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Permselective membranes in lithium–sulfur batteries
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ScienceDirect Permselective membranes in lithium–sulfur batteries Mahdokht Shaibani1,2, Anthony F Hollenkamp2, Matthew R Hill2,3 and Mainak Majumder1 Integration of permselective membranes – as a subset of the separator systems – in the configuration of lithium–sulfur battery, is a relatively simple solution to tackle the issue of polysulfide dissolution into the electrolyte. Cation-selective materials such as graphene oxide (GO) and Nafion have demonstrated to serve effectively as electrostatic shields for polysulfide anions, retarding their diffusion to the anode side of the battery. Looking into the future, the introducing of these materials in the configuration/composition of multifunctional/ multilayered separator systems will promote the high efficiency cycling of high content sulfur cathodes. Enhanced performance metrics and careful minimisation of the mass/volume of these extra components should result in enhanced volumetric/ gravimetric energy densities on a cell level for lithium–sulfur battery. Addresses 1 Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3168, Australia 2 CSIRO, Clayton, VIC 3168, Australia 3 Department of Chemical Engineering, Monash University, Clayton, VIC 3168, Australia Corresponding author: Majumder, Mainak (mainak.majumder@monash. edu)
Current Opinion in Chemical Engineering 2017, 16:31–38 This review comes from a themed issue on Separation engineering Edited by Mainak Majumder
to launching commercial products. While early works on Li–S date back to 1967, with a Li–S liquid cell [1] or the so-called Li-dissolved sulfur cell, extensive research has only been undertaken in the past 6–7 years. These efforts have yielded considerable progress, and improvements in performance are now highly anticipated. A typical Li–S cell benefits from two relatively light elements as the electrode materials: lithium metal as the anode and elemental sulfur as the cathode (Figure 1) [2]. With a theoretical-specific capacity of 1672 mAh g1 for elemental sulfur, this technology attracts immediate interest. The general reactions at the discharge of a Li–S cell are: Li ! Li+ + e (oxidation of lithium)
(1)
(2) S + 2e ! S2 (reduction of sulfur) Hence, the final discharge product is Li2S, and the overall discharge reaction of the Li–S battery is: (3) 2Li + S ! Li2S The electrochemical reduction of elemental sulfur during discharge, though, is a multistep reaction, so Reaction (2) is considered a series of reactions: 1/2 S8 + e ! 1/2 S82 (formation of Li2S8)
(4)
3/2 S82 + e ! 2 S62 (formation of Li2S6)
(5)
S62 + e ! 3/2 S42 (formation of Li2S4)
(6)
(7) 1/2 S42 + e ! S22 (formation of Li2S2) Followed by final reduction reaction which forms Li2S: http://dx.doi.org/10.1016/j.coche.2017.04.005 2211-3398/ã 2017 Published by Elsevier Ltd.
Introduction Ever since the commercialization of lithium-ion batteries (LIB), researchers have continued their effort to improve the energy storage characteristics of these devices and sought alternative battery technologies. Amongst the alternative battery chemistries, lithium–sulfur (Li–S) technology which replaces intercalation at the positive and negative electrodes with covalent chemical reactions, has received significant attention from both the scientific community and industry. Several companies such as Oxis Energy (UK) and Sion Power (USA) claim to be very close www.sciencedirect.com
1/2 S22 + e ! S2 (formation of Li2S) (8) The series of intermediates formed during Reactions (4)– (7) are known as lithium polysulfides with a general formula Li2Sx (2 x 8). High order Li2Sx species (4 x 8) are soluble in most of the organic battery electrolytes and the solubility increases with the length of the sulfide chain; the lower order reduction products (Li2S2 and Li2S) are insoluble [2]. There are a number of key challenges in realising the full potential of Li–S battery: (a) sulfur and its discharge products (Li2Sx) are insulating which results in low utilization of electrode active material and limits the high rate performance of the cell; (b) a large volumetric expansion/ contraction (76%) is associated with the conversion of sulfur to Li2S and vice versa and results in the disintegration of the cathode and rapid capacity fading upon Current Opinion in Chemical Engineering 2017, 16:31–38
32 Separation engineering
Figure 1
(1) confinement/trapping approaches by using functional host materials in the composition of sulfur cathode [8–10]; (2) electrolyte modification approaches by changing the composition and concentration of the electrolyte and the additives [11–13]; and (3) multifunctional membrane separators and or interlayers with permselectivity/localizing abilities [14,15,16]. Among these three main strategies, producing composite cathodes by immobilising sulfur particles within various host materials has been the most widely used, while the utilisation of permselective membranes has received the least attention (Table 1). Current Opinion in Chemical Engineering
Schematic illustration of a standard Li–S battery comprising a lithium anode and a sulfur cathode isolated from each other via a separator.
cycling; and (c) the higher order polysulfides (Li2Sx, 4 x 8) are highly soluble in common organic electrolytes resulting in dramatic loss of active material, capacity degradation and low coulombic efficiency [3–7]. Among these drawbacks, the concerns around the first two have been mitigated to a large extent by the use of a conductive agent and stable hosts to accommodate the strain generated during volume change [4]. On the other hand and in spite of marked improvements, the issue of polysulfides solubility has not been fully addressed.
Addressing the challenges of Li–S battery A number of strategies have been employed to mitigate the liberation of polysulfides in the electrolyte including
In spite of the outstanding progress in addressing the classic challenges of Li–S chemistry (solubility of polysulfides, volume change of electrode and insulating nature of sulfur), crucial commercialization parameters such as high fraction of active material in the cell, low passive weight of the cell and ease of fabrication have largely been overlooked in the literature. In order for future Li–S batteries to demonstrate high-performance metrics at competitive price points, the amount of sulfur in the cell should be maximised, while the weight and associated costs of the other components in the cell, such as the cathode host material, binder, conductive agent, current collector, separator and electrolyte should be minimized. A survey of the state of the art Li–S research shows that different methods to fabricate sulfur cathodes with fractions of sulfur between 10% and 80% (averaging 50%) has been accomplished [17], however, this low small fraction of active material in the sulfur composite cathodes, (compared to the corresponding values for LIBs
Table 1 Comparison of the performances of Li–S cells with permselective membranes in their configuration Rate (C)/ cycle (N)
Discharge capacity at nth cycle per mass of (mAh g1)
Sulfur fraction (%)/loading (mg cm2)
Membrane weight (mg cm2)/ thickness (mm)
Average coulombic efficiency (%)/LiNO3 in the electrolyte (Y/N)
70/?
?/50
?/50
815
570
>97//N
50/0.53
Sulfur Composite cathode Free-standing Nafion [15]
0.7/1.4
1 C/500
470
235
95.6/N
Vacuum filtered GO on the separator [16] 63/1–1.5
0.12
0.1 C/100
700
441
96.5/N
Celgard-coated Nafion [28 ]
GO/O-CNT on the separator [45]
65/1.2–1.4
0.3
1 C/100
750
487
98/Y
Polypropylene/graphene oxide/Nafion separator [29]
54/1.2
0.0532
0.1 C/100
850
459
95/N
Shear aligned GO membrane on the cathode [14]
35/1–1.2 70/1–1.2 80/1–1.2
0.05/0.75
0.5 C/100
1003 1190 1040
350 834 835
100/Y 98.3/Y 98.5/Y
GO/MOF [43]
56/0.6–0.8
?/?
0.5 C/500
799
448
100/Y
MMT (montmorillonite) on Celgard [39]
64/0.7
1.65/25
?/200
924
592
97/Y
Current Opinion in Chemical Engineering 2017, 16:31–38
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Permselective membranes in Li–S batteries Shaibani et al. 33
(>90%)), leads to low overall electrode capacity and consequently low energy density for the device. It is also questionable whether a Li–S cell with a sophisticated composite cathode can achieve attractive low-cost products, considering the complex manufacturing methods required to make the composite cathodes described in the literature. Although it is obvious that this latter issue would be a major concern for the manufacturing industry, particularly for large scale applications, it has been overlooked by the scientific community until very recently. To address the limitations of composite cathodes, functional membranes and films with various properties have been scrutinized as potential separators or interlayers in Li–S cells. These multifunctional separator systems include, but are not limited to, carbon-based interlayers/coatings with [18] or without [19] functional groups, non-porous Li ion conducting barriers [20], porous metal oxide coatings [21], gel polymer electrolytes [22,23], glass-ceramic electrolytes [24], and porous permselective membranes. Amongst these studies, introducing permselective membranes in the Li–S battery configurations has emerged as a powerful alternative strategy to retain the polysulfides in the cathode side of the cell, and significant progress has been made in the past 3–4 years. For this review, we highlight the role of permselective membranes, that is membranes which allow the transport of Lithium ions, but retards the transport of the charged and larger polysulfide species, review key articles and the battery performance characteristics reported in this growing research topic and subsequently provide our opinion on the strategies that can be adopted based on the constraints posed by the Li–S battery technology.
Permselective membranes in Li–S batteries Along with its essential function of preventing internal short circuits between the electrodes while enabling free ion flow, the separator should show adequate wettability, be porous, and possess minimum levels of chemical, thermal and dimensional stability. Ion-selectivity is not required in LIB technology but could be harnessed advantageously in certain energy storage devices where redox-active species are dissolved, dispersed, or suspended in the electrolyte such as flow batteries [14,25]. Permselective separators allow the transport of the electrolyte with desired ions while restricting the passage of other ions/species [26]. The efficiency of the process depends on the ability of the membrane to block the unwanted ions/species while enabling free transport of the other ion. Perfect permselectivity means the membrane completely excludes the unwanted species while allowing free movement of the electrolyte, while practical considerations also dictate that the membrane should be stable in the electrolyte used in the battery.
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Because of the ionic nature of the polysulfides, adding a permselective membrane to the configuration of a Li–S battery could retard the migration of these negatively charged species to a large extent, highly likely in a more efficient fashion than a functional host material used in a composite sulfur cathode [14,27,28]. To this end, several membrane designs based on the negative charge of various functional groups have been examined as polysulfide blocking layers in the configuration of Li–S battery, amongst which Nafion and graphene oxide have received the most attention. Utilisation of Nafion (sulfonated tetrafluoroethylene-based fluoropolymer–copolymer) cation selective membranes has been reported a number of times because of Nafion’s negatively charged SO3 functional groups, good Li+ conductivity and excellent stability [15,28,29,30,31,32]. Jin et al. [15] reported the application of a lithiated Nafion ionomer film fabricated by carrying out a lithium-ion exchange procedure on a Nafion-212 ionomer film in a solution of 1.0 M LiOH in H2O:ethanol (1:1 by weight) mixture. In spite of the low Li+ conductivity (2.1 105 S cm1), the Li–S cell configured with the lithiated Nafion film demonstrated an initial discharge capacity of 1185 mAh g1 and a retention of 69% across 50 cycles, a significant improvement over the performance of the cell configured with a typical Celgard 2400 separator which started at the same capacity yet retained only 46% over the same number of cycles [15]. As discussed by the authors, both the lithium ions and the polysulfide anions move freely inside the Celgard separator. On the other hand, the ionomer Nafion film has –OCF2CF(CF3)OCF2–CF2SO3Li pendent side chains and when Li dissociated from the chains, the SO3 groups take on a negative charge, allowing ion hopping of Li+ and rejecting the negatively charged polysulfides [15,27,28]. Additionally, the channels and clusters structure of the film serve as physical barriers towards the transport of polysulfides. Maintaining a Coulombic efficiency of above 97% in a LiNO3-free electrolyte, the lithiated Nafion was efficient at inhibiting the crossover of polysulfides, although a sharp capacity loss of 30% was also observed. Unfortunately, the loading of sulfur in the cathode and long-term rate capability data were not exclusively described. These missing data points, together with the limited number of presented cycles makes it difficult to conclude whether the approach of using a relatively thick free standing Nafion membrane (50 mm thick) could be transferred to a practical Li–S battery; however, these studies for the first time demonstrated the possible utility of a permselective membrane in a Li–S battery configuration. Nafion has also been successfully utilised in the fabrication of double-layer separators [28] and ternary-layered separators [29] to further improve the cycle life of Li–S. Huang et al. [28] applied a thin layer of Nafion on commonly used Celgard 2400 separators by a simple solution casting process. The 1.4 mm thick compact Current Opinion in Chemical Engineering 2017, 16:31–38
34 Separation engineering
Figure 2
High/Low order polysulfides Sulfur electrode
Ion selective Membrane
Routine membrane
Lithium ions
Lithium metal electrode
Ion selective membrane
1,600 100 1,200 80 800 60
400 Routine membrane Ion selective membrane
0
Sulfur cathode SX22X
S
Nafion layer GO barrier PP support Li+ Li+
Lithium anode
300 Cycle Number
40 500
400
100
1600
1200
75
800 50 400
PP/GO/Nafion ternary separator Routine PP separator 0
50
(c)
100
150
25
Coulombic efficiency (%)
Discharge capacity (mAh g-1)
(b)
Ternary separator
200
100
0
Coulombic Efficiency (%)
Routine Membrane
-1 Discharge capacity (mAh g )
(a)
200
Cycle number
Cathode
S
-1
1600
100
1400
90
1200 1000
(d)
80
800 600
70
400
GO coated cathode Un-coated cathode
200
0
100
0
60
200 Cycle Number (N)
300
HKUST-1 (nanosized) Diluted
GO
Filter membrane
+ so Lig lu an (re tion d pe at)
GO solutions
Peeled-off
50
400
1.2
1,600
1.0
1,400 1,200
0.8
1,000 0.6
800 600
0.4
400 MOF@GO separator C/2
200
0
0.2
Coulombic efficiency
Cu BTC
Capacity (mA h g-1)
1,800 2+
Coulombic efficiency (%)
Nematic GO
110
1800
Discharge capacity (mAh g )
Shear aligned GO membrane
Shear alignment tool
Nematic GO
0.0
0
50 100 150 200 250 300 350 400 450 500 Cycle numbers
Current Opinion in Chemical Engineering
(a) Schematic and cycling performance (at 1 C rate) comparison of lithium–sulfur battery configurations with routine membrane, in which polysulfides shuttle between the cathode and anode sides and with Nafion membrane, in which the polysulfide anions are limited to the cathode side. Reproduced from Ref. [28] with permission of The Royal Society of Chemistry; (b) schematic of a ternary PP/GO/Nafion separator and the cycling performance comparison of Li–S batteries with routine and ternary separators at 0.5 C rate. Reproduced from Ref. [29], with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (2015); (d) shear-alignment processing of nematic phase GO on a sulfur cathode and the performance comparison of a GO-coated cathode and an un-coated cathode at 1 C rate. Reprinted by permission from Macmillan Publishers Ltd: [Nature Energy] Ref. [43], copyright 2016; (c) schematic of the fabrication process to produce MOF@GO separators and the cycling performance of a Li–S battery configured with it at 0.5 C rate. Reproduced with permission from Ref. [14], Copyright ã2016 American Chemical Society.
Nafion layer (0.7 mg cm2) covered the macropores of the standard separator and proved successful in creating a polysulfide shield on the cathode side of the battery, retarding their crossover to the anode side. At a 1 C rate, Current Opinion in Chemical Engineering 2017, 16:31–38
the cell configured with the Nafion-coated separator started at 781 mAh g1 and retained over 60% of the initial capacity across 500 cycles, a significant improvement compared with only 34.4% retention for the cell www.sciencedirect.com
Permselective membranes in Li–S batteries Shaibani et al. 35
Bauer et al. [33] studied the relationship between the mass loading of Nafion on the Celgard separator and the resulting rate capability of the cell. Understandably, at the minimum Nafion loading (0.25 mg cm2), the rate performance matched that of cells with uncoated Celgard 2500 up to current densities of 3 mA cm2, while also maintaining the polysulfide rejection capability of the membrane. Zhuang et al. [29] minimised the loading of the Nafion layer by rationally introducing a ternary layered separator comprising a macroporous polypropylene layer, graphene oxide barrier layer and Nafion retarding layer (Figure 2b). The very low loading of graphene oxide (0.0032 mg cm2) and Nafion (0.05 mg cm2) compared to previous work [28] assisted with the faster diffusion of Li+ and improved the initial capacity [29]. Importantly and in spite of the very low thicknesses of the additional layers (30 nm and 100 nm for GO and Nafion layer, respectively), the ternary separator demonstrated robust mechanical properties, making it suitable for continuous roll-to-roll manufacturing [29].
limited Li+ conductivity of Nafion, in addition to its relatively high cost, Ma et al. [27] designed cross-linked PEGDMA membranes with pendant SO32 groups as separators with much enhanced abilities for electrolyte wetting and Li+ transport in Li–S cells. The membrane fabrication was carried out by copolymerization of poly (ethylene glycol)dimethacrylate (PEGDMA) and the vinylsulfonic acid salt (VS), where PEGDMA provides a cross-linked network to which vinylsulfonic groups are covalently attached as pendant/dangling entities. This was believed to enhance mobility and dissociation of the lithiated sulfonate groups. The fabricated membrane was then subjected to lithium ion exchange. The increased mobility of ions associated with the sulfonate groups results in some of the highest room temperature conductivity values for single ion conducting materials: 104 S cm1 when soaked in a 1:1 mixture of DOL:DME and 1.14 103 S cm1 when 1 M LiTFSI as the lithium salt was added, about two orders of magnitude higher than that of Nafion in the same environment (3.05 105 S cm1). Control experiments demonstrated that the immobilisation of polysulfide was as effective as Nafion while showing much improved transport for Li+. A sandwich-type separator in which the core layer is Celgard coated with the designed membrane containing 12.5% SO32 content on both sides with a coating thickness of 20 mm on each side demonstrated a high capacity of 1000 mAh g1 at 0.5 C rate after 100 cycles with an impressive Coulombic efficiency of above 98% in a LiNO3-free electrolyte. Similar membrane designs based on the polysulfide rejection ability of SO3 groups include, but are not limited to, polypropylene grafted with styrenesulfonate (PP-g-PLiSS) [36], sulfonated acetylene black modified separator [37] and perfluorinated ionomer electrolyte with lithium sulfonyl dicyanomethide functional groups as a functional separator [38].
Despite the lightweight of the ternary separator and the slight enhancement in both capacity and capacity retention, the initial capacity of the cell at the low 0.1 C rate was only 1057 mAh g1, much lower than that of Li–S cells configured with electrically conducting carbon coated separators [19,34,35]. This would mean that enhancement of initial capacity (a measure of the sulphur utilisation) could be enhanced by introducing a secondary layer ideally with minimal mass, but with good electrical conductivity to participate in electrochemical reactions.
Ahn et al. [39] introduced a montmorillonite (MMT) ceramic protective film as a negatively charged ion-selective membrane with enhanced stability and electrolyte wettability in the configuration of a Li–S battery. Despite the analytical confirmation of the ability of the MMTcoated separator to suppress the diffusion of the polysulfides to the anode side, the absence of rate capability data does not allow for further evaluation of the cycling performance of the Li–S battery configured with MMT film.
In spite of the impressive ability of the Nafion in retarding the shuttle of polysulfides, demonstrating high rate capability has been an issue, mainly due to increased resistance of the aprotic liquid electrolytes used in Li–S batteries in the presence of the Nafion membrane. The low room temperature permeability of the Nafion limits the power handling ability of a Li–S battery configured with a Nafion membrane, a major constraint in its commercial adaptation. To tackle the low permeability and
Another promising material which could serve effectively as an ion-selective membrane in the Li–S battery is graphene oxide (GO). Studies of the surface charge of GO sheets in aqueous colloidal suspensions show that they are negatively charged due to the ionisation of carboxylic groups [40,41]. The negatively charged GO sheets can repel the negatively charged partially reduced polysulfides, stopping their transport to the anode before full reduction, resulting in higher capacity and capacity
with an uncoated separator. A very high average Coulombic efficiency of 95.6% in a LiNO3-free electrolyte further confirms the effectiveness of the Nafion ionic shield (Figure 2a). In spite of the success in retarding the shuttle of polysulfides, low sulfur utilisation remains an issue in the cell configured with a Nafion-coated separator. In spite of the much lower thickness of the Nafion layer compared to the previous work [15], rate capability data demonstrates that capacities higher than 900 mAh g1 are not attainable even at a moderate cycling rate of 0.2 C [28]. Taking into account the low loading of sulfur (0.53 mg cm2) it can be concluded that the cell is suffering from large ohmic resistance and slow kinetics, which obviously is linked to the large mass transfer resistance of the dense Nafion films.
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Current Opinion in Chemical Engineering 2017, 16:31–38
36 Separation engineering
retention [3,14,16]. Taking advantage of this, Huang et al. [16] showed that a Li–S battery assembled with a GO coated separator fabricated by vacuum filtration of a GO solution through Celgard 2400 could be cycled at 0.1 C starting at 1000 mAh g1 and maintaining 70% capacity across 100 cycles. Further, the Coulombic efficiency of the Li–S battery configured with the GO coated separator was very high (95–98%) in a LiNO3-free electrolyte, a marked improvement over that of the battery with a bare separator (67–75%). This clearly demonstrates the ability of GO to prevent the crossover of polysulfides, minimising the loss of active material due to the unfavourable side reactions with the Lithium anode. Despite the improvement in the cyclic capacity decay rate compared to the control cell, this approach suffers from similar disadvantages as Nafion: (1) low electrolyte permeability, which is not surprising given the tortuous structure of the GO membrane fabricated by vacuum filtration lowers the flux of the electrolyte as the carrier of Li ions; (2) while GO is a relatively cheap material vacuum filtration is a time-consuming technique which is not industrially scalable, a potential limitation in its commercial adaptation by battery manufacturers. There are a number of recent reports on using GO to fabricate hybrid membranes [29,42,43,44,45]. For example Bai et al. [43] used the mechanically flexible and robust GO laminates to design an ion-selective membrane with an intrinsically brittle Metal Organic Framework (MOF) material. The MOF@GO membrane showed a remarkable capacity retention over long-term cycling, demonstrating a high efficiency in blocking the polysulfides while maintaining free pathways for Li ion transport (Figure 2d). In spite of the excellent long-term cycling performance, it is difficult to conclude whether this could be translated to a practical device due to the low sulfur loading in the cathode (56% S, 0.6– 0.8 mg cm2). Also, a current limitation to using porous crystalline nanoparticles of MOFs as the building blocks of ion-selective membranes is the access to high-quality inexpensive MOFs. Continuous flow production of MOFs and reducing solvent usage might address this limitation—cost projections range from $13/kg to $36/kg [46,47]. A similarity in all the works which used GO membranes in the configuration of a Li–S battery is the use of vacuumassisted filtration, a common practice in the literature for the fabrication of very thin membranes. In a different approach, Shaibani et al. [14] demonstrated a GO membrane fabricated by shear alignment of discotic nematic liquid crystals of GO, directly on the sulfur cathode, which assisted in retaining the polysulfides without unduly hindering ion-transport, that is without compromising the capacity and rate capability of the battery (Figure 2c). It was shown that a high degree of order and in-plane alignment is imparted to GO sheets Current Opinion in Chemical Engineering 2017, 16:31–38
upon applying shear to the nematic phase, as opposed to the much less ordered structure of the GO membrane formed by vacuum filtration [14,48,49]. With the aid of electrochemical impedance analysis the role of structural order of graphene sheets in different GO membranes on the kinetics of transport in Li–S battery was uncovered. This analysis showed the dramatic influence of the structural order and thinness of the GO layer in reducing ohmic polarisation of the device and highlighting the capability of shear-aligned GO membranes in Li–S batteries. From a practical point of view, the shear aligned processing can be adapted by battery manufacturers because of the high speed of the coating process and use of equipment available to battery manufacturers [14,48].
Summary and outlooks
There are several [29] technological advantages that can be provided by introducing permselective membranes – as a subset of multifunctional separator systems – in the configuration of a Li–S battery. First and foremost, the approach enables the utilisation of high content sulfur cathodes (e.g. 80% sulfur, 15% carbon black, 5% binder) [14] increases the overall electrode-specific capacity and is believed to significantly reduce the price of the cathode. Enhanced performance metrics and careful minimisation of the mass/volume of the extra layers should result in enhanced volumetric/gravimetric energy densities in a cell level. Last, but not least, it has been shown that these improvements to the performance of the Li–S system can be deployed easily, with techniques that are currently used on a routine basis in the manufacturing industry [14]. Nafion and GO have been most explored as ion-selective membranes in Li–S batteries. While Nafion provides a very strong ionic shield for the polysulfides, it imposes too great a restriction on ion movement which, in turn, lowers the high current response of the battery. Further, the very high cost of the Nafion limits its use to lab-scale purposes. Incorporation of lower loadings of Nafion in the composition of composite separators [50] or the configuration of multi-layered separators [29] might have opened up more efficient and practical uses for Nafion. GO, on the other hand, is more likely to be used in the development of better Li–S batteries with good performance and at a reasonable cost. It has been shown that GO membranes can be fabricated judiciously to avoid a loss in lithium ion conductivity and by techniques that can be easily adapted by battery manufacturers into existing high-speed coating equipment and manufacturing processes. Also, GO has proven to be a material which can be efficiently used in the configuration of multifunctional separators [29,43,45] and its lower cost and lightweight being predicts for its incorporation in the separator systems of future Li–S batteries. Unfortunately, most of www.sciencedirect.com
Permselective membranes in Li–S batteries Shaibani et al. 37
the cited works investigate the polysulfide rejection ability of their designed membranes when coupled with cathodes of low sulfur loading (<2 mg cm2) and in the presence of large amounts of electrolyte. As such it would be difficult to conclude whether they would be able to outperform Li-ion batteries.
13. Wu F, Lee JT, Nitta N, Kim H, Borodin O, Yushin G: Adv. Mater.2015, 27:101-108.
To successfully introduce a practical permselective membrane for the Li–S battery, researchers should consider certain criteria: (a) utilization of less sophisticated materials; (b) following methodologies that can be applied to existing battery manufacturing techniques; (c) evaluating the performance of the membrane in the presence of a practical sulfur cathode (ex., 80%, 5–6 mg cm2 S) and at practical cycling rates over a practical number of cycles; (d) minimizing the mass/volume contribution of any additional layer introduced to the configuration of the battery.
15. Jin Z, Xie K, Hong X, Hu Z, Liu X: J. Power Sources2012, 218:163167.
Designing composite membrane separators made of either different functional layers or mixture of unique functional materials are expected to result in better performance, although challenges in the processing of these type of membranes particularly in light of manufacturing techniques used by the industry need to be addressed.
Acknowledgement Mahdokht Shaibani would like to thank Monash Centre for Atomically Thin Materials (MCATM) for her top up scholarship.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest
14. Shaibani M, Akbari A, Sheath P, Easton CD, Banerjee PC, Konstas K, Fakhfouri A, Barghamadi M, Musameh MM, Best AS, Hill MR, Hollenkamp AF, Majumder M: ACS Nano2016, 10:77687779. First report to introduce an industrially adaptable technique for fabricating GO membranes with enhanced transport properties. Also, includes good cycling performance for 80% S cathodes.
16. Huang JQ, Zhuang TZ, Zhang Q, Peng HJ, Chen CM, Wei F: ACS Nano2015, 9:3002-3011. First report on using a GO membrane fabricated by vacuum filtration technique in the configuration of Li–S battery. 17. Hagen M, Hanselmann D, Ahlbrecht K, Maca R, Gerber D, Tubke J: Adv. Energy Mater.2015, 5. 18. Song J, Gordin ML, Xu T, Chen S, Yu Z, Sohn H, Lu J, Ren Y, Duan Y, Wang D: Angew. Chem.2015, 127:4399-4403. 19. Su Y-S, Manthiram A: Nat. Commun.2012, 3:1166. 20. Moy D, Narayanan SR: J. Electrochem. Soc.2017, 164:A560A566. 21. Zhang Z, Lai Y, Zhang Z, Zhang K, Li J: Electrochim. Acta2014, 129:55-61. 22. Marmorstein D, Yu TH, Striebel KA, McLarnon FR, Hou J, Cairns EJ: J. Power Sources2000, 89:219-226. 23. Rao M, Geng X, Li X, Hu S, Li W: J. Power Sources2012, 212:179185. 24. Hayashi A, Ohtomo T, Mizuno F, Tadanaga K, Tatsumisago M: Electrochem. Commun.2003, 5:701-705. 25. Li C, Ward AL, Doris SE, Pascal TA, Prendergast D, Helms BA: Nano Lett.2015, 15:5724-5729. 26. Geise GM, Cassady HJ, Paul DR, Logan BE, Hickner MA: PCCP2014, 16:21673-21681. 27. Ma L, Nath P, Tu Z, Tikekar M, Archer LA: Chem. Mater.2016, 28:5147-5154. A novel approach to design a cheaper replacement for Nafion, with improved ability for Li+ transport. 28. Huang J-Q, Zhang Q, Peng H-J, Liu X-Y, Qian W-Z, Wei F: Energy Environ. Sci.2014, 7:347-353. Demonstrating good long-term cycling performance and high Coulombic efficiency in the presence of a Nafion-coated separator in a LiNO3-free electrolyte.
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ScienceDirect Permselective membranes in lithium–sulfur batteries Mahdokht Shaibani1,2, Anthony F Hollenkamp2, Matthew R Hill2,3 and Mainak Majumder1 Integration of permselective membranes – as a subset of the separator systems – in the configuration of lithium–sulfur battery, is a relatively simple solution to tackle the issue of polysulfide dissolution into the electrolyte. Cation-selective materials such as graphene oxide (GO) and Nafion have demonstrated to serve effectively as electrostatic shields for polysulfide anions, retarding their diffusion to the anode side of the battery. Looking into the future, the introducing of these materials in the configuration/composition of multifunctional/ multilayered separator systems will promote the high efficiency cycling of high content sulfur cathodes. Enhanced performance metrics and careful minimisation of the mass/volume of these extra components should result in enhanced volumetric/ gravimetric energy densities on a cell level for lithium–sulfur battery. Addresses 1 Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3168, Australia 2 CSIRO, Clayton, VIC 3168, Australia 3 Department of Chemical Engineering, Monash University, Clayton, VIC 3168, Australia Corresponding author: Majumder, Mainak (mainak.majumder@monash. edu)
Current Opinion in Chemical Engineering 2017, 16:31–38 This review comes from a themed issue on Separation engineering Edited by Mainak Majumder
to launching commercial products. While early works on Li–S date back to 1967, with a Li–S liquid cell [1] or the so-called Li-dissolved sulfur cell, extensive research has only been undertaken in the past 6–7 years. These efforts have yielded considerable progress, and improvements in performance are now highly anticipated. A typical Li–S cell benefits from two relatively light elements as the electrode materials: lithium metal as the anode and elemental sulfur as the cathode (Figure 1) [2]. With a theoretical-specific capacity of 1672 mAh g1 for elemental sulfur, this technology attracts immediate interest. The general reactions at the discharge of a Li–S cell are: Li ! Li+ + e (oxidation of lithium)
(1)
(2) S + 2e ! S2 (reduction of sulfur) Hence, the final discharge product is Li2S, and the overall discharge reaction of the Li–S battery is: (3) 2Li + S ! Li2S The electrochemical reduction of elemental sulfur during discharge, though, is a multistep reaction, so Reaction (2) is considered a series of reactions: 1/2 S8 + e ! 1/2 S82 (formation of Li2S8)
(4)
3/2 S82 + e ! 2 S62 (formation of Li2S6)
(5)
S62 + e ! 3/2 S42 (formation of Li2S4)
(6)
(7) 1/2 S42 + e ! S22 (formation of Li2S2) Followed by final reduction reaction which forms Li2S: http://dx.doi.org/10.1016/j.coche.2017.04.005 2211-3398/ã 2017 Published by Elsevier Ltd.
Introduction Ever since the commercialization of lithium-ion batteries (LIB), researchers have continued their effort to improve the energy storage characteristics of these devices and sought alternative battery technologies. Amongst the alternative battery chemistries, lithium–sulfur (Li–S) technology which replaces intercalation at the positive and negative electrodes with covalent chemical reactions, has received significant attention from both the scientific community and industry. Several companies such as Oxis Energy (UK) and Sion Power (USA) claim to be very close www.sciencedirect.com
1/2 S22 + e ! S2 (formation of Li2S) (8) The series of intermediates formed during Reactions (4)– (7) are known as lithium polysulfides with a general formula Li2Sx (2 x 8). High order Li2Sx species (4 x 8) are soluble in most of the organic battery electrolytes and the solubility increases with the length of the sulfide chain; the lower order reduction products (Li2S2 and Li2S) are insoluble [2]. There are a number of key challenges in realising the full potential of Li–S battery: (a) sulfur and its discharge products (Li2Sx) are insulating which results in low utilization of electrode active material and limits the high rate performance of the cell; (b) a large volumetric expansion/ contraction (76%) is associated with the conversion of sulfur to Li2S and vice versa and results in the disintegration of the cathode and rapid capacity fading upon Current Opinion in Chemical Engineering 2017, 16:31–38
32 Separation engineering
Figure 1
(1) confinement/trapping approaches by using functional host materials in the composition of sulfur cathode [8–10]; (2) electrolyte modification approaches by changing the composition and concentration of the electrolyte and the additives [11–13]; and (3) multifunctional membrane separators and or interlayers with permselectivity/localizing abilities [14,15,16]. Among these three main strategies, producing composite cathodes by immobilising sulfur particles within various host materials has been the most widely used, while the utilisation of permselective membranes has received the least attention (Table 1). Current Opinion in Chemical Engineering
Schematic illustration of a standard Li–S battery comprising a lithium anode and a sulfur cathode isolated from each other via a separator.
cycling; and (c) the higher order polysulfides (Li2Sx, 4 x 8) are highly soluble in common organic electrolytes resulting in dramatic loss of active material, capacity degradation and low coulombic efficiency [3–7]. Among these drawbacks, the concerns around the first two have been mitigated to a large extent by the use of a conductive agent and stable hosts to accommodate the strain generated during volume change [4]. On the other hand and in spite of marked improvements, the issue of polysulfides solubility has not been fully addressed.
Addressing the challenges of Li–S battery A number of strategies have been employed to mitigate the liberation of polysulfides in the electrolyte including
In spite of the outstanding progress in addressing the classic challenges of Li–S chemistry (solubility of polysulfides, volume change of electrode and insulating nature of sulfur), crucial commercialization parameters such as high fraction of active material in the cell, low passive weight of the cell and ease of fabrication have largely been overlooked in the literature. In order for future Li–S batteries to demonstrate high-performance metrics at competitive price points, the amount of sulfur in the cell should be maximised, while the weight and associated costs of the other components in the cell, such as the cathode host material, binder, conductive agent, current collector, separator and electrolyte should be minimized. A survey of the state of the art Li–S research shows that different methods to fabricate sulfur cathodes with fractions of sulfur between 10% and 80% (averaging 50%) has been accomplished [17], however, this low small fraction of active material in the sulfur composite cathodes, (compared to the corresponding values for LIBs
Table 1 Comparison of the performances of Li–S cells with permselective membranes in their configuration Rate (C)/ cycle (N)
Discharge capacity at nth cycle per mass of (mAh g1)
Sulfur fraction (%)/loading (mg cm2)
Membrane weight (mg cm2)/ thickness (mm)
Average coulombic efficiency (%)/LiNO3 in the electrolyte (Y/N)
70/?
?/50
?/50
815
570
>97//N
50/0.53
Sulfur Composite cathode Free-standing Nafion [15]
0.7/1.4
1 C/500
470
235
95.6/N
Vacuum filtered GO on the separator [16] 63/1–1.5
0.12
0.1 C/100
700
441
96.5/N
Celgard-coated Nafion [28 ]
GO/O-CNT on the separator [45]
65/1.2–1.4
0.3
1 C/100
750
487
98/Y
Polypropylene/graphene oxide/Nafion separator [29]
54/1.2
0.0532
0.1 C/100
850
459
95/N
Shear aligned GO membrane on the cathode [14]
35/1–1.2 70/1–1.2 80/1–1.2
0.05/0.75
0.5 C/100
1003 1190 1040
350 834 835
100/Y 98.3/Y 98.5/Y
GO/MOF [43]
56/0.6–0.8
?/?
0.5 C/500
799
448
100/Y
MMT (montmorillonite) on Celgard [39]
64/0.7
1.65/25
?/200
924
592
97/Y
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Permselective membranes in Li–S batteries Shaibani et al. 33
(>90%)), leads to low overall electrode capacity and consequently low energy density for the device. It is also questionable whether a Li–S cell with a sophisticated composite cathode can achieve attractive low-cost products, considering the complex manufacturing methods required to make the composite cathodes described in the literature. Although it is obvious that this latter issue would be a major concern for the manufacturing industry, particularly for large scale applications, it has been overlooked by the scientific community until very recently. To address the limitations of composite cathodes, functional membranes and films with various properties have been scrutinized as potential separators or interlayers in Li–S cells. These multifunctional separator systems include, but are not limited to, carbon-based interlayers/coatings with [18] or without [19] functional groups, non-porous Li ion conducting barriers [20], porous metal oxide coatings [21], gel polymer electrolytes [22,23], glass-ceramic electrolytes [24], and porous permselective membranes. Amongst these studies, introducing permselective membranes in the Li–S battery configurations has emerged as a powerful alternative strategy to retain the polysulfides in the cathode side of the cell, and significant progress has been made in the past 3–4 years. For this review, we highlight the role of permselective membranes, that is membranes which allow the transport of Lithium ions, but retards the transport of the charged and larger polysulfide species, review key articles and the battery performance characteristics reported in this growing research topic and subsequently provide our opinion on the strategies that can be adopted based on the constraints posed by the Li–S battery technology.
Permselective membranes in Li–S batteries Along with its essential function of preventing internal short circuits between the electrodes while enabling free ion flow, the separator should show adequate wettability, be porous, and possess minimum levels of chemical, thermal and dimensional stability. Ion-selectivity is not required in LIB technology but could be harnessed advantageously in certain energy storage devices where redox-active species are dissolved, dispersed, or suspended in the electrolyte such as flow batteries [14,25]. Permselective separators allow the transport of the electrolyte with desired ions while restricting the passage of other ions/species [26]. The efficiency of the process depends on the ability of the membrane to block the unwanted ions/species while enabling free transport of the other ion. Perfect permselectivity means the membrane completely excludes the unwanted species while allowing free movement of the electrolyte, while practical considerations also dictate that the membrane should be stable in the electrolyte used in the battery.
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Because of the ionic nature of the polysulfides, adding a permselective membrane to the configuration of a Li–S battery could retard the migration of these negatively charged species to a large extent, highly likely in a more efficient fashion than a functional host material used in a composite sulfur cathode [14,27,28]. To this end, several membrane designs based on the negative charge of various functional groups have been examined as polysulfide blocking layers in the configuration of Li–S battery, amongst which Nafion and graphene oxide have received the most attention. Utilisation of Nafion (sulfonated tetrafluoroethylene-based fluoropolymer–copolymer) cation selective membranes has been reported a number of times because of Nafion’s negatively charged SO3 functional groups, good Li+ conductivity and excellent stability [15,28,29,30,31,32]. Jin et al. [15] reported the application of a lithiated Nafion ionomer film fabricated by carrying out a lithium-ion exchange procedure on a Nafion-212 ionomer film in a solution of 1.0 M LiOH in H2O:ethanol (1:1 by weight) mixture. In spite of the low Li+ conductivity (2.1 105 S cm1), the Li–S cell configured with the lithiated Nafion film demonstrated an initial discharge capacity of 1185 mAh g1 and a retention of 69% across 50 cycles, a significant improvement over the performance of the cell configured with a typical Celgard 2400 separator which started at the same capacity yet retained only 46% over the same number of cycles [15]. As discussed by the authors, both the lithium ions and the polysulfide anions move freely inside the Celgard separator. On the other hand, the ionomer Nafion film has –OCF2CF(CF3)OCF2–CF2SO3Li pendent side chains and when Li dissociated from the chains, the SO3 groups take on a negative charge, allowing ion hopping of Li+ and rejecting the negatively charged polysulfides [15,27,28]. Additionally, the channels and clusters structure of the film serve as physical barriers towards the transport of polysulfides. Maintaining a Coulombic efficiency of above 97% in a LiNO3-free electrolyte, the lithiated Nafion was efficient at inhibiting the crossover of polysulfides, although a sharp capacity loss of 30% was also observed. Unfortunately, the loading of sulfur in the cathode and long-term rate capability data were not exclusively described. These missing data points, together with the limited number of presented cycles makes it difficult to conclude whether the approach of using a relatively thick free standing Nafion membrane (50 mm thick) could be transferred to a practical Li–S battery; however, these studies for the first time demonstrated the possible utility of a permselective membrane in a Li–S battery configuration. Nafion has also been successfully utilised in the fabrication of double-layer separators [28] and ternary-layered separators [29] to further improve the cycle life of Li–S. Huang et al. [28] applied a thin layer of Nafion on commonly used Celgard 2400 separators by a simple solution casting process. The 1.4 mm thick compact Current Opinion in Chemical Engineering 2017, 16:31–38
34 Separation engineering
Figure 2
High/Low order polysulfides Sulfur electrode
Ion selective Membrane
Routine membrane
Lithium ions
Lithium metal electrode
Ion selective membrane
1,600 100 1,200 80 800 60
400 Routine membrane Ion selective membrane
0
Sulfur cathode SX22X
S
Nafion layer GO barrier PP support Li+ Li+
Lithium anode
300 Cycle Number
40 500
400
100
1600
1200
75
800 50 400
PP/GO/Nafion ternary separator Routine PP separator 0
50
(c)
100
150
25
Coulombic efficiency (%)
Discharge capacity (mAh g-1)
(b)
Ternary separator
200
100
0
Coulombic Efficiency (%)
Routine Membrane
-1 Discharge capacity (mAh g )
(a)
200
Cycle number
Cathode
S
-1
1600
100
1400
90
1200 1000
(d)
80
800 600
70
400
GO coated cathode Un-coated cathode
200
0
100
0
60
200 Cycle Number (N)
300
HKUST-1 (nanosized) Diluted
GO
Filter membrane
+ so Lig lu an (re tion d pe at)
GO solutions
Peeled-off
50
400
1.2
1,600
1.0
1,400 1,200
0.8
1,000 0.6
800 600
0.4
400 MOF@GO separator C/2
200
0
0.2
Coulombic efficiency
Cu BTC
Capacity (mA h g-1)
1,800 2+
Coulombic efficiency (%)
Nematic GO
110
1800
Discharge capacity (mAh g )
Shear aligned GO membrane
Shear alignment tool
Nematic GO
0.0
0
50 100 150 200 250 300 350 400 450 500 Cycle numbers
Current Opinion in Chemical Engineering
(a) Schematic and cycling performance (at 1 C rate) comparison of lithium–sulfur battery configurations with routine membrane, in which polysulfides shuttle between the cathode and anode sides and with Nafion membrane, in which the polysulfide anions are limited to the cathode side. Reproduced from Ref. [28] with permission of The Royal Society of Chemistry; (b) schematic of a ternary PP/GO/Nafion separator and the cycling performance comparison of Li–S batteries with routine and ternary separators at 0.5 C rate. Reproduced from Ref. [29], with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (2015); (d) shear-alignment processing of nematic phase GO on a sulfur cathode and the performance comparison of a GO-coated cathode and an un-coated cathode at 1 C rate. Reprinted by permission from Macmillan Publishers Ltd: [Nature Energy] Ref. [43], copyright 2016; (c) schematic of the fabrication process to produce MOF@GO separators and the cycling performance of a Li–S battery configured with it at 0.5 C rate. Reproduced with permission from Ref. [14], Copyright ã2016 American Chemical Society.
Nafion layer (0.7 mg cm2) covered the macropores of the standard separator and proved successful in creating a polysulfide shield on the cathode side of the battery, retarding their crossover to the anode side. At a 1 C rate, Current Opinion in Chemical Engineering 2017, 16:31–38
the cell configured with the Nafion-coated separator started at 781 mAh g1 and retained over 60% of the initial capacity across 500 cycles, a significant improvement compared with only 34.4% retention for the cell www.sciencedirect.com
Permselective membranes in Li–S batteries Shaibani et al. 35
Bauer et al. [33] studied the relationship between the mass loading of Nafion on the Celgard separator and the resulting rate capability of the cell. Understandably, at the minimum Nafion loading (0.25 mg cm2), the rate performance matched that of cells with uncoated Celgard 2500 up to current densities of 3 mA cm2, while also maintaining the polysulfide rejection capability of the membrane. Zhuang et al. [29] minimised the loading of the Nafion layer by rationally introducing a ternary layered separator comprising a macroporous polypropylene layer, graphene oxide barrier layer and Nafion retarding layer (Figure 2b). The very low loading of graphene oxide (0.0032 mg cm2) and Nafion (0.05 mg cm2) compared to previous work [28] assisted with the faster diffusion of Li+ and improved the initial capacity [29]. Importantly and in spite of the very low thicknesses of the additional layers (30 nm and 100 nm for GO and Nafion layer, respectively), the ternary separator demonstrated robust mechanical properties, making it suitable for continuous roll-to-roll manufacturing [29].
limited Li+ conductivity of Nafion, in addition to its relatively high cost, Ma et al. [27] designed cross-linked PEGDMA membranes with pendant SO32 groups as separators with much enhanced abilities for electrolyte wetting and Li+ transport in Li–S cells. The membrane fabrication was carried out by copolymerization of poly (ethylene glycol)dimethacrylate (PEGDMA) and the vinylsulfonic acid salt (VS), where PEGDMA provides a cross-linked network to which vinylsulfonic groups are covalently attached as pendant/dangling entities. This was believed to enhance mobility and dissociation of the lithiated sulfonate groups. The fabricated membrane was then subjected to lithium ion exchange. The increased mobility of ions associated with the sulfonate groups results in some of the highest room temperature conductivity values for single ion conducting materials: 104 S cm1 when soaked in a 1:1 mixture of DOL:DME and 1.14 103 S cm1 when 1 M LiTFSI as the lithium salt was added, about two orders of magnitude higher than that of Nafion in the same environment (3.05 105 S cm1). Control experiments demonstrated that the immobilisation of polysulfide was as effective as Nafion while showing much improved transport for Li+. A sandwich-type separator in which the core layer is Celgard coated with the designed membrane containing 12.5% SO32 content on both sides with a coating thickness of 20 mm on each side demonstrated a high capacity of 1000 mAh g1 at 0.5 C rate after 100 cycles with an impressive Coulombic efficiency of above 98% in a LiNO3-free electrolyte. Similar membrane designs based on the polysulfide rejection ability of SO3 groups include, but are not limited to, polypropylene grafted with styrenesulfonate (PP-g-PLiSS) [36], sulfonated acetylene black modified separator [37] and perfluorinated ionomer electrolyte with lithium sulfonyl dicyanomethide functional groups as a functional separator [38].
Despite the lightweight of the ternary separator and the slight enhancement in both capacity and capacity retention, the initial capacity of the cell at the low 0.1 C rate was only 1057 mAh g1, much lower than that of Li–S cells configured with electrically conducting carbon coated separators [19,34,35]. This would mean that enhancement of initial capacity (a measure of the sulphur utilisation) could be enhanced by introducing a secondary layer ideally with minimal mass, but with good electrical conductivity to participate in electrochemical reactions.
Ahn et al. [39] introduced a montmorillonite (MMT) ceramic protective film as a negatively charged ion-selective membrane with enhanced stability and electrolyte wettability in the configuration of a Li–S battery. Despite the analytical confirmation of the ability of the MMTcoated separator to suppress the diffusion of the polysulfides to the anode side, the absence of rate capability data does not allow for further evaluation of the cycling performance of the Li–S battery configured with MMT film.
In spite of the impressive ability of the Nafion in retarding the shuttle of polysulfides, demonstrating high rate capability has been an issue, mainly due to increased resistance of the aprotic liquid electrolytes used in Li–S batteries in the presence of the Nafion membrane. The low room temperature permeability of the Nafion limits the power handling ability of a Li–S battery configured with a Nafion membrane, a major constraint in its commercial adaptation. To tackle the low permeability and
Another promising material which could serve effectively as an ion-selective membrane in the Li–S battery is graphene oxide (GO). Studies of the surface charge of GO sheets in aqueous colloidal suspensions show that they are negatively charged due to the ionisation of carboxylic groups [40,41]. The negatively charged GO sheets can repel the negatively charged partially reduced polysulfides, stopping their transport to the anode before full reduction, resulting in higher capacity and capacity
with an uncoated separator. A very high average Coulombic efficiency of 95.6% in a LiNO3-free electrolyte further confirms the effectiveness of the Nafion ionic shield (Figure 2a). In spite of the success in retarding the shuttle of polysulfides, low sulfur utilisation remains an issue in the cell configured with a Nafion-coated separator. In spite of the much lower thickness of the Nafion layer compared to the previous work [15], rate capability data demonstrates that capacities higher than 900 mAh g1 are not attainable even at a moderate cycling rate of 0.2 C [28]. Taking into account the low loading of sulfur (0.53 mg cm2) it can be concluded that the cell is suffering from large ohmic resistance and slow kinetics, which obviously is linked to the large mass transfer resistance of the dense Nafion films.
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Current Opinion in Chemical Engineering 2017, 16:31–38
36 Separation engineering
retention [3,14,16]. Taking advantage of this, Huang et al. [16] showed that a Li–S battery assembled with a GO coated separator fabricated by vacuum filtration of a GO solution through Celgard 2400 could be cycled at 0.1 C starting at 1000 mAh g1 and maintaining 70% capacity across 100 cycles. Further, the Coulombic efficiency of the Li–S battery configured with the GO coated separator was very high (95–98%) in a LiNO3-free electrolyte, a marked improvement over that of the battery with a bare separator (67–75%). This clearly demonstrates the ability of GO to prevent the crossover of polysulfides, minimising the loss of active material due to the unfavourable side reactions with the Lithium anode. Despite the improvement in the cyclic capacity decay rate compared to the control cell, this approach suffers from similar disadvantages as Nafion: (1) low electrolyte permeability, which is not surprising given the tortuous structure of the GO membrane fabricated by vacuum filtration lowers the flux of the electrolyte as the carrier of Li ions; (2) while GO is a relatively cheap material vacuum filtration is a time-consuming technique which is not industrially scalable, a potential limitation in its commercial adaptation by battery manufacturers. There are a number of recent reports on using GO to fabricate hybrid membranes [29,42,43,44,45]. For example Bai et al. [43] used the mechanically flexible and robust GO laminates to design an ion-selective membrane with an intrinsically brittle Metal Organic Framework (MOF) material. The MOF@GO membrane showed a remarkable capacity retention over long-term cycling, demonstrating a high efficiency in blocking the polysulfides while maintaining free pathways for Li ion transport (Figure 2d). In spite of the excellent long-term cycling performance, it is difficult to conclude whether this could be translated to a practical device due to the low sulfur loading in the cathode (56% S, 0.6– 0.8 mg cm2). Also, a current limitation to using porous crystalline nanoparticles of MOFs as the building blocks of ion-selective membranes is the access to high-quality inexpensive MOFs. Continuous flow production of MOFs and reducing solvent usage might address this limitation—cost projections range from $13/kg to $36/kg [46,47]. A similarity in all the works which used GO membranes in the configuration of a Li–S battery is the use of vacuumassisted filtration, a common practice in the literature for the fabrication of very thin membranes. In a different approach, Shaibani et al. [14] demonstrated a GO membrane fabricated by shear alignment of discotic nematic liquid crystals of GO, directly on the sulfur cathode, which assisted in retaining the polysulfides without unduly hindering ion-transport, that is without compromising the capacity and rate capability of the battery (Figure 2c). It was shown that a high degree of order and in-plane alignment is imparted to GO sheets Current Opinion in Chemical Engineering 2017, 16:31–38
upon applying shear to the nematic phase, as opposed to the much less ordered structure of the GO membrane formed by vacuum filtration [14,48,49]. With the aid of electrochemical impedance analysis the role of structural order of graphene sheets in different GO membranes on the kinetics of transport in Li–S battery was uncovered. This analysis showed the dramatic influence of the structural order and thinness of the GO layer in reducing ohmic polarisation of the device and highlighting the capability of shear-aligned GO membranes in Li–S batteries. From a practical point of view, the shear aligned processing can be adapted by battery manufacturers because of the high speed of the coating process and use of equipment available to battery manufacturers [14,48].
Summary and outlooks
There are several [29] technological advantages that can be provided by introducing permselective membranes – as a subset of multifunctional separator systems – in the configuration of a Li–S battery. First and foremost, the approach enables the utilisation of high content sulfur cathodes (e.g. 80% sulfur, 15% carbon black, 5% binder) [14] increases the overall electrode-specific capacity and is believed to significantly reduce the price of the cathode. Enhanced performance metrics and careful minimisation of the mass/volume of the extra layers should result in enhanced volumetric/gravimetric energy densities in a cell level. Last, but not least, it has been shown that these improvements to the performance of the Li–S system can be deployed easily, with techniques that are currently used on a routine basis in the manufacturing industry [14]. Nafion and GO have been most explored as ion-selective membranes in Li–S batteries. While Nafion provides a very strong ionic shield for the polysulfides, it imposes too great a restriction on ion movement which, in turn, lowers the high current response of the battery. Further, the very high cost of the Nafion limits its use to lab-scale purposes. Incorporation of lower loadings of Nafion in the composition of composite separators [50] or the configuration of multi-layered separators [29] might have opened up more efficient and practical uses for Nafion. GO, on the other hand, is more likely to be used in the development of better Li–S batteries with good performance and at a reasonable cost. It has been shown that GO membranes can be fabricated judiciously to avoid a loss in lithium ion conductivity and by techniques that can be easily adapted by battery manufacturers into existing high-speed coating equipment and manufacturing processes. Also, GO has proven to be a material which can be efficiently used in the configuration of multifunctional separators [29,43,45] and its lower cost and lightweight being predicts for its incorporation in the separator systems of future Li–S batteries. Unfortunately, most of www.sciencedirect.com
Permselective membranes in Li–S batteries Shaibani et al. 37
the cited works investigate the polysulfide rejection ability of their designed membranes when coupled with cathodes of low sulfur loading (<2 mg cm2) and in the presence of large amounts of electrolyte. As such it would be difficult to conclude whether they would be able to outperform Li-ion batteries.
13. Wu F, Lee JT, Nitta N, Kim H, Borodin O, Yushin G: Adv. Mater.2015, 27:101-108.
To successfully introduce a practical permselective membrane for the Li–S battery, researchers should consider certain criteria: (a) utilization of less sophisticated materials; (b) following methodologies that can be applied to existing battery manufacturing techniques; (c) evaluating the performance of the membrane in the presence of a practical sulfur cathode (ex., 80%, 5–6 mg cm2 S) and at practical cycling rates over a practical number of cycles; (d) minimizing the mass/volume contribution of any additional layer introduced to the configuration of the battery.
15. Jin Z, Xie K, Hong X, Hu Z, Liu X: J. Power Sources2012, 218:163167.
Designing composite membrane separators made of either different functional layers or mixture of unique functional materials are expected to result in better performance, although challenges in the processing of these type of membranes particularly in light of manufacturing techniques used by the industry need to be addressed.
Acknowledgement Mahdokht Shaibani would like to thank Monash Centre for Atomically Thin Materials (MCATM) for her top up scholarship.
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