Directly Formed Alucone on Lithium Metal for High-Performance Li

Jan 30, 2018 - (14-19) However, key technical challenges prevent lithium anodes from commercial application. These include nonplanar electrodeposition...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

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Directly Formed Alucone on Lithium Metal for High-Performance Li Batteries and Li−S Batteries with High Sulfur Mass Loading Lin Chen,†,‡ Zhennan Huang,§ Reza Shahbazian-Yassar,§ Joseph A. Libera,† Kyle C. Klavetter,‡,∥ Kevin R. Zavadil,‡,∥ and Jeffrey W. Elam*,†,‡ †

Energy Systems Division and ‡Joint Center for Energy Storage Research (JCESR), Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States ∥ Materials Science and Engineering, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *

ABSTRACT: Lithium metal is considered the “holy grail” of next-generation battery anodes. However, severe parasitic reactions at the lithium−electrolyte interface deplete the liquid electrolyte and the uncontrolled formation of high surface area and dendritic lithium during cycling causes rapid capacity fading and battery failure. Engineering a dendrite-free lithium metal anode is therefore critical for the development of longlife batteries using lithium anodes. In this study, we deposit a conformal, organic/inorganic hybrid coating, for the first time, directly on lithium metal using molecular layer deposition (MLD) to alleviate these problems. This hybrid organic/inorganic film with high cross-linking structure can stabilize lithium against dendrite growth and minimize side reactions, as indicated by scanning electron microscopy. We discovered that the alucone coating yielded several times longer cycle life at high current rates compared to the uncoated lithium and achieved a steady Coulombic efficiency of 99.5%, demonstrating that the highly crosslinking structured material with great mechanical properties and good flexibility can effectively suppress dendrite formation. The protected Li was further evaluated in lithium−sulfur (Li−S) batteries with a high sulfur mass loading of ∼5 mg/cm2. After 140 cycles at a high current rate of ∼1 mA/cm2, alucone-coated Li−S batteries delivered a capacity of 657.7 mAh/g, 39.5% better than that of a bare lithium−sulfur battery. These findings suggest that flexible coating with high cross-linking structure by MLD is effective to enable lithium protection and offers a very promising avenue for improved performance in the real applications of Li−S batteries. KEYWORDS: molecular layer deposition, direct coating, lithium metal batteries, high mass loading, lithium−sulfur batteries

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INTRODUCTION Rechargeable batteries with high energy density, long cycle life, and excellent safety are needed for advanced portable electronic devices. More importantly, such batteries would facilitate the wide-scale adoption of electric vehicles to reduce the dependency on fossil fuels as well as carbon emissions for a sustainable future.1−8 The graphite anode used in conventional lithium-ion (Li-ion) batteries virtually eliminates the hazards posed by lithium dendrite formation, but the amount of intercalated lithium ions in graphite limits the theoretical capacity to only ∼370 mAh/g and cannot meet societal needs.5,9−11 Therefore, developing higher-energy-density anodes is of paramount importance for next-generation Li-ion batteries for electric vehicles and renewable energy storage.12,13 Lithium metal has been considered as the ideal anode for lithium batteries since the first reports in the 1970s due to its extremely high theoretical capacity (3860 mAh/g), very low electrochemical potential (−3.04 V vs standard hydrogen electrode), and low density (0.534 g/cm3).14−19 However, key technical challenges prevent lithium anodes from commercial © 2018 American Chemical Society

application. These include nonplanar electrodeposition of lithium upon cycling, resulting in the growth of long, needlelike dendrites that can pierce the separator and short the cell producing catastrophic failure,20,21 and parasitic reactions between the lithium metal and the electrolyte, leading to electrolyte depletion.22 These issues are applicable in lithium− sulfur (Li−S) batteries, which are promising for electric vehicles because of the high theoretical energy density of sulfur (2500 Wh/kg).23 For instance, the sulfur cathode in Li−S batteries can form polysulfides as intermediate products during cycling, which can dissolve into the electrolyte and migrate to the anode, where they react with Li metal and cause rapid capacity decay.4 Electrolyte depletion becomes acute in Li−S batteries because their commercial viability in electric vehicles demands a very low electrolyte volume-to-sulfur mass ratio (E/S, mL/g).24 As mentioned above, Li tends to electrodeposit with a rough, Received: October 18, 2017 Accepted: January 30, 2018 Published: January 30, 2018 7043

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic synthesis and characterization of alucone. (a) Schematic of the first cycle of alucone MLD beginning with a hydroxylated surface. (b) Facile MLD process directly coats alucone on lithium metal, forming highly cross-linking-structured alucone on the lithium foil. (c) Transmission electron microscopy (TEM) image of molecular-layer-deposited alucone film on 100 nm SiO2 nanospheres. (d) Energy-dispersive Xray spectroscopy (EDS) mapping of molecular-layer-deposited alucone film on 100 nm SiO2 nanospheres.

increases the modulus of the polymer electrolyte by several orders of magnitude. The authors also coupled their anode with intercalating cathodes for conventional Li-ion batteries. However, it is time-consuming by taking multiple steps to get the synthesized polymer. Molecular layer deposition (MLD) uses alternating exposures between precursor vapors and a surface to deposit thin films in an atomic layer-by-layer fashion, similar to atomic layer deposition (ALD), but yields organic films or organic/inorganic hybrid films.38,39 For instance, alternating exposures to ethylene glycol (EG) and trimethyl aluminum (TMA) yield alucone films that are an order of magnitude more flexible and soft compared to ALD Al2O3,40 while retaining the desirable attributes of conformality, continuity, and subnanometer precision. Previously, molecular-layer-deposited alucone was used to coat sodium metal anodes,41 but there has not yet been a report of molecular-layer-deposited alucone on lithium anodes. Herein, we propose a versatile approach by directly forming highly cross-linked organic/inorganic alucone thin films with MLD on lithium metal and evaluate the battery performance. Ethylene glycol was chosen as the precursor because EG is the monomer of poly(ethylene glycol) (PEO), a stable polymer with good ionic conductivity that is widely used for solid-state electrolytes.42 Thus, the deposited alucone

porous morphology, exposing fresh lithium and consuming electrolyte with each charge−discharge cycle. To tackle these problems, researchers have pursued a number of strategies, including (1) electrolyte additives25−29 designed to form a stable, low-impedance solid electrolyte interphase (SEI) on the Li anode28 and minimize parasitic reactions;29 (2) solid polymer electrolytes and high-modulus separators30−32 that impart compressive forces to stabilize the lithium anode during deposition; (3) thin film coatings to serve as artificial SEI layers.33−36 These coatings must exhibit high ionic conductivity and low electrical conductivity to ensure lithium electrodeposition beneath the coating.34,37 For instance, Yan et al. evaluated ultrathin hexagonal boron nitride and graphene coatings on Cu current collectors for lithium metal batteries (LMBs).21 These coatings increased the Coulombic efficiency (CE) of the LMBs due to their excellent chemical stability and great mechanical strength. One disadvantage of this approach is that the coated Cu must first be incorporated into a Li−Cu cell to deposit Li, after which the cell must be opened to harvest the Li-coated Cu electrode. Furthermore, the LMBs exhibited a CE of only ∼95%, insufficient for practical devices. In a different study, Archer et al. utilized a nanoparticle-based gel polymer as the electrolyte for LMBs.22 The nanoparticles in the polymer enabled cross-linking, which 7044

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

Research Article

ACS Applied Materials & Interfaces

Figure 2. Cycling stability of Li electrodeposition in symmetric cells and scanning electron microscopy (SEM) characterization. (a) Li stripping/ plating in different symmetric cells with 10 μL carbonate electrolyte at 1.0 mA/cm2 up to capacity of 1.0 mAh/cm2. Alucone on Li is ∼6 nm. (b) Li stripping/plating in different symmetric cells with only 5 μL carbonate electrolyte at 1.0 mA/cm2 up to capacity of 1.0 mAh/cm2. (c) SEM image of Li anode after 50 cycles in Li/Li symmetric cell with 20 μL carbonate electrolyte at 1.0 mA/cm2 up to capacity of 1.0 mAh/cm2. (d) SEM image of alucone-coated Li anode after 50 cycles in alucone−Li/alucone−Li symmetric cell with 20 μL carbonate electrolyte at 1.0 mA/cm2 up to capacity of 1.0 mAh/cm2. The scale bar is 10 μm.

MLD using trimethylaluminium (TMA) and ethylene glycol (EG).38 Detailed experimental procedures can be found in Methods. This diagram assumes that the starting surface is terminated with hydroxyl (OH) groups, which is characteristic of most metal oxide surfaces as well as metal surfaces that bear a native oxide. In the MLD process, TMA reacts with surface OH groups to form Al−O linkages and a methyl-terminated surface and liberates methane gas. Next, EG reacts with the Al−CH3 groups to form alkoxy (Al−O−R) linkages and methane, regenerating the hydroxyl-terminated surface. The alucone film continues to grow with each additional TMA/EG cycle, and cross-linking that occurs when TMA bridges neighboring R− OH groups provides great flexibility43 and good mechanical strength.44 Figure 1b presents our facile approach for directly coating alucone on lithium metal. The chemical structure of alucone after multiple MLD cycles, as shown in the inset of this figure, is a cross-linking one. To characterize the alucone thin films prepared by MLD, we performed 100 alucone MLD cycles at 150 °C onto 100 nm SiO2 nanospheres to deposit a film of ∼10 nm and performed

actually consists of the PEO structure that is cross-linked to aluminum as the framework in the hybrid organic/inorganic material. As the electrolyte volume plays a critical role in lithium metal performance, in this work, we tested different electrolyte contents for symmetric cells and Li−Cu cells and found that the ultrathin alucone coating not only stabilizes lithium metal anodes, but also dramatically extends the cycle life of lithium and provides a steady CE of ∼99.5%. We also evaluated alucone-protected Li as an anode for high-massloading Li−S batteries and observed 39.5% improvement in the capacity performance compared to bare Li in the electrolyte volume of 10 (E/S ratio).34 Further evaluation of electrolyte content (E/S = 5) was performed for Li−S batteries, which also showed that alucone is an excellent protective material on lithium for improved performance.

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RESULTS AND DISCUSSION Synthesis and Characterization of Molecular-LayerDeposited Alucone. Figure 1a shows a schematic diagram of the synthesis of hybrid organic−inorganic alucone films via 7045

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

Research Article

ACS Applied Materials & Interfaces

Figure 3. Coulombic efficiency and voltage profile of Li metal stripping/plating on Cu as the working electrode. (a) CE of Li electrodeposition on Cu with 20 μL ether electrolyte at 0.4 mA/cm2. (b) CE of Li electrodeposition on Cu with 20 μL ether electrolyte at 1.0 mA/cm2. (c) Voltage profile of Li/Cu and alucone-coated Li/Cu cells at 0.4 mA/cm2 for the second cycle. (d) Electrochemical impedance spectroscopy (EIS) data for bare Li and ∼6 nm alucone-coated Li paired with Cu as the working electrode in 20 μL ether electrolyte before cycling.

believe that the alucone thicknesses reported on lithium in this study are close to the stated values of ∼3 and ∼6 nm after 30 and 40 MLD cycles, respectively. Li Metal Cycling Stability during Electrodeposition. Symmetric Li|Li coin cells were fabricated with and without alucone coatings to investigate the stability of the Li surfaces during sequential stripping/plating cycles. Figure 2a presents the voltage profiles measured at 1.0 mA/cm2 to a capacity of 1.0 mAh/cm2 using 10 μL electrolyte volume for both ∼6 nm coated and bare Li coin cells. The voltage profile for the alucone-coated Li is much more stable versus time compared to the bare lithium. In contrast, the bare lithium shows a higher polarization during the first ∼10 cycles and the voltage is unstable. These voltage variations probably result from changes in surface morphology upon Li stripping/plating, which would produce a rough and thick solid electrolyte interphase (SEI) that continues to grow with each cycle as fresh Li is exposed. We speculate that the more stable voltage profile for the alucone-coated Li correlates with a stable and uniform SEI. The insets in Figure 2a show enlarged views of the voltage profiles for bare and coated Li coin cells at different periods. The overpotential of the bare Li cells increases with testing time, suggesting that the impedance was increasing due to electrolyte

transmission electron microscopy (TEM, Figure 1c). As we can see, the molecular-layer-deposited alucone is uniform, smooth, and conformal on the SiO2 nanospheres. Energy-dispersive Xray spectroscopy (EDS) mapping was performed to measure the spatial distributions of Al, C, Si, and O. Figure 1d shows a composite image of these elemental distributions and demonstrates that the Al signal is most intense around the perimeter of the particle as expected for the molecular-layerdeposited alucone film. Additional information is provided by elemental line scans in Figure S1, which clearly show O in the outer shell along with the Al and distributions of individual elements. The alucone thicknesses on lithium reported in this study are based on ellipsometric measurements of alucone films on silicon and TEM measurements of alucone films on silica nanoparticles. The reactivities of TMA and EG may be different on lithium compared to silicon and silica, and this will affect the alucone nucleation and the alucone film thickness. However, our previous study of Al2O3 ALD on lithium using TMA and H2O via in situ quartz crystal microbalance measurements revealed that nucleation lasted only four to five ALD cycles, after which the Al2O3 ALD on Li became very similar to that on conventional substrates such as silicon.34 For this reason, we 7046

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

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the anode divided by the amount of lithium stripped in the same cycle, is an effective indicator of side reactions between the Li and electrolyte. Significant parasitic reactions produce a low CE, and vice versa. Thus, CE can also be used to analyze SEI formation and stability. To calculate the CE on a cycle-bycycle basis, we prepared asymmetric Li|Cu coin cells incorporating both bare Li and ∼6 nm alucone-coated Li. Figure 3a shows CE versus charge/discharge cycles using a current rate of 0.4 mA/cm2. The CE is low for both the bare Li and alucone-coated Li cells during the first cycle, presumably due to side reactions between the Li and the electrolyte to form the SEI on both the Li and Cu electrodes. After several cycles, the CE reaches a stable value of ∼99.5%. For the bare Li cell, this high CE value is sustained until the 54th cycle when the CE drops rapidly and the cell fails at ∼60 cycles. This behavior is very similar to that of the Li|Li symmetric cell (Figure 2a), and we attribute this to electrolyte consumption by a rough, dendritic Li metal surface. Evidently, the SEI layer on bare Li is not strong enough to accommodate the volume changes of Li,20 especially after the growth of high-modulus lithium dendrites. In contrast, the molecular-layer-deposited alucone coating allows the CE of ∼99.5% to persist for over 160 cycles, more than 3 times that of uncoated Li. We hypothesize that the highly cross-linked structure of the alucone film is sufficiently robust to endure the Li volume changes. Consequently, the SEI is stable, which prevents rapid electrolyte consumption and promotes uniform and dendrite-free lithium electrodeposition. To evaluate the effectiveness of molecular-layer-deposited alucone protection at higher current rates, the Li−Cu cells were cycled at 1.0 mA/cm2 (Figure 3b). Under these conditions, the bare Li fails after only ∼30 cycles due to the rough lithium deposition caused by fast Li stripping/plating at higher current rates. It is interesting to note that the CE measured for the alucone-coated Li cell is ∼96% after 10 cycles and appeared to fluctuate between 10 and 53 cycles, after which it stabilized at ∼98.5%. These fluctuations may result from the alucone becoming lithiated during cycling because the lithiated alucone is expected to have a higher ionic conductivity than alucone, in analogy to lithiated Al2O3.34 At a relatively large current density of 1.0 mA/cm2, the driving force for lithium diffusion is huge, whereas alucone does not have excellent ionic conductivity at room temperature to realize this process, leading to some Li loss within the coating and a relatively low CE. As mentioned, alucone contains ether groups and should easily solvate Li+ to improve ionic conductivity in the amorphous phase, similar to PEO.15,45 After the alucone becomes lithiated, the ionic conductivity should increase yielding a higher and more stable CE. At a high current density of 1.0 mA/cm2, the aluconecoated Li survived more than 130 cycles, ∼4 times that of bare Li. The voltage difference (hysteresis) between the stripping and plating plateaus within a charge−discharge cycle can provide information about the nonideality of a battery. For instance, a larger hysteresis indicates a larger cell impedance and slower lithium-ion diffusion. Figure 3c depicts the voltage hysteresis recorded during the second charge−discharge cycle for Li−Cu asymmetric coin cells prepared with and without the molecularlayer-deposited alucone coating cycled at 0.4 mA/cm2 and using 20 μL electrolyte. The alucone-coated Li has a much smaller voltage difference (∼10 mV) in contrast to the bare Li (∼20 mV). This suggests that the molecular-layer-deposited alucone coating has a lower impedance than the SEI layer that forms on the bare Li surface during cycling. As discussed, we

consumption and thicker SEI formation. Conversely, the smaller overpotentials for the alucone-coated Li indicate small impedance changes and suggest that the electrolyte was preserved by the conformal alucone coating and SEI layer. During these measurements, the overpotential of the aluconecoated Li is consistently lower than that of the bare Li, illustrating that the alucone coating creates a stable system for lithium anode operation. Even after the bare lithium failed with short circuit, indicated by a sudden voltage drop at ∼158 h, the alucone-protected Li maintained stable operation. We examined several different alucone thicknesses on Li and compared their cycling stability using symmetric cells, as shown in Figure S2 (Supporting Information), in which 30 and 60 cycles of alucone correspond to ∼3 and ∼6 nm, respectively. Both alucone coatings were found to provide much better cycling stability compared to the uncoated Li anode, but the ∼6 nm alucone coating yielded a slightly lower overpotential over the entire testing period. Thus, ∼6 nm alucone was selected for all other measurements in this study except where noted. A recent study highlighted the need for high sulfur loadings and low electrolyte volumes for Li−S batteries to compete with conventional LIBs.24 Consequently, we studied the Li stripping/plating performance with and without the alucone coatings as a function of electrolyte volume. Figures 2b and S2 display voltage profiles of symmetric cells tested at a current density of 1.0 mA/cm2 to a capacity of 1.0 mAh/cm2 using 5 and 20 μL carbonate electrolytes, respectively. In both cases, the uncoated Li cell begins to exhibit voltage instability much sooner than the alucone-coated Li. In fact, the lifetime of the bare Li cells decreases with decreasing electrolyte volume, which is consistent with our interpretation that cell failure is caused by the consumption of the liquid electrolyte and associated short circuits. This problem is greatly reduced by the protective alucone coatings, indicating that these coatings reduce the rate of electrolyte consumption. Figure 2c,d shows scanning electron microscopy (SEM) images of bare Li metal and ∼6 nm alucone-coated Li, respectively, after 50 charge−discharge cycles in symmetric cells using the same testing conditions as in Figure S2. The bare Li surface is extremely rough and porous and shows lithium dendrites (Figure 2c), whereas the protected Li is smooth and relatively featureless (Figure 2d). High-resolution SEM images are shown in Figure S3. This observation argues that the conformal alucone protects the Li anode even after many cycles at a high current density of 1.0 mA/cm2. Moreover, the molecular-layer-deposited alucone film promotes planar electrodeposition of the Li without forming dendrites. We note that the 1.0 mAh/cm2 charge/discharge capacity corresponds to the movement of ∼5 μm Li during each charge or discharge compared to a molecular-layer-deposited alucone thickness of only ∼6 nm. Evidently, the highly cross-linked MLD film possesses sufficient mechanical strength and flexibility to withstand these large and fast volume changes. It should be mentioned that alucone-coated Li can also improve the electrolyte wettability that can help with uniform Li stripping/plating processes, as elaborated in our previous report.34 For batteries using lithium metal anodes, the Li continuously degrades the liquid electrolyte by parasitic chemical reactions and electrochemical reduction. Rough Li deposition and dendrite growth exacerbate the side reactions because fresh Li metal is exposed during every cycle. Coulombic efficiency (CE), calculated from the amount of lithium deposited onto 7047

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

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ACS Applied Materials & Interfaces

Figure 4. Electrochemical measurements of Li−S batteries with an E/S ratio of 10. (a) Cycling performance of Li−S batteries encompassing bare Li or alucone-coated Li with sulfur mass loading of ∼5 mg/cm2 and E/S = 10 at 0.1 C. (b) Voltage vs capacity profile at the 20th cycle. (c) Voltage vs capacity profile at the 60th cycle. (d) Voltage vs capacity profile at the 120th cycle.

high sulfur loading (∼5 mg/cm2). As discussed above, the electrolyte content in practical Li−S batteries must be very low (i.e., E/S < 11.1)24 and thus we used E/S values of 10 and 5 in these experiments. Figure 4a shows the cycling performance of bare Li−S and ∼6 nm alucone-coated Li−S batteries at a current density of ∼1 mA/cm2 (corresponding to 0.1 C) with E/S = 10. As seen in Figure 4a, the initial capacities for both cells are low because the sulfur electrode used in this work is very thick and so the electrolyte does not completely wet the active materials. After three to five cycles, the capacities drastically increase, which is likely due to more complete wetting of the active materials relocated well in the cathode during cycling. Both cells exhibit capacity fluctuations, and we attribute this behavior to the poor electronic and ionic conductivities of the thick sulfur laminates. The capacity becomes more stable with long-term cycling. For the Li−S battery with bare Li, it only retains a capacity of 471.6 mAh/g after 140 cycles, whereas the alucone-protected Li−S battery delivered a higher capacity of 657.7 mAh/g, representing a significant improvement of 39.5%. To assess the electrochemical reactions during cycling, voltage−capacity curves were measured for both cells at different cycle stages (Figure 4b−d). Voltage plateaus in the

speculate that the highly cross-linking structure and ether functional groups (−CH2−CH2−O−) of the alucone film effectively solvate Li+ similar to PEO15 and provide efficient pathways for Li+ to diffuse. The experiment was repeated using an electrolyte volume of only 10 μL to explore the cycling behavior under lean conditions. As shown in Figure S4, the alucone-coated Li still demonstrates much better stability, which does not degrade within 67 cycles, compared to initial failure of bare Li from the 18th cycle. These results are highly desirable for practical rechargeable batteries, such as Li−S batteries. Electrochemical impedance spectroscopy (EIS) measurements of uncoated and ∼6 nm alucone-coated Li paired with Cu in coin cells are displayed in Figure 3d. The semicircle at high frequency represents the resistance of charge transfer (Rct).46 Compared to uncoated Li, the alucone-coated Li has a larger Rct, indicating that alucone increases the electronic resistance. This is desirable for lithium protection because it prevents electrons from attacking the electrolyte34,47 and also induces the Li to redeposit underneath the alucone coating. High-Mass-Loading Li−S Batteries with Controlled Electrolyte Volume. To evaluate the performance of aluconecoated lithium in full cells, we prepared Li−S coin cells with 7048

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

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ACS Applied Materials & Interfaces

coating, exposing bare lithium to the electrolyte. Under the lean electrolyte conditions used in these experiments, the formation of new SEI on the bare Li causes localized electrolyte depletion and it takes time for fresh electrolyte to diffuse to these depleted regions. After ∼70 cycles, both cells return to a stable cycling behavior, but the alucone-protected Li−S battery capacity is ∼500 mAh/g, compared to a lower capacity of ∼450 mAh/g for the bare Li−S battery. It is surprising that the positive benefit of the ultrathin alucone coating persists after 150 charge−discharge cycles, some of which were performed at high rates.

charge process represent the conversion from lithium sulfide to sulfur, whereas the two plateaus during discharge are related to the change of sulfur to high-order polysulfides (Li2Sx, 4 ≤ x ≤ 8) at a voltage of ∼2.3 V and the reduction from high-order polysulfides to low-order polysulfides at ∼2.1 V.15 In Figure 4b, we can see that the second discharge plateau is at ∼1.85 V, much lower than at the expected value of ∼2.1 V, suggesting a voltage polarization for both cells, which we attribute to the large impedance of the sulfur cathodes. However, a relatively small voltage hysteresis (0.46 V) is found for the alucone-coated Li, in comparison to a larger polarization of 0.50 V for the bare Li. Given that all testing conditions are identical, the larger overpotential for the bare Li probably results from a thicker SEI and more polysulfides deposited on the anode. After 60 cycles, the potential difference between the charge and discharge processes for the conversion reactions of short-chain polysulfides (Li2Sx, 1 ≤ x < 4) and Li2S46 increases to 0.55 V for bare Li (Figure 4c). This indicates that the electrochemical reaction is slower because of more SEI and polysulfide deposition on the anode. In contrast, the voltage hysteresis remains constant at 0.46 V for the aluconecoated Li after 60 cycles, suggesting a more stable SEI and much less polysulfide deposition on the anode. After 120 charge−discharge cycles, the overpotential in the first plateau for alucone-coated Li increases slightly to 0.5 V (Figure 4d). However, the discharge voltage for the second plateau remains practically constant between 60 cycles (1.91 V) and 120 cycles (1.90 V). In fact, the specific capacity yielded a slight increase from 636 mAh/g at the 60th cycle to 641 mAh/g at the 120th cycle. It is clear that the increased overpotential has a negligible effect on the discharge capacity but a significant effect on the charge process. Meanwhile, it seems that the sulfur utilization is better with cycling instead of degradation. For the bare Li anode after 120 cycles, the overpotential increases, as expected, to 0.6 V. An extra discharge plateau can be observed at ∼1.7 V indicated by the blue arrow in Figure 4d, corresponding to the irreversible reduction of lithium nitrate in the electrolyte.48,49 This feature is usually only seen in the initial cycles,46 where lithium nitrate plays a key role to form a stable passivation layer on the lithium surface.40,50,51 Hence, it is surprising that this feature appears at 120 cycles. We speculate that it results from lithium dendrites breaking and exposing fresh lithium that reacts with the lithium nitrite to form a new SEI. It should be pointed out that the irreversible reduction of LiNO3 contributes to the final discharge capacity, which is not sustainable once this additive is consumed, which means that the actual enhancement of alucone-coated Li on the battery capacity is more than 34% compared to the bare Li cell. We further evaluated the rate performance of Li−S batteries with bare Li and alucone-coated Li (Figure S5). These experiments used E/S = 5 to explore realistic conditions for commercial Li−S batteries. The initial capacity for the ∼6 nm alucone-coated Li−S battery is 987 mAh/g compared to only 833 mAh/g for the bare Li cell. The lower capacity for the bare Li−S cell might be caused by parasitic reaction of polysulfides with bare Li. The measurements at different rates from 0.2 C (∼1 mA/cm2) to 2 C (∼10 mA/cm2) show similar capacity trends for both cells, probably due to the poor conductivity of the relatively thick sulfur cathodes. After cycling at 2 C, both batteries become unstable for about 40 cycles. We do not fully understand this behavior, but it might result from the high current rate (2 C, corresponding to ∼10 mA/cm2) driving fast Li+ stripping and platting that disrupts the SEI and the alucone

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CONCLUSIONS In summary, MLD has been employed to prepare ultrathin organic/inorganic alucone films on lithium for the first time in an effort to address the technical challenges associated with lithium metal anodes. We speculate that the conformal, flexible alucone coatings reduce undesirable surface reactions and promote a more uniform lithium electrodeposition and a more stable SEI layer. This coating permits several times longer cycling life compared to uncoated Li and yields a stable Coulombic efficiency of 99.5% even at a large current density. The alucone coating also reduces the voltage polarization during lithium stripping/plating in both Li−Cu cells and Li−S batteries. More significantly, the protected Li accomplishes much improved performance in Li−S batteries with a high sulfur mass loading of ∼5 mg/cm2 and a very low value of E/S = 5. The alucone-protected Li offers a promising avenue toward high-performance lithium anodes suitable for high-energydensity Li−S batteries and other energy-storage devices using lithium metal anodes.

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METHODS

Molecular-Layer-Deposited Alucone on Li. Commercial lithium was punched into lithium foil disks with a diameter of 7/16 in. in an Ar-filled glovebox (O2 and moisture levels lower than 0.5 ppm) and gently pressed to a flat surface. MLD was conducted in a custom, viscous flow reactor, with the reaction zone consisting of a heated 5 cm diameter stainless steel tube.52 The pressure within the reactor was maintained at ∼1 Torr using a 216 sccm flow of ultrahighpurity (99.999%) argon carrier gas, and the reactor temperature was 150 °C. The reactor was attached to a glovebag to avoid air exposure of Li foils. Alucone MLD was conducted using TMA and EG as precursors. TMA was dosed through a 100 μm diameter orifice to achieve a TMA partial pressure of ∼20 mTorr within the reactor. The alucone MLD timing sequences are expressed as: t1−t2−t3−t4, where t1 is the TMA exposure time, t3 is the EG source exposure time, and t2 and t4 are the respective purge times, with all times in seconds. The time sequence used here for alucone on Li is 1−5−1−15. Alucone coating thicknesses of ∼3 and ∼6 nm were prepared using 30 and 40 alucone MLD cycles, respectively. After MLD coatings, the samples were loaded into a sealed mason jar and immediately transferred to the glovebox. Characterization. Symmetric cells with 20 μL carbonate electrolyte after 50 cycles at 1 mA/cm2 and 1 mAh/cm2 were disassembled in the glovebox, and the Li anodes in the negative side were gently rinsed with 1,2-dimethoxyethane (DOL) three times to remove the electrolyte residue and then kept in vacuum, followed by treatment at 60 °C to dry out the solvent.23 A field emission scanning electron microscope (JEOL 7500 model) at Center for Nanoscale Materials (CNM) at Argonne was used for the microscopy study of lithium samples.34 To get TEM samples, 100 nm SiO2 nanoparticles were warped onto holy carbon grids and loaded into the reactor for alucone coating. The structure was analyzed using a spherical aberrationcorrected JEOL JEM-ARM200CF scanning transmission electron microscope equipped with a 200 kV cold field emission gun, annular 7049

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

Research Article

ACS Applied Materials & Interfaces bright-field (ABF) and high-angle annular dark-field detectors, and an energy-dispersive spectrometer using windowless EDS 100 mm Oxford detector. Sulfur Cathode Preparation. An 80:20 wt % mixture of sulfur (Sigma) and carbon nanotube (CNT, Cheap Tubes) was ground in a mortar and pestle until homogenized. The resulting mixture was transferred into a round-bottom flask, which was then evacuated to ∼10 mTorr. The mixture was then heated in a furnace to 150 °C for 18 h for sulfur infusion. After infusion, the S/CNT composite was ballmilled for 18 h to produce a fine homogenous powder. The PEO binder was fabricated by mixing 60 wt % PEO (Sigma) with 40 wt % lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Sigma) in acetonitrile. The final concentration of this mixture was 4 wt % PEO. This mixture was heated to 60 °C and mixed for 18 h. Cathode slurries were prepared by mixing 80 wt % S/CNT composite with 10 wt % Super P and 10 wt % PEO in excess acetonitrile to control viscosity. The slurries were mixed in a Cowles blender until a homogeneous slurry was obtained. The slurry was coated with a standard elcometer and doctor blade to 30 mil (∼750 μm) wet film thickness and allowed to dry. Dried coatings were then placed in a vacuum oven and dried under vacuum overnight. This process results in an average sulfur loading of 6 mg/cm2 with a composition of 64% S, 26% C, and 10% binder. Electrochemical Measurements. Bare Li foil and alucone-coated Li foils with a diameter of 7/16 in. were used, and 2032-type coin cells were fabricated in an Ar-filled glovebox for symmetric and asymmetric (Li−Cu) cell measurements. For symmetric cells, Li ions are employed on both sides as working and countering electrodes, where carbonate electrolyte with 1 M LiPF6 in ethylene carbonate/ethyl methyl carbonate (3:7) from BASF was used. Different current densities and capacities were set for Li−Li symmetric cells. As for Li−Cu tests, Cu functions as the working electrode and Li is the countering electrode, where 1 M LiTFSI with 0.18 M Li2S8 and 2 wt % LiNO3 in dioxolane (DOL)/DME (1:1, volume ratio) were used. Li−Cu cells were initially discharged at a current density of 1 mA/cm2 to 1 mAh/cm2, followed by being stripped from Cu to 1 V at 1 mA/cm2. As for Li−S battery measurements, uncoated and coated Li with a diameter of 7/16 in. were used to pair with sulfur cathode, where electrolyte is also 1 M LiTFSI with 0.18 M Li2S8 and 2 wt % LiNO3 in DOL/DME (1:1, volume ratio). The specific capacity is calculated on the basis of sulfur mass, and mass loadings of sulfur are ∼5 and ∼3 mg/cm2. The battery separator used in this work is Celgard 2325, and all cycling measurements were done in Arbin 2043 and LAND instrument. EIS of Li/Li cells before cycling was measured by a Solartron 1260 impedance/gain phase analyzer combined with a Solartron 1287 electrochemical interface. The frequency is from 1 MHz to 0.1 Hz with the amplitude of 5 mV.

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ization. Z.H. and R.S.-Y. conducted TEM determinations, and K.C.K. and K.R.Z. prepared sulfur cathodes and were also involved in insightful discussions. L.C. and J.W.E. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES). Use of the Center for Nanoscale Materials, including resources in the Electron Microscopy Center, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02−06CH11357. Dr. Christopher Johnson at Argonne and Dr. Jason Zhang at Pacific Northwest National Laboratory are highly appreciated for providing the carbonate electrolyte and the ether electrolyte, respectively. R.S.-Y. acknowledges the financial support from NSF-DMR award number 1620901.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15879. Scanning line detection by ABF TEM, EDX imaging, symmetric cell testing, asymmetric cell testing, lithium− sulfur battery testing at different cycling rates (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Reza Shahbazian-Yassar: 0000-0002-7744-4780 Jeffrey W. Elam: 0000-0002-5861-2996 Author Contributions

L.C. and J.W.E. conceived and designed the experiments with help from J.A.L., who prepared the samples. L.C. performed the electrochemical measurements and conducted SEM character7050

DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051

Research Article

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DOI: 10.1021/acsami.7b15879 ACS Appl. Mater. Interfaces 2018, 10, 7043−7051