Self-Formed Hybrid Interphase Layer on Lithium Metal for High

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Self-Formed Hybrid Interphase Layer on Lithium Metal for High-Performance Lithium-Sulfur Batteries Guoxing Li, Qingquan Huang, Xin He, Yue Gao, Daiwei Wang, Seong H. Kim, and Donghai Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08035 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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Self-Formed Hybrid Interphase Layer on Lithium Metal for High-Performance LithiumSulfur Batteries Guoxing Li1, Qingquan Huang1, Xin He2, Yue Gao1, Daiwei Wang1, Seong H. Kim2, Donghai Wang1* 1

Department of Mechanical and Nuclear Engineering, The Pennsylvania State University,

University Park, Pennsylvania 16802, USA. 2

Department of Chemical Engineering, The Pennsylvania State University, University Park,

Pennsylvania 16802, USA. E-mail: [email protected]

Abstract Lithium-sulfur (Li-S) batteries are promising candidates for high-energy storage devices due to high theoretical capacities of both the sulfur cathode and lithium (Li) metal anode. Considerable efforts have been devoted to improving sulfur cathodes. However, issues associated with Li anodes, such as low Coulombic efficiency (CE) and growth of Li dendrites, remain unsolved due to unstable solid-electrolyte interphase (SEI) and lead to poor capacity retention and short cycling life of Li-S batteries. In this work, we demonstrate a facile and effective approach to fabricate a flexible and robust hybrid SEI layer through co-deposition of aromatic-based organosulfides and inorganic Li salts using poly(sulfur-random-1,3-diisopropenylbenzene) as an additive in an electrolyte. The aromatic-based organic components with planar backbone conformation and π-π interaction in the SEI layers can improve the toughness and flexibility to promote stable and high efficient Li deposition/dissolution. The as-formed durable SEI layer can inhibit dendritic Li growth, enhance Li deposition/dissolution CE (99.1% over 420 cycles), and in turn enable Li-S batteries with good cycling stability (1000 cycles) and slow capacity decay. 1 ACS Paragon Plus Environment

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This work demonstrates a route to address the issues associated with Li metal anodes and promote the development of high-energy rechargeable Li metal batteries. Key Words: lithium metal anodes, solid-electrolyte interphase, lithium-sulfur battery, lithium organosulfides, lithium organopolysulfides.

Lithium-sulfur (Li-S) batteries as promising high energy storage devices have been attracting increased attention in recent years. Naturally abundant, low-cost sulfur cathodes and Li metal anodes which have the highest specific capacity (3860 mA h g-1) and lowest potential (-3.040 V as a standard hydrogen electrode) endow Li-S batteries with a high theoretical energy density.1-3 In spite of these advantages, practical application of Li-S batteries is still hampered by challenges from both cathodes and anodes. The issues associated with sulfur cathodes, such as poor conductivities of sulfur, dissolution of Li polysulfides, and irreversible deposition of Li2S, lead to the low Coulombic efficiency (CE), fast capacity decay, and short cycling life.4, 5 Many efforts have been made to overcome these issues associated with sulfur cathodes. Various carbon materials are utilized to enhance the overall conductivity and trap the soluble polysulfides.6-10 Electrolytes, additives and binders are optimized to mitigate the dissolution of polysulfides.11-16 Porous structure electrodes are fabricated to sustain the polysulfide dissolution.17 Interlayers and separators are used to block the shuttling of the polysulfides.18-20 Recently, our group reported a functional electrolyte system containing reactive organosulfides to promote an alternate electrochemical reaction pathway for sulfur cathodes by a formation of new redox intermediates and boost the capacity of Li-S batteries.21, 22

2 ACS Paragon Plus Environment

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Despite the significant progress made to improve the sulfur cathode, the challenges associated with Li metal anodes remain and hinder the application of the Li-S batteries, these Li anode issues are much more difficult to resolve and can directly lead to poor cycling stability and safety issues. As Li metal is highly reactive, fresh Li metal surfaces exposed during cycling react with electrolyte to form a solid-electrolyte interphase (SEI) layer, and the non-optimized SEI layer is intrinsically brittle and cracks during Li deposition/dissolution process,23,

24

leading to a

continuous consumption of electrolyte and low CE. Additionally, the Li deposition is not even due to an inhomogeneous local current density on the electrode,25-27 which leads to growth of dendritic Li, the Li dendrite can penetrate separators and cause serious safety issues. Among the many approaches developed to address the Li metal anode issues are electrolyte additives,28-34 optimized electrolyte,15,

35-38

artificially protective layer,39-44 and modification of current

collector.45-49 Based on these methods, relatively high Li deposition/dissolution CEs can be achieved at either a low deposition capacity or a low current density.44, 50 Recently, our group has developed a strategy to fabricate stable organic/inorganic hybrid SEI layer using a sulfurcontaining polymer to achieve dendrite-free Li deposition and high Li deposition/dissolution CE.51 Despite this promising result using the sulfur-containing polymer, it is desirable to optimize the polymer to further improve the CE and cycling stability at higher deposition capacity and current density to enable an extended cycling life of Li metal for high-energydensity Li-S battery. Here, we report an aromatic-based sulfur-containing polymer, i.e., poly(sulfur-random-1,3diisopropenylbenzene) (PSD) that can be chemically/electrochemically decomposed to self-form flexible and robust organic/inorganic hybrid SEI layer on Li metal. The aromatic-based organic components with planar backbone conformation and π-π interaction can facilitate the formation


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of flat morphology, improve the toughness and flexibility; the inorganic components (Li2S/Li2S2) provide Li conductive pathways and necessary mechanical hardness. The optimized stable and flexible SEI layer can effectively promote uniform deposition of Li metal and enhance CE. With this hybrid SEI layer, we demonstrate that a dendrite-free Li deposition/dissolution with high average CE of 99.1% can be maintained for 420 cycles (2 mA cm-2, 1 mA h cm-2). At higher capacities of 2 mA h cm-2 and 3 mA h cm-2 with a current density of 2 mA cm-2, the average CE can be as high as 99% and 98.9% over 250 cycles, respectively. The improved electrochemical performance over our previous work51 indicates the effectiveness of aromatic-based sulfurcontaining polymer in achieving high efficiency of Li metal anode. Based on this approach, the stable cycling stability (1000 cycles) and excellent capacity retention of Li-S battery are demonstrated.

Figure 1. Mechanism of self-formation of hybrid SEI layer. (a) Aromatic-based organic components and inorganic components generated from PSD after reacting with Li metal in the electrolyte.

(b)

Self-formation

of

the

stable

hybrid

SEI

layer

composed

organosulfides/organopolysulfides-Li2S/Li2S2 using the PSD as an electrolyte additive.


of

Li

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Results and Discussion PSD contains sulfur chains connected by the aromatic components (Figure 1a), and can be prepared through a direct copolymerization of liquid sulfur and 1, 3-diisopropenylbenzene (DIB).52,

53

PSD was used as an additive and added in the electrolyte. As PSD can

electrochemically release aromatic-based components (Li organosulfides/organopolysulfides) and inorganic Li components (Li2S/Li2S2) once contacting fresh Li metal surface (Figure 1a), an organic/inorganic hybrid SEI layer can, therefore, be self-formed through the co-deposition of the organic and inorganic components (Figure 1b). The aromatic-based components show planar backbone conformation (Figure S1a-b), which facilitates fabrication of the flat and uniform structure of SEI layer during the co-deposition. The π-π interaction between benzene rings can further improve the toughness and flexibility of SEI layer. PSDs with various sulfur contents (50 wt%, 70 wt% and 90 wt%, designated as PSD-50, PSD-70 and PSD-90, respectively) were prepared and used as additives in the electrolyte. The solubility of PSDs in electrolyte decreases with the increment in sulfur content of the polymer. When PSD-90 is added in the electrolyte, a mixture is formed. However, it becomes homogeneous after the first cycle of Li deposition/dissolution due to the chemically/electrochemically reaction with Li metal (Figure S2). The corresponding cycling performance of Li deposition/dissolution was first investigated. 1 M LiTFSI

(lithium

bis(trifluoromethanesulfonyl)imide)

and

1

wt%

LiNO3

in

dioxolane/dimethoxyethane (DOL/DME, V/V = 1) was used as an electrolyte here. It is found that the cells using PSD-90 as an additive exhibit the best cycling stability which drops when sulfur content of PSDs decreases (e.g., PSD-70, PSD-50), as shown in Figure S3. Therefore, the PSD-90 was used as the additive in the electrolyte to inhibit the growth of dendritic Li and enhance the CE. 1 M LiTFSI and 4 wt% LiNO3 in DOL/DME (V/V = 1) (named C-Ely) was


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used as a control electrolyte. 8 wt% PSD-90 was added in the control electrolyte (designated as PSD-90-Ely) and used in the following study. Figure 2 shows scanning electron microscopy (SEM) images of the deposited Li metal after 10 cycles of Li deposition/dissolution. For the CEly, both the top (Figure 2a-b) and the cross-section view (Figure 2c) show that dendritic and mossy Li grows on the whole electrode. When PSD-90-Ely was used, the deposited Li shows a continuous, uniform and highly packed morphology not accompanied by any dendritic and mossy Li (Figure 2d-e). The cross-section view displays the compact structure without any dendrite within Li layer (Figure 2f). Even after 100 cycles, the deposited Li still shows smooth surface and compact interior (Figure S4a-b). The growth of the compact and dendrite-free Li should be contributed to the property change of the SEI layer. In comparison to Li morphologies using PSD-90-Ely, much dendritic and mossy Li is observed using the C-Ely after 100 cycles (Figure S4c-d).


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Figure 2. SEM images of deposited Li metal in the C-Ely (a-c), and PSD-90-Ely (d-f) after 10 cycles of Li deposition/dissolution. Current density: 2 mA cm-2; deposition capacity: 2 mA h cm2

.

Figure 3. S 2p (a), C 1s (b), and F 1s (c) XPS spectra of PSD-90-SEI and C-SEI layers. The dotted lines in the S 2p spectra are the 2 p1/2 components of spin-orbit splitting. The composition of the SEI layers was investigated using X-ray photoelectron spectroscopy (XPS). The SEI layers were obtained from C-Ely and PSD-90-Ely after 100 cycles of Li deposition/dissolution and named C-SEI and PSD-90-SEI, respectively. Evidenced from S 2p XPS spectra (Figure 3a), two strong peaks at 160.5 and 161.7 eV for the PSD-90-SEI indicate the presence of Li2S and Li2S2;37, 54 and the peak located at 162.2 eV which corresponds to the S 2p3/2 for Li organosulfides (RS4Li4) confirms the Li organosulfides exist in the PSD-90-SEI layer.21 The peak at 163.3 eV is assigned to Li polysulfides (Li2Sx) and organopolysulfides (RSxLi4) which overlap and make the peak stronger.21 The 2p XPS spectra of PSD-90-SEI reflect the existence of Li organosulfides/organopolysulfides along with the Li2S/Li2S2 in the SEI layer. The C 1s XPS spectra of C-SEI (Figure 3b) contains a peak at ~292.1 eV assigning to the functional group –CF3 from the decomposition of LiTFSI. This peak almost disappears when


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PSD-90 is added in the electrolytes (PSD-90-Ely), indicating the decomposition of LiTFSI is dramatically suppressed in the presence of PSD-90. Moreover, the F 1s XPS spectra (Figure 3c) display a relatively lower intensity of peak assigned to –CF3 in the PSD-90-SEI, reconfirming the suppression of LiTFSI decomposition in the presence of PSD-90. Figure S5 shows that Fourier-transform infrared (FT-IR) spectra of C-SEI and PSD-90-SEI are quite different. The FT-IR spectra of PSD-90-SEI show prominent peaks at 1600, 1575 and 1558 cm-1, which correspond to the vibration of carbon-carbon double bond of the phenyl group. The peak at ~790 cm-1 is the vibration characteristic of C-H bond of the 1, 3-substituted phenyl groups, which can prove the presence of 1, 3-substituted phenyl groups. All of the peaks can also be found in the pure PSD-90, demonstrating that the organosulfides originate from the PSD-90.


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Figure 4. AFM images and indentation study of SEI layers obtained from C-Ely (a, c) and PSD90-Ely (b, d). SEM images of SEI layers obtained from C-Ely (e) and PSD-90-Ely (f). The scan size for AFM images is 10 × 10 µm2. The presence of aromatic-based organic components in the hybrid SEI layer changes its mechanical properties, which can be evidenced by atomic force microscopy (AFM). Surface morphologies of the C-SEI and PSD-90-SEI layers studied by AFM (Figure 4a and b) and SEM


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(Figure 4e and f) consistently show a porous, fractured, and loose structure observed in C-SEI (Figure 4a and e), in comparison, a mostly planar, smooth and uniform structure in PSD-90-SEI layer (Figure 4b and f). The root mean square (RMS) roughness of PSD-90-SEI layer is around 106 nm. The morphological difference indicates a continuous crack in C-SEI layer during Li deposition/dissolution and improvement of the integrity and robustness of PSD-90-SEI layer. Peak force tapping AFM mode was utilized to study the mechanical properties. Indentation curve of C-SEI (Figure 4c) shows the loading and unloading curves almost overlap and the slopes are very high, implying that the C-SEI layer is rigid and its viscoelasticity is negligible. The reduced modulus of the C-SEI is calculated to be 903 MPa from the Johnson-Kendall-Roberts (JKR) model fit (Table S1, Note S1 and Figure S6.). By contrast, a more substantial deformation and hysteresis between the loading and unloading curves of PSD-90-SEI layer (Figure 4d) reflects flexible characteristic under the mechanical deformation. The PSD-90-SEI layer exhibits a lower modulus of 340 MPa (Table S1), indicating this SEI layer becomes soft and viscoelastic. The different behaviour between the PSD-90-SEI and C-SEI is ascribed to the presence of organic Li organosulfides/organopolysulfides that improve the flexibility of PSD-90-SEI layer. Compared with the hybrid SEI layer originating from our recent development of poly(sulfur-randomtriallylamine) (PST) polymer,51 whose RMS roughness and reduce modulus are 157 nm and 367 MPa, respectively, PSD-90-SEI layer shows much more planar and smooth morphology and lower reduced modulus. This may be attributed to the benzene-ring-based components having planar backbone conformation (Figure S1a-b) rather than a branched structure of PST polymer (Figure S1c-d), facilitating the initial “growth” of a flat and uniform morphology of hybrid SEI layer and formation of a planar and smooth structure (as shown in Figure 4b). Meanwhile, the non-covalent π-π interaction between benzene rings can promote self-assembly of the aromatic


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components to further improve the toughness and flexibility of the SEI layer, and in turn, decrease the reduced modulus. Optical profilometry investigation (Figure S7) shows the thickness of PSD-90-SEI layer is low (~10 µm) and the coverage is around 98%. The high coverage reflects the PSD-90-SEI layer is stable to maintain the SEI integrity after cycles. All of these are beneficial for improving the Li deposition morphology, mitigating the growth of dendritic Li and thus enhancing CE.


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Figure 5. Cycling performances of Li deposition/dissolution using the PSD-90-Ely and C-Ely at a current density of 2 mA cm-2 and a deposition capacity of 1 (a), 2 (b) and 3 mA h cm-2 (c). (d) Voltage profiles (the first cycle) and (e) average voltage hysteresis of (b).


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The stable SEI layer fabricated from PSD-90-Ely enables a compact and uniform Li deposition

as

we

discussed

above,

and

in

turn,

dramatically

improves

the

Li

deposition/dissolution cycling performance. We further investigated the cycling performance of Li deposition/dissolution using the electrolytes with various amount of PSD-90. The CE of the Li deposition/dissolution is calculated by the ratio of Li stripped from substrates to that deposited during the same cycle.

37, 38

Electrolytes containing 2 wt%, 5 wt% and 10 wt% PSD-90 were

prepared and compared with PSD-90-Ely (8 wt% PSD-90). It is observed from Figure S8 that PSD-90-Ely shows the best cycling life among them, indicating the cycling performance is influenced by PSD-90 content of the electrolyte. The cycling performance of Li deposition/dissolution using PSD-90-Ely at different deposition capacities with a current density of 2 mA cm-2 was shown in Figure 5. At a capacity of 1 mA h cm-2, a high average CE of 99.1% can be maintained for 420 cycles (Figure 5a). At a higher capacity of 2 mA h cm-2, an enhanced average CE of 99% for 250 cycles can be obtained (Figure 5b). When a capacity as high as 3 mA h cm-2 was used, the cells still exhibit an impressive CE as high as 98.9% with a stable cycling over 250 cycles (Figure 5c). Notably, at an extremely high capacity and current density (4 mA h cm-2 and 4 mA cm-2), a high average CE of ~97% can be maintained over 150 cycles (Figure S9). Compared with our PST polymer,51 the CE and cycling stability is further improved, probably ascribed to the different structures of the aromatic organic species. In PSD-90-SEI layer, the benzene-ring-based components with planar backbone conformation originating from PSD-90 polymer facilitate the formation of more durable and flexible hybrid SEI layer with flat and uniform morphology (Figure 4b and d). The voltage profiles of Li deposition/dissolution (Figure 5d) indicate the cells using PSD-90-Ely show lower polarization than those with C-Ely. Moreover, in contrast to the cells using C-Ely, the batteries using PSD-90-Ely show more stable


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and smaller voltage hysteresis at ~47 mV over 200 cycles (Figure 5e). Both the low polarization and the modest and stable hysteresis are mainly due to the durable and flexible SEI layer that enables the compact and uniform Li deposition leading to thin and uniform SEI accumulated over the electrode surface. The impedance evolution in the cells during cycling was explored by electrochemical impedance spectroscopy (EIS), which can evidence the formation and accumulation of SEI layer. The depressed semicircle of Nyquist plot in Figure S10 corresponds to the formed SEI layer.55 As to the cells using PSD-90-Ely (Figure S10a), the diameter of semicircle has no distinct change with the increment in cycle number, implying the as-formed SEI is stable. Such durable SEI layer can enable an effective inhibition of dendritic Li growth and significant improvement of CE. By contrast, the cells using C-Ely show a considerable increase in the diameters of the semicircles (Figure S10b), which indicates the SEI layers formed from these electrolytes are not stable and continuously accumulate, causing the increase of impedance and polarization during cycling.

Figure 6. The electrochemical performance of the Li-S batteries using the C-Ely and PSD-90Ely at a rate of 1 C (1 C = 1672 mA g-1), respectively.


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A high-performance Li-S battery has been achieved using a stable Li metal anode enabled by the PSD-90-Ely. Sulfur cathode materials were prepared by thermally impregnating S (70 wt%) into Ketjen Black (KB) (30 wt%). As shown in Figure 6, the cells using PSD-90-Ely show a higher initial capacity of 1427 mA h g-1 than the cells using C-Ely (1182 mA h g-1), which can be ascribed to electrochemically active PSD-90 boosting the capacity of the cells (Figure S11). At the tenth cycle, the capacity drops to 1219 mA h g-1 as the PSD-90 is consumed to form the stable SEI layer. Then, the cells show relatively steady cycling performance and retain the capacity of 817 mA h g-1 with the CE of ~99.9% after 1000 cycles, corresponding to a low capacity decay (0.033% per cycle). By contrast, the cells using C-Ely show a very fast capacity decay with a drop from initial capacity to 350 mA h g-1 after 100 cycles. The superior cycling performance using PSD-90-Ely consolidates that PSD-90 as the additive can effectively protect the Li metal anode owing to the formation of flexible and robust hybrid SEI layer, and enable the Li-S batteries with stable cycling stability.

Conclusion In summary, we report the flexible and robust hybrid SEI layer fabricated through chemical/electrochemical decomposition of PSD to self-form a hybrid of aromatic-based organic components (Li organosulfides/organopolysulfides) and inorganic Li salts (Li2S/Li2S2) on the surface of Li. The aromatic-based organic components with planar backbone conformation and π-π interaction in the SEI layer can improve the toughness and flexibility. The co-existence of Li organosulfides/organopolysulfides and Li2S/Li2S2 in the SEI layer promotes compact and uniform Li deposition with the effective suppression of dendritic Li growth, and significantly improved cycling performance of Li deposition/dissolution. A Li-S battery based on this


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approach shows excellent cycling stability and slow capacity decay. This work provides a promising route to tackle the issues associated with Li metal anodes and brings promise for the practical application of high-energy-density Li metal batteries. Experimental Methods Materials and Chemicals. Sulfur (≥ 99.5%) was obtained from Sigma-Aldrich. 1, 3diisopropenylbenzene (DIB) (> 97%) was purchased from Tokyo Chemical Industry Co., Ltd.. The solvents and Li salts for preparing electrolytes were obtained from BASF. Ketjen Black (KB) EC60JD was obtained from AkzoNobel. General procedure for the preparation of PSD. Sulfur was added to a glass vial and heated to 185 oC until complete melting. DIB was then added to the liquid sulfur. The resulting mixture was stirred at 185 oC for 8–10 minutes and cooled to room temperature to form a red solid (PSD). (a) Preparation of PSD-90 (90 wt% S). PSD-90 was prepared by following the general procedure with 4.50 g sulfur (17.6 mmol) and 0.50 g DIB (3.16 mmol) (yield: 4.95 g). (b) Preparation of PSD-70 (70 wt% S). PSD-70 was prepared by following the general procedure with 3.50 g sulfur (13.7 mmol) and 1.50 g DIB (9.48 mmol) (yield: 4.90 g). (c) Preparation of PSD-50 (50 wt% S). PSD-50 was prepared by following the general procedure with 2.50 g sulfur (9.77 mmol) and 2.50 g DIB (15.8 mmol) (yield: 4.84 g). Electrochemical measurements. Electrochemical performance was performed using a Land tester system at room temperature. The cycling stability of Li deposition/dissolution was studied using an asymmetric coin cell (Li|stainless steel). The EIS measurements were carried out using a Solartron ModuLab with an amplitude of 5 mV over a frequency range of 100 kHz to 0.1 Hz. 70 wt% S and 30 wt% KB were sealed and heated at 160 oC for 10 h to get sulfur cathode materials (S/KB). The S/KB was mixed with Super C and poly(vinylidene fluoride) (PVDF) with


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a mass ratio of 80 : 10 : 10. N-methyl-2-pyrrolidone (NMP) was added to form a slurry that was then blade cast onto carbon-coated aluminum foil and dried at 50 oC overnight under vacuum. The sulfur cathode was assembled in a CR2016 coin cell. The electrolyte mixture was 1 M LiTFSI and 4 wt% LiNO3 in DOL/DME (V/V = 1) with/without adding PSD-90. The separator was a Celgard 2325 membrane. The sulfur-free cathode was prepared by replacing S/KB composite with KB in the slurry. Characterization. Morphological studies were conducted using a Nano630 FE-SEM scanning electron microscopy. XPS measurements were performed on a Kratos XSAM800 Ultra spectrometer. FT-IR data were collected on a Bruker Vertex V70 spectrometer.AFM images and mechanical properties of SEI layers are investigated on a Digital Instrument Multimode scanning probe microscope under an inert atmosphere.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Molecular conformation of aromatic-based Li organosulfide, SEM images, FT-IR spectra, reduced modulus of SEI layers, optical profilometry data, EIS data, charge-discharge profiles of Li-S batteries.

Acknowledgements The authors acknowledge financial support from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-EE0007795. We also thank the National Science Foundation (Grant No. CMMI-1435766) for supporting the AFM work.


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ACS Nano

Self-Formed Hybrid Interphase Layer on Lithium Metal for High-Performance LithiumSulfur Batteries Guoxing Li1, Qingquan Huang1, Xin He2, Yue Gao1, Daiwei Wang1, Seong H. Kim2, Donghai Wang1* 1

Department of Mechanical and Nuclear Engineering, The Pennsylvania State University,

University Park, Pennsylvania 16802, USA. 2

Department of Chemical Engineering, The Pennsylvania State University, University Park,

Pennsylvania 16802, USA. E-mail: [email protected]


Self-Formed Hybrid Interphase Layer on Lithium Metal for High

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