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Improved performance of Li-S battery with hybrid electrolyte by interface modification
Solid State Ionics 300 (2017) 67–72
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Improved performance of Li-S battery with hybrid electrolyte by interface modification Qingsong Wang a,b, Jing Guo a,b, Tian Wu a,b, Jun Jin a, Jianhua Yang a, Zhaoyin Wen a,⁎ a b
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 16 August 2016 Received in revised form 27 October 2016 Accepted 1 November 2016 Available online xxxx Keywords: Lithium-sulfur battery Hybrid electrolyte Solid state ionic conductor Interface modification
a b s t r a c t A carbon coating layer facing sulfur cathode is introduced on one side of the solid state ionic conductor in the ceramic-liquid hybrid electrolyte based Li-S cell. Both the cycling performance and the rate capability are improved as a result of enhanced electron transport for the conversion of the dissolved sulfur-containing active materials as well as the wettability of the ceramic electrolyte towards the liquid electrolyte. The interface modified hybrid Li-S cell exhibits an initial discharge capacity of 1409 mA h g−1 and remains at 1000 mA h g−1 which is 235 mA h g−1 higher than the pristine one, after 50 cycles at 0.2C rate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium metal is one of the most promising candidates as an anode material for next-generation energy storage systems due to its high specific capacity (3860 mA h g−1) and the low negative electrochemical potential (− 3.04 V versus the standard hydrogen electrode) [1,2]. When coupled with a sulfur cathode, a rechargeable lithium-sulfur (Li-S) battery is expected to yield high theoretical specific energy of 2600 W h kg−1, based on a redox reaction that reversibly interconverts sulfur (S8) and Li2S [3]. In addition, sulfur is low cost, highly natural abundant and environmental friendly. However, the practical realization of Li–S batteries is currently hindered by numerous scientific and technical challenges. The most critical problem is believed to be associated with the intermediate lithium polysulfides (Li2Sn, n = 4–8) that are highly soluble in many organic solvents based electrolytes [4–6]. The dissolved polysulfides, formed at the beginning of discharge, could shuttle between the cathode and anode through the commonly used porous polymer separator. This shuttle process induces irreversible loss of active materials from the cathode, corrosion of Li metal anode, low coulombic efficiency, rapid capacity fading, and high self-discharge rates [7,8]. Nevertheless, the dissolution of polysulfides is inevitable and essential for effective utilization of the active material in a Li-S cell [9,10]. In the discharge process, the soluble polysulfides dissolve into the electrolyte solution enabling the subsequent sulfur to be exposed to the conductive agent and the reduction process to progressively move forward. In the ⁎ Corresponding author. E-mail address: [email protected] (Z. Wen).
http://dx.doi.org/10.1016/j.ssi.2016.11.001 0167-2738/© 2016 Elsevier B.V. All rights reserved.
charge process, the dissolved polysulfide species can facilitate the electrochemical conversion of the insoluble discharge products Li2S2/Li2S [11,12]. We have previously reported a novel type of Li-S cell employing a hybrid electrolyte (HE) [13]. The hybrid electrolyte was composed of the liquid electrolyte and a lithium ion conductor that acts as the separator. The NASICON-type structured Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte, which physically block the dissolution and diffusion of polysulfides could solve the shuttling problem. Meanwhile, the liquid electrolyte takes advantage of the dissolved polysulfides which offer fast charge/discharge rates and favor fast electrochemical kinetics of the cathode. However, a redistribution of polysulfides on the electrode is unavoidable no matter what kind of treatment is done on the initial sulfur cathodes [14]. The redistribution of sulfur in the cell would cause the capacity fading once the soluble polysulfides detach from the cathode into the liquid electrolyte [15–17]. Gradually, sulfur loses intimate contact with carbon matrix. Electronically isolated and electrochemically inactive Li2S does not take part in the electrochemical oxidation during charge state owing to the lack of carbon matrix. The formation of irreversible Li2S becomes severe as cycling proceeds and the available conducting surface is decreased gradually, leading to the decreasing of discharge capacity. To make full use of the dissolved sulfur-containing active materials, a carbon coating layer facing sulfur cathode is introduced onto one side of the solid electrolyte. As shown in Fig. 1, the carbon-coating layer serves as an upper current collector to facilitate electron transport as well as to improve the wettability of solid electrolyte towards the liquid electrolyte. The migration of the polysulfides is physically blocked by the solid electrolyte and the shuttle effect is prevented. A lithium sulfur battery with enhanced electrochemical
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electronically conductive carbon is a mixture of acetylene black (AB) and carbon nano-tube (CNT) in the ratio of 3:1 (w/w). A uniform slurry composed of the mixture of carbon (AB&CNT) and polyvinylidene fluoride binder, with the weight ratio of 4:1 and N-Methyl pyrrolidone (AR, Aladdin Industrial Inc.) as the solvent, was directly sprayed onto LAGP ceramic plates by spin-coating method. After evaporation of the solvent, the carbon coated plates were annealed under argon atmosphere at 550 °C for 6 h to pyrolyze the polymer binder. 2.2. Preparation of the sulfur cathode
Fig. 1. Schematic illustration of the Li-S battery configuration with a carbon-coated LAGP, the active materials were recaptured on the conductive layer and enabled further redox reactions while the LAGP prevents the polysulfides from immigrating to the anode side.
The cathode composite was made from mixing sulfur powder (S8, spectrum pure) and commercial Ketjen black (KB, with a specific surface area of over 1300 m2 g−1) in the ratio of 3:2 (w/w) using ball milling and further heating at 155 °C for 12 h under vacuum. The electrodes were prepared by spraying a slurry of 80 wt% KB/S composite, 10 wt% vapor grown carbon fiber, 5 wt% Carboxy Methylated Cellulose and 5 wt% Styrene Butadiene Rubber binder dispersed in distilled water onto Al foil. The sulfur electrodes were dried at 60 °C under vacuum for 12 h. Finally, 1.13 cm2 cathode discs were punched for the battery tests. The sulfur mass loading is about 1.2–1.3 mg cm−2. 2.3. Characterization
performance is demonstrated with an initial discharge capacity of 1409 mA h g− 1 and a reversible specific capacity of 1000 mA h g− 1 after 50 cycles at 0.2C rate.
2. Experimental 2.1. Carbon coated Li1.5Al0.5Ge1.5(PO4)3 pellet fabrication The Li1.5Al0.5Ge1.5(PO4)3 ceramic electrolyte is synthesized by the solid state reaction method. The detailed preparation process is previously reported [13,18]. The obtained ceramic pellet, with the thickness of 0.7 mm and diameter of 17.4 mm, has a conductivity of 1.7 × 10−4 S/cm at room temperature. This value is in good agreement with results reported by several previous literatures [19–22]. The
The morphology of the cross section of the carbon coated LAGP ceramic electrolyte and the surface of the carbon coating layer was obtained by Scanning Electron Microscopy (SEM, HITACHI S-3400) at an accelerating voltage of 15 kV. Energy Disperse Spectroscopy (EDS, attached to the SEM) characterization was carried out on the surface of the carbon coating layer after the10th cycle. Transmission Electron Microscopy (TEM) image of the AB&CNT was taken on a Field Emission Microscope JEOL-2010CX. For the pore structure analysis of the AB&CNT, nitrogen sorption isotherms were carried out at 77 K with a Micromeritics Tristar 3000 analyzer (MICROMERITICS, U.S.). Specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method using the adsorption data in a relative pressure range of P/ P0 = 0.05–0.25. The pore volume was determined from the amounts of N2 adsorbed at a relative pressure (P/P0) of 0.99.
Fig. 2. (a) SEM image of the cross section of the carbon-coated LAGP; (b) TEM image of the AB&CNT; (c) SEM image of the surface of the carbon coating layer; (d) photograph of the carboncoated LAGP and the pristine LAGP.
Q. Wang et al. / Solid State Ionics 300 (2017) 67–72
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Fig. 3. (a) N2 adsorption–desorption isotherms for AB&CNT (BET surface area, 79.9 m2 g−1), (b) pore size distribution of AB&CNT (x-coordinate, double logarithmic reciprocal scale).
2.4. Electrochemical measurements The pristine LAGP pellet or the carbon coated LAGP pellet was sandwiched between the lithium metal anode and the as-prepared KB/S cathode which then was placed in a predesigned Teflon container held by the screw clamp. Liquid electrolyte, 30 μL LiN(CF3SO2)2 (LiTFSI, Sigma-Aldrich) dissolved in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich) with high thermal stability and moderate viscosity was used to improve the contact between the electrodes and the ceramic electrolyte. Teflon O-rings were used for sealing and stainless steel plates were employed as the current collectors. All cells were assembled and sealed in an argon filled glove box. After 2-h rest, the cells were tested by galvanostatic cycling in the voltage range of 1.5–3.0 V or 1.0–3.0 V vs. Li/Li+ using a LAND CT2001A battery test system (Wuhan, China). Specific capacities were calculated on the basis of sulfur mass. An Autolab PGSTAT302N Electrochemical Workstation (ECO CHEMIE B.V., Netherlands) was used to perform the Electrochemical impedance measurements of the cells by using a Frequency Response Analyzer (FRA) with a small perturbation voltage of 10 mV in the frequency range of 1 MHz to 0.1 Hz. Cyclic voltammetry measurements of the cells were also collected on the Electrochemical Workstation at a scan rate of 0.02 mV/s between 1.0 V and 3.0 V vs. Li/Li+. All electrochemical tests were conducted at room temperature (nominally 25 °C).
3. Results and discussion Fig. 2a presents cross-sectional image of the carbon coated LAGP ceramic electrolyte. The carbon coating layer is stacked well on the surface of the LAGP ceramic and is about 5 μm thick on average. The carbon
material is composed of a uniformly dispersed mixture of AB and CNT, as shown in the TEM image (Fig. 2b). Fig. 2c shows the top surface morphology of the carbon coating layer which shows a uniform porous structure without visible agglomeration. The digital photo of the pristine ceramic pellet and the carbon coated ceramic pellet is given in Fig. 2d. It can be seen that the carbon coated ceramic pellet has a black surface. Diameter of the ceramic pellets is about 17.4 mm. The carbon material has a large BET specific surface area of 79.9 m2 g−1 and is accompanied with 2 nm micropores (Fig. 3) which contributes to the adsorption of the dissolved sulfur species and the wettability of the ceramic electrolyte towards the liquid electrolyte [23]. The 0.0625 M Li2S8 solution was synthesized via reacting elemental sulfur and lithium sulfide (Li2S, Sigma-Aldrich) in the desired ratio in TEGDME at room temperature for 12 h under vigorous stirring inside an argon glove box. The obtained uniform deep brown Li2S8/TEGDME solution was employed to conduct the contact angle measurements. As expected, Fig. 4 shows that the carbon coating layer had a much lower contact angle than the uncoated ceramic electrolyte. Owing to the capture of polysulfides and promoting of electron transfer, the improved wettability of the ceramic electrolyte towards the liquid electrolyte will be beneficial in conversion of the dissolved polysulfides species at the three-phase boundary between carbon, sulfur, and electrolyte as shown in Fig. 1. Fig. 5a and b shows the impendence spectra of the hybrid electrolyte Li–S cells and inset is the proposed equivalent circuit. The impendence measurements were performed before the discharging–charging tests. As shown, both of the two type cells show a similar Nyquist plot which composed of two partially overlapped semicircles and a straight slopping line. Likely as reported by our previous works [13,24], the left extrapolated intercept of the semicircle with the real axis in the high frequency region reflects the resistance of the LAGP (RLAGP) solid
Fig. 4. Contact angle snapshots for Li2S8/TEGDME on (a) pristine LAGP and (b) carbon-coated LAGP. a-1 and b-1, time t = 0 s, correspond to the solution droplet just dropped down on the pellet surfaces. a-2 and b-2, time t = 10 s, correspond to the solution droplet stayed on the surface for 10 s.
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Fig. 5. Impedance spectra of the hybrid electrolyte Li-S cells measured at the region of 1 MHz to 0.1 Hz and inset is the proposed equivalent circuit (a) carbon-coated LAGP, and (b) the pristine LAGP; CV curves (at 0.02 mV/s) of the hybrid electrolyte Li-S cells (c) with carbon-coated LAGP, and (d) with the pristine LAGP; charge/discharge profiles (at different rates) of the hybrid electrolyte Li-S cells (e) with carbon-coated LAGP and (f) with the pristine LAGP.
electrolyte which remains unchanged after the carbon-layer modification. The depressed semicircles in the high frequency region and in the middle frequency region reflect the interphase contact resistance (Rint) caused by the solid–liquid hybrid electrolyte interface and the charge-transfer resistance (Rct), respectively. The sloping straight line in the low frequency region represents the diffusion impendence of the Li-ion diffusion process. It's clearly that the interphase contact resistance and the charge-transfer resistance are both decreased which results in enhanced performances and will be discussed below. The electrochemical performances of the hybrid Li-S cells containing the
carbon coated LAGP ceramic electrolyte and the pristine LAGP ceramic electrolyte were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling. The CV curves for the cell with carbon coated ceramic electrolyte are depicted in Fig. 5c. There are two cathodic peaks at 2.45 and 2.0 V corresponding to the change from elemental sulfur to high-order polysulfides (Li2Sx, x N 4) and the further reduction to low-order polysulfides (Li2Sx, x ≤ 4), respectively. The two distinguishable anodic peaks in the anodic sweep indicate the staged conversion of low-order polysulfides to high-order polysulfides and then to elemental sulfur [25,26]. Moreover, the shape and the position of the
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Fig. 6. Cycling performance of the Li-S cells, charge/discharge at 0.2C between 1.5 and 3.0 V.
anodic and cathodic peaks obtained during different CV cycles are basically met, attesting to the superior cell reversibility and stability of the Li-S redox reactions of the cell with the carbon coated ceramic electrolyte. However, there is only one anodic peak in the CV curves (Fig. 5d) for the cell with pristine uncoated ceramic electrolyte which suggests that the conversion from higher-order polysulfides to elemental sulfur is less active in the cell with uncoated ceramic electrolyte than that in the former one. In addition, the anodic oxidation peak voltage of the pristine cell is higher and the cathodic reduction peak voltage is lower compared to the cell with carbon coated LAGP, showing faster redox-reduction kinetics of the later cell. This is consistent with the effect of carbon coating layer in helping decrease in the interfacial and charge transfer resistances (Fig. 5a, b) by facilitating the electron transport in the conversion of the adsorbed dissolved polysulfides species [27]. Another important difference between the two cells is that all the peak currents in the cell with the carbon coated ceramic electrolyte are higher than those in the cell with uncoated ceramic electrolyte, indicating the increased ion and electron transport enabled by the carbon coating layer [27,28]. The representative charge/discharge profiles at different C rates are shown in Fig. 5e and f. The open circuit voltage of the as-assembled hybrid cells was about 3.2 V. Liquid electrolyte was employed to improve the contact between the ceramic electrolyte and the electrodes. The
polysulfides tend to dissolve in the liquid electrolyte at the beginning of the discharge. Galvanostatic voltage profiles of the hybrid cells represent the typical characterization of conventional Li-S batteries with liquid electrolyte. The hybrid electrolyte Li-S cell with the carbon coated ceramic electrolyte exhibits high discharge capacities of 1509 mA h g−1, 1454 mA h g−1, 1381 mA h g−1 and 1039 mA h g−1 at 0.05C, 0.1C, 0.2C and 0.5C, respectively. However, the cell with the pristine uncoated ceramic electrolyte shows much lower discharge capacities of 1316 mA h g− 1, 1276 mA h g− 1, 1172 mA h g−1 and 387 mA h g−1, respectively, at the same rates. As a result, the carbon coating layer helps to improve the rate capability of the hybrid Li-S cells. The larger capacities of the upper plateau of the cells with the carbon coated ceramic electrolyte reflect the improved conversion of the dissolved polysulfides species by the carbon coating layer. Moreover, the cells with the carbon coated ceramic electrolyte have much lower overpotentials than the later ones in both charge and discharge processes. This coincides with the lower impedance (Fig. 5a) and the smaller voltage gap between oxidation and reduction peaks in CV curves (Fig. 5c) of the cells with the carbon coated ceramic electrolyte, indicating that the cell is more electrochemically reversible than that of the cells with the pristine uncoated ceramic electrolyte. It is reported that the reduction in available electrochemical surface area due to Li2S precipitation in the discharge process plays a vital role in the increase in low-
Fig. 7. Surface EDS mapping and elemental identification of the coating layer after the10th cycle.
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plateau overpotential [29,30]. With the increase of charge-discharge current density, the decrease in capacity is mainly caused by the shortening of the lower discharge plateau which is associated with the reduction of polysulfides to the solid Li2S and the increase of its overpotential. Therefore, the length and the overpotential of the upper plateau are not strongly affected by the increasing discharge rates [8,31]. The cycling performance obtained in a voltage window of 1.5–3.0 V vs. Li/Li+ is shown in Fig. 6. The irreversible capacity loss in the first cycle, which is associated with the highly soluble polysulfide intermediates in the organic liquid electrolyte, is an important parameter to evaluate the reversibility of the cell [8,24,32]. As a consequence, the cell containing the pristine ceramic electrolyte shows a higher initial capacity loss. The cell containing the carbon coated ceramic electrolyte delivers an initial specific capacity of 1409 mA h g−1 and remains at 1000 mA h g− 1 after 50 cycles at 0.2C rate. Much higher capacities and greatly improved cycling stability are achieved for the cell with the carbon coated ceramic electrolyte owing to the facilitated conversion of Li-S redox reaction. Fig. 7 shows the surface EDS mapping and elemental identification of the coating layer after the 10th cycle. The appearance of elemental sulfur demonstrates the trapping of the active material in it [28,33]. Moreover, the uniform distribution of sulfur in the coating layer proves the effective reutilization of the trapped active material with the help of the efficient electron transfer of carbon [34,35]. It is worth mentioning that both cells exhibit good coulombic efficiency of about 100% since there is no shuttle effect in these hybrid Li-S cells. 4. Conclusion In summary, this work clearly demonstrates that by modifying the ceramic electrolyte with a carbon coating layer, a highly efficient interface is achieved in the ceramic-liquid hybrid Li-S cell. The carbon coating layer obviously increases the conducting surface between cathode and the ceramic electrolyte where high reutilization of the adsorbed dissolved polysulfide species is realized, leading to significant enhancement in the reversibility and rate capability. The resultant Li-S cell shows much improved cycling stability with an initial discharge capacity of 1409 mA h g−1 and remains at 1000 mA h g−1 after 50 cycles at 0.2C rate. Acknowledgements We are grateful to the support of the Natural Science Foundation of China (NSFC) No. 51402330, No. 51372262, No. 51472261 and the high resolution earth observation system major special project youth innovation foundation of China No. GFZX04060103. The authors thank Prof.
B. V. R. Chowdari (School of Materials science and Engineering, Nanyang Technological University, Singapore) for helpful discussion. References [1] J.M. Tarascon, M. Armand, Nature 414 (6861) (2001) 359. [2] J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, J.G. Zhang, Nat. Commun. 6 (2015) 6362. [3] Q. Pang, D. Kundu, M. Cuisinier, L.F. Nazar, Nat. Commun. 5 (2014) 4759. [4] A. Manthiram, Y. Fu, Y.S. Su, Acc. Chem. Res. 46 (5) (2013) 1125. [5] S. Evers, L.F. Nazar, Acc. Chem. Res. 46 (5) (2013) 1135. [6] S. Zhang, K. Ueno, K. Dokko, M. Watanabe, Adv. Energy Mater. 5 (16) (2015) (n/a). [7] S.E. Cheon, K.S. Ko, J.H. Cho, S.W. Kim, E.Y. Chin, H.T. Kim, J. Electrochem. Soc. 150 (6) (2003) A796. [8] S.E. Cheon, K.S. Ko, J.H. Cho, S.W. Kim, E.Y. Chin, H.T. Kim, J. Electrochem. Soc. 150 (6) (2003) A800. [9] S.S. Zhang, J. Power Sources 231 (2013) 153. [10] J. Scheers, S. Fantini, P. Johansson, J. Power Sources 255 (2014) 204. [11] Y. Yang, G. Zheng, S. Misra, J. Nelson, M.F. Toney, Y. Gui, J. Am. Chem. Soc. 134 (37) (2012) 15387. [12] Q. Wang, J. Zheng, E. Walter, H. Pan, D. Lv, P. Zuo, H. Chen, Z.D. Deng, B.Y. Liaw, X. Yu, X. Yang, J.G. Zhang, J. Liu, J. Xiao, J. Electrochem. Soc. 162 (3) (2015) A474. [13] Q. Wang, J. Jin, X. Wu, G. Ma, J. Yang, Z. Wen, Phys. Chem. Chem. Phys. 16 (39) (2014) 21225. [14] J. Zheng, M. Gu, C. Wang, P. Zuo, P.K. Koech, J.G. Zhang, J. Liu, J. Xiao, J. Electrochem. Soc. 160 (11) (2013) A1992. [15] Z. Lin, C. Liang, J. Mater. Chem. A 3 (3) (2015) 936. [16] F.Y. Fan, W.C. Carter, Y.M. Chiang, Adv. Mater. 27 (35) (2015) 5203. [17] J. Kulisch, H. Sommer, T. Brezesinski, J. Janek, Phys. Chem. Chem. Phys. 16 (35) (2014) 18765. [18] C.J. Leo, G.V. Subba Rao, B.V.R. Chowdari, J. Mater. Chem. 12 (6) (2002) 1848. [19] X.X. Xu, Z.Y. Wen, Z.H. Gu, X.H. Xu, Z.X. Lin, Solid State Ionics 171 (3-4) (2004) 207. [20] X. Xu, Z. Wen, X. Wu, X. Yang, Z. Gu, J. Am. Ceram. Soc. 90 (9) (2007) 2802. [21] C.R. Mariappan, C. Yada, F. Rosciano, B. Roling, J. Power Sources 196 (15) (2011) 6456. [22] J. Shi, Y. Xia, S. Han, L. Fang, M. Pan, X. Xu, Z. Liu, J. Power Sources 273 (0) (2015) 389. [23] H. Wei, J. Ma, B. Li, Y. Zuo, D. Xia, ACS Appl. Mater. Interfaces 6 (22) (2014) 20276. [24] Q. Wang, Z. Wen, J. Jin, J. Guo, X. Huang, J. Yang, C. Chen, Chem. Commun. (Camb.) 52 (8) (2016) 1637. [25] Y. Fu, Y.S. Su, A. Manthiram, Angew. Chem. Int. Ed. Engl. 52 (27) (2013) 6930. [26] L. Chen, Y.Z. Liu, M. Ashuri, C.H. Liu, L.L. Shaw, J. Mater. Chem. A 2 (42) (2014) 18026. [27] W. Lin, Y. Chen, P. Li, J. He, Y. Zhao, Z. Wang, J. Liu, F. Qi, B. Zheng, J. Zhou, C. Xu, F. Fu, J. Electrochem. Soc. 162 (8) (2015) A1624. [28] S.H. Chung, A. Manthiram, Adv. Mater. 26 (43) (2014) 7352. [29] T. Zhang, M. Marinescu, L. O'Neill, M. Wild, G. Offer, Phys. Chem. Chem. Phys. 17 (35) (2015) 22581. [30] T. Poux, P. Novák, S. Trabesinger, J. Electrochem. Soc. 163 (7) (2016) A1139. [31] M.R. Busche, P. Adelhelm, H. Sommer, H. Schneider, K. Leitner, J. Janek, J. Power Sources 259 (2014) 289. [32] M. Helen, M.A. Reddy, T. Diemant, U. Golla-Schindler, R.J. Behm, U. Kaiser, M. Fichtner, Sci. Rep. 5 (2015) 12146. [33] S.A. Abbas, M.A. Ibrahem, L.-H. Hu, C.-N. Lin, J. Fang, K.M. Boopathi, P.-C. Wang, L.-J. Li, C.-W. Chu, J. Mater. Chem. A 4 (24) (2016) 9661. [34] S.-H. Chung, A. Manthiram, J. Phys. Chem. Lett. 5 (11) (2014) 1978. [35] H.-J. Peng, D.-W. Wang, J.-Q. Huang, X.-B. Cheng, Z. Yuan, F. Wei, Q. Zhang, Adv. Sci. 3 (1) (2016) 1500268.
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Improved performance of Li-S battery with hybrid electrolyte by interface modification Qingsong Wang a,b, Jing Guo a,b, Tian Wu a,b, Jun Jin a, Jianhua Yang a, Zhaoyin Wen a,⁎ a b
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 16 August 2016 Received in revised form 27 October 2016 Accepted 1 November 2016 Available online xxxx Keywords: Lithium-sulfur battery Hybrid electrolyte Solid state ionic conductor Interface modification
a b s t r a c t A carbon coating layer facing sulfur cathode is introduced on one side of the solid state ionic conductor in the ceramic-liquid hybrid electrolyte based Li-S cell. Both the cycling performance and the rate capability are improved as a result of enhanced electron transport for the conversion of the dissolved sulfur-containing active materials as well as the wettability of the ceramic electrolyte towards the liquid electrolyte. The interface modified hybrid Li-S cell exhibits an initial discharge capacity of 1409 mA h g−1 and remains at 1000 mA h g−1 which is 235 mA h g−1 higher than the pristine one, after 50 cycles at 0.2C rate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Lithium metal is one of the most promising candidates as an anode material for next-generation energy storage systems due to its high specific capacity (3860 mA h g−1) and the low negative electrochemical potential (− 3.04 V versus the standard hydrogen electrode) [1,2]. When coupled with a sulfur cathode, a rechargeable lithium-sulfur (Li-S) battery is expected to yield high theoretical specific energy of 2600 W h kg−1, based on a redox reaction that reversibly interconverts sulfur (S8) and Li2S [3]. In addition, sulfur is low cost, highly natural abundant and environmental friendly. However, the practical realization of Li–S batteries is currently hindered by numerous scientific and technical challenges. The most critical problem is believed to be associated with the intermediate lithium polysulfides (Li2Sn, n = 4–8) that are highly soluble in many organic solvents based electrolytes [4–6]. The dissolved polysulfides, formed at the beginning of discharge, could shuttle between the cathode and anode through the commonly used porous polymer separator. This shuttle process induces irreversible loss of active materials from the cathode, corrosion of Li metal anode, low coulombic efficiency, rapid capacity fading, and high self-discharge rates [7,8]. Nevertheless, the dissolution of polysulfides is inevitable and essential for effective utilization of the active material in a Li-S cell [9,10]. In the discharge process, the soluble polysulfides dissolve into the electrolyte solution enabling the subsequent sulfur to be exposed to the conductive agent and the reduction process to progressively move forward. In the ⁎ Corresponding author. E-mail address: [email protected] (Z. Wen).
http://dx.doi.org/10.1016/j.ssi.2016.11.001 0167-2738/© 2016 Elsevier B.V. All rights reserved.
charge process, the dissolved polysulfide species can facilitate the electrochemical conversion of the insoluble discharge products Li2S2/Li2S [11,12]. We have previously reported a novel type of Li-S cell employing a hybrid electrolyte (HE) [13]. The hybrid electrolyte was composed of the liquid electrolyte and a lithium ion conductor that acts as the separator. The NASICON-type structured Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte, which physically block the dissolution and diffusion of polysulfides could solve the shuttling problem. Meanwhile, the liquid electrolyte takes advantage of the dissolved polysulfides which offer fast charge/discharge rates and favor fast electrochemical kinetics of the cathode. However, a redistribution of polysulfides on the electrode is unavoidable no matter what kind of treatment is done on the initial sulfur cathodes [14]. The redistribution of sulfur in the cell would cause the capacity fading once the soluble polysulfides detach from the cathode into the liquid electrolyte [15–17]. Gradually, sulfur loses intimate contact with carbon matrix. Electronically isolated and electrochemically inactive Li2S does not take part in the electrochemical oxidation during charge state owing to the lack of carbon matrix. The formation of irreversible Li2S becomes severe as cycling proceeds and the available conducting surface is decreased gradually, leading to the decreasing of discharge capacity. To make full use of the dissolved sulfur-containing active materials, a carbon coating layer facing sulfur cathode is introduced onto one side of the solid electrolyte. As shown in Fig. 1, the carbon-coating layer serves as an upper current collector to facilitate electron transport as well as to improve the wettability of solid electrolyte towards the liquid electrolyte. The migration of the polysulfides is physically blocked by the solid electrolyte and the shuttle effect is prevented. A lithium sulfur battery with enhanced electrochemical
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electronically conductive carbon is a mixture of acetylene black (AB) and carbon nano-tube (CNT) in the ratio of 3:1 (w/w). A uniform slurry composed of the mixture of carbon (AB&CNT) and polyvinylidene fluoride binder, with the weight ratio of 4:1 and N-Methyl pyrrolidone (AR, Aladdin Industrial Inc.) as the solvent, was directly sprayed onto LAGP ceramic plates by spin-coating method. After evaporation of the solvent, the carbon coated plates were annealed under argon atmosphere at 550 °C for 6 h to pyrolyze the polymer binder. 2.2. Preparation of the sulfur cathode
Fig. 1. Schematic illustration of the Li-S battery configuration with a carbon-coated LAGP, the active materials were recaptured on the conductive layer and enabled further redox reactions while the LAGP prevents the polysulfides from immigrating to the anode side.
The cathode composite was made from mixing sulfur powder (S8, spectrum pure) and commercial Ketjen black (KB, with a specific surface area of over 1300 m2 g−1) in the ratio of 3:2 (w/w) using ball milling and further heating at 155 °C for 12 h under vacuum. The electrodes were prepared by spraying a slurry of 80 wt% KB/S composite, 10 wt% vapor grown carbon fiber, 5 wt% Carboxy Methylated Cellulose and 5 wt% Styrene Butadiene Rubber binder dispersed in distilled water onto Al foil. The sulfur electrodes were dried at 60 °C under vacuum for 12 h. Finally, 1.13 cm2 cathode discs were punched for the battery tests. The sulfur mass loading is about 1.2–1.3 mg cm−2. 2.3. Characterization
performance is demonstrated with an initial discharge capacity of 1409 mA h g− 1 and a reversible specific capacity of 1000 mA h g− 1 after 50 cycles at 0.2C rate.
2. Experimental 2.1. Carbon coated Li1.5Al0.5Ge1.5(PO4)3 pellet fabrication The Li1.5Al0.5Ge1.5(PO4)3 ceramic electrolyte is synthesized by the solid state reaction method. The detailed preparation process is previously reported [13,18]. The obtained ceramic pellet, with the thickness of 0.7 mm and diameter of 17.4 mm, has a conductivity of 1.7 × 10−4 S/cm at room temperature. This value is in good agreement with results reported by several previous literatures [19–22]. The
The morphology of the cross section of the carbon coated LAGP ceramic electrolyte and the surface of the carbon coating layer was obtained by Scanning Electron Microscopy (SEM, HITACHI S-3400) at an accelerating voltage of 15 kV. Energy Disperse Spectroscopy (EDS, attached to the SEM) characterization was carried out on the surface of the carbon coating layer after the10th cycle. Transmission Electron Microscopy (TEM) image of the AB&CNT was taken on a Field Emission Microscope JEOL-2010CX. For the pore structure analysis of the AB&CNT, nitrogen sorption isotherms were carried out at 77 K with a Micromeritics Tristar 3000 analyzer (MICROMERITICS, U.S.). Specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method using the adsorption data in a relative pressure range of P/ P0 = 0.05–0.25. The pore volume was determined from the amounts of N2 adsorbed at a relative pressure (P/P0) of 0.99.
Fig. 2. (a) SEM image of the cross section of the carbon-coated LAGP; (b) TEM image of the AB&CNT; (c) SEM image of the surface of the carbon coating layer; (d) photograph of the carboncoated LAGP and the pristine LAGP.
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Fig. 3. (a) N2 adsorption–desorption isotherms for AB&CNT (BET surface area, 79.9 m2 g−1), (b) pore size distribution of AB&CNT (x-coordinate, double logarithmic reciprocal scale).
2.4. Electrochemical measurements The pristine LAGP pellet or the carbon coated LAGP pellet was sandwiched between the lithium metal anode and the as-prepared KB/S cathode which then was placed in a predesigned Teflon container held by the screw clamp. Liquid electrolyte, 30 μL LiN(CF3SO2)2 (LiTFSI, Sigma-Aldrich) dissolved in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich) with high thermal stability and moderate viscosity was used to improve the contact between the electrodes and the ceramic electrolyte. Teflon O-rings were used for sealing and stainless steel plates were employed as the current collectors. All cells were assembled and sealed in an argon filled glove box. After 2-h rest, the cells were tested by galvanostatic cycling in the voltage range of 1.5–3.0 V or 1.0–3.0 V vs. Li/Li+ using a LAND CT2001A battery test system (Wuhan, China). Specific capacities were calculated on the basis of sulfur mass. An Autolab PGSTAT302N Electrochemical Workstation (ECO CHEMIE B.V., Netherlands) was used to perform the Electrochemical impedance measurements of the cells by using a Frequency Response Analyzer (FRA) with a small perturbation voltage of 10 mV in the frequency range of 1 MHz to 0.1 Hz. Cyclic voltammetry measurements of the cells were also collected on the Electrochemical Workstation at a scan rate of 0.02 mV/s between 1.0 V and 3.0 V vs. Li/Li+. All electrochemical tests were conducted at room temperature (nominally 25 °C).
3. Results and discussion Fig. 2a presents cross-sectional image of the carbon coated LAGP ceramic electrolyte. The carbon coating layer is stacked well on the surface of the LAGP ceramic and is about 5 μm thick on average. The carbon
material is composed of a uniformly dispersed mixture of AB and CNT, as shown in the TEM image (Fig. 2b). Fig. 2c shows the top surface morphology of the carbon coating layer which shows a uniform porous structure without visible agglomeration. The digital photo of the pristine ceramic pellet and the carbon coated ceramic pellet is given in Fig. 2d. It can be seen that the carbon coated ceramic pellet has a black surface. Diameter of the ceramic pellets is about 17.4 mm. The carbon material has a large BET specific surface area of 79.9 m2 g−1 and is accompanied with 2 nm micropores (Fig. 3) which contributes to the adsorption of the dissolved sulfur species and the wettability of the ceramic electrolyte towards the liquid electrolyte [23]. The 0.0625 M Li2S8 solution was synthesized via reacting elemental sulfur and lithium sulfide (Li2S, Sigma-Aldrich) in the desired ratio in TEGDME at room temperature for 12 h under vigorous stirring inside an argon glove box. The obtained uniform deep brown Li2S8/TEGDME solution was employed to conduct the contact angle measurements. As expected, Fig. 4 shows that the carbon coating layer had a much lower contact angle than the uncoated ceramic electrolyte. Owing to the capture of polysulfides and promoting of electron transfer, the improved wettability of the ceramic electrolyte towards the liquid electrolyte will be beneficial in conversion of the dissolved polysulfides species at the three-phase boundary between carbon, sulfur, and electrolyte as shown in Fig. 1. Fig. 5a and b shows the impendence spectra of the hybrid electrolyte Li–S cells and inset is the proposed equivalent circuit. The impendence measurements were performed before the discharging–charging tests. As shown, both of the two type cells show a similar Nyquist plot which composed of two partially overlapped semicircles and a straight slopping line. Likely as reported by our previous works [13,24], the left extrapolated intercept of the semicircle with the real axis in the high frequency region reflects the resistance of the LAGP (RLAGP) solid
Fig. 4. Contact angle snapshots for Li2S8/TEGDME on (a) pristine LAGP and (b) carbon-coated LAGP. a-1 and b-1, time t = 0 s, correspond to the solution droplet just dropped down on the pellet surfaces. a-2 and b-2, time t = 10 s, correspond to the solution droplet stayed on the surface for 10 s.
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Fig. 5. Impedance spectra of the hybrid electrolyte Li-S cells measured at the region of 1 MHz to 0.1 Hz and inset is the proposed equivalent circuit (a) carbon-coated LAGP, and (b) the pristine LAGP; CV curves (at 0.02 mV/s) of the hybrid electrolyte Li-S cells (c) with carbon-coated LAGP, and (d) with the pristine LAGP; charge/discharge profiles (at different rates) of the hybrid electrolyte Li-S cells (e) with carbon-coated LAGP and (f) with the pristine LAGP.
electrolyte which remains unchanged after the carbon-layer modification. The depressed semicircles in the high frequency region and in the middle frequency region reflect the interphase contact resistance (Rint) caused by the solid–liquid hybrid electrolyte interface and the charge-transfer resistance (Rct), respectively. The sloping straight line in the low frequency region represents the diffusion impendence of the Li-ion diffusion process. It's clearly that the interphase contact resistance and the charge-transfer resistance are both decreased which results in enhanced performances and will be discussed below. The electrochemical performances of the hybrid Li-S cells containing the
carbon coated LAGP ceramic electrolyte and the pristine LAGP ceramic electrolyte were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling. The CV curves for the cell with carbon coated ceramic electrolyte are depicted in Fig. 5c. There are two cathodic peaks at 2.45 and 2.0 V corresponding to the change from elemental sulfur to high-order polysulfides (Li2Sx, x N 4) and the further reduction to low-order polysulfides (Li2Sx, x ≤ 4), respectively. The two distinguishable anodic peaks in the anodic sweep indicate the staged conversion of low-order polysulfides to high-order polysulfides and then to elemental sulfur [25,26]. Moreover, the shape and the position of the
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Fig. 6. Cycling performance of the Li-S cells, charge/discharge at 0.2C between 1.5 and 3.0 V.
anodic and cathodic peaks obtained during different CV cycles are basically met, attesting to the superior cell reversibility and stability of the Li-S redox reactions of the cell with the carbon coated ceramic electrolyte. However, there is only one anodic peak in the CV curves (Fig. 5d) for the cell with pristine uncoated ceramic electrolyte which suggests that the conversion from higher-order polysulfides to elemental sulfur is less active in the cell with uncoated ceramic electrolyte than that in the former one. In addition, the anodic oxidation peak voltage of the pristine cell is higher and the cathodic reduction peak voltage is lower compared to the cell with carbon coated LAGP, showing faster redox-reduction kinetics of the later cell. This is consistent with the effect of carbon coating layer in helping decrease in the interfacial and charge transfer resistances (Fig. 5a, b) by facilitating the electron transport in the conversion of the adsorbed dissolved polysulfides species [27]. Another important difference between the two cells is that all the peak currents in the cell with the carbon coated ceramic electrolyte are higher than those in the cell with uncoated ceramic electrolyte, indicating the increased ion and electron transport enabled by the carbon coating layer [27,28]. The representative charge/discharge profiles at different C rates are shown in Fig. 5e and f. The open circuit voltage of the as-assembled hybrid cells was about 3.2 V. Liquid electrolyte was employed to improve the contact between the ceramic electrolyte and the electrodes. The
polysulfides tend to dissolve in the liquid electrolyte at the beginning of the discharge. Galvanostatic voltage profiles of the hybrid cells represent the typical characterization of conventional Li-S batteries with liquid electrolyte. The hybrid electrolyte Li-S cell with the carbon coated ceramic electrolyte exhibits high discharge capacities of 1509 mA h g−1, 1454 mA h g−1, 1381 mA h g−1 and 1039 mA h g−1 at 0.05C, 0.1C, 0.2C and 0.5C, respectively. However, the cell with the pristine uncoated ceramic electrolyte shows much lower discharge capacities of 1316 mA h g− 1, 1276 mA h g− 1, 1172 mA h g−1 and 387 mA h g−1, respectively, at the same rates. As a result, the carbon coating layer helps to improve the rate capability of the hybrid Li-S cells. The larger capacities of the upper plateau of the cells with the carbon coated ceramic electrolyte reflect the improved conversion of the dissolved polysulfides species by the carbon coating layer. Moreover, the cells with the carbon coated ceramic electrolyte have much lower overpotentials than the later ones in both charge and discharge processes. This coincides with the lower impedance (Fig. 5a) and the smaller voltage gap between oxidation and reduction peaks in CV curves (Fig. 5c) of the cells with the carbon coated ceramic electrolyte, indicating that the cell is more electrochemically reversible than that of the cells with the pristine uncoated ceramic electrolyte. It is reported that the reduction in available electrochemical surface area due to Li2S precipitation in the discharge process plays a vital role in the increase in low-
Fig. 7. Surface EDS mapping and elemental identification of the coating layer after the10th cycle.
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plateau overpotential [29,30]. With the increase of charge-discharge current density, the decrease in capacity is mainly caused by the shortening of the lower discharge plateau which is associated with the reduction of polysulfides to the solid Li2S and the increase of its overpotential. Therefore, the length and the overpotential of the upper plateau are not strongly affected by the increasing discharge rates [8,31]. The cycling performance obtained in a voltage window of 1.5–3.0 V vs. Li/Li+ is shown in Fig. 6. The irreversible capacity loss in the first cycle, which is associated with the highly soluble polysulfide intermediates in the organic liquid electrolyte, is an important parameter to evaluate the reversibility of the cell [8,24,32]. As a consequence, the cell containing the pristine ceramic electrolyte shows a higher initial capacity loss. The cell containing the carbon coated ceramic electrolyte delivers an initial specific capacity of 1409 mA h g−1 and remains at 1000 mA h g− 1 after 50 cycles at 0.2C rate. Much higher capacities and greatly improved cycling stability are achieved for the cell with the carbon coated ceramic electrolyte owing to the facilitated conversion of Li-S redox reaction. Fig. 7 shows the surface EDS mapping and elemental identification of the coating layer after the 10th cycle. The appearance of elemental sulfur demonstrates the trapping of the active material in it [28,33]. Moreover, the uniform distribution of sulfur in the coating layer proves the effective reutilization of the trapped active material with the help of the efficient electron transfer of carbon [34,35]. It is worth mentioning that both cells exhibit good coulombic efficiency of about 100% since there is no shuttle effect in these hybrid Li-S cells. 4. Conclusion In summary, this work clearly demonstrates that by modifying the ceramic electrolyte with a carbon coating layer, a highly efficient interface is achieved in the ceramic-liquid hybrid Li-S cell. The carbon coating layer obviously increases the conducting surface between cathode and the ceramic electrolyte where high reutilization of the adsorbed dissolved polysulfide species is realized, leading to significant enhancement in the reversibility and rate capability. The resultant Li-S cell shows much improved cycling stability with an initial discharge capacity of 1409 mA h g−1 and remains at 1000 mA h g−1 after 50 cycles at 0.2C rate. Acknowledgements We are grateful to the support of the Natural Science Foundation of China (NSFC) No. 51402330, No. 51372262, No. 51472261 and the high resolution earth observation system major special project youth innovation foundation of China No. GFZX04060103. The authors thank Prof.
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