Polydopamine-coated separator for high

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Mar 6, 2015 - at 0.5 C after 200 cycles, which is higher than that of the cell with routine separator. Additionally, the coulombic efficiency of the cell is also im-.
Polydopamine-coated separator for highperformance lithium-sulfur batteries

Zhian Zhang, Zhiyong Zhang, Jie Li & Yanqing Lai

Journal of Solid State Electrochemistry Current Research and Development in Science and Technology ISSN 1432-8488 Volume 19 Number 6 J Solid State Electrochem (2015) 19:1709-1715 DOI 10.1007/s10008-015-2797-8

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Author's personal copy J Solid State Electrochem (2015) 19:1709–1715 DOI 10.1007/s10008-015-2797-8

ORIGINAL PAPER

Polydopamine-coated separator for high-performance lithium-sulfur batteries Zhian Zhang & Zhiyong Zhang & Jie Li & Yanqing Lai

Received: 3 January 2015 / Revised: 10 February 2015 / Accepted: 16 February 2015 / Published online: 6 March 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract The polydopamine-coated membrane is used as a separator to enhance the electrochemical performance of lithium-sulfur battery. The lithium-sulfur cell employing the polydopamine-coated separator displays a high specific capacity of 670 mAh g−1 at 0.5 C after 200 cycles, which is higher than that of the cell with routine separator. Additionally, the coulombic efficiency of the cell is also improved obviously after using the polydopamine-coated separator. The improved performance can be attributed to the physical inhibition by the polydopamine coating layer. Besides, due to the highly hydrophilic property of polydopamine, a hydrophilic surface chemical gradient is constructed to trap the polysulfides. Keywords Separator modification . Polydopamine-coated separator . Polysulfides . Lithium-sulfur battery

Introduction Advanced energy storage systems are highly desired to meet the increasing demands of high energy density batteries [1, 2]. Accordingly, lithium-sulfur (Li-S) batteries have attracted great attention as one of the most promising systems for the next generation high energy density rechargeable lithium batteries because of their high theoretical specific capacity (1675 mAh g−1) and energy density (2600 Wh kg−1) [3, 4]. As a cathode active material, sulfur also has advantages of non-toxicity and abundance in nature [5]. However, the Z. Zhang (*) : Z. Zhang : J. Li : Y. Lai (*) School of Metallurgy and Environment, Central South University, Changsha, Hunan 410083, China e-mail: [email protected] e-mail: [email protected]

practical applications of Li-S batteries are still hindered by some major obstacles. One of the major challenges is the shuttle effect. Polysulfides (Li2Sx, 4 ≤ x ≤ 8) produced in discharge/charge processes can dissolve into organic electrolyte, then diffuse to the anode and can be reduced to lower order polysulfides at the interface of the lithium anode. These reduced products will migrate back to the cathode where they may be reoxidized. This process takes place repeatedly, creating polysulfide shuttle, which can cause loss of active materials and the low coulombic efficiency of Li-S batteries, eventually resulting in rapid capacity fading [6, 7]. In addition, sulfur and its final discharge products (Li2S2, Li2S) are electrical insulators, which can cause poor electrochemical accessibility, leading to a low utilization of the active material. In order to prevent polysulfides shuttling in organic electrolyte, various approaches have been proposed by research teams over the latest three decades. One of the most effective strategies is to confine sulfur into porous frameworks, such as porous carbon materials [8–10], metal oxide matrix [11], and polymer matrix [12, 13]. The porous networks can significantly improve the capacities of the cathodes while the shuttle effect has not been fully addressed that causes fast capacity decay. Another promising approach is the modification of cell configuration, which means building a physical barrier to prevent the migration of polysulfides. The cell modification can be classified into three aspects, that is, surface coating of the sulfur cathode [14, 15], insertion of free-standing interlayer (carbon interlayer [16–18], polypyrrole interlayer [19]), and modification of separator [20, 21]. The first two methods of cell modification have been proven to be effective to enhance the performance of lithium-sulfur batteries, while the study on the separation modification of lithium-sulfur batteries is still at a very early stage. Separator, the basic component of the lithium-sulfur battery, is a porous membrane (e.g., polypropylene, polyethylene, glass fibers), which serves solely as an electronic insulator and

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does not influence the transportation of ions through the membrane [22, 23]. Polysulfides can diffuse freely through the membrane and react with the anode, which can cause the degradation of the Li-S battery. Therefore, the inhibition of shuttle effect by modification of separator will be an effective method to improve the performance of lithium-sulfur batteries. Dopamine, a commercially available chemical, containing both catechol and amine groups, is advantageous to electrolyte wetting, electrolyte uptake, and ionic conductivity [24, 25]. Besides, it has been used to modify the separators’ surface properties for lithium-ion batteries [26, 27]. The results of the research by Zhang [24] and Choi [25] have proved that the polydopamine coating layer can enhance hydrophilic surface properties of the separator and increase the electrochemical performance of lithium-ion battery. In this work, we were motivated to introduce polymer coating layer on routine separator to suppress the polysulfide diffusion and to improve cycling stability, as well as increase the discharging capacities. The advantages of such a modified separator are shown as follows: (1) It can act as a physical barrier to block the shuttle of the polysulfides and (2) the

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surface of the modified separator becomes highly hydrophilic, and a chemical barrier is constructed so that the polysulfides can preferentially be trapped. The lithium-sulfur batteries with polydopamine-coated separator show a higher coulombic efficiency and an improved electrochemical performance, with a specific capacity of 670 mAh g−1 after 200 cycles at 0.5 C. These results indicate that the polydopamine-coated separator is more suitable for lithium-sulfur battery applications.

Experimental Preparation and characterization of polydopamine-coated separator The modification of the PP/PE/PP separator (Celgard 2320) was achieved by simple immersion of the separator into the dopamine solution (10 mM) that used methanol and Tris buffer solution (pH 8.5) as co-solvents (CH3OH:buffer=1:1 by vol). After overnight soaking, the PP/PE/PP separator were taken out, rinsed with DI water and acetone several times

Fig. 1 Schematic of polydopamine-coated separator preparation (a), suggested structure for polydopamine-coated separator (b)

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and dried at 55 °C for 24 h under vacuum. A schematic illustration of the fabrication of the polydopamine-coated separator is shown in Fig. 1. For brevity, the polydopamine-coated separator will be referred as PDA-coated separator, and the cell with the polydopamine-coated separator will adopt the same notation. Cell assembly and characterization The sulfur cathodes were prepared by conventional slurry coating method with a doctor blade. The cathode slurry was prepared by mixing 80 wt% mesoporous carbon/sulfur composite (80 wt%, sulfur), 10 wt% carbon black (Super P Timcal), and 10 wt% PVDF (6020 Solef) in NMP solution. Fig. 2 SEM photographs of routine PP/PE/PP separator (a, b); polydopamine-coated PP/PE/PP separator (c, d). XPS spectra of the routine separator and polydopamine-coated separator (e)

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Then, the slurry was spread onto aluminum foil (20 μm) and dried at 60 °C under vacuum overnight. The sulfur loading density was almost 1.3 mg cm−2. Coin-type (CR2025) cells were assembled in an argon-filled glove box (Universal 2440/750) in which oxygen and water contents were less than 1 ppm. Sulfur electrode was used as the cathode, lithium foil as the counter and reference electrode, Celgard 2320 or the PDA-coated membrane as the separator, and 1.5 mol L−1 lithium bis(trifluoromethane sulfonyl)imide (LiTFSI, 99.95 %, Aldrich) in a solvent mixture of 1,3-dioxolane and 1,2dimethoxyethane (1:1, in volume) as the electrolyte with 0.1 M lithium nitrate (LiNO3) as an additive. Electrochemical impedance spectroscopy measurements were conducted using Solartron 1470E cell test. The

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electrochemical impedance spectroscopy (EIS) of the working electrode was carried out in three-electrode system, and the sulfur cathode was used as the working electrode; metallic lithium was used as the reference electrode and the counter electrode. EIS measurements were carried out at open-circuit potential in the frequency range between 100 kHz and 0.01 Hz with a perturbation amplitude of 5 mV. The galvanostatic charge/discharge tests were carried out at a constant current density of 838 mA g−1 (0.5 C) in the potential range of 1.8 to 2.8 V under a LAND CT2001A charge–discharge system. All experiments were conducted at room temperature. Morphology characterization of the polydopaminecoated separator was observed with scanning electron microscopy (FEI Quanta 200). In order to optically determine the retention of polysulfide species by the introduced polydopamine coating layer, we carried out a simple test. Thereby, 3 ml solution of 0.5 M Li2S6 (normal concentration) in 1,2-dimethoxyethane:1,3dioxolane (DME:DOL; 1:1, v:v) was placed inside the glass tube, whereas the opposite side of the separator was filled with a pristine mixture of 5 ml DME:DOL (1:1, v:v). During the experiment, the solutions rested without movement to exclude external influence on the diffusion test of polysulfides through the routine separator and the PDA-coated separator. The resulting color change was evaluated by visual examination.

Results and discussion As shown in Fig. 1, the polydopamine-coated separator was achieved by simple immersion of routine separator in the aqueous-buffered dopamine solution at 25 °C and pH 8.5. After the self-polymerization for 24 h, the routine separator turned its color from white to dark brown, which is consistent with the previous report [25]. Scanning electron micrographs of the surface of the routine separator and the PDA-coated separator are presented in Fig. 2a–d. The routine separator (Fig. 2a–b) exhibits a uniformly interconnected submicron pore structure, which maintains the ionic pathway and blocks the transfer of electrons between the cathode and anode [28]. And, the size of the pore is around 100 nm, which allow the rapid transportation of ions for electrochemical reactions. As shown in Fig. 2c, d, the PDA-coated separator has a unique coating layer on the surface of routine separator. The channels in the routine separator are almost completed covered by the polydopamine coating layer, which will build a physical barrier for the diffusion of polysulfides. The coating amount of polydopamine is 0.04 mg cm−2. Figure 2e shows the XPS pattern of modified routine separator. The data of XPS confirms that the surface composition changes resulted from the polydopamine coating, which is similar to the results by Choi [25]. While the routine separators exhibits only C 1 s peaks, the PDA-coated

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Fig. 3 Cycle performance and coulombic efficiency of Li-S cells with routine separator and polydopamine-coated separator at the current density of 838 mA g−1 (0.5 C)

separator exhibits newly appeared N 1 s and O 1 s, located in 399–401 and 531–533 eV, respectively, which indicates that polydopamine has been adhered to the routine separator. In order to demonstrate the advantageous electrochemical properties of the cell using the PDA-coated separator, cycle performance test was carried out at the current density of 838 mA g−1 (0.5 C) between 1.8 and 2.8 V, as shown in Fig. 3. It can be seen that the capacity of both the sulfur cathodes decrease with an increased number of cycles. The discharge capacity of the cell with the routine separator decreases from 846 to 523 mAh g−1 after 200 cycles, which is similar to the result reported in previous literature [29, 30]. Whereas, the initial discharge capacity of the cell with the PDA-coated separator is 885 mAh g−1 and a high reversible capacity of 670 mAh g−1 is retained after 200 cycles, showing a great improvement in cyclability, especially relatively higher specific capacity and higher capacity retention rate. As shown in Fig. 3, the coulombic efficiency of the cell with polydopamine-coated separator is almost 99 % at the first

Fig. 4 Rate performance of Li-S cells with routine separator and polydopamine-coated separator

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Fig. 5 Nyquist plots for the fresh Li-S cells with routine separator and polydopamine-coated separator

5 cycles, which is higher than that of the cell with routine separator. In the next 195 cycles, the coulombic efficiency of the cell with polydopamine-coated separator is close to 99 % all the way. However, the coulombic efficiency of the cell with routine separator decreases from 99 to 95 %. The difference in the coulombic efficiency indicates that the polydopaminecoated separator can effectively suppress the shuttle effect. The polydopamine layer covered on the channels of the routine separator can act as a physical barrier for the diffusion of polysulfides. In addition, considering that polydopamine is highly hydrophilic, a highly hydrophilic surface chemical gradient is formed to trap the polysulfides preferentially [31–33]. To identify whether the introduction of PDA-coated separator would influence the rate performance of lithium-sulfur cells, the rate performances were also evaluated as shown in Fig. 4. The discharge capacity gradually decreases as the current rate rising from 0.5 to 2 C for both cells. Compared with the cell using the routine separator with a specific capacity of 440 mAh g−1 at 2 C, similar to the research by Xu [34], a

Fig. 7 Test setup for polysulfide diffusion experiments (a); the optical images for the diffusion of polysulfides: b, c with routine separator and d, e with polydopamine-coated separator

satisfactory capacity of 530 mAh g−1 can be obtained for the cell with the PDA-coated separator. Moreover, the cell with the PDA-coated separator can recover most of the capacity when the current rate was reduced back to 0.5 C. Such improved electrochemical performance of the cell with the PDAcoated separator can be attributed to that the polysulfides is

Fig. 6 Cycle performance of the Li-S cells with the routine separator and the polydopamine-coated separator: at a current density of 1 C (a) and at a current density of 2 C (b)

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limited in the area between the cathode and the PDA-coated separator by physical obstruction. To understand the improved electrochemical performance of the cells using the PDA-coated separator, EIS analysis was carried out. The Nyquist plots and the fitting results of the fresh cells with PDA-coated separator and routine separator are presented in Fig. 5. Both impedance plots are composed of a depressed semicircle at high frequency, which corresponds to the charge transfer resistance (Rct) of the sulfur electrode [35] and an inclined line at low frequency, which reflects the Li ion diffusion into the active mass [36]. Besides, the intercept at real axis Z, corresponding to the combination resistance Re, associates with the ionic conductivity of electrolyte, the intrinsic resistance of the cathode, separator, and anode, the contact resistance at the active material/current collector interface. It is clear that the charge transfer resistance (Rct) of the cell using PDA-coated separator (33 Ω) is slightly lower than that of the cell using routine separator (37.8 Ω). It can be attributed to the better wetting properties and the higher electrolyte uptake [25, 26]. To further investigate the effect of polydopamine-coated separator on the cycle performance of the lithium-sulfur cell at high current density, cycle performance test was carried at 1 and 2 C. As shown in Fig. 6, the cell with the polydopaminecoated separator shows better cycle stability than the cell with the routine separator. The discharge capacity of the cell with the routine separator is 704 mAh g−1 for the initial cycle and decreases to 485 mAh g−1 after 100 cycles at 1 C. Whereas, the discharge capacity of the cell with the polydopaminecoated separator is 752 mAh g−1 for the initial cycle, and a high reversible capacity of 603 mAh g−1 is retained after 100 cycles with the capacity retention ratio of 80 %, showing a great improvement in cyclability. It can be seen from Fig. 6b that the specific capacity of lithium sulfur cell is also improved after using the polydopamine-coated separator at 2 C, with a specific capacity of 450 mAh g−1 after 100 cycles. In order to optically determine the retention of polysulfide species by the introduced polydopamine coating layer, we carried a simple test by the setup shown in Fig. 7a. As we expected (shown in Fig. 7b, c), the routine separator did not suppress the diffusion of polysulfides; thus, the color of the DME/DOL mixture solution already changed from colorless to yellow after 6 h. In contrast, the PDA-coated separator is effective to limit polysulfides within the cathode side, and no obvious change in the color of DME/DOL mixture solution can be observed (Fig. 7e), indicating that the PDA-coated separator play a vital role as a shield for polysulfide anions. It can be attributed ascribed to two aspects. Firstly, most channel of the routine separator is coated by polydopamine which blocks the pathway of polysulfides. Secondly, the surface of routine separator becomes more hydrophilic after polydopamine coating, and a highly hydrophilic surface

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chemical gradient is constructed so that the polysulfides would preferentially be trapped [31–33].

Conclusions In summary, we prepared a polydopamine-coated separator by a cheap and simple dipping process for lithium-sulfur batteries. The polydopamine covered on the channels of the separator forms a physical barrier to block the pathway of polysulfides. Besides, due to the highly hydrophilic property of polydopamine, a hydrophilic surface chemical gradient is constructed to trap the polysulfides preferentially. Consequently, the lithium-sulfur cell employing the polydopamine-coated separator displays a high specific capacity with 670 mAh g−1 at 0.5 C after 200 cycles and excellent rate performance from 0.5 to 2 C. Additionally, the coulombic efficiency of the cell was also improved obviously after using the polydopamine-coated separator. The excellent results prove that the modification of the separator will be a promising strategy to promote the development of the lithiumsulfur batteries.

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