¼ 0.64 Å, Ti4þ ¼ 0.61 Å), as reported in literature [25,27]. Based on the XRD pattern of NTO15s, a small trace of rutile TiO2 is found in NTO15s. Fig. 2a presents the structure of the cell with the MCMB/S/ NTO15, comprising an Al foil, an MCMBesulfur layer, and an NTO15 layer. The SEM images in Fig. 2b and c shows that the MCMB/S/ NTO15 is covered by NTO15s over the entire surface of the MCMBesulfur layer. Furthermore, EDS mapping results presented in Fig. 2d clearly show that some NTO15s interfuse with the sulfurrich layer. The same binder and solvent are used in each layer, suggesting that NTO15s possibly penetrate the MCMBesulfur layer during the casting and drying process. Therefore, NTO15s can form a thin protective layer and additives for MCMB/S-layer. Furthermore, the binder could rebind these two layers together to provide better contact. 3.2. Interaction between NTO15s and the lithium polysulfide The reaction between NTO15s and LiPS was examined by XPS. L2S8 was prepared by combining sulfur powder with Li2S and chosen as the LiPS representative. Fig. 3a and b presents the Ti 2p spectra of NTO15s before and after stirring with L2S8. A small shift (
0.2 eV) to a lower binding energy in the Ti 2p spectrum of NTO15s/L2S8, and a lower Ti3þ oxidation state are observed. The same phenomenon is observed in the Nb 3d spectrum (Fig. 3d), which shifts to a lower binding energy as compared to the NTO15s spectrum (Fig. 3c). The peak of Nb4þ is evaluated at 206.5 (3d5/2) and 209.3 eV (3d3/2) in the NTO15s/L2S8 spectrum. As shown in Fig. 3e, the S 2p spectrum of Li2S8 exhibits two 2p3/2 peaks at 161.1 and 163.0 eV referred to terminal (ST
1) and bridge sulfur (S0B). A 3:1 ratio between these two 2p3/2 peaks is consistent with the Li2S8 composition. In the S 2P spectrum (Fig. 3f), post stirring with NTO15s, ST
1 and S0B move to 161.8 and 163.4 eV, respectively, indicating a shift (þ0.7 eV; þ0.4 eV) to a higher binding energy. The XPS study represents oxygen atoms binding with Nb or Ti, which may oxidize sulfur atoms in Li2S8 and form SeO bonds. The replacement of lower electronegativity sulfur results in the shifting peaks [26]. Based on Tao et al.'s DFT calculations [27], LiPSs tend to 18 W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22 Fig. 1. The characterization of NTO15s: (a) SEM image of NTO15s; (b) the XRD pattern of NTO15s. Fig. 2. Scheme of the dual-layer Li
S battery and the morphology of MCMB/S and MCMB/S/NTO15: (a) schematic for the LieS battery with the MCMB/S/NTO15; SEM top views of the (b) MCMB/S and (c) MCMB/S/NTO15; (d) EDS mapping results of the MCMB/S/NTO15. W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22 19 Fig. 3. Demonstration of the interaction between the LiPS and NTO15s: XPS Ti 2p spectrum of (a) NTO15s and (b) NTO15s/Li2S8; Nb 3d spectrum of (c) NTO15s and (d) NTO15s/Li2S8; S 2P spectrum of (e) Li2S8 and (f) NTO15s/Li2S8 (black line: experimental value, red line: sum of fitted value, and lines in other colors: fitted individual components). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Table 1 The XPS binding energies (eV) of NTO15s, NTO15s/Li2S8, and Li2S8 in Ti 2p, Nb 3d, and S 2p spectrum. Spectrum Ti Sample Peak 1 Peak 2 NTO15s 4þ 4þ NTO15s/Li2S8 Nb NTO15s NTO15s/Li2S8 S Li2S8 NTO15s/Li2S8 Ti 2p3/2 459.4 Ti3þ2p3/2 458.2 Nb5þ3d5/2 207.6 Nb4þ3d5/2 206.5 S
1T 2p3/2 161.1 S
1T 2p3/2 161.8 Ti 2p1/2 465.1 Ti4þ2p3/2 459.2 Nb5þ3d3/2 210.4 Nb5þ3d5/2 207.4 S
1T 2p1/2 162.3 S
1T 2p1/2 162.6 react with the bridging oxygen atoms instead of Ti. The XPS results of this study are in accordance with Tao's observation, wherein no TieS or NbeS binding formation is observed. Table 1 presents the corresponding details. 3.3. Electrochemical behavior of MCMB/S/NTO15 cathodes To evaluate the electrochemical performances of the MCMB/S/ NTO15, all batteries were measured between 1.6 and 2.6 V (vs. Li/ Liþ), then compared to the performances of the MCMB/S under the same conditions. As shown in Fig. 4, the cells were activated by a multichannel testing system under 0.05 C (1C ¼ 1675 mAh g
1) for 3 cycles before the CV test, conducted at a sweep rate of Peak 3 Peak 4 Sulfite Sulfate e e e e 3þ 4þ Ti 2p1/2 463.8 e Ti 2p1/2 464.9 e e e e e Nb4þ3d3/2 209.3 S0B 2p3/2 163.0 S0B 2p3/2 163.4 Nb5þ3d3/2 210.1 S0B 2p1/2 164.1 S0B 2p1/2 164.4 e e 166.4 167.7 166.5 167.3 (2p3/2) (2p1/2) (2p3/2) (2p1/2) e 168.2 (2p3/2) 169.3 (2p1/2) 0.075 mV s
1. For the cell with the MCMB/S/NTO15, two cathodic peaks are observed at 2.35 and 2.02 V, corresponding to the reduction of elemental sulfur to LiPSs (Li2Sx, 4  x  8) and longchain LiPSs to insoluble discharge products (Li2S2 and Li2S). Two overlapped anodic peaks at 2.41 and 2.36 V are attributed to the oxidation of LiPSs [28,29]. The peaks at 1.96 and 1.75 V belong to the delithiation and lithiation of NTO15s, respectively [27]. For the cell with the MCMB/S, a lower current intensity, a broader anodic peak at 2.43 V, and a larger peak separation between the first redox couple (DV ¼ 0.1 V) indicate higher polarization. Fig. 5a presents the EIS results. The EIS studies show that the resistance of the cell with the MCMB/S/NTO15 is significantly lower than that of the cell with MCMB/S owing to the better electron- 20 W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22 the MCMB/S/NTO15. The MCMB/S/NTO15 exhibits three longer plateaus (2.6e2.3 V, 2.3e2.1 V, and 2.1e1.9 V), which are primarily caused by the reactions as expressed through Eqs. (3)e(5) [33]. Fig. 4. CV curves of Li
S batteries with the MCMB/S and MCMB/S/NTO15. conductivity network [30]. The lithium ion diffusion coefficient (D) can be calculated by the slope ðsÞ of the fitting lines of the Zre vs. u
0.5 diagram (Fig. 5b) at low frequency, using Eqs. (1) and (2) [31]: Zre ¼ Rs þ Rct þ su
0:5 D¼ R2 T 2 2A2 n4 F 4 C 2 s2 (1) (2) where u is frequency, R is the gas constant (8.314 J mol
1 K
1), T is room temperature (298 K), A is the electrode area (1.37 cm2), n is the number of electrons transferred in the reaction (n ¼ 2), F is Faraday constant (96485 C mol
1), and C is the molar concentration of Li ions (1 M of LiTFSI in the electrolyte; C ¼ 0.001 mol cm
3). The lithium ion diffusion in the MCMB/S/NTO15 (D ¼ 1.11  10
9 cm2 s
1) is an order of magnitude faster than that in the MCMB/S (D ¼ 4.83  10
10 cm2 s
1). In Fig. 6a, the discharging curves depict two plateaus. The upper plateau indicates that S8 is reduced to long-chain LiPSs, and the lower plateau indicates that long-chain LiPSs are further reduced to Li2S2 and Li2S [32]. It is observed that the additional capacity contributed by NTO15s results in a sluggish slope at about 1.7 V in S8 þ 2Liþ þ 2e
/ Li2S8 (3) 2Li2S6 þ 2Liþ þ 2e
/ 3Li2S4 (4) Li2S4 þ 2Liþ þ 2e
/ 2Li2S2 (5) More soluble Li2S8, Li2S6, and Li2S4 products are immobilized on the MCMB/S/NTO15 than those on the MCMB/S and continue participating in the charging and discharging process. To further investigate the electrochemical performance improvement associated with the NTO15s additive layer, the lower-voltage capacity (Qlow), referring to short-chain polysulfides is used to divide the higher-voltage capacity (Qhigh), as a parameter [34,35]. The parameters can eliminate the error caused by the weight of cathodes. The higher Qlow/Qhigh ratio indicates that LiPSs can be effectively trapped on the cathode, contributing to the capacity. In Table 2, the MCMB/S/NTO15 displays a higher Qlow/Qhigh ratio in the 30th and 100th cycle than that of the MCMB/S, suggesting the potential suppression of the shuttle effect. The relatively large difference between Qlow/Qhigh in the 30th and 100th cycle implies the rapid dissolution of active materials into the electrolyte and/or conversion to insulated Li2S, which is in accordance with the XPS study, wherein LiPSs bind with NTO15s. Fig. 6b shows the rate capabilities of the cells, as measured by increasing the charge/discharge current density from 0.1 to 2 C. The MCMB/S delivers discharge capacities of 966, 664, 520 and 408 mAh g
1 (vs. the active material mass of the electrode) at 0.1, 0.2, 1, and 2 C, respectively. By contrast, the MCMB/S/NTO15 achieves considerably higher discharge capacities of 1717, 1069, 815, and 761 mAh g
1 at 0.1, 0.2, 1, and 2 C, respectively. However, high irreversibility of capacity is observed during the initial cycles. This phenomenon is also observed during long term cycling under the current rate of 0.1 C (Fig. 6c), wherein the lithiation of NTO15s causes the decay [25]. The MCMB/S/NTO15 demonstrates an initial capacity of 1883 mAh g
1, which is maintained over 894 mAh g
1after 100 cycles. However, the MCMB/S exhibits a lower capacity of 793 mAh g
1 during the 1st cycle and a final capacity of 246 mAh g
1. Moreover, retention of the MCMB/S/NTO15 represented by comparing the capacity at the 100th (Q100th) and 20th cycles (Q20th) Fig. 5. (a) Nyquist plots of the cells with MCMB/S and MCMB/S/NTO15 at 2.15 V. (b) The fitting lines of Zre vs. u
0.5 at low frequency region. W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22 21 Fig. 6. Electrochemical performances of LieS batteries with MCMB/S and MCMB/S/NTO15: (a) charging and discharging profiles with the current density of 0.1 C at 30th cycle and 100th cycle; (b) rate performances from 0.1 to 2 C; (c) long term cycling capability at 1 C for 100 cycles. Table 2 The parameter of capacity ratio of Qlow/Qhigh in the 30th and 100th cycle for the MCMB/S and MCMB/S/NTO15. Q 30th high Q 30th low Q 30th low Q 100th high Q 100th low Q 30th high MCMB/S MCMB/S/NTO15 122.39 277.88 229.98 612.70 1.88 2.20 Q 100th low Q 100th high 81.00 228.73 137.19 514.14 1.69 2.24 Additionally, the shifts and different oxidation states from the XPS results indicate chemical interaction between NTO15s and LiPSs. Moreover, the polysulfide shuttle effect can be effectively mitigated by the chemisorption of LiPSs on the NTO15 additive layer. Therefore, by fabricating NbeTiO2 additiveelayer cathodes, the cyclability, c-rate performance, and specific capacity of LieS batteries can be enhanced. Acknowledgments is significantly improved (78% vs. 53%). The improvement in discharge capacity, cycle life, and rate capability is attributed to the protective layer of NTO15s, which is an excellent absorbent to LiPSs. The shuttle reaction can be suppressed owing to the chemical attraction between LiPSs and NTO15s. The authors would like to acknowledge the financial support from Nice Success International Ltd, Hong Kong and the Ministry of Science and Technology, Taiwan, under the Contract No. MOST 1062811-E-007-012. References 4. Conclusion In summary, NTO15s was synthesized via the hydrothermal method and cast on the MCMBesulfur layer using the doctorblading method to construct an NTO15s additive layer. The MCMBesulfur layer was physically confined to prevent direct contact between the electrolyte and the sulfur-rich region. [1] X. 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