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A Nanophase-separated, Quasi-solid State Polymeric Single Ion Conductor: Polysulfide Exclusion for Lithium Sulfur Batteries Jinhong Lee, Jongchan Song, Hongkyung Lee, Hyungjun Noh, YunJung Kim, Sung Hyun Kown, Seung Geol Lee, and Hee-Tak Kim ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on April 30, 2017

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ACS Energy Letters

A Nanophase-separated, Quasi-solid State Polymeric Single Ion Conductor: Polysulfide Exclusion for Lithium Sulfur Batteries Jinhong Leea, Jongchan Songa, Hongkyung Leeb, Hyungjun Noha, Yun-Jung Kima, Sung Hyun Kwonc, Seung Geol Leec, Hee-Tak Kima* a

J. Lee, Dr. J. Song, Dr. H. Lee, H. Noh, Y. Kim, Prof. H.-T. Kim Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea b H.Lee Energy and Environment Directorate Pacific Northwest National Laboratory 902 Battelle Boulevard, Richland, WA 99354, USA c S. H. Kwon, Prof. S. G. Lee Department of Organic Material Science and Engineering, Pusan National University, Busandaehak-ro 63 beon-gil, Geumjeong-gu, Busan 46241, South Korea

ABSTRACT. Formation of soluble polysulfide (PS), which is a key feature of lithium sulfur (Li-S) battery, provides a fast redox kinetic based on liquid-solid mechanism, however, it imposes a critical problem of PS shuttle. Here, we address the dilemma by exploiting a solvent-swollen polymeric single ion conductor (SPSIC) as the electrolyte medium of Li-S battery. The SPSIC consisting of a polymeric single ion conductor and lithium salt-free organic solvents provides a Li ion hopping by forming a nanoscale conducting channel and suppresses PS shuttle according to Donnan exclusion principle when being employed for Li-S battery. The organic solvents at the interface of sulfur/carbon composite and SPSIC eliminate the poor interfacial contact, and function as a soluble PS reservoir for maintaining liquidsolid mechanism. Furthermore, the quasi-solid state SPSIC allows the fabrication of bipolartype stack, which promise a realization of high voltage and energy dense Li-S battery.

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Recently, lithium–sulfur (Li–S) batteries have been considered as a promising candidate to succeed LIBs because of their high theoretical energy density of 2500 Wh·kg-1, non-toxic nature, low cost, and natural abundance.1,

2

The charge and discharge of the sulfur cathode

are based on the conversion chemistry between the solid sulfur and solid Li2S. It can proceed in a reversible manner despite the electronically insulating nature of the sulfur and Li2S through the formation of soluble polysulfides (PSs) which enables a solid(S8)-liquid(PSs)solid(Li2S) redox reaction mechanism to work.3 However, ironically, the soluble PSs cause challenges including low sulfur utilization, a short cycle life, low cycling efficiency, and a high self-discharge rate. These are mainly attributed to a process known as the PS shuttle 4-7; the soluble PSs diffuse to the Li anode where they directly react with the Li metal to produce lower order PS species, which diffuse back to the sulfur cathode to regenerate higher PS forms.5, 6 The PS shuttle leads to incomplete charging of the sulfur electrode and corrosion of the Li electrode, thus resulting in poor battery performance. Prevention of the PS shuttle is therefore extremely important for the practical use of Li–S batteries.7 In recent years, there have been many efforts to address the PS shuttle problem, which can be classified into two major directions according to their underlying physics. One direction is the kinetic approach, which is based on the kinetic suppression of the PS dissolution and diffusion by spatially confining the PS in porous hosts or by imposing a barrier to PS diffusion between the sulfur cathode and anode.8-11 In these cases, the possibility of PS 2 ACS Paragon Plus Environment

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ACS Energy Letters

dissolution and diffusion cannot be excluded for extended operation because these are still thermodynamically favored, and furthermore, any defects in the host or barriers can cause spontaneous PS dissolution and diffusion. Additionally, the kinetic approach is accompanied by the introduction of additional inactive materials such as sulfur hosts and PS barriers to LiS batteries, which lower the energy density of the corresponding Li-S batteries. The other direction is the thermodynamic approach, which features new electrolyte designs to thermodynamically reduce the PS solubility of the electrolyte medium providing a sustainable PS rejection at the electrode/electrolyte interface. One example is a liquid electrolyte with a high Li salt concentration which has a low PS solubility according to the common ion effect,12,13 and another example is ionic liquids with an intrinsic low PS solubility.14,15 This approach does not impose additional weight or volume on Li-S batteries, which is beneficial in designing an energy dense battery. However, the lowered PS shuttle is compensated by a large overpotential due to slow redox kinetics.13,14,16 Another example is the introduction of an inorganic solid electrolyte with high Li ion conductivity and nearly zero PS solubility. However, this approach has a large polarization due to the lack of a fast solid-liquid-solid mechanism and a poor ionic connection between the inorganic solid electrolyte and solid state electrodes with a large dimensional change of the sulfur cathode.1720

Taking these issues into consideration, we report a novel thermodynamic approach to suppress the PS shuttle with a solvent-swollen polymeric single ion conductor (SPSIC) as an electrolyte medium for a Li-S battery for the first time. The SPSIC, which consists of a polymeric single ion conductor swollen with lithium salt-free organic solvent, provides Li+ hopping through a nanoscale conducting channel and suppresses PS dissolution according to the Donnan exclusion principle (Scheme 1). This approach is motivated by a theoretical consideration of the thermodynamic PS partitioning between the liquid electrolyte phase and 3 ACS Paragon Plus Environment


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the liquid electrolyte-swollen PFSA in contact described in Supplementary note 1. An important conclusion drawn from this consideration is that the PS concentration in the SPSIC should be lower than that of the liquid electrolyte due to the Donnan potential difference, and as such, the Donnan exclusion effect is more intensified in the absence of bi-ionic lithium salt. Therefore, to strengthen the PS rejection function, the SPSIC should be designed without any bi-ionic Li salts. In this work, lithiated perfluorinated sulfonic acid (Li-PFSA) polymer and a sulfolane/diglyme mixture were strategically used as a proof-of-concept. Being different from a conventional Li-S battery with liquid electrolytes, the SPSIC-based Li-S battery features the use of the SPSIC for both the electrolyte medium of the sulfur cathode and the polymer electrolyte membrane between the two electrodes (Scheme 1a, b). Previously, polymers with a fixed negative charge were used for Li-S batteries as a constituent of the sulfur cathode21-22 or as a separator.23-26 However, the current work is conceptually different from the previous works in that the SPSIC is free of bi-ionic lithium salt and used not as a cathode binder or an additional PS blocking layer but as the whole electrolyte medium shown in Scheme 1b (see Table S1, Supporting Information for a more detailed comparison). Due to the quasi-solid state characteristic of the SPSIC, the ionic connection between the sulfur/carbon composite and the SPSIC phase can be a problem. However, it is addressed by filling the pores of the sulfur/carbon (S/C) composite with the organic solvent. The solvent confined in the pores eliminates the poor interfacial contact and functions as a soluble PS reservoir which enables the solid-liquid-solid mechanism for the sulfur redox reaction (Scheme 1c). Therefore, the corresponding Li-S battery includes two electrolyte phases; one is a liquid phase in the pores of the S/C composite and the other a quasi-solid state phase in the electrolyte medium, which is a concept suggested for the first time in this field. Furthermore, the SPSIC enables the fabrication of a bipolar-type stacked cell because of its quasi-solid state offering promise in achieving a high voltage Li-S battery with a superior energy density. 4 ACS Paragon Plus Environment

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ACS Energy Letters

Scheme 1. Schematics of the cell configuration for (a) conventional Li-S battery and (b) SPSIC-based Li-S battery. Schematic of (c) Li ion transport and PS rejection at the interface between the SPSIC and sulfur/carbon composite.

The SPSIC presented in this work was inspired by hydrated perfluorinated sulfonic acid (PFSA) polymers which feature a heterogeneous membrane structure of a non-conducting hydrophobic polymer phase and a nano-scale solvent-filled phase surrounded by the fixed sulfonate anion groups within which ionic transport takes place27; due to the nano-phase separated morphology, the PFSA membrane can have both high proton conductivity and mechanical strength. The pristine PFSA membrane was lithiated for the use in SPSIC; the SO3H group in the pristine membrane was converted to SO3-Li+ by the LiOH treatment, which was confirmed by the appearance of the peak from SO3-Li+ at 1640 cm-1 in the FTIR spectra of Li-PFSA after the lithiation (Figure S2, Supporting information). The CF2 moieties (1211, 1153 cm-1) from the main chain and side chain and –COC- groups (984, 966 cm-1) which connect the main chain and side chain were also identified in the FTIR spectra of the pristine PFSA and Li-PFSA.28 To design an organic solvent-based SPSIC that can dictate the morphological feature and exploit the benefits of the hydrated PFSA polymers, the Li5 ACS Paragon Plus Environment


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PFSA polymer was equilibrated in various organic solvents frequently used for lithium batteries, and their lithium ion conductivities and solvent uptakes were measured (Table S2, Supporting Information). Among the solvents, a mixture of EC/PC showed the highest ionic conductivity (1.5 x 10-4 S cm-1) and the highest solvent up-take amount (91%) at 25oC; however, carbonate solvents were excluded for use in the Li-S battery because of their irreversible reaction with high order PS.29 DME and DOL, generally used in Li-S batteries, are not compatible with the Li-PFSA polymer; they have low swelling ratios and consequently low Li+ conductivities. Interestingly, sulfolane showed a high swelling of 71% and a reasonably high Li+ conductivity above its melting point of 27oC. To provide room temperature Li+ conductivity, diglyme, which is compatible with sulfur and lithium metal electrodes, was used as a co-solvent. The SPSICs fully swollen with sulfolane/diglyme mixtures at various mixing ratios (0/100 (0S/100D), 20/80 (20S/80D), 50/50 (50S/50D), and 80/20 (80S/20D) in volume ratio) exhibited an increased solvent uptake from 14 to 69 % and an increased Li+ conductivity from 0.595 to 3.88 x 10-5 S cm-1 with increasing sulfolane content from 0 to 80%. These SPSICs are free-standing and mechanically tough as shown in Figure 1a and Figure S3 (Supporting Information). The Young’s modulus and tensile strength are in the range of 0.183~0.377 and 7.08~10.7 MPa depending on the sulfolane/diglyme ratio, respectively. Due to their mechanical toughness, the SPSICs enable the removal of a conventional separator for the corresponding Li-S batteries. According to the linear sweep voltammetry test for the SPSIC (Figure S4, Supporting information), the SPSIC is electrochemically stable under 4V (vs Li/Li+), which is acceptable for the use in LiS batteries. And the SPSIC is high stable against Li-metal, which is confirmed by the invariant impedances with storage time for the SPSIC-50S/50D-containing Li/Li symmetric cell. (Figure S5, Supporting information). The high stability of Li-PFSA based polymer electrolytes were also reported in the previous research.30 The SPSICs have a nanophase6 ACS Paragon Plus Environment

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ACS Energy Letters

separated morphology as intended, which was verified by SAXS analysis. The diameter of the solvent cluster was determined from the SAXS patterns (Figure 1b) of the SPSICs and from a model of spherical clusters which is usually adopted for the morphological analysis of hydrated PFSA polymers.31 The detailed analysis procedure is described in Supplementary note 2. These were calculated to be 2.95, 3.74, 5.98 and 6.51 nm for SPSIC-0S/100D, 20S/80D, 50S/50D, and 80S/20D, respectively. The increased solvent domain size with the sulfolane content is in good agreement with the solvent uptake data. Additionally, the size of the solvent phase is strongly correlated with their Li+ conductivity, shown in Figure 1c, indicating that it has a critical role in the Li+ conduction mechanism. The temperature dependency of the ionic conductivities follows the Arrhenius equation, which is typical of the hopping mechanism (Figure 1d).32 The behavior is contrasted by a curved temperature dependency known as the VTF behavior, which is associated with a redistribution of free volume in the electrolyte medium and usually observed for liquid and gel polymer electrolytes. Despite the presence of the solvent phase, the SPSICs did not show any VTF behavior, which implies Li+ conduction along the wall of the solvent domain not through the solvent medium. To support this postulation, we performed a fully atomistic molecular dynamics (MD) simulation for SPSIC-50S/50D to understand the special distribution of Li+ from the interface of the solvent phase and the polymer phase where SO3groups are densely populated. The last frame of the model clearly shows that Li+ resides in the vicinity of SO3- (Figure 1e). From the equilibrated trajectory of the MD simulation, we analyzed the pair correlation function of SO3- and the Li+ pair to find the probability density of finding Li+ around the SO3- groups at a distance r averaged moves not through the interior of the solvent cluster but along the wall of the distance ranging from 0.28 ~ 0.33 nm from the SO3-. The value is much smaller than the diameter of the solvent cluster (5.98 nm). Therefore, it is highly probable that Li+ over the equilibrium trajectory. Using this function, it is possible 7 ACS Paragon Plus Environment


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to characterize the Li+ distribution in the system and to determine what environment the Li+ are located in. As shown in Figure 1f, the main peak of the pair correlation function is shown at 0.31 nm. This result indicates that most of the Li+ is likely to be located around SO3- at a solvent cluster. More detailed information on the MD simulation is presented in Supplementary note 3.

Figure 1. (a) Digital photo images of the dry Li-PFSA polymer and SPSIC swollen with an organic solvent mixture (50S/50D) (i and ii, respectively) and (iii) an SEM image of the LiPFSA polymer surface. (b) Small angle invariant X-ray scans for the SPSICs versus Bragg spacing. (c) Li+ ion conductivities as a function of the solvent cluster diameter of the SPSICs. (d) Arrhenius plot of the SPSICs with different organic solvents. (e) The last frame of the equilibrated SPSIC-50S/50D from the MD simulation. The size of Li+ and S of SO3- are enlarged for a clear view. (f) Pair correlation function of the SO3- and Li+ pair (g) Preexponential factor as a function of the solvent cluster diameter of the SPSICs. (h) Equilibrium concentrations of Li2S8 in the SPSIC-50S/50Ds after being equilibrated in the Li2S8 and LiTFSI containing-50S/50D solution with different LiTFSI salt concentrations.

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Interestingly, the activation energies were found to have a narrow range of 24.5~25.1 KJ mol-1 despite the differences in the sulfolane/diglyme ratio. This result indicates that the interaction between the solvated Li+ and fixed SO3- and the hopping distance between the neighboring SO3- group, which mainly influences the activation energy for Li+ hopping, are not much different among these SPSICs. The large difference in ion conductivity with the sulfolane/diglyme ratio is mainly attributed to an increase in the pre-exponential factor by 8 times with increasing sulfolane content from 0 to 80% (Figure 1g), which is known to depend on the number of charge carriers and the connectivity of the conducting domain for a heterogeneous polymer electrolyte.32 Considering the invariant activation energy with the sulfolane/diglyme ratio, it is not likely that the difference in the pre-exponential factor is attributed to a difference in the degree of Li-SO3 dissociation. Rather, the influence of the percolation of the solvent phase would mainly be reflected in the pre-exponential factor. For a heterogeneous polymer electrolyte, the connection of the conducting domain is highly important to its ionic conductivity as shown by the hydrated PFSA polymers. According to the values for the cluster size and inter-cluster spacing (Bragg spacing) determined by the SAXS analysis, the conducting domains expand with increasing solvent uptake, and above the percolation threshold, they become overlapped and connected. To demonstrate the Donnan exclusion effect of the SPSICs, the PS concentration in the Li-PFSA membrane equilibrated with 0.1M Li2S8 sulfolane/diglyme (50/50) solutions was measured by varying the LiTFSI concentration in the polysulfide solution with UV spectroscopy (Figure S7, Supporting Information). As shown in Figure 1h, the equilibrium PS concentration in the SPSIC decreased with the decreasing LiTFSI salt concentration, which agrees with the Donnan exclusion theory. In the SPSIC-based sulfur cathode, the S/C composite, carbon fiber, and Li-PFSA were uniformly mixed and densely packed shown in the SEM images (Figure 2a). Carbon fiber 9 ACS Paragon Plus Environment


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was introduced to improve the mechanical integrity of the cathode and to form a wellconnected electric network in the cathode. Li-PFSA occupies the interstitial space between the S/C composites; however, it does not fill the micro- and meso-pores of the S/C composite. The as-prepared sulfur cathodes were not ionically conducting, and to impart a Li+ conductivity to the cathodes, the cathodes were equilibrated with sulfolane/diglyme solvents during the cell assembly. The equilibrium solvent uptakes for the sulfur cathode were higher than the equilibrium swelling of the Li-PFSA in the cathodes compared in Figure 2b. Furthermore, the difference was nearly unchanged with the sulfolane/diglyme ratio. The behavior indicates that the micro- and meso-pores of the S/C composites are not occupied by the Li-PFSA and can accommodate the solvents, as shown in Figure 1c, connecting the solid state S/C composite and quasi-solid state SPSIC. The role of the organic solvent included in the S/C composite on the electrochemical activity of the sulfur cathode was investigated with the electrochemical impedance spectroscopy (EIS) technique (Figure 2c). The AC impedances of the as-prepared cathode were quite large and did not exhibit a semi-circle from the charge-transfer reaction, indicating that the sulfur cathode is not electrochemically active in the absence of the solvent. After the cathode was equilibrated with the 50S/50D solvent, the sulfur cathode became electrochemically active as indicated by the reduced impedances and the appearance of the semi-circle, which would be attributed to the Li+ conduction through the SPSIC medium in the cathode. Because the solvent included in the pores is not ionically conducting due to the absence of Li salt before the cell operation, the charge transfer resistance would originate from the part of the S/C composite in keen contact with the SPSIC phase. Interestingly, the cathode semi-circle was markedly reduced after the first cycle, which indicates a facile Li ion conduction by the dissociated PSs in the solvent-filled pores after the first cycle. Assuming that all the sulfur molecules included in the cathode are converted to Li2S4 and the formed 10 ACS Paragon Plus Environment

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ACS Energy Letters

Li2S4 are solubilized in the solvent phase in the pores and dissociated, the Li ion concentration can be as high as 1.45M. Since the sulfur utilizations are less than 55%, the solvent phase can include considerable amount of dissociated Li+, therefore, Li+ conduction in the solvent does not impede the cathode redox reactions, as evidenced by the small impedance after the 1st cycle. Li-S batteries based on the SPSICs with four different sulfolane/diglyme contents (0S/100D, 20S/80D, 50D/50D, and 80S/20D) were fabricated with the composite sulfur cathode and a 16 µm thick-Li-PFSA membrane.

Figure 2. (a) SEM image of the SPSIC-based sulfur cathode with a top view and crosssection view. (b) Swelling test for the Li-PFSA polymer and sulfur cathode with various sulfolane/diglyme solvents. (c) Nyquist plots of the impedances for the sulfur cathode before and after solvent addition prior to cycling, and after the 1st cycle

Figure 3a compares the first discharge and charge curves for the three SPSICs with different sulfolane/diglyme ratios (20S/80D, 50S/50D, and 80S/20D). Although, the discharge capacity was profoundly varied with the sulfolane/diglyme ratio, the discharge voltage profiles commonly exhibited two distinctive voltage plateaus. The upper and lower 11 ACS Paragon Plus Environment


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voltage plateaus represent the formation of soluble PS from elementary sulfur and Li2S formation through the liquid-solid mechanism, respectively. Therefore, the appearance of the two voltage plateaus indicates that the SPSIC-based Li-S batteries enable the formation of soluble PS formation within the sulfur cathodes. For the SPSIC-20S/80D cell, the discharge voltage rapidly dropped due to its low Li+ conductivity delivering a very low capacity of 130 mAh g-1. Such an abrupt voltage drop was also observed for SPSIC-0S/100D which has quite a low Li+ conductivity of 5.95 x 10-6 S cm-1 (Table S3, Supporting Information) On the other hand, the SPSIC-50S/50D, for which the Li+ conductivity is more than three times higher than that of the SPSIC-20S/80D, showed a high initial discharge capacity of 900 mAh g-1. The comparisons show the importance of the Li+ conductivity in achieving a high discharge capacity for the quasi-solid state Li-S battery. However, the discharge capacity of the SPSIC-80S/20D (400 mAh g-1) was found to be smaller than that of the SPSIC-50S/50D despite its 1.5 times higher Li+ conductivity, which can be understood in terms of the PS solubility difference in the solvent phases of the S/C composite. As discussed earlier, the pores in the S/C composite can accommodate the solvents, which enable the formation of soluble PSs and subsequently Li2S formation through the soluble PSs. The Li2S8 solubility for 20S/80D, 50S/50D, and 80S/20D solvents was measured to be 1.34, 0.772, and 0.395 M, respectively (Figure S8, Supporting Information). The decrease in the PS solubility with increasing sulfolane content can be explained by the difference in the donor number between the sulfolane (14.8) and diglyme (19.2). A solvent with a high donor number can effectively stabilize the dissociated LiPS as well as enable an increase in the PS solubility which results in higher sulfur utilization. The shorter upper voltage plateau of the SPSIC-80S/20S compared with the SPSIC-50S/50D also shows that the formation of soluble PS in the S/C composite pores is limited by the low PS solubility. Because of the limited PS accommodation of the 80S/20D solvent in the pores, the PSs generated are readily reduced to 12 ACS Paragon Plus Environment

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ACS Energy Letters

Li2S, by passivating the carbon surface at an earlier discharge stage, and consequently results in the short and lower voltage plateau. For comparison, corresponding liquid electrolyte cells were constructed by replacing the SPSIC membrane with a PE separator and 1M LiTFSI sulfolane/diglyme electrolytes. The liquid electrolyte cells had an overcharging behavior during charging and a shortening of the upper and lower plateaus with cycling shown in Figure 3b (50S/50D electrolyte) and Figure S9 (20S/80D and 80S/20D electrolytes, Supporting Information), indicating a significant PS loss due to PS dissolution from the sulfur cathode.34 In contrast, the SPSIC-based Li-S batteries based on SPSIC-50S/50D showed a profoundly improved capacity retention than that of the liquid electrolyte-based references; for the SPSIC-50S/50D cell, the discharge and charge profiles were nearly unchanged with cycling, shown in Figure 3c, confirming the benefit of the SPSIC approach. Figures 3d and e compare the discharge capacities and coulombic efficiencies of the three SPSIC-based batteries and three liquid electrolyte-based batteries, respectively. The liquid cells were characterized by the fast capacity fade in the early stage of cycling; however, the SPSIC cells maintained more than 79% of their initial capacities at 100 cycles. In particular, the SPSIC-50S/50D cell delivered a high discharge capacity of 720 mAh g-1 even after 100 cycles. The SPSIC-based Li-S batteries (20S/80D, 50S/50D, and 80S/20D) had much more coulombic efficiencies than those of the liquid electrolyte based batteries. However, the coulombic efficiency for the SPSIC-50S/50D gradually decreased, which is contrasted by to the stable retention of coulombic efficiencies with cycling for the SPSIC-20S/80D and -80S/20D. It would is attributed to the larger capacity of the SPSIC-50S/50D. For Li-S batteries, coulombic efficiency loss generally comes from PS shuttle effect. For the SPSIC-50S/20D, the amount of soluble PS formed during charging should be larger due to the larger charge capacity and the time for PS diffusion toward Li metal electrode be longer due to the consequent longer charging time. 13 ACS Paragon Plus Environment


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Therefore, the negative effect by few PSs dissolved into the SPSIC can be more pronounced for the SPSIC-50S/50D.

Figure 3. (a) Galvanostatic charge/discharge voltage profiles of three SPSIC cells containing 20S/80D, 50S/50D, and 80S/20D solvent mixtures, respectively. (b) Voltage profiles of the control cell using 1 M LiTFSI 50S/50D liquid electrolyte at the first, tenth and thirtieth cycles. (c) Voltage profiles of the SPSIC-50S/50D cell at the first, tenth and thirtieth cycles. (d) Discharge capacity retentions and (e) Coulombic efficiencies for the 50S/50D liquid electrolyte cell and the SPSIC-20S/80D, 50S/50D, and 80S/20D cells during 100 cycles at a 0.1 C-rate.

In terms of structural battery design, the SPSICs enable a bipolar type stack configuration design due to their quasi-solid state. The bipolar battery structure is known to minimize IR losses between adjacent cells in a stack and provides a more uniform current and potential distribution over the active area of each cell.34 In this regard, achieving a bipolar stack configuration is highly important to gain uniform Li deposition/dissolution at the Li metal electrode and to improve the power density of the Li-S battery for high power applications such as electric vehicles and drones. As shown in Figure 4a, a bipolar stacked Li-S battery was fabricated by stacking two single cells and packaging them in a CR2032 type coin cell. Because of the lack of a free 14 ACS Paragon Plus Environment

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ACS Energy Letters

liquid electrolyte, the electrolyte phases of the two cells are spatially separated and ionically disconnected. Therefore, the shunt current due to an ionic connection between the cells, which is unavoidable for liquid electrolyte-based bipolar batteries, can be eliminated. As shown in Figure 4b, the bipolar stacked bi-cell has a voltage of 4.1V, which is exactly two times higher than the single cell voltage. This result clearly shows that the two stacked cells in a single package did not have a shunt current. Furthermore, the discharge capacity of the bipolar stacked bi-cell (940 mAh g-1) was nearly the same as that of the single cell (920 mAh g-1) showing that the sulfur cathode performance did not vary due to the bipolar structure. As shown in the inset of Figure 4b, the bipolar stacked bi-cell has coulombic efficiencies higher than 92% indicating that the PSs can be effectively confined in the sulfur cathodes even for a bipolar stack configuration. It should be stressed here that such a bipolar stacked structure has not been proposed yet in the Li-S battery technology sector. Although bipolar type LIBs based on a bi-ionic polymer electrolyte were recently reported,36,37 the use of a polymeric single ion conductor system is reported for the first in this paper. The SPSIC-based bipolar cell design can be extended to other lithium secondary batteries, which is currently on going in our group. It should be noted here that although this work suggests a new design strategy based on the Donnan exclusion principle, the sulfur loading of the sulfur cathode used in this work was as low as 0.2 mg cm-2 because the ionic conductivity of the SPSIC is not high enough to be operated at high current densities. A new SPSIC design with a higher conductivity is therefore needed for practical applications.



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Figure 4. (a) Schematic illustration of the bipolar stack type Li-S battery (bi-cell). (b) Voltage profiles of the single- and double-stacked Li-S battery at the 7th cycle (inset: initial cycling stability of the bipolar-stacked cell at a 0.1 C-rate).

In summary, we demonstrated the concept of a quasi-solid state SPSIC, which permits Li+ conduction while excluding PS anions based on the Donnan exclusion principle, and its application in a Li-S battery. The SPSIC featuring Li+ hopping through a nanoscaleinterconnected conduction phase and an intensified Donnan exclusion effect was successfully fabricated by incorporating a sulfolane/diglyme mixture in Li-PFSA. By replacing the conventional separator with a SPSIC membrane and using SPSIC as the electrolyte medium for the sulfur cathode, a quasi-solid state Li-S battery was achieved with high performance. The SPSIC-based Li-S battery exhibited stable cycling and effective PS shuttle prevention. In addition, by exploiting the quasi-solid state of the SPSIC, a bipolar stack-type Li-S battery was successfully demonstrated for the first time. Therefore, we believe that building a quasi16 ACS Paragon Plus Environment

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solid state Li-S cell with SPSIC is a breakthrough academic strategy which can motivate the development of new battery structures and new polymeric single ion conducting materials with both a strong Donnan potential and high ionic conductivity.

ASSOCIATED CONTENT SEM, SAXS characterization of Li-PFSA and electrode, galvanostatic charge discharge of the Li-S cells.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by a National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2014R1A1A2056199) and a National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF2016M1B3A1A01937431)

Reference 17 ACS Paragon Plus Environment


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