Performance-Enhancing Asymmetric Separator for LithiumâSulfur

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Performance-Enhancing Asymmetric Separator for Lithium–Sulfur Batteries Joanna Maria Conder, Antoni Forner Cuenca, Elisabeth Müller Gubler, Lorenz Gubler, Petr Novák, and Sigita Trabesinger ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04662 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 4, 2016

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ACS Applied Materials & Interfaces

Performance-Enhancing Asymmetric Separator for Lithium–Sulfur Batteries Joanna Conder1, Antoni Forner-Cuenca1, Elisabeth Müller Gubler2, Lorenz Gubler1, Petr Novák1, Sigita Trabesinger1,*

1

Paul Scherrer Institute, Electrochemistry Laboratory, CH-5232 Villigen PSI, Switzerland 2

Paul Scherrer Institute, Laboratory of Biomolecular Research, CH-5232 Villigen PSI, Switzerland

ABSTRACT Asymmetric separators with polysulfide barrier properties consisting of porous polypropylene grafted with styrene sulfonate (PP-g-PLiSS) were characterized in lithium–sulfur cells to assess their practical applicability. Galvanostatic cycling at different C-rates with and without an electrolyte additive, and cyclic voltammetry were used to probe the electrochemical performance of the cells with the PP-g-PLiSS separators, and to compare it with the performance of the cells utilizing state-of-the-art separator, Celgard 2400. Overall, it was found that regardless of the applied cycling rate, the use of the grafted separators greatly enhances the coulombic efficiency

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of the cell. An appropriate Li-exchange-site (-SO3-) concentration at and near the surface of the separator were found to be essential to effectively suppress the polysulfide shuttle without sacrificing the Li-ion mobility through the separator, and to improve the practical specific charge of the cell.

KEYWORDS: Polysulfide rejection; Li–S battery; separators; polysulfide shuttle control; grafted separators.

1. INTRODUCTION The lithium–sulfur (Li–S) battery, which pairs metallic lithium with a sulfur / carbon electrode, can significantly extend the range of electric cars and / or provide longer operating times for many daily-use electronic devices, owing to the high theoretical specific charge of sulfur of 1675 mAh g-1 and high projected specific energy of the cell of 500 Wh kg-1.1,2 However, despite decades of on-going research, the chemistry of the Li–S battery still remains challenging since the battery cycle life is compromised by the dissolution of sulfur-based species, polysulfides Li2Sn (2 < n < 8), formed during each discharge.3 Any of these species, soluble in the electrolyte, can diffuse through the separator, reaching the lithium negative electrode, where they can undergo irreversible reduction to a solid precipitate (Li2S).1,4 This leads to the detrimental phenomenon referred to as ’polysulfide shuttle’, which entails a permanent loss of active material, resulting in rapidly declining specific charge and poor coulombic efficiency of the cells.3,5


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Thus far particular attention has been dedicated to the sulfur electrode, more specifically to the confinement of the sulfur in various conductive porous carbon matrices.6-10 This confinement of the sulfur improved the conductivity of the electrode quite remarkably, at the same time minimizing the loss of active material caused by the dissolution of the polysulfides in the liquid electrolyte.11 The advances offered by this approach are rather limited as it does not eliminate the polysulfide shuttle.12 It has been shown, however, that effective confinement of the polysulfides can be achieved by an ion-conductive polymer coating or a polymer layer formation on the surface of the sulfur based electrode or the separator. A number of Li-ion conductive polymers such as polyacrylonitrile13 and poly(ethylene oxide)7,8 have been considered for this purpose. This strategy has shown promising results leading to good cycling stability with a high specific charge.7 Polymers containing sulfonic acid groups, and in particular Nafion, have been considered as candidates for polymeric coating onto one side of a Celgard separator.14-16 With the optimal Li–Nafion loading, both good rate capability and improved charge efficiency, especially at low C-rates, were achieved.14 Moreover, a 1 µm thick Nafion film that corresponds to a loading of 0.7 mg cm-2 was sufficient to establish an ion-selective layer acting as an anion shield against polysulfides, and thus significantly improve the cyclability of the Li–S cells. After 500 cycles, 60% of the initial specific charge was retained and an average coulombic efficiency of 96% was attained, which is much higher than the values obtained in case of the reference cell with Celgard (< 90%).15 A poly(styrene sulfonate)-based (PSS) conductive coating on mesoporous carbon and sulfur composite proposed by Yang et al.17 also led to significant improvements, especially mitigating specific charge loss. Recently, we proposed a novel microporous separator based on polypropylene grafted with styrene sulfonate (PP-g-PLiSS) as an approach to tame the ‘polysulfide shuttle’.18 The advantage


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of the PP-g-PLiSS over Nafion-coated separators is that the grafted layer can be much thinner and the PSS grafts can be effectively distributed not only at but also near the surface of the separator, owing to the 3D porous microstructure of the base polymer. While these novel separators effectively blocked the diffusion of polysulfide species,18 their advantages, limitations, and practical applicability for Li–S batteries remained yet to be established. Here we report a systematic study of the electrochemical performance of cells with PP-g-PLiSS separators. In this study optimal and easy to control parameters for the PSS grafted layer design are identified, yielding the desired balance between practical specific charge and coulombic efficiency. 2. EXPERIMENTAL 2.1 Cell assembly. Grafted PP-g-PLiSS separators in the form of an asymmetric porous membrane were prepared by a one-step plasma-induced graft copolymerization. The details of the synthesis of the PP-g-PLiSS separators are given elsewhere.18 In the present study PP-gPLiSS separators with 5, 8, 20, and 30% graft level, which is defined as the mass of the graft component added with respect to the base polymer (here PP) mass,19 were chosen for electrochemical tests. Celgard 2400 (Celgard) was used as an unmodified reference separator. Standard sulfur composite electrodes, consisting of 60 wt% sulfur (Sigma Aldrich), 30 wt% carbon black Super-P (Imerys) and 10 wt% poly(ethylene oxide) (Mw = 4 000 000 Da, Fluka) binder suspended in acetonitrile (Sigma Aldrich), were prepared as described previously10 and doctor-bladed onto carbon-pre-coated aluminum foil. After 24 h of air-drying at room temperature, circular electrodes of 13 mm diameter were punched out and transferred to an Arfilled glove box. Coin-type cells with lithium metal as the negative electrode and the positive carbon / sulfur electrode were assembled. On average, the positive electrode had a sulfur loading


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of 2.0 ± 0.5 mg cm-2. Prior to assembly, each PP-g-PLiSS separator was soaked for 10 min in liquid organic electrolyte (BASF): 1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI) in a 2:1 weight ratio of dimethoxyethane (DME) and 1,3-dioxolane (Diox). Subsequently, it was introduced into the cell with the grafted side facing the positive electrode and additional 30 µl of electrolyte were added on top of it to ensure complete wetting. Celgard was not pre-soaked in the electrolyte due to its open porosity. It was placed on top of the positive electrode and directly wetted with 50 µl of the electrolyte. In some experiments (where indicated), an electrolyte, containing 0.4 M LiNO3 (Fluka) as an additive, was used. Overall, the experimental electrolyte / sulfur ratio amounted to 25 and ca. 40 ml g-1 for experiments with Celgard and the grafted separators, respectively. 2.2 Electrochemical tests. Electrochemical cycling was performed galvanostatically at C/5 and C/20 rates between 1.5 and 3.0 V vs. Li+/Li, and at C/40 rate between 1.8 and 2.7 V vs. Li+ / Li. The latter potential window was also used for the cells with the electrolyte containing LiNO3 as the additive. The cell specific charge, and thus, the current values were calculated assuming the theoretical sulfur’s specific charge of 1672 mAh g-1 and the amount of active material (sulfur) in each electrode. The cyclic voltammetry (CV) measurements were performed on a VMP3 multichannel potentiostat at a sweep rate of 100 µV s-1 and 20 µV s-1 between 1.8 and 2.7 V vs. Li+ / Li. In all cases, the magnitude of the peaks was normalized to the loading of the sulfur. The deconvolution of the peaks was performed using the Pseudo-Voigt function. Throughout this paper, all the potentials are with reference to the Li+ / Li electrode. 2.3 Effective conductivity. Electrochemical impedance spectroscopy was employed to determine the effective conductivity with different separators. Measurements were performed on


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a VMP3 multichannel potentiostat on symmetric Li / Li cells over the frequency range from 1 MHz down to 50 mHz. The effective conductivity σeff was calculated as follows: =

; ∙

where L is the thickness of the separator, A is the geometrical area of the separator and Rel is the ohmic resistance of the electrolyte. 2.4 Ex situ characterization. The surface morphology of the separators after cycling was examined with a scanning electron microscope (SEM, Ultra 55, Carl Zeiss) operated at an accelerating voltage of 3 kV. For this purpose, the cells were disassembled in an Ar-filled glove box after the 40th charge cycle. An in-house developed transfer chamber was used to transport the samples from the glove box to the microscope without exposure to air. For comparison, images of the separators before cycling were taken after sputter-coating them with chromium. Additionally, the separators extracted from the cells and washed with DME were characterized by Fourier transform infrared spectroscopy (FTIR) using a 2000 FTIR spectrophotometer (Perkin Elmer) equipped with a diamond attenuated total reflectance (ATR, Specac) accessory. The spectra were acquired in air at 4 cm-1 resolution between 700 and 4000 cm-1. 2.5 PSS layer characterization. In order to estimate the maximum thickness of the grafted PSS layer without losing asymmetry (grafting through the separator), a depth profile of the 30% grafted separator was prepared using a focused ion-beam (FIB) (Zeiss NVision). As the grafted PP-g-PLiSS are rather non-conductive and beam-sensitive, a protective carbon layer was deposited onto the surface of the sample prior to milling. The deposition was done in two steps in a manner described previously20. First, a thin layer of carbon was electrodeposited onto the separator (5 kV, C(e–)). Subsequently, on top of it a thicker second layer of carbon was deposited by using an ion beam (10 kV, C(Ga)). In order to minimize the heating of the sample, the milling


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was performed in the deposition mode, where the ion-beam constantly scans the whole to-beetched area. The elemental composition across the separator was determined by energy dispersive X-ray spectroscopy (EDX, EDAX TSL, AMETEK, Ultra 55, Carl Zeiss). 2.6 Wetting dynamics. The wettability of the grafted and reference (Celgard, PP base polymer) separators (Scheme 1a) was investigated in air with the help of an in-house developed contact angle goniometer. For this, a 5 µl droplet of the electrolyte was placed onto the separator and its wetting dynamics were tracked. In order to mimic the conditions during the assembly of the Li–S cells, the separator was placed on top of the positive electrode. Two digital cameras (Conrad Digitale Mikroskop kamera) were used: one recording parallel to the separator plane, allowing the measurement of the variables h(t) and the contact angles (Scheme 1c, top); the second one positioned above the sample, allowing for the simultaneous measurement of the lateral spreading variables a(t) and b(t) (Scheme 1c, bottom). Since the initial volume of the droplet may slightly differ between two tests, the variables h, a, and b were normalized to their initial values (h0, a0, and b0). The measurements of these quantities have been used to compare the wetting dynamics of the PP-g-PLiSS separators with different GLs and to assess the optimal time needed for their complete wetting prior to cell assembly. The images were processed as described in the Supplementary Information.


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Scheme 1 Schematic representation of the wettability measurement setup for (a) grafted, and (b) reference separators. (c) Schematic side and top view of the experiment; the variables h(t), a(t), and b(t) represent the height of the droplet, its shorter, and longer spreading direction, respectively, as a function of time.

3. RESULTS AND DISCUSSION 3.1 Thickness and the composition of PSS layer. Figure 1c and d show the depth profile of the 30% grafted PP-g-PLiSS separator after FIB milling. As can be seen from the image, the nonporous layer that covers the modified side of the separator (Figure 1b) was no longer observed close to the surface of the PP-g-PLiSS (Figure 1c). The layer underneath exhibited some porosity. As expected, at larger depths the material revealed the porous, branch-like structure (Figure 1d) similar to the pristine PP base polymer.18


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Figure 1 (a) Illustration of the FIB milling in a single pass, resulting in a depth profile of the sample. (b) SEM micrograph of the surface of the 30% grafted PP-g-PLiSS. Cross-sectional micrographs (c) at smaller and (d) larger depths of (b) after milling. (e) The distribution of the sulfur in the milled sample obtained by EDX line scan is indicated by the arrow. All images were taken at the same magnification and, thus, have the same scale bar.

Subsequently, the elemental composition of the milled sample (Figure 1d) was analyzed by EDX using line scan mode. The bulk content of the sulfur, corresponding to SS grafts, was of particular interest, as it indicates the extent of grafting into the depth of the separator. Other elements present in the monomer used for grafting were not selected, since they cannot be detected by EDX (lithium), or they are also present in the base polymer and / or the SEM carbon support tape (carbon and oxygen). The result indicates that there is indeed a gradient of the sulfur content across the 30% grafted separator (Figure 1e). The sulfur content is relatively high and almost constant within first 1 µm from the surface of the separator (the thickness of the whole separator – GL dependence can be found in the Supplementary Information, Figure S3d), after which it starts to decrease (Figure 1e). This suggests that the poly(styrene sulfonate) constituents are mostly located at and near the surface of the separator, and lower amount of grafts can be found underneath, in the depth of the PP sub layer. The presence of the grafts therein is not surprising, as previously reported ATR - FTIR results have indicated the loss of asymmetry with high graft levels (GLs).18 This may, however, play a role in the electrochemistry, leading to an increase in the cell internal resistance, and, thus, to a decrease in the Li-ion transfer rate through the highly-grafted PP-g-PLiSS separator.


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3.2. Wettability of the separators. The thorough wetting of the separator by the liquid electrolyte is of vital importance for efficient functioning of the battery. Slow and / or low absorption of the electrolyte by the separator may directly impair the ionic transport through the separator and increase the cell internal resistance,21-23 leading either to several formation cycles with low specific charge, or cause a direct cell failure. Wetting, and the imbibition dynamics of a droplet on a porous separator, is governed by the interplay of two competing processes, namely, the spreading of the droplet over the already wetted region of the separator (increase of the droplet radius, b(t) / b0), and the imbibition of the electrolyte from the droplet into the separator with the expansion of the wetted region into the material (absorption of the droplet, h(t) / h0).24,

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Both processes

depend, among others, on the interaction between the polymer and the electrolyte (surface tension), the porosity of the polymer and the pore sizes, and can be modified, for instance, by introducing ion-exchange groups.23 Grafting of the styrene sulfonate groups at and near the surface of the PP leads to a change in the wetting behavior of the separator compared to the PP base polymer and Celgard (Figure 2). In the following discussion of the results, the wetting dynamics of Celgard are not included, as this reference separator has a distinctly different pore structure than the PP base polymer and its grafted derivatives. Since the cycling performance of the cells with the PP base polymer as a separator is similar to that of the cells with Celgard,26 the former was chosen as a reference for the measurement of the wetting dynamics. A closer look at the lateral spreading of the electrolyte droplet and its absorption by the separator reveals that the wetting dynamics are not a monotonous function of the GL. The


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droplet spreads over the PP base polymer much faster than over the grafted separators (Figure 2a). The 3D open structure with randomly distributed pores in the former case promotes the spreading of the electrolyte, while a change in the surface porosity and overall surface energy of the polymer in the case of PP-g-PLiSS leads to its slowdown. For low GLs, this slowdown is not yet critical, and should not impair the ionic transport through the separator. For high GLs, where the asymmetry of the PP-g-PLiSS separator is almost lost and the porosity is strongly reduced,18 the spreading of the electrolyte droplet is markedly impeded. A similar trend is observed for the absorption of the electrolyte by the separators with different GLs (Figure 2b), with the exception of the 8% grafted PP-gPLiSS. In this latter case, the electrolyte droplet is absorbed as fast as in the case of the PP base polymer. Apparently, a relatively low amount of SO3- groups does not disturb the absorption of the electrolyte. As the GL increases, so does the thickness of the PSS layer, which affects not only the spreading of the electrolyte droplet over the surface of the separator, but also its absorption. As a result, for the 30% grafted PP-g-PLiSS separator a longer soaking time is required to wet completely this highly-grafted separator. b) a) 1.6

a) b) b(t)

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h(t) // hh00 h(t)

b(t) b(t) // bb00

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Figure 2(a) Lateral spreading dynamics of the electrolyte droplet over the surface of the PP base polymer (▬), 8% (▬), and 30% (▬) grafted separator over time. (b) Absorption of the droplet by different separators monitored by the decrease of its height as a function of time. For comparison, wetting and imbibition dynamics of Celgard reference is also plotted (− −).

3.3 Cycling performance. In a preliminary attempt, three different C-rates, namely C/5, C/20, and C/40, have been evaluated to select the optimal cycling conditions for Li–S cells with 8 and 30% grafted separators. Due to the expected increase in the polarization when using grafted separators at C/5 and C/20 rates, the cells were tested within a broader potential window (1.5 - 3 V). It can be seen that at C/5 rate (Figure 3a) the practical specific charge of the cell with 8% grafted PP-g-PLiSS separator is very similar to the reference cell (initial specific charge of 1070 and 1011 mAh g-1, respectively), and has an analogous specific charge fading behavior upon repetitive cycling. After 30 cycles, the cell with 8% grafted PP-g-PLiSS retained about 67% of its initial specific charge (ca. 720 mAh g-1), which is 2% higher than the value of the reference cell (65%, ca. 660 mAh g-1). In contrast, the use of the highly-grafted separator led to higher specific charge loss at the beginning of cycling compared to the reference cell. The initial specific charge in the former case amounted only to 550 mAh g-1. After 10 cycles, it decreased further, below 400 mAh g-1, and continued to drop until it attained a stable value of 270 mAh g-1 after 30 cycles. Although at this cycling rate there was no clear improvement in the practical specific charge when using grafted separators, the overall coulombic efficiencies for the cells with the PP-g-PLiSS separators were significantly higher than the reference cell. With the highly-grafted separator, the efficiency reached nearly 100%


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in the first cycle, and remained stable upon cycling, indicating significant polysulfide shuttle suppression. Lowering the cycling rate to C/20 (Figure 3b) has improved the performance of the Li–S cells with the PP-g-PLiSS separators. The cell assembled with the 8% grafted PP-g-PLiSS delivered high initial specific charge close to 1300 mAh g-1. It then insignificantly decreased within the first few cycles, and stabilized at around 911 mAh g-1 after 30 cycles. In comparison, the initial specific charge of the cell assembled with Celgard was only about 950 mAh g-1. With the 30% grafted separator, the initial specific charge was almost twice the value obtained at C/5 (1046 mAh g-1). However, after 20 cycles, it slowly decreased to about half of this value, ending up near the specific charge of the cell with Celgard. Overall, at C/20, the cells with PP-g-PLiSS separators showed much higher and more stable coulombic efficiencies (≥ 95%) than the reference cell (ca. 80%). Given sufficient time for Li-ion diffusion through the two different layers of the PP-g-PLiSS separators, the polarization at the slower cycling rate of C/20 should have been much lower than at the C/5 rate to result in such significant increase in the practical specific charge, and its much better retention. To confirm this working hypothesis, we also performed tests at an even slower rate, C/40 (Figure 3c). It is known that a lower current density promotes the dissolution of polysulfides27 and this can be seen in the case of the reference cell with Celgard. Herein, cycling at a low rate resulted in rapid specific charge fading from the initial 966 mAh g-1 to 650 mAh g-1 after 20 cycles, and to an overall lower coulombic efficiency. In the first 20 cycles, the average coulombic efficiency of the reference cell was around 75%. However, after 40 cycles it reached 87%, and explanation for this increase has not been


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found. This lower coulombic efficiency indicates that the polysulfide shuttle plays a significant role at low currents. Since Celgard is porous and non-grafted, a large amount of the polysulfides can diffuse to the lithium side, where they take part in parasitic reactions, thus leading to both lower practical specific charge and significant coulombic efficiency loss. Once again, cells with the PP-g-PLiSS separators showed coulombic efficiencies close to 100%, a value which is usually only obtained with the help of an electrolyte additive.10 Additionally, even at a rate as slow as C/40, the cell with the 8% grafted separator showed the best performance among the tested cells, resulting in an average practical specific charge higher by about 230 mAh g-1. To identify the most suitable separator, we assembled Li–S cells with PP-g-PLiSS separators having GLs other than 8 and 30%, and tested them at the C/20 rate (Figure 3d). The use of separators with a GL lower than 8% did not show further improvement in the overall practical specific charge compared to the most promising 8% grafted separator identified so far. Presumably, the amount of SO3- groups at and near the surface of the separator was not sufficient to suppress the polysulfide shuttle effectively. Cells assembled with the PP-g-PLiSS separators with intermediate GL, such as 20%, showed a performance close to the one obtained with the 30% grafted separator, indicating an excessive amount of SO3- groups.


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Figure 3 Specific charge (squares) and coulombic efficiency (stars) of the Li–S cells tested with Celgard (▬), 8% (▬), and 30% (▬) grafted PP-g-PLiSS separators at (a) C/5, (b) C/20, and (c) C/40. (d) Comparison of the cell performance after 10 cycles at C/20 rate as a function of the graft level of the separator.

In order to compare the polarization of the cells and its evolution upon cycling at different C-rates, and, thus, to get more insight into the reaction mechanism when using grafted separators, a more detailed evaluation of the galvanostatic curves at C/5 and C/20 was performed (Figure 4). A typical discharge profile of the Li–S cell consists of two welldefined potential plateaus, the first appearing at ca. 2.3 V (upper-voltage plateau) and a second one at ca. 2.1 V (lower-voltage plateau).28 This typical shape can also be observed for the cell with 8% grafted separator and the reference cell at C/5 rate. At C/20 rate,


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however, even if the galvanostatic curves for both the cells have a similar shape, the length of the plateaus varies significantly. With the 8% grafted separator, the uppervoltage plateau is 10% shorter, and thus, the subsequent lower-voltage plateau is 10% longer than that of the reference cell (non-normalized curves are shown in the Supplementary Information in Figure S4). It may thus indicate a better conversion of sulfur to Li2S. No significant increase in the polarization of the cell with 8% grafted separator is observed along cycling, which is in line with the effective conductivity with this separator being close to the one with Celgard (0.4 and 0.8 mS cm-1, respectively). For the cell with the 30% grafted PP-g-PLiSS separator, the upper-voltage plateau is not well defined during the first lithiation at C/5 as with other separators. Moreover, the polarization in the middle of the lower-voltage plateau is about 180 mV higher than that of the reference cell (Figure 4a). On further cycling the polarization continuously builds up reaching approximately 270 mV after 30 cycles (Figure 4b). At the same time, the discharge curve no longer resembles the one observed for the reference cell (Figure 4a). Since the transfer of the polysulfides through this highly-grafted separator is impeded, we suspect that at this relatively fast cycling rate, the concentration of the Sn2- species near the positive electrode, and thus, the local viscosity of the electrolyte28 increases significantly at the beginning of cycling. Meanwhile, the Li-ion transfer through the separator is also hindered due to a drop in the effective conductivity with this 30% grafted separator by one order of magnitude (0.03 mS cm-1), compared to the one with Celgard (0.8 mS cm-1), as well as the assumed change in the ion-mobility within the two different layers of the PP-g-PLiSS separator. All phenomena eventually lead to the gradual increase in the internal resistance of the cell during cycling, and, thus, to the significant


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loss of practical specific charge (Figure 3a). At the lower C/20 rate, the shape of voltage profile during the first delithiation for the cell with the 30% grafted PP-g-PLiSS separator resembles that of the reference cell (Figure 4c). Interestingly, the initial polarization of the former cell is slightly lower compared to the reference cell. However, the polarization increases after 30 cycles and, additionally, at the very end of the lithiation, at ca. 1.9 V, a new phenomenon is observed (also for the 8% grafted separator). Most probably it corresponds to the solid-solid equilibrium between Li2S2 and Li2S.29

Figure 4 Normalized galvanostatic curves for 1st (a, c) and 30th (b, d) cycle of the Li–S cells with Celgard (▬), 8% (▬), and 30% (▬) grafted PP-g-PLiSS at C/5 (a, b) and C/20 rate (c, d).


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In order to understand the outstanding cyclability of the cells with 8% grafted separator, the galvanostatic cycling results were complemented by cyclic voltammetry (CV) experiments. In a typical cyclic voltammogram of a Li–S cell (Figure 5, right) up to three cathodic peaks are found upon lithiation, matching with the potential plateaus in the galvanostatic curve (Figure 5, left). The first cathodic peak corresponds to the reduction of the elemental sulfur to the high-order Li2S8 and Li2S6 polysulfides (region I). The second and third peaks correspond to the reduction of the high-order polysulfides to low-order Li2S4 polysulfides (regions II and III), followed by their disproportionation in the electrolyte and further reduction to Li2S3, Li2S2, and Li2S (regions III and IV).30, 31 In the subsequent anodic sweep, two partially overlapping peaks are attributed to the stepwise oxidation from Li2S / Li2S2, through low- and high-order polysulfides, to the elemental sulfur. 32, 33

Figure 5 A typical discharge and charge potential profile of a Li–S cell (left), a concept adopted from reference [34], and the corresponding typical cyclic voltammogram (right). The Roman numerals on both the graphs indicate the different equilibrium regions of the Li–S system.


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As can be seen in Figure 6, regardless of the separator used, two main peaks at ca. 2.27 and 1.94 V are visible in the first cathodic sweep. In the case of the reference cell, during the second and third cycle the position of the maximum of both peaks tends to shift towards higher potentials. We also observe a shift of these peaks for the cell with the 8% grafted PP-g-PLiSS separator towards lower potentials. Moreover, the peak at ca. 1.96 V becomes broader and less defined with cycling. The change in the shape of this peak may be due to the kinetic barrier — the presence of the PSS grafts at and near the surface of the separator. Owing to the confining properties of the PP-g-PLiSS separator and lower Li-ion mobility in the polymeric matrices, partial lithium depletion may arise at a given sweep rate. It may slow down the reaction processes and lead to peak broadening, unlike in the reference cell, where there is no obstacle for the Li-ion mobility. In the reverse sweep, in both cases, two partially overlapping peaks appear at ca. 2.43 and 2.54 V. As already mentioned, this pair of peaks corresponds to the stepwise oxidation process taking place in the Li–S cell. In this part of the oxidation process, we also observe some differences between the cells tested with the reference and the grafted separator. For the reference cell, the peak appearing at 2.43 V associated with the conversion of Li2S2 and / or Li2S to low-order polysulfides shifts slightly towards lower potentials. Meanwhile, the peak at 2.54 V remains nearly constant. For the cell with the grafted separator, the position of the 2.43 V peak does not change with the cycle number. However, its intensity gradually decreases. The position of the other anodic peak changes with increase in the cycle number: initially, it is centered at 2.53 V, and then shifts continuously towards higher potentials, most likely for the same reason as the negative shift upon sulfur reduction. To test the hypothesis of kinetic hindrance for Li-ion transfer through the grafted separator, a CV at much slower sweep rate of 20 µV s-1 was performed (Supplementary Information, Figure S5). At this sweep rate, the redox


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peaks are closer to the equilibrium potential of the corresponding reaction,14 but are still much broader than the peaks observed for the reference cell. This confirms the slower kinetics due to the presence of the PSS grafts. Nevertheless, in a complex electrochemical system combining kinetic influence, concentration variations and the loss of active material it is difficult to draw the confident conclusions from the analysis of the peaks positions.

Figure 6 Cyclic voltammograms of Li–S cells with (a) Celgard, and (b) 8% grafted PP-g-PLiSS at a sweep rate of 100 µV s-1.

The CV curves were further analyzed in terms of the charge involved in each phenomenon in the course of a single cycle. This was determined by an integration of the area under the corresponding peaks in the given potential range (Figure 7). It can be seen that although the total charge delivered by the cell with the 8% grafted separator is mostly similar to the reference cell with Celgard, the amount of charge per phenomenon varies significantly. Upon the first lithiation, in both cases, the integration of the cathodic peak at 2.27 V (peak 1) yields ca. 300 mAh g-1. After three cycles, this value decreases by about 50 and 30 % for the reference cell and the cell with 8% grafted separator, respectively. When Celgard is used as a separator the


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charge associated with the reduction of high-order to low-order polysulfides (peaks 2 + 3) also decreases in the following cycles, resulting in a loss of charge of approximately 225 mAh g-1. In contrast, with the 8% grafted separator, this value remains constant and amounts to ca. 960 mAh g-1. A similar trend is observed for the anodic peaks. Upon the first delithiation, the integration of the peak at 2.43 V (peak 4) yields a specific charge of 668 and 518 mAh g-1 for the reference cell and the cell with 8% grafted separator, respectively. In both cases, the charge decays with repetitive cycles, yet the loss is much higher when Celgard is used as a separator. The contribution from the second anodic peak at 2.54 V (peak 5) is about 250 mAh g-1 higher for the cell with the grafted separator. Overall, the deconvolution of the peaks implies better charge reversibility, and thus, better sulfur utilization in the case of the 8% grafted separator. The CV results are in good agreement with the galvanostatic cycling results presented in the previous section.


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Figure 7 First CV cycle at a sweep rate of 100 mV s-1 with de-convoluted peaks for the cell with (a) Celgard, and (b) 8% grafted separator. Corresponding amounts of charge consumed per peak for the first and third cycles are summarized in the table.

Next, the 8% grafted PP-g-PLiSS was tested in the Li–S cell containing the electrolyte additive LiNO3. Often LiNO3 salt is chosen as an electrolyte additive because it can facilitate the formation of a protective passivation layer on the metallic lithium anode, thereby, impeding the electrochemical reduction of the polysulfides on its surface. Consequently, it can significantly suppress the polysulfide shuttle resulting in much better coulombic efficiency of the cell.35, 36 Coupling the polysulfide rejection ability of the grafted separator with the Li anode protection by the addition of LiNO3 was believed to further enhance the performance and the stability of Li–S cells.

Figure 8 (a) Specific charge (squares) and the coulombic efficiency (stars) of the Li–S cells with Celgard (▬) and 8% grafted PP-g-PLiSS (▬) separators at C/20 with electrolyte containing the additive. (b) Comparison of the performance of the Li–S cells with 8% grafted PP-g-PLiSS separator at C/20 with electrolyte with additive (▬), without (solid symbols), and with (empty symbols) additional potentiostatic step, to its performance with the standard electrolyte (▬).


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Although the presence of LiNO3 in the electrolyte drove the coulombic efficiency close to 100% for both, the cells containing Celgard and the one with the 8% grafted PP-g-PLiSS separator, it did not lead to any further improvement in the practical specific charge (Figure 8a). Both cells attained a similar initial specific charge of ca. 1120 mAh g-1. The values started to diverge notably from the third cycle onwards, leading to a difference of 160 mAh g-1 after 40 cycles in favor of the cell with the grafted separator (800 mAh g-1). However, it was not as tremendous as the specific charge gain for the cell with 8% grafted sample without LiNO3 (ca. 950 mAh g-1, Figure 8b). The difference might be influenced by either the discharge cut-off potential or the lack of synergy between the grafted separator and the electrolyte additive, or both. When LiNO3 is used as an electrolyte additive, the discharge cut-off potential should not be lower than 1.8 V to avoid its irreversible reduction on the sulfur cathode.37 As previously discussed, due to the higher polarization of the cells with the grafted separators, the operating potential range for the standard galvanostatic cycling (without the electrolyte additive) has been extended. Most importantly, the discharge cut-off potential has been set to 1.5 V. In order to compensate the 200 mV potential loss when using LiNO3, and to recover the remaining specific charge, we added a potentiostatic step of 2 h to the lithiation stage (Figure 8b). This additional step in the cycling protocol, however, did not improve the overall specific charge of the sulfur electrode in the cell with the PP-g-PLiSS separator. In fact, the LiNO3 additive protects the lithium anode from any chemical reaction with the dissolved polysulfides.38,

39

Indeed, our grafted separator

indirectly also plays this role: polysulfides that are continuously rejected by the SO3- groups mostly do not reach the lithium anode, and hence, are not further reacting therein. Nevertheless, after a certain number of cycles, the PP-g-PLiSS separator may start aging and become less


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efficient in rejecting the polysulfides. This can be seen in Figure 8b, where after ca. 20 cycles, the cycling performance of the cell without the additive almost coincides with the performance of the one where the electrolyte additive was used.

3.4. Post-mortem characterizations. The SEM images of Celgard and the 8% grafted separator before and after 40 cycles at C/20 rate are shown in Figure 9. The morphology of Celgard before cycling is shown in Figure 9a. whereas the one of the 8% grafted separator is shown in Figure 9b. It is important to note that the grafted separator is based on, other than Celgard, polypropylene (TreoPore) used as a base polymer for grafting (SEM micrograph can be found in Supplementary Information, Figure S3a). After cycling (without the LiNO3 additive), a fairly dense layer with some large spherical agglomerates is formed on the surface of Celgard (Figure 9c). Upon longer exposure to the electron beam, these agglomerates tend to melt. Overall, the original structure of Celgard with regular arrays of pores (Figure 9a) cannot be distinguished anymore, unless the surface is washed with DME (Figure 9c). Thus, we concluded that the surface layer could have been composed of the species soluble in DME, including the LiTFSI salt. In contrast, the grafted side of the 8% grafted PP-g-PLiSS separator after cycling under the same conditions is devoid of this layer (Figure 9e). Only some randomly distributed irregular agglomerates appeared on its otherwise mostly clean surface. Higher magnification reveals original branch-like features of this grafted separator (Figure 9f). These results further confirm that the presence of the SO3- groups at and near the surface of the PP-based separator efficiently prevents the accumulation of inactive species on the separator.


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Figure 9 SEM images of (a - c) Celgard, and (d - f) 8% grafted PP-g-PLiSS separator (a, d) before, and (b, c, e and f) after 40 cycles.

Post-mortem ATR - FTIR analysis (Figure 10) of the grafted side of the 8% grafted separator showed that the poly(styrene sulfonate) groups are still present at and near the surface of the separator after 40 cycles. Characteristic absorption of the benzene ring was observed next to the polypropylene bands at 1011 cm-1 18, 40 confirming the stability of the grafts during the cycling.


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SS

1020

PP

1000

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PP

980

960

Wavenumber (cm-1) Figure 10 ATR - FTIR spectra of Celgard (▬) and the 8% grafted PP-g-PLiSS separator (▬) extracted from the Li–S cells after cycling, normalized to the height of the PP peak at 1376 cm-1. For comparison, the spectrum of the PP-g-PLiSS separator before cycling is also shown (−·−).

4. Conclusions At all cycling rates studied, we found that using PP-g-PLiSS separators instead of stateof-the-art Celgard 2400 significantly improved coulombic efficiencies, as a result of successfully inhibiting the detrimental redox shuttling by partially reduced soluble polysulfides. The graft level of the separator affects both the practical specific charge and the cycling-rate performance, by enhancing the former and limiting the latter. Among all separators tested, the PP-g-PLiSS separator with a graft level of 8% yielded the highest specific charge when cycling rates were rather low, C/20 and C/40. The use of it did not lead to specific-charge enhancement at higher rates. When separators with higher graft levels were used, increased overpotentials, which in turn led to poor specific charge, were observed.


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The results of this study establish that asymmetrically functionalized separator can be designed and its properties can be tuned to meet the requirements of selective transport in practical electrochemical cells. In the present case, separator containing styrene sulfonate grafts reduces redox shuttling in the Li–S battery, and therefore enhances its performance.

ASSOCIATED CONTENT Supporting Information. Principle of image processing, comparison of the morphology and the thickness of the PP base polymer and PP-g-PLiSS separators, appendix to Figure 4 — nonnormalized galvanostatic curves and appendix to Figure 6 — CV curves at two different sweep rates. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the financial support from the Swiss National Science Foundation (project no 200021_144292). We also would like to thank Prof. Renaud Bouchet for his invaluable help with the impedance data analysis. Finally, we would like to acknowledge our colleagues from the Battery Materials group for fruitful discussions and valuable comments.


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Performance-Enhancing Asymmetric Separator for LithiumâSulfur

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