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Fabricating Multifunctional Nanoparticle Membranes by a Fast Layer-by-Layer Langmuir-Blodgett Process: Application in Lithium-sulfur Batteries

The Langmuir-Blodgett (LB) technique is a powerful, widely used method for preparing coatings of amphiphilic molecules at air/water interfaces with thickness control down to a single molecule. Here we report two new LB techniques designed to create ordered, multifunctional nanoparticle films on any non-reactive support. The methods utilize Marangoni stresses produced by surfactants at a fluid/solid/gas interface and self-assembly of nanoparticles to facilitate rapid creation of dense monolayers of multi-wall carbon nanotubes (MWCNT), metal-oxide nanoparticles, polymers, and combinations of these materials in a layer-by-layer configuration. Using the polyolefin separator in a lithium sulfur (Li-S) electrochemical cell as an example, we illustrate how the method can be used to create structured membranes for regulating mass and charge transport. We show that a layered MWCNT/SiO2/MWCNT nanomaterial created in a clip-like configuration, with gravimetric areal coverage of ~130 mg cm-2 and a thickness of ~3 micron, efficiently adsorbs dissolved lithium polysulfide (LiPS) species and efficiently reutilize them for improving Li-S battery performance.


Introduction
The Langmuir-Blodgett (LB) technique is a method for preparing coatings of amphiphilic molecules at air/water interfaces with thickness of one molecule [1][2][3] . The method is attractive for a variety of reasons, including its ability to precisely control the thicknesses of coatings down to molecular dimensions, for the versatility of substrates that can be coated, and for its scalability.
Ever since its discovery in the 1920s and its rise in popularity following Irving Langmuir's receipt of the Nobel prize in chemistry in 1932, it has been applied in numerous fields of science and technology to form thin micro patterns, 4,5 molecular-thick thin films, 6,7 and more recently monoparticle layers based on spheres, 8,9 rods, 10,11 and nanotubes, 12 which can be easily transferred onto various substrates. Recently, Nie et al proposed an electrospray method that extends the LB technique to yield high coverage of metallic nanospheres at the surface of water 13 . With LB assembly it is therefore now possible to produce monolayers of colloidal films that broadly range in particle sizes and shapes, which may be used to advantage for tuning film properties 14 as well as for coating applications to create thin film devices 15,16 .
This article reports two new and versatile LB coating approaches -Langmuir-Blodgett sequential dip coating (LBSDC) and the Langmuir-Blodgett scooping (LBS), which facilitate efficient creation of multifunctional, layer-by-layer coatings of carbon, metal-oxides, polymers, and combinations of these materials on any non-reactive substrate. Unlike the conventional LB method, which uses a mechanical force applied to a disordered material at the air/water interface to create well-ordered assemblies of molecules or particles, LBSDC and LBS utilize surfactant and self-assembly, respectively, to create ordered coatings that can be transferred to a solid or porous support. This difference allows highly organized coatings to be created in a fraction of the time and using any containment vessel (i.e. a LB mechanical barrier is not needed). The speed with which ordered monolayer coatings are created, the high quality and low thickness of the transferred coatings, and versatility of the process by which the coatings are formed mean that LBSDC and LBS can be applied in a repetitive fashion to create multi-functional coatings in a layer-by-layer format that enable design of new materials with surface features able to regulate mass and charge transport. Because the assembly occurs at a sharp gas/liquid interface, the methods nonetheless benefit from the inherent attributes of the LB technique -precise control over film thickness and structure, as well as the versatility of substrate choices. Moreover, numbers and positions of suspension injection nozzle and water surface area can be altered and customized to scale up the coating process.
The utility of the LBSDC and LBS approaches is illustrated in the present study using the polyolefin separator membrane of a standard Lithium-Sulfur (Li-S) electrochemical cell as a substrate. This choice is motivated by the promise such cells offer for cost-effective storage of large quantities of electrical energy and by the stubborn challenges associated with solubility and diffusion of long-chain (Li 2 S x ; x ≥4), lithium polysulfide (LiPS) species, to the electrolyte that limit performance of Li-S batteries.  We report that using LBSDC and LBS it is possible to create multifunctional coatings in multiple designs that enable conventional membranes to overcome the most difficult challenges. We further report a novel "clip" separator membrane configuration in which a well-formed, but incomplete layer of structures of one chemistry is sandwiched between complete layers of another chemistry. This coating morphology allows one to engineer the surface of a membrane to simultaneously trap an undesired material (e.g. LiPS) and to maintain electrochemical access to it. In so doing, we show that it is possible to preserve the favorable attributes of the Li-S cell and address some of its most serious weaknesses.

Results and Discussions
The LBSDC and LBS coating methods enable creation of well-defined layers of materials in various physical forms and chemistries on a conventional polypropylene separator, without the need for chemical binders. LBSDC is a discontinuous process that utilizes a sodium dodecyl sulfate (SDS) surfactant, inducing Marangoni effect, to lower the surface tension of water and to provide a unidirectional force on floating particles or to a particulate LB film at the air-water interface to form a dense, close-packed structure. Care is needed in this step, for on the small length scales of these monolayer films the pressure provided by the surfactant can easily exceed the stability of the self-assembled LB film, causing it to rupture due to too strong surface tension gradient (Supplementary Figure S1). Grains that exceed 200 nm in size exhibit the greatest film stability and are able to form the most densely packed coatings through the LBSDC technique.
The LBS method, on the other hand, is a continuous process that uses constant injection of a particle suspension during the coating process to maintain a closely-packed LB film by a self-assembly mechanism, which is induced by the simple spreading and mixing of the water miscible fluid. This approach enables particles smaller than 200 nm in size to be coated on a mobile substrate due to the absence of the extra surface tension gradient provided by the surfactant and may be implemented in a roll-to-roll manufacturing process. The LBS method is therefore more flexible than the LBSDC method, but requires constant injection of the suspension during the coating process. Surface pressure profiles obtained using the LBSDC and LBS approaches to assemble monoparticle layers of ~350 nm diameter sized nanospheres and multi-walled carbon nanotubes (MWCNT) at the air/water interface are reported in Supplementary Figure S2. Figure 1 compares the pressure profiles to those obtained using a conventional LB trough. Three different surface pressure points are chosen in Figure 1a to investigate the packing densities of the colloidal film onto a separator achieved with each of the approaches. The points A, B, and C correspond to the surface pressure of 60 mN m -1 , 33 mN m -1 , and 2 mN m -1 , respectively: Points A and C represent the surface pressure where the colloids are overly packed and inadequately packed, and point B represents the starting surface pressure from the LBSDC method. The inflection point of the surface pressure profile represents a transition point where the folding of the film starts, which is consistent with what is observed in the SEM images at point A. Point B shows the most uniform coating, which indicates that the surface pressure between 33 mN m -1 and 38 mN m -1 will yield the highest quality LB film. The surface pressure profile of the colloids from the LBSDC method starts at 33 mN m -1 , which represents the amount of pressure exerted by one 5µL drop of 3wt% SDS surfactant on the film. This pressure from the surface tension gradient allows the colloids to be packed closely and remain stationary, and therefore, no inflection point is observed from the LBSDC profile. To confirm the packing density of the colloids using the LBSDC method, the colloids are coated onto the separator using the LB trough at 33 mN m -1 and using the LBSDC method. A coating quality of good agreement is observed from SEM images at point B and from LBSDC, which confirms that LBSDC starts from a highly packed colloidal LB film. To understand the role of the surfactant in the LBSDC method, the maximum amount of the pressure acting on the film is measured and its stability is observed (See Supplementary Figure   S3a & S3b). The maximum pressure that the surfactant can provide is ~34 mN m -1 and tends to slowly fade over time. The maximum pressure exerted by the surfactant matches the starting surface pressure of the colloids using the LBSDC method, where the increased pressure is the same as obtained from one drop of the surfactant.
In Figure 1b, the surface pressure profiles of the MWCNT is compared from conventional LB trough and LBS methods, and three different surface pressure points, A', B', and C', are chosen at 61 mN m -1 , 37 mN m -1 , and 1 mN m -1 , respectively, to observe the coating quality. The MWCNT film tends to fold as shown in the SEM at point A', and poor coverage of the film is seen at point C'. A long compression region of the film is observed from the profile by comparing the area before and after the inflection point. This is because of the elastic behavior of the self-assembled MWCNT film as the film is comprised of nanotubes. Based on the geometry of the particle, different trends of the surface pressure profiles can be obtained (See Supplementary Figure S3c). To investigate where the coating quality of the LBS lies, the surface pressure profile of the LBS method is measured and compared to the profile obtained from the LB trough. The LBS method requires a constant injection of the suspension to maintain high packing density by self-assembly mechanism induced by spreading and mixing of the suspension solvent (ethanol) with water. To observe how much pressure is exerted during the self-assembly, the surface pressure was measured by saturating the surface of the trough with MWCNT. The measured pressure is 37 mN m -1 , which is the pressure exerted from the self-assembly. Since the spreading velocity depends on the distance traveled by the particle, the area on the trough is set to around 25 cm 2 , which is a similar surface area for our experimental coating process. No inflection point is shown from the profile, which confirms that the fibers are closely packed and compressed from the  Figure S1). The above results support that the LBSDC and LBS coating methods start in an optimized packing condition and yield high-quality LB films. Figure 2a illustrates the simplicity and effectiveness of the LBSDC and LBS methods.
Multilayer coatings using the two approaches are reported in Figure 2b & 2c. The coatings and their processes are important not only because they exhibit an excellent close-packed morphology, but also because they are the thinnest and highest fidelity coatings reported on a battery separator. For example, thickness variations for silica nanospheres and MWCNT within a single monolayer of the silica particle size and ~80 nm of MWCNT (Supplementary Figure S4a) are achieved. This means that these coatings will add very little mass to a battery separator or electrode, yet significantly optimize the electrochemical performance in the batteries due to the uniform and densely-packed coating layers.
The polypropylene Celgard TM membrane used as a separator in the Li-S battery is chosen as a substrate to illustrate the utility of the LBSDC and LBS approach for at least three reasons. First, the rechargeable Li-S battery is arguably one of the most important platforms for storing large amounts of electrical energy at a moderate cost. The redox reaction between lithium and sulfur, 16Li + S 8 ó 8Li 2 S, occurs spontaneously, is reversible, and produces up to two electrons per formula unit of sulfur, without intervention with catalysts or other means. These features endow the Li-S battery with high theoretical specific energy, 2600 Wh kg -1 , and low material and operating costs [17][18][19] .
Second, in practice Li-S cells fail to deliver on these high expectations for two stubborn, Dissolution of LiPS in an electrolyte means that a substantial fraction of the active material can be lost before it is fully reduced to Li 2 S, if the LiPS diffuses too far from the conductive substrate in the cathode. An even greater concern is that once in the electrolyte, LiPS can diffuse to the lithium metal anode and undergo chemical reduction to form polysulfides of lower order, some of which are insoluble and deposit on the anode, causing time-dependent loss of both lithium and sulfur in a parasitic process termed shuttling.
Finally, a variety of approaches have been investigated for controlling LiPS dissolution, diffusion, and shuttling in Li-S cells. Methods ranging from sequestering the sulfur in porous carbon structures in nanospheres 21 , nanotubes/nanofibers 22 , graphene/graphene oxide sheets 23,24 all utilize the strong affinity of sulfur for carbon-based materials to limit dissolution. Other workers have shown that strong specific interactions of LiPS with amine-containing molecular 25,26 and inorganic chalcogenide 27 , particulate additives can be used to reduce sulfur loss to the electrolyte 28,29 . Even in the best cases, however, there is a finite, equilibrium concentration of LiPS dissolved in the electrolyte such that chemical potential of LiPS in the cathode is equal to that in the electrolyte 18,25, . As a result, the dissolved LiPS are still able to diffuse to the Li anode, react with it, and increase the interfacial resistance of the anode. Very recently, Hendrickson et al showed that a substantial amount of LiPS is also lost by adsorption in the pores of the separator and that this source of loss can be removed in model Li-S cells run in a separator-/membrane-free configuration, but at the price of very high interfacial impedances at the anode 30 . Other works have shown that incorporation of carbon, 31-42 metal-oxide, 43 with pristine Celgard separator. Specifically, the capacity retention after 100 cycles is improved from 31% for the pristine separator to 63%, 71%, 63%, and 49% for ten coating layers of MWCNT, KB, SP, and silica nanospheres, respectively. The initial capacity for the pristine separator, 10LR MWCNT, 10LR KB, 10LR SP, and 10LR are 1067 mAh g -1 , 1535 mAh g -1 , 1594 mAh g -1 , 1541 mAh g -1 , and 1588 mAh g -1 , respectively at 0.5C for the first four and 0.2C for the last.
Our results show that carbon-coated Celgard is far more effective than the silica-coated material in stabilizing cycling performance of Li-S cells. We attribute this behavior to the stronger adsorption of LiPS on the SiO 2 coating layer by physical and covalent bonds 28   previously demonstrated that a Li-S battery separator coated with a mixture of ceramic nanoparticles and Super-P (SP) carbon, using the doctor-blade coating method, yields cells with poorer electrochemical performance than those in which a simple SP coating layer was used.
Here, we take advantage of the spatial control afforded by the LBS and LBSDC coating strategy to create a multifunctional separator coating with the configuration shown in Figure 3a  In order to investigate the electrochemical performance of the clip-coated separator, three different cathodes, infused sulfur cathode (ISC), vapor infused sulfur cathode (VISC), and ballmilled sulfur cathode (BMSC), have been used in this study. Figure 4a reports results from galvanostatic cycling studies of the materials in a 1M LiTFSI DOL/DME electrolyte and with ISC. The lithium metal anode used in the study was pretreated in the manner reported in reference 30 , by soaking it in an electrolyte containing LiNO 3 for 24 hours followed by rigorous drying in an Ar environment. It is immediately apparent that they offer significant advantages.
Specifically, in the control case with the pristine (uncoated) separator, the capacity dropped to 360 mAh g -1 after 100 cycles at a current rate of 0.5C (838 mA g -1 ). However, when the separator was coated using the aforementioned clip configuration, a capacity of ~1000 mAh g -1 was obtained. The cells with the clip configuration also exhibit superior performance at high current rates. High initial capacities of 1290 mAh g -1 and 1160 mAh g -1 were obtained at 1C and 2C with high capacity retentions of 63% and 65%, respectively. Figure 4b   Longer cycling performance of clip coated separators with ISC and VISC without LiNO 3 in the electrolyte is shown in Figure 4k, and stable performance is observed >200 cycles at 0.5 C, which is remarkable with such high sulfur loading and LiNO 3 free electrolyte. In order to investigate the effect of each compartment in the clip configuration, we compared the electrochemical performance of the clip with five layers of MWCNT coating, three layers of silica, and a clip configuration without the final electrical path MWCNT layer, which is the equivalent structure shown in Figure 3d (Figure 5a). Significantly, it is noted that without the final MWCNT coating to complete the clip, similar electrochemical performances are observed in Li-S cells using the multifunctional MWCNT-SiO 2 coatings, compared to those based on separators coated with three monolayers of silica. These results underscore the importance of the clip configuration in complementing LiPS adsorption achieved with SiO 2 coatings, with utilization of the trapped LiPS made possible by the MWCNT coating layers. They also validate our hypothesis that a good electrical conductive path is required to efficiently entrap and utilize dissolved LiPS. Figure 5b compares the voltage profiles of the cell with the clip configuration done in different steps. Consistent with the previous observation, the capacity is seen to increase progressively as the clip components are sequentially added to complete the structure: 561 mAh g -1 at 100 th cycle, 596 mAh g -1 at 100 th cycle, 785 mAh g -1 at 100 th cycle, and 1214 mAh g -1 at 100 th cycle when 3LR silica, the multifunctional MWCNT-SiO 2 , 5LR MWCNT, and clip are coated on the separator, respectively. Another important observation is that the overpotential of the cell substantially declines when the final layer is involved, which confirms our hypothesis that the silica surface traps LiPS in the separator, which overtime reduces the electrolyte conductivity. Reutilization of the LiPS in the clip configuration eliminates this problem and reduces the overpotential correspondingly.

Figure 6 | Clip coated separator morphology and elemental mappings after several cycles. a,
Cross sectional SEM image of the clip coated separator with ISC after 10 th discharge with carbon, sulfur, and silicon maps. b, Top view SEM image of the clip coated separator with ISC after 10 th discharge/charge with carbon and sulfur maps. c, Cross sectional SEM image of the clip coated separator with VISC after 100 th discharge with carbon, sulfur, and silicon maps.
Finally, we investigated the morphology of the coating surface on the clip-coated separator after cycling using SEM. Figure 6a shows the morphology of the cross sectional separator after 10 th discharge and corresponding energy-dispersed X-ray spectroscopy (EDXS) measurements of elemental mapping of carbon, sulfur, and silicon, in which silicon layer and sulfur distribution in the clip structure is clearly seen. The low x-ray intensity for carbon is due to high coverage of sulfur after the discharge, indicating carbon layers are efficiently trapping dissolved LiPS. The structure of the clip coating is preserved upon discharge and charge, indicating the robust properties of the coating in both mechanical and chemical aspects. Also EDXS measurements of top view of the clip coated separator show a uniform distribution of carbon and sulfur elements ( Figure 6b). The fact that the amount of sulfur decreased during the charge process further substantiates the ability of the coating to reutilize the adsorbed species. In addition, the morphology of the clip coated separator with VISC after 100 th cycles is shown in Figure 6c. This double confirms that the multifunctional coatings remain robust after large number of cycles with the presence of high loading and content of sulfur in the Li-S cells. We have also observed decrease in internal resistance of the Li-S cell for clip coated separator compared to pristine separator (See Supplementary Figure S20). The decreased impedance indicated that the clip design is able to facilitate the electron transfer even the insulating silica particles are involved.

Conclusion
In summary, we have demonstrated two new versatile coating methods, LBSDC and LBS, for creating surface films that utilize the surface tension gradient to create well-ordered monolayer films at an air/water interface. The methods allow multifunctional coatings to be created in a range of designs using a wide selection of individual materials, as well as material combinations, on a variety of substrates, without the need for chemical binders. The utility of the approach is illustrated using the polypropylene membrane separator, Celgard, commonly employed in Li-S batteries as a substrate. Through systematic studies, it is shown how mono-functional coatings based on different metal oxides and carbon influence reutilization of dissoleved lithium polysulfide species. An unusual coating configuration termed the "clip", created by stacking an incomplete, but well-formed layer of SiO 2 particles between two complete layers of carbon is used to illustrate both the versatility of the method to create multifunctional coatings with good spatial control and the effectiveness of such coatings in battery separators. In particular, the clip coated separator is observed to exhibit largely improved active material utilization, enhanced capacity retention over extended charge/discharge cycling, and attractive high rate capability.
These observations are explained in terms of the ability of the multifunctional coatings to simultaneously adsorb and trap LiPS created at the cathode, without loosing electrochemical access to the materials. The new coating approach and configurational design of coating materials synergistically work together to advance Li-S cells and allow us to investigate and optimize the different coating structures.

Synthesis
Silica nanospheres: Silica nanospheres were synthesized by the Stober synthesis technique 49  The coated slurry is then dried in a convection oven at 60 o C for 5 hours. The prepared electrode is cut into a circular disk, and the electrode has sulfur loading of 3.5 mg cm -2 with 70 wt% of active sulfur per cathode. VISC -this is a carbon-sulfur cathode created by infusion of sulfur in the vapor phase into a carbon fiber matrix. 21 Sulfur cathodes are prepared by coating the composite material onto a carbon coated Al foil. The cathode has a high sulfur loading (68w%) and a high areal density of sulfur (3.5 mg cm -2 ). After including conductive carbon components in the clip-coated separator (excluding the mass of silica nanospheres), the sulfur contents for BMSC and VISC are 69.31% and 67.35%, respectively. All measurements reported in the study utilize a cathode size of 1.26 cm 2 .

Coating process
Separator: The commercial polypropylene separator (Celgard 2500) was cut into a 1.6 cm diameter circular disk. The separator was placed onto a 1.8 x 1.8 cm microscope cover glass and the ends taped with Kapton tape to control the coating location (Supplementary Figure S22).

Single component separator coating:
Mono/multi layers of Silica nanospheres, multi-walled cabon nanotubes, Ketjen Black carbon, and Super P carbon were coated on the separator using LBS method. Prepared separators are washed with tap water to flush out any impurities stuck onto the surface. One drop of isopropanol (IPA) is applied onto the separator or separator with coating layers to uniformly wet the surface of the separator with water, and the excess IPA on the separator is diluted with water. Then, the fully wetted separator is immersed in water. One of the suspensions is then injected at the surface of the water until more than half of the surface is saturated with the desired nanomaterial; the separator is subsequently raised up to transfer the film followed by a constant injection of the suspension. After that, the coated separator is dried on a hot plate at 110 o C for 30 seconds. Note that no IPA wetting step is required for the silica nanosphere coatings. The single layer coating process is repeated until the desired number of layers is achieved. After the final layer coating, the whole separator is dried on the hot plate for one minute at 110 o C.   Figure S2). The trough was cleaned using pure ethanol and DI water and fully dried with nitrogen gas. The trough was filled with DI water and a 0.5 ml of suspension is injected at the ends of the trough to float particles.
After the injection of the suspension, rest time of ~7 minutes was needed to evaporate excess ethanol from the suspension. Then the resulting film is compressed at the rate of 3 mm min -1 to collect the surface pressure profiles. For obtaining the pressure profile of LBDSC and LBS, ~35 cm 2 and ~25 cm 2 areas are set to mimic actual coating process occurs at the surface from the 50 ml glass beaker. To collect LBDSC surface profile, ~35 cm 2 of the surface is saturated with the silica colloids and rested about 5 minutes to evaporate remaining ethanol. Then, 5 µL of the surfactant is added and the pressure profile is collected at the compression rate of 3 mm min -1 .
For LBS surface profiles, ~25 cm 2 area is fully covered by MWCNT, KB, and SP, and without the rest time, the surface pressure profiles are collected at the compression rate of 3 mm min -1 .
The surfactant surface pressure profile is measured at four different areas (7 cm 2 , 11 cm 2 , 19 cm 2 , 38 cm 2 ) without compressing the barriers, and the 5 µL of the surfactant surface pressures are measured over time.
Galvanostatic charge/discharge: Neware battery testing system is used to perform cycling testing of the Li-S cells. 1.5V to 2.6V and 1.7V to 2.6V voltage windows are used for without/with LiNO 3 electrolyte systems, respectively. 1.7V to 2.6V voltage window is chosen for the LiNO 3 system to preserve LiNO 3 passivation layer on the Li. Supplementary Figure S6: Uniform substrate coating demonstration. Images of fluorescent 1µm silica nanosphere and Super P carbon coatings on various uniform substrates using LBS coating method are shown. Note that other materials can also be coated on these substrates. Figure S7: Non-uniform substrate coating demonstration. Images of Super P carbon and fluorescent 1µm silica nanosphere coatings on razor blade, rough metallic strip, metallic foam, paper, wood toothpicks, curved plastic, tip of glass vials, and micropipette tips using LBS coating method are shown. Note that other materials can also be coated on these substrates. Figure S9: Single-sided separator coating demonstration. Images of zero to ten coating layers of multi-walled carbon nanotube (MWCNT), Ketjen Black carbon (KB), and Super P carbon (SP) on the separator using LBS method are shown.