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A new polysulfide blocker - poly(acrylic acid) modified separator for improved performance of lithium-sulfur battery
Journal of Membrane Science 563 (2018) 277–283
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A new polysulfide blocker - poly(acrylic acid) modified separator for improved performance of lithium-sulfur battery
T
Shili Song, Lei Shi, Shiyao Lu, Yuanchao Pang, Yuankun Wang, Min Zhu, Dawei Ding, ⁎ Shujiang Ding Department of Applied Chemistry, Xi'an Key Laboratory of Sustainable Energy Materials Chemistry, School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, 710049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photografting Permselective Polysulfide Separator Lithium-sulfur batteries
Lithium-sulfur (Li-S) batteries gain great popularity due to its high theoretical energy density, low cost, and natural abundance of sulfur active elements, whereas, "shuttle effect" of soluble polysulfide seriously deteriorates their electrochemical performance, hindering their practical applications. Herein, we demonstrate a strategy that graft poly(acrylic acid) (PAA) on the surface of polypropylene (PP) separator to improve the electrochemical performance of Li-S batteries by impeding the “shuttle effect” of polysulfide. The PP grafted with PAA (PP-gPAA) separator allows Li+ migration while rejects polysulfide anions by electrostatic repulsion. Consequently, this permselective separator restrains the polysulfide anions on the cathode side. In addition, the total reaction process of preparing PP-g-PAA separator is facile and low cost. Owing to the strong structure stability between PP and grafted PAA, a uniform and consistent blocking of the polysulfide shuttle during the charge-discharge process could be realized. An ultra-low decay value of 0.074% per cycle for the first 600 cycles is realized in this work. The electrochemical properties are significantly improved compared to the cell with PP separator. Such a permselective separator can be used in various electrodes and working conditions, which is promising for the construction of high performance batteries.
1. Introduction Lithium-sulfur (Li-S) batteries gain grand popularity since its high theoretical energy density (~2600 W h kg−1), low cost, and natural abundance of sulfur active elements [1–3]. They are considered as the next generation of high-energy-density electrochemical energy storage devices [4–6]. Nevertheless, "shuttle effect" is one of the key problems which hindered the practical applications of rechargeable Li-S batteries. The shuttle effect derives from the polysulfide’ diffusion between anode and cathode, results in loss of capacity, decrease of coulombic efficiency and serious self-discharge of the cells [7–9]. In order to suppress the adverse effects caused by shuttle effect, lots of investigations focused on inhibiting the “shuttle” of soluble polysulfide have been reported. The main strategies can be concluded as two aspects: (a) cathode functionalization, using materials such as carbonaceous materials [10–14], metallic compounds [15–18] and polymers [19–23] to adsorb polysulfide physically or chemically; (b) constructing size selective or electrostatic repulsive separators [24–34], which trap the polysulfide in the cathode side. In particular, functionalization of separator is an efficient and reliable strategy. For
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Corresponding author. E-mail address: [email protected] (S. Ding).
https://doi.org/10.1016/j.memsci.2018.05.050 Received 7 March 2018; Received in revised form 22 April 2018; Accepted 24 May 2018
Available online 25 May 2018 0376-7388/ © 2018 Published by Elsevier B.V.
examples, Zhou and co-workers presented a metal-organic framework (MOF, HKUST-1)-based separator, acting as an ionic sieve (~0.9 nm) in the Li-S battery, which allows Li+ get through and blocks the largesized polysulfide in the cathode side. There is only a capacity decay rate of 0.019% per cycle after 1500 cycles and the capacity remains almost constant in the first 500 cycles [24]. Zhang and co-workers prepared a Nafion coated ion selective membrane to suppress the "shuttle effect" of polysulfide and improve the cell efficiency of the cells. The capacity loss of per cycle was only 0.08% during the first 500 charge/discharge processes [25]. Manthiram and co-workers modified the commercial polypropylene (PP) separator by forming a facile carboxyl functional group on the PP skeleton through a sequence of hydroxylation, graft, and hydrolyzation process to achieve significantly enhanced cycling performances [30]. However, these separators still suffer from some defects such as complex fabrication process, high cost and poor structure stability. As for battery separators modification, numerous attentions have been paid on the Nafion due to its special character [35–41]. Nevertheless, the high cost of Nafion polymer restricts its wide range application for Li-S batteries. In this work, we prepared a series of carboxyl group functionalized
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Fig. 1. (a) Illustration of the preparation method of the reaction mechanism of the PP-g-PAA separator. (b) Schematic of lithium-sulfur battery with PP-g-PAA separator, in which the polysulfide anions are limited to the cathode side. (c) The photographic images of the Nafion coated separator (left) and the PP-g-PAA2 separator (right). (d) FT-IR spectra of separators.
separator by a facile, low cost process. Photografting method was used to introduce carboxyl functional group to modify commercial PP separator [42]. The poly(acrylic acid) (PAA) presents analogous function as Nafion, which could suppress the diffusion of the polysulfide anions between the cathode and anode by electrostatic repulsion. To obtain the best modified separator, different UV illumination time were regulated, and through a series of characterization and electrochemical tests, the PAA modified separator of 11.5% grafting rate reveals best cycling capability of a long-term charge-discharge process with a cycle decay rate of 0.074% per cycle over 600 cycles at 0.5 C.
DG =
W1 − W0 W0
× 100 %
Where DG is grafting rate, W0 is the weight of primitive Celgard 2325 separator, W1 is the weight of the PAA modified separators. 2.2. Preparation of sulfur cathodes The sulfur was mixed with GO in a ratio of 70:30 wt% heat treated under N2 protection at the 155 °C for 12 h to acquire the S/C composite. The cathode consists of 70 wt% S/C composite, 20 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF). Then mixture was dissolved in N-methyl-2-pyrrolidinone (NMP) to acquire black slurry. The slurry was then coated on aluminum foil to achieve cathode film. The membrane was dried at 60 °C in vacuum oven over 10 h and cut into 12 mm disks with the sulfur loading of around 1.2 mg cm−2.
2. Experimental section 2.1. Preparation of the PAA grafted separators Photopolymerization experiments were conducted under the photografting equipment (UV Cure). The UV lamp was used at a distance of 15 cm from the sample. The schematic diagram of experimental method is shown in the Fig. S1. Photopolymerization experiments: First, Celgard 2325 separator of PP (5 cm * 10 cm) was extracted with acetone for 1 h to remove the residual additive before photografting of PAA; and then PP membrane was immersed in an acetone solution containing benzophenone (BP) for 1 h and dried out at the room temperature. Later, the pretreated PP membrane was sandwiched with quartz plates and irradiated by UV light for 10 mins. After then, 20 mL of acrylic acid (AA) aqueous solution with 20 wt% concentration was added into the quartz plates and irradiated by UV light for 5 min, 10 min and 15 min to produce PP-g-PAA1, PP-g-PAA2 and PP-g-PAA3. At last, the grafted PP membrane was washed thoroughly with water and dried to a constant weight at room temperature under vacuum conditions. The grafting rate was determined by gravimetric method [42]:
2.3. Cell assembly and electrochemical testing The CR2025 cells were assembled and sealed in an Ar-filled glovebox with concents of oxygen and moisture blow 0.1 ppm. The electrolyte contains 1 mol L−1 bis(trifluoromethanesulfonyl)imide lithium (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 v/v) solvent，and with 1 wt% LiNO3 additives. The Celgard 2325 separator and PAA modified separators were utilized, lithium metal was utilized as anode, S/C composite was utilized as cathode. The coin cells were galvanstaically cycled by NEWARE within a voltage range of 1.7–2.8 V. The data of cyclic voltammetry (CV) were collected with a Princeton Ametek MC electrochemistry workstation at 0.5 mV s−1. Electrochemical impedance spectra (EIS) were also measured by it in the 100 kHz to 0.1 Hz frequency range. The LiNO3-free electrolyte was employed for the shuttle current measurement. First, the cells were galvanostatically charged of 100% state to 2.8 V under 278
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Fig. 2. SEM and Contact angle images. (a) (e) PP separator. (b) (f) PP-g-PAA1 separator. (c) (g) PP-g-PAA2 separator. (d) (h) PP-g-PAA3 separator.
shuttle of polysulfide between the cathode and anode can be effectively suppressed. A variate about reaction time was carried out, three groups of experiments were conducted parallelly at 5 min, 10 min and 15 min respectively, the grafting rate of poly(acrylic acid) increased gradually with the increase of lighting time. The final data were taken the average of three groups showed in Table S1, consequently, the diameter and mass of pristine separator are 19 mm and 3.95 mg, ultimately the grafting rate are 6.4%, 11.5%, 15.2%, respectively, which are named PP-g-PAA1, PP-g-PAA2, PP-g-PAA3. The Nafion layer was generated on the pristine separator with a coating amount of 1 mg cm−2 as previous method (Fig. 1c) [35]. It can be seen that the Nafion forms a film when the aqueous solution evaporated out. Unfortunately, the Nafion layer is easy to fall off due to the poor interaction between Nafion and separator. In our work (Fig. 1c), the grafted PAA is hard to shed because the PAA is closely linked with PP through strong chemical bond. Hence the cell with PP-g-PAA separator could better inhibit the polysulfide shuttle during the charge and discharge process. Additionally, as the photografting reaction proceeds on the one surface of the PP separator, thin PAA layer chemically bonded to the PP separator. So Fourier-transform infrared spectra (FT-IR) analysis was carried out to manifestly ascertain the formation of PAA on the separator (Fig. 1d). It can be seen that the pristine PP separator has four main characteristic adsorption peaks in 2800–3000 cm−1. The characteristic peaks around 2951 cm−1, 2870 cm−1 are the asymmetric and symmetrical stretching vibration of –CH3, the characteristic peaks around 2918 cm−1, 2837 cm−1 are stretching vibration of –CH2 [43]. Besides, the peaks at 1456 cm−1 and 1372 cm−1 could be corresponding to the asymmetric and symmetrical bending vibration of –CH3. After grafting of PAA, two new absorption peaks at 3000–3500 cm−1 and 1710 cm−1 appear, which can attributed to the hydroxyl and carbonyl band [44], respectively. Proving that PAA is successfully grafted onto the separator, and the hydroxyl increases after grafting the acrylic. Scanning electron microscopy (SEM) was characterized to manifest the microstructure of the grafted polypropylene microporous surface (Fig. 2a). The pristine separator shows channels with a flat surface throughout the whole separator, and the pore sizes are approximately 100 nm, which allows the fast transmission of ions for electrochemical reactions. On the contrary, the Fig. 2b and Fig. 2c are the PAA modified separators with grafting rates of 6.4% and 11.5%, it can be seen that the pore size of the membrane are reduced after grafting. The porosity is significantly decreased because the grafted PAA clogs or covers
Fig. 3. The shuttle currents of Li-S cells with PP, PP-g-PAA1, PP-g-PAA2, PP-gPAA3 separator.
the current density of 0.5 C, after that, the cells were discharged to 2.38 V, as Zhang’ group indicated previously, shuttle current reached maximum and then switched to potentiostatic mode [48]. 2.4. Characterization Field-emission scanning electron microscopy (FESEM, Gemini SEM 500) was used to characterize the micro morphology of the separators. The optical contact angle measuring instrument (DSA 100, Germany) was used to measure the wettability of the membrane to water and lithium sulfur electrolyte. FTIR spectra were obtained by FTIR spectrometer (BIO-RAD FTS6000) in the range of 4000–400 cm−1. 3. Result and discussion Fig. 1a shows the preparation process of the PP-g-PAA separator. The experimental setup is shown in the Fig. S1. Benzophenone (BP) was used as photoinitiator, acrylic acid used as monomer, and Celgard 2325 (PP) used as substrate. The grafting polymerization process is completed by two steps. Firstly, benzophenone captures hydrogen from the PP substrate to aggregate surface radical and semipinacol radicals. Then the AA monomer solutions are added to the active substrate, and the surface initiators trigger the graft polymerization under UV irradiation. Fig. 1b illustrates the PAA modified separator with the carboxyl functional group, which can prevent the diffusion of polysulfide anion, but allow Li+ get through freely by electrostatic interaction. Thus the 279
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Fig. 4. (a) Cycling performance of the cell with PP separator, PP-g-PAA1 separator, PP-g-PAA2 separator, PP-g-PAA3 separator at 0.2 C with voltage range of 2.8–1.7 V. (b) Initial charge-discharge curves of Li-S batteries with PP separator, PP-g-PAA2 separator. (c) Rate performance of cell with PP separator, PP-g-PAA2 separator. (d) The electrochemical impedance spectroscopy of cell with PP separator, PP-g-PAA2 separator. (e) Cycling performance of the cell with PP-g-PAA2 separator at the current of 0.5 C.
So as to reveal the ability of suppressing the polysulfide shuttle by the modified separator, electrochemical cells was constructed with diverse separators and LiNO3-free electrolytes (Fig. 3). As a result, the cells with PP-g-PAA separator generate a reduced shuttle current due to the electrostatic repulsion effect. According to Zhang’ work, shuttle current is largest at a potentiostatic charging voltage of 2.38 V in a working Li-S cell [47,48]. The cell with PP-g-PAA2 separator presents the shuttle current approximately of 4.16 × 10−3 mA cm−2，which is merely one-sixth of the cell with PP separator (2.55 × 10−2 mA cm−2). In addition, the cell with PP-g-PAA1, PP-g-PAA3 shows the shuttle current of 1.52 × 10−2 mA cm−2, 8.31 × 10−3 mA cm−2, respectively. It indicates that the cell with PP-g-PAA2 could availably block polysulfide and restrain the shuttle effect in Li-S cell. With the introduction of the PP-g-PAA separator, the electrochemical performances of lithium sulfur battery are significantly improved. The electrochemical performances of Coin-type half cells (2025 R type) with our PP-g-PAA separators were evaluated in different systems (Fig. 4). The cycle performance of different separators is shown in the Fig. 4a, the black curve in Fig. 4a corresponds to the cell with PP separator, which shows a primary discharge capacity of 612 mA h g−1, and the capacity quick fading to 300 mA h g−1 over 50 cycles. On the
membrane microspores, but there are a considerable amount of microspores. However, there is almost no channel on the membrane when the grafting rate is 15.2%. By grafting PAA, the channels of the PP separator are partially covered to prevent the pass of high order polysulfide anions. As PP-g-PAA separators can obtain hydrophilic properties of surface, the hydrophily is enhanced with the increased grafting rate. The contact angel gradually decreases of the PP-g-PAA separator against the water (Fig. S2), which proved that PAA successfully grafted onto the PP separator. Nevertheless, the wettability is pivotal among separator and liquid electrolyte in the electrochemical property of the cell. The weak wettability of the separator detrimentally aggrandizes the cell resistance, and consequently lessens the rate capability [45,46]. So the contact angel of the PP separator and the PP-g-PAA separator against the Li-S batteries electrolyte has been measured as 26.5°, 30.1°, 32.3°, 36.7° respectively. Grafted PAA increases the contact angle between the separator and the ether electrolyte, but it is not changed so much, just the modified separator with the highest graft rate was 10° higher than the PP separator. Through the later electrochemical impedance spectroscopy test can also proves that the modified separator almost has little effect on the cell resistance. 280
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Fig. 5. Diffusion tests of polysulfide with PP separator (a-d) and the PP-g-PAA2 (e-h) separator at different permeation times.
plateaus are in accordance with the CV curves. Besides, the initial discharge capacities of Li-S cells with the PP-g-PAA2 separator and the PP separator are about 786 mA h g−1 and 612 mA h g−1 at 0.2 C, respectively, it is obvious that the cell with the PP-g-PAA2 separator provides higher capacity. Which is attributed to the ability of the cell with PP-g-PAA2 separator suppresses the shuttle of polysulfide and hence insures subsequent utilization of elemental sulfur for batter cyclic stability. The rate properties were also evaluated with different separators (Fig. 4c). the cells with PP-g-PAA separator deliver the specific capacity of 713, 599, 548, 502, 442, 373, 580 mA h g−1 at 0.1 C, 0.2 C, 0.3 C, 0.5 C, 1 C, 2 C, and 0.1 C rate, respectively, which exceed the Li-S cells with a PP separator (693, 574, 522, 193, 151, 106, 545 mA h g−1). There are not many differences of the discharge capacities in low-power working conditions, yet the Li-S cells with PP-g-PAA2 separator show higher capacities at the high current densities. When the discharge/ charge current was converted to 0.1 C, the cell with PP-g-PAA2 separator demonstrates capacity retention of 81% superior to the cell with PP separator (78% capacity retention). We ascribe the better rate capability to the ability of PP-g-PAA suppressing the shuttle of polysulfide. In order to make a thorough inquiry of the electrode process in the cell, electrochemical impedance spectrum (EIS) was conducted with frequency from 0.01 Hz to 100 k Hz. As shown in the EIS (Fig. 4d and Fig. S5) [49,50]. The cells with PP-g-PAA1 separator, PP-g-PAA2 separator and PP separator slightly display a similar resistance value (about 70 Ω), thus no significant impact on the charge transfer resistance, probably due to the ultrathin thickness of PAA membrane. Since grafted much more PAA, the impedance of the cell with PP-gPAA3 separator is 80 Ω, which is a little larger than the cell with PP separator. The cycle performance test was investigated at the current density of 0.5 C (Fig. 4e), the cell with the PP-g-PAA2 separator exhibits an excellent cycle performance, which displays an initial discharge capacity of 562 mA h g−1, it can also retains the capacity of 311 mA h g−1 after 600 cycles, retaining 55% of initial discharge capacity and the average capacity fading rate is only 0.074% per cycle, which demonstrates the PAA obstructs the polysulfide to the anode. In order to test the ability of the separator resisting polysulfide, the
contrary, batteries configured with the PP-g-PAA separators exhibits higher capacity in all cycles, the cycling becomes much more stable. This is attributed to the cell with PP separator with carboxyl functional group could suppress the shuttle of polysulfide and increases the availability of elemental sulfur. Moreover, the cell with PP-g-PAA2 separator shows optimum cyclic stability and highest capacity, after 250 cycles it also remains the capacity of 580 mA h g−1, which confirms its ability to localize polysulfide lithium diffusion near the cathode for maximum active material utilization and minimal losses by parasite reactions with the anode. It is also noticed that the separator with the higher carboxyl group does not necessarily lead to better performance. Li-S battery based on the PP-g-PAA3 separator has a low coulombic efficiency of 96–97% compared to others, and the discharge capacity is lower than the PP-g-PAA2 separator. Maybe the separator grafted too much PAA limits rate capacity due to the insufficient ionic conductivity. So the PP-g-PAA2 separator expresses better electrochemical cycling performance, it is consistent to the result of the shuttle current test. Thus the following electrochemical performance tests were measured by cells with the PP separator and PP-g-PAA2 separator. Fig. S3 exhibits CV curves of cells with PP separator and the PP-g-PAA separator in the range of 1.7–2.8 V at a scan rate of 0.5 mV s−1. The cathodic peaks at 2.25 and 2.03 V represent that the elemental sulfur from S8 reduced to higher-order polysulfide (Li2SX, x = 4–8), and then transferred to lower-order L2S2 / L2S. While a broad peaks appeared in the anodic sweep, which indicates the stage of short-chain lithium polysulfide or lithium sulfide translate to long-chain lithium polysulfide (Li2SX, x = 4–8) to elemental sulfur. It is clear that the cell with PP-g-PAA2 separator shows negligible change in first three cycles, which implies the cell with the PP-g-PAA2 separator has better electrochemical reversibility and capacity retention compared to the cell with PP separator. Meanwhile, the cell with the PP-g-PAA separator shows no influence on the electrochemical reactions, hence the peak positions almost the same. The first three charge-discharge curves of Li-S cells with different separators are exhibited in the Fig. S4, the discharge and charge plateaus are well in accordance with the CV curves. Besides, the initial charge-discharge curve of Li-S cells with the PP-g-PAA2 separator displays two voltage plateaus in Fig. 4b, the discharge and charge 281
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polysulfide permeability of the separator samples was tested (Fig. 5). Li2S6 was first dissolved in the 5 mL of the DME/DOL (1:1, by volume) and then added to the left glass tubes. As time goes by, the polysulfide all gradually diffuses under the pressure/concentration gradient. The PP separator shows a faster diffusion than the PP-g-PAA2 separator, and the right glass tube turns deep yellow after 24 h. Meanwhile, the PP-gPAA2 separator has such a slower polysulfide permeation rate, where there are almost no color changes, indicating a good polysulfide blocking ability of the PP-g-PAA2 separator, and the carboxyl group excludes polysulfide at the cathode by electrostatic interaction.
[11] [12]
[13]
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4. Conclusion
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In this work, the permselective PP-g-PAA separators were fabricated with different grafting rate by photografting process and applied in the Li-S batteries, respectively. The unique PP-g-PAA separators substantially suppressed shuttle effect of soluble lithium polysulfide, and elongated the cycle life of Li-S batteries. Due to physical confinement and electrostatic repulsion to polysulfide species, the resultant battery cell showed an obvious improved cycling stability with a cyclic decay of 0.074% per cycle over first 600 cycles at 0.5 C. Moreover, the rate capability of the S/C composite cathode with PP-g-PAA separator exhibited a higher capacity even at a high current density of 2 C than that of pristine separator. It also contributed to high lifespan of the Li-S batteries because of the effective inhibition of the shuttle effect. Unlike the previous approach to block the polysulfide with physical barriers, the functional polymer can provide a chemopselective strategy to enhance the performance of Li-S batteries. Furthermore, this approach can be extended to other Li ion batteries, Li air batteries, even fuel cells that require confinement for high reactivity and sturdy cyclability.
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Acknowledgments [24]
This work was supported by the National Natural Science Foundation of China (No. 51773165), State Key Laboratory of Electrical Insulation and Power Equipment (EIPE18205). We thank Miss Zijun Ren at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with field-emission scanning electron microscopy (SEM) analysis. We also thank Miss Dan Li at School of Chemical Engineering and Technology for the optical contact angle measuring instrument (DSA 100).
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Appendix A. Supporting information
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Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2018.05.050.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A new polysulfide blocker - poly(acrylic acid) modified separator for improved performance of lithium-sulfur battery
T
Shili Song, Lei Shi, Shiyao Lu, Yuanchao Pang, Yuankun Wang, Min Zhu, Dawei Ding, ⁎ Shujiang Ding Department of Applied Chemistry, Xi'an Key Laboratory of Sustainable Energy Materials Chemistry, School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, 710049, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Photografting Permselective Polysulfide Separator Lithium-sulfur batteries
Lithium-sulfur (Li-S) batteries gain great popularity due to its high theoretical energy density, low cost, and natural abundance of sulfur active elements, whereas, "shuttle effect" of soluble polysulfide seriously deteriorates their electrochemical performance, hindering their practical applications. Herein, we demonstrate a strategy that graft poly(acrylic acid) (PAA) on the surface of polypropylene (PP) separator to improve the electrochemical performance of Li-S batteries by impeding the “shuttle effect” of polysulfide. The PP grafted with PAA (PP-gPAA) separator allows Li+ migration while rejects polysulfide anions by electrostatic repulsion. Consequently, this permselective separator restrains the polysulfide anions on the cathode side. In addition, the total reaction process of preparing PP-g-PAA separator is facile and low cost. Owing to the strong structure stability between PP and grafted PAA, a uniform and consistent blocking of the polysulfide shuttle during the charge-discharge process could be realized. An ultra-low decay value of 0.074% per cycle for the first 600 cycles is realized in this work. The electrochemical properties are significantly improved compared to the cell with PP separator. Such a permselective separator can be used in various electrodes and working conditions, which is promising for the construction of high performance batteries.
1. Introduction Lithium-sulfur (Li-S) batteries gain grand popularity since its high theoretical energy density (~2600 W h kg−1), low cost, and natural abundance of sulfur active elements [1–3]. They are considered as the next generation of high-energy-density electrochemical energy storage devices [4–6]. Nevertheless, "shuttle effect" is one of the key problems which hindered the practical applications of rechargeable Li-S batteries. The shuttle effect derives from the polysulfide’ diffusion between anode and cathode, results in loss of capacity, decrease of coulombic efficiency and serious self-discharge of the cells [7–9]. In order to suppress the adverse effects caused by shuttle effect, lots of investigations focused on inhibiting the “shuttle” of soluble polysulfide have been reported. The main strategies can be concluded as two aspects: (a) cathode functionalization, using materials such as carbonaceous materials [10–14], metallic compounds [15–18] and polymers [19–23] to adsorb polysulfide physically or chemically; (b) constructing size selective or electrostatic repulsive separators [24–34], which trap the polysulfide in the cathode side. In particular, functionalization of separator is an efficient and reliable strategy. For
⁎
Corresponding author. E-mail address: [email protected] (S. Ding).
https://doi.org/10.1016/j.memsci.2018.05.050 Received 7 March 2018; Received in revised form 22 April 2018; Accepted 24 May 2018
Available online 25 May 2018 0376-7388/ © 2018 Published by Elsevier B.V.
examples, Zhou and co-workers presented a metal-organic framework (MOF, HKUST-1)-based separator, acting as an ionic sieve (~0.9 nm) in the Li-S battery, which allows Li+ get through and blocks the largesized polysulfide in the cathode side. There is only a capacity decay rate of 0.019% per cycle after 1500 cycles and the capacity remains almost constant in the first 500 cycles [24]. Zhang and co-workers prepared a Nafion coated ion selective membrane to suppress the "shuttle effect" of polysulfide and improve the cell efficiency of the cells. The capacity loss of per cycle was only 0.08% during the first 500 charge/discharge processes [25]. Manthiram and co-workers modified the commercial polypropylene (PP) separator by forming a facile carboxyl functional group on the PP skeleton through a sequence of hydroxylation, graft, and hydrolyzation process to achieve significantly enhanced cycling performances [30]. However, these separators still suffer from some defects such as complex fabrication process, high cost and poor structure stability. As for battery separators modification, numerous attentions have been paid on the Nafion due to its special character [35–41]. Nevertheless, the high cost of Nafion polymer restricts its wide range application for Li-S batteries. In this work, we prepared a series of carboxyl group functionalized
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Fig. 1. (a) Illustration of the preparation method of the reaction mechanism of the PP-g-PAA separator. (b) Schematic of lithium-sulfur battery with PP-g-PAA separator, in which the polysulfide anions are limited to the cathode side. (c) The photographic images of the Nafion coated separator (left) and the PP-g-PAA2 separator (right). (d) FT-IR spectra of separators.
separator by a facile, low cost process. Photografting method was used to introduce carboxyl functional group to modify commercial PP separator [42]. The poly(acrylic acid) (PAA) presents analogous function as Nafion, which could suppress the diffusion of the polysulfide anions between the cathode and anode by electrostatic repulsion. To obtain the best modified separator, different UV illumination time were regulated, and through a series of characterization and electrochemical tests, the PAA modified separator of 11.5% grafting rate reveals best cycling capability of a long-term charge-discharge process with a cycle decay rate of 0.074% per cycle over 600 cycles at 0.5 C.
DG =
W1 − W0 W0
× 100 %
Where DG is grafting rate, W0 is the weight of primitive Celgard 2325 separator, W1 is the weight of the PAA modified separators. 2.2. Preparation of sulfur cathodes The sulfur was mixed with GO in a ratio of 70:30 wt% heat treated under N2 protection at the 155 °C for 12 h to acquire the S/C composite. The cathode consists of 70 wt% S/C composite, 20 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF). Then mixture was dissolved in N-methyl-2-pyrrolidinone (NMP) to acquire black slurry. The slurry was then coated on aluminum foil to achieve cathode film. The membrane was dried at 60 °C in vacuum oven over 10 h and cut into 12 mm disks with the sulfur loading of around 1.2 mg cm−2.
2. Experimental section 2.1. Preparation of the PAA grafted separators Photopolymerization experiments were conducted under the photografting equipment (UV Cure). The UV lamp was used at a distance of 15 cm from the sample. The schematic diagram of experimental method is shown in the Fig. S1. Photopolymerization experiments: First, Celgard 2325 separator of PP (5 cm * 10 cm) was extracted with acetone for 1 h to remove the residual additive before photografting of PAA; and then PP membrane was immersed in an acetone solution containing benzophenone (BP) for 1 h and dried out at the room temperature. Later, the pretreated PP membrane was sandwiched with quartz plates and irradiated by UV light for 10 mins. After then, 20 mL of acrylic acid (AA) aqueous solution with 20 wt% concentration was added into the quartz plates and irradiated by UV light for 5 min, 10 min and 15 min to produce PP-g-PAA1, PP-g-PAA2 and PP-g-PAA3. At last, the grafted PP membrane was washed thoroughly with water and dried to a constant weight at room temperature under vacuum conditions. The grafting rate was determined by gravimetric method [42]:
2.3. Cell assembly and electrochemical testing The CR2025 cells were assembled and sealed in an Ar-filled glovebox with concents of oxygen and moisture blow 0.1 ppm. The electrolyte contains 1 mol L−1 bis(trifluoromethanesulfonyl)imide lithium (LiTFSI) in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 v/v) solvent，and with 1 wt% LiNO3 additives. The Celgard 2325 separator and PAA modified separators were utilized, lithium metal was utilized as anode, S/C composite was utilized as cathode. The coin cells were galvanstaically cycled by NEWARE within a voltage range of 1.7–2.8 V. The data of cyclic voltammetry (CV) were collected with a Princeton Ametek MC electrochemistry workstation at 0.5 mV s−1. Electrochemical impedance spectra (EIS) were also measured by it in the 100 kHz to 0.1 Hz frequency range. The LiNO3-free electrolyte was employed for the shuttle current measurement. First, the cells were galvanostatically charged of 100% state to 2.8 V under 278
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Fig. 2. SEM and Contact angle images. (a) (e) PP separator. (b) (f) PP-g-PAA1 separator. (c) (g) PP-g-PAA2 separator. (d) (h) PP-g-PAA3 separator.
shuttle of polysulfide between the cathode and anode can be effectively suppressed. A variate about reaction time was carried out, three groups of experiments were conducted parallelly at 5 min, 10 min and 15 min respectively, the grafting rate of poly(acrylic acid) increased gradually with the increase of lighting time. The final data were taken the average of three groups showed in Table S1, consequently, the diameter and mass of pristine separator are 19 mm and 3.95 mg, ultimately the grafting rate are 6.4%, 11.5%, 15.2%, respectively, which are named PP-g-PAA1, PP-g-PAA2, PP-g-PAA3. The Nafion layer was generated on the pristine separator with a coating amount of 1 mg cm−2 as previous method (Fig. 1c) [35]. It can be seen that the Nafion forms a film when the aqueous solution evaporated out. Unfortunately, the Nafion layer is easy to fall off due to the poor interaction between Nafion and separator. In our work (Fig. 1c), the grafted PAA is hard to shed because the PAA is closely linked with PP through strong chemical bond. Hence the cell with PP-g-PAA separator could better inhibit the polysulfide shuttle during the charge and discharge process. Additionally, as the photografting reaction proceeds on the one surface of the PP separator, thin PAA layer chemically bonded to the PP separator. So Fourier-transform infrared spectra (FT-IR) analysis was carried out to manifestly ascertain the formation of PAA on the separator (Fig. 1d). It can be seen that the pristine PP separator has four main characteristic adsorption peaks in 2800–3000 cm−1. The characteristic peaks around 2951 cm−1, 2870 cm−1 are the asymmetric and symmetrical stretching vibration of –CH3, the characteristic peaks around 2918 cm−1, 2837 cm−1 are stretching vibration of –CH2 [43]. Besides, the peaks at 1456 cm−1 and 1372 cm−1 could be corresponding to the asymmetric and symmetrical bending vibration of –CH3. After grafting of PAA, two new absorption peaks at 3000–3500 cm−1 and 1710 cm−1 appear, which can attributed to the hydroxyl and carbonyl band [44], respectively. Proving that PAA is successfully grafted onto the separator, and the hydroxyl increases after grafting the acrylic. Scanning electron microscopy (SEM) was characterized to manifest the microstructure of the grafted polypropylene microporous surface (Fig. 2a). The pristine separator shows channels with a flat surface throughout the whole separator, and the pore sizes are approximately 100 nm, which allows the fast transmission of ions for electrochemical reactions. On the contrary, the Fig. 2b and Fig. 2c are the PAA modified separators with grafting rates of 6.4% and 11.5%, it can be seen that the pore size of the membrane are reduced after grafting. The porosity is significantly decreased because the grafted PAA clogs or covers
Fig. 3. The shuttle currents of Li-S cells with PP, PP-g-PAA1, PP-g-PAA2, PP-gPAA3 separator.
the current density of 0.5 C, after that, the cells were discharged to 2.38 V, as Zhang’ group indicated previously, shuttle current reached maximum and then switched to potentiostatic mode [48]. 2.4. Characterization Field-emission scanning electron microscopy (FESEM, Gemini SEM 500) was used to characterize the micro morphology of the separators. The optical contact angle measuring instrument (DSA 100, Germany) was used to measure the wettability of the membrane to water and lithium sulfur electrolyte. FTIR spectra were obtained by FTIR spectrometer (BIO-RAD FTS6000) in the range of 4000–400 cm−1. 3. Result and discussion Fig. 1a shows the preparation process of the PP-g-PAA separator. The experimental setup is shown in the Fig. S1. Benzophenone (BP) was used as photoinitiator, acrylic acid used as monomer, and Celgard 2325 (PP) used as substrate. The grafting polymerization process is completed by two steps. Firstly, benzophenone captures hydrogen from the PP substrate to aggregate surface radical and semipinacol radicals. Then the AA monomer solutions are added to the active substrate, and the surface initiators trigger the graft polymerization under UV irradiation. Fig. 1b illustrates the PAA modified separator with the carboxyl functional group, which can prevent the diffusion of polysulfide anion, but allow Li+ get through freely by electrostatic interaction. Thus the 279
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Fig. 4. (a) Cycling performance of the cell with PP separator, PP-g-PAA1 separator, PP-g-PAA2 separator, PP-g-PAA3 separator at 0.2 C with voltage range of 2.8–1.7 V. (b) Initial charge-discharge curves of Li-S batteries with PP separator, PP-g-PAA2 separator. (c) Rate performance of cell with PP separator, PP-g-PAA2 separator. (d) The electrochemical impedance spectroscopy of cell with PP separator, PP-g-PAA2 separator. (e) Cycling performance of the cell with PP-g-PAA2 separator at the current of 0.5 C.
So as to reveal the ability of suppressing the polysulfide shuttle by the modified separator, electrochemical cells was constructed with diverse separators and LiNO3-free electrolytes (Fig. 3). As a result, the cells with PP-g-PAA separator generate a reduced shuttle current due to the electrostatic repulsion effect. According to Zhang’ work, shuttle current is largest at a potentiostatic charging voltage of 2.38 V in a working Li-S cell [47,48]. The cell with PP-g-PAA2 separator presents the shuttle current approximately of 4.16 × 10−3 mA cm−2，which is merely one-sixth of the cell with PP separator (2.55 × 10−2 mA cm−2). In addition, the cell with PP-g-PAA1, PP-g-PAA3 shows the shuttle current of 1.52 × 10−2 mA cm−2, 8.31 × 10−3 mA cm−2, respectively. It indicates that the cell with PP-g-PAA2 could availably block polysulfide and restrain the shuttle effect in Li-S cell. With the introduction of the PP-g-PAA separator, the electrochemical performances of lithium sulfur battery are significantly improved. The electrochemical performances of Coin-type half cells (2025 R type) with our PP-g-PAA separators were evaluated in different systems (Fig. 4). The cycle performance of different separators is shown in the Fig. 4a, the black curve in Fig. 4a corresponds to the cell with PP separator, which shows a primary discharge capacity of 612 mA h g−1, and the capacity quick fading to 300 mA h g−1 over 50 cycles. On the
membrane microspores, but there are a considerable amount of microspores. However, there is almost no channel on the membrane when the grafting rate is 15.2%. By grafting PAA, the channels of the PP separator are partially covered to prevent the pass of high order polysulfide anions. As PP-g-PAA separators can obtain hydrophilic properties of surface, the hydrophily is enhanced with the increased grafting rate. The contact angel gradually decreases of the PP-g-PAA separator against the water (Fig. S2), which proved that PAA successfully grafted onto the PP separator. Nevertheless, the wettability is pivotal among separator and liquid electrolyte in the electrochemical property of the cell. The weak wettability of the separator detrimentally aggrandizes the cell resistance, and consequently lessens the rate capability [45,46]. So the contact angel of the PP separator and the PP-g-PAA separator against the Li-S batteries electrolyte has been measured as 26.5°, 30.1°, 32.3°, 36.7° respectively. Grafted PAA increases the contact angle between the separator and the ether electrolyte, but it is not changed so much, just the modified separator with the highest graft rate was 10° higher than the PP separator. Through the later electrochemical impedance spectroscopy test can also proves that the modified separator almost has little effect on the cell resistance. 280
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Fig. 5. Diffusion tests of polysulfide with PP separator (a-d) and the PP-g-PAA2 (e-h) separator at different permeation times.
plateaus are in accordance with the CV curves. Besides, the initial discharge capacities of Li-S cells with the PP-g-PAA2 separator and the PP separator are about 786 mA h g−1 and 612 mA h g−1 at 0.2 C, respectively, it is obvious that the cell with the PP-g-PAA2 separator provides higher capacity. Which is attributed to the ability of the cell with PP-g-PAA2 separator suppresses the shuttle of polysulfide and hence insures subsequent utilization of elemental sulfur for batter cyclic stability. The rate properties were also evaluated with different separators (Fig. 4c). the cells with PP-g-PAA separator deliver the specific capacity of 713, 599, 548, 502, 442, 373, 580 mA h g−1 at 0.1 C, 0.2 C, 0.3 C, 0.5 C, 1 C, 2 C, and 0.1 C rate, respectively, which exceed the Li-S cells with a PP separator (693, 574, 522, 193, 151, 106, 545 mA h g−1). There are not many differences of the discharge capacities in low-power working conditions, yet the Li-S cells with PP-g-PAA2 separator show higher capacities at the high current densities. When the discharge/ charge current was converted to 0.1 C, the cell with PP-g-PAA2 separator demonstrates capacity retention of 81% superior to the cell with PP separator (78% capacity retention). We ascribe the better rate capability to the ability of PP-g-PAA suppressing the shuttle of polysulfide. In order to make a thorough inquiry of the electrode process in the cell, electrochemical impedance spectrum (EIS) was conducted with frequency from 0.01 Hz to 100 k Hz. As shown in the EIS (Fig. 4d and Fig. S5) [49,50]. The cells with PP-g-PAA1 separator, PP-g-PAA2 separator and PP separator slightly display a similar resistance value (about 70 Ω), thus no significant impact on the charge transfer resistance, probably due to the ultrathin thickness of PAA membrane. Since grafted much more PAA, the impedance of the cell with PP-gPAA3 separator is 80 Ω, which is a little larger than the cell with PP separator. The cycle performance test was investigated at the current density of 0.5 C (Fig. 4e), the cell with the PP-g-PAA2 separator exhibits an excellent cycle performance, which displays an initial discharge capacity of 562 mA h g−1, it can also retains the capacity of 311 mA h g−1 after 600 cycles, retaining 55% of initial discharge capacity and the average capacity fading rate is only 0.074% per cycle, which demonstrates the PAA obstructs the polysulfide to the anode. In order to test the ability of the separator resisting polysulfide, the
contrary, batteries configured with the PP-g-PAA separators exhibits higher capacity in all cycles, the cycling becomes much more stable. This is attributed to the cell with PP separator with carboxyl functional group could suppress the shuttle of polysulfide and increases the availability of elemental sulfur. Moreover, the cell with PP-g-PAA2 separator shows optimum cyclic stability and highest capacity, after 250 cycles it also remains the capacity of 580 mA h g−1, which confirms its ability to localize polysulfide lithium diffusion near the cathode for maximum active material utilization and minimal losses by parasite reactions with the anode. It is also noticed that the separator with the higher carboxyl group does not necessarily lead to better performance. Li-S battery based on the PP-g-PAA3 separator has a low coulombic efficiency of 96–97% compared to others, and the discharge capacity is lower than the PP-g-PAA2 separator. Maybe the separator grafted too much PAA limits rate capacity due to the insufficient ionic conductivity. So the PP-g-PAA2 separator expresses better electrochemical cycling performance, it is consistent to the result of the shuttle current test. Thus the following electrochemical performance tests were measured by cells with the PP separator and PP-g-PAA2 separator. Fig. S3 exhibits CV curves of cells with PP separator and the PP-g-PAA separator in the range of 1.7–2.8 V at a scan rate of 0.5 mV s−1. The cathodic peaks at 2.25 and 2.03 V represent that the elemental sulfur from S8 reduced to higher-order polysulfide (Li2SX, x = 4–8), and then transferred to lower-order L2S2 / L2S. While a broad peaks appeared in the anodic sweep, which indicates the stage of short-chain lithium polysulfide or lithium sulfide translate to long-chain lithium polysulfide (Li2SX, x = 4–8) to elemental sulfur. It is clear that the cell with PP-g-PAA2 separator shows negligible change in first three cycles, which implies the cell with the PP-g-PAA2 separator has better electrochemical reversibility and capacity retention compared to the cell with PP separator. Meanwhile, the cell with the PP-g-PAA separator shows no influence on the electrochemical reactions, hence the peak positions almost the same. The first three charge-discharge curves of Li-S cells with different separators are exhibited in the Fig. S4, the discharge and charge plateaus are well in accordance with the CV curves. Besides, the initial charge-discharge curve of Li-S cells with the PP-g-PAA2 separator displays two voltage plateaus in Fig. 4b, the discharge and charge 281
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polysulfide permeability of the separator samples was tested (Fig. 5). Li2S6 was first dissolved in the 5 mL of the DME/DOL (1:1, by volume) and then added to the left glass tubes. As time goes by, the polysulfide all gradually diffuses under the pressure/concentration gradient. The PP separator shows a faster diffusion than the PP-g-PAA2 separator, and the right glass tube turns deep yellow after 24 h. Meanwhile, the PP-gPAA2 separator has such a slower polysulfide permeation rate, where there are almost no color changes, indicating a good polysulfide blocking ability of the PP-g-PAA2 separator, and the carboxyl group excludes polysulfide at the cathode by electrostatic interaction.
[11] [12]
[13]
[14]
4. Conclusion
[15]
In this work, the permselective PP-g-PAA separators were fabricated with different grafting rate by photografting process and applied in the Li-S batteries, respectively. The unique PP-g-PAA separators substantially suppressed shuttle effect of soluble lithium polysulfide, and elongated the cycle life of Li-S batteries. Due to physical confinement and electrostatic repulsion to polysulfide species, the resultant battery cell showed an obvious improved cycling stability with a cyclic decay of 0.074% per cycle over first 600 cycles at 0.5 C. Moreover, the rate capability of the S/C composite cathode with PP-g-PAA separator exhibited a higher capacity even at a high current density of 2 C than that of pristine separator. It also contributed to high lifespan of the Li-S batteries because of the effective inhibition of the shuttle effect. Unlike the previous approach to block the polysulfide with physical barriers, the functional polymer can provide a chemopselective strategy to enhance the performance of Li-S batteries. Furthermore, this approach can be extended to other Li ion batteries, Li air batteries, even fuel cells that require confinement for high reactivity and sturdy cyclability.
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
Acknowledgments [24]
This work was supported by the National Natural Science Foundation of China (No. 51773165), State Key Laboratory of Electrical Insulation and Power Equipment (EIPE18205). We thank Miss Zijun Ren at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with field-emission scanning electron microscopy (SEM) analysis. We also thank Miss Dan Li at School of Chemical Engineering and Technology for the optical contact angle measuring instrument (DSA 100).
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Appendix A. Supporting information
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Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2018.05.050.
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